Project topics and materials
Get Complete Project Material File(s) Now! »
Naturally browsed trees
Regrowth on trees naturally damaged by elephants was similar to the simulated treatment trees. Shoots and leaves were significantly longer than on control trees (Wilcoxon matched-pairs test: P < 0.01; shoots: 18.2 ± 1.34 versus 7.03 ± 0.341 cm; leaves: 62.6 ± 1.65 versus 49.6 ± 1.07 mm), while there was no difference in the number of shoots/50 cm length of branch (P = 0.859), nor in the tannin:protein ratio and total polyphenolic content of the foliage (P = 0.507, P = 0.249 respectively; Fig. 3.3).
Natural caterpillar defoliation, however, had a greater effect on plant responses than simulated defoliation. Both shoot and leaf lengths were significantly shorter on previously defoliated trees than control trees (Wilcoxon matched-pairs test: P < 0.01; shoots: 3.70 ± 0.135 versus 7.15 ± 0.231 cm; leaves: 45.4 ± 1.36 versus 57.3 ± 1.79 mm) and foliar tannin:protein ratio and total polyphenolic content were significantly reduced (tannin:protein ratio: 0.348 ± 0.027 versus 0.443 ± 0.027; polyphenols: 52.4 ± 3.67 versus 63.9 ± 3.39 mg/gDW; Wilcoxon matched-pairs test: P < 0.05; Fig. 3.3). Only shoot density was unaffected (P = 0.211).
Discussion
Both mopane caterpillars and elephants cause extensive damage to individual mopane trees, yet the two types of herbivory affect plant responses in significantly different ways.
Pruning by elephants resulted in the production of longer shoots and leaves, while defoliation by caterpillars had the opposite effect. Foliar chemical composition was only found to change after natural caterpillar defoliation, where an increase in nutritional value occurred. The difference in plant responses found here supports results from numerous other studies on browsing (e.g. Bryant et al. 1991; Danell et al. 1994; Lehtilä et al. 2000), and results are as expected considering the differential affect each damage type has on resource availability and resource allocation within individual plants.
According to the sink/source hypothesis (SSH), for example, damage affects plant growth primarily by changing the ability of meristems to compete for resources (Honkanen & Haukioja 1994). In undamaged individuals, sink-source relationships determine resource allocation among organs, with sink strength determining the degree of resource accumulation. Damage through browsing often removes sinks and/or sources, however, thereby altering relationships and modifying allocation patterns (Stowe et al. 2000). Because defoliation and pruning affect functionally different plant tissues (physiological sources or sinks), it is hence not surprising that they have variable effects on growth. Defoliation, for example, weakens sink strength of meristems formed immediately after damage, thereby leading to poor plant growth, while branch/stem removal destroys apical meristem dominance (strong physiological sinks) of entire branches, thereby resulting in the redirection of resources towards lateral meristems, which would otherwise remain dormant.
While changes in sink/source relationships explains how damage could affect the distribution of growth activity, the overall regrowth of a plant is also dependent upon the quantity of nutrient reserves in the plant (i.e. resource availability). During summer, photoassimilates are accumulated and transferred to stems and roots, where they are stored for growth the following spring. Removal of storage organs reduces these carbon and mineral reserves, thereby reducing overall plant growth (Quiring & McKinnon 1999).
In mopane woodland, the timing of the main defoliation event (November/December) is such that plants would have utilized stored resources for new shoot production (i.e. acting as sinks), but would most likely not have had sufficient time to replenish the used reserves through photosynthesis. Leaf removal at this time would therefore result in an overall decrease in resources available compared to at the start of the first flush. Enhanced by the increase in shoot number after defoliation, less resources are then available per shoot, resulting in a decrease in shoot and leaf size. A similar response has been observed for other southern African deciduous species, such as Acacia tortilis, Grewia flavescens and Dichrostachys cinerea (Bryant et al. 1991). The response to elephant utilization, which was opposite to defoliation, is also as expected considering the probable changes in resource availability within the plant. The high intensity of branch/stem breakage had a significant impact on the root/shoot ratio, as up to 75% of the canopy biomass was removed. When part of the photosynthetic material of a plant is removed, potentially more water and nutrients are then available for the remaining photosynthetic material, resulting in increased shoot and leaf growth (Alados et al. 1997). Similarly, the shorter shoot length on trees flushing for the second time (E’02) compared to those flushing for the first time (E’03) can be explained in this way, as the root/shoot ratio would have decreased after the first flush.
In accordance with results from other studies is the greater negative impact of late season (February) defoliation on regrowth found here (Maschinski & Whitham 1989; Danell et al. 1994). According to Tiffin (2002), early and late-season damage is more detrimental than mid-season herbivore damage for various reasons. During early season defoliation, leaves may be removed while still growing and acting as sinks, and are therefore removed before having a chance to act as sources and replenish resources used for growth. Similarly, if defoliation takes place too late in the growing season, time for regrowth and replacement of lost resources before the dry season may be insufficient (Maschinski & Whitham 1989; Lennartsson et al. 1998). Consequently, growth the following season is retarded. This explains the reduced growth after February (i.e. lateseason) defoliation here, as very few mopane trees were observed being able to flush again before the dry season, while November-defoliated (i.e. mid-season) trees re-flushed readily. It should be noted, however, that the summer during which late-season treatments were applied (February 2003) was hot with little rainfall after February (see Fig. 2.3), resulting in a very poor mopane caterpillar crop (most died of desiccation) and minimal defoliation of trees. The following season (2003/2004) experienced good rainfall into March, however, and stands of trees were observed to re-flush each time after three complete defoliation events (once was by puss moth larva). It may therefore have been an unusual situation for mopane trees to incur defoliation and reduced resource availability simultaneously, as in more arid areas there is naturally only one generation of caterpillars (Oberprieler 1995). Growth of mopane is known to be dependant upon water availability, as nitrogen mineralistaion requires the soil to be moist (Henning & White 1974). The already short period for regrowth and nutrient replenishment after the February defoliation would then have been made even worse by the low late-season rainfall in 2003.
CHAPTER 1: General introduction
CHAPTER 2: Study sites and species
2.1 Study sites
2.2 Mopane trees
2.3 Mopane moths and caterpillars
2.4 African elephants in mopane woodland
2.5 References
CHAPTER 3: Differential effects of defoliation by mopane caterpillars and pruning by african elephants on the regrowth of Colophospermum mopane foliage
3.1 Introduction
3.2 Methods
3.3 Results
3.4 Discussion
3.5 References
CHAPTER 4: Effects of pruning by elephants and defoliation by mopane caterpillars on reproduction in Colophospermum mopane
4.1 Introduction
4.2 Methods
4.3 Results
4.4 Discussion
4.5 References
CHAPTER 5: Intraspecific host preferences of mopane moths (Imbrasia belina) in mopane (Colophospermum mopane) woodland
5.1 Introduction
5.2 Methods
5.3 Results
5.4 Discussion
5.5 References
CHAPTER 6: Elephants and mopane caterpillars : interactions through a shared resource
6.1 Introduction
6.2 Method s
6.3 Results
6.4 Discussion
6.5 References
CHAPTER 7: Elephant browsing, caterpillar defoliation and fluctuating asymmetry in Colophospermum mopane leaves
7.1 Introduction
7.2 Methods
7.3 Results
7.4 Discussion
7.5 References
CHAPTER 8: General discussion
