Demography of perennial grassland plants: survival, life expectancy and life span

1. Survival, life expectancy and life span are key demographic parameters that are essential for understanding life-history evolution and forecasting population dynamics, but empirical data on these parameters is extremely limited for herbaceous species. 2. We used long-term data from annually mapped permanent quadrats in a Kansas, USA, grassland to estimate survival, life expectancy and life span for 29 perennial forbs (herbaceous dicots) and 11 perennial grasses. In the cases of both forbs and grasses, they were the most common species at the research site. 3. We developed computer programs to track the identity of individual genets based on their spatial locations in the permanent quadrats. The programs distinguished between new recruits and surviving individuals, and calculated the ages and life spans of the survivors. 4. Most herbaceous perennials die young; life expectancy at age 1 year ranged from 0.6 to 6.5. However, forbs are more likely to die young than grasses. Survival from age 1 to 2 years for forbs ranged from 0.11 to 0.49 with an average of 0.30, whereas for grasses it ranged from 0.30 to 0.63 with an average of 0.44. Maximum observed life spans ranged from 3 to 25 years for forbs and 5 to 39 years for grasses. 5. All species tended towards Type III survivorship curves, but grasses were more strongly Type III while many forbs had relatively constant survival rates with age. Therefore, population models must account for increasing survival with age, especially for grasses. 6. Age was a better predictor of grass survival than size, raising questions about the use of size-based methods to indirectly estimate survival and life span. 7. Maximum observed life span was positively and significantly related with species importance. 8. Synthesis. The higher survival, life expectancy and life span of grasses compared to forbs may provide a demographic explanation for community-level differences in the dominance and turnover of these two functional groups in grassland plant communities.