Biodiversity increases the resistance of ecosystem productivity to climate extremes.

It remains unclear whether biodiversity buffers ecosystems against climate extremes, which are becoming increasingly frequent worldwide. Early results suggested that the ecosystem productivity of diverse grassland plant communities was more resistant, changing less during drought, and more resilient, recovering more quickly after drought, than that of depauperate communities. However, subsequent experimental tests produced mixed results. Here we use data from 46 experiments that manipulated grassland plant diversity to test whether biodiversity provides resistance during and resilience after climate events. We show that biodiversity increased ecosystem resistance for a broad range of climate events, including wet or dry, moderate or extreme, and brief or prolonged events. Across all studies and climate events, the productivity of low-diversity communities with one or two species changed by approximately 50% during climate events, whereas that of high-diversity communities with 16-32 species was more resistant, changing by only approximately 25%. By a year after each climate event, ecosystem productivity had often fully recovered, or overshot, normal levels of productivity in both high- and low-diversity communities, leading to no detectable dependence of ecosystem resilience on biodiversity. Our results suggest that biodiversity mainly stabilizes ecosystem productivity, and productivity-dependent ecosystem services, by increasing resistance to climate events. Anthropogenic environmental changes that drive biodiversity loss thus seem likely to decrease ecosystem stability, and restoration of biodiversity to increase it, mainly by changing the resistance of ecosystem productivity to climate events.

Lamb performance in hardwood silvopastures, II: animal behavior in summer.

Integrating trees into pastures, a practice known as silvopasture, may benefit livestock in the summertime through the provision of shade. The purpose of this project was to compare the behavioral patterns of sheep grazing in silvopastures and open pastures. Black walnut (Juglans nigra L.) and honeylocust (Gleditisia triacanothose L.) based silvopasture systems were compared with open pastures in a randomized complete block design with three blocks over two summers. Behavior measures were recorded within a replicate within a week, and these measures were taken sequentially within three experimental periods. Ewe lambs (n = 3) within each experimental unit were equipped with a wideband audio-recording device to detect prehension events. Time-lapse cameras documented sheep behavior every 60 s. In the silvopastures, the lambs spent over 90% of daylight hours within shade from trees. Lambs in silvopastures spent more time lying down than animals in the open pastures (P ≤ 0.01), while lambs in the open pastures spent more than 2 h longer each day standing (P < 0.0001). Lambs in the black walnut silvopastures spent more time grazing (488 ± 14 min · d-1) than lambs in the honeylocust silvopastures (438 ± 14 min · d-1; P = 0.0493) and lambs in the open pastures (417 ± 14 min · d-1; P = 0.0026). There was no difference in grazing time for lambs in the latter two systems (P = 0.5597). Spectral analysis of the imagery revealed that the lambs in the black walnut silvopastures grazed more frequently than the lambs in the other systems for both years. The acoustic analysis, though limited by recorder durability to 47 complete recordings, revealed no difference in total bites taken per day (P ≥ 0.7222) or in the morning (P ≥ 0.2069), afternoon (P ≥ 0.5816), and evening periods (P ≥ 0.9337). Silvopastures provide an opportunity to improve lamb comfort in the summer.

Lamb performance in hardwood silvopastures, I: animal gains and forage measures in summer.

The integration of trees into pasture systems can have variable effects on forage and animal growth. Some reports of these systems have indicated that animal gains are similar or better even when tree presence lowers forage yield. Forage production and animal performance were compared in black walnut (Juglans nigra L.)-based and honeylocust (Gleditisia triacanthose L.)-based silvopasture systems and open pastures in a randomized complete block design with three blocks over three summers. Cool season-based, mixed grass pastures were rotationally stocked with four to seven lambs depending on available forage. A rising plate meter was used to estimate pre- and post-graze forage mass. Forage samples of the mixed sward were collected and analyzed for nitrogen (N) and neutral detergent fiber (NDF) concentrations. Species percent cover was estimated using a modified Daubenmire approach at the same 12 points within each experimental unit every 4 wk during the study. Pre-graze herbage mass was similar (P = 0.0717) in honeylocust silvopastures (5020 ± 30 kg·ha-1) and open pastures (4930 ± 30 kg·ha-1) and lowest (P < 0.0001) in the black walnut silvopastures (3560 ± 30 kg·ha-1). Forages in the black walnut and honeylocust silvopastures had similar (P = 0.4867) N concentrations (23.3 ± 0.4 and 23.9 ± 0.4 g·kg-1, respectively), which was greater (P ≤ 0.0003) than that of the forages in the open pastures (21.0 ± 0.4 g·kg-1). Forages in the honeylocust silvopasture had lower (P ≤ 0.0042) NDF concentrations (507 ± 3 g·kg-1) than forages in the black walnut silvopasture and open pastures (mean = 525 ± 3 g·kg-1). Forage species present in the black walnut silvopastures differed from those present in the open and honeylocust systems, which had similar composition. Despite differences in stocking rates, total lamb weight gains per system did not differ (P ≥ 0.7592) among black walnut, honeylocust, and open pasture systems (10 ± 2, 12 ± 2, and 10 ± 2 kg·d-1, respectively). Silvopasture practices can improve land productivity when incorporated into cool season forage pastures.

Evaluating mob stocking for beef cattle in a temperate grassland.

Mob stocking is a type of livestock management method where high densities of animals are restricted to a small area of grassland for short periods of time (e.g., 12-24 hr.) before being moved to new forage. Use of mob stocking has generated considerable interest among forage-livestock professionals in recent years, but questions remain about its purported benefits to cattle and forage plants. To address questions about the possible benefits of mob stocking, a 3-yr study (2014-2016) was conducted in Virginia, USA comparing mob, rotational, and continuous stocking methods in a temperate grassland common to that region. The main objective of this study was to evaluate how mob-stocking management affected selected forage variables, cattle performance, and legume/weed abundance. Herbage mass and nutritive value were measured monthly. Cow and calf weights and body condition score (BCS) were used as indicators of animal performance. Legumes (red and white clover) were over-seeded prior to the study, and their abundance along with weeds were evaluated annually thereafter. Mean herbage mass and forage nutritive values were similar across mob, rotational and continuously stocked systems despite extra-long rest periods that allowed grasses to grow tall and over-mature under mob stocking. Cow weights going into winter were lower (P < 0.05) under mob stocking (619 kg) compared with continuous stocking (688 kg) possibly because many tall grasses were trampled and not grazed. Lastly, we found mob stocking can favor establishment of erect-growing red clover (Trifolium pretense L.), but it had no effect on weed abundance. Overall, we found few compelling reasons why mob stocking should be adopted for season-long forage and livestock production over other stocking methods in this environment.

Physiological Effect of Cutting Height and High Temperature on Regrowth Vigor in Orchardgrass.

Producers of orchardgrass (Dactylis glomerata L.) hay in the Mid-Atlantic US have experienced a reduction in regrowth vigor and a decline in the persistence of their swards. The common management practice for the region is to harvest the first growth of hay by cutting at 2.5-7.5 cm height in May or June. We hypothesize that high temperature and low cutting height interact to limit the regrowth rate. To test this, orchardgrass plants were cut to either 2.5 or 7.5 cm and then placed into environmentally controlled chambers with a constant temperature of 20 or 35°C. Stubble was harvested on days 0, 1, 3, and 11 following cutting and subjected to metabolite analysis. Photosynthetic parameters were measured in the regrown leaves on days 3 and 11, and regrowth biomass was recorded on day 11. Under optimal growth temperature (20°C), vegetative regrowth upon defoliation was significantly enhanced when more stubble tissue remained. However, this advantage was not observed under heat stress. Defoliation generally decreases the abundance of carbohydrate reserves in stubble. Interestingly, high temperature stimulated the accumulation of starch and ethanol-soluble carbohydrates in plants cut to 7.5 cm. The similar trends were also observed in protein, amino acids, nitrate, and ammonium. These responses were not pronounced in plants cut to 2.5 cm, presumably due to inhibited photosynthesis and photosystem II photochemistry. Overall, we anticipated that heat-activated metabolite accumulation is part of adaptive response to the stress. However, modified allocation of carbohydrate and nitrogen reserves leads to reduced vegetative regrowth upon defoliation. These data suggest that cutting height management for orchardgrass may be more effective for its regrowth vigor and productivity in cool seasons or when cool weather follows hay harvest.

Shifts in grassland invasibility: effects of soil resources, disturbance, composition, and invader size.

There is an emerging recognition that invasibility is not an intrinsic community trait, but is a condition that fluctuates from interactions between environmental forces and residential characters. Elucidating the spatiotemporal complexities of invasion requires inclusion of multiple, ecologically variable factors within communities of differing structure. Water and nutrient amendments, disturbance, and local composition affect grassland invasibility but no study has simultaneously integrated these, despite evidence that they frequently interact. Using a split-plot factorial design, we tested the effects of these factors on the invasibility of C3 pasture communities by smooth pigweed Amaranthus hybridus L., a problematic C4 forb. We sowed seeds and transplanted 3-week old seedlings of A. hybridus into plots containing monocultures and mixtures of varying composition, subjected plots to water, soil disturbance, and synthetic bovine urine (SBU) treatments, and measured A. hybridus emergence, recruitment, and growth rate. Following SBU addition, transplanted seedling growth increased in all plots but differed among legume and nonlegume monocultures and mixtures of these plant types. However, SBU decreased the number and recruitment rate of emerged seedlings because high residential growth reduced light availability. Nutrient pulses can therefore have strong but opposing effects on invasibility, depending on when they coincide with particular life history stages of an invader. Indeed, in SBU-treated plots, small differences in height of transplanted seedlings early on produced large differences in their final biomass. All facilitative effects of small-scale disturbance on invasion success diminished when productivity-promoting factors were present, suggesting that disturbance patch size is important. Precipitation-induced invasion resistance of C3 pastures by a C4 invader was partly supported. In grazed grasslands, these biotic and environmental factors vary across scales and interact in complex ways to affect invasibility, thus a dynamic patch mosaic of differential invasion resistance likely occurs in single fields. We propose that disturbance patch size, grazing intensity, soil resource availability, and resident composition are inextricably linked to grassland invasions and comment on the utility of community attributes as reliable predictors of invasibility. Lastly, we suggest temporal as well as spatial coincidences of multiple invasion facilitators dictate the window of opportunity for invasion.

Herbivore influence on soil microbial biomass and nitrogen mineralization in a northern grassland ecosystem: Yellowstone National Park.

Microorganisms are largely responsible for soil nutrient cycling and energy flow in terrestrial ecosystems. Although soil microorganisms are affected by topography and grazing, little is known about how these two variables may interact to influence microbial processes. Even less is known about how these variables influence microorganisms in systems that contain large populations of free-roaming ungulates. In this study, we compared microbial biomass size and activity, as measured by in situ net N mineralization, inside and outside 35- to 40-year exclosures across a topographic gradient in northern Yellowstone National Park. The objective was to determine the relative effect of topography and large grazers on microbial biomass and nitrogen mineralization. Microbial C and N varied by almost an order of magnitude across sites. Topographic depressions that contained high plant biomass and fine-textured soils supported the greatest microbial biomass. We found that plant biomass accurately predicted microbial biomass across our sites suggesting that carbon inputs from plants constrained microbial biomass. Chronic grazing neither depleted soil C nor reduced microbial biomass. We hypothesize that microbial populations in grazed grasslands are sustained mainly by inputs of labile C from dung deposition and increased root turnover or root exudation beneath grazed plants. Mineral N fluxes were affected more by grazing than topography. Net N mineralization rates were highest in grazed grassland and increased from dry, unproductive to mesic, highly productive communities. Overall, our results indicate that topography mainly influences microbial biomass size, while mineral N fluxes (microbial activity) are affected more by grazing in this grassland ecosystem.