C. van der Waal

Nitrogen prediction in grasses: effect of bandwidth and plant material state on absorption feature selection

We analysed stability and predictive capabilities of known nitrogen absorption features between plant material prepared for NIRS (dried) and RS (fresh) studies. Grass spectra were taken of the plant canopy, and again after the grass sample was dried and ground. Models were derived using stepwise multiple linear regression (sMLR). Regression values (adj.r 2) produced using the dried material were greater than those produced using canopy material. For dried material only wavebands from the SWIR region were selected. Wavebands selected by sMLR on canopy material were located in both the VNIR and SWIR regions. Using wavebands selected for dried material models produced low adj.r 2 values when applied to canopy plant material; differences in adj.r 2 values are smaller when wavebands selected in canopy material models are applied to dried material. Widening of nitrogen features produced higher adj.r 2 values for both dried and canopy material. This work shows that obtaining models with high predictive capabilities for nitrogen concentration is possible, but waveband selection should not be limited to features identified by NIRS studies. To accommodate for variability in absorption features, and instrument errors, absorption features should be widened.

Induced chemical defences in Colophospermum mopane trees

A field experiment was conducted in which Mopane (Colophospermum mopane) trees were subjected to three different canopy treatments:felling of trees at a mean height of 0.7m above ground level, felling at a height of 2m, or pruning of selected branches. These treatments were intended to simulate wood harvesting by local communities, pollarding by elephants, and browsing by herbivores, respectively. The foliar concentrations of total phenol, condensed tannin and protein-precipitating tannin were tracked for three consecutive years after treatments were effected. Coppicing, pollarding and pruning induced chemical defences in Mopane tree foliage. The order of magnitude of the response was positively related with impact severity: coppicing > pollarding > pruning > control. Differences between treatments remained the same for the duration of the study. The effects of canopy treatments on phenol and tannin concentrations were small compared to the seasonal and inter-seasonal fluctuations. The protein precipitation capacity of the secondary metabolites increased significantly with increasing impact. We concluded that secondary metabolite concentrations in Mopane foliage are dynamic and can be explained satisfactorily by season, year and treatment severity, with a high explanatory power (r2 = 0.94–0.98). Findings of this study have consequences for current harvesting, animal production and conservation practices.

Kudu foraging behaviour: Influenced by animal density?

In deciduous savanna a marked decline in browse availability characterises the late dry season and apparently regulates populations of large browser species such as kudu. The dry season utilisation patterns of two woody species, Acacia tortilis and Boscia albitrunca, in two comparable sites but subjected to different kudu densities, were studied to determine if kudu adapt their feeding behaviour in response to animal density. Shoots of plants in the high-density area were vertically more uniformly utilised compared to shoots in the low-density area where browsing was more intense in certain height strata. Thicker shoots were selected in the high-density area and smaller plants selected than in the low-density area. It is concluded that kudu feeding behaviour during the dry season was significantly altered as a result of animal density differences. It is suggested that a high browsing pressure during times of low browse availability probably affects the productivity and recruitment of critical resources and questions the sustainability of such systems.In deciduous savanna a marked decline in browse availability characterises the late dry season and apparently regulates populations of large browser species such as kudu. The dry season utilisation patterns of two woody species, Acacia tortilis and Boscia albitrunca, in two comparable sites but subjected to different kudu densities, were studied to determine if kudu adapt their feeding behaviour in response to animal density. Shoots of plants in the high-density area were vertically more uniformly utilised compared to shoots in the low-density area where browsing was more intense in certain height strata. Thicker shoots were selected in the high-density area and smaller plants selected than in the low-density area. It is concluded that kudu feeding behaviour during the dry season was significantly altered as a result of animal density differences. It is suggested that a high browsing pressure during times of low browse availability probably affects the productivity and recruitment of critical resources and questions the sustainability of such systems.