| Plants affect, and are affected by biotic and abiotic factors, and these relationships may differ between grasses and trees. These differences may have ecosystem-level consequences in areas undergoing woody plant invasion, afforestation or deforestation. In an aspen forest-grassland ecotone at the northern edge of Great Plains, I assessed nitrogen contributions from root and shoot production, and examined the relationships among plants, soil water and temperature, and Collembola (Insecta).; N inputs from total production were significantly greater in forest than grassland, but such differences were due to the quality rather than the quantity of litter produced. Total (root + shoot) production was similar in aspen forest and grassland, and most (75--90%) of total production and N input to the soil was accounted by fine roots.; Forest and grassland also differed in their effects on soil temperature and water. Relative to grasslands, forest soils were cooler and moister near the surface but drier in deeper layers. Vertical patterns of soil water depletion were closely related to vertical patterns of root length. On the other hand, the seasonality of soil water and temperature seemed to control the timing of plant production, and these controls varied between roots and shoots. Shoot production, for example, was positively related to soil moisture, but root production was negatively related with moisture and positively related with soil temperature. Thus, the increasing temperature and seasonal droughts forecasted to occur as a result of global climate change may increase root: shoot ratios, and alter plant effects on resources. To control for soil effects on water and plant production, I also studied these relationships in monocultures of five grasses and five woody plants growing on a common soil type. In general, data from monocultures exhibited the same patterns as natural vegetation, suggesting that plants and not site conditions generate the differences between vegetation types.; Collembola, the most abundant soil microarthropods, can affect root-mycorrhiza associations and, consequently, plant growth. Collembola density was three times greater in forest than grassland, and negatively related to mycorrhizae and root production, likely due to Collembola grazing. Such negative effects, however, were compensated by other factors, because the long-term relationship between Collembola and mycorrhizal roots was positive. Greater Collembola density may also promote N mobilization and availability in forest soils.; Overall, a combined above-belowground approach allowed me to establish the relative contribution of belowground processes in grassland and forest. My results also underline the importance of differentiating between shoot and root responses in predicting vegetation and biogeochemical changes resulting from climate change. |
