Functional variation of leaf succulence in a cold rainforest epiphyte

Oscar Godoy,Ernesto Gianoli

doi:10.5091/plecevo.2013.800

Abstract

Background and aims – Succulence, a common attribute of floras in dry regions and of species living in microenvironments with transient water shortage, has been typically viewed as an adaptive plant feature for surviving in (semi-)arid conditions. The existence of leaf succulence in a temperate cold rainforest challenges the view of its adaptive value. We studied leaf functional variation in Sarmienta repens Ruiz & Pav. (Gesneriaceae), an epiphyte living in the Valdivian forest of southern Chile. Material and methods – We measured leaf thickness, absolute leaf water content, specific leaf area and leaf anatomy (epidermis, palisade parenchyma, and spongy parenchyma) in two distinct light microenvironments: shaded understory versus border of canopy gaps. We also characterized micro-environmental conditions in terms of light availability, temperature and water evaporation. Key results – We show that leaves from sun conditions, the environment with higher water demand, have lower SLA (specific leaf area), thicker epidermis and store more water due to a thicker spongy parenchyma, than leaves from shade conditions. Conclusions – We found high phenotypic variation in S. repens at intraspecific level in response to contrasting environmental conditions. This variation reflects a two-fold strategy common in epiphytes: increase water storage and reduce water loss. Furthermore, it suggests that leaf succulence has an adaptive value even in a temperate cold rainforest. We discuss that the occurrence of succulence on a cold rainforest might be explained by a combination of ecological, biogeographic and phylogenetic factors.

Highlights

Plant features commonly observed in ecological communities are often the result of current selective pressures and prove their adaptive value when they are associated with plant success in function, distribution or abundance (Niklas 2007)
Leaf thickness and aLWC were greater on plants exposed to sun conditions
2.56 ns SLA tomical layers studied were significantly thicker at sun conditions, the spongy parenchyma contributed the most to the observed leaf succulence patterns

Summary

Introduction

Plant features commonly observed in ecological communities are often the result of current selective pressures and prove their adaptive value when they are associated with plant success in function, distribution or abundance (Niklas 2007). Apparently adaptive characters or syndromes that are rare in a community may be explained by past evolutionary processes that took place long time before and/or faraway from the current plant environment, where the selective pressure is no longer operating (Larson & Losos 1996). It is not easy to elucidate whether rare phenotypic traits are evolutionary anachronisms or actual adaptive features because a large amount of ecological, biogeographic and phylogenetic information is needed (Larson & Losos 1996). If it is shown that (i) there is field-based evidence of a functional relation between the observed phenotype and the putative selective factor, supported by ecophysiological knowledge (e.g. Saldaña et al 2005) and/or (ii) variation in the candidate trait is significantly associated with plant fitness in the field (e.g. Saldaña et al 2007), it may be inferred that such a plant feature has an adaptive value in the current ecological scenario

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Discussion

Conclusion