The relative importance of intra- and interspecific trait variation in the maintenance of plant species diversity

Background/Question/Methods

Despite the well-recognized importance of intraspecific variability in evolutionary biology, its role in the maintenance of species coexistence and diversity is poorly known. The limiting similarity principle predicts that as species richness increases, intraspecific variation will decrease. In contrast, some recent empirical evidence as well as theoretical developments have suggested that large intraspecific variation may promote species diversity. To test these differing hypotheses we utilized a trait-based approach to test the relative contribution of intra- and interspecific trait variation in promoting species coexistence. In this study, we quantify variation in leaf function within and across tree communities along elevational gradients. We also assessed the potential differential responses of temperate versus tropical forests by sampling three mountain gradients across a latitudinal gradient: Area de Conservación Guanacaste (Costa Rica), the Santa Catalina Mountains (Arizona), and the Siskiyou Mountains (Oregon). Two key leaf traits related to plant growth and resource use were considered: specific leaf area and leaf succulence.

Results/Conclusions

First, in temperate systems, interspecific variance was negatively correlated to elevation; however, in the tropical site interspecific variance was not correlated to elevation. Conversely, intraspecific variation was not correlated to elevation in temperate systems and negatively correlated to elevation in the tropical system. Interestingly, neither intra- nor interspecific variation was correlated to species richness in either temperate or tropical sites which suggests that limiting similarity may not be a primary driver of trait or species diversity. In addition, the ratio of intraspecific to interspecific variation was greatest at high elevations in the temperate systems and least at high elevations in the tropical system, suggesting a strong environmental effect. Finally, interspecific variation was not correlated to intraspecific variation in any site which further suggests that niche packing effects may not be as important as other environmental or abiotic drivers in the maintenance of diversity. The patterns seen here provide insight into the underlying mechanisms that promote species coexistence across latitudinal and elevational gradients and the relative importance of intra- and interspecific variation in the maintenance of species diversity in temperate and tropical forests.