Using functional traits to predict demographic parameters of herbaceous perennial plants

Background/Question/Methods

An important assumption that underlies trait-based models of community assembly is that plant functional traits influence population dynamics in different environments. This assumption allows trait-based models to bypass the complexities associated with population-based models. If it can be shown that traits are good predictors of demographic parameters, then perhaps we can use traits to predict demographic parameters for use in population-based models. Using data collected annually from 2002 to 2010 from 98 permanent 1m² quadrats we obtained estimates of survival rates and life expectancy for 17 common herbaceous perennial species in northern Arizona. We used linear regression to quantify the strength of the relationship between these demographic parameters and functional traits associated with resource acquisition (specific leaf area (SLA), foliar nitrogen concentration, plant height and seed mass). Specifically we  hypothesized that: 1) SLA and nitrogen content would be a good predictor of year 2 survival rates and life expectancy at age 1 since these traits are associated with relative growth rates; 2) seed mass would be a good predictor of year 1 survival rates because species with large endosperm reserves may have higher probabilities of establishment.

Results/Conclusions

With respect to hypothesis one we found that specific leaf area (SLA) was negatively correlated with life expectancy at age 1 (R² = 0.34, P = 0.012). Given that species adapted to this semi-arid environment have relatively low SLA we would expected them to have higher life expectancies once established. SLA was weakly negatively correlated with year 2 survival (R² = 0.23, P = 0.050). Foliar nitrogen content was not correlated with either year 2 survival (R² = 0.01, P = 0.5) or life expectancy at age 1 (R² = 0.1, P = 0.2). Seed mass was weakly positively correlated with year 1 survival (R² = 0.23, P = 0.050). One explanation for this would be that having larger endosperm reserves would increase the chances of survival once established. These results confirm some of the theorized relationships between functional traits and survival probabilities proposed in the literature. However, given that mean trait values did not strongly predict these demographic parameters, we caution the use of traits to parameterize population-based models at this stage.