C ONTRASTS BETWEEN WHOLE-PLANT AND LOCAL NUTRIENT LEVELS DETERMINE ROOT GROWTH AND DEATH IN A ILANTHUS ALTISSIMA (SIMAROUBACEAE) 1

Soil resources are distributed heterogeneously ( Jackson and Caldwell, 1993 ; Stewart et al., 2000 ). Root plastic responses to environmental heterogeneity may be critical for effective nutrient uptake by plants ( Hodge, 2004 , 2006 ). Plant roots are able to show several forms of phenotypic plasticity in response to spatial and temporal resource variability ( Drew and Saker, 1975 ; Gersani and Sachs, 1992 ; Gross et al., 1993 ; Mou et al., 1997 ; Wang et al., 2006 ). Plant growth aboveground is modular ( Harper, 1977 ), and modules are often semiautonomous ( Preston and Ackerly, 2004 ). There is evidence that plant roots can also behave as modules that respond independently to their own local resource environment ( de Kroon et al., 2005 ). There has been an ongoing debate about the degree of module autonomy ( Watson, 1986 ) vs. whole-plant integration ( Novoplansky et al., 1989 ) of aboveground organs. There is now a similar debate with respect to root plasticity. Based upon modularity and cost-benefi t theory, Eissenstat and Yanai (1997) proposed the concept of root effi ciency, defi ned as nutrient uptake per unit C cost (root construction + maintenance per unit of root length or mass). Following this line of reasoning, and assuming that the C cost per unit root length (or mass or surface area) is relatively constant, we hypothesized that growth or death of a root segment is determined by the local resource level with a response time lag, and we recently tested this hypothesis in a split-root experiment ( Hu and Mou, 2013 ). Consistent with previous work, the results supported the prediction that root growth was promoted by higher local N level. But root death was not observed in low nutrient patches, even when the N levels in the patches were extremely low. The responses of a root to a resource patch may depend not only on the nutrient availability in the patch but also on the contrasts among the nutrient contents in other patches or overall ( Friend et al., 1990 ; Wijesinghe and Hutchings, 1999 ; Blouin and Puga-Freitas, 2011 ). Most studies examining root plasticity have focused on the responses to the level of resource in patches ( Campbell and Grime, 1989 ; Einsmann et al., 1999 ; Wang et al., 2006 ), but little attention has been paid to the possible effects of contrasts among nutrient levels experienced by different roots of the plant ( Kotliar and Wiens, 1990 ; Lamb et al., 2004 ). For example, a plant root may respond to a low resource patch differently if other roots from the same plant are in higher resource environments than if the other roots are also experiencing low resource levels, as has been shown for shoots ( Snow, 1931 ; Novoplansky et al., 1989 ). A few modeling studies have considered the effects of contrasts among patch nutrient levels. Some modeling studies predict that organisms will allocate more efforts to patches where local resource levels are high or when the overall resource level is low ( McNickle et al., 2009 ). Other modeling studies predict that plants will place more roots into 1 Manuscript received 22 March 2014; revision accepted 31 March 2014. The authors thank Y. Yang, M. Gu, L. Pan, J. Ren, X. Liu, P. Wang, Y. Li, J. Dong, Z. Zhang, and H. Chen for help with the experiments and two anonymous reviewers for helpful comments on the manuscript. This work was supported by the National Science Foundation of China (grants 30830024 and 30770330). 5 Author for correspondence (e-mail: ppmou@bnu.edu.cn)