Life form influences survivorship patterns for 109 herbaceous perennials from six semi-arid ecosystems

Questions: What factors explain the variation in plant survival parameters across species and ecosystems? Location: Western North America. Methods: We compiled six long-term data sets from western North America to test for ecosystem-dependent demographic responses for forbs and grasses. Based on these data, we characterized 123 survivorship curves for 109 species. Three demographic parameters were extracted from these survivorship curves: survival rate at age 1, life expectancy at age 1, and a parameter describing the shape of the survivorship curve. We used a mixed effects model to compare the differences in demographic parameters between life forms (forbs or grasses) and among ecosystems, incorporating 'ecosystem' as a random factor, with life form treated as a categorical factor, and mean annual precipitation and mean annual temperature treated as continuous variables. Results: Grasses had higher survival and longer life expectancy than forbs at 1 yr of age. Both forbs and grasses followed Type III survivorship curves, although forbs were closer to Type II compared to the grasses. Averaging across species, hazard ratios for whole survivorship curves differed among most ecosystems. While mean annual precipitation had no effect on any demographic parameter, mean annual temperature had a significantly negative effect on both first year survival rates and life expectancy for forbs. Conclusions: Our results demonstrate that life form exerts a strong influence on demographic parameters and their response to temperature variation among ecosystems. This unprecedented information on the age-specific demography of herbaceous plants has implications for population modelling and research on life-history evolution and senescence.