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

由于气候变暖，北方森林越来越多地遭受夏季干旱，而最初它们受到夏季气温低和大部分分布范围内氮供应量低的限制。由于夏季干旱导致广泛的生长限制和减少每年的干增量与渐进的气候变暖的机制是不够knowed.In拟议的项目中，我们要阐明木材形成的干旱限制背后的因果关系。因此，我们希望在高时间分辨率下研究形成层活动（即细胞分裂）和木质部分化（包括细胞扩大，壁增厚和木质化）。这项研究将在蒙古的亚洲内陆北方森林的南部边缘进行，那里的森林与森林草原交错带的草原接壤。在这一地区，大多数北方森林历来干旱有限，自世纪末以来，由于气候变暖，森林健康和生产力一直在下降。我们计划每周采集一次树干的微芯，以便详细分析木质部的形成。这些调查相结合的小气候分析，树水的关系（拍摄水势，木质部液流）和存储的非结构性碳水化合物（NSC）在干的水平。形成层的活动进一步与物候观测，特别是芽的断裂。研究将在西伯利亚落叶松（Larix sibirica）和西伯利亚银（Betula platyphylla）纯林和混交林中进行。落叶松是占主导地位的顶极树种，而桦树是占主导地位的阔叶先锋树种在最南端的北方针叶林在Mongolia.Key假设测试地址背后的因果关系，观察到的干旱相关的长期趋势减少早材和晚材形成的两个树种。我们假设，在生长季节开始时，由于缺乏前一个生长季节的同化物储存，茎材的形成是有限的。温暖和干燥的夏季应导致年度碳增量减少，导致冬季NSC储存减少，这被认为是导致随后生长季节早材形成减少的关键因素。我们进一步假设，形成层的活动和木质部分化在生长季节后期的低水分供应直接受到限制。白桦树，这是促进自然和人为干扰的存在下，假设增加落叶松所经历的干旱胁迫，由于不对称的种间竞争的水。

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.