JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Facilitation of tree establishment by nurse shrubs, which ameliorate otherwise unfavorable microenvironmental conditions, is a widely studied phenomenon. However, relatively little is known about how facilitative influences change in relation to interannual climatic variability. In northern Patagonia, Argentina, we examined influences of potential nurse shrubs on the establishment of the conifer Austrocedrus chilensis and assessed the significance of those influences to establishment during years of contrasting climate. We also experimentally investigated the effects of nurse shrubs and different water availability on tree seedling emergence and survival.A strong spatial association of Austrocedrus juveniles with shrubs, both beneath shrub canopies and near shrub canopies, indicates that shrubs favorably influence tree regeneration and that in some habitats and time periods nurse plants appear to be required for successful tree seedling establishment. Protection from direct sunlight was the main factor contributed by shrubs that enhanced the germination and survival of Austrocedrus. During the 1995-1996 experiment, no seedlings survived in the unwatered interspaces between shrubs, whereas maximal survival was obtained by watering seedlings at shaded sites.The results of this study indicate that in the Patagonian ecotone the strength of facilitative associations between shrubs and Austrocedrus juveniles closely tracks annual climatic variability. During extremely warm dry years, recruitment of Austrocedrus is nil with or without protection by nurse shrubs. During cool wet years, establishment may occur both beneath shrubs and in open interspaces; however, during average years, which are still years with substantial drought stress, establishment of Austrocedrus appears to require nurse shrubs.

Phenology is central to understanding vegetation response to climate change, as well as vegetation effects on plant resources, but most temporal production data is based on shoots, especially those of trees. In contrast, most production in temperate and colder regions is belowground, and is frequently dominated by grasses. We report root and shoot phenology in 7-year old monocultures of 10 dominant species (five woody species, five grasses) in southern Canada. Woody shoot production was greatest about 8 weeks before the peak of root production, whereas grass shoot maxima preceded root maxima by 2-4 weeks. Over the growing season, woody root, and grass root and shoot production increased significantly with soil temperature. In contrast, the timing of woody shoot production was not related to soil temperature (r 5 0.01). The duration of root production was significantly greater than that of shoot production (grasses: 22%, woody species: 54%). Woody species produced cooler and moister soils than grasses, but growth forms did not affect seasonal patterns of soil conditions. Although woody shoots are the current benchmark for phenology studies, the other three components examined here (woody plant roots, grass shoots and roots) differed greatly in peak production time, as well as production duration. These results highlight that shoot and root phenology is not coincident, and further, that major plant growth forms differ in their timing of above-and belowground production. Thus, considering total plant phenology instead of only tree shoot phenology should provide a better understanding of ecosystem response to climate change.

Summary 1.The phenology of temperate vegetation is advancing in association with climate warming. Most phenology data, however, comes from flowers and tree leaves. We tested the generality of results from shoot phenology by expanding data collection in two new directions. We related forest leaf phenology to root phenology, and to phenology in a second habitat, grassland. 2. We measured leaf and root phenology simultaneously in aspen forest and adjacent native grassland. Root growth accounts for 80-90% of productivity in these habitats. Seasonal variation in soil moisture and temperature were also measured. 3. Forest leaf production was greatest about 45 days before peak root production, resulting in a significant negative correlation between leaf and root production in forest. Grassland leaf production was greatest about 15 days before peak root production, and grassland leaf and root production were significantly positively correlated. The duration of root production was 40% greater than that of shoot production. 4. Forest leaf production increased significantly with increasing soil moisture, but not temperature. In contrast, the production of forest roots, grassland roots and grassland leaves increased significantly with soil temperature. 5. Synthesis . The most commonly measured aspect of phenology, forest leaves, is out of step with the majority of production in forest, as well as phenology in grassland. The invasion of grassland by woody vegetation is characterized by a decoupling of root and shoot phenology, a result that has not been reported previously. Given the global nature of woody plant encroachment, this decoupling may influence our general understanding of productivity and carbon sequestration in response to warming.

Forest expansion at the northern edge of the Great Plains is associated with increased availability of soil nitrogen (N). Studies of N dynamics typically focus on aboveground litter production, but in semiarid ecosystems, fine-root production greatly exceeds shoot production. We explored the contribution of root and shoot litter to N cycling in adjacent grassland and aspen (Populus tremuloides) forest at the northern edge of the Great Plains. We used a new approach to measure N inputs from root litter production: we combined root productivity data from minirhizotron images with N content data from several root diameter and color classes. We also measured the production and N content of aboveground litter. The novel contribution of our study comes from the simultaneous measurements of above-and belowground productivity and N input from litter production in adjacent forest and grassland habitats. Aboveground litter production was threefold greater in forest than in grassland (330 vs. 136 g·m Ϫ2 ·yr Ϫ1 ), but fine roots accounted for 80-90% of total litter production. As a result, total production was not significantly different between habitats, and the N contribution from total litter production was surprisingly similar between grassland (16.8 g·m Ϫ2 ·yr Ϫ1 ) and forest (17.1 g·m Ϫ2 ·yr Ϫ1 ). Thus in spite of great differences between habitats in aboveground litter production, N inputs from total litter production cannot explain the higher availability of N in forest soils. However, we found differences between habitats in root litter quality (forest, 1.14% N; grassland, 0.81% N, as well as in the seasonal and vertical distribution of root production. Grassland root production was significantly greater than forest root production in early summer in the top 20 cm of soil. Conversely, forest produced more root length at the end of the growing season in deeper soil layers (Ͼ50 cm). These differences may increase available N in forest soils, but this increase is not attributable to differences in total litter production between habitats.