Data from Lundell et al. 2022 Ecology. This includes plant biomass data from five species used in a growth chember experiment. These species were seeded together into pots inoculated with live and sterile soils collected from 12 grassland sites that had been subjected to drought treatments. The pots were also exposed to another drought treatment over the course of the study.
Drought has the most significant impact on arid grassland ecosystems. Managed grazing, including the timing and intensity of defoliation, may interact with drought to differentially affect processes related to soil organic matter decomposition. Extracellular enzyme activity (EEA) provides integrated measure of soil microbial activity which affects nutrient cycling. This study examined EEAs in response to five defoliation regimes and drought at seven grasslands across temperate grasslands of Canada. All sites were dominated by perennial grasses and forbs, but differed in plant species, climate, and soils. Soil samples were analyzed for five EEAs involved in carbon (C), nitrogen (N) and phosphorus (P) cycling. Drought reduced activity of enzymes involved in C cycling, β-glucosidase and β-cellobiosidase by 16 and 17%, respectively, P cycling (acid phosphatase) by 11%, and N cycling (N-acetyl-β-glucosaminidase) by 12%. β-xylosidase showed close association with, and was not affected by drought, suggesting a reduction in C turnover under future drought. β-glucosidase activity was reduced by intermediate defoliation relative to both control and heavy. Acid phosphatase and N-acetyl-β-glucosaminidase were affected by three-way interaction of drought, defoliation and mean growing season precipitation, highlighting the complex mechanism underlying EEA responses. Findings suggest that EEA was affected by drought, but defoliation effects were largely dependent upon drought and local climate.
We examined the development of understorey forage plant communities in relation to tree density in an experimental ponderosa pine (Pinus ponderosa) stand. We used a 45-year-old ponderosa pine spacing trial near Westwold, British Columbia, Canada, with five spacing treatments (1.22, 2.44, 3.66, 4.88, and 6.10 m) to sample understorey biomass and diversity, with a focus on pinegrass (Calamagrostis rubescens) and rough fescue (Festuca campestris)—two regionally important forage grasses. We predicted that there would be a positive correlation between tree spacing and understorey biomass and a compositional shift from pinegrass to rough fescue under increased tree spacing. We found that rough fescue, the preferred forage species, grew only under tree spacings equal to or greater than 3.66 m, with the greatest biomass at 4.88 and 6.10 m spacings, whereas pinegrass was equally abundant under all spacings. We believe that silvopasture principles could be applied to similar ponderosa pine stands to optimize and maintain both timber and forage productivity.
The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.
Abstract Questions Inter‐annual variability in precipitation is expected to increase in grasslands, potentially causing additional stress to systems already impacted by anthropogenic activities such as livestock grazing, which can induce changes to grassland vegetation. Yet, the sensitivity of key ecosystem functions to these co‐occurring factors is often overlooked. Here, we determine: (a) the effects of grazing on the sensitivity of above‐ground net primary productivity (ANPP sensitivity) to inter‐annual variation in water‐year precipitation (the sum of precipitation from September through to the following August); (b) whether ANPP sensitivity to precipitation is associated with shifts induced by grazing in functional group biomass (grass vs forb) contribution to total ANPP, litter, and species richness, and mean annual water‐year precipitation; and (c) whether the impacts of grazing on ANPP vary between dry and wet years. Location Native grasslands in Alberta, Canada. Methods We used long‐term (14–28 years) ANPP and precipitation data from 31 grazed grasslands, each with a paired non‐grazed livestock exclosure. ANPP was sampled annually within exclosures and adjacent grazed locations at each site. Results We found that grazing increased ANPP sensitivity to inter‐annual changes in precipitation. Increased ANPP sensitivity to precipitation in grazed, relative to non‐grazed locations was associated with both an increase in the contribution of forbs to total ANPP and a decrease in the contribution of grasses to total ANPP; reduced litter also increased ANPP sensitivity to precipitation. Species richness was not associated with ANPP sensitivity in both grazed and non‐grazed locations. Arid grasslands were more sensitive to inter‐annual variation in precipitation when grazed than were mesic grasslands. Similarly, grazing reduced ANPP during dry years but had no effect during wet years. Conclusions Overall, these findings suggest that grazed grasslands are more vulnerable to reductions in primary productivity in dry years, which may present a challenge for maintaining ecosystem services in an era of increasing precipitation variability.
Soil biota are critical drivers of plant growth, population dynamics, and community structure and thus have wide-ranging effects on ecosystem function. Interactions between plants and soil biota are complex, however, and can depend on the diversity and productivity of the plant community and environmental conditions. Plant-soil biota interactions may be especially important during stressful periods, such as drought, when plants can gain great benefits from beneficial biota but may be susceptible to antagonists. How soil biota respond to drought is also important and can influence plant growth following drought and leave legacies that affect future plant responses to soil biota and further drought. To explore how drought legacies and plant community context influence plant growth responses to soil biota and further drought, we collected soils from 12 grasslands varying in plant diversity and productivity where precipitation was experimentally reduced. We used these soils as inoculum in a growth chamber experiment testing how precipitation history (ambient or reduced) and soil biota (live or sterile soil inoculum) mediate plant growth and drought responses within an experimental plant community. We also tested whether these responses differed with the diversity and productivity of the community where the soil was collected. Plant growth responses to soil biota were positive when inoculated with soils from less diverse and productive plant communities and became negative as the diversity and productivity of the conditioning community increased. At low diversity, however, positive soil biota effects on plant growth were eliminated if precipitation had been reduced in the field, suggesting that diversity loss may heighten climate change sensitivity. Differences among species within the experimental community in their responses to soil biota and drought suggest that species benefitting from less drought sensitive soil biota may be able to compensate for some of this loss of productivity. Regardless of the plant species and soil origin, further drought eliminated any effects of soil biota on plant growth. Consequently, soil biota may be unable to buffer the effects of drought on primary productivity or other ecosystem functions as extreme events increase in frequency.
Adaptive multi-paddock (AMP) grazing is a form of rotational grazing in which small paddocks are grazed with high densities of livestock for short periods, with long recovery periods prior to regrazing. We compared the fluxes of greenhouse gases (GHGs), including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), from soils of AMP-grazed grasslands to paired neighboring non-AMP-grazed grasslands across a climatic gradient in Alberta, Canada. We further tested GHG responses to changes in temperature (5 °C vs. 25 °C) and moisture levels (permanent wilting point (PWP), 40% of field capacity (0.4FC), or field capacity (FC)) in a 102-day laboratory incubation experiment. Extracellular enzyme activities (EEA), microbial biomass C (MBC) and N (MBN), and available-N were also measured on days 1, 13, and 102 of the incubation to evaluate biological associations with GHGs. The 102-day cumulative fluxes of CO2, N2O, and CH4 were affected by both temperature and moisture content (p < 0.001). While cumulative fluxes of N2O were independent of the grazing system, CH4 uptake was 1.5 times greater in soils from AMP-grazed than non-AMP-grazed grasslands (p < 0.001). There was an interaction of the grazing system by temperature (p < 0.05) on CO2 flux, with AMP soils emitting 17% more CO2 than non-AMP soils at 5 °C, but 18% less at 25 °C. The temperature sensitivity (Q10) of CO2 fluxes increased with soil moisture level (i.e., PWP < 0.4FC ≤ FC). Structural equation modelling indicated that the grazing system had no direct effect on CO2 or N2O fluxes, but had an effect on CH4 fluxes on days 1 and 13, indicating that CH4 uptake increased in association with AMP grazing. Increasing soil moisture level increased fluxes of GHGs—directly and indirectly—by influencing EEAs. Irrespective of the grazing system, the MBC was an indirect driver of CO2 emissions and CH4 uptake through its effects on soil EEAs. The relationships of N-acetyl-β glucosaminidase and β-glucosidase to N2O fluxes were subtle on day 1, and independent thereafter. AMP grazing indirectly affected N2O fluxes by influencing N-acetyl-β glucosaminidase on day 13. We conclude that AMP grazing has the potential to mitigate the impact of a warmer soil on GHG emissions by consuming more CH4 compared to non-AMP grazing in northern temperate grasslands, presumably by altering biogeochemical properties and processes.
