Reconstruction of climate-mediated crop yield

In addition to reconstructing climate variability, tree-ring material can be suitable to reconstruct climate-sensitive grain yield fluctuations.¹⁰¹ The crop yield ratio re-construction presented in Article III is indicative to yield ratio responses to climate variability. The full reconstruction (760–2000) indicated several intervals of potential-ly low and high yield ratios, which provides a novel perspective to the agricultural history of the region. The period of highest yield ratios was observed from 760–1106.

This was followed by a period of lower yield ratios and increased yield variability for 1107–1451. The yield ratios further decreased in 1452–1694. The reconstruction indicates a period of lowest yield ratios and highest variability for 1695–1911. After that, the yield ratio rose, especially from the 1920s onwards, to values comparable to the period of high yield ratios of 760–1106. All the interval shifts coincide with peri-ods of agricultural transformation, as previously suggested by pollen analysis and historical studies (Figure 3).

Based on pollen analyses, the cultivation of cereals started in Finland at the be-ginning of the Iron Age, in c. 500 bc, although signs of cultivation can also be found

97 Wigley et al. 1984, 201–213; Büntgen et al. 2012, 263–269.

98 Jutikkala 1994, 11; Lappalainen 2012, 32.

99 Leroy 2006, 6; Izdebski et al. 2016, 10–11, 13–14.

100 See, e.g., Kelly & Ó Gráda 2014.

101 Therrell et al. 2006, 487–499.

from the Neolithic period onwards.¹⁰² Yet, in the area that the reconstruction covers, i.e. central and northern Finland, the earliest evidence of continuous crop cultivation is largely found between the 6th and 10th centuries ad.¹⁰³ The reconstruction suggests that permanent agriculture was not introduced to the ‘Kingdom of Frost’. Instead, crop cultivation was established slowly in a land where and when climate supported high crop yields. Over the second half of the first millennium, climate conditions did not pose such a threat to crop cultivation as they did in the following centuries.

Coinciding with the first significant drop in the reconstructed values, in the first half of the 12th century, pollen analyses have evidence of agricultural intensification and increased use of fire.¹⁰⁴ This suggest that people might have coped with the deterio-rating climate and increased risk of poor harvest by expanding cultivated areas and by increasing slash-and-burn cultivation, which provided higher yields than arable cultivation.¹⁰⁵

The next sudden drop in temperatures and yields took place in the mid-15th cen-tury. The rapid cooling, which followed a major atmospheric circulation change over the North Atlantic and coincided with the culmination of the Spörer solar minimum, has been evidenced in various summer and winter season reconstructions of the region.¹⁰⁶ Similarly, coinciding with this drop, pollen evidence and written sources suggests a rapid expansion of agricultural area.¹⁰⁷ Moreover, at the beginning of this third interval, a new rye variety suitable for slash-and-burn fields in the coniferous forests was introduced to the area.¹⁰⁸ The culmination of the LIA in Finland has been commonly dated to the late-17th and early-18th centuries, which is synchronous with the onset of the phase of the lowest yield ratios in the reconstruction. The Maunder solar minimum (c. 1645–1715) and several volcanic eruptions preceded the culmi-nation.¹⁰⁹ Coinsiding with the onset of the period of lowest yield ratios, slash-and-burn cultivation increased extensively in the eastern parts of Finland.¹¹⁰ Furthermore, based on the tithe series collected from the region to assess the weighting of the yield reconstruction (Article III), winter rye replaced barley as the main cultivated crop in large parts of the studied area at the beginning of this interval. Winter rye provided steadier yields and decreased the risk of frost damage, as this autumn-sown rye rip-ened earlier than the spring-sown barley.¹¹¹ It was only in northernmost Finland that rye was not established as the principal bread crop because of considerable wintering damages due to the long winters.¹¹²

102 Alenius et al. 2009, 2013; Lahtinen & Rowley-Conwy 2013; Josefsson et al. 2014.

103 Baudou et al. 1991; Wallin & Segerström 1994; Taavitsainen et al. 1998, 2007; Ojala 2001; Alenius et al.

2004, 2008, 2009, 2013; Tiljander 2005; Alenius & Laakso 2006; Augustsson et al. 2013.

104 Alenius et al. 2004, 2008, 2013; Alenius & Laakso 2006.

105 Soininen 1975, 169.

106 Meeker and Mayewski 2002, 261; Miyahara et al. 2006, 1–5; Haltia-Hovi et al. 2007, 686; Klimenko &

Solomina 2010, 83.

107 Baudou et al. 1991; Taavitsainen et al. 1998; Ojala 2001; Tiljander 2005; Korpela 2012; Augustsson et al.

2013.

108 Korpela et al. 2012, 279–280.

109 Shindell et al. 2003, 4102–4105.

110 Taavitsainen et al. 1998, 232.

111 Tornberg 1989, 68.

112 Mukula & Rantanen 1989, 3–8; Solantie 2012, 137.

Extensive increase of slash-and-burn

cultivation

-4 -3

-2

-1 0 1 2 3 4

Reconstructed yield ratio anomalies (z-scores)

Establishment of continous agriculture

Agricultural intensification,

increased use of fire

Rapid expansion of cultivated area

800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Figure 3. Reconstructed climate-mediated yield ratio anomalies 760–2000 and periods of ag-ricultural transformation in Finland. The black dashed line shows significant (0.05 level) shifts in the reconstructed values.

Source: Article III.

Thus, it appears that medieval and early modern farmers adapted to the deterio-rating climate and lowering yield ratios by expanding the area of cultivation and favouring higher yielding methods.

Although the reconstruction is indicative of yield fluctuations in central and northern Finland, the intervals of potentially low and high yield ratios correspond also to the patterns of agricultural development in the north-west Russia. Also there, intensive agricultural expansion took place when the reconstruction indicates the period of the highest yield ratios, c. 950–1100.¹¹³ Yet, when the Finnish peasant pop-ulation responded to the sudden cooling of the mid-15th century by expanding the cultivated area, the population in north-west Russia gradually switched from agricul-ture to maritime trade.¹¹⁴ In addition, Article II also demonstrates that further south, in the city of Novgorod, a decade long period of high grain prices and scarcity of

113 Klimenko 2016, 368–369.

114 Ibid. 372–373.

food coincided with this sudden cooling of the mid-15th century. The Finnish adap- tation strategies, continuous clearing of new fields and high-yielding methods that required large land areas, like slash-and-burn cultivation, were not, possible every-where in the northern margin of agriculture. Consequently, the intense colonisation of the Russian north stopped and many farms were deserted in Sweden in the latter half of the 15th century.¹¹⁵

Interestingly, the reconstruction can perhaps also provide material to better un-derstand the general northern health decline observed in previous research, as the long-term trends in the reconstructed values correspond with the long-term height decline of the northern people.¹¹⁶ By analysing estimated average adult male heights in northern Europe (Britain, Denmark, Iceland, Netherlands, Norway, and Sweden) Richard H. Steckel (2004) found that the average male height was the highest in the 9–11th centuries, approximately 173.4 centimetres. This is the period when the re-construction indicates the highest yields. The stature of the northern males declined in the 12–14th centuries to 171.5 centimetres, corresponding with the declining yield ratios of the reconstruction. The shortest average male heights, 166.2 centimetres, were found from the 18th century. Again, also the reconstruction indicates the same century had the lowest yield ratios over the full reconstruction period. Thus, lower food production due to deteriorating climate may have contributed to the demon-strated health decline.

Yet, above all, the climate-mediated yield ratio reconstruction provides insight to the long-term development of the agriculture in the north. Already Le Roy Ladurie (1972a) noted that temperature largely dictates harvest fluctuations in the northern margin of agriculture. Nevertheless, he was sceptical whether climate had any long-term influences:

In the short or relatively short term (on an intradecennary, decennary, and in certain cases interdecennary scale), agricultural history is vulnerable to the caprices of me-teorology which produce bad harvests and used to produce food crises. But in the long term the human consequences of climate seem to be slight, perhaps negligible, and certainly difficult to detect.¹¹⁷

Here, the reconstructed climate-mediated yield ratio may provide new material to detect and better understand the long-term human consequences of climate. It con-tributes to the discussion on why agricultural changes, as previously demonstrated in pollen analyses and archaeological and historical studies, took place. Adaptation to the cooling climate and resulting lowering yield ratios likely underlie these chang-es. Consequently, it provides a source to explore the long-term processes of adapta-tion, which may not be tracable in the traditional sources of historical reseach.¹¹⁸

115 Vahtola 2003, 574–565; Klimenko 2016, 374.

116 Steckel 2004, 216.

117 Le Roy Ladurie 1972a, 119.

118 De Vries 1980, 630.