Performance of tropical forest seedlings under shade and drought: an interspecific trade-off in demographic responses

Stefan J Kupers,Bettina M J Engelbrecht,Nadja Rüger,Christian Wirth,S Joseph Wright,Richard Condit,Andrés Hernández

doi:10.1038/s41598-019-55256-x

Abstract

Seedlings in moist tropical forests must cope with deep shade and seasonal drought. However, the interspecific relationship between seedling performance in shade and drought remains unsettled. We quantified spatiotemporal variation in shade and drought in the seasonal moist tropical forest on Barro Colorado Island (BCI), Panama, and estimated responses of naturally regenerating seedlings as the slope of the relationship between performance and shade or drought intensity. Our performance metrics were relative height growth and first-year survival. We investigated the relationship between shade and drought responses for up to 63 species. There was an interspecific trade-off in species responses to shade versus species responses to dry season intensity; species that performed worse in the shade did not suffer during severe dry seasons and vice versa. This trade-off emerged in part from the absence of species that performed particularly well or poorly in both drought and shade. If drought stress in tropical forests increases with climate change and as solar radiation is higher during droughts, the trade-off may reinforce a shift towards species that resist drought but perform poorly in the shade by releasing them from deep shade.

Highlights

Seedlings in moist tropical forests must cope with deep shade and seasonal drought
When comparing relatively closed moist forests with relatively open dry forests on a regional scale, there is a stronger contrast in environmental conditions which should result in a stronger trade-off, i.e. in moist forests species are well adapted to shade but poorly to drought, while in dry forests species are well adapted to drought but poorly to shade[24]
We assessed the interspecific relationship of species responses to shade and drought in a naturally regenerating tropical seedling community

Summary

Introduction

Seedlings in moist tropical forests must cope with deep shade and seasonal drought. the interspecific relationship between seedling performance in shade and drought remains unsettled. Various studies have evaluated the relationship between shade and drought tolerance in different ecosystems, but there is no conclusive answer as to which relationship emerges under which environmental conditions (see Table 1) Most of these studies used functional traits or species distributions as proxies for shade and drought tolerance (Table 1), even though whole-plant performance determines population dynamics[17]. The relationship between light and water availability gradients in these forests determines the adaptive pressures acting on plant communities This relationship varies depending on the scale at which the gradients are compared and local climatic conditions. In these forests species would either need to cope with low light or low soil moisture availability Since these environmental differences are relatively modest, one would expect a relatively weak interspecific trade-off between performance in shaded versus dry conditions. There is a positive relationship between adaptation to shade and drought, i.e. a division between conservative evergreen species that specialize on coping with shade and drought and acquisitive deciduous species that avoid shade and take advantage of optimal growing conditions in the wet season[25,26]

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