New potential of old proxies: Using wood anatomy to help unravel the complex interplay of tree growth and climate in boreal forest

点击翻译按钮获取中文摘要

Boreal forests are the worlds largest land biome, tightly coupled to the global climate system through albedo changes and carbon dynamics. However, boreal forests are undergoing complex ecological changes, showing increasing signs of non-linear behaviour and thus have been identified as a policy relevant tipping point in the global climate system. Due to the tight coupling between tree-growth in boreal forests and climate, these trees have been extensively used to reconstruct past climate variations. But it is increasingly unclear, whether these reconstructions and future growth projections are reliable, because the influence of recent climate change on tree growth seems to strongly vary in time. This has been termed the divergence effect in dendroclimatology and has recently led to widespread boreal browning, or growth decreases with warming temperatures. Their reasons and responsible mechanisms are hotly contested and so far unresolved, because we are lacking information on environmental controls on xylogenesis in general and their temporal and spatial variations in boreal forests in particular. Here we propose to narrow this knowledge gap by applying a combination of classical dendrochronological and new wood anatomical approaches. From each tree, we will develop 13 proxy time series (ranging from traditional annual rind width, via density, to cell anatomical time series). This combination is state-of-the-art and will allow resolving intra-annual dynamics of growth processes and promises increased understanding of the processes leading to the divergence effect and boreal browning. As model systems, we have chosen Picea glauca, an important boreal North American tree species with strong prevalence to browning and Pinus sylvestris, an important Eurasian tree species showing less prevalence to browning. We will work along spatial gradients of increasing continentality to cover the naturally occurring climatic variability at the northern distribution edge of each species (eleven sites in total), using mainly available tree cores samples with some additional sampling. Our work program is divided into three related work packages: 1) Field and laboratory work, 2) Analyses of spatio-temporal pattern of browning, 3) The effects of extreme climatic events and physiological thresholds on xylogenesis. We will synthesize our results with a projection of future growth performance of both species at their northern species limit, which seems far more complex than simple species advance northwards with warming climate.