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Applying scaling laws to Psychotria: A test of 19 species in a tropical forest .
Hardesty, Britta Denise1, Hubbell, Stephen1,2, Wright, Joseph2, 1 2
ABSTRACT- Recent studies comparing plant species across multiple biomes from arctic tundra to tropical rain forests suggest that scaling laws may define universal relationships between leaf lifespan, net photosynthetic rate, leaf conductance, leaf nitrogen, and specific leaf area. Here, we test the hypothesis that these same scaling relationships also occur among closely related species in the genus Psychotria that occur in a single forest community on Barro Colorado Island, Panama. Ranging from small shrubs to larger treelets, these 19 species occupy a variety of microsites with variable light, water, and nutrient availability. We compared maximum photosynthetic rate, leaf nitrogen, leaf conductance, 13C, leaf lifespan and specific leaf area to test whether relationships among pairs of traits were linearly related, as has been previously reported. If these scaling- law patterns also apply at a local scale within a single forest community, then maximum photosynthetic rate should b! e negatively correlated with leaf life span and positively correlated with leaf nitrogen and specific leaf area. Our analyses demonstrate a lack of relationship between these and other tested pairs of leaf traits. We suggest the scaling law relationships observed at larger spatial scales are not applicable within a group of closely related species that occupy a wide range of environmental conditions, despite large differences between species for several traits (leaf lifespan, maximum photosynthetic rate, leaf nitrogen, and specific leaf area). Evolutionary constraints may prevent the expression of these scaling laws.
KEY WORDS: scaling laws, Psychotria, ecophysiological traits