Abstract
Trees are characterized by the large number of seeds they produce. Although most of those seeds will never germinate, plenty will. Of those which germinate, many die young, and eventually, only a minute fraction will grow to adult stage and reproduce. Is this just a random process? Do variations in germination and survival at very young stages rely on variations in adaptations to microgeographic heterogeneity? and do these processes matter at all in determining tree species distribution and abundance?We have studied these questions with the Neotropical Symphonia tree species. In the Guiana shield, Symphonia are represented by at least two sympatric taxa or ecotypes, Symphonia globulifera found almost exclusively in bottomlands, and a yet undescribed more generalist taxon/ecotype, Symphonia sp1. A reciprocal transplantation experiment (510 seeds, 16 conditions) was set up and followed over the course of 6 years to evaluate the survival and performance of individuals from different ecotypes and provenances.Germination, survival, growth, and herbivory showed signs of local adaptation, with some combinations of ecotypes and provenances growing faster and surviving better in their own habitat or provenance region. S. globulifera was strongly penalized when planted outside its home habitat but showed the fastest growth rates when planted in its home habitat, suggesting it is a specialist of a high‐risk high‐gain strategy. Conversely, S. sp1 behaved as a generalist, performing well in a variety of environments.The differential performance of seeds and seedlings in the different habitats matches the known distribution of both ecotypes, indicating that environmental filtering at the very early stages can be a key determinant of tree species distributions, even at the microgeographic level and among very closely related taxa. Furthermore, such differential performance also contributes to explain, in part, the maintenance of the different Symphonia ecotypes living in intimate sympatry despite occasional gene flow.
Highlights
Trees may produce millions of seeds over their lifespan (Moles, Leishman, Falster, & Westoby, 2004), yet the vast majority of those seeds will never become adults
Our results show a pattern consistent with an adaptive advantage of western S. sp1 to the driest conditions included in the experiment, which may be caused by differences in rooting system structure, water use efficiency (Baltzer, Thomas, Nilus, & Burslem, 2005), or a variety of other traits related to response to drought
Our Reciprocal transplant experiments (RTE) experiment has given us insights into the ecological mechanisms governing differential germination and survival of cohorts of individuals in their own and foreign natural environments
Summary
Trees may produce millions of seeds over their lifespan (Moles, Leishman, Falster, & Westoby, 2004), yet the vast majority of those seeds will never become adults. Differences in water drainage have been long identified as a main ecological factor driving the tree community composition on the Guiana shield (Barthes, 1991; Sabatier et al, 1997; ter Steege, Jetteer, Polak, & Werger, 1993), allowing to position the species along a gradient of tolerance to prolonged water saturation of soil porosity and a gradient of tolerance to temporary water saturation (Pélissier, Dray, & Sabatier, 2002) All these studies show that the most striking variations in tree species distribution at local scale result from the widespread gradient between seasonally flooded (SF) habitats along streams and all other surrounding habitats on slopes and hilltops (HT). The two ecotypes are differentiated by their leaf traits (Figure 1), overall size of their leaves, flowers, and fruits, the texture of their bark, the presence of pneumatophores or prop roots (Baraloto et al, 2007; Schmitt, Hérault, et al, 2020), as well as differences in maximum
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