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Summary 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 〈 50 cm windows. Kaplan‐Meier (productlimit) 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 χ 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.

Summary 1 Optimality theory suggests that roots should be kept alive until the efficiency of resource acquisition is maximized (i.e. a maximum ratio of benefits to costs). Because root efficiency may vary with environmental conditions, ecological distributions of plant species may be linked to different patterns of root demography. 2 In a greenhouse study, we investigated fine root turnover (growth and death) for three woody species from the fall-line sandhills of the south-eastern USA. Pinus palustris Mill. is a generalist in this habitat, whereas Quercus marilandica Muenchh. occurs in more fertile, mesic habitats relative to Quercus laevis Walt. Seedlings were grown under four resource treatments (water + nutrients) for 7 months: high resources, low resources, and short-term exposure (last 2 months of the study) to resource enrichment or depletion. 3 Increasing fine root longevity may be optimal in resource-poor sites because root efficiency may be maximized by less root turnover and resource loss. As expected, fine root death and growth was less in species from xeric habitats (Q. laevis), but greater in species from mesic habitats (Q. marilandica). The generalist species P. palustris exhibited high growth but little death of fine roots. 4 When soil resources decrease, less root turnover may reduce resource loss. Fine root growth of all three species decreased at low resources. Fine root death decreased at low resources in Q. marilandica, the only species with significant root death. 5 Demographic responses differed between fine roots and leaves. Although leaf and fine root death were greater in Q. marilandica than in Q. laevis, leaf death, unlike root death, did not change with resource availability. Short-term resource enrichment or depletion affected leaf production but not fine root demography in the Quercus species. 6 Given that fine root morphology affects root maintenance and construction costs, we expected greater fine root growth and death in species with thinner roots of high specific root length (SRL) and low density. However, the species with the greatest root turnover, Q. marilandica, had thick and dense roots of low SRL. 7 Results from our study with woody species indicate that fine root demography, but not fine root morphology, was linked to ecological distribution in a narrow geographical range. Differences in fine root turnover under different resource availability were consistent with the optimality theory, and may reflect trade-offs between tolerance and competitive ability below ground: less root turnover of xeric species under low fertility may conserve resources, whereas faster root growth of mesic species potentially maximizes resource uptake in more fertile soils.

Seasonal patterns of water stress (determined from predawn xylem pressure potentials) and relative use of summer precipitation (determined from hydrogen isotope composition) varied intraspecifically and interspecifically in a Great Basin shrub community. The development of water stress during the dry season from June to July was positively correlated with the use of summer precipitation in August. The smallest plants generally developed the greatest water stress and took up the most summer precipitation, presumably due to being more dependent on shallow roots. Among mature shrubs examined, moisture from summer precipitation was taken up by Chrysothamnus viscidiflorus and Artemisia tridentata, but not by C. nauseosus, Juniperus osteosperma and Tetradymia canescens (...)

Ecophysiological characteristics of plants from different size and life-history classes were investigated in a field population of Crysothamnus nauseosus. Juvenile plants (>1-year-old but pre-reproductive) had higher rates of photosynthesis, stomatal conductance and transpiration than reproductive adults, even though pre-dawn xylem pressure potentials of juveniles were slightly lower. Juveniles were also less water-use efficient than adults based on instantaneous gas exchange (photosynthesis/transpiration) and carbon isotope discrimination (...)