Root longevity and phenology differences between two co-occurring savanna bunchgrasses with different leaf habits

1. Aristida stricta and Schizachyrium scoparium are C-4 perennial bunchgrass species native to Pinus palustris savannas in the south-eastern USA. Species differences in growth rate, tissue nutrient content and distribution suggest that A. stricta is more stress-tolerant (sensu Grime) than S. scoparium. In addition, A. stricta retains leaves all year, whereas S. scoparium is winter deciduous. Based on these observations, we wanted to determine (1) whether root longevity was higher for A. stricta as compared to S. scoparium; and (2) whether the leaf habits of these species had consequences for seasonal patterns of root production and death. 2. Using in situ rhizotrons, we recorded root number and length of these species over the course of 2.5 years on flat, transparent 50 x 50 cm windows. Kaplan-Meier (product-limit) analysis was used to produce survival functions and to estimate median lifespans for the right-censored root longevity data. These survival functions were compared using log-rank chi(2) statistics, and a proportional hazards model was used to examine the effects of season and species on the survival of each cohort. Final demographic variables were analysed using anova. 3. The median lifespan of A. stricta was 777 days whereas that of S. scoparium was 374 days. This significant difference in median lifespan was consistent between species for all cohorts in both years. Also, coincident with winter leaf senescence, root production nearly stopped for S. scoparium whereas A. stricta produced roots all year. This is probably due to the winter photosynthetic capacity of A. stricta . Root death did not exhibit any seasonal pattern in either species. A pulse of roots produced in apparent response to the 2000 drought was evident for S. scoparium but not for A. stricta . This cohort, however, rapidly died during the dormant season, probably due to the lack of winter photosynthesis in that species. 4. Aristida stricta and S. scoparium exhibited root production, death and longevity patterns that were predicted on the basis of ecological and leaf habit differences between them. These findings suggest species-specific differences in performance and element cycling in these nutrient-poor soils.