Fine root demography and morphology in response to soil resources availability among xeric and mesic sandhill tree species

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 Querecus 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.