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

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.

The triangular space of abiotic stress tolerance in woody species: a unified trade-off model

“…Previously published trade-off analyses have been characterized by either a low explanatory power of the bivariate relationships under consideration, or by relatively low sample sizes, leading to inconsistency in results obtained from different studies. As a consequence, trade-offs between environmental tolerances have not been regarded as absolute constraints on combinations between different stress tolerances [20].…”

“…What is still lacking is a mechanistic understanding of stress tolerance strategies, and therefore of trade-offs, in woody species [20]. The STS revealed by this analysis provides a unique baseline from which to examine further ecological questions, such as the way in which non-redundant stress tolerance axes relate to plant trait spectra [9] and to species' niche breadths, and how the STS is modified by the inclusion of biotic interactions such as herbivory, competition and symbiosis.…”

“…For example, adaptations that allow plants to grow in shade have traditionally been thought to compromise their ability to grow in dry conditions [10]. Support for such trade-offs has been provided by some studies [11][12][13][14][15], but not by all [16][17][18][19][20]. An extensive list of ways in which species can escape trade-offs between tolerances has been proposed by [21].…”

“…However, it is also possible that scatter of data in bivariate relationships is caused by multivariate trade-offs, in which additional factors prevent the observed points from lying on the twodimensional plane close to the trend line (see [21][22][23] for further discussion). The possible impacts of such multivariate influences is so far untested [20].…”

Response of whole plant water use to limiting light and water conditions are independent of each other in seedlings of seasonally dry tropical forests

“…Large statured trees with dense canopies in wet forests result in diminished light availability in the understory, and conversely, low plant cover and open canopies allow greater availability of light in dry forests (Kitajma et al 1994, Johnson et al 2017, Comita et al 2014. It has been proposed that plants face a trade-off in responding to limiting water and light as a result of contrasting physiological and morphological adaptations required for dealing with each of these limiting resources separately (Smith and Huston 1989, Holmgren et al 2012, Kupers et al 2019. However, results from other studies suggest that plant responses to limiting water may be independent of light (Sack et al 2004, Sánchez-Gómez et al 2006.…”