Complementarity of gymnosperm and angiosperm trees in space and time

森林为人类提供了多种基本服务。长期以来，由于木材作为一种商品的价值，以及最近由于森林在全球碳循环中的中心作用，森林生产力一直受到人们的关注。混合树种生产力的提高，或超产，往往与具有不同特性或特征的不同树种对光或养分的互补利用有关。特别是，针叶树(常绿)和落叶(阔叶)混交林的互补性与管理林和田间试验中的超产有关。拟议研究计划的长期目标是提高我们对树木多样性、生产力和树木生长弹性之间关系的理解。在短期内，现有的野外实验将被用来解决以下目标：(1)确定树木生物量估计的异速生长不确定性对多样性效应的影响；(2)量化空间和时间互补对树木群落超产的作用；(3)测试多样性如何影响群落生产力的弹性和时间稳定性。为了解决(1)，将为混交物中的每个物种建立将树木尺寸(直径、高度)与生物量联系起来的异速生长方程，并考察这些方程的精确度如何影响对选择和互补效应的解释。为了解决(2)，我们将通过测量树冠的重叠，然后计算群落的重叠指数来量化冠层空间的互补性。我们还将通过量化树冠在春季落叶和秋季落叶的时间，将这一指数扩展到包括时间互补性。最后，我们将测试包括冲洗和脱落信息是否有助于我们更好地预测功能多样化的树木群落的增长。为了解决(3)，将在一个长期的白杨-云杉混交林试验中研究种内和种间竞争对树木生长水平和变异性的影响。拟议的研究将为基本问题提供新的知识，这些问题对于理解树木多样性和森林生产力之间的关系至关重要。这些研究产生的实用知识将有助于在面临全球变暖压力的情况下管理森林，告知应促进哪些林分组成，以减少干旱对树木生长的影响。

Forests provide a multitude of essential services to humanity. Forest productivity has long been of interest due to the value of wood as a commodity, and more recently due to the central role of forests in the global carbon cycle. Enhanced productivity of mixtures of tree species, or overyielding, has often been linked to the complementary use of light or nutrients by different tree species possessing different traits or characteristics. In particular, complementarity in mixtures of conifer (evergreen) and deciduous (broadleaf) species has been linked to overyielding in both managed forests and in field experiments. The long-term objective of the proposed research program is to improve our understanding of the relationships among tree diversity, productivity and the resilience of tree growth. In the short-term, existing field experiments will be used to address the following objectives (1) to determine the influence of allometric uncertainty of tree biomass estimation on diversity effects, (2) to quantify the roles of spatial and temporal complementarity for overyielding in tree communities, and, (3) to test how diversity influences the resilience and temporal stability of community productivity. To address (1), allometric equations that relate tree dimensions (diameter, height) to biomass will be developed for each species in a mixture, and how the precision of these equations influence interpretations of selection and complementarity effects will be examined. To address (2) we will quantify complementarity in canopy space by measuring the overlap of tree crowns and then calculating an index of the overlap for the community. We will also extend this index to include temporal complementarity, by also quantifying when tree crowns flush leaves in the spring and shed leaves in the autumn. Finally, we will test if including the flushing and shedding information helps us make better predictions of the growth of functionally diverse tree communities. To address (3), the effects of within-species and between-species competition on the level of tree growth and the variability tree growth will be studied in a long-running experiment with aspen-spruce mixtures. The proposed research will contribute new knowledge to fundamental questions that are central to understanding relationships between tree diversity and forest productivity. Practical knowledge resulting from these studies will help manage forests in the face of stress due to global warming, by informing what stand compositions should be promoted to reduce drought impacts on tree growth.