Complementarity of gymnosperm and angiosperm trees in space and time

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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.