A meta-analysis of the global distribution pattern of condensed tannins in tree leaves

Condensed tannins (CT's) are a dominant class of plant secondary metabolites that play important roles in regulating ecosystem processes such as herbivory, decomposition, and nutrient cycling. However, the factors shaping the global distribution pattern of the abundance of CT's in tree leaves are still poorly understood. The purpose of this study was to assess the global distribution pattern of CT abundance in tree leaves in order to examine the effects of climatic parameters (air temperature and precipitation) and latitude as well as the effects of methodologies used for the measurement of CT concentration (extract solutions and standard substance). Through a database search, we collected 805 individual values of CT concentration in tree leaves from 72 study sites, which were reported in 26 individual studies as well as mean annual temperature, annual precipitation, and latitude. We also collected the information of extract solution and standard substance if available. We found that mean annual temperature positively correlated with the foliar CT abundance. On the other hand, annual precipitation and latitude did not have significant influences on CT abundance. We did not find significant effects of the extract solution and standard substance on the CT abundance, neither. A higher allocation of carbon to CT's may be advantageous in warmer regions, where the potential herbivore pressure may be relatively higher than that in cooler regions; however, this hypothesis could not be directly tested in this study. Our finding that foliar CT abundance is significantly associated with mean annual temperature supports the 'classical' picture of the stronger plant defenses in warmer climates. A comprehensive empirical comparison using standardized methods is still required to improve our understanding of CT abundance in tree leaves and the mechanism of ecosystem processes.