Abstract
Abstract. Leaves in tropical forests come in an enormous variety of sizes and shapes, each of which can be ultimately viewed as an adaptation to the complex problem of optimising the capture of light for photosynthesis. However, the fact that many different shape "strategies" coexist within a habitat demonstrate that there are many other intrinsic and extrinsic factors involved, such as the differential investment in support tissues required for different leaf lamina shapes. Here, we take a macrogeographic approach to understanding the function of different lamina shape categories. Specifically, we use 106 permanent plots spread across the Amazon rainforest basin to: 1) describe the geographic distribution of some simple metrics of lamina shape in plots from across Amazonia, and; 2) identify and quantify relationships between key environmental parameters and lamina shape in tropical forests. Because the plots are not randomly distributed across the study area, achieving this latter objective requires the use of statistics that can account for spatial auto-correlation. We found that between 60–70% of the 2791 species and 83 908 individual trees in the dataset could be classified as having elliptic leaves (= the widest part of the leaf is on an axis in the middle fifth of the long axis of the leaf). Furthermore, the average Amazonian tree leaf is 2.5 times longer than it is wide and has an entire margin. Contrary to theoretical expectations we found little support for the hypothesis that narrow leaves are an adaptation to dry conditions. However, we did find strong regional patterns in leaf lamina length-width ratios and several significant correlations with precipitation variables suggesting that water availability may be exerting an as yet unrecognised selective pressure on leaf shape of rainforest trees. Some support was found for the hypothesis that narrow leaves are an adaptation to low nutrient soils. Furthermore, we found a strong correlation between the proportion of trees with non-entire laminas (dissected, toothed, etc.) and mean annual temperature once again supporting the well documented association that provides a basis for reconstructing past temperature regimes.
Highlights
Leaves come in a huge variety of shapes and sizes, from needle-like spikes to almost perfect disks
Our main source for exploring spatial variation in leaf shape across the Amazon rainforest is the RAINFOR database (Peacock et al, 20071), which has been constructed using information from 115 permanent plots distributed across eight South American countries, to which we added 26 data plots from Guyanas and Suriname compiled by the ATDN network
Leaf lamina shape varied considerably between species, and even within species found within the same plot, there is a strong tendency for Amazonian trees to possess leaves that are roughly elliptic to oblong
Summary
Leaves come in a huge variety of shapes and sizes, from needle-like spikes to almost perfect disks. Certain leaf shapes might act as physical barriers to insect herbivory or influence the density or diversity of herbivores attacking a leaf (Rivero-Lynch et al, 1996). An obovate (egg-shaped) leaf, in which the majority of the weight of the leaf is situated far from the leaf base, requires a far greater investment in supporting tissues than a cordate (heart-shaped) leaf, where the inverse occurs. Givnish and Vermeij (1976) have argued that the higher proportion of cordate leaves on vines in forests in Costa Rica is evidence of how this shape is more likely to evolve when investment in woody support tissue is not required An obovate (egg-shaped) leaf, in which the majority of the weight of the leaf is situated far from the leaf base, requires a far greater investment in supporting tissues than a cordate (heart-shaped) leaf, where the inverse occurs. Givnish and Vermeij (1976) have argued that the higher proportion of cordate leaves on vines in forests in Costa Rica is evidence of how this shape is more likely to evolve when investment in woody support tissue is not required
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