Growth performance of P. tremula

3.1.1 Growth performance of seedlings under (simulated) climate change

In this thesis, I studied the height, diameter, and biomass growth in response to elevated CO2 concentration and temperature in P. tremula for a single growing season in greenhouses (I), and the variation in height and diameter growth rates in response to elevated temperature and UVB radiation during a three-year growth period in a field experiment (II). In the greenhouse experiment, the height growth of P. tremula decreased significantly under elevated levels of CO2 concentration (I). Although numerous studies have reported increased growth in plants exposed to elevated CO2 concentrations (McDonald et al. 1999; Lavola et al. 2013), some studies have shown a decrease or no response (Coley et al. 2002; Yazaki et al. 2004). Here, the CO2 -fertilized P. tremula seedlings had a shorter apical portion of the main stem and more branches compared with the plants grown in the ambient CO2

condition, and this weak apical dominance is likely because of the effects of the higher CO2 concentration on hormonal production or transportation in the shoot apex (Conroy et al. 1990). On the other hand, elevated CO2

concentration did not affect diameter and biomass growth; however, diameter growth was considerably higher in the CO2-fertilised plants grown under elevated temperature (I). Elevated temperature might have increased the light-saturated rate of CO2 uptake in the plants, and this might have resulted in greater diameter growth under the combined treatments of elevated CO2 concentration and temperature (Farrar and Williams 1991;

Morison and Lawlor 1999; Dieleman et al. 2012).

Elevated temperature stimulated the height, diameter, leaf, and stem biomass of P. tremula seedlings when compared with the control plants (I).

These results are consistent with earlier studies. Many previous studies

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conducted in greenhouses and open fields for a single growing season have reported that elevated temperature increased growth in broad-leaf tree species in the northern hemisphere (Veteli et al. 2002; Huttunen et al. 2007;

Randriamanana et al. 2015), since tree growth is mainly limited by the below-optimal temperature in this region (Way and Oren 2010; Stinziano and Way 2014; Kellomäki 2017). In the field experiment, elevated temperature increased the height and diameter growth rates of P. tremula during the three-year growth period, however the stimulating effect of elevated temperature on the diameter growth rate became weaker as the plants aged (II). This result is in accordance with Nybakken et al. (2012), who also revealed a decrease of proportional increment in growth in response to elevated temperature in the second growing season when compared to the first one in S. myrsinifolia. The P. tremula seedlings might have acclimated in this study to the higher temperatures over the duration of the experiment, as long-term exposure of plants to a warmer growth environment commonly leads to thermal acclimation, which results in a decline in the proportional increment in carbon gain through photosynthesis in plants over time (Way and Yamori 2014; Way et al. 2015; Smith et al. 2016).

In response to elevated UVB radiation, the height and diameter growth rates of P. tremula did not vary over the entire three-year growing period or in any specific year (II). Although some experimental studies have revealed a cumulative effect of elevated UVB radiation on different growth traits in perennial plants (Sullivan et al. 1994; Tegelberg et al. 2001), there are also studies consistent with the findings of this thesis. For example, after seven years of exposure, no deleterious effect of elevated UVB radiation was detected on growth performance including dry weight, leaf thickness, and leaf area in the slow-growing S. polaris (Wahlenb.) plants (Rozema et al.

2006). In line with other contemporary studies (Sullivan 2005; Ballaré et al.

2011), it can be suggested that elevated UVB radiation may have a weak or no effect on growth increment in P. tremula young plants or seedlings and saplings in the case of ecologically relevant UVB radiation.

39 stimulated the height growth (except in temperature-treated male individuals), leaf, stem, and total aboveground biomass in P. tremula saplings when compared to the intact plants in all climatic treatments (III). Moreover, diameter growth also tended to be greater under all climatic treatments in bud-removed plants compared with intact individuals (III). Stowe et al. (2000) and Jacquet et al. (2013) suggested that damaged plants usually respond by overcompensation in growth after a low to moderate level of tissue loss. In this study, the removal of ~5% of the lateral buds was interestingly sufficient to alter the growth in P. tremula saplings. Overall, the removal of lateral buds increases the source-to-sink ratio by reducing the non-photosynthetic organs and increases the carbon supply to the remaining sinks (Honkanen et al. 1994; Ozaki et al. 2004). Therefore, in the bud-removed individuals, an increase in the source-to-sink ratio might have facilitated higher assimilation and induced greater growth.

There was no interaction between bud removal and UVB treatment on growth increment, however the magnitude of the increment in the leaf and total aboveground biomass due to bud removal was significantly higher in the temperature-treated plants (III). Moreover, after bud removal, UVA+T-treated plants had significantly greater stem biomass due to the additive effects of bud removal and elevated temperature (III). Due to bud removal, the plants might have improved their physiological competency and might have a greater ability to acquire resources under elevated temperature. It was found in the previous studies that tissue-damaged plants under warming conditions had higher concentrations of nutrients, greater water-use efficiency, and an increased photosynthetic rate (Huttunen et al. 2007;

Lemoine et al. 2013). Therefore, based on those and our findings, we may assume that if plants are subjected to low-to-moderate levels of tissue damage as a consequence of increased levels of insects and other

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herbivores due to climate warming, this may result in overcompensation in plant growth (Lehtilä 2000; Stowe et al. 2000).

3.1.3 Sex-dependent variation in growth performance

The sex-dependent variation in growth traits in P. tremula was studied both in greenhouse (I) and field (II, III) conditions. In the greenhouse experiment, there was a sex-dependent variation in males and females of P. tremula, since male individuals had significantly greater height growth than their female counterparts (I). In addition, leaf, stem, and total aboveground biomass tended to be higher in males when compared with females (I).

These results agree with the earlier studies that reported the differences between females and males in growth traits in dioecious tree species in greenhouse conditions (Nybakken and Julkunen-Tiitto 2013;

Randriamanana et al. 2014). In the field experiment, however, both female and male individuals of P. tremula maintained a similar level of growth (II, III).

Some earlier studies also detected the absence or only a few sex-specific differences in growth traits of S. myrsinifolia and P. tremuloides grown in field conditions (Nybakken et al. 2012; Nissinen et al. 2018; Cope et al. 2019).

Nissinen et al. (2018) suggested that plants grown in field conditions often face different stresses – for example, herbivory, plant diseases, competition by other plants, heat, heavy rain, and strong wind – all of which could play a part in maintaining a balance between males and females. However, in greenhouse experiments, plants are grown in a controlled growth environment. Therefore, the inherent secondary sexual dimorphism become very distinctive in such a controlled environment.

Under elevated temperature in the greenhouse experiment, height and diameter growth were considerably greater in females than in males (I). In the field experiment, on the other hand, height growth tended to be greater in males for temperature-treated and bud-removed individuals, but diameter growth was significantly higher in females (III). Moreover, leaf and stem biomass tended to be higher in temperature-treated and bud-removed females (III). These results indicate that under favorable conditions females also preferred to grow more, although plant ecological theory

41 suggests that males are growth biased (Ågren et al. 1999; Obeso 2002).

These additive effects of elevated temperature and bud removal may also make dioecious females more susceptible to herbivores, although male-biased herbivory is frequently reported (Hjältén 1992; Ågren et al. 1999).

3.2 Phenolic concentration in stem bark, whole stems, and leaves