Local range boundaries vs. large‐scale trade‐offs: climatic and competitive constraints on tree growth

Species often respond to human‐caused climate change by shifting where they occur on the landscape. To anticipate these shifts, we need to understand the forces that determine where species currently occur. We tested whether a long‐hypothesised trade‐off between climate and competitive constraints explains where tree species grow on mountain slopes. Using tree rings, we reconstructed growth sensitivity to climate and competition in range centre and range margin tree populations in three climatically distinct regions. We found that climate often constrains growth at environmentally harsh elevational range boundaries, and that climatic and competitive constraints trade‐off at large spatial scales. However, there was less evidence that competition consistently constrained growth at benign elevational range boundaries; thus, local‐scale climate‐competition trade‐offs were infrequent. Our work underscores the difficulty of predicting local‐scale range dynamics, but suggests that the constraints on tree performance at a large‐scale (e.g. latitudinal) may be predicted from ecological theory.     

The Enemy of My Enemy Hypothesis: Why Coexisting with Grasses May Be an Adaptive Strategy for Savanna Trees

Savannas are characterized by the coexistence of trees and flammable grasses. Yet, tree–grass coexistence has been labeled as paradoxical—how do these two functional groups coexist over such an extensive area, despite being generally predisposed to excluding each other? For instance, many trees develop dense canopies that limit grass growth, and many grasses facilitate frequent/intense fires, increasing tree mortality. This study revisits tree–grass coexistence with a model of hierarchical competition between pyrogenic grasses, “forest trees” adapted to closed-canopy competition, and “savanna trees” that are inferior competitors in closed-canopy communities, but more resistant to fire. The assumptions of this model are supported by empirical observations, including a systematic review of savanna and forest tree community composition reported here. In general, the model simulations show that when savanna trees exert weaker competitive effects on grasses, a self-reinforcing grass community is maintained, which limits forest tree expansion while still allowing savanna trees to persist (albeit as a subdominant to grasses). When savanna trees exert strong competitive effects on grasses, savanna trees cover increases initially, but as grasses decline their inhibitory effect on forest trees weakens, allowing forest trees to expand and exclude grasses and savanna trees. Rather than paradoxical, these results suggest that having weaker competitive effects on grasses may be advantageous for savanna trees, leading to greater long-term abundance and stability. We label this the “enemy of my enemy hypothesis,” which might apply to species coexistence in communities defined by hierarchical competition or with species capable of generating strong ecological feedbacks.     

Savanna tree–grass competition is modified by substrate type and herbivory

Question: Woody plant and grass interactions in savannas have frequently been studied from the perspective of the response of one growth form on the other but seldom evaluated as two‐way interactions. What causes woody plant encroachment in semi‐arid savannas and what are the competitive responses of tree seedlings and grasses on rocky and sandy substrates? Methods: In this greenhouse study, we investigated the influence of substrate and grazing on responses to interspecific competition by tree seedlings and grasses. We measured competitive/facilitative responses on biomass and nutrient status of tree seedlings and grasses grown together. Results: Interspecific competition suppressed growth of trees and grasses. Tree seedlings and uncut grass accumulated double the biomass when grown without competition relative to when they competed. Competitive responses varied on different substrates. Grass biomass on rocky substrate showed no response to tree competition, but appeared to be facilitated by trees on sandy substrate. Grass clipping resulted in higher tree seedling biomass on rocky substrate, but not on sandy substrate. There was a positive response of grass nutrient status to competition from tree seedlings. Conclusion: Selective grass herbivory in the absence of browsing or suppression of shade‐intolerant grasses by trees are commonly cited reasons behind bush encroachment in savannas. We show that grazing may confer a competitive advantage to tree seedlings and promote bush encroachment more readily on rocky substrates. This may be due to the imposed sharing of the soil depth niche on rocky substrates, whereas possible niche separation on sandy substrates minimizes the advantage conferred by reduced competition.     

Disentangling the relative importance of climate,size and competition on tree growth in Iberian forests: implications for forest management under global change

Most large‐scale multispecies studies of tree growth have been conducted in tropical and cool temperate forests, whereas Mediterranean water‐limited ecosystems have received much less attention. This limits our understanding of how growth of coexisting tree species varies along environmental gradients in these forests, and the implications for species interactions and community assembly under current and future climatic conditions. Here, we quantify the absolute effect and relative importance of climate, tree size and competition as determinants of tree growth patterns in Iberian forests, and explore interspecific differences in the two components of competitive ability (competitive response and effect) along climatic and size gradients. Spatially explicit neighborhood models were developed to predict tree growth for the 15 most abundant Iberian tree species using permanent‐plot data from the Spanish Second and Third National Forest Inventory (IFN). Our neighborhood analyses showed a climatic and size effect on tree growth, but also revealed that competition from neighbors has a comparatively much larger impact on growth in Iberian forests. Moreover, the sensitivity to competition (i.e. competitive response) of target trees varied markedly along climatic gradients causing significant rank reversals in species performance, particularly under xeric conditions. We also found compelling evidence for strong species‐specific competitive effects in these forests. Altogether, these results constitute critical new information which not only furthers our understanding of important theoretical questions about the assembly of Mediterranean forests, but will also be of help in developing new guidelines for adapting forests in this climatic boundary to global change. If we consider the climatic gradients of this study as a surrogate for future climatic conditions, then we should expect absolute growth rates to decrease and sensitivity to competition to increase in most forests of the Iberian Peninsula (in all but the northern Atlantic forests), making these management considerations even more important in the future.     

EVOLUTION OF THE STORAGE EFFECT

The storage effect, a mechanism that promotes species coexistence in temporally variable environments, poses a dilemma to evolutionary ecologists. Ecological studies have demonstrated its importance in natural communities, but evolutionary models have predicted that selection either impedes coexistence or diminishes the storage effect if there is coexistence. Here, we develop a lottery model of competition in which two species experience a trade‐off in competitive ability between two types of years. We use an adaptive evolution framework to determine conditions favoring the evolution of the storage effect. Storage evolves via divergence of relative performance in the two environments under a wide range of biologically realistic conditions. It evolves between two initially identical species (or lineages) when the trade‐off in performance is strong enough. It evolves for species having different initial trade‐offs for both weak and strong trade‐offs. Our simple 2‐species‐2‐environment scenario can be extended to multiple species and environmental conditions. Results indicate that the storage effect should evolve in a broad range of situations that involve a trade‐off in competitive ability among years, and are consistent with empirical observations. The findings show that storage can evolve in a manner and under conditions similar to other types of resource partitioning.     

Seasonality and facilitation drive tree establishment in a semi-arid floodplain savanna

A popular hypothesis for tree and grass coexistence in savannas is that tree seedlings are limited by competition from grasses. However, competition may be important in favourable climatic conditions when abiotic stress is low, whereas facilitation may be more important under stressful conditions. Seasonal and inter-annual fluctuations in abiotic conditions may alter the outcome of tree–grass interactions in savanna systems and contribute to coexistence. We investigated interactions between coolibah (Eucalyptus coolabah) tree seedlings and perennial C₄ grasses in semi-arid savannas in eastern Australia in contrasting seasonal conditions. In glasshouse and field experiments, we measured survival and growth of tree seedlings with different densities of C₄ grasses across seasons. In warm glasshouse conditions, where water was not limiting, competition from grasses reduced tree seedling growth but did not affect tree survival. In the field, all tree seedlings died in hot dry summer conditions irrespective of grass or shade cover, whereas in winter, facilitation from grasses significantly increased tree seedling survival by ameliorating heat stress and protecting seedlings from herbivory. We demonstrated that interactions between tree seedlings and perennial grasses vary seasonally, and timing of tree germination may determine the importance of facilitation or competition in structuring savanna vegetation because of fluctuations in abiotic stress. Our finding that trees can grow and survive in a dense C₄ grass sward contrasts with the common perception that grass competition limits woody plant recruitment in savannas.     

Trait‐abundance relation in response to nutrient addition in a Tibetan alpine meadow: The importance of species trade‐off in resource conservation and acquisition

In competition‐dominated communities, traits promoting resource conservation and competitive ability are expected to have an important influence on species relative abundance (SRA). Yet, few studies have tested the trait‐abundance relations in the line of species trade‐off in resource conservation versus acquisition, indicating by multiple traits coordination. We measured SRA and key functional traits involving leaf economic spectrum (SLA, specific leaf area; LDMC, leaf dry matter content; LCC, leaf carbon concentration; LNC, leaf nitrogen concentration; LPC, leaf phosphorus concentration; Hs, mature height) for ten common species in all plots subjected to addition of nitrogen fertilizer (N), phosphorus fertilizer (P), or both of them (NP) in a Tibetan alpine meadow. We test whether SRA is positively related with traits promoting plant resource conservation, while negatively correlated with traits promoting plant growth and resource acquisition. We found that species were primarily differentiated along a trade‐off axis involving traits promoting nutrient acquisition and fast growth (e.g., LPC and SLA) versus traits promoting resource conservation and competition ability (e.g., large LDMC). We further found that SRA was positively correlated with plant height, LDMC, and LCC, but negatively associated with SLA and leaf nutrient concentration irrespective of fertilization. A stronger positive height‐SRA was found in NP‐fertilized plots than in other plots, while negative correlations between SRA and SLA and LPC were found in N or P fertilized plots. The results indicate that species trade‐off in nutrient acquisition and resource conservation was a key driver of SRA in competition‐dominated communities following fertilization, with the linkage between SRA and traits depending on plant competition for specific soil nutrient and/or light availability. The results highlight the importance of competitive exclusion in plant community assembly following fertilization and suggest that abundant species in local communities become dominated at expense of growth while infrequent species hold an advantage in fast growth and dispersals to neighbor meta‐communities.     

Variable rainfall has a greater effect than fire on the demography of the dominant tree in a semi‐arid Eucalyptus savanna

Rainfall, fire and competition are emphasized as determinants of the density and basal area of woody vegetation in savanna. The semi‐arid savannas of Australia have substantial multi‐year rainfall deficits and insufficient grass fuel to carry annual fire in contrast to the mesic savannas in more northern regions. This study investigates the influence of rainfall deficit and excess, fire and woody competition on the population dynamics of a dominant tree in a semi‐arid savanna. All individuals of Eucalyptus melanophloia were mapped and monitored in three, 1‐ha plots over an 8.5 year period encompassing wet and dry periods. The plots were unburnt, burnt once and burnt twice. A competition index incorporating the size and distance of neighbours to target individuals was determined. Supplementary studies examined seedling recruitment and the transition of juvenile trees into the sapling layer. Mortality of burnt seedlings was related to lignotuber area but the majority of seedlings are fire resistant within 12 months of germination. Most of the juveniles (≤1 cm dbh) of E. melanophloia either died in the dry period or persisted as juveniles throughout 8.5 years of monitoring. Mortality of juveniles was positively related to woody competition and was higher in the dry period than the wet period. The transition of juveniles to a larger size class occurred at extremely low rates, and a subsidiary study along a clearing boundary suggests release from woody competition allows transition into the sapling layer. From three fires the highest proportion of saplings (1–10 cm dbh) reduced to juveniles was only 5.6% suggesting rates of ‘top‐kill’ of E. melanophloia as a result of fire are relatively low. Girth growth was enhanced in wet years, particularly for larger trees (>10 cm dbh), but all trees regardless of size or woody competition levels are vulnerable to drought‐induced mortality. Overall the results suggest that variations in rainfall, especially drought‐induced mortality, have a much stronger influence on the tree demographics of E. melanophloia in a semi‐arid savanna of north‐eastern Australia than fire.     

Grazing and neighborhood interactions limit woody encroachment in wet subtropical savannas

Woody encroachment in savannas is a worldwide concern, and there is growing consensus that anthropogenic activities play a central role in changing tree – grass interactions. We evaluated the influence of livestock grazing and neighborhood interactions on seedling emergence and survival of the native tree Acacia caven in wet savannas of northeastern Argentina. We hypothesized that grazing and grass competition act as biotic barriers limiting tree recruitment, but the relative magnitude of such barriers differs according to grass patch type. In two consecutive years (cohort 1 and 2) we sowed seeds and transplanted seedlings of Acacia in two grass patch types (prostrate/palatable and tussock/unpalatable grasses) in both, grazed and ungrazed plots. Each grass patch type was further manipulated to create three levels of grass competition (unclipped control, above-ground biomass removal and total biomass removal).Cattle grazing diminished seedling emergence of both cohorts and seedling survival of cohort 1. The effect of grass competition changed according to grass patch type. Prostrate grass cover enhanced emergence but lowered early survival, while tussock grass cover and also its total biomass removal facilitated early survival. During the second year, a severe drought drastically reduced Acacia recruitment, and it was strong enough to eliminate any grazing effects although the effect of grass competition on seedling establishment remained significant.Our results suggest that grazing and grass competition additively diminished the risk of woody establishment in this wet savanna. However, the stocking rate should be carefully balanced, thus contributing to the maintenance of a competitive grass cover to limit tree recruitment.     

Life‐history constraints in grassland plant species: a growth‐defence trade‐off is the norm

Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade‐off possibilities. The competition‐defence hypothesis posits a trade‐off between competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth‐defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth‐defence trade‐off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life‐history groups and within the majority of individual sites. Thus, a growth‐defence trade‐off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint.     

The competition–dispersal trade‐off exists in forbs but not in graminoids: A case study from multispecies alpine grassland communities

Much theoretical evidence has demonstrated that a trade‐off between competitive and dispersal ability plays an important role in facilitating species coexistence. However, experimental evidence from natural communities is still rare. Here, we tested the competition–dispersal trade‐off hypothesis in an alpine grassland in the Tianshan Mountains, Xinjiang, China, by quantifying competitive and dispersal ability using a combination of 4 plant traits (seed mass, ramet mass, height, and dispersal mode). Our results show that the competition–dispersal trade‐off exists in the alpine grassland community and that this pattern was primarily demonstrated by forbs. The results suggest that most forb species are constrained to be either good competitors or good dispersers but not both, while there was no significant trade‐off between competitive and dispersal ability for most graminoids. This might occur because graminoids undergo clonal reproduction, which allows them to find more benign microenvironments, forage for nutrients across a large area and store resources in clonal structures, and they are thus not strictly limited by the particular resources at our study site. To the best of our knowledge, this is the first time the CD trade‐off has been tested for plants across the whole life cycle in a natural multispecies plant community, and more comprehensive studies are still needed to explore the underlying mechanisms and the linkage between the CD trade‐off and community composition.     

Termites facilitate and ungulates limit savanna tree regeneration

Both large herbivores and termites are key functional groups in savanna ecosystems, and in many savanna areas, large termite mounds (termitaria) are associated with distinct woody clusters. Studies on the effect of large mammals on tree regeneration are few, and the results are conflicting. Large herbivores have been found to be important seedling predators in some areas, but facilitate tree regeneration by outcompeting small mammals and reducing grass cover in other areas. Through the use of the experimental fencing of termite mounds and adjacent savanna areas in this study, we investigated how termites and large herbivores influence tree regeneration. Termite mounds had a higher number of seedlings, more species richness, more alpha diversity (OD) and lower evenness (E) than savanna plots. Large herbivores did not significantly affect overall seedling density, species richness, OD or E. Beta diversity was higher in savanna areas than on termitaria, and beta diversity decreased in savanna areas when herbivores were excluded. Herbivore exclusion increased the density of the 12 (40 %) most common seedling species, representing 79 % of all seedlings, and fenced plots had relatively taller seedlings than open plots. Thus, termites were the main determinants of tree regeneration in our study area, but large mammals regulated the most common species. Although our study confirms previous work suggesting that large herbivores affect tree regeneration, we found that termites were an even more important determinant. Termite impacts on tree regeneration deserve increased attention by savanna ecologists.     

Spatial pattern of dry rainforest colonizing unburnt Eucalyptus savanna

Abstract The spatial pattern of dry rainforest and savanna tree species was analysed in a 1.56‐ha plot within an unburnt eucalypt savanna woodland in north Queensland, Australia. Rainforest colonization constituted only 1.3% of the basal area and mostly consisted of individuals less than 3 m high. The distribution of rainforest trees was highly clumped around the large savanna eucalypt trees. Ecological mechanisms generating the clumped distribution are discussed in light of evidence from this study and the literature. Herbaceous biomass was not reduced under trees, suggesting that relief from grass competition has not favoured rainforest colonization under tree crowns. Edaphic facilitation through nutrient enrichment under savanna tree crowns appears to be only minor on the moderate fertility soils of the area. The highly clumped pattern of colonizing dry rainforest may be a consequence of seeds dropped from birds roosting in savanna trees.     

Tolerance to apical and leaf damage of Raphanus raphanistrum in different competitive regimes

Tolerance to herbivory is an adaptation that promotes regrowth and maintains fitness in plants after herbivore damage. Here, we hypothesized that the effect of competition on tolerance can be different for different genotypes within a species and we tested how tolerance is affected by competitive regime and damage type. We inflicted apical or leaf damage in siblings of 29 families of an annual plant Raphanus raphanistrum (Brassicaceae) grown at high or low competition. There was a negative correlation of family tolerance levels between competition treatments: plant families with high tolerance to apical damage in the low competition treatment had low tolerance to apical damage in the high competition treatment and vice versa. We found no costs of tolerance, in terms of a trade‐off between tolerance to apical and leaf damage or between tolerance and competitive ability, or an allocation cost in terms of reduced fitness of highly tolerant families in the undamaged state. High tolerance bound to a specific competitive regime may entail a cost in terms of low tolerance if competitive regime changes. This could act as a factor maintaining genetic variation for tolerance.     

An idealized model for tree–grass coexistence in savannas: the role of life stage structure and fire disturbances

1.  We discuss a simple implicit-space model for the competition of trees and grasses in an idealized savanna environment. The model represents patch occupancy dynamics within the habitat and introduces life stage structure in the tree population, namely adults and seedlings. A tree can be out-competed by grasses only as long as it is a seedling.
2.  The model is able to predict grassland, forest, savanna and bistability between forest and grassland, depending on the different characteristics of the ecosystem, represented by the model's parameters.
3.  The inclusion of stochastic fire disturbances significantly widens the parameter range where coexistence of trees and grasses is found. At the same time, grass-fire feedback can induce bistability between forest and grassland.
4.   Synthesis . These results suggest that tree–grass coexistence in savannas can be either deterministically stable or stabilized by random disturbances, depending on prevailing environmental conditions and on the types of plant species present in the ecosystem.     

Interspecific effects on plant size inequality: evidence from a temperate savanna community

The influences of intraspecific competition on plant size inequality have been well documented, but interspecific effects on this topic remain little understood. Here we examined the effects of canopy shading and fine roots of the trees (Elaeagnus angustifolia) on size inequality of the grasses (Achnatherum splendens) in a temperate savanna community in northwest China. Three study plots of 400 m² were divided into 4-m² quadrats, within each of which (1) canopy shading was quantified by modeling cumulative direct solar radiation (CDSR) and (2) the root effect was quantified using an empirical relationship between tree fine root density (TFRD) and relative distance to tree bases. Morphological traits were measured to represent grass size. Redundancy analysis (RDA) was conducted to examine the relative influences of grass density, CDSR and TFRD on the coefficient of variation of grass size. Results showed that no significant correlation occurred between grass density and grass size inequality. Both CDSR and TFRD had significant negative correlations with grass size inequality, suggesting that canopy shading and the presence of fine roots of trees can, respectively, increase and reduce grass size inequality. Canopy shading and TFRD played competitive roles in determining grass size inequality, where the root effect was a stronger factor than canopy shading. The tree effects can substantially alter the intensity of water stress. In response, size inequality of the grasses could be influenced through size-specific growth/mortality and slowed size divergence. These mechanisms could operate together in the savanna community.     

Effects of increased N and P availability on biomass allocation and root carbohydrate reserves differ between N‐fixing and non‐N‐fixing savanna tree seedlings

In mixed tree‐grass ecosystems, tree recruitment is limited by demographic bottlenecks to seedling establishment arising from inter‐ and intra‐life‐form competition, and disturbances such as fire. Enhanced nutrient availability resulting from anthropogenic nitrogen (N) and phosphorus (P) deposition can alter the nature of these bottlenecks by changing seedling growth and biomass allocation patterns, and lead to longer‐term shifts in tree community composition if different plant functional groups respond differently to increased nutrient availability. However, the extent to which tree functional types characteristic of savannas differ in their responses to increased N and P availability remains unclear. We quantified differences in above‐ and belowground biomass, and root carbohydrate contents in seedlings of multiple N‐fixing and non‐N‐fixing tree species characteristic of Indian savanna and dry forest ecosystems in response to experimental N and P additions. These parameters are known to influence the ability of plants to compete, as well as survive and recover from fires. N‐fixers in our study were co‐limited by N and P availability, while non‐N‐fixers were N limited. Although both functional groups increased biomass production following fertilization, non‐N‐fixers were more responsive and showed greater relative increases in biomass with fertilization than N‐fixers. N‐fixers had greater baseline investment in belowground resources and root carbohydrate stocks, and while fertilization reduced root:shoot ratios in both functional groups, root carbohydrate content only reduced with fertilization in non‐N‐fixers. Our results indicate that, even within a given system, plants belonging to different functional groups can be limited by, and respond differentially to, different nutrients, suggesting that long‐term consequences of nutrient deposition are likely to vary across savannas contingent on the relative amounts of N and P being deposited in sites.     

Trait–environment relations for dominant grasses in South African mesic grassland support a general leaf economic model

Question: Following the framework of Suding et al. (2003), we examined whether morphological traits (organismal response), tolerance and competitive effect (specific process response) were associated with grass dominance (abundance response) on burning, mowing, fertilization and soil depth gradients in KwaZulu‐Natal (KZN), South Africa. Location: University of KwaZulu‐Natal, Pietermaritzburg, South Africa. Methods: Using several pot experiments involving 29 grass species in total, we determined the vegetative traits, competitive effect and response, and tolerance to shading for grasses common in closed, tufted mesic grassland in KZN. Results: The primary axis of grass–trait variation was most strongly related to a negative correlation (trade‐off) between growth rate and specific leaf area (SLA), with broad‐leaved, rapidly‐growing grasses (high SLA) occupying one extreme and narrow‐leaved, slow‐growing grasses (low SLA) the other extreme of the first principal component. The low SLA, slow‐growth strategy was found to be a relatively general strategy among grasses dominant in undisturbed, high litter grassland, as well as those adapted to moisture‐stressed habitats. In contrast, grasses dominant in highly productive habitats with some form of disturbance, e.g. mowing, had a broad‐leaved, rapid‐growth strategy. Intermediate combinations of the SLA–growth rate trade‐off were common among grasses dominant under other combinations of disturbance and soil resource availability. Conclusions: Distinct patterns of organismal (SLA, growth rate) and specific process (competitive effect and response, as well as tolerance of shading) responses appeared to be associated with grasses dominant on gradients of burning, mowing, fertilization and soil depth. These organismal and specific process responses were similar to those for North American and European grasses dominant under the same environmental influences, suggesting that some general trait–environment patterns exist at an inter‐continental scale. This general trait–environment relationship appears to be driven by functional adaptive selection along the SLA–environment continuum and its unavoidable trade‐off with growth rate.     

Herbivory effects on saplings are influenced by nutrients and grass competition in a humid South African savanna

Woody plant encroachment is a common consequence of disturbance in savannas. Grazers and browsers interfere with sapling establishment dynamics by direct consumption of plant tissue, changing soil nutrient status (through fertilization and trampling) and grass competition. Studies evaluating the effects of herbivory on sapling establishment have mostly been extrapolated from single species. In a controlled field experiment, we studied the effects of clipping (simulating grazing and browsing), nutrients, grass competition, and their interactive effects on sapling survival and growth of four dominant humid and four dominant mesic savanna species. We conducted this experiment in a humid South African savanna. We found no effects on sapling survival by the treatments provided. However, clipped saplings of all species increased their investment in relative growth rate of stem length (RGR_L). Clipping had a greater negative impact on relative growth rate of more humid than mesic species in terms of stem diameter (RGR_D), total dry biomass and proportion of leaf biomass. Nutrients had a positive effect on the RGR_L and sapling biomass of three mesic species. Positive effects of nutrients on RGR_L of one humid and two mesic species were observed in their clipped saplings only. Grass competition had a strong negative impact on all growth parameters measured. Clipped saplings of one humid and two mesic species had lower RGR_L with grass competition whereas intact saplings showed no significant response. After clipping, humid savanna species were more vulnerable to grass competition than mesic species, with reduced ability to use nutrients. In conclusion, herbivory increases sapling vulnerability to grass competition, with humid species being more susceptible than mesic species, indicating that woody-plant control strategies are more likely to be effective in humid savannas.     

Effects of moisture,nitrogen, grass competition and simulated browsing on the survival and growth of Acacia karroo seedlings

The effects of irrigation, nitrogen fertilization, grass competition and clipping were investigated for one growing season at the research farm of the University of Fort Hare in the Eastern Cape Province of South Africa. The aim of the experiment was to assess the short‐term performance of Acacia karroo seedlings under different environmental conditions and the implications of such factors on the long‐term recruitment of plant species in savanna rangelands. There were no significant treatment effects on the survival of A. karroo seedlings. Using stem length and basal diameter as growth parameters, it was observed that irrigation enhanced both variables, while nitrogen fertilization did not have any significant effects. Clipping, grass competition and their interaction greatly suppressed the growth of the seedlings. Clipping increased the mean stem length when they were irrigated and fertilized. Control and fertilized plants had the highest stem length in the absence of grass competition, while grass competition combined with clipping resulted in the lowest stem length in both irrigated and nonirrigated plants. It was concluded that in the presence of grass competition, controlled browsing could be a viable solution to the problem of bush encroachment in savanna rangelands.