Surviving and growing amidst others : the effect of environmental factors on germination and establishment of savanna trees

Savanna ecosystems are characterized by a continuous grass layer intermixed with a discontinuous layer of trees and shrubs. A complex set of environmental drivers, such as water, soil nutrients, solar radiance, fire and herbivory, determines vegetation structure and composition in savannas.Such environmental drivers are expected to be strongly affected by future global climatic and land-use changes, potentially modifying savanna vegetation, and consequently savanna fauna. The ability to predict changes in plant community composition is therefore importantfor management and conservation of savannas. However, the mechanisms controlling plant establishment and growth in savannas are still unclear. Germination and seedling establishment are critical recruitment stages in the life cycle of plants and can influence plant community composition. A better understand of the factors influencing plant species recruitment and their ecology is needed. This thesis focuses on seedling recruitment of several savanna tree species. Water stress is probably the single greatest constraint to tree seedling survival in savanna systems: tree seedling recruitment and survival are hypothesized to be limited by soil moisture availability. Shade by established adult trees may facilitate tree seedling recruitment by maintaining high soil moisture availability. Chapter 2 deals with germination and early seedling establishment of several tree species. I expected that tree species would germinate and establish best under high moisture conditions (high water and shade), while under stress conditions (i.e. low soil moisture due to low water supply and full sun, and in the presence of grasses) plants would suffer. The observed variability of seedling performance among the tree species under stress conditions may be explained by differences in functional traits. Higher soil moisture mostly benefited germination of species with seeds with high calcium concentration and low water content. On the other hand, low soil moisture conditions benefited germination of tree species with seeds with higher magnesium and phosphorus concentration and water content. Furthermore, under low soil moisture availability, grass presence facilitated germination of most tree species but its effect on early survival (positive or negative) differed among species. The findings of this chapter confirm a large difference in the tree species responses to environmental variation during early recruitment, which potentially affect theplant community composition and dynamics under different environmental conditions in savannas.Seed trait differences among the species partly contribute to explain such variability. Therefore, considering inter-specific variation among tree species and information on seed traits can improve the ability to predict and manage the impacts of environment changes. For later stages of seedling development (up to 9 months), the effect of environmental variation (water, nutrient and light supply, as well as grass presence or absence) on survival and establishment of semi-arid savanna tree seedlings differed between species (Chapter 3). All species were expected to respond positively to higher resource availability, and negatively to the presence of grass. Indeed, the results of this chapterclearly show that grass presence strongly suppressed seedling establishment. However, recruitment strategies varied among species, particularly under high stress conditions (water stress or low light). In some of the studied tree species, light shortage (i.e., shade) reduced the negative effects of the presence of grass on growth. Furthermore, nutrient availability also reduced the negative effect of grasses, although for certain species (broad-leaf species) this effect occurred only under natural rainfall, while for others (fine-leaf species) it occurred only under regular water provision. Increased atmospheric nitrogen deposition, intensification of agricultural fertilizer use and large herbivore management interventions are on-going processes that increase soil nutrient levels in many savannas. As nutrient concentration in the leaves (i.e., plant quality for herbivores) also depends on soil nutrient availability, I expected in Chapter 4 that both biomass production and leaf nutrient concentration would increase with increasing soil nutrient availability. Contrary to my expectations, differences in soil nutrient levels (low vs. high) did not affect biomass production of any of the tree species, independently of water availability (uneven vs. even water supply). However, leaf nutrient content of the seedlings did differ significantly with different water and nutrient levels. Soil nutrient input increased leaf nutrient content, but only when water was applied regularly, indicating that plant nutrient uptake strongly depends on water availability. Under irregular rainfall patterns, nutrient input significantly reduced leaf quality. Given that large herbivore populations depend on plant nutrient content for their nutritional requirements, increases in nutrient deposition and rainfall levels will likely impact herbivore populations and their browsing patterns, altering the functional structure of ecosystems even if overall plant biomass remains unaffected. In Chapter 5, the effect of fluctuations of environmental conditions on above and belowground growth of juveniles of three savannas tree species (<em>Acacia karroo</em>, <em>A. nigrescens</em> and <em>Colophospermum mopane</em>) during the first 18 months was tested. While it was expected that low soil resource availability would result in high biomass allocation towards roots, experimental simulation of dry eventswithin the wet season or pulses of nutrient availability did not have a clear effect on the seedlings’ aboveground and belowground growth. Furthermore, the results of this chapter demonstrated that browsing stimulated stem regrowth and root elongation of savanna tree seedlings, suggesting that the three studied species have compensatory growth in response to frequent herbivory, quickly recovering the loss of biomass. This result puts in question the usefulness of herbivory or human land-cleaning in controlling invasive woody species in tropical grasslands and savannas. In conclusion, the results of this thesis demonstrated that savanna tree species are generally able to cope with differences in resource availability during seedling establishment, being mostly limited by grass competition for resource. Furthermore, this study shows that during early stages of the life-cycle, when exposed to the same environmental conditions, tree species within a plant community differ in their responses, only having advantages over other species under specific conditions.This inter-specific variation may allow tree species coexistence, explaining the diversity of plant species in savannas.