If environmental stress provides conditions under which positive relationships between plant species richness and productivity become apparent, then species that seem functionally redundant under constant conditions may add to community functioning under variable conditions. Using naturally co-occurring mosses and liverworts, we constructed bryophyte communities to test relationships between species diversity (1, 2, 4, 8, 16, 24, or 32 species) and productivity under constant conditions and when exposed to experimental drought. We found no relationship between species richness and biomass under constant conditions. However, when communities were exposed to experimental drought, biomass increased with species richness. Responses of individual species demonstrated that facilitative interactions rather than sampling effects or niche complementarity best explained results-survivorship increased for almost all species, and those species least resistant to drought in monoculture had the greatest increase in biomass. Positive interactions may be an important but previously underemphasized mechanism linking high diversity to high productivity under stressful environmental conditions. T he potential for loss of ecosystem functioning (physical and chemical processes occurring within ecosystems) with declining species diversity has prompted a number of recent studies that experimentally examine relationships between plant species richness and ecosystem processes, especially productivity (e.g., refs. 1-4). Although some studies have found that increased plant species richness resulted in increased productivity (1, 2, 4), the existence of positive relationships between diversity and productivity and the extent to which they are caused by simple ''sampling effects'' (increased probability of including highly productive dominant species in diverse communities), as opposed to more elaborate mechanisms such as niche complementarity (reduced interspecific compared with intraspecific competition), remain highly contentious (refs. 5-9; http:͞͞ www.sciencemag.org͞cgi͞content͞full͞289͞5483͞1255a). However, most of these experiments were conducted under fairly constant conditions, where the value of high species richness may not be apparent. Instead, species that seem to be ''redundant'' under one set of conditions may provide additional services under different conditions. Such a scenario has been linked most often to the ''insurance hypothesis'' (10-14), a more complicated sampling-effects model suggesting that in communities with high levels of diversity at least some of the species will be highly productive after environmental disturbance (e.g., a drought or flood). However, this pattern also could arise through niche complementarity or even positive interactions among species in more diverse communities. Demonstrating compensatory abilities of apparently redundant species within a community would provide a powerful argument for the maintenance of high diversity in natural systems (12, 15), and also may be able to explain the inco...
Constraints on nitrogen fixation are the ultimate causes of N limitation of primary production, but hypotheses concerning limitations to N 2 fixation remain largely untested in natural terrestrial ecosystems. We examined limitations to N 2 fixation by thinleaf alder (Alnus tenuifolia) in two stages of primary forest succession on the Tanana River floodplain (interior Alaska, USA) and focused on the hypothesis that N 2 fixation was limited by low soil P availability. Paired control and P fertilized plots were established at four replicate early successional alder stands and four later successional poplar (Populus balsamifera) stands (dense alder understories with mature poplar overstories) and N 2 fixation was estimated with an acetylene reduction assay. In alder stands, P fertilization increased total nodule dry biomass and increased total ecosystem N inputs, but it had little effect on nitrogenase activity per unit nodule dry mass (specific acetylene reduction activity, ARA). Specific ARA increased only in late July when soil temperature and ARA were at their maximum values. In contrast, fertilization had no effect on these measures in poplar stands where reduced soil moisture may have superseded limitation by P. We detected no differences in specific ARA, total nodule biomass, or N inputs, between alder and poplar stands but all of these measures were highly variable. Leaf area of the alder canopy emerged as the best predictor of ecosystem inputs of fixed N among control plots. Alders resorbed high amounts of P but little N (consistent with low P availability and a high P demand and a high N availability in alder), and P fertilization reduced P resorption but had no effect on N resorption. The timing of N 2 fixation and N resorption indicate that late-season increases in leaf N, following a midseason reduction in leaf N, were driven by N 2 fixation in excess of plant N demands as nodules continued fixing N while alder leaves senesced. These results have shown that P limits N 2 fixation in alder stands in this nitrogen-limited sere, but that factors limiting N 2 fixation can change over short successional time scales.
Constraints on nitrogen fixation are the ultimate causes of N limitation of primary production, but hypotheses concerning limitations to N2 fixation remain largely untested in natural terrestrial ecosystems. We examined limitations to N2 fixation by thinleaf alder (Alnus tenuifolia) in two stages of primary forest succession on the Tanana River floodplain (interior Alaska, USA) and focused on the hypothesis that N2 fixation was limited by low soil P availability. Paired control and P fertilized plots were established at four replicate early successional alder stands and four later successional poplar (Populus balsamifera) stands (dense alder understories with mature poplar overstories) and N2 fixation was estimated with an acetylene reduction assay. In alder stands, P fertilization increased total nodule dry biomass and increased total ecosystem N inputs, but it had little effect on nitrogenase activity per unit nodule dry mass (specific acetylene reduction activity, ARA). Specific ARA increased only in late July when soil temperature and ARA were at their maximum values. In contrast, fertilization had no effect on these measures in poplar stands where reduced soil moisture may have superseded limitation by P. We detected no differences in specific ARA, total nodule biomass, or N inputs, between alder and poplar stands but all of these measures were highly variable. Leaf area of the alder canopy emerged as the best predictor of ecosystem inputs of fixed N among control plots. Alders resorbed high amounts of P but little N (consistent with low P availability and a high P demand and a high N availability in alder), and P fertilization reduced P resorption but had no effect on N resorption. The timing of N2 fixation and N resorption indicate that late‐season increases in leaf N, following a midseason reduction in leaf N, were driven by N2 fixation in excess of plant N demands as nodules continued fixing N while alder leaves senesced. These results have shown that P limits N2 fixation in alder stands in this nitrogen‐limited sere, but that factors limiting N2 fixation can change over short successional time scales.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.