Pervasive effects of drought on tree growth across a wide climatic gradient in the temperate forests of the Caucasus

Abstract

Aim The Caucasus is a global biodiversity hotspot that includes a wide diversity of temperate forests, from xeric to mesic and rain forest. Little is known about their vulnerability to climate change. We aimed to identify the major climate constraints on tree growth. Location Western Caucasus of Georgia, Russia and Turkey (40–43° N, 41–43° E). Time period Twentieth century. Major taxa studied Trees, angiosperms and gymnosperms. Methods We used a new network of 35 tree‐ring width chronologies from four angiosperm and four gymnosperm species across an elevational gradient of > 2,000 m. We used correlations to identify the major climate factors (temperature, precipitation and drought) at monthly and seasonal scales affecting tree growth and to assess whether their effects change over time. To explore common response patterns among species, we used self‐organizing maps, a type of artificial neural network. Results Spring or summer drought reduced radial growth of most tree species, despite large differences in elevation. As expected, drought was particularly detrimental at warm, low‐elevation sites. Besides drought, growth of conifers at high elevations was also limited by cold winters and summers. Important species‐specific climate–growth responses were also evident. In general, climate–growth relationships were stable over time, except at some cold‐limited sites, where positive responses to summer and winter temperatures have diminished over the last few decades. Main conclusion Growth responses to precipitation and drought among species were more similar than they were to temperature, even at humid sites, providing further evidence of drought vulnerability in mesic forests. The productivity of high‐elevation conifer forests, limited by summer drought and low temperatures, will depend on the balance between temperature and precipitation. Given that climate change is expected to induce larger climatic gradients in the region, the potential reduction of forest cover at a regional scale would make the conservation of these mesic forests more essential.

Past, present and future distributions of Oriental beech (Fagus orientalis) under climate change projections

“…Towards the east, climatic conditions become drier, but the great vertical gradient remains, thus providing a dynamic environment for tree species to respond to changing environments by vertical movements. A recent study by Martin-Benito et al [21] supports the notion that due to little drought response and positive effects of spring-summer warmth, midto high-elevation sites in the Caucasus might be potential major climatic refugia for Fagus orientalis in the future.…”

“…This species occupies a narrow ecological niche being sensitive to late spring frosts and summer drought [18]. Consequently, it has been identified as especially vulnerable to climate change effects [19][20][21]. Since F. orientalis acts as a keystone species in its habitat and serves as an important resource to ecosystem services such as plywood, particleboard, furniture, flooring veneer, mining poles, railway tiles, paper, and firewood [22,23], it is of paramount importance to understand the fate of this species under climate change.…”

Contrasting resistance and resilience to extreme drought and late spring frost in five major European tree species

“…For example, severe summer drought might be compensated by wet spring or previous winter conditions (Vitali, Büntgen, & Bauhus, 2018). However, spring droughts are probably the most detrimental to the growth of temperate trees as they need substantial water to supply emerging and growing leaves (Lévesque, Rigling, Bugmann, Weber, & Brang, 2014;Martin-Benito et al, 2018). After testing different time intervals, we selected the period from March to June for the calculation of the SPEI as (a) it maximizes the Pearson correlation between SPEI and the site-and species-specific chronologies of tree-ring indices across species ( Figure S1), (b) it integrates the period and the pre-period of the beginning of the growing season for trees in this area when typically high production rates of xylem cells are observed (Dietrich, Zweifel, & Kahmen, 2018), and (c) the largest fraction of water taken up by trees during the growing season comes from early spring (Brinkmann et al, 2018).…”

A Framework for Determining Population-Level Vulnerability to Climate: Evidence for Growth Hysteresis in Chamaecyparis thyoides Along Its Contiguous Latitudinal Distribution

“…While the central populations appear vulnerable to summer warming, winter warming might also increase growth such that it could offset some of this potential negative impact on growth. Warm winter temperatures have been found to be associated with increased growth in the Northern Hemisphere for decades (e.g., Brubaker, 1980;Graumlich, 1993;Pederson et al, 2004;Babst et al, 2013;Martin-Benito et al, 2018;Weigel et al, 2018;Alexander et al, 2019;LeBlanc et al, accepted). Mechanisms that explain how dormant season temperatures might influence growth in conifers are, however, not an area of consensus.…”