tiedekunta
Faculty of Science and Forestry
HEIGHT GROWTH AND SURVIVAL OF TWO SILVER BIRCH (Betula pendula Roth.) CONTAINER SEEDLING TYPES
Emmi Lehtinen
MASTER’S THESIS IN FOREST SCIENCE TRANSFOR-M- PROGRAM SPECIALIZATION IN FOREST MANAGEMENT AND ECOSYSTEMS
JOENSUU 2018
CONTENTS
Abstract ... 3
1 INTRODUCTION ... 6
1.1 Background ... 6
1.2 Aim of this research ... 10
2 MATERIALS AND METHODS ... 11
2.1 The study material ... 11
2.2 Experiment I: Autumn and spring planting dates ... 13
2.3 Experiment II: Planting date of freezer stored seedlings ... 15
2.4 The statistical data analysis ... 17
3 RESULTS ... 17
3.1 Experiment I: the morphology test ... 17
3.2 Experiment I: the main test ... 18
3.3 Experiment I: the rooting test ... 20
3.4 Experiment II: the freezer storage duration test ... 20
4 DISCUSSION ... 22
4.1 Experiment I: Autumn and spring planting dates ... 22
4.2 Experiment II: Planting date of freezer stored seedlings ... 27
4.3 Concluding remarks ... 28
References ... 29
Attachments ... 32
Lehtinen, Emmi. 2018. “Height growth and survival of two silver birch (Betula pendula Roth.) container seedling types”. University of Eastern Finland, Faculty of Science and Forestry, School of Forest Sciences, Master’s thesis in Forest Science, specialization in forest management and ecosystems. 35 pages.
Abstract
The interest for smaller silver birch (Betula pendula Roth.) container seedlings has increased in forestry. As a result of more efficient seedling production, unit prices decrease, which is an important consideration. In addition, both machine planting or planting with a tube can only be done using relatively small seedlings.
In this study, silver birch seedlings were planted and observed to determine if different container types (PL25 & PL81F) and planting dates affected their height growth and survival.
In addition, their rooting characteristics were inspected. Morphological features were measured before planting. PL25 is a usual container type for silver birch seedlings in Finland, whereas the use of PL81Fs is increasing, but there is little recorded data on it with this particular species and planting time window. The test site was a nursery field in Suonenjoki, Finland. There were seven planting dates, three in the late summer and autumn of 2014, and four in the spring and early summer of 2015. Seedlings’ root egress was determined three weeks after each planting date.
Based on this study, the seedling type did not have a significant effect on the total height growth or survival during the 3-year monitoring period. However, due to the initial differences in planting height, the PL25- seedlings were on average 50 cm taller in the end of the 3-year monitoring period.
Also, the appropriate planting time window for freeze-stored PL81F-seedlings was determined in a separate test with different storage durations and planting dates. The later the planting, the more time the seedlings spend in freezing storage, and the remaining growing season after planting may be insufficient. There were seven planting dates for this test as well, from mid- May to mid-August, 2015. The seedlings were taken out to thaw a week before each planting date. Seedlings’ root egress was determined in October 2015, from all planting dates at the same time.
For the freeze-stored PL81F-seedlings, a safe planting window is from the start of the growing season to mid-June at the very latest. The root egress decreased by 43 % between the first (May 12th) and the second (May 25th) planting date, which is a dramatic change. The downward trend is more gradual after that, with virtually no new roots detected after the final planting session.
This research caters to the need for information on the PL81F- silver birch seedlings’ success, there is still quite little research on the matter. Since their growth is as good or better as compared to the PL25s, their use is encouraged. However, the use of PL25- silver birch seedlings is still the best option in the most nutrient-rich sites due to their planting height.
Further research is still needed for the growth and success of PL81F- seedlings in long term field tests and on forest sites.
Keywords: silver birch, Betula pendula, container seedling, height growth, PL81F, PL25, freezing storage tolerance, rooting capacity, root egress, morphology
Lehtinen, Emmi. 2018. Pituuskasvu ja taimien elossapysyvyys rauduskoivun (Betula pendula Roth.) taimilla kahdella eri paakkukoolla. Itä-Suomen yliopisto, metsätieteiden osasto, metsätieteen pro gradu, erikoistumisala metsien hoito ja metsäekosysteemit. 35 sivua.
Tiivistelmä
Kiinnostus käyttää pienikokoista rauduskoivun (Betula pendula Roth.) paakkutyyppiä on lisääntynyt sekä taimitarhoilla, taimien istuttajilla, että metsänomistajien keskuudessa.
Tehokkaampi taimikasvatus alentaa taimi- ja istutuskustannuksia. Putkella tai koneella istuttaminen on lisäksi helpompaa pieniä taimia käyttäen.
Tässä tutkimuksessa tutkittiin kahden eri paakkutyypin (PL25 & PL81F) ja eri istutusaikojen vaikutusta rauduskoivun taimien pituuskasvuun, juurtumiseen ja elävyyteen.
Morfologiatunnukset otettiin ennen istutusta. PL25 ja vastaavat paakkukoot ovat yleisesti käytössä rauduskoivun istutuksissa Suomessa, kun taas PL81F ja muut pienemmät paakkukoot ovat vasta yleistymässä. Tässä työssä tutkittu koe perustettiin taimitarhapellolle Luonnonvarakeskuksen Suonenjoen tutkimusasemalle vuonna 2014. Istutusaikakokeessa oli seitsemän ajankohtaa, kolme loppukesällä ja syksyllä 2014, neljä muuta keväällä ja alkukesästä 2015.
Paakkutyyppi ei vaikuttanut merkittävästi taimien kokonaispituuskasvuun tai elävyyteen kokeen kolmevuotisen tarkastelujakson aikana. Kuitenkin istutuspituuden eroista johtuen PL25- koivut olivat vielä kolmen kasvukauden kuluttua keskimäärin 50 cm pidempiä kuin PL81F- koivut.
Istutusaikakokeen lisäksi PL81F- koivuilla tehtiin pakkasvarastoinnin kestokoe. Näin pyrittiin selvittämään sopiva istutusaikaikkuna pakkasvarastoiduille taimille. Mitä myöhempi istutusaika, sitä pidempään taimet siis olivat pakkasvarastossa. Tällöin myöhemmän istutuksen jälkeisen kasvukauden pituus voi olla riittämätön taimien kasvuun ja selviytymiseen. Tässä kokeessa oli myös seitsemän istutusajankohtaa, toukokuun puolivälistä elokuun puoliväliin 2015. Taimet otettiin sulamaan viikkoa ennen kutakin istutusajankohtaa.
Juurtumistaimimittaukset tehtiin lokakuussa 2015, kaikkien istutuskertojen juuritaimille yhtä aikaa.
Pakkasvarastoidut koivun PL81F- taimet tulee tulosten perusteella istuttaa viimeistään kesäkuun puolivälissä, mutta mitä aikaisemmin, sen parempi. Näiden taimien juurtuminen väheni jokaisella istutuskerralla toukokuusta elokuuhun.
Koska PL81- koivuntaimet kasvavat yhtä hyvin tai keskimäärin jopa paremmin kuin isommat PL25- taimet, niiden käyttöä voitaneen lisätä. Toisaalta PL25- taimien käyttö ravinteikkaimmilla kasvupaikoilla on järkevää tästä huolimatta. Lisää tutkimustietoa PL81- koivuntaimilla tarvitaan nimenomaan käytännön kenttäkokeista ja maastomenestyksestä.
Avainsanat: rauduskoivu, Betula pendula, paakkutaimi, pituuskasvu, PL81F, PL25, pakkasvarastonkestävyys, juurtuminen, morfologia
FOREWORD
The data used in this research was collected by the Natural Resources Institute Finland (Luke) in Suonenjoki research station, Finland. The research process was planned and coordinated by special scientist, Dr. Jaana Luoranen (Agr.and For.), who is also the main instructor of this thesis. Thank you to Jaana for her wisdom, guidance, and patience. Dr. Heli Peltola (Agr.and For.), professor in silvicultural sciences in the University of Eastern Finland, is the second instructor, whom I also thank for her comments and guidance.
Thank you to the coordinators of the TRANSFOR-M- program, Dr. Marjoriitta Möttönen (University of Eastern Finland) and Dr. Andreas Hamann (University of Alberta). I am grateful for the lessons and experiences I had while studying in Canada, and hope that the program has a great future.
Thank you to my family for their support, and to my friends from the class K-30.
In Parkano, on April 29th, 2018 Emmi Lehtinen
1 INTRODUCTION
1.1 Background
The silver birch (Betula pendula Roth.) can be found in many kinds of sites differing in soil type, nutrient content and water availability. However, it generally thrives in nutritious, fine moraine-dominated mineral soils, or in mixed soils which are rich in organic matter. The soil needs to be sufficiently aerated; dense or waterlogged soils are not ideal for silver birch (Niemistö et al., 2008a, Viherä-Aarnio et al., 2008a).
Until recently, the practice of planting birch in the dormant stage during a few weeks both in May and in September was common (Luoranen et al. 2003). Nowadays, it is also planted in June, July and August – or even later in some studies (Luoranen et al., 2003; Nilsson et al., 2010). This has been done using seedlings that have been sown on the spring of their planting year, making them only a few months old at the time of the planting, and clearly smaller than the previously preferred seedlings. These so called summer birches are typically planted in leaf, in active growth stage. In Finland, the optimal time window for birch planting is considered to be from the summer solstice to the end of August, peaking in the month of July (Luoranen et al., 2003).
Silver birch seedling production peaked in the 1990’s with ca. 18 million seedlings delivered annually. At the time, they were largely planted on former agricultural land. Since then, the current demand of ca. 4.5 million seedlings per year has been somewhat steady for years now (Finnish Forest Research Institute, 2014). In 2013, birch was planted on 2 587 hectares, making up 3.4 % of the entire planting stock (Finnish Forest Research Institute, 2014). Prior to the mid- 1990’s, the seedlings produced were mainly bare root seedlings (Rikala, 1996). Since then, the demand for container seedlings increased immensely, and nowadays virtually every seedling planted in Finland is a container seedling. In 2012, only 0.09 % of silver birches delivered in Finland were bare root seedlings (Finnish Forest Research Institute, 2014).
To ensure the quality and the growth potential of the silver birch seedlings, nurseries use tested seed sources. The main qualities that birch breeding aims to achieve are better growth, smaller branch thickness, better survival and little stem narrowing (Viherä-Aarnio et al., 2008a, b).
Once the seeds are sown in nurseries, it usually takes them no longer than two weeks to grow to a stage where they can be pricked out – although seed coating is nowadays more common.
After this, they are left to grow. They can either be planted during the same year or freeze stored for next spring (Viherä-Aarnio et al., 2008b).
With birch planting, soil preparation by mounding is strongly recommended: mounding creates small rises in which the seedlings are planted. Appropriate soils are fine and sandy tills, fine sand and coarse silt. Soils with over 20 % organic matter are also naturally suitable, but there is a higher risk for the trees to develop thick branches. The recommended planting density is ca. 1600 seedlings per hectare. Soil nutrient level needs to be at least Myrtillus- type (Niemistö et al., 2008a).
Natural regeneration is also an option for establishing a new generation of silver birches. It is cheaper, but there are more insecurity factors involved then. Even though silver birch releases plenty of seeds every year, the quality fluctuates. Less valuable downy birch (Betula pubescens Ehrh.) may also release much seeds on the same site. Therefore, natural regeneration is the riskier option. Surplus seedlings will most likely develop naturally in the stand nonetheless, which is why planting is usually the best option for silver birch (Niemistö et al., 2008a).
Typically, the rotation period for an all-birch site is 60-70 years, with two or three thinnings done before the clearcut (Niemistö et al., 2008b, Äijälä et al., 2014).
Birch seedlings will encounter many biotic and abiotic risks in the boreal conditions. The risk for fungal damages increases with improper storage or on-site handling. The thawing stage before planting is very risky in this regard – the root plug cannot be frozen when planted, but once the seedlings are thawed, the plugs need to be watered generously and kept in shade if they are not planted immediately. Even then the on-site storage duration should not be longer than three days, and a week is an absolute maximum (Niemistö et al., 2008a; Rikala, 2002).
Generally, a good rule of thumb is that water should be dripping out of the root plug when squeezed before planting. In addition to watering, proper aeration is crucial: the storage boxes or bags need to be opened while they are left to thaw. The more time the seedlings are stored on-site, waiting to be planted, the more likely it is that they dry out or develop mold (Niemistö et al., 2008a; Perälä et al., 1999; Viherä-Aarnio et al., 2008b).
In the planting process, tears, wounds, and breaks in the seedling can occur (Renou et al., 2007).
The silver birch seedlings are fragile and lanky, even if their early-stage elasticity protects them.
The seedlings planted in full leaf must be handled with special care, since the leaves are easily ripped or damaged. Transportation and handling in general must be done carefully, seedlings
in active growth stage are the most vulnerable to damages at this part of the process (Perälä et al., 1999).
Two disease-inducing agents may cause a disease and related damages more commonly to younger birches, one for the leaves, and the other for the stem. Birch rust, caused by Melampsoridium betulinum, is a leaf-damaging fungal disease that does not cause damage to the wood material. It is, however, an extra stressor for the birch. It may be found both in nurseries and on site in all stages of the birches’ lifecycle (Ericson & Burdon, 2008). There is also a genus of microbes, Phytophthora spp., which causes black spots or areas on the seedling stem and may induce leaf yellowing and abscission, or top dying. It has been a problem in nurseries since the 1990’s, but it can be controlled with correct spacing, watering, and aeration (Lilja et al. 2010). Neither one of the aforementioned pathogens cause significant economic damage.
Herbivory is the most prominent one out of the biotic risks – moose (Alces alces) grazing damages being the most probable form of it in Finland. Field vole (Microtus agrestis) and blue hare (Lepus timidus) are also a great threat to freshly planted seedlings, no matter the tree species. Damages caused by these species can be decreased by keeping herbs and grasses to a minimum and maintaining a proper density of seedlings (Viherä-Aarnio et al., 2008a; Niemistö et al., 2008a). Fortunately, not many insects cause significant damage for birch seedlings. In addition, suppressing insect folivory is usually either futile or extremely difficult (Heikkilä et al., 2008).
Longer drought periods lasting over two weeks may weaken the planting results, since it is vital for the seedlings to have access to sufficient water immediately after planting (Niemistö et al, 2008a). Seedlings in active growth cannot handle drought and physical stress as well as in their dormant stage during springtime (Luoranen et al., 1999). Colder periods in late spring may cause frost damages. The risk for frost damages is the greatest when the leaf buds have just opened (Viherä-Aarnio et al., 2008a).
In the northern boreal zone, due to climate change (elevated atmospheric temperature and CO2), the growth of trees will most likely increase – the growing season will be longer and warmer (Huttunen et al., 2006). Currently the growing season for birches is from early or mid-May well into August (Viherä-Aarnio et al., 2008a). Studies show that deciduous trees in particular can benefit from climate change (Kellomäki, 2016). However, the warmer the climate, the more
time and reproduction possibilities there are also for herbivores and fungi. Predicting the final results is therefore difficult. Especially Norway spruce (Picea abies (L.)) monocultures are expected to suffer under climate warming and associated drought especially in the southern boreal conditions on soil with low water holding capacity (Kellomäki, 2016). Promoting silver birch cultivation or mixing it with Norway spruce would increase biodiversity and resilience (Viherä-Aarnio et al., 2008a; Heikkilä et al., 2008).
Usually, summertime planting will be less costly, since the seedlings used can be smaller than the generally used seedlings. Smaller seedlings are also easier to handle and can be planted with a planting tube (Luoranen et al., 1999). Mechanization of planting has already started, and smaller seedlings are easier to plant with a machine – it will be a great advantage as time passes and these machines continue to develop. Summertime planting evens out the peaks in labor needed in forestry, thus speeding up the entire forest regeneration process (Luoranen et al., 1999). If the smaller seedlings are proven to be equally or more successful than the larger ones in respect of their growth and survival, it would be beneficial for the seedling-producing industry. The seedlings would be cheaper to raise, occupying less space and taking less time to grow. Their use is also profitable for those who organize and implement planting, and for forest owners as well, due to reduced planting costs.
In addition, the best root egress and height growth is achieved with summertime planting (Luoranen et al., 2003; Luoranen, 2018). It takes roughly +10 ℃ soil temperatures before tree roots start growing in the spring (Rikala, 2002). If the soil is frosty and the spring is cold, the onset of growth can be delayed until the end of May, even in Southern Finland. Birches start developing leaves before the root plug is anchored to the soil, so the leaves act as somewhat of a sail that may catch the wind (Rikala, 2002). Sudden bursts of wind can therefore break the seedling, or tip it over, if the roots have not grown yet. Therefore, the sooner the seedlings starts anchoring to the soil after planting, the better.
It is important to determine a safe planting time window for seedlings of different size. For example, with freezer storage of seedlings, the risks of outdoor storage are avoided. In addition, seedling delivery in the spring is more flexible (Lilja et al., 2010). Based on previous research and instructions given by researchers, freeze-stored summer birches need to be planted during May (Luoranen et al., 2003). The remaining growing season after that is considered insufficient for these seedlings.
1.2 Aim of this research
The goal of this study is to further inspect the effect of extended planting season on a new type of birch seedling versus the older, larger one. This is done by monitoring the rooting and growth of silver birch seedlings of two different sizes planted in seven different times. Hypothetically, the seedlings planted in September and October will be less successful than the ones planted the following spring – the best success is expected from the seedlings planted in the summer.
Special attention is paid to leafless birch seedlings (the PL81Fs): earlier, only larger leafless seedlings (PL25s) have been planted in the springtime. This way the effect of leaf budding stage on seedling survival is studied. Sections on both morphological and rooting properties will be included.
Another monitoring viewpoint is to look into the appropriate planting window for the PL81F- seedlings after freeze storage: this is measured through the survival rate of the seedlings once planted. The later the planting, the more time the seedlings spend in freezing storage, and the remaining growing season after planting may be insufficient. There were seven planting dates for this test as well, from mid-May to mid-August, 2015. Similarly to the planting date test, rooting of said freeze-stored seedlings is also studied to get a better understanding of the planting date’s effect on growth.
Ultimately, this study aims to find out whether or not current planting practices for birch can be improved by switching to a smaller seedling size (experiment I), while optimizing the planting windows for smaller seedlings (experiment II).
2 MATERIALS AND METHODS
2.1 The study material
There were two types of container-grown seedlings used in experiment I, the PL25 and the PL81F. The trays’ product name is Plantek, hence the abbreviation “PL”, and the manufacturing company is called BCC. The PL25- plugs are the larger ones, and the seedlings in PL25- trays are therefore grown larger than in PL81F- trays. The mean planting height for the PL25- seedlings was 58.8 cm, and 25.7 cm for the PL81Fs. The dimensions of the growing units can be found in Table 1.
Table 1. Dimensions of the two types of growing trays used in experiment I (information from Rikala (2012)).
Properties PL25 PL81F
Container volume (cm³) 380 85
Cell density (pcs/m²) 156 549
Cells/unit (pcs/tray) 5 x 5 9 x 9
Tray size, width x length x height, (mm)
400 x 400 x 90 385 x 385 x 73 Single cell size (mm) 80 x 80 x 90 41 x 41 x 73
The seedlings were raised in a nursery like normal market seedlings. The PL25- seedlings were sown on May 21th, 2014, and pricked out June 2nd-6th, 2015. The PL25s were transferred outside and set more sparsely on July 9th, 2014, and also treated with Aliette, a fungicide that is used for preventing Phytophthora spp. damages. The PL81Fs were sown on June 20th, 2014, and pricked out on July 3rd of the same year. The PL81Fs were set to grow more sparsely on July 22th, 2014, and simultaneously treated with Aliette. On August 1st, the PL81F- seedlings were transferred outside. The seedlings were in the outdoor growing area until each planting date in autumn 2014. In late October, the PL81F- seedlings were packed in cardboard boxes and the PL25s in plastic bags and then transferred to a freezing storage (-2 – -3 °C) for planting dates in the following spring and summer.
The seedlings for planting date IV were then taken out to thaw on May 4th, 2015, and seedlings for time V on April 22nd, 2015. They needed to be leafless for time IV, and budding for time V.
They were kept in +5 °C for a week and transferred to an unheated greenhouse on April 28th, 2015. Seedlings of both types were then transferred to seedling container trays, and watered until planting. On May 7th the seedlings had already grown small leaves, and they were moved
outside on the field – the goal was to have small leaves or burst buds on the seedlings before planting. For time VI, with open leaf buds, they were kept in a freezer storage until May 26th, the leafless PL81Fs needed for time VII were taken to thaw on May 2nd, 2015.
Also, the rooting capacity and morphology of the seedlings were measured in a separate test.
66 seedlings (30 for the main test, 18 for rooting tests, 18 for morphology tests) were needed per both seedling types for each planting date. The test consisted of three parts: growth, rooting capacity, and morphology. Freezing storage survival test (experiment II) was carried out with the smaller seedlings.
The seedlings were measured and checked for damages every autumn during the three years of growth and monitoring for the planting date test – the seedlings of experiment I were checked in the fall of 2015 and finally in the fall of 2016. In every measurement session, the seedlings were categorized in different classes based on their appearance (Table 2.). The same categories were used both in experiment I and experiment II. Based on these classes and the overall condition of the seedling, they were categorized binomially into two groups: dead/in poor condition = 0, and alive/in good condition = 1.
Table 2. Seedling evaluation chart for measuring sessions. These classes were used to evaluate the appearance of the seedling to determine their condition. An ideal seedling would be a one- topped, healthy seedling with no damaging agent code.
Amount/condition of tops
Seedling condition
Damaging agent
1 One top Healthy Drought
2 Two tops Weak Frost
3 Dry terminal bud Withering Hare
4 Dry at the top Dead Other
(specified) 5 The whole seedling is
dry
- Unknown
6 The top is broken - Lawnmower
2.2 Experiment I: Autumn and spring planting dates
The test site was a nursery field, and the soil was quite sandy (Picture 1.). Planting date details are provided in Table 3.
Table 3. Seedling properties for the seven planting dates in experiment I.
Time Seedling types Leaf type Planting date
I PL25 & PL81F Full 14.8.2014
II PL25 & PL81F Yellowing 11.9.2014 III PL25 & PL81F Mainly fallen 14.10.2014 IV PL25 & PL81F Leafless 12.5.2015
V PL25 & PL81F Buds burst 12.5.2015 VI PL25 & PL81F Open leafs 9.6.2015 VII Just PL81Fs (rooting) Leafless 9.6.2015
The PL25s were planted with a hoe, whereas the PL81Fs were planted with a planting tube.
The seedlings are planted 1.5 m apart from each other, in ca. 3 cm depth. The planting site was fertilized right after each planting date, otherwise they were left alone. The N-P-K- rate was 12-4-13.3, with 0.15 % of boron. The dosage was 60 g/m². The only precaution taken was a fence surrounding the field, which was put up in the fall of 2014. Still, the fence was not put up early enough, and a few seedlings were somewhat damaged by hares.
Picture 1. Planting the PL25- birches (left) and the PL81F- birches (right). Pictures by: Pekka Voipio/Luke.
Rooting test seedlings were planted between the test seedlings (Picture 2.). The test structure was a split-plot design. Main plot was the planting date, and subplot the container type. There were 10 blocks in which three test seedlings were planted during each planting time. Times I-
III were planted in the fall of 2014, times IV-VII in the spring of 2015. Time VII was done with only the PL81Fs to determine their rooting compared to the time VI. Subplots were randomized to seedling types. Full design of the test area is provided in Attachment 1.
Immediately after planting, the following measurements were taken: seedling’s height (1 cm accuracy), and seedlings diameter at 5 cm height (0.1 mm accuracy). These are the parameters that are measured after the growth has ended every fall during the next three years.
Picture 2. A view to the test area. The spaces between the seedling rows were mowed if necessary. Picture by: Jaana Luoranen/Luke
In addition to the main test, root growth and morphology of the seedlings were inspected. 18 seedlings for both rooting and morphology tests are selected randomly from the same trays as the seedlings for the main test. So, 36 seedlings in total need to be reserved for this purpose with each planting date. These seedlings are taken before the actual main test seedlings. From each of the three PL25-trays six seedlings are taken randomly to gather the 18 seedlings. The stems of the rooting test seedlings as well as the outgrown roots are dried and weighted. Leaves, if there were any, were dried and weighed.
Rooting test seedlings are planted between the main test seedlings, two seedlings per each seedling type and each planting session for each block. The rooting test seedlings for this part
of the study were lifted three weeks after planting to see the amount and length of new roots.
The number of new roots as well as their combined length were measured.
Information gathered from morphology seedlings include height, diameter, the amount of resin papilli, and the number of branches. Dry mass of roots, stem, and leaves – if there are any - are weighed with 0,001 g accuracy after 48 hours in a 60 °C- oven. In June, morphology samples are also taken from time VII. The purpose of these morphology seedling
measurements is to make sure that seedlings of each seedling type are as similar as possible between planting times.
2.3 Experiment II: Planting date of freezer stored seedlings
If seedlings are kept in a freezer storage far into spring, the remaining growing season may be insufficient for them to form new roots, grow, and prepare for overwintering. Seedlings also lose some of their carbohydrate reserve during freezing storage, and the longer it lasts, the more they lose. Therefore, a section concerning a safe planting time window was included for the freeze-stored PL81F- seedlings. Storing them for too long is risky, especially because of their smaller size.
After the seedlings have been taken out from the freezer storage, they need to be planted within a week, and the root plug needs to be properly thawed. In this study, seedlings of different leaf budding stages were tested to find the optimal stage of planting.
Planting dates for experiment II are provided in the following table (Table 4.). The structure of the test area was similar to the planting date test’s structure (Picture 3.). Since the test only concerned the PL81F-seedlings, the subplot of container type does not apply.
Table 4. Planting dates of the PL81F- seedlings for freezing storage duration test.
Time Planting date
I 12.5.2015
II 25.5.2015 III 9.6.2015 IV 23.6.2015
V 7.7.2015
VI 28.7.2015 VII 18.8.2015
The rooting of these freeze-stored seedlings was also studied, similarly to the main test: the number of new roots and their combined length were measured. These rooting test seedlings were lifted on the same day, October 7th, 2015 – in contrast to experiment I, where the rooting test seedlings were lifted three weeks after each planting date.
Picture 3. Experiment II- test area in August 5th, 2015. Picture by: Jaana Luoranen/Luke
Seedlings’ measurements were taken with the same accuracy as in the planting date test:
seedling height with 1 cm accuracy, diameter from 5 cm height with 0.1 mm accuracy. The seedlings were planted 1.5 m apart from each other, with the rooting test seedlings planted between two test seedlings again. There were nine blocks and seven planting dates. Unlike in the planting date test, where the rooting test seedlings were lifted three weeks after each planting time, these rooting test seedlings were lifted at the same time on October 7th, 2015, after the leaves had yellowed and the growth had ended. Blueprint of the test area is provided in Attachment 2.
2.4 The statistical data analysis
Results were analyzed using the statistical analysis program IBM SPSS Statistics 23. The analysis method used was mixed linear model. Generalized linear mixed model was only used to analyze the condition of the seedlings, since it was a binomial variable.
For the planting date test, fixed factors were the seedling type (PL25 & PL81F) and the planting time number (I-VI(I)). Their interaction was also included. Random factors were: seedling tray;
section; seedling type(planting date); seedling type(planting date(section)). Dependent variables were height and diameter growth on each year of inspection and in total, number and weight of new roots, and overall survival.
For the freezing storage duration test, only the planting time number (I-VII) was used as a fixed factor, since only the PL81Fs were of interest in this test. Random factors were therefore only section; planting date(section). Dependent variables were growth in height and diameter on planting year and the following year. number and weight of new roots, weight of the stem, and overall survival.
The results of both experiments were interpreted on the 95 % significance level. So, if the p- value was 0.05 or lower, the result is considered statistically significant.
3 RESULTS
3.1 Experiment I: the morphology test
The stem dry weight was statistically dependent on both the planting date and on the seedling type. In total, planting date statistically affected the stem dry mass, which means that there were differing seedlings with each planting time. Between planting times, however, all times statistically differed from each other (p-value > 0.05) except for the first time.
Similarly to the stem dry weight, the dry weight of the roots was statistically dependent on the planting date, and also on the seedling type. Individually, all planting times differed from each other in terms of seedling dry weight (p-value > 0.05). Both the height and diameter of morphology seedlings follows this pattern, too: seedling type and planting date are statistically
significant in explaining the overall variation, but individually the significance of planting dates differ from each other. Therefore, the seedlings were somewhat different between planting times concerning these variables.
The amount of resin papilli was not statistically dependent on neither the planting date nor the seedling type (p-value for both < 1). This also applied to the leaf dry mass. The number of branches on the seedlings was statistically dependent on the planting date (p-value < 0.018) but not on the seedling type (p-value < 0.129). Therefore, the seedling material per each seedling type was similar between planting times concerning the resin papilli, leaf dry mass, and number of branches. Full results for morphology test seedlings can be found in attachment 3.
3.2 Experiment I: the main test
The absolute height growth was about the same with both seedling types for the first two years of monitoring, the differences were statistically non-significant. The p-value was <0.119 for the 1st year’s growth, and <0.731 for the 2nd year. On the first growing season after planting, the height growth of the seedlings of planting time III was the only one statistically differing from the others. On the second growing season, only the height growth of the seedlings of planting time IV were statistically different from the others (p-value < 0.085). On the third growing season, the height growth differences were statistically dependent on both the seedling type and planting time. At that time, all planting times except for time I statistically differed from each other. Differences in total height growth were also statistically significant between seedling types, as the PL25s were taller on every planting date (Picture 4.). The PL25s were up to 50 cm taller than the simultaneously planted PL81Fs after three growing seasons. However, on average, the PL81Fs grew better: during the 3-year period, the PL25s grew on average 289 cm, whereas the PL81Fs grew 295 cm.
Figure 1. Growth of test seedlings from experiment I. Mean error bars included for each annual growth bar.
The best overall growth was seen on PL25-seedlings planted in mid-August, on which time the best growth out of the PL81F- seedlings was also documented. Best growth was documented on the second year of monitoring (green parts of the bars, Figure 1).
Planting date was statistically significant in explaining the differences in height growth throughout the monitoring period. Planting date was also statistically significant in explaining the differences in diameter growth for the first two years, diameter width was no longer monitored on the third year. Seedling type explained the variation of diameter growth significantly on the first year, but not any longer on the second year of monitoring.
Throughout monitoring, the planting date was statistically significant in explaining the total height growth. This was the case for each planting time individually. The seedling type was not statistically significant in explaining the total height growth.
Neither planting date nor seedling type significantly explained the variation in seedling survival. This was the case in all years of monitoring. By the measurements of 2017, only eight seedlings were dead, and they were distributed evenly across seedling types and planting times.
Full results of experiment I are provided in attachment 3.
3.3 Experiment I: the rooting test
The number and length of new roots egressing from the plug significantly depended on the planting date. However, the seedling type’s effect on the number of new roots was insignificant, but significant for the length of new roots (Figure 2.). The overall rooting clearly decreased after the first planting date of Aug.14th, 2014 - there was no difference between the seedling types, however. Again, on planting time VI with budding leaves on June 9th, the number of new roots grew nearly 20-fold as compared to time V.
Figure 2. The number of new roots emerging from the root plug, as dependent of container type and planting date.
3.4 Experiment II: the freezer storage duration test
As stated, the storage endurance test included only the PL81Fs, in order to confirm their appropriate freeze storage duration window. Between the first (May 12th, 2015) and the second (May 25th, 2015) planting dates, there were 43 % more roots formed after the first time compared to the second (Figure 3). The decrease slows after the second time, but the root formation rate shows a clear, gradual decline. Most of the seedlings were alive the fall after all
the plantings were done. In the fall of 2016, however, the condition of the seedlings did statistically depend on the planting date. After the fourth planting date, the survival rate more than halved (Figure 3).
Figure 3. The mean dry mass of new roots egressing from the root plugs (upper), and the survival rate of the freezer-stored seedlings (lower) according to the planting date.
The planting date had a statistically significant effect on the dry mass of new roots emerging from the root plug. The weight of the new roots also statistically depended on the planting date.
Similarly, the stem weight was statistically dependent on planting date.
Similarly, the height growth of the seedlings (Figure 4) have similar trends than in the survival rate graph. The growth of both height and diameter in 2015 and 2016 were statistically dependent on the planting date. The total height was also significantly dependent on the planting date. Planting height was random, but the growth of both 2015 and 2016 can be seen to decrease with later planting dates. From figures 3 and 4 it is clear to see that the seedlings of planting date VII were virtually all dead in 2016. Full results of experiment II are provided in attachment 4.
Figure 4. Height development of the freezing storage tolerance test seedlings.
4 DISCUSSION
4.1 Experiment I: Autumn and spring planting dates
The best growth for the PL81Fs was reached with seedlings planted in mid-August, which grew on average 103 cm in the summer of 2016 – and the PL25s were not too far from that with an average of 96 cm at the same time. Similar findings have come up in previous research. It seems that the best results over time are achieved by planting the seedlings during the summer, while there are photosynthesizing leaves on the seedlings. Luoranen et al. (2003) discovered that the
planting period for silver birch can be safely extended to at least July and early August – given that the site is appropriate for silver birch seedlings. The seedlings planted during the summer months grew better on the following growing seasons than the seedlings planted after mid- August in the fall, or the ones planted in the following spring (Luoranen et al., 2003). Still, the traditional course of action, planting either in the spring or in the fall, is still valid and does not lead to higher mortality: the best growth was simply observed with summer planting.
On the seedlings of the fourth (leafless) and fifth (with leaves) planting time, the best growth out of the two was documented on the PL25s of the fifth time. There were no visible differences on the PL81Fs during the same time window. Therefore, the PL81Fs are not so sensitive to this matter, which can be considered as an advantage.
Labor distribution problems and mechanization of planting have also brought up the need for extending the season of conifer planting. Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) planting accounts for the clear majority of tree planting in Finland, since silver birch makes up to only 3 % of the annual planting stock (Finnish Forest Research Institute, 2014). In a study by Luoranen (2018), Scots pine and Norway spruce seedlings were planted on a time window similar to this study. Normally, the planting season for these seedlings is from May to early June for Scots pine, and from May to the end of September for Norway spruce. In the study of Luoranen (2018), the planting period was extended to mid- October for both species. After mid-September, rooting ceased for both Norway spruce and Scots pine. This led to poorer root egress also in the following spring. The differences in root growth between autumn and the following spring did not significantly affect the field performance. Consequently, conifer seedlings planted in late autumn may grow slower, but there were no significant increases in mortality (Luoranen 2018). The same result was also found in this research – the poorer initial rooting of autumn plantings did not affect the survival rate.
Regardless of the initial seedling size, the two different seedling types will face the same risks once they are planted on an actual forest site. For example, herbivory, fungi, and snow press can affect seedlings of both sizes. The planted seedlings, regardless of their size, theoretically have very similar risks in the timespan of this study. Therefore, even if the PL25s are taller after three growing seasons, they are still at risk for moose damages, for example. Weighing the pros and cons of both types in contrast to the costs is still the responsibility of the forest owner.
A few seedlings in this research were damaged by hares before there was a fence around the test area. The preferences of herbivores concerning the qualities of a given food plant is described using the term ‘palatability’. The more resin papilli there are on a birch seedling, the more likely the hare is to reject it. The same also applies to voles, but not as clearly as to hares (Heikkilä et al., 2008).
Renou et al. (2007) concluded that larger initial size led to better overall growth after five growing seasons. Their test took place on an Irish cutaway peatland, where mild climatic conditions prevail with an average annual temperature of +9 ℃. Using both Betula pendula and B. pubescens, and bare-root and container seedlings, they tested the survival, growth, attributes and form of the seedlings. Larger seedlings grew better no matter the species. As compared to this study, the growth was the same for both seedling sizes. However, the PL25- seedlings were still taller after three growing seasons, and in Renou et al. (2007), the larger seedlings were taller after even five growing seasons. Therefore, the question of when the differences might even out between the two seedling types remains unanswered. Future research should address to this dilemma, while making sure that the follow-up period is at least ten years.
One may ask why the PL25s continued to be the dominant type for so long, since the growth and success rates of PL81F- seedlings are satisfactory to say the least. To an extent, it is a matter of custom and familiarization. There is still a market for PL25s, for the larger size gives it a noticeable advantage in the most nutrient-rich sites: it is a stronger competitor against herbs and grasses that start to sprout very soon after a clear-cut. Taller seedlings still have this clear advantage when selections are made between these two seedling types.
Results in Luoranen et al. (2003, 2006a, b) were consistent with this study: the best growth and root egress was achieved with summer planting, the peak of height growth achieved in July and early August. Luoranen et al. (2006a) studied the success of hybrid aspen (Populus tremula x tremuloides) with seedlings that were grown in PL25- trays and planted when the height of seedlings was 20-25 cm. These seedlings were planted from July to September, and also the following May (Luoranen et al., 2006a). In Luoranen et al. (2006b), summer planting of Norway spruce seedlings was done on seven different times over the course of three years.
Seedlings planted in July and early-August had better root egress and height growth than in the seedlings planted after mid-August, which supports summer planting of Norway spruce (Luoranen et al., 2006b). In addition, later studies show that spruce seedlings planted after mid- September suffer from poor rooting, and the growth is slower in comparison to seedlings
planted earlier (Luoranen, 2018). This was also the case in this study – birch seedlings planted in mid-August grew the best no matter the seedling type.
In this study, the smaller size of the PL81F- seedlings was expected to be somewhat insignificant in the end, since birches grow rapidly once they have established their roots – given that the conditions are favorable. Therefore, whatever significance there is or is not to the initial seedling size would fade out in time. But, as stated, the PL81Fs were smaller after three growing seasons. Whether or not it matters depends on the forest owner, their goals, and the damage risks of the planting area.
Naturally, the seedling experiments must be founded on a proper site, i.e. mineral soil with adequate water flow and nutrient balance. In another relevant study, only 26 % of birches planted on sites without soil preparation were alive five years after planting. Best viability after five years, 67 %, was proven to be on mounded sites (Raulo & Rikala, 1981). The birch seedlings’ height growth in mounds was almost twice that of sites with unbroken soil. The crucial role of soil preparation was also seen in Karlsson (2002), where B.pendula and B.
pubescens seedlings were planted at only 5-10 cm tall. After three years, the tallest seedlings were found on sites with topsoil retained in the soil profile – in other words, on sites that were mounded (Karlsson, 2002). Concerning the matter of the seedlings’ size, Karlsson (2002) concluded that 5-10 cm tall seedlings are generally too small to achieve a satisfactory level of survival and growth – at best, the seedlings’ mean height after three growing seasons was 50- 70 cm. In comparison to this study, the seedlings in Karlsson (2002) were clearly smaller than PL81Fs at the planting moment. The survival rate was the same between the PL25s and the PL81Fs, but the height was significantly different after three growing seasons. One could therefore contemplate that the planting size predetermines future height for many years to come.
This would be somewhat of a hindrance, at least to the mechanization of planting, where the seedlings need to be very small.
The purpose of this research was to inspect the differences in the seedlings’ growth in different planting dates. Whenever this type of research is done, special attention needs to be addressed to correct spacing and overall spatial arrangement of the seedlings. Alternatively, timely stand tending measures can be done as one would on a normal seedling stand. Competition exists in reforestation sites, but in order to achieve control results of the seedlings’ success, competition is one of the factors that needs to be controlled. Arrangement that is too sparse or dense may skew the results greatly, especially with a fast-growing species such as the birches.
In this study, field performance was not dependent on the seedling type. The seedling type did, however, affect the morphology. This research focused on the container type, and there were no differences in survival rates between seedling types. Container size, i.e. the seedling type did affect morphology in evident ways, since seedlings grown in smaller containers are younger and smaller. Hence, they weigh less, for example. However, some morphological variables that should in theory be random in contrast to planting date were not: root and stem dry masses, number of branches and seedling height and diameter all significantly depended on the planting date as an entity, even if they did not individually in some cases. Aphalo & Rikala (2003) observed the field performance of silver birch seedlings grown at different spacings and in containers of different volume (190 cm³ and 300 cm³). It was discovered that stand density had a large effect on seedlings’ morphology, yet had little influence on their field performance.
Container size affected both the morphology and the field performance. However, the clearest correlation was found between field performance and mean dry weight of the stems – so, the sturdier the stem, the more likely it is to survive and thrive.
When it comes to the planting date, previous research supports the conclusion that silver birch seedlings planted during June, July and early August grow better in length. The best time window for planting is from mid-June to mid-August, the optimum time being the month of July (Luoranen et al., 1999; Luoranen et al., 2006a). One-year-old seedlings started growing roots fairly slowly in the spring following their planting. Faster root growth started after the leaves were fully developed in June (Rikala, 1996). This further implies that birch seedlings, no matter their developmental stage, get the best start when planted during midsummer. Root establishment is the key to survival, and summer planting provides the ideal conditions for it.
However, in this context it needs to be stated that seedlings must be planted on their correct developmental stage. For birches, the leaves and shoots have to be in their correct state considering the time of year. In this test for example, the seedlings were planted on May 9th, 2015 with both open and closed leaf buds in order to get a more complete idea of the correct planting stage.
4.2 Experiment II: Planting date of freezer stored seedlings
The most important finding from experiment II was that with freezer stored PL81F- birch seedlings, planting should be done as soon as the weather allows for it. The dry weight of new roots nearly halved in two weeks time between the first and the second planting dates in May.
Still, the survival rate remained steady, or close to 1, until about mid-June. Between the seedlings of the fourth (June 23rd) and the fifth (July 7th) planting time, the survival rate decreased by 54 %. This implies that the sooner the seedlings are planted, the better, but they should be planted no later than mid-June. The growth in height shows similar variation:
between said planting dates, the growth of 2016 decreased by 60 %. Height growth is similar for the seedlings planted in the first three planting dates.
There is little research done on small birch seedlings’ freezing storage tolerance. More data is available with coniferous seedlings and deciduous seedlings of larger size. For example in Luoranen et al. (2005), research was done with Norway spruce seedlings that were freezer stored and planted from mid-May to mid-July or the end of August. There were both actively growing seedlings and freezer stored, dormant seedlings used in the study. Dormant seedlings could be planted from May to mid-June without losses in growth or survival (Luoranen et al., 2005). This result is consistent with the findings of this study.
Moreover, in Luoranen et al. (2006a), it was found that hybrid aspen seedlings grown in PL25- containers can also be freezer stored without significant damages. The aspens used for springtime planting were packed in plastic bags once the leaves were fallen, and stored in a freezer storage at -3 °C. The two springtime plantings were done on May 14th, 1999, and on May 5th, 2000. Compared to the seedlings of the same study planted in the summer and fall, there were no differences in survival, but height growth was better on seedlings planted in the summer (Luoranen et al., 2006a). Since these freezer stored seedlings were only planted in May, there are no comparable results further on in the summer. Still, this further indicates that May is a safe and secure planting time for freezer stored seedlings.
For this research, in the follow-up inspections of experiment II in 2016, it was found that seedling mortality increased dramatically on the seedlings planted after June 23rd. In Hänninen et al. (2009), freezer storage tolerance was also tested on Norway spruce seedlings. Temperature sum models and long-term air temperature data were used in computer simulations to solve how the freezer-storage termination date affected autumn frost damage in the long term. The
risks were calculated for 60 consecutive termination dates, i.e. the months of June and July.
Great year-to-year variation was detected. For Central Finland for example, the frost damage risk started to increase quite soon after the storage termination was delayed in early June.
Storage terminations on June 15th and on June 22nd caused frost damages every tenth and every fifth year, respectively (Hänninen et al., 2009). In other words, a week of difference between freezer storage termination dates caused the damages to double. Naturally, there are various uncertainties with simulations, but these results correlate with the ones found in this research:
freezer-stored seedlings need to be planted no later than in mid-June.
4.3 Concluding remarks
The current practice of planting in July using summer seedlings seems like the best method.
The risks are more or less the same for both types of seedlings. One advantage that summer seedlings have is that they get “a running start”, unlike the seedlings planted in dormant stage later on in the summer and autumn. They start the rooting process quicker, and are therefore safer from frost heave, for example.
All in all, PL81F- seedlings have proven to be just as potent in growth as the PL25s, and can for the most part replace them. Silver birch can be planted as late as in October, but freezer stored PL81F- birches need to be planted no later than in mid-June. In addition, the mechanization of planting means that smaller seedlings must be produced in larger quantities.
Finnish seedling producing industry has started to lean towards this direction in their investments. This study was carried out in a somewhat secure environment, i.e. on a nursery field, to get an understanding of how the seedling material differed in contrast to the planting date. This test design excluded competitive vegetation and, to some degree, herbivory. In addition, the soil type was considerably drier than the average forest soil favorable for silver birch. Further research about the PL81Fs should therefore be aimed at collecting practical data from the field.
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Attachments
Attachment 1. Experimental design of the planting test area (experiment I).
Attachment 2. Experimental design of the freezing storage tolerance test area (experiment II).
