Corli Coetsee

Nitrogen availability is not affected by frequent fire in a South African savanna

There is a perception that sustained frequent fires cause nitrogen limitation over the long term (50-100 y) by volatilizing the nitrogen in soil, plant biomass and litter. Here we test this perception in a South African savanna located in the Kruger National Park. At our study site we compare the effects of 50 y of fire exclusion, season (August and February) and frequency (triennial and annual August and triennial February) of burn on nitrogen cycling and availability. We do this using three different methods to determine nitrogen mineralization; in situ incubations, laboratory incubations and ion-exchange resin bags. On each treatment we established two parallel transects 100 m apart with 10 sampling points per treatment along these transects. Daily mineralization rates for in situ incubations were determined monthly from August 2004 to June 2005 at each of the sampling points. Ion-exchange resin bags were buried (5 cm) at the same points and left in the field from August 2004 to August 2005. In February 2005 five randomly located soil samples from each of the four treatments were collected for laboratory incubations using a 7-cm-diameter soil auger. Regardless of method used our results show that there are no significant differences in daily nitrogen mineralization rates after 50 y of different burning treatments from annual burning to fire exclusion. In fact, both in situ and laboratory incubations show that nitrogen availability is higher on the annual burn than the fire exclusion (0.16 μg g -1 soil d -1 vs. 0.11 μg g -1 soil d -1 and 0.46 μg g -1 soil d -1 vs. 0.30 μg g -1 soil d -1 respectively). Perceived negative effects of fire on ecosystem functioning has curbed the use of fire as a management tool with fire often actively suppressed in savanna. The results of our study show that fire can be used more vigorously in mesic African savanna to manipulate tree:grass ratios without negatively affecting the nitrogen cycle.

Leaf traits of African woody savanna species across climate and soil fertility gradients: evidence for conservative versus acquisitive resource‐use strategies

Summary 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. 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. 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. 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. 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.

An overview of nitrogen cycling in a semiarid savanna: some implications for management and conservation in a large African park.

Nitrogen (N) is a major control on primary productivity and hence on the productivity and diversity of secondary producers and consumers. As such, ecosystem structure and function cannot be understood without a comprehensive understanding of N cycling and dynamics. This overview describes the factors that govern N distribution and dynamics and the consequences that variable N dynamics have for structure, function and thresholds of potential concern (TPCs) for management of a semiarid southern African savanna. We focus on the Kruger National Park (KNP), a relatively intact savanna, noted for its wide array of animal and plant species and a prized tourist destination. KNP’s large size ensures integrity of most ecosystem processes and much can be learned about drivers of ecosystem structure and function using this park as a baseline. Our overview shows that large scale variability in substrates exists, but do not necessarily have predictable consequences for N cycling. The impact of major drivers such as fire is complex; at a landscape scale little differences in stocks and cycling were found, though at a smaller scale changes in woody cover can lead to concomitant changes in total N. Contrasting impacts of browsers and grazers on N turnover has been recorded. Due to the complexity of this ecosystem, we conclude that it will be complicated to draw up TPCs for most transformations and pools involved with the N cycle. However, we highlight in which cases the development of TPCs will be possible.

Defence strategies in African savanna trees.

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.