Phenotypic variation of Guzmania triangularis functional traits along precipitation and elevation gradients in northern Colombian cloud forests

Abstract

El cambio climático y su impacto sobre los diferentes ecosistemas, es un tema de gran importancia para la conservación de la biodiversidad y del medio ambiente a nivel global. Recientemente, se ha discutido sobre la consideración del cambio climático como la mayor amenaza en la prevalencia de los ecosistemas. Sin embargo, este debe ser entendido como la confluencia de un complejo de factores que interactúan en simultaneidad y no como factores independientes (Thomas et al. 2004). Además de su impacto sobre el aumento de la temperatura, el cambio climático influye sobre los regímenes de precipitación, la química atmosférica y la dinámica y formación de las nubes, entre otros (Helmer et al. 2019; Poveda and Mesa 1999). Por esto, los ecosistemas de montaña que dependen de las nubes como fuente principal o importante de agua, se van a ver gravemente afectados, principalmente los bosques nublados neotropicales y los páramos (Helmer et al. 2019). Por otra parte, las nubes y la neblina confieren a la atmósfera propiedades amortiguadoras sobre los efectos de otros parámetros climáticos como por ejemplo las temperatura, y es por ello es que conocer las relaciones entre biota y nubes y neblina, es fundamental para predecir la supervivencia de los ecosistemas de montaña bajo escenarios de Cambio Climático. Es así como, estudiar la respuesta biótia a las variaciones en los regímenes de precipitación que son modulados por procesos orográficos, es relevante para la iden'ficación de microrefugios potenciales bajo futuros escenarios de climas más calientes. Por consiguiente, en este estudio, queremos estudiar el impacto de las variaciones en la humedad rela'va (precipitación horizontal y vertical) en la plasticidad morfofisiológica de la bromelia epífita Guzmania triangularis a lo largo de un gradiente de elevación en un bosque nublado en el norte de los Andes occidentales colombianos.

Tropical species occupy narrower climatic niches compared to species in temperate zones, which renders them more vulnerable to climate change. Most research addressing resilience has emphasised their response towards predicted changes in temperature; much less evidence is available on the impacts of covarying hydric parameters, which are arguably crucial for many life forms and entire ecosystems. The tropical montane cloud forests (TMCF) are unique and species-rich ecosystems, with high levels of endemism and a strong dependence on high environmental humidity. The high abundance of epiphytes is one of the TMCF hallmarks and a key factor to sustain its high levels of biodiversity. Therefore, the study of epiphytes phenotypic plasticity should improve our ability to predict the effect of climatic variation on TMCF. We studied the relationship between environmental humidity and morphophysiological functional traits in the epiphyte bromeliad Guzmania triangularis along an altitudinal gradient, and across windward and leeward slopes, in a montane cloud forest of the Colombian northwestern Andes. To characterise climate, we installed field stations recording humidity, rain and run-off precipitation, and temperature. To examine associated variation in bromeliad functional traits, we combined correlative and manipulative approaches, including a translocation experiment among slopes and elevation. As output variables, we examined bromeliad traits associated to water balance and photosynthetic performance, namely the trichome and stomata densities, and the colouration and morphology of the leaves. As expected, daily and elevational variation in temperature was much larger (between 10-20 C°) than concomitant variation in relative humidity, which remained most of the time at the saturation point. The ratio of rain to run-off precipitation was unrelated to elevation, and more variable within the windward compared to the leeward slope. Among the plant functional traits, the trichome densities covaried negatively with elevation on the windward slope, but positively on the leeward slope. An opposite interaction term was found regarding stomata densities on the base leaves underside: the plants exhibited more stomata at higher elevation on the windward slope, but less at higher elevation on the leeward slope. After plants translocation, we observed that stomata densities on the apex underside and the morphology of the leaves varied due to the slope change and the elevational gradient. These variations can be attributed to phenotypic plasticity in response to microclimatic variations. This intraspecific variation in bromeliads functional traits is related to both elevation and precipitation, and would thereby affect their ecological performance under scenarios of climate change.