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Abstract Changes in climate and land management over the last half‐century have favoured woody plants native to grasslands and led to the rapid expansion of woody species. Despite this being a global phenomenon, it is unclear why some woody species have rapidly expanded while others have not. We assessed whether the most abundant woody encroaching species in tallgrass prairie have common growth forms and physiology or unique traits that differentiate their resource‐use strategies.We characterized the abundance, above‐ground carbon allocation, and leaf‐level physiological and structural traits of seven woody encroaching species in tallgrass prairie that span an order of magnitude in abundance. To identify species‐specific increases in abundance, we used a 34‐year species composition dataset at Konza Prairie Biological Station (Central Great Plains, USA). We then compared biomass allocation and leaf‐level traits to determine differences in carbon and water use strategies among species.While all focal species increased in abundance over time, encroachment in this system is primarily driven by three species:Cornus drummondii,Prunus americanaandRhus glabra. The most dominant species,Cornus drummondii, had the most extreme values for several traits, including the lowest leaf:stem mass ratios, lowest photosynthetic capacity and highest turgor loss point.Two of the most abundant species,Cornus drummondiiandRhus glabra, had opposing growth forms and resource‐use strategies. These species had significantly different above‐ground carbon allocation, leaf‐level drought tolerance and photosynthetic capacity. There were surprisingly few interspecific differences in specific leaf area and leaf dry matter content, suggesting these traits were poor predictors of species‐level encroachment.Synthesis. Woody encroaching species in tallgrass prairie encompass a spectrum of growth forms and leaf physiology. Two of the most abundant woody species fell at opposite ends of this spectrum. Our results suggest niche differences among a community of woody species facilitate the rapid encroachment by a few species. This study shows that woody encroaching species do not conform to a ‘one‐size‐fits‐all’ strategy, and a diversity of growth forms and physiological strategies may make it more challenging to reach management goals that aim to conserve or restore grassland communities. 

Abstract Abstract. Using herbarium specimens spanning 133 years and field-collected measurements, we assessed intraspecific trait (leaf structural and stomatal) variability from grass species in the Great Plains of North America. We focused on two widespread, closely related grasses from the tribe Paniceae: Dichanthelium oligosanthes subsp. scribnerianum (C3) and Panicum virgatum (C4). Thirty-one specimens per taxon were sampled from local herbaria from the years 1887 to 2013 to assess trait responses across time to changes in atmospheric [CO2] and growing season precipitation and temperature. In 2021 and 2022, the species were measured from eight grasslands sites to explore how traits vary spatially across natural continental precipitation and temperature gradients. Δ13C increased with atmospheric [CO2] for D. oligosanthes but decreased for P. virgatum, likely linked to increases in precipitation in the study region over the past century. Notably, this is the first record of decreasing Δ13C over time for a C4 species illustrating 13C linkages to climate. As atmospheric [CO2] increased, C:N increased and δ15N decreased for both species and %N decreased for D. oligosanthes. Across a large precipitation gradient, D. oligosanthes leaf traits were more responsive to changes in precipitation than those of P. virgatum. In contrast, only two traits of P. virgatum responded to increases in temperature across a gradient: specific leaf area (increase) and leaf dry matter content (decrease). The only shared significant trend between species was increased C:N with precipitation. Our work demonstrates that these closely related grass species with different photosynthetic pathways exhibited various trait responses across temporal and spatial scales, illustrating the key role of scale of inquiry for forecasting leaf trait responses to future environmental change. 

Abstract To predict ecological responses at broad environmental scales, grass species are commonly grouped into two broad functional types based on photosynthetic pathway. However, closely related species may have distinctive anatomical and physiological attributes that influence ecological responses, beyond those related to photosynthetic pathway alone. Hyperspectral leaf reflectance can provide an integrated measure of covarying leaf traits that may result from phylogenetic trait conservatism and/or environmental conditions. Understanding whether spectra‐trait relationships are lineage specific or reflect environmental variation across sites is necessary for using hyperspectral reflectance to predict plant responses to environmental changes across spatial scales. We measured hyperspectral leaf reflectance (400–2400 nm) and 12 structural, biochemical, and physiological leaf traits from five grass‐dominated sites spanning the Great Plains of North America. We assessed if variation in leaf reflectance spectra among grass species is explained more by evolutionary lineage (as captured by tribes or subfamilies), photosynthetic pathway (C₃or C₄), or site differences. We then determined whether leaf spectra can be used to predict leaf traits within and across lineages. Our results using redundancy analysis ordination (RDA) show that grass tribe identity explained more variation in leaf spectra (adjustedR² = 0.12) than photosynthetic pathway, which explained little variation in leaf spectra (adjustedR² = 0.00). Furthermore, leaf reflectance from the same tribe across multiple sites was more similar than leaf reflectance from the same site across tribes (adjustedR² = 0.12 and 0.08, respectively). Across all sites and species, trait predictions based on spectra ranged considerably in predictive accuracies (R² = 0.65 to <0.01), butR²was >0.80 for certain lineages and sites. The relationship between Vc_max, a measure of photosynthetic capacity, and spectra was particularly promising. Chloridoideae, a lineage more common at drier sites, appears to have distinct spectra‐trait relationships compared with other lineages. Overall, our results show that evolutionary relatedness explains more variation in grass leaf spectra than photosynthetic pathway or site, but consideration of lineage‐ and site‐specific trait relationships is needed to interpret spectral variation across large environmental gradients. 

Abstract 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 mopanestems 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 assessedC. mopanephysiological 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 onC. mopanesurvival in the first 2–3 years after initial clearing, but 80% of individuals were dead after 7 years. Repeatedly cutC. mopanehad lower belowground starch concentrations and used water from shallower soil depths thanC. mopanein 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 Woody encroachment—the expansion of woody shrubs into grasslands—is a widely documented phenomenon with global significance for the water cycle. However, its effects on watershed hydrology, including streamflow and groundwater recharge, remain poorly understood. A key challenge is the limited understanding of how changes to root abundance, size and distribution across soil depths influence infiltration and preferential flow. We hypothesised that woody shrubs would increase and deepen coarse‐root abundance and effective soil porosity, thus promoting deeper soil water infiltration and increasing soil water flow velocities. To test this hypothesis, we conducted a study at the Konza Prairie Biological Station in Kansas, where roughleaf dogwood (Cornus drummondii) is the predominant woody shrub encroaching into native tallgrass prairie. We quantified the distribution of coarse and fine roots and leveraged soil moisture time series and electrical resistivity imaging to analyse soil water flow beneath shrubs and grasses. We observed a greater fraction of coarse roots beneath shrubs compared to grasses, which was concurrent with greater saturated hydraulic conductivity and effective porosity. Half‐hourly rainfall and soil moisture data show that the average soil water flow through macropores was 135% greater beneath shrubs than grasses at the deepest B horizon, consistent with greater saturated hydraulic conductivity. Soil‐moisture time series and electrical resistivity imaging also indicated that large rainfall events and greater antecedent wetness promoted more flow in the deeper layers beneath shrubs than beneath grasses. These findings suggest that woody encroachment alters soil hydrologic processes with cascading consequences for ecohydrological processes, including increased vertical connectivity and potential groundwater recharge. 

Summary Savannas cover a significant fraction of the Earth's land surface. In these ecosystems, C 3 trees and C 4 grasses coexist persistently, but the mechanisms explaining coexistence remain subject to debate. Different quantitative models have been proposed to explain coexistence, but these models make widely contrasting assumptions about which mechanisms are responsible for savanna persistence. Here, we show that no single existing model fully captures all key elements required to explain tree–grass coexistence across savanna rainfall gradients, but many models make important contributions. We show that recent empirical work allows us to combine many existing elements with new ideas to arrive at a synthesis that combines elements of two dominant frameworks: Walter's two‐layer model and demographic bottlenecks. We propose that functional rooting separation is necessary for coexistence and is the crux of the coexistence problem. It is both well‐supported empirically and necessary for tree persistence, given the comprehensive grass superiority for soil moisture acquisition. We argue that eventual tree dominance through shading is precluded by ecohydrological constraints in dry savannas and by fire and herbivores in wet savannas. Strong asymmetric grass–tree competition for soil moisture limits tree growth, exposing trees to persistent demographic bottlenecks.