Using forest gap models and experimental data to explore long-term effects of tree diversity on the productivity of mixed planted forests

Abstract

Key message In this exploratory study, we show how combining the strength of tree diversity experiment with the long-term perspective offered by forest gap models allows testing the mixture yielding behavior across a full rotation period. Our results on a SW France example illustrate how mixing maritime pine with birch may produce an overyielding (i.e., a positive net biodiversity effect). Context Understanding the link between tree diversity and stand productivity is a key issue at a time when new forest management methods are investigated to improve carbon sequestration and climate change mitigation. Well-controlled tree diversity experiments have been set up over the last decades, but they are still too young to yield relevant results from a long-term perspective. Alternatively, forest gap models appear as appropriate tools to study the link between diversity and productivity as they can simulate mixed forest growth over an entire forestry cycle. Aims We aimed at testing whether a forest gap model could first reproduce the results from a tree diversity experiment, using its plantation design as input, and then predict the species mixing effect on productivity and biomass in the long term. Methods Here, we used data from different forest experimental networks to calibrate the gap model ForCEEPS for young pine (Pinus pinaster) and birch (Betula pendula) stands. Then, we used the refined model to compare the productivity of pure and mixed pine and birch stands over a 50-year cycle. The mixing effect was tested for two plantation designs, i.e., species substitution and species addition, and at two tree densities. Results Regarding the comparison with the experiment ORPHEE (thus on the short term), the model well reproduced the species interactions observed in the mixed stands. Simulations showed an overyielding (i.e., a positive net biodiversity effect) in pine-birch mixtures in all cases and during the full rotation period. A transgressive overyielding was detected in mixtures resulting from birch addition to pine stands at low density. These results were mainly due to a positive mixing effect on pine growth being larger than the negative effect on birch growth. Conclusion Although this study remains explorative, calibrating gap models with data from monospecific stands and validating with data from the manipulative tree diversity experiment (ORPHEE) offers a powerful tool for further investigation of the productivity of forest mixtures. Improving our understanding of how abiotic and biotic factors, including diversity, influence the functioning of forest ecosystems should help to reconsider new forest managements optimizing ecosystem services.

Effect of tree mixtures and water availability on belowground complementarity of fine roots of birch and pine planted on sandy podzol

“…The latter is confirmed as fine root biomass per species was proportional to its basal area (Table 2). It is logical to expect that since pine grew faster than birch (Morin et al 2020), it would also take up water faster, and would outcompete the relatively slower growing birch trees, leaving less available water in the soil. In addition, birch has finer roots than pine, and it might be more sensitive to unfavourable moisture conditions, which might occur more frequently in the very permeable sandy soils that "hold" less water.…”

Tackling unresolved questions in forest ecology: The past and future role of simulation models

“…By virtually manipulating the composition of simulated forest communities, forest IBMs have proven useful in exploring the effect of species richness and functional composition on ecosystem properties (e.g., Fischer et al, 2018). Simulations reproduced positive relationships between (species or functional) diversity and productivity or biomass, in agreement with observed patterns (Maréchaux & Chave, 2017;Morin et al,2011Morin et al, , 2020, further motivating a finer-grained representation of diversity in DGVMs. These studies demonstrated how competition for light can induce this positive effect in heterogeneous forests.…”

Tree diversity effects on forest productivity: disentangling the effects of tree species addition vs. substitution

“…The copyright holder for this preprint this version posted May 9, 2021. ; https://doi.org/10.1101/2021.05.07.443133 doi: bioRxiv preprint time (Morin et al, 2020). Oaks species with their slower growth rates and varying drought and shade tolerances will gradually establish into stands, leading to a stand stratification possibly suitable to mixed stands, even though at this point, there is no consensus on the stage of forest development for which the positive effect of diversity peak (Jucker et al, 2020;Taylor et al, 2020).…”