Climate control of wood formation measured at high precision in a southern boreal pioneer and a climax tree species (Larix sibirica and Betula platyphylla)

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Boreal forests suffer increasingly from summer drought as the result of climate warming, whereas originally they were limited by low summer temperatures and low nitrogen availability in most of their distribution range. The mechanisms leading to widespread growth limitations due to summer drought and to reduced annual stem increment with progressive climate warming are insufficiently known.In the proposed project, we want to elucidate the causalities behind the drought limitation of wood formation. We therefore want to study cambial activity (i.e. cell division) and xylem differentiation (including cell enlargement, wall thickening, and lignification) at high temporal resolution. The study will be carried out at the southern edge of the boreal forest in Inner Asia in Mongolia, where forests border on grasslands in the forest-steppe ecotone. In this region, most boreal forests are drought-limited since ever and forest health and productivity has been declining since the late 20th century due to climate warming. We plan to collect microcores of stemwood weekly for the detailed analysis of xylem formation. These investigations are combined with analyses of microclimate, tree water relations (shoot water potential, xylem sap flow) and the levels of stored non-structural carbohydrates (NSC) in the stem. Cambial activity is further related to phenological observations, especially to bud break. The studies will be carried out in pure and mixed stands of Siberian larch (Larix sibirica) and Siberian silver birch (Betula platyphylla). Larch is the dominant climax tree species, whereas birch is the dominant broadleaved pioneer tree species at disturbed sites of the southernmost boreal forests in Mongolia.The key hypotheses tested address the causality behind the observed drought-related long-term trends toward reduced earlywood and latewood formation in the two tree species. We hypothesize that stemwood formation is limited at the start of the growing season by the lack of stored assimilates from the previous growing season. Warmer and drier summers should result in reduced annual carbon gain, leading to reduced NSC storage over winter, which is thought to be a key factor causing reduced earlywood formation in the subsequent growing season. We further hypothesize that cambial activity and xylem differentiation in the late growing season are limited directly by low water availability. The presence of birch trees, which are promoted by natural and anthropogenic disturbances, is hypothesized to increase the drought stress experienced by larch due to asymmetric interspecies competition for water.