COS 20-4 - Functional traits predict demographic fitness in perennial graminoid species.

Abstract: Functional traits determine a plant species’ ability to persist in a given environment. Differences in these functional traits can simultaneously influence multiple demographic rates, rendering individual demographic rates inadequate metrics for fitness. Holistic estimates of fitness can be obtained as population growth rates, which are derived from all components of fitness. Here, we use 21 years of demographic census data paired with functional trait measurements to evaluate the net effects of functional traits on the population fitness of dominant perennial graminoids. These populations are located within 16 sites outside of Flagstaff, AZ representing a significant gradient of temperature, precipitation, and soil properties.

We detected significant inter- and intraspecific variation in functional trait values corresponding to changes in environmental conditions across sites. We also found differences in population growth rates calculated using integral projection models (IPMs). Fitness could be explained through significant interactions between traits and environments. Specifically, we found that higher values of specific leaf area (SLA; g/cm²) were predicted to convey fitness benefits (higher population growth rates) in resource-rich environments, but were detrimental at resource-poor locations. Root nitrogen content (RootN; %) displayed similar tradeoffs. Higher RootN values were beneficial to populations located where mean summer temperatures were warmer and soil sand content was higher; however, the inverse was found in populations at cooler, less sandy sites. Our results demonstrate that functional traits can explain interspecific variation in population growth rates along environmental gradients using a long-term demographic dataset. Testing these empirical models on new species under experimental conditions will provide even stronger tests of the importance of traits for population persistence.