Mixed stands of spruce and beech under drought stress and stress recovery. From pattern to process.

挪威云杉（Picea abies， L.）喀斯特)和欧洲山毛榉（Fagus sylvatica L.）在中欧森林中占主导地位。干旱胁迫对这两种树种生长的影响是显著的，但其背后的机制尚未完全了解，特别是当它们生长在混交林时。我们在KROOF联合项目中的目标是了解挪威云杉和欧洲山毛榉的单一栽培和混合栽培在干旱胁迫和胁迫恢复下的树木和林分生长过程。KROOF项目的所有合作伙伴的共同基础是通过瀑布排除实验（TEE）和沿着巴伐利亚（PGR）的降水梯度的一系列地块，它们都提供了独特的机会来研究土壤过程（例如土壤水分、养分有效性）、资源有效性（例如光、水、养分）和树木和林分生长之间的相互作用。在分析了第一个资助期的干旱试验及其对树木和林分生长的影响后，我们对第二个资助期的零假设为：(1)挪威云杉和欧洲山毛榉对年度干旱期和延长干旱期的反应相似；(2)挪威云杉和欧洲山毛榉在种内和种间邻居中对干旱的生长反应没有差异；(3)挪威云杉和欧洲山毛榉的水分利用效率在年度干旱期和延长干旱期以及种内和种间邻居中都没有差异；(4)干旱不会改变茎冠异速生长。(5)挪威云杉和欧洲山毛榉的生长恢复在沿PGR的干燥和潮湿地点相似，在种内和种间环境中；(6)生长恢复与树木的大小和竞争地位无关，挪威云杉和欧洲山毛榉的生长恢复与种内和种间环境相似；(7)干旱胁迫与种群中树木的相对大小无关；(8)在干旱和对照条件下，木材密度以及树轮宽度与木材密度之间的关系相等，混合林分与单一林分之间没有差异；(9)混合林分与单一林分相比，在PGR沿线的超产相等，且不受各自生长限制资源（水、养分、光照）的影响。因此，我们的工作计划旨在六个目标，即回顾性分析TEE和PGR生长和资源测量时间序列中记录的干旱反应（O1），分析2015年干旱胁迫沿PGR的生长恢复（O2），分析TEE站点极端干旱胁迫下的生长恢复（O3），改变干旱胁迫下种群内不同大小树木之间的生长分区（O4），分析干旱和混合两种树种对其木材密度的影响（O5），以及光、水和营养供应作为混合树种林分与单一树种林分过度产量的潜在原因的审查（O6）。

Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) dominate Central European forests. The effects of drought stress on the growth of these two species are significant, while the mechanisms behind are not yet fully understood, particularly when they grow in mixed stands. Our aim within the joint project KROOF is to understand the processes of tree and stand growth in monocultures and mixtures of Norway spruce and European beech under drought stress and stress recovery. The common base of all partners of the KROOF project is a throughfall-exclusion-experiment (TEE) and a series of plots along a precipitation gradient across Bavaria (PGR), which both offer the unique opportunity to study the interactions between soil processes (e.g. soil water, nutrient availability), resource availability (e.g. light, water, nutrients) and tree and stand growth. After analyzing the drought experiments and their effects on tree and stand growth within the first funding term our null hypotheses for the second funding term are: (1) Norway spruce and European beech react similarly to annual and extended drought periods, (2) both, Norway spruce and European beech do not show different growth responses to drought in intra- and inter-specific neighbourhood, (3) the water use efficiency of Norway spruce and European does neither differ in annual and extended drought periods nor in intra- and inter-specific neighbourhood, (4) drought does not modify stem and crown allometry, (5) growth recovery is similar on dry and moist sites along the PGR for Norway spruce and European beech, and in intra- and interspecific environments, (6) growth recovery is independent of tree size and competitive status, similarly for Norway spruce and European beech, in intra- and interspecific environments, (7) drought stress is independent of the relative size of trees within the population, (8) wood density as well as the relationship between tree ring width and wood density is equal under drought and control conditions and does not differ between mixed-species stands and monocultures and (9) overyielding in mixed stands compared to monocultures is equal along the PGR and independent of the respective growth limiting resource (water, nutrient, light). Consequently, our work plan aims at six objectives, i.e. the retrospective analyses of the drought reactions documented in the time series of growth and resource measurements on TEE and PGR (O1), the analysis of the growth recovery from the 2015 drought stress along the PGR (O2), the analysis of the growth recovery from extreme drought stress at the TEE site (O3), the modification of growth partitioning among trees of different sizes within a population under drought stress (O4), the analysis of the effects of drought and mixing both tree species on their wood density (O5), and the scrutiny of light, water, and nutrient supply as potential causes of overyielding in mixed-species stands versus monocultures (O6).