Abstract
Plant structural diversity is usually considered as beneficial for ecosystem functioning. For instance, numerous studies have reported positive species diversity-productivity relationships in plant communities. However, other aspects of structural diversity such as individual size inequality have been far less investigated. In forests, tree size inequality impacts directly tree growth and asymmetric competition, but consequences on forest productivity are still indeterminate. In addition, the effect of tree size inequality on productivity is likely to vary with species shade-tolerance, a key ecological characteristic controlling asymmetric competition and light resource acquisition. Using plot data from the French National Geographic Agency, we studied the response of stand productivity to size inequality for ten forest species differing in shade tolerance. We fitted a basal area stand production model that included abiotic factors, stand density, stand development stage and a tree size inequality index. Then, using a forest dynamics model we explored whether mechanisms of light interception and light use efficiency could explain the tree size inequality effect observed for three of the ten species studied. Size inequality negatively affected basal area increment for seven out of the ten species investigated. However, this effect was not related to the shade tolerance of these species. According to the model simulations, the negative tree size inequality effect could result both from reduced total stand light interception and reduced light use efficiency. Our results demonstrate that negative relationships between size inequality and productivity may be the rule in tree populations. The lack of effect of shade tolerance indicates compensatory mechanisms between effect on light availability and response to light availability. Such a pattern deserves further investigations for mixed forests where complementarity effects between species are involved. When studying the effect of structural diversity on ecosystem productivity, tree size inequality is a major facet that should be taken into account.
Highlights
Over the last few decades, the influence of structural diversity of plant communities on the functioning of ecosystems has been studied for the most part through the lens of species diversity [1,2,3,4]
We explored the respective role of light interception efficiency (LIE) and light use efficiency (LUE) in explaining the effect of tree size inequality on productivity with Samsara2, a forest dynamics model using a detailed algorithm of light interception by trees
We found that plots with low levels of tree size inequality were generally more productive than plots with high levels of tree size inequality
Summary
Over the last few decades, the influence of structural diversity of plant communities on the functioning of ecosystems has been studied for the most part through the lens of species diversity [1,2,3,4]. Evenaged stand management strategies reduce size inequalities by regenerating a stand in just a few years and by growing a single cohort of trees, whereas uneven-aged stand strategies increase tree size inequality by regenerating a stand continuously, with young trees growing in small gaps between older trees [16] Despite these theoretical and applied dimensions, the effects of tree size inequality on productivity at the population or community scales have been little studied explicitly. In a coniferous forest, Liang et al [22,23] found for different species a decrease in diameter growth and recruitment or an increase in mortality associated with increased size diversity These different studies indicate that it is crucial for productivity to disentangle the effects of species diversity and tree size inequality. A first step would be to analyze the effect of tree size inequality in monospecific stands for a larger set of species than the one used in previous studies
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