Herbaceous vegetation responses to experimental fire in savannas and forests depend on biome and climate.

Fire-vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest ecosystems should have differential responses to fire, but fire response data for herbaceous vegetation have yet to be synthesized across biomes. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in herbaceous vegetation abundance for burned versus unburned plots. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C4 grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests.

Land use intensification destabilizes stream microbial biodiversity and decreases metabolic efficiency.

Urbanization and agricultural intensification can transform landscapes. Changes in land-use can lead to increases in storm runoff and nutrient loadings which can impair the health and function of stream ecosystems. Microorganisms are an integral component of stream ecosystems. Due to the sensitivity of microorganisms to perturbations, changes in hydrology and water chemistry may alter microbial activity and structure. These shifts in microbial community dynamics may alter stream metabolism and water quality, potentially impacting higher trophic levels. Here we examine the effects of land-use and associated changes in water chemistry on sediment microbial communities by studying the West Run Watershed (WRW) a mixed-land-use system in West Virginia, USA. Streams were sampled throughout the growing season at six sites within the WRW spanning different levels of land use intensification. The proportion of land impacted by agricultural and urban development was positively correlated with temporal variation in stream sediment microbial community composition (adj R2 = 0.65), suggesting development can destabilize microbial communities. Moreover, streams in developed watersheds had an increased metabolic quotient (20-50% higher), this indicates that microorganisms have greater respiration per unit biomass and signifies reduced metabolic efficiency. Further, our results suggest that land use associated changes in water chemistry alter microbial function both directly and indirectly via changes in microbial community composition and biomass. Taken together our results suggest that highly developed watersheds with elevated conductivity, metal ion concentration, and pH impose stress on microbial communities resulting in reduced microbial efficiency and elevated respiration.

Decadal changes in fire frequencies shift tree communities and functional traits.

Global change has resulted in chronic shifts in fire regimes. Variability in the sensitivity of tree communities to multi-decadal changes in fire regimes is critical to anticipating shifts in ecosystem structure and function, yet remains poorly understood. Here, we address the overall effects of fire on tree communities and the factors controlling their sensitivity in 29 sites that experienced multi-decadal alterations in fire frequencies in savanna and forest ecosystems across tropical and temperate regions. Fire had a strong overall effect on tree communities, with an average fire frequency (one fire every three years) reducing stem density by 48% and basal area by 53% after 50 years, relative to unburned plots. The largest changes occurred in savanna ecosystems and in sites with strong wet seasons or strong dry seasons, pointing to fire characteristics and species composition as important. Analyses of functional traits highlighted the impact of fire-driven changes in soil nutrients because frequent burning favoured trees with low biomass nitrogen and phosphorus content, and with more efficient nitrogen acquisition through ectomycorrhizal symbioses. Taken together, the response of trees to altered fire frequencies depends both on climatic and vegetation determinants of fire behaviour and tree growth, and the coupling between fire-driven nutrient losses and plant traits.

Stream sediment bacterial communities exhibit temporally-consistent and distinct thresholds to land use change in a mixed-use watershed.

Freshwater ecosystems are susceptible to biodiversity losses due to land conversion. This is particularly true for the conversion of land from forests for agriculture and urban development. Freshwater sediments harbor microorganisms that provide vital ecosystem services. In dynamic habitats like freshwater sediments, microbial communities can be shaped by many processes, although the relative contributions of environmental factors to microbial community dynamics remain unclear. Given the future projected increase in land use change, it is important to ascertain how associated changes in stream physico-chemistry will influence sediment microbiomes. Here, we characterized stream chemistry and sediment bacterial community composition along a mixed land-use gradient in West Virginia, USA across one growing season. Sediment bacterial community richness was unaffected by increasing anthropogenic land use, though microbial communities were compositionally distinct across sites. Community threshold analysis revealed greater community resilience to agricultural land use than urban land use. Further, predicted metagenomes suggest differences in potential microbial function across changes in land use. The results of this study suggest that low levels of urban land use change can alter sediment bacterial community composition and predicted functional capacity in a mixed-use watershed, which could impact stream ecosystem services in the face of global land use change.

Characterization of sub-watershed-scale stream chemistry regimes in an Appalachian mixed-land-use watershed.

An exploratory study was conducted in an urbanizing, mixed-land-use Appalachian watershed. Six study sites, characterized by contrasting land use/land cover, were instrumented to continuously monitor stream stage. Weekly grab samples were collected from each site and analyzed for elemental composition via spectrometric and spectrophotometric methods. Additional physico-chemical parameters were measured in situ. Data were analyzed using a suite of statistical methods, including hypothesis testing, correlation analysis, and principal components analysis (PCA). Significant differences (p < 0.05) between study sites were identified for every measured parameter except Co, Cu, Pb, and Ti concentrations. However, different parameters showed significant differences (p < 0.05) between site pairings. PCA results highlight consistent spatial differences between elemental composition and physico-chemical characteristics of streamwater samples. Results from correlation analyses indicated varying significant (p < 0.05) relationships between chemical parameters and hydroclimate metrics, with certain elements (e.g., Ca and Sr) and physico-chemical parameters (e.g., specific conductance) displaying greater sensitivity to hydroclimate at mixed-land-use sites, as compared to predominately urban, agricultural, or forest sites. Given the geological, topographical, and climatological similarities between the sites, and their close proximity, it was concluded that land use characteristics and associated hydrologic regime contrasts were the primary factors contributing to the observed results. Results comprise valuable information for land and water managers seeking to mitigate the impacts of land use practices on water resources and aquatic ecosystem health. The applied methodology can be used to more effectively target sub-watershed-scale remediation/restoration efforts within mixed-use watersheds, thereby improving the ultimate efficacy of management practices.

Comparing the influence of wildfire and prescribed burns on watershed nitrogen biogeochemistry using 15N natural abundance in terrestrial and aquatic ecosystem components.

We evaluated differences in the effects of three low-severity spring prescribed burns and four wildfires on nitrogen (N) biogeochemistry in Rocky Mountain headwater watersheds. We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire. To better understand fire effects on the entire watershed ecosystem, we measured N concentrations and δ15N in both the terrestrial and aquatic ecosystems components, i.e., soil, understory plants in upland and riparian areas, streamwater, and in-stream moss. In addition, we measured nitrate reductase activity in foliage of Spiraea betulifolia, a dominant understory species. We found increases of δ15N and N concentrations in both terrestrial and aquatic ecosystem N pools after wildfire, but responses were limited to terrestrial N pools after prescribed burns indicating that N transfer from terrestrial to aquatic ecosystem components did not occur in low-severity prescribed burns. Foliar δ15N differed between wildfire and prescribed burn sites; the δ15N of foliage of upland plants was enriched by 2.9 ‰ (difference between burned and unburned watersheds) in the first two years after wildfire, but only 1.3 ‰ after prescribed burns. In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations. S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns. These responses are consistent with less altered N biogeochemistry after prescribed burns relative to wildfire. We concluded that δ15N values in terrestrial and aquatic plants and streamwater nitrate concentrations after fire can be useful indicators of the magnitude and duration of fire effects and the fate of post-fire available N.