Abstract
Tropical hyperseasonal savannas provide a rare example of a tropical climax community dominated by graminoid species. Species living in such savannas are frequently exposed to excess heat and light, in addition to drought and waterlogging, and must possess traits to avoid or tolerate these stress factors. Here we examine the contrasting heat and light stress adaptations of two dominant savanna sedges: Lagenocarpus guianensis, which is restricted to the sheltered forest edge, and Lagenocarpus rigidus, which extends from the forest edge to the open savanna. An ecotone extending from the forest edge to the open savanna was used to assess differences in a range of physiological traits (efficiency of photosystem II, cell membrane thermostability, stomatal conductance, leaf surface reflectance and canopy temperature depression) and a range of leaf functional traits (length : width ratio, specific leaf area and degree of folding). Lagenocarpus guianensis showed significantly less canopy temperature depression than L. rigidus, which may explain why this species was restricted to the forest edge. The range of leaf temperatures measured was within the thermal tolerance of L. guianensis and allowed photosystem II to function normally, at least within the cool forest edge. The ability of L. rigidus to extend into the open savanna was associated with an ability to decouple leaf temperature from ambient temperature combined with enhanced cell membrane thermostability. The high degree of canopy temperature depression seen in L. rigidus was not explained by enhanced stomatal conductance or leaf reflectance, but was consistent with a capacity to increase specific leaf area and reduce leaf length: width ratio in the open savanna. Plasticity in leaf functional traits and in cell membrane thermostability are key factors in the ability of this savanna sedge to survive abiotic stress.
Highlights
As the effects of global climate change become more apparent, there is increasing interest in understanding how plants will respond to a warmer environment (Reynolds et al 2001; Larkindale et al 2005; Allakhverdiev et al 2008; Mittler et al 2012)
The forest edge was characterized by the highest canopy shading; there was a marked drop in shading at 14 m from the edge, a smaller drop at 28 m, after which shading remained low throughout the rest of the ecotone (Fig. 1B)
Leaf temperatures of L. guianensis were observed to be more closely coupled to that of the environment, with leaves heating up substantially by midday (Fig. 3A). In both cultivated and noncultivated species, canopy temperature depression has been observed to be positively correlated with stomatal conductance (Burke and Upchurch 1989; Radin et al 1994; Amani et al 1996; Lu et al 1998; Nagler et al 2003); the lower potential of L. guianensis to achieve canopy temperature depression may in part be attributable to the rapid reduction in stomatal conductance observed during the morning period, 0800–1200, the absolute rate & The Authors 2013
Summary
As the effects of global climate change become more apparent, there is increasing interest in understanding how plants will respond to a warmer environment (Reynolds et al 2001; Larkindale et al 2005; Allakhverdiev et al 2008; Mittler et al 2012). By understanding the responses of species that have evolved in environments frequently exposed to heat stress, we will be better placed to manage the transition to a warmer environment in both wild and cultivated systems. John-Bejai et al — Physiological responses to heat and light in two tropical savanna sedges are segregated into two distinct habitats: open savanna and seasonally flooded marsh forest (Beard 1953; Richardson 1963). The abiotic character of the open savanna poses a challenge to plant species, as illustrated by the markedly lower floral diversity of these areas as compared with adjacent marsh forest (Richardson 1963). The influence of soil drainage on species composition is observed to be greatest in the open savannas as the shallow clay pan depths that characterize these areas result in more pronounced periods of inundation and subsequent drought (Richardson 1963)
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