Adaptation to seasonal drought in two closely related species of Neotropical Costus (Costaceae)

Abstract

Seasonal drought has been shown to greatly influence the distributions and species composition of plants in tropical rain forests. By conducting a series of field, greenhouse, and growth chamber experiments, we examined how Costus villosissimus, a forest edge species, has adapted to drought and differentiated from C. allenii, its closely related species in the understory. We hypothesize that delayed seed germination and high drought tolerance may lead to habitat differentiation and thus reproductive isolation between closely related plant species in the tropics. Abstract in Spanish is available with online material.

Flexible drought deciduousness in a neotropical understory herb

“…To assess whole‐plant drought responses, we conducted a dry‐down experiment in the MSU greenhouses on 45 individuals of each species. Stem data from this experiment (previously published by Chen and Schemske, 2019) revealed that C. villosissimus retained living stems for up to 9 months longer than C. allenii under common garden drought conditions and that once individuals lost all stems, they were dead and unable to resprout with rewatering. Here, we monitored leaf loss during the dry‐down to determine whether C. villosissimus followed leaf retention patterns typical of drought tolerance or drought avoidance.…”

“…This level of die‐off contrasts with that of C. villosissimus , for which comparatively little die‐off occurs during the same dry seasons (Chen and Schemske, 2015). Chen and Schemske (2019) further explored the role of drought adaptation in habitat isolation and found through reciprocal seed transplants in the wild that seed dormancy in C. villosissimus enables drought escape, whereas C. allenii seeds germinate immediately and then die during the wet season. The same study also showed clear differential survival of adults during a greenhouse dry down, which is consistent with greater adult C. allenii die‐off during dry seasons in natural C. villosissimus habitat (Chen and Schemske, 2015, 2019).…”

“…Chen and Schemske (2019) further explored the role of drought adaptation in habitat isolation and found through reciprocal seed transplants in the wild that seed dormancy in C. villosissimus enables drought escape, whereas C. allenii seeds germinate immediately and then die during the wet season. The same study also showed clear differential survival of adults during a greenhouse dry down, which is consistent with greater adult C. allenii die‐off during dry seasons in natural C. villosissimus habitat (Chen and Schemske, 2015, 2019). These studies demonstrate that drought adaptation is a key component of differential habitat adaptation and therefore reproductive isolation between C. allenii and C. villosissimus .…”

“…Drought adaptation of tropical herbs is also likely to differ from that of temperate herbs, especially in tropical forests where the vapor pressure deficit is generally lower than in temperate systems (Cunningham, 2005). Such differences are underexplored because drought adaptation has received less attention in the tropics than in temperate systems, despite being a key component of tropical species distributions (Engelbrecht et al, 2007; Chen and Schemske, 2019).…”

“…Costus villosissimus (Maas) and C. allenii (Jacq.) are closely related species that show divergent adaptation along a steep gradient in rainfall seasonality across the Panamanian isthmus (Chen and Schemske, 2015, 2019). By comparing a species that occurs in seasonal moist forests ( C. villosissimus ) to a close relative restricted to perennially wet forests ( C. allenii ), we can assess what traits are essential to drought adaptation.…”