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Abstract Plants adjust their allocation to different organs based on nutrient supply. In some plant species, symbioses with nitrogen‐fixing bacteria that live in root nodules provide an alternate pathway for nitrogen acquisition. Does access to nitrogen‐fixing bacteria modify plants' biomass allocation? We hypothesized that access to nitrogen‐fixing bacteria would have the same effect on allocation to aboveground versus belowground tissues as access to plentiful soil nitrogen. To test this hypothesis and related hypotheses about allocation to stems versus leaves and roots versus nodules, we conducted experiments with 15 species of nitrogen‐fixing plants in two separate greenhouses. In each, we grew seedlings with and without access to symbiotic bacteria across a wide gradient of soil nitrogen supply. As is common, uninoculated plants allocated relatively less biomass belowground when they had more soil nitrogen. As we hypothesized, nitrogen fixation had a similar effect as the highest level of fertilization on allocation aboveground versus belowground. Both nitrogen fixation and high fertilization led to ~10% less biomass allocated belowground (~10% more aboveground) than the uninoculated, lowest fertilization treatment. Fertilization reduced allocation to nodules relative to roots. The responses for allocation of aboveground tissues to leaves versus stems were not as consistent across greenhouses or species as the other allocation trends, though more nitrogen fixation consistently led to relatively more allocation to leaves when soil nitrogen supply was low. Synthesis: Our results suggest that symbiotic nitrogen fixation causes seedlings to allocate relatively less biomass belowground, with potential implications for competition and carbon storage in early forest development. 

Abstract Climate models predict increases in the frequency and intensity of extreme‐weather events. The impacts of these events may be modulated by biotic agents in unpredictable ways, yet few experiments cover sufficient spatiotemporal scales to measure the interactive effects of multiple extreme events.We used 15 years of a 28‐year experiment spanning several significant droughts to investigate how rainfall, large herbivores, and soil‐engineering termites affect understorey vegetation in a semi‐arid savanna.Herbivory was the dominant influence on community structure—decreasing cover, increasing species richness, and favouring occurrence of annuals relative to perennials—but these effects were contingent on rainfall and termitaria in non‐additive (hence unpredictable) ways.A separate experiment showed that resource enrichment, mimicking the effects of termitaria, does not straightforwardly compensate for top‐down effects of herbivory.Synthesis. Our study highlights the potency of top‐down forcing in African savannas. It suggests impressive robustness to drought and underscores the value of multi‐decadal experiments for studying interactions among multiple drivers of ecosystem dynamics. 

Abstract Forbs (“wildflowers”) are important contributors to grassland biodiversity but are vulnerable to environmental changes. In a factorial experiment at 94 sites on 6 continents, we test the global generality of several broad predictions: (1) Forb cover and richness decline under nutrient enrichment, particularly nitrogen enrichment. (2) Forb cover and richness increase under herbivory by large mammals. (3) Forb richness and cover are less affected by nutrient enrichment and herbivory in more arid climates, because water limitation reduces the impacts of competition with grasses. (4) Forb families will respond differently to nutrient enrichment and mammalian herbivory due to differences in nutrient requirements. We find strong evidence for the first, partial support for the second, no support for the third, and support for the fourth prediction. Our results underscore that anthropogenic nitrogen addition is a major threat to grassland forbs, but grazing under high herbivore intensity can offset these nutrient effects. 

Allometric equations are often used to estimate plant biomass allocation to different tissue types from easier-to-measure quantities. Biomass allocation, and thus allometric equations, often differs by species and sometimes varies with nutrient availability. We measured biomass components for five nitrogen-fixing tree species ( Robinia pseudoacacia , Gliricidia sepium , Casuarina equisetifolia , Acacia koa , Morella faya ) and three non-fixing tree species ( Betula nigra , Psidium cattleianum , Dodonaea viscosa ) grown in field sites in New York and Hawaii for 4–5 years and subjected to four fertilization treatments. We measured total aboveground, foliar, main stem, secondary stem, and twig biomass in all species, and belowground biomass in Robinia pseudoacacia and Betula nigra , along with basal diameter, height, and canopy dimensions. The individuals spanned a wide size range (<1–16 cm basal diameter; 0.24–8.8 m height). For each biomass component, aboveground biomass, belowground biomass, and total biomass, we determined the following four allometric equations: the most parsimonious (lowest AIC) overall, the most parsimonious without a fertilization effect, the most parsimonious without canopy dimensions, and an equation with basal diameter only. For some species, the most parsimonious overall equation included fertilization effects, but fertilization effects were inconsistent across fertilization treatments. We therefore concluded that fertilization does not clearly affect allometric relationships in these species, size classes, and growth conditions. Our best-fit allometric equations without fertilization effects had the following R 2 values: 0.91–0.99 for aboveground biomass (the range is across species), 0.95 for belowground biomass, 0.80–0.96 for foliar biomass, 0.94–0.99 for main stem biomass, 0.77–0.98 for secondary stem biomass, and 0.88–0.99 for twig biomass. Our equations can be used to estimate overall biomass and biomass of tissue components for these size classes in these species, and our results indicate that soil fertility does not need to be considered when using allometric relationships for these size classes in these species. 

Excluding large native mammals is an inverse test of rewilding. A 25-year exclosure experiment in an African savanna rangeland offers insight into the potentials and pitfalls of the rewilding endeavor as they relate to the native plant community. A broad theme that has emerged from this research is that entire plant communities, as well as individual plants, adjust to the absence of herbivores in ways that can ill-prepare them for the return of these herbivores. Three lines of evidence suggest that these “naïve” individuals, populations, and communities are likely to initially suffer from herbivore rewilding. First, plots protected from wild herbivores for the past 25 years have developed rich diversity of woody plants that are absent from unfenced plots, and presumably would disappear upon rewilding. Second, individuals of the dominant tree in this system, Acacia drepanolobium , greatly reduce their defences in the absence of browsers, and the sudden arrival of these herbivores (in this case, through a temporary fence break), resulted in far greater elephant damage than for their conspecifics in adjacent plots that had been continually exposed to herbivory. Third, the removal of herbivores favoured the most palatable grass species, and a large number of rarer species, which presumably would be at risk from herbivore re-introduction. In summary, the native communities that we observe in defaunated landscapes may be very different from their pre-defaunation states, and we are likely to see some large changes to these plant communities upon rewilding with large herbivores, including potential reductions in plant diversity. Lastly, our experimental manipulation of cattle represents an additional test of the role of livestock in rewilding. Cattle are in many ways ecologically dissimilar to wildlife (in particular their greater densities), but in other ways they may serve as ecological surrogates for wildlife, which could buffer ecosystems from some of the ecological costs of rewilding. More fundamentally, African savannah ecosystems represent a challenge to traditional Western definitions of “wilderness” as ecosystems free of human impacts. We support the suggestion that as we “rewild” our biodiversity landscapes, we redefine “wildness” in the 21 st Century to be inclusive of (low impact, and sometimes traditional) human practices that are compatible with the sustainability of native (and re-introduced) biodiversity. 

As droughts become longer and more intense, impacts on terrestrial primary productivity are expected to increase progressively. Yet, some ecosystems appear to acclimate to multiyear drought, with constant or diminishing reductions in productivity as drought duration increases. We quantified the combined effects of drought duration and intensity on aboveground productivity in 74 grasslands and shrublands distributed globally. Ecosystem acclimation with multiyear drought was observed overall, except when droughts were extreme (i.e., ≤1-in-100-year likelihood of occurrence). Productivity losses after four consecutive years of extreme drought increased by ~2.5-fold compared with those of the first year. These results portend a foundational shift in ecosystem behavior if drought duration and intensity increase, from maintenance of reduced functioning over time to progressive and profound losses of productivity when droughts are extreme.