Grazing alters the sensitivity of plant productivity to precipitation in northern temperate grasslands

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 interannual 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.

Multi‐year drought alters plant species composition more than productivity across northern temperate grasslands

“…For example, we observed the following patterns of response to drought in this experiment: immediate changes in soil moisture content, followed by shifts in species composition (though not richness or evenness), but inconsistent effects on ANPP. This may chiefly be through variation in the abundance of graminoids, which contribute most to the stability of productivity to precipitation in the region (Batbaatar et al, 2021;Bork et al, 2019). The positive link between the effects of reduced precipitation on species composition and evenness further suggests that changes in the abundance of graminoids may have led to differences in species composition between the ambient and reduced precipitation treatments.…”

Antecedent climatic conditions spanning several years influence multiple land-surface phenology events in semi-arid environments

“…For example, only a few thousand of over a million records in the evergreen forest land cover type were within fire perimeters. In addition to fire, grazing can also lead to possible phenological impacts through changes in composition, productivity, and sensitivity to precipitation variability (Fuhlendorf and Engle, 2001;Briske et al, 2005;Beever et al, 2008;Batbaatar et al, 2021). Data for grazing intensity do not exist at the scale of our study, but this metric should be considered in future assessments.…”

Ecosystem Gross Primary Productivity After Autumn Snowfall and Melt Events in a Mountain Meadow

“…This notion is consistent with a hypothesis that montane grassland photosynthesis is resilient to late‐season snow events and can quickly re‐establish previous function after melt, much in the same way that grass photosynthesis tends to re‐establish quickly following harvesting (Novick et al., 2004; Stoy et al., 2008), grazing (Parsons et al., 1983), and heat waves (Hoover et al., 2014). Alternately, canopy photosynthesis in mountain meadows could simply be resistant to snow disturbances following a growing body of cold‐season research that demonstrates that photosynthesis can occur under snow (Lundell et al., 2008; Starr & Oberbauer, 2003; Street et al., 2012) and the notion that grass productivity is resistant to minor disturbances like mild drought (Batbaatar et al., 2021). In this scenario, ecosystem carbon uptake would not meaningfully differ before and after snow events and ecosystem carbon uptake would have little response to snow apart from the limitations to photosynthesis caused by cold conditions and diminished light availability under the snowpack.…”