Functional traits can be used to understand and predict a number of ecological processes including community assembly and response to environmental changes. However, relatively few studies have explicitly linked traits to long-term plant performance, reproductive fitness, or population growth rates. To better understand which traits are linked to fitness under different precipitation regimes, we measured root traits, leaf traits, and two fitness indicators (above-ground growth, seed number) for four annual and four perennial species from a coastal sage scrub community in California. We predicted that, in perennial species, individuals with high resource-use efficiency would have high fitness in drier conditions. Conversely, in annual species, we predicted that individuals with high resource acquisition (i.e., a drought-escape strategy) would be more successful in drier conditions. Plants were grown in field plots under conditions of 50%, 100%, and 150% ambient precipitation. We measured leaf nitrogen (N) concentration, leaf mass per unit area (LMA), photosynthetic capacity, stomatal conductance, transpiration rate, water-use efficiency, mid-day leaf water potential, specific root length, root length density, root diameter, canopy volume, and reproductive output.
Results/Conclusions
Water treatments impacted a number of traits in three of the four annual species (Avena barbata, Bromus madritensis, Medicago polymorpha). The other five species showed few trait differences across water treatments. Across species and treatments, photosynthetic rate, stomatal conductance, and transpiration rate were positively correlated with fitness, while root length density (standardized by canopy volume) was negatively correlated with fitness. However, the correlation between some traits and fitness varied across species; for example, LMA was negatively correlated with fitness in most annuals and positively correlated with fitness in two perennial species. In the low water treatment, fitness was negatively correlated with root length density (standardized by canopy volume) and positively correlated with transpiration rate and LMA in perennial species. In contrast, fitness was negatively correlated with LMA and positively correlated with leaf N concentration in annual species under low water conditions. Our results demonstrate that the functional significance of traits varies across species and resource environment. We found some support that annual species with resource acquisition traits (e.g., low LMA, high N) fare better under low water conditions. Overall, our data do not support the idea that drought-deciduous shrubs with resource conservation traits perform better under low water conditions.