Spatial distribution and functional significance of leaf lamina shape in Amazonian forest trees

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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.

Figures

  • Fig. 1. Categorization system derived from the Royal Horticultural Society Dictionary (1997). The figure shows 14 ordinal categories (the four lobed categories are not shown).
  • Fig. 2. Leaf Architecture Working Group (1999) categorization system.
  • Table 1. Overview of variables used in this study. The qualitative variables are: the 18 nominal categories from the Royal Horticultural Society (ShapeCAT1); the 4 nominal Leaf Architecture Working Group categories (ShapeCAT2); and lamina margin classification (Margin). The length-width ratio of a leaf (I1) and the length-width ratio of a leaf without leaf tip (I2) are the basis for the quantitative variables. Different metrics are used on the quantitative analyses: the mean values for the I1,2; proportion of trees following Traiser’s et al. (2005) categories; and proportion of broad/narrow leaves in relation to the Amazonian context (see Methods). In addition, we also used a shape variable controlling for the leaf size category. Analyses were performed in three main groups: 1) general description of the data/patterns (descriptive), 2) study of the regional variation of lamina shape (regional distribution), and 3) study of correlations between lamina shape and environmental variables (environmental correlations).
  • Table 2. Proportion of leaf lamina shape nominal categories (shapeCAT1) among Amazonian species (n=2791) and trees (n=83 868) present in the RAINFOR dataset. This categorization is derived from the Royal Horticultural Society Dictionary (1997) (Fig. 1).
  • Table 3. Leaf lamina shape physiognomy of the most species rich families (>50 species) in the RAINFOR dataset. The first four columns present the distribution of the categorical lamina system (shapeCAT2: ovate, elliptic, obovate and oblong; the Leaf Architecture Working group), the three final columns represent the mean, the standard deviation and standard error of length-width ratios (I1).
  • Fig. 3. Distribution of the abundance of lamina shape typesshapeCAT2 (elliptic, oblong, obovate and ovate) in 106 plots. This is calculated as the relative proportion of each lamina type in relation to the total of described lamina shapes within a plot. The top of each box represents the 75th percentile, the bottom represents the 25th percentile, and the line in the middle represents the 50th percentile (median). The whiskers represent the highest and lowest values that are not outliers or extreme values. Circles represent outliers and asterisks represent extreme values.
  • Fig. 4. Map of the proportion of relative narrow-leaves trees (>1 SD=I2,b) in each plot organized by frequency categories. For the purposes of clear visualisation, the positions of some plots within clusters have been adjusted, and thus may not correspond to exact geographic location.
  • Fig. 5. Map of the proportion of narrow-leaved trees (>1 SD of the mean) with big leaves (mesophyll and above) in each plot organized by frequency categories. For the purposes of clear visualisation, the positions of some plots within clusters have been adjusted, and thus may not correspond to exact geographic location

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CITATION STYLE

APA

Malhado, A. C. M., Whittaker, R. J., Malhi, Y., Ladle, R. J., Ter Steege, H., Butt, N., … A Ramírez, H. (2009). Spatial distribution and functional significance of leaf lamina shape in Amazonian forest trees. Biogeosciences, 6(8), 1577–1590. https://doi.org/10.5194/bg-6-1577-2009

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