Grassland Responses Research Articles

The response of root traits to precipitation change of herbaceous species in temperate steppes

Abstract Plasticity of root traits plays an important role in plant growth and survival under changing climate. Shift in precipitation is one of the most pertinent global change factors driving changes in structure and function of grasslands. However, few studies have investigated intraspecific variation of root traits in response to precipitation change under field conditions. We conducted a 10‐year simulated increased precipitation experiment in a temperate grassland and a 700‐km regional scale transect along a precipitation gradient ranging from 144.23 to 412.29 mm in northern China. The morphological, chemical and anatomical traits of the first two‐order roots were determined on 15 common herbaceous species in the manipulation experiment and two regionally common species (Leymus chinensis, Artemisia frigida) along a precipitation gradient. We found that most of the root traits of the herbaceous species exhibited no significant responses to water addition. The two regionally common species adjusted their root traits at sites with the annual precipitation lower than certain value, that is 250 and 160 mm for L. chinensis and A. frigida, respectively. These results indicate that root traits of the herbaceous species exhibit little plasticity in response to precipitation change and that the adjustment of root traits occurs when the range of annual precipitation exceeds a certain threshold. Root traits of L. chinensis and A. frigida varied differently both in manipulation experiment and along the precipitation gradient. Root traits of L. chinensis were relatively constant, while A. frigida adjusted their morphological root traits in response to water addition. Moreover, L. chinensis showed higher specific root length (SRL) and area, and root N contents at sites with annual precipitation lower than c. 250 mm. In contrast, A. frigida displayed thicker roots with lower SRL and area at sites with annual precipitation lower than c. 160 mm. Our results showed that root traits of herbaceous species in temperate grasslands exhibited little plasticity and that different species have evolved diverse adaptive strategies in response to precipitation change. These novel findings provide valuable information to predict responses of temperate grasslands to future climate change.

Density-Dependent Plant-Plant Interactions Triggered by Grazing.

Plant species performance in rangelands highly depends on the effect of grazing and also on the occurrence of unpalatable benefactor species that can act as biotic refuges protecting neighboring plants from herbivores. The balance between facilitation and competition may changes with the benefactor density. Despite the high number of studies on the role of biotic refuges, the density dependent effects of unpalatable herbaceous plants on the performance of other species, and on the habitat heterogeneity of rangelands are still unclear. Therefore, we performed a study to test the following hypotheses: (i) Performances of understory species follow a humped-back relationship along the density gradient of the unpalatable benefactor species. (ii) Small-scale heterogeneity of the vegetation decreases with increasing benefactor density. We studied meadow steppes with medium intensity cattle grazing in Hungary. We surveyed understory species’ performance (number of flowering shoots and cover scores) along the density gradient of a common, native unpalatable species (Althaea officinalis). Our findings supported both hypotheses. We found unimodal relationship between the benefactor cover and both the flowering success and richness of understory species. Moreover, small-scale heterogeneity declined with increasing benefactor cover. In this study we detected a humped-back pattern of facilitation along the density gradient of an herbaceous benefactor in pastures. Indeed, this pattern was predictable based on such conceptual models like “consumer pressure-abiotic stress model,” “humped-back model,” “intermediate disturbance hypothesis,” and “disturbance heterogeneity model”; but until now the validity of these relationships has not been demonstrated for herbaceous species. By the demonstration of this effect between herbaceous species we can better forecast the responses of grasslands to changes in management.

Responses of vegetation growth to temperature during 1982-2015 in Xinjiang, China.

Combined with the normalized difference vegetation index (NDVI) dataset, vegetation type data, and meteorological data, we revealed the variation of vegetation growth responses to air temperature in the growing-season during 1982-2015 in Xinjiang, using the moving-windows based partial correlation analysis, the unitary linear regression analysis and GIS spatial analysis. Results showed that, in the whole growing-seasons of study period, there was a significant downturn trend in the responses of vegetation growth to temperature. At the seasonal scale, the downturn trend was obvious especially in summer and autumn, while it was in adverse in spring. During the whole gro-wing season, the responses of different vegetation types to air temperature change showed a decreasing trend. Seasonally, the responses of grassland and forest to temperature change showed a significant increase, while that of shrubland and desert were exactly the opposite in spring. The responses of all natural vegetation (grassland, shrubland, desert and forest) to temperature change showed a significant decreasing trend in summer, whereas their responses in autumn had no significant statistical characteristics. Spatially, the decreasing influence of temperature on the vegetation growth during the growing season in Xinjiang was universal, which might be due to the change in precipitation and solar radiation.

Short-term plant community response to grassland management intended to improve forage quality for the endangered Pampas deer

The Pampas deer is a medium-sized neotropical cervid whose population has been affected by the advancement of the agricultural frontier, poaching and depredation by introduced species. As a consequence, the population of the deer has dramatically decreased and its distribution has contracted. At present, Samborombón Bay remains one of the last populations of this deer in Argentina. The management of natural pastures to improve forage quality is one of the policies adopted by authorities at this site for the conservation of this cervid species. In this study, we assessed the efficacy of grassland management practices (burning and cutting) to promote better forage quality for Pampas deer. The work was carried out in the Campos del Tuyú National Park where two plots were managed to improve the available natural pastures. These plots were burned at the beginning of autumn and intensely defoliated at the end of spring, followed by mechanical cuts in the following autumn and late spring. Multivariate analyses revealed that structure and composition varied between treatments. Green biomass, total plant cover, and forage quality of grasses increased in the plots under management for winter pastures, although no significant differences were detected between summer pastures. The results indicate that grassland management would be a useful tool to improve habitat quality for Pampas deer; however, managed grassland also lost structural heterogeneity. Despite the fact that this study reveals a short-time response of grassland to management, our study shows promising results to continue and expand this practice for the improvement of the habitat quality for Pampas deers.

A synthesis of ecosystem aboveground productivity and its process variables under simulated drought stress

Abstract Projected increases in drought duration and intensity under climate change considerably affect aboveground productivity (ANPP) and associated process variables (photosynthesis rates (Pn), stomatal conductance (gs), soil respiration (Rs) and soil water content (SWC)). Although ANPP has been extensively studied across the ecosystems, there is a little consensus on how the spatiotemporal patterns of ANPP will be altered with increasing drought stress. Here, we present a global meta‐analysis of ANPP and the four variables (610 observations from 78 studies) for drought duration, intensity and their combination. Forest‐ANPP had stronger negative responses to long‐term drought (34.44%; ≥4 years) than short‐term drought (10.78%; ≤1 year) and decreased more in Mediterranean forests than in tropical forests. Decreases in Pn and gs were strongest under long‐term moderate drought. In the short term, Rs increased by 5.66% under light drought, but decreased by 14.12% and 28.43% under moderate and severe droughts. Grass‐ANPP showed a nonlinear decrease with extended duration and the rate slowed. Within light to severe intensities, ANPP decreased linearly, but became stable under extreme condition. In the short term, ANPP reduced more seriously with increasing drought intensity (12.01%‐30.34%). With aggravation of drought stress, the reductions in Rs and SWC increased. There was significant heterogeneity in grassland responses to drought stress. The greatest decreases in ANPP, Pn and gs were observed in North America, and the reductions in Rs and SWC were greater in Western Europe. Shrub‐ANPP showed stronger negative responses to long‐term moderate drought (12.59%). Pn and gs declined significantly with increasing drought intensity. Variations in Rs to drought duration, intensity and their combination were more complex, either showing positive or negative responses (dominated). Synthesis. Forest‐ANPP shows high sensitivity to long‐term moderate drought, whereas grass‐ANPP is more responsive to short‐term drought. Compared to forests and grasslands, shrub‐ANPP exhibits less sensitivity to droughts. Different responses of ecosystems were predominantly driven by physiological mechanisms or species differences in turnover time, community architecture and drought adaptation strategies. Given these findings, future studies should focus on nonlinear patterns, response thresholds and adaptation mechanisms when predicting and modeling feedback between ecosystems and climate change.

Drought offsets the positive effect of summer heat waves on the canopy greenness of mountain grasslands

Heat waves are becoming more frequent in a warming climate and understanding the impacts of these extreme events on terrestrial ecosystems remains a major challenge. Seasonally snow-covered mountain grasslands are temperature-limited ecosystems and one may question whether the summer heat waves there have the same negative effect on primary productivity as they have on lowland ecosystems. Here, we examined the remotely-sensed phenology of mountain grasslands in the French Alps with a particular focus on the four unusually warm summers that occurred in 2003, 2012, 2015, and 2016. Our results showed that an early and rapid senescence and a lack of late-season regrowth were the most significant responses in 2003 and 2012 when heat waves coincided with climatic water deficit, i.e. negative anomalies of precipitation minus reference evapotranspiration. Mountain grasslands located below 2350 m were the most affected. In contrast, we found low senescence and moderate to high regrowth in 2015 and 2016 which were years unaffected by severe water deficit. When water was non-limiting, warm and even very warm summers have a positive effect on the canopy greenness of the highest grasslands, particularly by extending the green period in late summer and early fall. Our study highlights the pivotal role that summer water balance has in the phenological response of mountain grasslands exposed to heat waves.

Modeling the carbon dynamics of alpine grassland in the Qinghai-Tibetan Plateau under scenarios of 1.5 and 2 °C global warming

Alpine grassland occupies two-thirds of the Qinghai-Tibetan Plateau (QTP). It is vital to project changes of this vulnerable ecosystem under different climate change scenarios before taking any mitigation or adaptation measures. In this study, we used a process-based ecosystem model, driven with output from global circulation models under different Representative Concentration Pathways (RCPs), to project the carbon dynamics of alpine grassland. The results showed the following: 1) Vegetation carbon (C) on the QTP increased by 22–38 gC m−2 during periods of 1.5 and 2 °C warming under different RCPs when compared to the baseline period (1981–2006), while soil C increased by 85–122 gC m−2. 2) The increases of vegetation C and soil C at the period of 1.5 °C warming were about 15 gC m−2 and 40 gC m−2 smaller than those at the period of 2 °C warming, respectively; increase of C was greater for alpine meadow than for alpine steppe. 3) Precipitation, radiation, and permafrost changed significantly and showed heterogeneous spatial patterns, and caused heterogeneous response of C dynamics. For alpine meadow in regions transformed from permafrost to seasonally frozen soil with medium annual precipitation (200–400 mm), vegetation C and net primary production decreased by 18.7 gC m−2 and 3.1 gC m−2 per year during 2 °C warming under RCP 4.5, respectively. This decrease can be attributed to the disappearing impermeable permafrost. Different from previous studies that indicated an unfavorable response of alpine grassland to climate warming, this study showed a relatively favorable response, which is mainly attributed to CO2 fertilization.

Effects of multiple climate change factors on the spring phenology of herbaceous plants in Inner Mongolia, China: Evidence from ground observation and controlled experiments

AbstractIn a natural ecosystem or as climatic climax community, grassland is relatively dry with a strongly seasonal climate, and sensitive to climatic changes. Most of the previous studies used remote sensing data to investigate the phenological response of grassland to climate change, while ground‐based studies covering a large geographic area were limited. In this study, using the long‐term phenological data (1981–2012) of 16 herbaceous species observed at 20 stations in Inner Mongolia, China, we first investigated the trends in three spring phenophases, including the dates of bud‐burst, first leaf unfolding, and 50% of leaf unfolding. Subsequently, multiple linear regressions between phenophases and four climatic factors (spring temperature, soil moisture, chilling temperature, and insolation) were performed to determine the relative importance of each factor on the spring phenology. To validate the resulted regression coefficients, we developed a controlled environment to investigate the factors regulating the leaf unfolding time of one dominant species (Leymus chinensis). The results showed that the study area became warmer and drier from 1981 to 2012. However, the overall changes in spring phenophases were not apparent, as there was a similar proportion of significant earlier or later trends. Such phenological changes were driven by multiple climatic factors. The warmer temperature would advance the spring phenophases, while lower soil moisture would delay them. The impact of soil moisture was significant in the experimental data, but not significant in the observation data. In addition, leaf unfolding became faster when L. chinensis experienced more chilling days, but this effect was difficult to be detected in observation data due to the weak sensitivity of the leaf unfolding time to chilling days. These findings can help us to understand how the spring phenology of typical herbaceous species responds to multiple climatic factors under the background of climate change.

Ecological responses to heavy rainfall depend on seasonal timing and multi-year recurrence.

Heavy rainfall events are expected to increase in frequency and severity in the future. However, their effects on natural ecosystems are largely unknown, in particular with different seasonal timing of the events and recurrence over multiple years. We conducted a 4yr manipulative experiment to explore grassland response to heavy rainfall imposed in either the middle of, or late in, the growing season in Inner Mongolia, China. We measured hierarchical responses at individual, community and ecosystem levels. Surprisingly, above-ground biomass remained stable in the face of heavy rainfall, regardless of seasonal timing, whereas heavy rainfall late in the growing season had consistent negative impacts on below-ground and total biomass. However, such negative biomass effects were not significant for heavy rainfall in the middle of the growing season. By contrast, heavy rainfall in the middle of the growing season had greater positive effects on ecosystem CO2 exchanges, mainly reflected in the latter 2yr of the 4yr experiment. This two-stage response of CO2 fluxes was regulated by increased community-level leaf area and leaf-level photosynthesis and interannual variability of natural precipitation. Overall, our study demonstrates that ecosystem impacts of heavy rainfall events crucially depend on the seasonal timing and multiannual recurrence. Plant physiological and morphological adjustment appeared to improve the capacity of the ecosystem to respond positively to heavy rainfall.

Simulated Runoff and Sediment Yield Responses to Land-Use Change Using the SWAT Model in Northeast China

Land-use change is one key factor influencing the hydrological process. In this study, the Hun River Basin (HRB) (7919 km2), a typical alpine region with only four gauge meteorological stations, was selected as the study area. The China Meteorological Assimilation Driving Datasets for the SWAT model (CMADS), widely adopted in East Asia, was used with the Soil and Water Assessment Tool (SWAT) model to simulate runoff and sediment yield responses to land-use change and to examine the accuracy of CMADS in the HRB. The criteria values for daily/monthly runoff and monthly sediment yield simulations were satisfactory; however, the validation of daily sediment yield was poor. Forestland decreased sediment yield throughout the year, increased water percolation, and reduced runoff during the wet season, while it decreased water percolation and increased runoff during the dry season. The responses of grassland and forestland to runoff and sediment yield were similar, but the former was weaker than the latter in terms of soil and water conservation. Cropland (urban land) generally increased (increased) runoff and increased (decreased) sediment yield; however, a higher sediment yield could occur in urban land than that in cropland when precipitation was light.

Growth controls over flowering phenology response to climate change in three temperate steppes along a precipitation gradient

Our understanding of how simultaneous climate warming and changing precipitation influence plant phenology in grasslands is still limited. As part of a field transplant experiment, this study was conducted to explore the flowering phenology of the dominant species in three temperate steppes (i.e. a desert, typical, and meadow steppe) along a precipitation gradient in response to simulated night warming and precipitation manipulation (i.e. decreased, ambient, and increased precipitation). Of all monitored species and across three growing seasons (2015–2017), night warming advanced flowering date in the desert and typical steppes, but delayed it in the meadow steppe. Decreased precipitation postponed flowering date but increased precipitation advanced it in all the three steppes. Plant growth largely determined the changes in flowering date under night warming and changing precipitation across and in each of the three steppes. The dominant role of plant growth in modulating reproductive phenology provides mechanistic understanding in interpreting phenological response and facilitate the projections of response of temperate grasslands under future climate change scenarios.

Threshold responses of grassland and sagebrush birds to patterns of disturbance created by an ecosystem engineer

Burrowing mammals play a role in rangeland disturbance worldwide, enhancing habitat for certain species while negatively affecting others. However, little is known concerning effects of disturbance spatial pattern on co-occuring fauna. In the North American Great Plains, colonial black-tailed prairie dogs (Cynomys ludovicianus) may enhance habitat for one suite of birds while degrading habitat for others. We examined the influence of prairie dogs on birds in a mosaic grassland–shrubland landscape. We evaluated how birds associated with shortgrass, midgrass, and sagebrush (Artemisia spp.) plant communities respond to spatial pattern of prairie dog disturbance and identified thresholds where abundance changes. We surveyed bird abundance on prairie dog colonies of varying sizes and shapes, across colony edges into undisturbed habitat, and within undisturbed sagebrush in northeastern Wyoming. We modeled species responses to colony presence, distance to colony edge, and total area and edge density of colonies at four spatial scales (100 m, 225 m, 500 m, 1000 m). Sagebrush specialists like Brewer’s sparrow (Spizella breweri) and sage thrasher (Oreoscoptes montanus) were 4.5 times more abundant in undisturbed shrublands. Conversely, the shortgrass-specialist mountain plover (Charadrius montanus) was abundant on colonies but showed a non-linear response to colony edge, increasing in abundance up to 600 m from edges then declining further towards colony cores. While some species may be broadly intolerant to disturbance, disturbance-dependent birds can display a “goldilocks syndrome” relative to disturbance size. As such, management for multiple species of conservation concern can be optimized relative to other goals by identifying thresholds associated with the effect of disturbance.

Deciphering impacts of climate extremes on Tibetan grasslands in the last fifteen years

Climate extremes have emerged as a crucial driver of changes in terrestrial ecosystems. The Tibetan Plateau, facing a rapid climate change, tends to favor climate extremes. But we lack a clear understanding of the impacts of such extremes on alpine grasslands. Here we show that extreme events (drought, extreme wet, extreme cold and extreme hot) occurred at a frequency of 0.67–4 months decade−1 during 2001–2015, with extreme precipitation predominantly occurring in June-to-August and extreme temperatures in May. Drought and extreme wet cause opposite and asymmetric effects on grassland growth, with drought-induced reductions greater than increases due to extreme wet. Grassland responses to extreme temperatures, which predominantly occur in May, show a dipole-like spatial pattern, with extreme hot (cold) events enhanced (reduced) growth in the eastern plateau but slightly reduced (enhanced) growth in the western plateau. These opposite responses to extreme temperatures over the eastern plateau are explained by the possibility that the occurrence of extreme cold slows the preseason temperature accumulation, delaying the triggering of spring phenology, while extreme hot hastens the accumulation. In the western plateau, in contrast, positive responses to extreme cold are induced by accompanying high precipitation. Furthermore, high extremeness of climate events generally led to a much lower extremeness in growth response, implying that the Tibetan grasslands have a relatively high resistance to climate extremes. The ecosystem models tested could not accurately simulate grassland responses to drought and extreme temperatures, and require re-parameterization before trust can be placed in their output for this region.

Drought-induced decline of productivity in the dominant grassland species Lolium perenne L. depends on soil type and prevailing climatic conditions

Severe constraints on grasslands productivity, ecosystem functions, goods and services are expected to result from projected warming and drought scenarios under climate change. Negative effects on vegetation can be mediated via soil fertility and water holding capacity, though specific mechanisms are fairly complex to generalise. In field drought experiments, it can be difficult to disentangle a drought effect per se from potential confounding effects related to vegetation or soil type, both varying along with climate. Furthermore, there is the need to distinguish the long-term responses of vegetation and soil to gradual climate shift from responses to extreme and stochastic climatic events. Here we address these limitations by means of a factorial experiment using a single dominant grassland species (the perennial ryegrass Lolium perenne L.) grown as a phytometer on two soils types with contrasted physicochemical characteristics, placed at two elevation sites along a climatic gradient, and exposed to early or late-season drought during the plant growing season.Warmer site conditions and reduced precipitation along the elevational gradient affected biogeochemistry and plant productivity more than the drought treatments alone, despite the similar magnitude in volumetric soil moisture reduction. Soil type, as defined here by its organic matter content (SOM), modulated the drought response in relation to local site climatic conditions and, through changes in microbial biomass and activity, determined the seasonal above and belowground productivity of L. perenne. More specifically, our combined uni- and multivariate analyses demonstrate that microbes in a loamy soil with low SOM are strongly responsive to change in climate, as indicated by a simultaneous increase in their C,N,P pools at high elevation with cooler temperatures and wetter soils. Contrastingly, microbes in a clay-loam soil with high SOM are mainly sensitive to temperature, as indicated by a strong increase in microbial biomass under warmer temperatures at low elevation and a concomitant increase in C:N, C:P and N:P ratios. High SOM promoted a better annual yield of the phytometer grass under warmer climate and the effect of drought on productivity was transient. In contrast, low SOM reduced cumulative yield under warmer conditions and root production strongly decreased, enduring a lasting drought effect. Microbes in soils with high organic matter remained more active during warmer and drier conditions, ensuring soil fertility and stimulating a higher overall plant nutrient availability and productivity.Our study highlights the important role of soil type for grassland responses to both stochastic climatic extremes and long-term climate change. Management practices enhancing SOM accumulation via organic residue incorporation seem a promising way to mitigate the effects of increased temperature and drought on plants and soil microbes alike promoting thereby a sustainable ecosystem functioning.

Remotely sensed agricultural grassland productivity responses to land use and hydro-climatic drivers under extreme drought and rainfall

Climate change is expected to increase the frequency and intensity of drought globally with potentially significant consequences for grasslands. We examined grassland responses to a long-term drought on the Darling Downs, eastern Australia, using the Enhanced Vegetation Index (EVI), a remotely sensed measure of primary productivity. This extreme drought period had rainfall deficits comparable to the hottest and driest projected climate change scenarios for 2030 and was followed by extreme rainfall. This juxtaposition allowed investigation of grassland dynamics (decline and recovery) under extreme climatic variability. Our aim was to determine whether factors associated with grassland decline during extreme drought are the same as those that drive recovery post drought. There is limited knowledge about whether the determinants of grassland decline and recovery are consistent, but this information is important for understanding how best to reduce grassland decline, without inhibiting recovery. We calculated EVI (Enhanced Vegetation Index) trends at 2549 grassland sites situated in an agricultural landscape and used boosted regression trees to model these against multiple hydro-climatic and land use factors. As anticipated, hydro-climatic variables were key drivers of EVI trends in both the drought and wet phases, with higher soil moisture corresponding to less decline in the drought phase and enhanced recovery in the wet phase; however, land use and plant trait variables were also important predictors of EVI trends. Higher proportions of dryland agriculture in the local landscape, high C3:C4 ratios and lower proportions of woody vegetation in the local landscape were associated with negative EVI trends (i.e. greater decline) during drought, but had inverse or negligible effects during the post drought recovery phase. Our results suggest that mitigating decline and fostering grassland recovery following drought requires considering multiple hydro-climatic, land use and plant trait drivers and how their importance changes under drought and wet phases.

Extreme flood events at higher temperatures exacerbate the loss of soil functionality and trace gas emissions in grassland

The frequency and intensity of extreme weather events (e.g. flood, drought) are predicted to increase for the foreseeable future and it is expected that these will negatively impact upon agroecosystem functioning. Our understanding of how grassland ecosystems respond to extreme weather events occurring at different times of the year, however, is lacking. To better understand the seasonal response of grassland to flooding, we subjected an agricultural grassland to an 8-week extreme flood event at three different temperatures (5 °C-winter, 15 °C-spring/autumn and 25°C-summer) and then followed its subsequent recovery for 9 weeks after floodwater removal. We focused on key indicators of ecosystem functioning including primary production, nutrient cycling, greenhouse gas (GHG) emissions, ammonia (NH3) volatilization, and soil microbial communities. The experiment used intact soil mesocosms (1 kg) with indigenous vegetation collected from a grassland with no previous history of flooding. Flooding reduced biomass production by 18% at 5 °C, 50% at 15 °C and 95% at 25 °C. Flooding also significantly disrupted elemental cycling (nitrogen, phosphorus and carbon) as evidenced by an increased release of P, Fe and NH4+ into the soil and overlying floodwater and large amounts of CH4 and NH3 released to the atmosphere (mainly during the flooding). These effects were more pronounced at higher temperatures (e.g. 45–700 kg CH4C ha−1 and 1–5 kg NH3N ha−1 at 15 and 25 °C, respectively). In addition, after floodwater removal this NH4+ was rapidly nitrified leading to large losses of N2O (1.0–14.2 kg N2ON ha−1 at 5–25 °C, respectively). Especially at higher temperatures, flooding resulted in a reduction in soil microbial biomass (more than 58% of the equivalent unflooded treatment at 25 °C) and changes in microbial community structure (assessed by PLFAs). Further, some of these changes persisted after flood removal including a loss of actinomycetes, arbuscular mycorrhizal fungi and fungi. Overall, we conclude that ecosystem responses to extreme weather events are critically dependent on temperature with those occurring at higher temperatures having a greater negative impact than those at the lowest temperature (5 °C). The large potential release of CH4 and N2O also suggests that flood events should be considered as a potential source of GHGs when comparing top-down and bottom-up calculations of national inventories, and that further work is needed to better refine GHG emission estimates for these events.

Carbon exchange responses of a mesic grassland to an extreme gradient of precipitation.

Growing evidence indicates that ecosystem processes may be differentially sensitive to dry versus wet years, and that current understanding of how precipitation affects ecosystem processes may not be predictive of responses to extremes. In an experiment within a mesic grassland, we addressed this uncertainty by assessing responses of two key carbon exchange processes-aboveground net primary production (ANPP) and soil respiration (Rs)-to an extensive gradient of growing season precipitation. This gradient comprised 11 levels that specifically included extreme values in precipitation; defined as the 1st, 5th, 95th, and 99th percentiles of the 112-year climate record. Across treatments, our experimental precipitation gradient linearly increased soil moisture availability in the rooting zone (upper 20cm). Relative to ANPP under nominal precipitation amounts (defined as between the 15th and 85th percentiles), the magnitude of ANPP responses were greatest to extreme increases in precipitation, with an underlying linear response to both precipitation and soil moisture gradients. By contrast, Rs exhibited marginally greater responses to dry versus wet extremes, with a saturating relationship best explaining responses of Rs to both precipitation and soil moisture. Our findings indicate a linear relationship between ANPP and precipitation after incorporating responses to precipitation extremes in the ANPP-precipitation relationship, yet in contrast saturating responses of Rs. As a result, current linear ANPP-precipitation relationships (up to ~ 1000mm) within mesic grasslands appear to hold as appropriate benchmarks for ecosystems models, yet such models should incorporate nonlinearities in responses of Rs amid increased frequencies and magnitudes of precipitation extremes.

Montane valley grasslands are highly resistant to summer wildfire

AbstractAimsUnderstanding the ecological role of fire in shaping plant communities in fire‐prone ecosystems is needed for ecosystem restoration, preservation and management. We investigated the effects of wildfire on plant community structure and the biotic and abiotic factors that are most influential in stabilizing and/or driving change before and after burning in high‐elevation montane grasslands dominated by C3 species receiving a mean annual precipitation of 545 mm.LocationValles Caldera National Preserve, Jemez Mountains, New Mexico, USA.MethodsLong‐term data (10 year pre‐fire; 5 year post‐fire) on plant community composition of nine burned and seven unburned grassland sites were used to determine the response of montane grasslands to a summer wildfire. We used multivariate and univariate analyses to determine changes in plant community composition and structure, pre‐ and post‐wildfire on burned and unburned grasslands.ResultsThe montane grassland community consisted of 155 plant taxa, including 44 perennial grasses, one annual grass, 92 perennial forbs and nine annual forbs. We found that these fire‐adapted plant communities were highly resilient to fire; fire had no significant effects on composition or structure beyond the normal range of inter‐annual variability. Instead, seasonal precipitation had the largest influence on plant community dynamics over time, with lower average plant canopy height and diversity during drought periods.ConclusionOur results show that fire has very limited effects on the composition and structure of these C3‐dominated montane grasslands. Our results support efforts by land managers to reintroduce fire on these historically burned landscapes.

Legacy Effects of Climate Extremes in Alpine Grassland.

Climate change is particularly apparent in many mountainous regions, with warming rates of more than twice the global average being reported for the European Alps. As a result, the probability of climate extremes has increased and is expected to rise further. In an earlier study, we looked into immediate impacts of experimentally imposed heat waves in alpine grassland, and found that these systems were able to cope with heat as long as enough water was available. However, concomitant drought led to increased stress, and reduced aboveground biomass production and green plant cover. Here, we studied the legacy effects (lag-effects) of the imposed climate extreme to see whether delayed responses occurred and how fast the alpine grassland could rebound from the initial changes. Green cover continued to be suppressed the two following years in communities that had been exposed to the most intense hot drought, while aboveground biomass production had returned to control levels by year 2. The initial lower resistance of the forb fraction in the communities was not compensated by faster recovery later on. This resulted in alpine communities that became (and remained) relatively enriched with graminoids, which resisted the original extreme better. The responses of alpine grassland to heat extremes with or without drought observed in this study resemble those typically found in lowland grassland in the short term. However, alpine grassland exhibited longer legacy effects from an annual perspective, with delayed recovery of aboveground production and persistent changes in community composition. This suggests that once initial resistance thresholds are exceeded, impacts may be longer-lasting in alpine grassland, where recovery is constrained by both the short growing season and difficult seedling establishment.

Evaluating Landscape Degradation Along Climatic Gradients During the 1930s Dust Bowl Drought From Panchromatic Historical Aerial Photographs, United States Great Plains

The U.S. Great Plains (USGP) are some of the most productive rangelands globally and a significant carbon sink for the atmosphere, but grassland response to precipitation is highly variable and poorly constrained over time and space. There is a rich historical aerial photographic record of the USGP which provides an unparalleled view of past landscapes and allows for evaluation of surficial response to drought beyond the satellite record, such as during the 1930s Dust Bowl Drought (DBD). This study classified the extent and loci of surficial denudation from seamless mosaics of radiometrically-corrected and georectified digitized aerial negatives acquired in the late 1930s from six counties distributed across USGP ecoregions. The dominant sources of degradation found for sites east of the 100th meridian are cultivated fields and fluvial deposits, associated with woody vegetation response to water availability in uncultivated areas. For sites to the west, denuded surfaces are predominantly eolian sandsheets and dunes, correlated with intensity of drought conditions and reduced plant diversity. Discrete spatial signatures of the drought are observed not only within the classically recognized southern Dust Bowl area, but also in the northern and central plains. Statistical analyses of site variability suggest landscape response to the DBD is most strongly influenced by the arid–humid divide and severity of precipitation and temperature anomalies. With a projected increase 21st century aridity, eolian processes cascading across western grasslands, like during the Dust Bowl, may significantly impact future dust particle emission and land and carbon storage management.