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.)主宰着中欧森林。干旱胁迫对这两个物种生长的影响是显著的，但其背后的机制尚不完全清楚，特别是当它们生长在混交林中时。我们在联合项目KROOF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种植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穿透雨排除实验(TEE)和沿巴伐利亚降水梯度(PGR)的一系列小区，它们都为研究土壤过程(例如土壤水分、养分有效性)、资源可用性(例如光、水、养分)与树木和林分生长之间的相互作用提供了独特的机会。在分析了第一个资助期内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无效假设是：(1)挪威云杉和欧洲山毛榉对年度和长期干旱的反应相似，(2)挪威云杉和欧洲山毛榉在种内和种间对干旱没有不同的生长反应，(3)挪威云杉和欧洲山毛榉的水分利用效率在年度和长期干旱期间以及种内和种间没有差异，(4)干旱不改变树干和树冠的异速生长，(5)挪威云杉和欧洲山毛榉在PGR沿线干湿地的生长恢复相似，在种内和种间环境中，(6)生长恢复与树的大小和竞争地位无关，挪威云杉和欧洲山毛榉在种内和种间环境中也是如此，(7)干旱胁迫与种群内树木的相对大小无关，(8)在干旱和对照条件下，木材密度以及年轮宽度与木材密度的关系是相等的，混交林和单一林的木材密度没有差异；(9)混交林的超产与单一林相比是相同的，与各自的生长限制资源(水、养分、光)无关。因此，我们的工作计划针对六个目标，即TEE和PGR(O1)的生长和资源测量时间序列中记录的干旱反应的回顾分析，沿PGR(O2)的2015年干旱胁迫的生长恢复分析，TEE地点极端干旱胁迫的生长恢复分析(O4)，在干旱胁迫下不同大小的树木之间的生长分区的修改(O4)，干旱和混合对木材密度的影响的分析(O5)，以及对光、水、与单一栽培相比，养分供应是混交林超产的潜在原因(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).