N and P Cycling in Tanzanian Humid Savanna: Influence of Herbivores, Fire, and N2-Fixation

Availabilities of nitrogen (N) and phosphorus (P) have a strong influence on plant growth and the species composition of savannas, but it is not clear how these availabilities depend on factors such as fire, N₂-fixation, and activities of wild herbivores and cattle. We quantified soil N and P availabilities in various ways (extractable pools, mineralization, resin adsorption) along vegetation gradients within a recently abandoned cattle ranch and a former game reserve in Tanzania (both areas now part of the Saadani National Park). We also assessed annual N and P balances to evaluate how long-term availabilities of N and P are affected by large herbivores, symbiotic N₂-fixation, and fire. The results show that cattle ranching led to a spatial re-distribution of nutrients, with the local accumulation of P being stronger and more persistent than that of N. In the former game reserve, intensively grazed patches of short grass tended to have elevated soil N and P availabilities; however, because quantities of nutrients removed through grazing exceeded returns in dung and urine, the nutrient balances of these patches were negative. In dense Acacia stands, N₂-fixation increased N availability and caused a net annual N input. Fire was the major cause for nutrient losses from tallgrass savanna, and estimated N inputs from the atmosphere and symbiotic N₂-fixation were insufficient to compensate for these losses. Our results call into question the common assumption that N budgets in annually burned savanna are balanced; rather, these ecosystems are a mosaic of patches with both N enrichment and impoverishment, which vary according to the vegetation type.     

Increases of Soil C, N, and P Pools Along an Acacia Tree Density Gradient and Their Effects on Trees and Grasses

Nitrogen (N) fixing trees including many species of Acacia are an important though variable component of savanna ecosystems. It is known that these trees enrich the soil with carbon (C) and N, but their effect on the combined C:N:P stoichiometry in soil is less well understood. Theory suggests that they might reduce available phosphorus (P), creating a shift from more N-limited conditions in grass-dominated to more P-limited conditions in tree-dominated sites, which in turn could feed back negatively on the trees’ capacity to fix N. We studied the effects of Acacia zanzibarica tree density upon soil and foliar N:P stoichiometry, and the N₂-fixation rates of trees and leguminous herbs in a humid Tanzanian savanna. Foliar N:P ratios and N₂-fixation rates of trees remained constant across the density gradient, whereas soil C, N and organic P pools increased. In contrast, the N:P ratio of grasses increased and N₂-fixation rates of leguminous herbs decreased with increasing tree density, indicating a shift towards more P-limited conditions for the understory vegetation. These contrasting responses suggest that trees and grasses have access to different sources of N and P, with trees being able to access P from deeper soil layers and perhaps also utilizing organic forms more efficiently.     

Frequent fire affects soil nitrogen and carbon in an African savanna by changing woody cover

When tropical and sub-tropical ecosystems burn, considerable amounts of N present in the biomass fuel may be released. This ultimately results in a loss of fixed N to the atmosphere. It is often assumed that this volatilization loss of N with frequent fire will result in a reduction of plant-available N and total system N. By changing the amount of woody biomass fire may, however, also have indirect effects on N and C dynamics. Here we consider the effects of 50 years of frequent fire on total soil N and soil organic C (SOC) and total soil N in a mesic savanna in the Kruger National Park, South Africa. We also determine how changes in woody biomass may affect total soil N and SOC. We measured soil and fine root N and C concentrations as well as total soil N and SOC pools in four burning treatments, including fire exclusion, of a long-term fire experiment. Our results show that regardless of soil depth, fire treatment had no significant effect on total soil N and SOC. Our results also show that under trees total soil N and SOC concentrations of the surface soil increase, and pools of N and SOC increase to a depth of 7 cm. However, the extent to which soil N and C dynamics differed under canopies and away from canopies was dependent on fire treatment. Our results show that the effect of fire on soil N and C is mediated both through the indirect effect of changes in woody cover and the direct effects of fire (volatilization losses of nutrients). We suggest that woody thickening in this mesic savanna will have pronounced effects on long-term N and C dynamics.     

Woody encroachment reduces nutrient limitation and promotes soil carbon sequestration

During the past century, the biomass of woody species has increased in many grassland and savanna ecosystems. As many of these species fix nitrogen symbiotically, they may alter not only soil nitrogen (N) conditions but also those of phosphorus (P). We studied the N‐fixing shrub Dichrostachys cinerea in a mesic savanna in Zambia, quantifying its effects upon pools of soil N, P, and carbon (C), and availabilities of N and P. We also evaluated whether these effects induced feedbacks upon the growth of understory vegetation and encroaching shrubs. Dichrostachys cinerea shrubs increased total N and P pools, as well as resin‐adsorbed N and soil extractable P in the top 10‐cm soil. Shrubs and understory grasses differed in their foliar N and P concentrations along gradients of increasing encroachment, suggesting that they obtained these nutrients in different ways. Thus, grasses probably obtained them mainly from the surface upper soil layers, whereas the shrubs may acquire N through symbiotic fixation and probably obtain some of their P from deeper soil layers. The storage of soil C increased significantly under D. cinerea and was apparently not limited by shortages of either N or P. We conclude that the shrub D. cinerea does not create a negative feedback loop by inducing P‐limiting conditions, probably because it can obtain P from deeper soil layers. Furthermore, C sequestration is not limited by a shortage of N, so that mesic savanna encroached by this species could represent a C sink for several decades.     

Soil phosphorus does not keep pace with soil carbon and nitrogen accumulation following woody encroachment

Soil carbon, nitrogen, and phosphorus cycles are strongly interlinked and controlled through biological processes, and the phosphorus cycle is further controlled through geochemical processes. In dryland ecosystems, woody encroachment often modifies soil carbon, nitrogen, and phosphorus stores, although it remains unknown if these three elements change proportionally in response to this vegetation change. We evaluated proportional changes and spatial patterns of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) concentrations following woody encroachment by taking spatially explicit soil cores to a depth of 1.2 m across a subtropical savanna landscape which has undergone encroachment by Prosopis glandulosa (an N₂ fixer) and other woody species during the past century in southern Texas, USA. SOC and TN were coupled with respect to increasing magnitudes and spatial patterns throughout the soil profile following woody encroachment, while TP increased slower than SOC and TN in topmost surface soils (0–5 cm) but faster in subsurface soils (15–120 cm). Spatial patterns of TP strongly resembled those of vegetation cover throughout the soil profile, but differed from those of SOC and TN, especially in subsurface soils. The encroachment of woody species dominated by N₂‐fixing trees into this P‐limited ecosystem resulted in the accumulation of proportionally less soil P compared to C and N in surface soils; however, proportionally more P accrued in deeper portions of the soil profile beneath woody patches where alkaline soil pH and high carbonate concentrations would favor precipitation of P as relatively insoluble calcium phosphates. This imbalanced relationship highlights that the relative importance of biotic vs. abiotic mechanisms controlling C and N vs. P accumulation following vegetation change may vary with depth. Our findings suggest that efforts to incorporate effects of land cover changes into coupled climate–biogeochemical models should attempt to represent C‐N‐P imbalances that may arise following vegetation change.     

Forage nutritive quality in the Serengeti ecosystem: the roles of fire and herbivory

Fire and herbivory are important determinants of nutrient availability in savanna ecosystems. Fire and herbivory effects on the nutritive quality of savanna vegetation can occur directly, independent of changes in the plant community, or indirectly, via effects on the plant community. Indirect effects can be further subdivided into those occurring because of changes in plant species composition or plant abundance (i.e., quality versus quantity). We studied relationships between fire, herbivory, rainfall, soil fertility, and leaf nitrogen (N), phosphorus (P), and sodium (Na) at 30 sites inside and outside of Serengeti National Park. Using structural equation modeling, we asked whether fire and herbivory influences were largely direct or indirect and how their signs and strengths differed within the context of natural savanna processes. Herbivory was associated with enhanced leaf N and P through changes in plant biomass and community composition. Fire was associated with reduced leaf nutrient concentrations through changes in plant community composition. Additionally, fire had direct positive effects on Na and nonlinear direct effects on P that partially mitigated the indirect negative effects. Key mechanisms by which fire reduced plant nutritive quality were through reductions of Na-rich grasses and increased abundance of Themeda triandra, which had below-average leaf nutrients.     

Patterns of CO<Subscript>2</Subscript> exchange and productivity of the herbaceous vegetation and trees in a humid savanna in western Kenya

Factors governing the dynamics between woody and herbaceous vegetation in the savanna are of ecological interest since they determine ecosystem productivity and stability. Field measurements were conducted in a humid savanna in the Lambwe valley, western Kenya, to compare CO2 exchange of the herbaceous vegetation and trees and its regulation. Soil characteristics and root distribution patterns under tree canopies and in the open locations dominated by the herbaceous vegetation were profiled in 1-m-deep soil layers. Soil water content (SWC) was measured at 30 cm depth both in the herbaceous vegetation and also under the tree canopies. The mean maximum monthly gross primary production (GPPmax) in the herbaceous vegetation was determined from chamber measurements, while daily GPP (GPPday) in both the grass and tree canopies was simulated using the PIXGRO model. The highest mean GPPmax in the herbaceous vegetation was 26.2 ± 3.7 μmol m-2 s-1 during April. Seasonal fluctuations of GPP in the herbaceous vegetation were explained by soil water availability (R ² = 0.78) within the upper 30-cm soil profile. Seasonal GPPday fluctuations were larger (between 1 gC m-2 d-1 and 10 gC m-2 d-1) in the herbaceous vegetation compared to the trees, which fluctuated around 4.3 ± 0.3 gC m-2 d-1 throughout most of the measurement period. Daily tree canopy transpiration (Ec), canopy conductance (Gc), and GPPday were decoupled from SWC in the top 30-cm soil profile. On average, ecosystem GPPday (mean of tree and herbaceous vegetation) was 14.3 ± 1.2 gC m-2 d-1 during the wet period and 6.1 ± 0.9 gC m-2 d-1 during drought. Differences between the herbaceous and tree canopy responses were attributed to soil moisture availability.     

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

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

Self‐thinning and Tree Competition in Savannas

This paper examines the feasibility of applying self‐thinning concepts to savannas and how competition with herbaceous vegetation may modify self‐thinning patterns among woody plants in these ecosystems. Competition among woody plants has seldom been invoked as a major explanation for the persistence of herbaceous vegetation in mixed tree–grass ecosystems. On the contrary, the primary resource‐based explanations for tree–grass coexistence are based on tree–grass competition (niche‐separation) that assumes that trees are inferior competitors unless deeper rooting depths provide them exclusive access to water. Alternative nonresource‐based hypotheses postulate that trees are the better competitors, but that tree populations are suppressed by mortality related to fire, herbivores, and other disturbances. If self‐thinning of woody plants can be detected in savannas, stronger evidence for resource‐limitation and competitive interactions among woody plants would suggest that the primary models of savannas need to be adjusted. We present data from savanna sites in South Africa to suggest that self‐thinning among woody plants can be detected in low‐disturbance situations, while also showing signs that juvenile trees, more so than adults, are suppressed when growing with herbaceous vegetation in these ecosystems. This finding we suggest is evidence for size‐asymmetric competition in savannas.     

Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation

In savannas, the tree-grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.     

Woody overstorey effects on soil carbon and nitrogen pools in South African savanna

Abstract Woody plant encroachment in savannas may alter carbon (C) and nitrogen (N) pools over the long‐term, which could have regional or global biogeochemical implications given the widespread encroachment observed in the vast savanna biome. Soil and litter %C and %N were surveyed across four soil types in two encroached, semi‐arid savanna landscapes in northern South Africa. Litter at sampling points with a woody component had a higher %N and lower C : N ratio than litter at solely herbaceous points. Severely encroached areas had lower C : N ratios throughout the soil profile than less encroached areas. Soil %C and %N were highly influenced by soil texture but were also influenced by the presence of a woody overstorey, which increased surface soil %C on three soil types but decreased it on the most heavily encroached soil type. Soil C sequestration may initially increase with bush encroachment but then decline if bush densities become so high as to inhibit understorey grass growth.     

Fire rather than nitrogen addition affects understory plant communities in the short term in a coniferous‐broadleaf mixed forest

Increasing fire risk and atmospheric nitrogen (N) deposition have the potential to alter plant community structure and composition, with consequent impacts on biodiversity and ecosystem functioning. This study was conducted to examine short‐term responses of understory plant community to burning and N addition in a coniferous‐broadleaved mixed forest of the subtropical‐temperate transition zone in Central China. The experiment used a pair‐nested design, with four treatments (control, burning, N addition, and burning plus N addition) and five replicates. Species richness, cover, and density of woody and herbaceous plants were monitored for 3 years after a low‐severity fire in the spring of 2014. Burning, but not N addition, significantly stimulated the cover (+15.2%, absolute change) and density (+62.8%) of woody species as well as herb richness (+1.2 species/m², absolute change), cover (+25.5%, absolute change), and density (+602.4%) across the seven sampling dates from June 2014 to October 2016. Light availability, soil temperature, and prefire community composition could be primarily responsible for the understory community recovery after the low‐severity fire. The observations suggest that light availability and soil temperature are more important than nutrients in structuring understory plant community in the mixed forest of the subtropical‐temperate transition zone in Central China. Legacy woody and herb species dominated the understory vegetation over the 3 years after fire, indicating strong resistance and resilience of forest understory plant community and biodiversity to abrupt environmental perturbation.     

Why is Abundance of Herbaceous Legumes Low in African Savanna? A Test with Two Model Species

Although fire is frequent in African savanna ecosystems and may cause considerable loss of nitrogen (N), N₂-fixing herbaceous legumes—which could be expected to benefit from low N conditions—are usually not abundant. To investigate possible reasons for this scarcity, we conducted a pot experiment using two common plants of humid African savannas as model species, the legume Cassia mimosoides and the C₄ grass Hyperthelia dissoluta. These species were grown at different levels of water, N and phosphorus (P), both in monoculture and in competition with each other. In the monocultures, yields were significantly increased by the combined addition of N and P in pots receiving high water supply. In pots with interspecific competition, the legume grew poorly unless P was added. Foliar δ¹⁵N values of legume plants grown in mixtures were considerably lower than those in monocultures, suggesting that rates of symbiotic N-fixation were higher in the presence of the grass. Grass δ¹⁵N values, however, were also lower in mixtures, while N concentrations were higher, indicating a rapid transfer of N from the legume to the grass. We conclude that the main reason for the low abundance of C. mimosoides is not low P availability as such, but a greater ability of H. dissoluta to compete for soil N and P, and a much higher N-use efficiency. If other C₄ grasses have a similar competitive advantage, it could explain why herbaceous legumes are generally sparse in African savannas. We encourage others to test these findings using species from other types of savanna vegetation.     

Woody cover in African savannas: the role of resources, fire and herbivory

Aim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas. Location Savanna grasslands of East, West and Southern Africa. Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique. Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall‐determined levels. Woody cover showed a complex, non‐linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N‐deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold. Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N‐enrichment and elevated levels of CO₂ are likely to have opposing, and potentially interacting, influences on the tree–grass balance in savannas.     

Fire and Grazing Change Herbaceous Species Composition and Reduce Beta Diversity in the Kalahari Sand System

After rainfall and soils, fire and herbivory are two of the main determinants of savanna ecosystems. Although the interactive effects of fire and herbivores on soil and vegetation are widely acknowledged few studies have addressed these two factors in concert, and none of the studies has focused on the Kalahari sand system. We experimentally studied how annual late dry season fires and grazing affect herbaceous plant species composition, above- and belowground biomass, and soil and grass nutrient concentrations in the nutrient-poor semi-arid Kalahari system in northern Botswana. Four treatments (fire, grazing, fire + grazing, and no-fire–no-grazing) were applied for two consecutive years in the late dry season. Plant species composition was affected by treatment and year. The no-fire–no-grazing treatment was distinctly different from all the other treatments in terms of species composition. Beta diversity was lower on the fire treatment and grazing treatment, but not where fire and grazing were combined. Fire and grazing alone or in combination did not have a substantial effect on biomass, soil and plant nutrients or plant species alpha diversity. Plant nitrogen was the only element that differed between treatments, with high concentrations on all the grazed treatments in the first year and low levels on the fire-alone treatment during the second year. The results show that fire and grazing mainly affect species composition and large-scale biodiversity patterns as indicated by the no-fire–no-grazing treatment being distinctly different from other treatments, suggesting the evolutionary adaptation of this dystrophic Kalahari sand system to herbivory and fire.     

Fire in sub‐Saharan Africa: The fuel,cure and connectivity hypothesis

Aim

Past analyses of satellite‐based fire activity in tropical savannas support the intermediate fire–productivity hypothesis (IFP), which posits a close correlation with estimates of total net primary productivity in drier savannas and declines towards the extremes. However, these analyses ignore the distinct roles played by herbaceous and woody vegetation in fire ignition and spread. We hypothesize that, as herbaceous vegetation provides the primary fuel, fire activity in African savannas is asymptotically correlated with herbaceous production. Conversely, woody production affects fires indirectly through effects on herbaceous production and its connectivity. In contrast to the IFP, we propose the fuel, cure and connectivity (FCC) conceptual model for tropical fire activity. The FCC model makes explicit the distinct role of herbaceous and woody fuels, avoiding the confounding interpretation of the role of total production, while providing opportunities to quantify fuel curability, effects of trees on herbaceous fuel growth and connectivity, and human management.

Location

Sub‐Saharan Africa (SSA).

Time period

2003–2015.

Major taxa studied

Woody and herbaceous vegetation.

Methods

We used boosted regression tree analysis to test competing models explaining fire activity: (a) aggregate fuel loads; and (b) partitioned woody and herbaceous fuel loads; both derived from MODIS leaf area index.

Results

Herbaceous fuel load was consistently most influential, providing more explanatory power than overall biomass in fire activity. Fuel curability rated second, then human population density (HPD), and woody biomass was least important. We observed an asymptotic relationship between herbaceous fuel load and fire activity consistent with the FCC model; trees promote fires at low densites but suppress fires at higher densities; fires were rare in wetter regions, emphasizing the need for fuel to cure; and fires were concentrated in areas of low human population, underscoring the crucial role of land management.

Conclusions

The proposed FCC framework provides a more nuanced understanding of fire activity in tropical ecosystems, where herbaceous biomass is the key determinant of fire activity.     

Nitrous oxide and methane fluxes from soils of the Orinoco savanna under different land uses

The study investigates the effect of land‐use change on nitrous oxide (N₂O) and methane (CH₄) fluxes from soil, in savanna ecosystems of the Orinoco region (Venezuela). Gas fluxes were measured by closed static chambers, in the wet and dry season, in representative systems of land management of the region: a cultivated pasture, an herbaceous savanna, a tree savanna and a woodland (control site). Higher N₂O emissions were observed in the cultivated pasture and in the herbaceous savanna compared with the tree savanna and the woodland, and differences were mainly related to fine soil particle content and soil volumetric water content measured in the studied sites. Overall N₂O emissions were quite low in all sites (0–1.58 mg N₂O‐N m^?2 day^?1). The cultivated pasture and the woodland savanna were on average weak CH₄ sinks (?0.05±0.07 and ?0.08±0.05 mg CH₄ m^?2 day^?1, respectively), whereas the herbaceous savanna and the tree savanna showed net CH₄ production (0.23±0.05 and 0.19±0.05 mg CH₄ m^?2 day^?1, respectively). Variations of CH₄ fluxes were mainly driven by variation of soil water‐filled pore space (WFPS), and a shift from net CH₄ consumption to net CH₄ production was observed at around 30% WFPS. Overall, the data suggest that conversion of woodland savanna to managed landscape could alter both CH₄ and N₂O fluxes; however, the magnitude of such variation depends on the soil characteristics and on the type of land management before conversion.     

Solutions to fire and shade: resprouting,growing tall and the origin of Eurasian temperate broadleaved forest

Tree species of Eurasian broadleaved forest possess two divergent trait syndromes with contrasting patterns of resource allocation adapted to different selection environments: short-stature basal resprouters that divert resources to a bud bank adapted to frequent and severe disturbances such as fire and herbivory, and tall trees that delay reproduction by investing in rapid height growth to escape shading. Drawing on theory developed in savanna ecosystems, we propose a conceptual framework showing that the possession of contrasting trait syndromes is essential for the persistence of broadleaved trees in an open ecosystem that burns. Consistent with this hypothesis, trees of modern Eurasian broadleaved forest bear a suite of traits that are adaptive to surface and crown-fire regimes. We contend that limited opportunities in grassland restricts recruitment to disturbance-free refugia, and en masse establishment creates a wooded environment where shade limits the growth of light-demanding savanna plants. Rapid height growth, which involves investment in structural support and the switch from a multi-stemmed to a monopodial growth form, is adaptive in this shaded environment. Although clustering reduces surface fuel loads, these establishment nuclei are vulnerable to high-intensity crown fires. The lethal effects of canopy fire are avoided by seasonal leaf shedding, and aerial resprouting enhances rapid post-fire recovery of photosynthetic capacity. While these woody formations satisfy the structural definition of forest, their constituents are clearly derived from savanna. Contrasting trait syndromes thus represent the shift from consumer to resource regulation in savanna ecosystems. Consistent with global trends, the diversification of most contemporary broadleaved taxa coincided with the spread of grasslands, a surge in fire activity and a decline in wooded ecosystems in the late Miocene–Pliocene. Recognition that Eurasian broadleaved forest has savanna origins and persists as an alternative state with adjacent grassy ecosystems has far-reaching management implications in accordance with functional rather than structural criteria. Shade is a severe constraint to the regeneration and growth of both woody and herbaceous growth forms in consumer-regulated ecosystems. However, these ecosystems are highly resilient to disturbance, an essential process that maintains diversity especially among the species-rich herbaceous component that is vulnerable to shading when consumer behaviour is altered.     

Effect of woody vegetation clearing on nutrient and carbon relations of semi-arid dystrophic savanna

Throughout the savanna biome, woody vegetation is cleared to increase productivity of herbaceous pasture. While clearing can result in increased pasture production of semi-arid dystrophic savannas in the short term, it is uncertain whether production is sustained in the long term. There is insufficient knowledge of how clearing affects soil nutrient and organic carbon (SOC) stocks. Using cleared-uncleared site pairs, we evaluated techniques for time-integrated assessment of nutrient and carbon relations in Australian savanna. Short-term in situ resin incubation showed that soil at cleared sites had a higher time-integrated availability of ammonium and nitrate, indicating that nitrogen (N) may turn over faster and/or is taken up slower at cleared sites than uncleared savanna. Nitrate and ammonium availability was approximately 2-fold higher in spring than in summer, likely due to greater uptake and/or loss of nitrate during summer rains. Nitrate was a prominent N source for evergreen trees, especially before summer rain, pointing to a role of trees as permanent N sinks. Stable isotope signatures of soil and vegetation indicate that N input occurs via N₂ fixing microbiotic crusts and Acacia species. 30 years after clearing, SOC contained more C₄ grass-derived carbon than uncleared savanna, but this shift in C source was not associated with the net C gain often observed in grasslands. Interactions between altered nutrient and C relations and composition of the understorey should be assessed in context of introduced buffelgrass (Cenchrus ciliaris) which had higher macronutrient concentrations than native grasses. Heterogeneity of the studied soils highlights the need for replication at several spatial scales to infer long-term dynamics with space-for-time chronosequences. We conclude that the techniques presented here are useful for gaining knowledge of the biogeochemical processes governing savannas and the systems that result from clearing.