Macroecology and Forest Dynamics ¶

Project #1:

Paper: Complementarity of gymnosperms and angiosperms along an altitudinal temperature gradient (2018).  Oikos. doi.org/10.1111/oik.05360

Authors: John Caspersen, Esther Thürig, Andreas Rigling, Niklaus Zimmermann.

Abstract: In seasonal tropical forests, evergreen–deciduous mixtures are more productive than monocultures because they intercept more light throughout the year, reflecting complementary resource use by functional groups possessing different traits. This suggests that temperate and boreal forests may also exhibit overyielding, due to the difference in phenology between gymnosperms and angiosperms. However, complementarity could also arise from differences in morphology between needle leaves and broad leaves, or facilitation by N‐fixing species. Alternatively, mixtures may be more productive simply because interspecific competition is less intense than intraspecific competition. We used forest inventory data to assess the complementarity of the main functional groups in Switzerland. We employed a trait‐based analysis of competition to determine whether: 1) trait differences reduce the intensity of competition between complementary functional groups, 2) complementarity is observed along a broad altitudinal temperature gradient. N‐fixing species facilitated the growth of non‐fixing species, such that 50/50 mixtures were 50% more productive than monocultures, though half the overyielding was due to the alleviation of intraspecific competition. In contrast, we found no evidence of complementarity between evergreen and deciduous species. For example, in stands where larch was mixed with other gymnosperms, there was no reduction in heterospecific competition among evergreen species, even though evergreens cast shade on one another throughout the year. In cold montane forests, broadleaf species reduced the suppression of needleleaf species, and vice versa. Thus, 50/50 mixtures of needleleaf and broadleaf species were 15% more productive than needleleaf monocultures. However, in warm lowland forests, broadleaf species exacerbated the suppression of needleleaf species, completely offsetting the positive effect that needleleaf species had on broadleaf species. In summary, we found no evidence of complementarity between evergreen and deciduous species, but needleleaf–broadleaf mixtures exhibited overyielding in cold montane forests, which is consistent with the stress gradient hypothesis, though the underlying mechanisms remain uncertain.

Project #2

Paper: Allometric equations for integrating remote sensing imagery into forest monitoring programmes (2017). Global Change Biology, 23(1), 177-190.

Authors: Tommaso Jucker, John Caspersen, Jérôme Chave, Cécile Antin, Nicolas Barbier, Frans Bongers, Michele Dalponte, Karin Y. van Ewijk, David I. Forrester,  Matthias Haeni, Steven I. Higgins, Robert J. Holdaway, Yoshiko Iida, Craig Lorimer,  Peter L. Marshall, Stéphane Momo, Glenn R. Moncrieff, Pierre Ploton, Lourens Poorter,  Kassim Abd Rahman, Michael Schlund, Bonaventure Sonké, Frank J. Sterck,  Anna T. Trugman, Vladimir A. Usoltsev, Mark C. Vanderwel, Peter Waldner, Beatrice M. M. Wedeux, Christian Wirth, Hannsjörg Wöll, Murray Woods, Wenhua Xiang, Niklaus E. Zimmermann, David A. Coomes.

Abstract: Remote sensing is revolutionizing the way we study forests, and recent technological advances mean we are now able – for the first time – to identify and measure the crown dimensions of individual trees from airborne imagery. Yet to make full use of these data for quantifying forest carbon stocks and dynamics, a new generation of allometric tools which have tree height and crown size at their centre are needed. Here, we compile a global database of 108753 trees for which stem diameter, height and crown diameter have all been measured, including 2395 trees harvested to measure aboveground biomass. Using this database, we develop general allometric models for estimating both the diameter and aboveground biomass of trees from attributes which can be remotely sensed – specifically height and crown diameter. We show that tree height and crown diameter jointly quantify the aboveground biomass of individual trees and find that a single equation predicts stem diameter from these two variables across the world's forests. These new allometric models provide an intuitive way of integrating remote sensing imagery into large‐scale forest monitoring programmes and will be of key importance for parameterizing the next generation of dynamic vegetation models.

Dr. Tommaso Jucker, CSIRO, Australia