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Many savannas are experiencing increased cover of trees and shrubs, resulting in reduced herbaceous productivity, shifts in savanna functional structure and potential reductions in ecotourism. Clearing woody plants has been suggested as an effective management strategy to mitigate these effects and restore these systems to an open state with higher rates of grass production and herbivory. This study investigated the effectiveness of repeated shrub clearing as a tool to mitigate bush encroachment in a semi‐arid savanna in southern Africa.We present data from a 7‐year experiment in the Mthimkhulu Game Reserve bordering Kruger National Park, South Africa. Colophospermum mopane stems and resprouting shoots were basally cut 2–3 times per year (2015–2022) in three pairs of treatment and control plots of 60 × 60 m. We monitored changes in soil moisture, grass biomass and herbivore activity via dung counts. We assessed C. mopane physiological responses to repeated cutting using non‐structural carbohydrates and stable water isotopes to infer changes to energy storage and functional rooting depth, respectively.The cleared treatment had higher soil moisture and grass biomass than the control treatment. Dung counts showed impala and buffalo visited the cleared treatment more frequently than the control treatment.Repeated cutting had limited effects on C. mopane survival in the first 2–3 years after initial clearing, but 80% of individuals were dead after 7 years. Repeatedly cut C. mopane had lower belowground starch concentrations and used water from shallower soil depths than C. mopane in control plots.Synthesis and applications. Repeated cutting increased soil moisture availability and grass biomass, and attracted charismatic grazing herbivores. While more costly than once‐off clearing methods, this practice created more employment opportunities for a neighbouring rural community. Transforming portions of the ecosystem to a grass‐dominated state may increase ecotourism potential through improved game viewing in open systems. [ABSTRACT FROM AUTHOR]

In savannas, partitioning of below‐ground resources by depth could facilitate tree–grass coexistence and shape vegetation responses to changing rainfall patterns. However, most studies assessing tree versus grass root‐niche partitioning have focused on one or two sites, limiting generalization about how rainfall and soil conditions influence the degree of rooting overlap across environmental gradients.We used two complementary stable isotope techniques to quantify variation (a) in water uptake depths and (b) in fine‐root biomass distributions among dominant trees and grasses at eight semi‐arid savanna sites in Kruger National Park, South Africa. Sites were located on contrasting soil textures (clayey basaltic soils vs. sandy granitic soils) and paired along a gradient of mean annual rainfall.Soil texture predicted variation in mean water uptake depths and fine‐root allocation. While grasses maintained roots close to the surface and consistently used shallow water, trees on sandy soils distributed roots more evenly across soil depths and used deeper soil water, resulting in greater divergence between tree and grass rooting on sandy soils. Mean annual rainfall predicted some variation among sites in tree water uptake depth, but had a weaker influence on fine‐root allocation.Synthesis. Savanna trees overlapped more with shallow‐rooted grasses on clayey soils and were more distinct in their use of deeper soil layers on sandy soils, consistent with expected differences in infiltration and percolation. These differences, which could allow trees to escape grass competition more effectively on sandy soils, may explain observed differences in tree densities and rates of woody encroachment with soil texture. Differences in the degree of root‐niche separation could also drive heterogeneous responses of savanna vegetation to predicted shifts in the frequency and intensity of rainfall. [ABSTRACT FROM AUTHOR]

Frequent fire and grazing by megafauna are important determinants of tallgrass prairie plant community structure. However, fire suppression and removal of native grazers have altered these natural disturbance regimes and changed grassland plant communities with potential long‐term consequences for soil carbon (C) and nitrogen (N) storage. We investigated multidecade changes in soil C and N pools in response to contrasting long‐term burning and grazing treatments. Fire suppression with or without grazers and exclusion of grazers in annually burned prairie increased soil C content and shifted the δ13C signature of soil C over time, concomitant with changes in plant community composition. Soil δ13C values indicated that increased soil C content was associated with an increased contribution from plants using a C3 photosynthetic pathway (i.e., woody shrubs) under fire suppression. Soil N content also increased when fire was suppressed, relative to frequently burned grassland, but the rate of increase was slower when grazers were present. Additionally, changes in δ15N values suggested that grazing increased the openness of the N cycle, presumably due to greater N losses. By coupling long‐term fire and grazing treatments with plant community data and soil samples archived over three decades, we demonstrate that human‐caused changes to natural disturbance regimes in a tallgrass prairie significantly alter soil C and N cycles through belowground changes associated with shifts in the plant community. Since natural disturbance regimes have been altered in grasslands across the world, our results are relevant for understanding the long‐term biogeochemical consequences of these ongoing land use changes. Plain Language Summary: The tallgrass prairie of the central United States was historically maintained by frequent fire and bison grazing. However, human‐caused fire suppression and bison removal has altered the plant community composition with cascading effects on elemental cycling. By analyzing soil collected over a 30‐year period, we investigated the long‐term effects of contrasting fire and grazing regimes on soil carbon and nitrogen. Soil carbon content increased over time if fire was suppressed and/or bison were absent. Soil nitrogen content increased only if fire was suppressed. Plant community data and soil isotopic evidence indicated that soil carbon was highest when woody plants were contributing more carbon to the soil in the fire suppression treatments. On the other hand, soil carbon was highest in the annually burned and grazed treatment when warm‐season grasses were contributing more carbon to the soil. Since many grasslands around the world are subject to similar land use changes, our results are important for predicting the long‐term effects of those changes on the ecosystem. Additionally, while our results indicate that the encroachment of woody plants into grasslands may increase soil carbon, land managers should consider the negative trade‐offs of woody encroachment on grassland diversity and ecosystem services. Key Points: We identified the longitudinal effects of 30 years of contrasting burning and grazing regimes on soil C and N in tallgrass prairieIncreased soil C concentrations were associated with increased woody plants under fire suppression or C4 grasses under annual burningSoil N concentration increased over time when fire was suppressed; N was recycled less tightly if bison were present [ABSTRACT FROM AUTHOR]

Biodiversity enhances a variety of ecosystem processes, and yet the underlying mechanisms through which these relationships occur remain a critical knowledge gap. Here, we used the natural abundance of stable isotopes to measure depth of water uptake in five common grassland species (Asclepias tuberosa, Lespedeza capitata, Liatris aspera, Schizachyrium scoparium and Sorghastrum nutans) growing across an experimental grassland diversity gradient. Using this approach, we addressed the following questions: 1) does the depth‐specific provenance of water uptake differ among species and/or do interspecific differences in water source manifest with increasing community diversity? 2) Does the isotopic niche space occupied by plants change with increasing diversity? 3) Is plasticity in water uptake depth across a diversity gradient associated with functional plant responses? We found that the depth of soil water used by plants was inherently different among species when grown in monocultures. All species used less shallow soil water and more intermediate‐depth soil water in mixed assemblages than in monocultures, resulting in similar interspecific differences in water source across the diversity gradient. However, plasticity in the locations of water used were positively associated with increases in plant growth in higher diversity treatments. These results indicate that plasticity in water‐use may contribute to positive biodiversity–productivity relationships commonly observed in temperate grasslands. [ABSTRACT FROM AUTHOR]

A significant fraction of the terrestrial biosphere comprises biomes containing tree-grass mixtures. Forecasting vegetation dynamics in these environments requires a thorough understanding of how trees and grasses use and compete for key belowground resources. There is disagreement about the extent to which tree-grass vertical root separation occurs in these ecosystems, how this overlap varies across large-scale environmental gradients, and what these rooting differences imply for water resource availability and tree-grass competition and coexistence. To assess the extent of tree-grass rooting overlap and how tree and grass rooting patterns vary across resource gradients, we examined landscape-level patterns of tree and grass functional rooting depth along a mean annual precipitation (MAP) gradient extending from ~ 450 to ~ 750 mm year in Kruger National Park, South Africa. We used stable isotopes from soil and stem water to make inferences about relative differences in rooting depth between these two functional groups. We found clear differences in rooting depth between grasses and trees across the MAP gradient, with grasses generally exhibiting shallower rooting profiles than trees. We also found that trees tended to become more shallow-rooted as a function of MAP, to the point that trees and grasses largely overlapped in terms of rooting depth at the wettest sites. Our results reconcile previously conflicting evidence for rooting overlap in this system, and have important implications for understanding tree-grass dynamics under altered precipitation scenarios. [ABSTRACT FROM AUTHOR]

Hydraulic lift, the passive movement of water through plant roots from wet to dry soil, is an important ecohydrological process in a wide range of water-limited ecosystems. This phenomenon may also alter plant functioning, growth, and survival in mesic grasslands, where soil moisture is spatially and temporally variable. Here, we monitored diurnal changes in the isotopic signature of soil and plant xylem water to assess (1) whether hydraulic lift occurs in woody and herbaceous tallgrass prairie species ( Rhus glabra, Amorpha canescens, Vernonia baldwinii, and Andropogon gerardii), (2) if nocturnal transpiration or grazing by large ungulates limits hydraulic lift, and (3) if a dominant grass, A. gerardii, utilizes water lifted by other tallgrass prairie species. Broadly, the results shown here suggest that hydraulic lift does not appear to be widespread or common in this system, but isolated instances suggest that this process does occur within tallgrass prairie. The isolated instance of hydraulic lift did not vary by grazing treatment, nor did they result in facilitation for neighboring grasses. We suggest that the topographic complexity of this tallgrass prairie and the high rates of nocturnal transpiration observed in this study likely limit the frequency and occurrence of hydraulic lift. These results suggest that hydraulic lift can be a patchy process, particularly in heterogeneous landscapes. [ABSTRACT FROM AUTHOR]

Background and Aims: Fire and grazing are important disturbances in grasslands, yet we know little about how they impact a variety of plant physiological processes such as plant ecohydrology. Here, we assessed the impact of fire history and grazing by Bison bison on the source of water uptake and niche overlap in common grassland species at the Konza Prairie Biological Station, a temperate mesic grassland located in northeastern Kansas, USA. Methods: We used the stable isotopic signature of soil and xylem water to evaluate water uptake in Andropogon gerardii, Vernonia baldwinii, Amorpha canescens, and Rhus glabra within varying grazing (grazed, ungrazed), fire (0,1,2 or 3 years since last burn), topography (upland, lowland), and month (July, August) contrasts over 3 years (2013-2015). Results: The presence of grazers, not fire history, altered water uptake patterns in these common grassland species. Particularly, grazing increased the proportion of shallow water utilized by A. gerardii and R. glabra, reducing niche overlap with other co-occurring species. However, these responses varied intra-annually and were often modulated by topography. Conclusions: These results suggest that grazing can alter aspects of grassland ecohydrology at small scales, which may extend to impact community and ecosystem processes at larger spatial scales. [ABSTRACT FROM AUTHOR]

It is still far from clear whether and to what extent trees and grasses partition soil moisture in tropical savannas. A major reason for this is that we don't know how snapshot data on rooting differences translate into ecologically relevant patterns of water use at seasonal scales. We used stable isotopes in soil and stem water to quantify functional rooting profiles in grasses and two tree species in a South African savanna. Concurrently, we measured tree sap-flow velocity, grass canopy temperature (a transpiration correlate), and soil moisture content at multiple depths over the course of a growing season. We used lasso regression to identify the dominant soil moisture layers driving daily variation in tree and grass water-use metrics while controlling for weather variables. We found clear rooting depth differences between grasses (shallow) and trees (deep) from the isotopic data, and these patterns were strongly supported by the water-use data, which showed that grasses and trees predominantly responded to soil moisture availability at 5 and 40 cm depth, respectively. Our results provide a rare example of mechanistic support for the resource partitioning hypothesis in savannas, with important implications for our understanding of tree-grass dynamics under altered precipitation regimes. [ABSTRACT FROM AUTHOR]

Nitrogen (N) metabolism is a major research target for increasing productivity in crop plants. In maize (Zea mays L.), yield gain over the last few decades has been associated with increased N absorption and utilization efficiency (i.e. grain biomass per unit of N absorbed). However, a dynamical framework is still needed to unravel the role of internal processes such as uptake, allocation, and translocation of N in these adaptations. This study aimed to 1) characterize how genetic enhancement in N efficiency conceals changes in allocation and translocation of N, and 2) quantify internal fluxes behind grain N sources in two historical genotypes under high and low N supply. The genotypes 3394 and P1197, landmark hybrids representing key eras of genetic improvement (1990s and 2010s), were grown under high and low N supply in a two-year field study. Using stable isotope 15N labelling, post-silking nitrogen fluxes were modeled through Bayesian estimation by considering the external N (exogenous-N) and the pre-existing N (endogenous-N) supply across plant organs. Regardless of N availability, P1197 exhibited greater exogenous-N accumulated in leaves and cob-husks. This response was translated to a larger amount of N mobilized to grains (as endogenous-N) during grain-filling in this genotype. Furthermore, the enhanced N supply to leaves in P1197 was associated with increased post-silking carbon accumulation. The overall findings suggest that increased N utilization efficiency over time in maize genotypes was associated with an increased allocation of N to leaves and subsequent translocation to the grains. • Bayesian modelling and 15N analysis were combined to estimate N allocation in maize. • Modern maize plants evidenced an improved post-silking N partitioning to leaves. • Greater N allocation to leaves resulted in an increased post-silking carbon fixation. • Internal translocation to the grains was improved due to larger supply from leaves. [ABSTRACT FROM AUTHOR]

Water availability strongly governs grassland primary productivity, yet this resource varies dramatically in time (seasonally) and space (with soil depth and topography). It has long been assumed that co-occurring species differ in their partitioning of water use by depth, but direct evidence is lacking. We report data from two growing seasons (2004–2005) in which we measured the isotopic signature of plant xylem water from seven species (including C3 forbs and shrubs and C4 grasses) growing along a topographic gradient at the Konza Prairie Biological Station. Plant xylem stable oxygen isotope ratio (δ18O) values were compared to soil water δ18O profiles, recent rainfall events, and groundwater. Species varied in both their temporal patterns of water use and their responses to seasonal droughts in both years. During wet periods, species differences in water use were minimal, with common dependency on recent rainfall events stored in the upper soil layers. However, during dry periods, most C3 species used proportionally more water from deeper portions of the soil profile relative to the C4 grasses. Plants in uplands used more shallow soil water compared to those in lowlands, with the greatest differences across the topographic gradient occurring during dry periods. While the documented vertical root distribution varies by species and growth form in this grassland, each of the species we measured appeared to compete for the same surface layer soil moisture when water was not limiting. Thus, our results suggest that variation in precipitation history and landscape positions are greater determinants of water-use patterns than would be expected based on absolute rooting depth. [ABSTRACT FROM AUTHOR]

• Inverse Lagrangian analysis can be used to study isotope exchange in plant canopies. • Canopy flow decoupling affected the Lagrangian analysis estimates. • The inverse dispersion analysis had better performance for short canopies than for the forest canopy. Multi-layer Lagrangian models could be useful techniques for studying stable isotope exchange within and just above plant canopies. The main objective of this study was to evaluate the use of an analytical Lagrangian analysis (localized near-field theory, LNF), to study 13CO 2 and C18OO isotope exchange in different plant canopies by comparing the LNF estimates with those provided by the eddy covariance (EC) technique and the isotope flux ratio method (IFR). Mixing ratios of stable isotopes of CO 2 were measured within and above a temperate deciduous forest, tallgrass prairie and corn field using a multi-port sampling system and the tunable diode laser spectroscopy technique. Wind velocity data and the net CO 2 ecosystem exchange (NEE) were measured above the plant canopies using an EC system. The wind velocity data and CO 2 stable isotope mixing ratios were combined with the LNF theory to infer NEE and source/sinks of isotopes inside canopies. The LNF NEE estimates were likely affected by the flux decoupling in the forest canopy, resulting in a low correlation (R2 ranging from 0.03 to 0.35) between LNF and EC NEE estimates. On the other hand, LNF NEE estimates for corn and grassland canopies showed better correlation with EC NEE estimates (R2 ranging from 0.58 to 0.85), suggesting better coupling between in and above canopy air flows. Although, both LNF and IFR estimates showed large variability, our results show that the LNF approach reduced the uncertainties of the isotope compositions of NEE when compared to the IFR approach. These results suggest that LNF is a useful tool to study CO 2 isotope exchange within short canopies where flux measurements are more challenging than inside tall canopies. [ABSTRACT FROM AUTHOR]