Woody plant increase in grassy biomes has been widely reported over the last century. Increases have been attributed to local drivers associated with land use change, such as heavy grazing or fire suppression, or, controversially, to global drivers such as increased atmospheric carbon dioxide (CO 2 ). Here, we report a comparison of woody increase since the 1930s in three neighbouring areas with highly contrasting land use systems to help distinguish between local and global causes of woody increase. Aerial photography was used to measure changes in tree cover for three time intervals (1937, 1960, 2004) for three adjacent 25 km 2 sites which remained under radically different tenure (conservation, commercial farms, and communal rangeland) over the study period. From previous studies on drivers affecting savanna dynamics, we predicted a decrease in tree cover for the conservation and communal sites and an increase in tree cover at the commercial site. The analyses showed highly significant increases in tree cover at all sites. Total tree cover increased from 14% in 1937 to 58% in 2004 at the conservation site, 3-50% in the commercial ranching area and 6-25% in the communal farming area. Reconstruction of past land use practices showed large differences in stocking rates, herbivore species, burning practices, human population densities and natural resource harvesting between the three sites. These land use differences are reflected in differences in woody cover among the three sites in 2004. However, despite major differences in land use, tree cover also increased significantly in all three areas. This suggests global drivers favouring woody plant increase in grassy vegetation regardless of land use practises. In our study area the most likely candidates are increased CO 2 and/or atmospheric nitrogen deposition.

This study determined the effects of land‐use practice had on the rate and extent of bush encroachment in a mesic savanna in KwaZulu‐Natal, South Africa. Changes in woody cover were measured for 1 km2 sites in areas under communal, commercial and conservation land‐use systems for the period between 1937 and 2000. Land users from each area were interviewed to gain the histories of each area and to determine how the changes in woody cover had impacted them and whether anything was being done to counteract the spread of trees and shrubs on their land. Bush encroachment occurred across all three of the land‐use types in the 67‐year period between 1937 and 2000. The results showed that land‐use practice had enormous impacts on the process of bush encroachment. The communal site showed a decrease in grass (21%) and tree (5%) cover and an increase in shrub cover (13%). At the commercial site, there was a considerable decrease in grass cover (46%) and moderate increase in shrub cover (10%) and a massive increase in tree cover (36%). The area under conservation showed a substantial decrease in grass cover (47%), a slight decrease in shrub cover (19%) and a massive increase in tree cover (66%). The perceived causes of these changes were fairly similar amongst the different land users. The changes were mostly not perceived to be a problem for the communal land users. The main advantages mentioned were increased woody resources for building and firewood and increased browse availability. The commercial and conservation land users perceived the changes to have significant negative connotations including the loss of grazing land and biodiversity and secondary invasion of encroached areas by alien plant species. Despite these perceptions, very little has been done to combat bush encroachment in the commercial and conservation land use systems.

In frequently burnt savannas, saplings face the formidable challenge of both recovering from, and eventually growing tall enough to escape from, frequent fire damage. The aim of this study was to explore how saplings allocate carbon to achieve these ends through carbon partitioning, storage and remobilization. Lignotuber total non-structural carbohydrate (TNC) concentrations and d 13 C values of Acacia karroo (Fabaceae; Mimosoideae) were determined in plants from two different juvenile stages. These were one year after a fire when the plant consisted of numerous leafy shoots or coppices (''coppicing'' stage), and three years after a fire when the plant consisted of one pole-like stem (''Gulliver'' stage). Gulliver lignotubers were found to have significantly larger TNC pools (150 g vs. 97 g) and larger TNC concentrations (33% vs. 24%, w/w) than coppice lignotubers showing that post-coppice Gullivers recharged TNC in the lignotuber. d 13 C values from the stems of plants in the Gulliver stage were significantly enriched ([1%) in 13 C compared to both coppicing (P \ 0.01) and adult (P \ 0.05) plants.Changes in both the amount of stored carbon and in the d 13 C values indicated dependence on stored carbon reserves, and partially heterotrophic growth for initial resprouting. The plants appeared to use both current photosynthate and stored carbon reserves for growth of the Gulliver stem. The use of stored carbon is hypothesized to promote fast stem growth rates to a height where saplings escape fire injury.

Summary Dimensions of tree root systems in savannas are poorly understood, despite being essential in resource acquisition and post‐disturbance recovery. We studied tree rooting patterns in Southern African savannas to ask: how tree rooting strategies affected species responses to severe drought; and how potential rooting depths varied across gradients in soil texture and rainfall. First, detailed excavations of eight species in Kruger National Park suggest that the ratio of deep to shallow taproot diameters provides a reasonable proxy for potential rooting depth, facilitating extensive interspecific comparison. Detailed excavations also suggest that allocation to deep roots traded off with shallow lateral root investment, and that drought‐sensitive species rooted more shallowly than drought‐resistant ones. More broadly across 57 species in Southern Africa, potential rooting depths were phylogenetically constrained, with investment to deep roots evident among miombo Detarioids, consistent with results suggesting they green up before onset of seasonal rains. Soil substrate explained variation, with deeper roots on sandy, nutrient‐poor soils relative to clayey, nutrient‐rich ones. Although potential rooting depth decreased with increasing wet season length, mean annual rainfall had no systematic effect on rooting depth. Overall, our results suggest that rooting depth systematically structures the ecology of savanna trees. Further work examining other anatomical and physiological root traits should be a priority for understanding savanna responses to changing climate and disturbances.

The role of grazers in determining vegetation community compositions and structuring plant communities is well recognised in grassy systems. The role of browsers in affecting savanna woody plant communities is less clear. We used three long-term exclosures in the Kruger National Park to determine the effect of browsers on species compositions and population structures of woody communities. Species assemblages, plant traits relating to browsing and soil nutrients were compared inside and outside of the exclosures. Our results showed that browsers directly impact plant species distributions, densities and population structures by actively selecting for species with traits which make them desirable to browsers. Species with high leaf nitrogen, low total phenolic content and low acid detergent lignin appeared to be favoured by herbivores and therefore tend to be rare outside of the exclosures. This study also suggested that browsers have important indirect effects on savanna functioning, as the reduction of woody cover can result in less litter of lower quality, which in turn can result in lower soil fertility. However, the magnitude of browser effects appeared to depend on inherent soil fertility and climate.Conservation implications: Browsers were shown to have significant impacts on plant communities. They have noticeable effects on local species diversity and population structure, as well as soil nutrients. These impacts are shown to be related to the underlying geology and climate. The effects of browsers on woody communities were shown to be greater in low rainfall, fertile areas compared to high rainfall, infertile soils.

Southern African savannas are commonly polarised into two broad types based on plant functional types and defences; infertile savannas dominated by broad-leaved trees typically defended by nitrogen-free secondary compounds and fertile savannas dominated by fine-leaved trees defended by structural defences. In this study, we use trait and other data from 15 wooded savanna sites in Southern Africa and ask if broad-leaved and fine-leaved species dominate on nutrient-poor and nutrient-rich soils, respectively. We then test if there is there any evidence for trade-offs in chemical (i.e., condensed tannins and total polyphenols) vs. structural defences on different soil types. We did not find strong evidence for a general divide in fine- vs. broad-leaved savannas according to soil fertility, nor for a simple trade-off between chemical and structural defences. Instead, we found savanna species to cluster into three broad defence strategies: species were high in leaf N and either (A) highly defended by spines and chemicals or (B) only structurally defended, or (C) low in leaf N and chemically defended. Finally, we tested for differences in browser utilisation between soil types and among plant defence strategies and found that browsing by meso-herbivores was higher on nutrient-rich soils and targeted species from groups A and B and avoided C, while browsing by elephants was mostly not affected by soil type or defence strategy. We propose a framework that can be used as a basis for asking strategic questions that will help improve our understanding of plant defences in savannas.

1. Establishing trade-offs among traits and the degree to which they covary along environmental gradients has become a key focal point in the effort to develop community ecology into a predictive science. While there is evidence for these relationships across global data sets, they are often too broad in scale and do not consider the particularities of local to regional species pools. This decreases their usefulness for developing predictive models at scales relevant for conservation and management. 2. We tested for trade-offs between traits and relationships with environmental gradients in trees and shrubs sampled across southern African savannas and explored evidence for acquisitive versus conservative resource-use strategies using a phylogenetically explicit approach. 3. We found a distinct trade-off between two major poles of specialization indicative of acquisitive (high leaf nitrogen concentration, leaf phosphorus concentration, leaf N:P, specific leaf area and average leaf area) and conservative resource-use strategies (high leaf carbon to nitrogen ratios (C:N), tensile strength and leaf dry matter content). Although we found that trait variance and species occurrence were constrained by phylogeny, phylogenetically informed analyses did not contradict non-phylogenetic analyses, strengthening relationships in most cases. 4. The high intrasite trait variability and weak relationships with soils and climate may in part be explained by the high levels of deciduousness and disturbance (i.e. fire and herbivory) inherent in African savannas. 5. Synthesis. The relationships between traits and between traits and environmental gradients were far weaker than, and often contradictory to, broad-scale studies that compare these relationships across biomes and growth forms, cautioning against making generalizations about relationships at specific sites based on broad-scale analyses.The effort to develop community ecology into a predictive science over the last decade has seen a strong focus on Supporting InformationAdditional Supporting Information may be found in the online version of this article: Appendix S1. Calculation and interpretation of lambda (k), kappa (j) and delta (d) estimates.

2020. Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna.Abstract. Although studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remain unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semiarid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C 3 vs. C 4 ) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8 m 2 /ha), and woody canopy cover (31%), translating to ã 8.5 t/ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5 t/ha) in total soil carbon to 30-cm depth, with~71% of this derived from C 4 grasses (vs.~76% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C 4 grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g., Acacia spp.) on grass cover and productivity in this semiarid system.