Grass Species Richness Research Articles

Wild herbivores and cattle have differing effects on postfire herbaceous vegetation recovery in an African savanna.

Fire and herbivory have profound effects on vegetation in savanna ecosystems, but little is known about how different herbivore groups influence vegetation dynamics after fire. We assessed the separate and combined effects of herbivory by cattle and wild meso- and megaherbivores on postfire herbaceous vegetation cover, species richness, and species turnover in a savanna ecosystem in central Kenya. We measured these vegetation attributes for five sampling periods (from 2013 to 2017) in prescribed burns and unburned areas located within a series of replicated long-term herbivore exclosures that allow six different combinations of cattle and wild meso- and megaherbivores (elephants and giraffes). Vegetation cover (grasses, mainly) and species richness were initially reduced by burning but recovered by 15-27 months after fire, suggesting strong resilience to infrequent fire. However, the rates of recovery differed in plots accessible by different wild and domestic herbivore guilds. Wildlife (but not cattle) delayed postfire recovery of grasses, and the absence of wildlife (with or without cattle) delayed recovery of forbs. Herbivory by only cattle increased grass species richness in burned relative to unburned areas. Herbivory by cattle (with or without wildlife), however, reduced forb species richness in burned relative to unburned areas. Herbivory by wild ungulates (but not cattle) increased herbaceous species turnover in burned relative to unburned areas. Megaherbivores had negligible modifying effects on these results. This study demonstrates that savanna ecosystems are remarkably resilient to infrequent fires, but postfire grazing by cattle and wild mesoherbivores exerts different effects on recovery trajectories of herbaceous vegetation.

Habitat modifies the relationship between grass and herbivore species richness in a South African savanna.

The savanna ecosystem is dominated by grasses, which are a key food source for many species of grazing animals. This relationship creates a diverse mosaic of habitats and contributes to the high grass species richness of savannas. However, how grazing interacts with environmental conditions in determining grass species richness and abundance in savannas is still insufficiently understood. In the Kruger National Park, South Africa, we recorded grass species and estimated their covers in 60 plots 50 × 50 m in size, accounting for varying proximity to water and different bedrocks. To achieve this, we located plots (i) near perennial rivers, near seasonal rivers, and on crests that are distant from all water sources and (ii) on nutrient-rich basaltic and nutrient-poor granitic bedrock. The presence and abundance of large herbivores were recorded by 60 camera traps located in the same plots. Grass cover was higher at crests and seasonal rivers than at perennial rivers and on basalts than on granites. The relationship between grass species richness and herbivore abundance or species richness was positive at crests, while that between grass species richness and herbivore species richness was negative at seasonal rivers. We found no support for controlling the dominance of grasses by herbivores in crests, but herbivore-induced microsite heterogeneity may account for high grass species richness there. In contrast, the decrease in grass species richness with herbivore species richness at seasonal rivers indicates that the strong grazing pressure over-rides the resistance of some species to grazing and trampling. We suggest that the relationships between grasses and herbivores may work in both directions, but the relationship is habitat-dependent, so that in less productive environments, the effect of herbivores on vegetation prevails, while in more productive environments along rivers the effect of vegetation and water supply on herbivores is more important.

Erratum: The influence of fire presence and absence on grass species composition and species richness at Mountain Zebra National Park

No abstract available.

The contemporary distribution of grasses in Australia: A process of immigration, dispersal and shifting dominance

AbstractAimLittle is known about the distribution of grasses throughout Australia. Using endemism as a basis for understanding biogeographical distributions, we hypothesised that contemporary species richness would be the result of environmental factors and dynamic ecological interactions spanning more than 25 Ma.LocationAustralia.TaxonGrasses.MethodsWe mapped the distribution of all Australian grass species and modelled climatic and landscape correlates according to photosynthetic type (C3 or C4), endemism, age in Australia, phylogenetic lineage and traits linked to dominance, using height as a proxy. Age classes comprised ‘Ancient’ (Gondwanan), and three others related to migration during the Sunda‐Sahul Interchange (SSI): Early, Mid or Recent. In some analyses, ‘Ancient’, ‘Early SSI’ and ‘Mid SSI’ were combined into ‘Pre‐Recent SSI’.ResultsOverall, species richness of C4 grasses increased with warmer mean annual temperatures, while richness of C3 grasses was higher in cooler areas. Recent SSI species had strong associations with summer rains and were dominant in the continent's northeast, with Pre‐Recent SSI species concentrated in the southeast, a pattern largely reflecting photosynthetic type (C4 and C3 respectively). Endemic and shared species distribution patterns support a migration sequence in which most C3 Pooideae and Panicoideae genera arrived in Australia before the Pliocene aridifications, followed by C4 Chloridoideae as aridification increased, with C4 Andropogoneae immigrating most recently across Lake Carpentaria's open habitats in the later Pleistocene. Recent SSI shared species were significantly taller than Pre‐Recent SSI endemic grasses.Main ConclusionsThe few grasses present in Australia before the Pliocene grew in cooler areas. The influx of taller Recent SSI grasses contributed to dramatic environmental changes—including creation of the northern savannas—with repercussions for resident taxa. Contemporary methods of fuel management could be promoting invasion by grass, thereby jeopardising the conditions suited to ancient taxa and threatening the region's evolutionary history.

Fine‐scale variation in soil and topography influences herbaceous vegetation and the distribution of large mammalian herbivores

AbstractCurrent understanding of the distribution of vegetation and large mammalian herbivores (LMH) is based on a combination of biogeographic studies and highly controlled field experiments, but a more complete understanding of these patterns requires study of their natural co‐occurrence patterns at intermediate spatial scales. The study was conducted in the 120‐ha Mpala Forest Global Earth Observatory (ForestGEO) plot, Kenya. We examined differences in herbaceous plant communities and habitat use by LMH among three topographic habitats with distinct soil types, namely steep slopes, valley and plateau. Each pair of habitats differed in plant and animal composition. The steep slopes and plateau respectively had ≥1‐fold higher percentage herbaceous cover than the valley, whereas the steep slopes and valley had >1.5‐fold greater grass species richness and diversity than the plateau. The activity of LMH was ≥1.7‐fold higher in the valley than the steep slopes and plateau, reflecting a positive relationship between LMH activity index and richness and diversity of grass species. Results indicate that fine‐scale variation in topography and soil are associated with both the distribution of herbaceous vegetation and LMH, suggesting a need to account for local habitat characteristics when examining the distributions of plants, animals, and plant‐herbivore interactions in natural systems.

The influence of fire presence and absence on grass species composition and species richness at Mountain Zebra National Park

It is well known that fire is a common driver in many biomes and it plays a vital role in maintaining ecosystem functioning in many South African biomes. This ecosystem process is an important determinant of plant community composition and diversity, and can result in changes in structural composition and ecosystem functioning. The main objectives of this study are to determine the influence of fire on grass species richness, diversity and composition in Mountain Zebra National Park. Using satellite imagery, the park’s fire history was determined between 2000 and 2020. Eighty plots (approximately 20 m × 20 m; 100 m apart) were laid out purposively across different fire regimes. There was no significant difference in both species richness and diversity in burned and unburned sites. However, there was a difference in species composition between burned and unburned sites and between different fire frequencies. The unburned site had higher moribund material and unpalatable grasses compared to the burned area.Conservation implications: The results of this study will help in the completion of the fire management plan for the park which will enable conservation managers to make better decisions with regard to fire management in mountainous grassland at Mountain Zebra National Park. Consequently, this will lead to improved veld condition and vegetation structure.

Long-Term Anthropogenic Management and Associated Loss of Plant Diversity Deeply Impact Virome Richness and Composition of Poaceae Communities.

Modern agriculture has influenced plant virus emergence through ecosystem simplification, introduction of new host species, and reduction in crop genetic diversity. Therefore, it is crucial to better understand virus distributions across cultivated and uncultivated communities in agro-ecological interfaces, as well as virus exchange among them. Here, we advance fundamental understanding in this area by characterizing the virome of three co-occurring replicated Poaceae community types that represent a gradient of grass species richness and management intensity, from highly managed crop monocultures to little-managed, species-rich grasslands. We performed a large-scale study on 950 wild and cultivated Poaceae over 2 years, combining untargeted virome analysis down to the virus species level with targeted detection of three plant viruses. Deep sequencing revealed (i) a diversified and largely unknown Poaceae virome (at least 51 virus species or taxa), with an abundance of so-called persistent viruses; (ii) an increase of virome richness with grass species richness within the community; (iii) stability of virome richness over time but a large viral intraspecific variability; and (iv) contrasting patterns of virus prevalence, coinfections, and spatial distribution among plant communities and species. Our findings highlight the complex structure of plant virus communities in nature and suggest the influence of anthropogenic management on viral distribution and prevalence. IMPORTANCE Because viruses have been mostly studied in cultivated plants, little is known about virus diversity and ecology in less-managed vegetation or about the influence of human management and agriculture on virome composition. Poaceae (grass family)-dominated communities provide invaluable opportunities to examine these ecological issues, as they are distributed worldwide across agro-ecological gradients, are essential for food security and conservation, and can be infected by numerous viruses. Here, we used multiple levels of analysis that considered plant communities, individual plants, virus species, and haplotypes to broaden understanding of the Poaceae virome and to evaluate host-parasite richness relationships within agro-ecological landscapes in our study area. We emphasized the influence of grass diversity and land use on the composition of viral communities and their life history strategies, and we demonstrated the complexity of plant-virus interactions in less-managed grass communities, such as the higher virus prevalence and overrepresentation of mixed virus infection compared to theoretical predictions.

Shrub-mediated effects on soil nitrogen determines shrub-herbaceous interactions in drylands of the Tibetan Plateau.

Shrub promotes the survival, growth and reproduction of understory species by buffering the environmental extremes and improving limited resources (i.e., facilitation effect) in arid and semiarid regions. However, the importance of soil water and nutrient availability on shrub facilitation, and its trend along a drought gradient have been relatively less addressed in water-limited systems. We investigated species richness, plant size, soil total nitrogen and dominant grass leaf δ13C within and outside the dominant leguminous cushion-like shrub Caragana versicolor along a water deficit gradient in drylands of Tibetan Plateau. We found that C. versicolor increased grass species richness but had a negative effect on annual and perennial forbs. Along the water deficit gradient, plant interaction assessed by species richness (RIIspecies) showed a unimodal pattern with shift from increase to decrease, while plant interaction assessed by plant size (RIIsize) did not vary significantly. The effect of C. versicolor on soil nitrogen, rather than water availability, determined its overall effect on understory species richness. Neither the effect of C. versicolor on soil nitrogen nor water availability affected plant size. Our study suggests that the drying tendency in association with the recent warming trends observed in drylands of Tibetan Plateau, will likely hinder the facilitation effect of nurse leguminous shrub on understories if moisture availability crosses a critical minimum threshold.

Spatial distribution and effects of Lantana camara invasion on soil properties and native vegetation community composition in Tugwi‐Mukosi Recreational Park, Masvingo, Zimbabwe

AbstractThis study investigated spatial distribution and effects of Lantana camara invasion on soil properties and vegetation composition in Tugwi‐Mukosi Recreational Park. Supervised classification with the Random Forest classifier was used to map L. camara in the park. The study area was stratified into two categories based on soil type and the extent of L. camara invasion. Stratified random sampling was used in data collection to assess differences in species diversity and soil properties in the park. Results revealed that L. camara covered an area of 1772 ha. Using Random Forest classification, the study obtained an accuracy score of 92.1% and an F1 score of 86.25%. Two‐way ANOVA showed significant effect of L. camara invasion on soil moisture (F = 28.143, p = 0.000), organic matter (F = 13.377, p = 0.003), pH (F = 1272.369, p = 0.000), total nitrogen (F = 51.762, p = 0.000), total phosphorus (F = 5.000, p = 0.045), woody species density (F = 4.987, p = 0.027), basal area (F = 10.393, p = 0.001), grass species richness (F = 196.258, p = 0.000) and grass cover (F = 3.637, p = 0.042). These results suggest that L. camara was modifying the soil and vegetation properties of the ecosystem which has implications on biodiversity.

Effects of Late-Summer Prescribed Fire on Botanical Composition, Soil Cover, and Forage Production in Caucasian Bluestem-Infested Rangeland in the Kansas Smoky Hills: Year 4 of 5

Objective: Our objective was to determine effects of late-summer prescribed fire on frequency of Caucasian bluestem (Bothriochloa bladhii) in the Kansas Smoky Hills as well as changes in soil cover, botanical composition, and forage production associated with fire treatment. Study Description: The study was in Ellsworth County, KS. Eighteen one-acre plots were randomly assigned to one of three treatments: no burn, one burn (August 14, 2019), and two burns (August 14, 2019, and August 11, 2021). Soil cover, botanical composition, forage production, and Caucasian bluestem frequency were measured annually beginning in 2019. The Bottom Line: These data suggest that regular application of late-summer prescribed fire may be an effective method to reduce Caucasian bluestem frequency while improving overall grass-species richness.

Do native grasses emerge and establish in areas rehabilitated using vetiver grass?

Species-rich grasslands provide important ecosystem services, and in South Africa, approximately 40% of these grasslands are degraded. Vetiver grass (from India) is often used during rehabilitation efforts to restore soil function without a thorough understanding of the potential negative ecological impacts. Hence, a study was initiated to investigate vetiver’s ecological impacts during grassland rehabilitation. Firstly, a field survey was conducted using a contiguous quadrat method to evaluate the extent of grass secondary succession in these rehabilitated sites. Secondly, the effect of vetiver competition and seed sowing method on the recruitment of two native grasses (Eragrostis curvula and Megathyrsus maximus) was examined using pot trials. The field survey results showed no evidence of grass secondary succession, but rather the abundance of bare ground around vetiver, and a marked increase in grass species richness with increasing distance from planted vetiver. Subsequently, in the pot trial, vetiver facilitated emergence in both native grasses, and soil surface sowing of indigenous grass seeds showed greater emergence than other sowing methods. However, vetiver inhibited native grass seedling establishment, even when root competition was excluded. This study suggests that areas rehabilitated using vetiver are unlikely to become productive grasslands with good grazing, because vetiver inhibits colonisation by native grasses.

Effects of Native and Non-Native Invasive Grasses on Northern Bobwhite Habitat in South Texas

Usable space for northern bobwhite (Colinus virginianus) has been reduced across South Texas rangelands because of the spread of non-native, invasive grasses. A native grass, tanglehead (Heteropogon contortus), has also rapidly expanded its dominance in South Texas within the past 25 yr. It has formed high-density monocultures, similar to non-native grasses, which are associated with losses of herbaceous diversity and bare ground, both key components of bobwhite habitat. The objectives of our research were to 1) determine selection-avoidance of invasive grass cover by bobwhites and 2) determine effects of invasive grass cover on vegetation characteristics.We hypothesized the effects of tanglehead on bobwhite would be negative and similar to the effect of non-native grasses in the region. We recorded covey locations via helicopter surveys in December 2014 and 2015, with field measurements of bobwhite habitat at covey and random locations taken each spring. We measured grass and forb species richness, herbaceous vegetation height, woody-plant cover, tanglehead canopy cover, and non-native grass canopy cover at all covey detections and an equal number of random locations. We developed continuous selection ratios using Simple Saddlepoint Approximations to determine habitat selection by bobwhites. We also used quantile regression, at the 10th, 50th, and 90th quantiles, to determine relationships between invasive grass cover and vegetation factors. Bobwhites avoided areas of high canopy cover (> 20%) of all invasive grasses measured. We found negative relationships between native and non-native invasive grass cover and forb and grass species richness, as well as bare ground, and a positive relationship with herbaceous vegetation height at all quantiles modeled. Our results demonstrate the negative effects of increased tanglehead cover on native rangeland habitats, which aligns with similar non-native invasive grasses. Further expansion by tanglehead has the potential to significantly reduce usable space for bobwhites in South Texas.

Understanding the efficacy of different techniques to manage Chromolaena odorata L., an Invasive Alien Plant in the sub-Himalayan tall grasslands: Toward grassland recovery

Research on control efficacy of Chromolaena odorata is rare worldwide. Given the species' impact on internationally recognized protected areas, such research is important. Management method to control invasion and promote grassland recovery is poorly studied, particularly of C. odorata. In the present study, we test the efficacies of different management methods- manual uprooting, cutting, and cutting and burning of C. odorata plants to restore native grassland habitat in the sub-Himalayan grassland ecosystem in Manas National Park. We conducted experiments in three 1-ha plots, each plot was subjected to different treatments, where we monitored the recovery of native grass species. Based on our experiment we found that the species richness, density and cover of native grasses increased significantly by the third year (t2) in the manually uprooted treatment site compared to other two treatments; while the density and cover of C. odorata decreased. Our data highlight the efficacy of the manual uprooting treatment in reviving the grassland habitat. It is an ecologically-effective strategy as it can restore the entire native grass-community if maintained and monitored for at least three to five years. However, we caution that manually uprooting the C. odorata and reviving natural grass-communities requires long-term investment of resources and time. With this tried and tested management methodwe have reduced invasion by approximately 95% in the selected prime grassland habitat of around 5 km2 area in the central and eastern ranges of the Park in subsequent years. We have encouraged the use of C. odorata as natural dyeing agent among the local communities, as a complementary control action. The findings of the study will aid the forest managers to devise a comprehensive and pragmatic invasive species management plan in order to ensure long-term conservation of grassland habitat and its dependent species.

Un aporte al entendimiento de la lignificación de los pastizales/sabanas del Chaco Semiárido sudamericano

The grassland/savannas in the South American Semiarid Chaco have been massively transformed into shrubland. However, the ecology of this phenomenon is little known and restoration initiatives almost non-existent. We provide information about 1) the differences in plant structure and diversity along a woody encroachment gradient, and 2) the association among plant structural and functional attributes with environmental factors. We utilized 38 sample units and obtained tree and shrub cover, herbaceous biomass, species richness and composition of grasses and woody plants and several herbaceous functional types (mainly grasses). The environmental factors were historic grazing intensity and fire frequency. Related to item 1, we found: a) shrub cover was significant and progressively greater along the woody encroachment gradient and largely exceeded the tree cover, b) the herbaceous biomass was significantly smaller, with a high woody cover (x̄=89.7%), c) the grass species richness was maximum at intermediate levels of woody cover (x̄=39.3% and 64%), d) the dissimilarity in grass species composition was maximum between very low and high levels of woody cover (x̄=15.8% and 89.7%), and e) woody species richness and composition were uniform along the woody encroachment gradient. In relation to item 2, we observed: 1) the increase in historic grazing intensity was significant and strongly associated with the reduction of fire frequency, which, in turn, was significantly associated with the increase in tree and shrub cover and the decrease in herbaceous biomass, suggesting that overgrazing reduces or eliminates the grassland fires and promotes the woody encroachment, and 2) several grass functional types were significantly associated with the historic grazing intensity and fire frequency. Our results are useful both to anticipate the grassland/savannas responses to woody encroachment processes and to plan restoration and management strategies.

Grass species composition in tropical forest of southern India

Grass composition was assessed by plot method (1 m2; n= 1,749) in three habitats (dry deciduous-DDF, moist deciduous-MDF, and thorn forest-TF) at Mudumalai Tiger Reserve, southern India across different seasons from Jan 2004 to Dec 2007. The grass species richness and availability (per cent composition) varied significantly with habitats. Seventy-four species of grasses and sedges were recorded in all three habitats, with a few species common in all habitats. Grass availability varied significantly in different habitats across seasons and was positively influenced by precipitation. Among biotic factors, regeneration and shrub density had a primary influence on grass availability, followed by herb, sedge and weed density. The principal coordinate analysis revealed seven major associations in the tropical forest. There were considerable changes in the composition and association of grasses when compared to the past. Fire resistant species such as Themeda triandra, Heteropogan contortus and T. cymbaria dominated in the DDF. Grass species Aristida/Eragorstis were recorded in the TF, which were considered as indicators of heavy grazing pressure. Grass species that were reported rare and sporadic in the earlier study were not recorded, which emphasizes better pasture management in the tropical forest. Grass species composition and availability was threatened by invasion of weeds.

Both fencing duration and shrub cover facilitate the restoration of shrub-encroached grasslands

Fully understanding community assembly mechanisms is crucial to better predict grassland community dynamic under restoration. The community assembly over time is more complicated in shrub-encroached than in non-encroached degraded grasslands because the presence and size of shrub could regulate community processes and diminish effects of restoration time. However, how shrub size influences community assembly under restoration in shrub-encroached communities remains poorly understood. Therefore, in Caragana microphylla shrub-encroached grasslands in the Inner Mongolia typical steppe region, three plots with different shrub cover (low, medium and high) were set up in each of three sites with different fencing duration (15-, 35- and 39-year fencing). We assessed the assembly mechanisms of shrub-interspace herbaceous communities by estimating standardized effect size (SES) of functional dispersion (FDis) based on the first two principal components (PC1 and PC2) of traits characterizing resource acquisition strategies. According to the SES of FDisPC1, the assembly mechanisms shifted from stochastic processes to competitive exclusion over time of fencing in the plots of low and medium shrub cover and across a shrub cover gradient in the sites of 15-year and 35-year fencing, with the increase of soil moisture, species richness, Shannon-Wiener diversity and relative density of perennial grasses. Competitive exclusion dominated the communities in the site of 39-year fencing and in the plots of high shrub cover. According to the SES of FDisPC2, the competitive exclusion dominated all of the 9 communities investigated in this study. By variance component analysis, shrub cover showed the highest contribution in explaining the variance in community-weighted mean (CWM), and shrub cover and fencing duration showed the first two highest contributions in explaining the variance in SES of FDis. The present findings highlight the significance of shrub cover in influencing the community assembly in shrub-encroached grasslands under restoration in the European-Asian typical steppe region.

Selection of tree species by principal component analysis for abandoned farmland in southeastern Horqin Sandy Land, China

With changes in global climate and land use, the area of desertified farmland in southeastern Horqin Sandy Land (HSL) has increased in recent years, and farmlands are being abandoned. These abandoned farmlands (AFs) negatively impact the local ecology. Therefore, the aim of the present study was to select suitable trees and shrubs for those AFs to prevent and control the desertification tendency. In this study, three AFs were fenced for 2 years, then 37 arbor and shrub species or varieties of 21 families were planted in the fenced AFs and grown for 10 years. The ecological adaptability of the species was evaluated and ranked using a principal component analysis. The results showed that the biodiversity of the AFs significantly improved after 2 years of fencing; the Shannon–Wiener index and species richness of perennial grasses and forbs were 1.45 and 3.6 times higher, respectively, than for the unfenced AF. Among all species planted in fenced AFs, nine tree species had positive comprehensive F (CF) values; Pinus sylvestris (Russian Shira steppe provenance), Populus alba ‘Berolinensis’ and Gleditsia triacanthos had CF greater than 1, and the first (PC1), second (PC2) and third (PC3) principal component values (F1, F2, F3) were all positive. Among the shrubs, only Lespedeza bicolor and Rosa xanthina f. normalis had CF greater than 0. All these results suggest that fencing improves biodiversity and that planting trees and shrubs that have higher CF values on the basis of fencing is an effective way to green and beautify AFs in HSL.

Grazing by over-abundant native herbivores jeopardizes conservation goals in semi-arid reserves

Relaxation of predation pressure following the removal of mammalian predators has led to the irruption of large herbivores and excessive levels of herbivory in many regions of the Earth. Conservation reserves in dryland regions aim to restore ecological processes and facilitate biodiversity conservation by functioning as refuges from grazing by livestock. However, grazing pressure in conservation reserves may be high and hamper conservation objectives because wild herbivore populations have irrupted in the surrounding landscape due to predator control as well as the absence of hunting and competition with livestock within reserves. Here, we assess the relative abundance of native herbivores (kangaroos) and introduced herbivores (rabbits) using driving transects and dung surveys, and use selective exclosures to assess wild herbivores’ impacts on vegetation and soils in four conservation reserves in semi-arid Australia. Kangaroos were the dominant herbivore in each reserve. Grazing by kangaroos and/or rabbits was linked to reduced complexity of understorey vegetation, grass cover, species richness of grasses, forbs and shrubs, the depletion of soil carbon and phosphorous, and increased soil bulk density. The marked impacts of grazing by wild herbivores, particularly kangaroos, on vegetation and soils that we report are symptomatic of overgrazing. Our study provides evidence that grazing by kangaroos may jeopardize conservation efforts across a large region of semi-arid Australia. We contend that managing the total grazing pressure exerted by wild herbivores is crucial to prevent overgrazing in dryland conservation reserves where herbivore populations are not regulated by predators.

Grass species richness decreases along a woody plant encroachment gradient in a semi-arid savanna grassland, South Africa

Savanna grasslands are undergoing rapid land cover transformation as a consequence of woody plant encroachment. It still remains unclear how increasing woody plant density and cover in savannas influences tree-grass-soil interactions. This study determined the effect of increasing tree density on grass species richness, evenness, diversity and assessed how nutrients change along an encroachment gradient of a shallow plinthic savanna soil. Vegetation was quantified and soil samples analysed from eighteen randomly distributed 10 m × 10 m plots along a tree density gradient spanning from open, transitioning into intermediate and intensively encroached savanna grassland in northern South Africa. We found that an increase in tree density along the encroachment gradient decreased grass richness (GR) by 53% in the intensively woody-encroached grassland. Higher tree density and canopy cover led to a 90% dominance of Panicum maximum grass species. Conversely, higher tree density increased concentrations of carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil layer. This study highlights that woody plant encroachment decreases grass species richness and composition, but increases nutrients in the uppermost layer of a plinthic savanna soil. Such information is critical to improve understanding of the mechanisms underlying tree-grass interactions in savannas.

Shade alters savanna grass layer structure and function along a gradient of canopy cover

AbstractAimIn savannas, a grass‐dominated ground layer is key to ecosystem function via grass–fire feedbacks that maintain open ecosystems. With woody encroachment, tree density increases, thereby decreasing light in the ground layer and potentially altering ecosystem function. We investigated how light availability can filter individual grass species distributions and whether different functional traits are associated with response to a shade gradient in a landscape experiencing woody encroachment.LocationSavanna–forest mosaic in the Cerrado domain, southeastern Brazil.MethodsAlong an encroachment gradient of increasing tree leaf area index (LAI) and shade, we determined how changing light availability alters grass diversity and ground layer structure relative to grass cover and grass functional traits (photosynthetic pathway, underground storage organs, bud protection and traits related to grass shape, size and leaf dimensions).ResultsIncreasing shade led to a decrease in grass cover and grass species richness, and also compositional and functional changes. We found that where tree LAI reached 1, grass cover was reduced by 50% and species richness by 30%. While C4 grass species abundances decreased with increasing shade, the opposite pattern was true for C3 grasses. There were only small differences in light preferences among C4 subtypes, with phosphoenolpyruvate carboxykinase (PCK) species tolerating slightly more shaded conditions. Persistence of some C4 species under more shaded conditions was possible, likely due to an ability to store starch reserves via underground storage organs.ConclusionsWoody encroachment changes diversity and structure of the grassy layer that is critical to the functioning of savanna ecosystems, highlighting the dependence of the diverse grass layer on open and sunny conditions. Our results suggest a threshold of tree cover close to LAI ≈ 1 as being critical to cerrado grassy layer conservation.