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，北方森林越来越受到夏季干旱的影响，而最初它们受到夏季温度低和大部分分布范围内氮素有效性低的限制。由于夏季干旱导致的普遍生长限制以及随着气候逐渐变暖而导致树干年生长量减少的机制还不够清楚。在拟议的项目中，我们想要阐明干旱限制木材形成背后的因果关系。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、壁加厚和木质化)。这项研究将在内蒙古北部针叶林的南缘进行，那里的森林与森林草原交错带的草原接壤。在该地区，由于气候变暖，自20世纪末以来，大多数北方森林都受到干旱的限制，森林健康和生产力一直在下降。我们计划每周收集茎木材的微芯，用于木质部形成的详细分析。这些研究结合了对小气候、树木水分关系(地上部水势、木质部液流)和茎中储存的非结构性碳水化合物(NSC)水平的分析。形成层的活动与物候学观察，特别是与花蕾的破裂有进一步的关系。研究将在西伯利亚落叶松(Larix Siberica)和西伯利亚白桦(Betula Plyphy Lla)的纯林和混交林中进行。落叶松是蒙古最南端北方森林受干扰地区的主要顶层树种，而桦树是主要的阔叶先锋树种。测试的关键假设解决了观察到的与干旱相关的长期趋势背后的因果关系，这两个树种的早材和晚材形成减少。我们假设，在生长季节开始时，由于缺乏前一个生长季节储存的同化物，茎的形成受到限制。更温暖和更干燥的夏季应该会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期形成减少的关键因素。我们进一步假设，在生长后期，形成层活动和木质部分化直接受到低水分供应的限制。在自然和人为干扰的推动下，白桦树的存在被认为是由于物种间不对称的水分竞争而增加了落叶松所经历的干旱胁迫。

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.