Savanna Plant Communities Research Articles

Tillage agriculture and afforestation threaten tropical savanna plant communities across a broad rainfall gradient in India

Abstract The consequences of land‐use change for savanna biodiversity remain undocumented in most regions of tropical Asia. One such region is western Maharashtra, India, where old‐growth savannas occupy a broad rainfall gradient and are increasingly rare due to agricultural conversion and afforestation. To understand the consequences of land‐use change, we sampled herbaceous plant communities of old‐growth savannas and three alternative land‐use types: tree plantations, tillage agriculture and agricultural fallows (n = 15 sites per type). Study sites spanned 457 to 1954 mm of mean annual precipitation—corresponding to the typical rainfall range of mesic savannas globally. Across the rainfall gradient, we found consistent declines in old‐growth savanna plant communities due to land‐use change. Local‐scale native species richness dropped from a mean of 12 species/m2 in old‐growth savannas to 8, 6 and 3 species/m2 in tree plantations, fallows and tillage agriculture, respectively. Cover of native plants declined from a mean of 49% in old‐growth savannas to 27% in both tree plantations and fallows, and 4% in tillage agriculture. Reduced native cover coincided with increased cover of invasive species in tree plantations (18%), fallows (18%) and tillage agriculture (3%). In analyses of community composition, tillage agriculture was most dissimilar to old‐growth savannas, while tree plantations and fallows showed intermediate dissimilarity. These compositional changes were driven partly by the loss of characteristic savanna species: 65 species recorded in old‐growth savannas were absent in other land uses. Indicator analysis revealed 21 old‐growth species, comprised mostly of native savanna specialists. Indicators of tree plantations (nine species) and fallows (13 species) were both invasive and native species, while the two indicators of tillage agriculture were invasive. As reflective of declines in savanna communities, mean native perennial graminoid cover of 27% in old‐growth savannas dropped to 9%, 7%, and 0.1% in tree plantations, fallows and tillage agriculture, respectively. Synthesis. Agricultural conversion and afforestation of old‐growth savannas in India destroys and degrades herbaceous plant communities that do not spontaneously recover on fallowed land. Efforts to conserve India's native biodiversity should encompass the country's widespread savanna biome and seek to limit conversion of irreplaceable old‐growth savannas.

Root vascular anatomy predicts maximum growth rates in savanna trees and grasses

AbstractRoot‐based functional traits are relatively overlooked as drivers of savanna plant community dynamics, an important gap in water‐limited ecosystems. Recent work has shed light on patterns of trait coordination in roots, but less is known about the relationship between root functional traits, water acquisition, and plant demographic rates. Here, we investigated how fine‐root vascular and morphological traits are related in two dominant PFTs (C3 trees and C4 grasses from the savanna biome), whether root traits can predict plant relative growth rate (RGR), and whether root trait multivariate relationships differ in trees and grasses. We used root data from 21 tree and 18 grass species grown under greenhouse conditions, and quantified a suite of vascular and morphological root traits. We used a principal components analysis (PCA) to identify common axes of trait variation, compared trait correlation matrices between the two PFTs, and investigated the relationship between PCA axes and individual traits and RGR. We found that there was no clear single axis integrating vascular and morphological traits, but found that vascular anatomy predicted RGR in both trees and grasses. Trait correlation matrices differed in trees and grasses, suggesting potentially divergent patterns of trait coordination between the two functional types. Our results suggested that, despite differences in trait relationships between trees and grasses, root conductivity may constrain maximum growth rate in both PFTs, highlighting the critical role that water relations play in savanna vegetation dynamics and suggesting that root water transport capacity is an important predictor of plant performance in the savanna biome.

Should we burn the Cerrado? Effects of fire frequency on open savanna plant communities

AbstractAimsTropical savanna evolved in the presence of fire, depending on it to maintain its physiognomies and biodiversity. Open ecosystems of tropical savanna burn frequently, and thus changes in fire frequency can affect their plant communities. In this study we aimed to analyze the effects of different fire frequencies on the composition, structure and post‐fire regeneration of open savanna plant communities of the Cerrado.LocationOpen savannas, Central Brazil (13°35–38′S and 47°45–51′W).MethodsWe conducted surveys in areas with different fire frequencies (fire exclusion, burned annually and burned biennially for 6 years, from 2013). Vegetation sampling was carried out before application of fire treatments in 2017, and then 1 and 2 years after. Species cover was estimated visually and later grouped into graminoids, shrubs and forbs.ResultsEvaluation of the plant communities after 6 years of fire treatment indicated that fire frequency did not affect species richness. Fire frequency did, however, promote alteration in species occurrence and abundance, resulting in three dissimilar plant communities. Regarding post‐fire regeneration, annually burned plots showed consistency in species richness and growth form cover over 6 years of annually applied fire. Biennially burned plots tended to be more variable than the other treatments, with a large increase in species richness and growth form cover in the first year after fire, succeeded by a decrease in the second year. Not all species that were recruited just after fire were able to persist in the above‐ground community in the second year after fire.ConclusionsOpen savannas of the Cerrado are highly resilient to fire, even to annual fires, with 6 years of annual fire treatments not affecting post‐fire species diversity or growth form cover. However, communities differed in species composition, which can influence their functional diversity and lead to alterations in ecosystem services. Our findings encourage long‐term studies to evaluate the resilience of areas frequently burned, as well as longer fire exclusion periods.

Effects and response of the Cerrado ground-layer to frost along the canopy cover gradient.

Frost effects on savanna plant communities have been considered as analogous to those from fire, both changing community structure and filtering species composition. However, while frost impacts have been well-studied for the woody component of savannas, it is still poorly explored for the ground-layer community. Here, we investigated effects of frost in the Cerrado along a gradient of tree cover, focusing on ground-layer plant species, near the southern limit of the Cerrado in Brazil. We aimed to elucidate if the pattern already described for the tree layer also extends to the ground layer in terms of mimicking the effects of fire on vegetation structure and composition. We assessed how damage severity differs across species and across the tree-cover gradient, and we examined the recovery process after frost in terms of richness and community structure along the canopy cover gradient. Frost caused immediate and widespread dieback of the perennial ground-layer, with greatest impact on community structure where tree cover was lowest. However, frost did not reduce the number of species, indicating community resilience to this natural disturbance. Although frost mimicked the effects of fire in some ways, in other ways it differed substantially from fire. Unlike fire, frost increases litter cover and decreases the proportion of bare soil, likely hindering crucial processes for recovery of plant populations, such as seed dispersal, seed germination and plant resprouting. This finding calls attention to the risk of misguided conclusions when the ground layer is neglected in ecological studies of tropical savannas and grasslands.

Savanna plant communities in the wetter parts of the Indonesian archipelago

Savanna plant communities in the wetter parts of the Indonesian archipelago

Direct and indirect effects of global change on mycorrhizal associations of savanna plant communities

Mycorrhizal fungi are key components of whole‐plant adaptive strategies to cope with different abiotic and biotic constraints. Although they are particularly sensitive to different global change drivers, there are still many gaps on the mechanisms underpinning shifts in mycorrhizal associations under different climatic and management scenarios.We carried out a field manipulative experiment of rainfall exclusion and increased temperature aimed to evaluate the impact of forecasted warming and drying on mycorrhizal associations of savanna plant communities subjected to different grazing history. Additionally, we compiled detailed information on the abiotic and biotic environment with the final aim of disentangling the direct and indirect effects of climate change on this widespread mycorrhizal symbiosis.Our results suggest that climate change could induce relevant changes in mycorrhizal associations, primarily promoted by warming, which decreased the abundance of mycorrhizae but induced higher activity of nutrient exchange between the host–plant and the mycosymbiont. Temperature did not only affect this symbiosis in a direct way, but also exerted relevant indirect effects via changes in soil functioning and other root‐colonizing microorganisms such as dark septate endophytes. Grazing history influenced the allocation of fungal structures inside the host‐root, but its effect differed as a function of the climatic treatment.Results from this study suggest that mycorrhizal fungi might become less prevalent in plant communities inhabiting savanna ecosystems under future scenarios of increasing aridity. In agreement with this finding, higher temperatures also promoted the predominance of plants with root traits that favour efficient resource acquisition by themselves without the help of a mycorrhizal partner. Our results provide new insights into the interactive effects of the two main threats facing Mediterranean savanna ecosystems, as well as potentially useful information to be applied in ecologically‐based management strategies aimed at attenuating the potential impact of global change on mycorrhizal fungi.

Biotic interactions and limitations to explain their effects on a neotropical savanna plant community

Ecological communities result from the interaction of different sorts of biotic and abiotic processes. Here, we assess the effects of two target plants that show different interactions with their surrounding (neighborhood) vegetation, on species richness, abundance, and diversity of communities, to address the influence of different biological processes in the formation of a plant community. We compared communities of savannas in Central Brazil, South America, in plots with Hancornia speciosa or with Himatanthus obovatus (Apocynaceae), or none of them (Control plots). We analyzed the communities from different stratum (herb and woody) and with different species abundance (rare, common, and abundant). We used two cut-off methods to define classes of species abundance, one based on the number of individuals and other on the percentage of species sampled (quartile of abundances). We collected 16,979 individuals, representing 161 species and 48 families. Regardless of the cut-off method, communities with H. speciosa or H. obovatus showed more rare species than communities without them (Control plots). Our findings confirm the ecological effects of Hancornia speciosa and Himatanthus obovatus shaping the structure of surrounding vegetation. We also found that the cut-off methods for abundance classes directly influence our perception of the rare species composition in a community and, consequently, affects our interpretations about the dynamics that produce rarity. Thus, it is important that community studies justify the application of the cut-off method in the definition of rare species, according to their aims and the spatial scale in question.

Pine savanna plant community disassembly after fire suppression

AbstractQuestionsBiodiversity is being lost rapidly due to anthropogenic changes in land use, climate, and other environmental conditions. In fire‐maintained ecosystems, altered fire regimes accelerate native species loss — community disassembly — and promote recruitment of fire‐sensitive species. In this study, we ask whether fire suppression results in changes over time in functional trait composition of ground‐layer species and whether these changes differ in longleaf pine savannas invaded by hardwoods from those invaded by sand pines.LocationFive Floridian locations on the southeastern coastal plain of the United States.MethodsAt each location we selected a fire‐maintained and fire‐suppressed savanna and measured percent plant cover by ground‐story species in 1,000 m2 plots. For 102 of these species, we measured 10 functional traits — height, growth form, specific leaf area, leaf dry matter content (LDMC), leaf water content, leaf ignition time, leaf mass consumed by fire, light compensation points (LCPs), non‐structural carbohydrate concentrations in under‐ground organs, and seed mass.ResultsFire exclusion was associated with reductions in both functional diversity and species richness. We identified 38 species exclusive to frequently burned sites: these species showed high LDMC values, high LCPs, high leaf mass consumed, and low leaf ignition time values. Lack of fire was associated with loss of 6 of the 12 C4 native grass species. Species and functional trait composition were affected by both time‐since‐fire and whether post‐fire communities were invaded by broadleaved trees (Quercus spp. and Liquidambar styraciflua) or by sand pine (Pinus clausa).ConclusionsWe demonstrated the effects of altered disturbance regimes on savanna plant species and functional trait composition. This trait‐based approach advanced our understanding of how altered disturbance regimes can alter plant communities. Although 38 shade‐intolerant and flammable native savanna species were absent by 10 years since fire, 12 such species persisted even 40 years after fire exclusion.

Short communication: The dissimilarity in plant species composition of savanna ecosystem along the elevation gradient on Flores Island, East Nusa Tenggara, Indonesia

Abstract. Sutomo, Darma IDP, Iryadi R. 2020. Short communication: The dissimilarity in plant species composition of savanna ecosystem along the elevation gradient on Flores Island, East Nusa Tenggara, Indonesia. Biodiversitas 21: 492-496. Savannas in Indonesia are located from west to the east across the archipelago. The objective of this research was to investigate the dissimilarity of floristic composition among savannas at different elevations in Flores. Sixteen sampling plots, each measuring 20 x 20 m were spread over the lowland, midland, and upland savannas. We analyzed the differences in plant community composition among the savannas using NMDS ordination and SIMPER analyses available in PRIMER V.6. As many as 41 species of plants were found in all of the sampling plots. The lowland savanna plant community consisted of Themeda arguens, Zoesya sp., Chromolaena odorata, Crotalaria sp., Adenanthera pavonina, Ocimum sp., Lantana camara in the groundcover layer, and Ziziphus jujube, and Borassus flabellifer at the tree layer. The midland savanna had almost similar composition, except the occurrence of Imperata cylindrica and Leucaena leucocephala, and the upland savanna plant species were Cyperus sp., Polygala paniculata, I. cylindrica, Melastoma malabathricum, C. odorata, Centella asiatica, Vaccinium sp., and Cymbopogon sp. The savannas in Flores were invaded by invasive exotic species, namely L. camara and C. Odorata which may pose serious threat to the existence of savannas.

Supervised versus un‐supervised classification: A quantitative comparison of plant communities in savanna vegetation

AbstractQuestionWhat are the differences between plant communities recognised using supervised versus un‐supervised methods?LocationNortheastern Australia.MethodsTwo classifications of savanna plant communities were formed independently with two different approaches: supervised and un‐supervised (using agglomerative hierarchical clustering). Each approach used the same vegetation datasets and, importantly, classification criteria. The communities occur on two different landscapes, with differing environmental gradients, covering an area of 53,500 km2. We compared the internal characteristics of plant communities between approaches and landscapes using four evaluation criteria: identifiability, distinctiveness, similarity of internal heterogeneity and predictability of species foliage cover. Additionally, we compared the central floristic concepts and compositional boundaries of communities identified by each approach.ResultsSupervised and un‐supervised approaches recognised similar floristic community concepts. Compositional boundaries between communities were similar on the landscape with steeper environmental gradients but significantly different on the landscape with gradual environmental gradients. However, communities distinguished using supervised methods were significantly less distinct and identifiable, worse at predicting species foliage cover and significantly more variable in species composition than those identified using un‐supervised methods.ConclusionsUsing supervised rather than un‐supervised approaches to distinguish plant communities can result in less recognisable communities, possibly reducing their usefulness for land management planning. Importantly, we found a large disparity between the two approaches in delineating compositional boundaries between communities on landscapes with gradual environmental gradients. This is particularly relevant to communities in biomes such as the savanna which comprises 20% of the Earth's landmass. Ecologists can be more confident using a supervised approach on landscapes with steep environmental gradients but should target landscapes with gradual environmental gradients for un‐supervised classification.

Sixty years of community change in the prairie-savanna-forest mosaic of Wisconsin.

Biodiversity loss is a global concern, and maintaining habitat complexity in naturally patchy landscapes can help retain regional diversity. A mosaic of prairie, savanna, and forest historically occurred across central North America but currently is highly fragmented due to human land conversion. It is unclear how each habitat type now contributes to regional diversity. Using legacy data, we resurveyed savanna plant communities originally surveyed in the 1950s to compare change in savannas to that in remnant forests and prairies. Savanna community structure and composition changed substantially over the past 60 years. Tree canopy density nearly doubled and many prairie and savanna specialist species were replaced by forest and non‐native species. All three habitats gained and lost many species since the 1950s, resulting in large changes in community composition from local colonizations and extinctions. Across all three habitats, regional species extinctions matched that of regional colonization resulting in no net change in regional species richness. Synthesis—Despite considerable species turnover within savannas, many species remain within the broader prairie–savanna–forest mosaic. Both regional extinctions and colonizations were high over the past 60 years, and maintaining the presence of all three community types—prairie, savanna and forest—on the landscape is critical to maintaining regional biodiversity.

CO2 enrichment does not entirely ameliorate Vachellia karroo drought inhibition: A missing mechanism explaining savanna bush encroachment

There is theoretical, empirical and modeled evidence that atmospheric enrichment of carbon dioxide (CO2) is driving the conversion of open savannas to woodlands (i.e., woody thickening or bush encroachment). This study measured the impacts of available CO2 and water limitation on seedling growth and physiology to determine how these two environmental conditions may be influencing savanna seedling establishment. We hypothesized that a model savanna tree, Vachellia karroo, would experience positive growth responses to increasing CO2 and that CO2 enrichment would partially offset slowed growth and physiological inhibition imposed by drought. To test this hypothesis, we grew V. karroo in open-top chambers at the Kirstenbosch Research Center (Cape Town, South Africa). The seedlings were divided equally into four CO2 treatments: 180 ppm, 270 ppm, 400 ppm, and 800 ppm. Seedlings were grown in their CO2 treatments for 35 days before drought was initiated. The drought was initiated by removing the irrigation from half of the pots. Leaf gas-exchange, stem water potential, leaf non-structural carbohydrate concentrations, and plant morphology were measured at drought day-30 and drought day-60. The results showed significant benefits from CO2 enrichment to aboveground plant structures, including larger leaves and stems, when water was abundant (p < 0.05). CO2-related increases to photosynthetic parameters, such as instantaneous water-use efficiency, were significantly diminished by water limitation (p < 0.05). Our results present the first experimental evidence of the relationship between the two driving factors structuring the large-scale spatial patterns of savanna vegetation productivity and cover. These results suggest that atmospheric enrichment of CO2 will significantly affect aspects of plant growth and physiology, yet prolonged drought will remain an overarching determinant of savanna plant community dynamics.

Land Cover Change in Northern Botswana: The Influence of Climate, Fire, and Elephants on Semi-Arid Savanna Woodlands

Complex couplings and feedback among climate, fire, and herbivory drive short- and long-term patterns of land cover change (LCC) in savanna ecosystems. However, understanding of spatial and temporal LCC patterns in these environments is limited, particularly for semi-arid regions transitional between arid and more mesic climates. Here, we use post-classification analysis of Landsat TM (1990), ETM+ (2003), and OLI (2013) satellite imagery to classify and assess net and gross LCC for the Chobe District, a 21,000 km2 area encompassing urban, peri-urban, rural, communally-managed (Chobe Enclave), and protected land (Chobe National Park, CNP, and six protected forest reserves). We then evaluate spatiotemporal patterns of LCC in relation to precipitation, fire detections (MCD14M, 2001–2013) from the Moderate Resolution Imaging Spectroradiometer (MODIS), and dry season elephant (Loxodonta africana) aerial survey data (2003, 2006, 2012, 2013). Woodland cover declined over the study period by 1514 km2 (16.2% of initial class total), accompanied by expansion of shrubland (1305 km2, 15.7%) and grassland (265 km2, 20.3%). Net LCC differed importantly in protected areas, with higher woodland losses observed in forest reserves compared to the CNP. Loss of woodland was also higher in communally-managed land for the study period, despite gains from 2003–2013. Gross (class) changes were characterized by extensive exchange between woodland and shrubland during both time steps, and a large expansion of shrubland into grassland and bare ground from 2003–2013. MODIS active fire detections were highly variable from year to year and among the different protected areas, ranging from 1.8 fires*year−1/km2 in the Chobe Forest Reserve to 7.1 fires*year−1/km2 in the Kasane Forest Reserve Extension. Clustering and timing of dry season fires suggests that ignitions were predominately from anthropogenic sources. Annual fire count was significantly related to total annual rainfall (p = 0.009, adj. R2 = 0.50), with a 41% increase in average fire occurrence in years when rainfall exceeded long-term mean annual precipitation (MAP). Loss of woodland was significantly associated with fire in locations experiencing 15 or more ignitions during the period 2001–2013 (p = 0.024). Although elephant-mediated damage is often cited as a major cause of woodland degradation in northern Botswana, we observed little evidence of unsustainable pressure on woodlands from growing elephant populations. Our data indicate broad-scale LCC processes in semi-arid savannas in Southern Africa are strongly coupled to environmental and anthropogenic forcings. Increased seasonal variability is likely to have important effects on the distribution of savanna plant communities due to climate-fire feedbacks. Long-term monitoring of LCC in these ecosystems is essential to improving land use planning and management strategies that protect biodiversity, as well as traditional cultures and livelihoods under future climate change scenarios for Southern Africa.

Are cattle surrogate wildlife? Savanna plant community composition explained by total herbivory more than herbivore type.

The widespread replacement of wild ungulate herbivores by domestic livestock in African savannas is composed of two interrelated phenomena: (1) loss or reduction in numbers of individual wildlife species or guilds and (2) addition of livestock to the system. Each can have important implications for plant community dynamics. Yet very few studies have experimentally addressed the individual, combined, and potentially interactive effects of wild vs. domestic herbivore species on herbaceous plant communities within a single system. Additionally, there is little information about whether, and in which contexts, livestock might functionally replace native herbivore wildlife or, alternatively, have fundamentally different effects on plant species composition. The Kenya Long-term Exclosure Experiment, which has been running since 1995, is composed of six treatment combinations of mega-herbivores, meso-herbivore ungulate wildlife, and cattle. We sampled herbaceous vegetation 25 times between 1999 and 2013. We used partial redundancy analysis and linear mixed models to assess effects of herbivore treatments on overall plant community composition and key plant species. Plant communities in the six different herbivore treatments shifted directionally over time and diverged from each other substantially by 2013. Plant community composition was strongly related (R2 =0.92) to residual plant biomass, a measure of herbivore utilization. Addition of any single herbivore type (cattle, wildlife, or mega-herbivores) caused a shift in plant community composition that was proportional to its removal of plant biomass. These results suggest that overall herbivory pressure, rather than herbivore type or complex interactions among different herbivore types, was the main driver of changes in plant community composition. Individual plant species, however, did respond most strongly to either wild ungulates or cattle. Although these results suggest considerable functional similarity between a suite of native wild herbivores (which included grazers, browsers, and mixed feeders) and cattle (mostly grazers) with respect to understory plant community composition, responses of individual plant species demonstrate that at the plant-population-level impacts of a single livestock species are not functionally identical to those of a diverse group of native herbivores.

Nutrient dynamics and plant assemblages of Macrotermes falciger mounds in a savanna ecosystem

Termites through mound construction and foraging activities contribute significantly to carbon and nutrient fluxes in nutrient-poor savannas. Despite this recognition, studies on the influence of termite mounds on carbon and nitrogen dynamics in sub-tropical savannas are limited. In this regard, we examined soil nutrient concentrations, organic carbon and nitrogen mineralization in incubation experiments in mounds of Macrotermes falciger and surrounding soils of sub-tropical savanna, northeast Zimbabwe. We also addressed whether termite mounds altered the plant community and if effects were similar across functional groups i.e. grasses, forbs or woody plants. Mound soils had significantly higher silt and clay content, pH and concentrations of calcium (Ca), magnesium (Mg), potassium (K), organic carbon (C), ammonium (NH4+) and nitrate (NO3−) than surrounding soils, with marginal differences in phosphorus (P) and sodium (Na) between mounds and matrix soils. Nutrient enrichment increased by a factor ranging from 1.5 for C, 4.9 for Mg up to 10.3 for Ca. Although C mineralization, nitrification and nitrification fraction were similar between mounds and matrix soils, nitrogen mineralization was elevated on mounds relative to surrounding matrix soils. As a result, termite mounds supported unique plant communities rich and abundant in woody species but less diverse in grasses and forbs than the surrounding savanna matrix in response to mound-induced shifts in soil parameters specifically increased clay content, drainage and water availability, nutrient status and base cation (mainly Ca, Mg and Na) concentration. In conclusion, by altering soil properties such as texture, moisture content and nutrient status, termite mounds can alter the structure and composition of sub-tropical savanna plant communities, and these results are consistent with findings in other savanna systems suggesting that increase in soil clay content, nutrient status and associated changes in the plant community assemblage may be a general property of mound building termites.

Savanna plant community types at El Palmar National Park (Argentina)

Since 1965, El Palmar National Park, Entre Rios, protects the largest remnant of Butia yatay palm savanna, a species-rich ecosystem threatened by human activity. To study the functioning of this ecosystem and to manage its preservation, it is necessary to have an adequate picture of its vegetation heterogeneity. In this paper, we present a phytosociological classification of the plant communities of this savanna, describe their structure, species composition and richness, and provide their distribution in the landscape. To stratify our data acquisition, we first mapped the major geomorphic units in this landscape. In the spring of 1999 and the following summer, we conducted complete inventories of plant species in 73 savanna stands with different fire histories distributed throughout the different geomorphic units. We used multivariate analysis and numerical classification techniques to group inventories in plant community types and species in floristic groups. Finally, we summarized the compositional differences among community types as a metric multidimensional scaling ordination. We defined seven community types, recognizable by combinations of 20 floristic groups. The pattern of similarities among these community types suggests that floristic heterogeneity is organized along two gradients, one associated with soil texture differences, and the other with differences in water input. Three of the community types, closely related to each other, are distributed on elevated fluvial terraces and interfluves, in correspondence with gradual differences in topography and soil. These community types undergo successional change due to woody encroachment triggered by cattle exclusion and maintained by scarcity of natural fires. The remaining four community types are strictly associated with rocky outcrops, sand deposits, or runoff pathways that interrupt the landscape matrix, with no apparent woody encroachment.

Fire frequency, agricultural history and the multivariate control of pine savanna understorey plant diversity

AbstractQuestionHuman‐altered disturbance regimes and agricultural land uses are broadly associated with reduced plant species diversity in terrestrial ecosystems. In this study, we seek to understand how fire frequency and agricultural land‐use history influence savanna understorey plant diversity through complex relationships (i.e. indirect effects) among multiple biophysical variables.LocationFort Bragg, NC, US, Savannah River Site, SC, US and Fort Stewart, GA, US.MethodsWe use structural equation modelling (SEM) to evaluate the relationships among six groups of predictor variables and their influence on local‐scale species richness in pine savannas at 256 sites from three locations in the southeastern USA. In the model, fire frequency and agricultural history are hypothesized to control richness through a combination of direct effects, and indirect effects mediated by resource availability, tree abundance, understorey plant abundance and the O horizon (litter and duff depth).ResultsFrequent fires promote richness by limiting tree abundance, which increases understorey abundance and reduces the O horizon. Frequent fires also limit the O horizon independent of tree abundance. Of the total positive effect of fire on richness, 70% is attributable to reductions in the O horizon and 30% to reduced tree abundance. Agricultural history has a negative effect on richness through a positive correlation with tree abundance, which decreases understorey abundance and increases the O horizon. Agricultural history has a modest negative effect on richness by reducing resource availability as well as a strong direct negative effect (38% of the total effect) that is unrelated to other modelled variables.ConclusionsThrough a multivariate framework and large‐scale data set, this study unites and tests our understanding of the factors that control plant species diversity in a fire‐dependent ecosystem. We show that the effects of fire frequency and agricultural history on richness are largely mediated through other ecosystem attributes, including vegetation structure (i.e. tree and understorey abundance), resource availability and the O horizon. Persistent, negative effects of agricultural history demonstrate the slow rates of savanna plant community recovery on post‐agricultural land and highlight the conservation value of frequently burned savanna remnants.

Short-term effect of nutrient availability and rainfall distribution on biomass production and leaf nutrient content of savanna tree species.

Changes in land use may lead to increased soil nutrient levels in many ecosystems (e.g. due to intensification of agricultural fertilizer use). Plant species differ widely in their response to differences in soil nutrients, and for savannas it is uncertain how this nutrient enrichment will affect plant community dynamics. We set up a large controlled short-term experiment in a semi-arid savanna to test how water supply (even water supply vs. natural rainfall) and nutrient availability (no fertilisation vs. fertilisation) affects seedlings’ above-ground biomass production and leaf-nutrient concentrations (N, P and K) of broad-leafed and fine-leafed tree species. Contrary to expectations, neither changes in water supply nor changes in soil nutrient level affected biomass production of the studied species. By contrast, leaf-nutrient concentration did change significantly. Under regular water supply, soil nutrient addition increased the leaf phosphorus concentration of both fine-leafed and broad-leafed species. However, under uneven water supply, leaf nitrogen and phosphorus concentration declined with soil nutrient supply, this effect being more accentuated in broad-leafed species. Leaf potassium concentration of broad-leafed species was lower when growing under constant water supply, especially when no NPK fertilizer was applied. We found that changes in environmental factors can affect leaf quality, indicating a potential interactive effect between land-use changes and environmental changes on savanna vegetation: under more uneven rainfall patterns within the growing season, leaf quality of tree seedlings for a number of species can change as a response to changes in nutrient levels, even if overall plant biomass does not change. Such changes might affect herbivore pressure on trees and thus savanna plant community dynamics. Although longer term experiments would be essential to test such potential effects of eutrophication via changes in leaf nutrient concentration, our findings provide important insights that can help guide management plans that aim to preserve savanna biodiversity.

Effects of Fire on Seedling Diversity and Plant Reproduction (Sexual vs. Vegetative) in Neotropical Savannas Differing in Tree Density

AbstractLittle is known about the effects of fire on the structure and species composition of Neotropical savanna seedling communities. Such effects are critical for predicting long‐term changes in plant distribution patterns in these ecosystems. We quantified richness and density of seedlings within 144 plots of 1 m2located along a topographic gradient in long‐unburned (fire protected since 1983) and recently burned (September 2005) savannas in Brazil. These savannas differ in tree density and canopy cover. Sites along the gradient, however, did not differ in species composition prior to the fire. In recently burned savannas we also evaluated the contribution of vegetative reproduction relative to sexual reproduction by quantifying richness and density of root suckers. Finally, we tested seed tolerance to pulses of high temperatures—similar to those occurring during fires on the soil surface and below—of five dominant savanna tree species. Seedlings were more abundant and diverse in unburned than in burned savannas. Seedling species composition differed among unburned and burned savannas probably reflecting early differences in root: shoot biomass allocation patterns. In recently burned savannas, root suckers were more abundant and diverse than seedlings. Relatively long exposures (&gt;10 min) of temperatures of 90 °C reduced seed germination in all studied species suggesting a negative effect of fire on germination of seeds located at or aboveground level. Because vegetative reproduction contributes more than sexual reproduction in burned environments, frequent fires are likely to cause major shifts in species composition of Neotropical savanna plant communities, favoring clonally produced recruits along tree density/topographic gradients.

Tree species from different functional groups respond differently to environmental changes during establishment

Savanna plant communities change considerably across time and space. The processes driving savanna plant species diversity, coexistence and turnover along environmental gradients are still unclear. Understanding how species respond differently to varying environmental conditions during the seedling stage, a critical stage for plant population dynamics, is needed to explain the current composition of plant communities and to enable us to predict their responses to future environmental changes. Here we investigate whether seedling response to changes in resource availability, and to competition with grass, varied between two functional groups of African savanna trees: species with small leaves, spines and N-fixing associations (fine-leaved species), and species with broad leaves, no spines, and lacking N-fixing associations (broad-leaved species). We show that while tree species were strongly suppressed by grass, the effect of resource availability on seedling performance varied considerably between the two functional groups. Nutrient inputs increased stem length only of broad-leaved species and only under an even watering treatment. Low light conditions benefited mostly broad-leaved species' growth. Savannas are susceptible to ongoing global environment changes. Our results suggest that an increase in woody cover is only likely to occur in savannas if grass cover is strongly suppressed (e.g. by fire or overgrazing). However, if woody cover does increase, broad-leaved species will benefit most from the resulting shaded environments, potentially leading to an expansion of the distribution of these species. Eutrophication and changes in rainfall patterns may also affect the balance between fine- and broad-leaved species.