Understanding the effects of changing climate and land use on woody plant encroachment in South African grasslands and savannas.

Abstract

The increase in dominance of woody species, observed in many grasslands and savannas worldwide, highlights the sensitivity of woody herbaceous biomass ratios to changed environmental conditions. While the major factors driving the tree–grass continuum have been identified as precipitation, nutrients, increased CO2 concentrations, fire and herbivory, the interaction of these driving forces determines the tree: grass ratio, and ultimately the occurrence/ absence of woody plant encroachment. Furthermore, with forecasted alterations to the global climate (such as an increase in the frequency and magnitude of climate extremes), and increased nutrient availability (either through extensive use of fertilizer or increased nitrogen deposition), more knowledge is required on the factors which promote woody plant performance. We conducted a series of greenhouse experiments to manipulate these factors, and a two-year field trial to disentangle the effects of varying climate and land use on woody plant encroachment. These studies are summarized below: (1) We investigated the effects of manipulated water availability and passive warming on the productivity of the commonly occurring grass species Eragrostis curvula, and the emergence and performance of seedlings of the woody encroacher, Acacia sieberiana. We simulated a typical savanna microhabitat by planting tree species within a grass matrix. Watering treatments altered to represent drought (minus 40% of the average), normal (same amount as the long-term average of the area), and excess (addition of 40% of the long term average) precipitation. Passive warming was achieved using open-top chambers. Tree seedling emergence was monitored in the first two weeks. Thereafter, weekly tree growth rate measurements were recorded. The final above-ground grass biomass, tree root: shoot ratio and specific leaf area were quantified. We found that water availability was the main limiting factor for grass productivity, with low grass biomass attained under the drought treatments. Warming had no significant effect on grass productivity, but had a large positive effect on tree growth and performance when coupled with excess or normal water treatments. Tree seedling root:shoot ratios were highest in the drought treatment, possibly as an adaptation to low water supply. Higher specific leaf area was recorded with warming, further elucidating the positive effect of increased temperature for tree growth. (2) Using a completely randomized experimental design in the greenhouse, we investigated the effects of fertilizer, fire and grazing on the survival, growth and biomass of Acacia sieberiana. We simulated a typical savanna microhabitat by planting a common savanna grass species, Eragrostis curvula, together with saplings of A. sieberiana. Treatments of fertilizer, simulated grazing and fire were applied. Weekly measurements of relative growth rate (RGRheight and RGRdiameter) were recorded. At the end of 24 weeks, survival, total above- and below-ground biomass, and key functional traits (specific leaf area and average thorn length) were recorded to assess differences in tree performance. We found that fertilization was beneficial to grasses only, increasing their total biomass. Consequently, we observed increased tree sapling mortality with high grass biomass. Fire increased the saplings’ investment in shoot growth and stem diameter. However, we found no evidence of differential post-fire allocation to roots or shoots when grass competition was absent. Grass biomass was also found to decrease the specific leaf area of tree saplings. (3) We conducted a two-year field fertilizer trial, using pairs of open grassland and encroached plots at four sites across a precipitation gradient, ranging from 300–1500 mm mean annual precipitation (MAP). We assessed the effects of encroachment, N, P and N+P addition on soil N stocks and soil P, litter % N and C:N ratio (index of decomposition), plant productivity and species richness. We found that soil N stocks were on average 15 times higher in the higher precipitation sites regardless of nutrient addition or encroachment. Larger variation in soil P was noted at the high precipitation sites, with some evidence of P-limitation in the encroached area with the highest precipitation. We found significant positive effects of encroachment on litter % N (quality), C:N ratios and forb cover across the precipitation gradient. Fertilization increased grass biomass and reduced species richness in the high precipitation sites only. The purportedly beneficial effects of encroachment on litter and plant species richness was more pronounced at the high-precipitation sites. Overall, increased nutrient availability was found to be of importance in high precipitation sites only, possibly due to water scarcity at the low-precipitation sites. (4) To elucidate the effects of fertilizer addition, simulated fire and grazing on the rate of nitrogen (N) fixation and soil respiration we conducted a greenhouse experiment using a woody encroaching species, Acacia sieberiana, and the commonly occurring grass species Eragrostis curvula. Treatments of fertilizer, simulated grazing and fire were applied. Thereafter the Acetylene Reduction Assay was used to determine the rate of biological Nitrogen fixation (BNF). We found a significant decrease in BNF with fertilizer addition, and increases in BNF after fire application. Soil respiration increased with fertilizer addition and decreased after fire application. Grazing had no independent effect on any of the response variables. However, decreased grass biomass resulted in increased BNF across all treatments. Furthermore we found that larger saplings achieved a higher rate of BNF, with a positive correlation between the rate of BNF and both the number and weight of root nodules. The implications of these studies are: (1) Future climate-change predictions of increased drought may constrain grass biomass, thereby promoting woody plant success. Predicted warming is likely to further enhance woody plant performance. (2) Increased nutrient availability, whether as fertilizer addition or increased nitrogen deposition, may promote the competitive ability of the grass component, thereby limiting woody plant invasion. This is based on the assumption that grass productivity is not limited by frequent fire or intensive grazing. (3) Encroachment of leguminous woody plants is purportedly beneficial in higher precipitation areas, due to increased soil nitrogen, higher grass foliar quality under trees, greater forb cover and increased decomposition. Hence, encroachment control should be prioritized in low precipitation sites, particularly in rangelands where woody plants are undesirable. (4) Low intensity or infrequent fire is ineffective at controlling woody plant proliferation, particularly if the invader is able to resprout and is capable of biological nitrogen fixation.