Claudia Hartl-Meier

Patterns of drought tolerance in major European temperate forest trees: climatic drivers and levels of variability

The future performance of native tree species under climate change conditions is frequently discussed, since increasingly severe and more frequent drought events are expected to become a major risk for forest ecosystems. To improve our understanding of the drought tolerance of the three common European temperate forest tree species Norway spruce, silver fir and common beech, we tested the influence of climate and tree-specific traits on the inter and intrasite variability in drought responses of these species. Basal area increment data from a large tree-ring network in Southern Germany and Alpine Austria along a climatic cline from warm-dry to cool-wet conditions were used to calculate indices of tolerance to drought events and their variability at the level of individual trees and populations. General patterns of tolerance indicated a high vulnerability of Norway spruce in comparison to fir and beech and a strong influence of bioclimatic conditions on drought response for all species. On the level of individual trees, low-growth rates prior to drought events, high competitive status and low age favored resilience in growth response to drought. Consequently, drought events led to heterogeneous and variable response patterns in forests stands. These findings may support the idea of deliberately using spontaneous selection and adaption effects as a passive strategy of forest management under climate change conditions, especially a strong directional selection for more tolerant individuals when frequency and intensity of summer droughts will increase in the course of global climate change.

Mountain forest growth response to climate change in the Northern Limestone Alps

Key messageGrowth response to climate differs between species and elevation. Fir is the most drought-tolerant species. The mountain forests are robust to the climatic changes until now.AbstractAlpine mountain forests provide a wide range of ecological and socio-economic services. Climate change is predicted to challenge these forests, but there are still considerable uncertainties how these ecosystems will be affected. Here, we present a multispecies tree-ring network of 500 trees from the Berchtesgaden Alps (Northern Limestone Alps, Southeast Germany) in order to assess the performance of native mountain forest species under climate change conditions. The dataset comprises 180 spruce, 90 fir, 110 larch and 120 beech trees from different elevations and slope exposures. We analyse the species with respect to: (1) the general growth/climate response; (2) the growth reaction (GR) during the hot summer in 2003 and (3) the growth change (GC) resulting from increasing temperatures since the 1990s. Spruce is identified as the most drought-sensitive species at the lower elevations. Fir shows a high drought tolerance and is well suited with regard to climate change. Larch shows no clear pattern, and beech remains unaffected at lower elevations. The unprecedented temperature increase of the last decades did not induce any distinct GC. The mountain forests of the Berchtesgaden Alps appear to be robust within the climatic changes until now.

Uniform climate sensitivity in tree-ring stable isotopes across species and sites in a mid-latitude temperate forest

Tree-ring stable isotopes, providing insight into drought-induced eco-physiological mechanisms, are frequently used to reconstruct past changes in growing season temperature and precipitation. Their climatic response is, however, still not fully understood, particularly for data originating from non-extreme, mid-latitude environments with differing ecological conditions. Here, we assess the response of δ(13)C, δ(18)O and tree-ring width (TRW) from a temperate mountain forest in the Austrian pre-Alps to climate and specific drought events. Variations in stem growth and isotopic composition of Norway spruce, common beech and European larch from dry, medium and moist sites are compared with records of sunshine, temperature, moisture, precipitation and cloud cover. Results indicate uniform year-to-year variations in δ(13)C and δ(18)O across sites and species, but distinct differences in TRW according to habitat and species. While the climate sensitivity of TRW is overall weak, the δ(13)C and δ(18)O chronologies contain significant signals with a maximum sensitivity to cloud cover changes (r = -0.72 for δ(18)O). The coherent inter-annual isotopic variations are accompanied by substantial differences in the isotopic signatures with offsets up to ∼3‰ for δ(13)C, indicating species-specific physiological strategies and varying water-use efficiencies. During severe summer drought, beech and larch benefit from access to deeper and moist soils, allowing them to keep their stomata open. This strategy is accompanied by an increased water loss through transpiration, but simultaneously enables enhanced photosynthesis. Our findings indicate the potential of tree-ring stable isotopes from temperate forests to reconstruct changes in cloud cover, and to improve knowledge on basic physiological mechanisms of tree species growing in different habitats to cope with soil moisture deficits.

Effects of host abundance on larch budmoth outbreaks in the European Alps

Outbreaks of the larch budmoth (LBM) in the European Alps are among the most documented population cycles and their historical occurrence has been reconstructed over 1200 years. Causes and consequences of cyclic LBM outbreaks are poorly understood and little is known about populations near the margin of the hosts distribution range. In the present study, we quantify historical LBM outbreaks and associated growth reductions in host trees (European larch). Tree‐ring data collected from 18 sites between approximately 500 and 1700 m a.s.l. in the Northern pre‐Alps are compared with data from the Western Alps and Tatra Mountains, as well as with nonhost Norway spruce. Highly synchronized host and nonhost growth in the Northern pre‐Alps shows that periodic LBM outbreaks are largely absent near the distributional limit of larch. By contrast, growth patterns in the Western Alps LBM core region are indicative of LBM events. Although climatic conditions in the Northern pre‐Alps and Tatra Mountains would allow LBM outbreaks, low host plant abundance is likely the key driver for the absence of cyclic outbreaks in these regions. The results obtained in the present study suggest that, in addition to the climatic conditions, host‐species abundance is critically important for the occurrence of periodic LBM outbreaks and the determination of the respective outbreak range.

Temperature covariance in tree-ring reconstructions and model simulations over the past millennium

Spatial covariance in the simulated temperature evolution over the past millennium has been reported to exceed that of multiproxy-based reconstructions. Here we use tree ring-based temperature reco ...