Effect of intensity and frequency of defoliation on aerial growth and carbohydrate reserve levels inAcacia karrooplants

Abstract

Acacia karroo trees were defoliated by goats at two intensities and four frequencies; 2, 4, 8 and 12-weekly. Leaf accumulation and carbohydrate reserve levels were compared to a non-defoliated control, and to plants (defoliation controls) which were defoliated for the first time that season each time a frequency treatment was defoliated. These plants are activated by defoliation in such a manner that successive defoliations can result in this activation being additive. There is clearly a defoliation level below which they are not activated. Activation appears to be negated to a degree by defoliations at 2 and 4-weekly frequencies, relative to the 8-weekly defoliation frequency. The 12-weekly frequency at heavy defoliation produced less than the same defoliation at 8-weekly frequency. The 2-weekly frequency treatments produced as much leaf as the 4 and 12-weekly defoliations at the same defoliation intensity. The more frequently plants were defoliated, the more carbohydrate reserves dropped. However, plants adjusted to cope with very frequent defoliations. There was no connection between leaf accumulation and carbohydrate reserve levels following the different frequencies and intensities of defoliation.Acacia karroo borne is deur bokke benut teen twee vlakke van intensiteit en teen vier vlakke van frekwensie naamlik; 2, 4, 8 en 12-weekliks. Blaarakkumulasie is met 'n onbenutte kontrole en met plante ('benutte kontrole') wat vir die eerste keer in daardie seisoen teen elke vlak van frekwensie ontblaar is, vergelyk. Die plante word sodanig deur ontblaring geaktiveer, dat opeenvolgende ontblarings tot gevolg het dat hierdie aktivering kumulatief word. Daar is duidelik 'n vlak van ontblaring waaronder A. karroo plante nie geaktiveer word nie. Dit lyk of aktiveering tot 'n mate weerspreek word deur ontblaring teen frekwensies van 2 en 4-weekliks relatief tot die 8-weeklikse frekwensie se ontblaring. Die 12-weeklikse frekwensie, teen die hoe ontblaring, produseer minder as die 8-weeklikse frekwensie, teen dieselfde ontblaring. Die 2-weeklikse frekwensie behandelings het dieselfde hoeveelheid blare geakkumuleer as die 4 en 12-weeklikse frekwensie behandelings teen dieselfde ontblaringsintensiteit. Hoe meer dikwels die plante ontblaar is, hoe laer het die koolhidraatvlakke gedaal. Nogtans het die plante aangepas om tred te hou met veelvuldige ontblarings. Daar was geen ooreenkoms tussen blaarakkumulasie en koolhidraatreserwesvlakke, indien die verskillende frekwensies en ontblaringsintensiteite nagevolg is nie.

Physical and chemical response of juvenile Acacia tortilis trees to browsing. Experimental evidence

“…1), similar to that following natural browsing (e.g. Pellew 1984;Teague 1989a). However, leaf biomass in new shoots of pruned plants did not increase significantly and the proportion of the foliage in new shoots was slightly reduced when compared with unpruned trees (Table 1).…”

“…In this study, a test was made of whether the plants compensate for pruning by producing more biomass of new shoots (Teague 1989a;du Toit et al 1990) and leaves of enhanced nutritional quality (du Toit et al 1990), and whether pruning leads to an increase in the plant's physical defences against browsing (Milewski et al 1991). A further test was to see whether there is a relation between resource allocation and growth, chemical and physical defence, and the concentrations of nitrogen in leaves following pruning, as this relation is crucial to understand the outcome of the response of the plant.…”

“…Pellew 1983Pellew , 1984, some evidence of the plant responses to herbivory has been gathered within the genus Acacia. Browsing may stimulate leaf accumulation, at the expense of non-structural carbohydrates of the plant (Teague 1989a) and lead to compensatory growth of new shoots and enhanced nutritional value of foliage (du Toit et al 1990) but plants or plant parts growing within reach of browsing ungulates also show significantly higher spine density and longer spines (Young 1987;Milewski, Young & Madden 1991).…”

“…Increased biomass production and enhanced nutritional value of leaves (Teague 1989a;du Toit et al 1990) may lead to increased browsing whenever the nutritional value of the foliage is not negatively related to its availability, whereas higher production of spines may result in higher costs of foraging (Cooper & Owen-Smith 1986;Belovsky et al 1991;Gross et al 1993) and a reduction of the accessibility of leaves (Young 1987;Milewski et al 1991). Thus, it is important to study experimentally whether these changes are true responses to browsing and how they are linked to each other to evaluate their influence on future herbivory on the plant.…”

Growth responses of an African savanna tree, Bauhinia thonningii Schumacher, to defoliation, fire and climate

“…It has also been suggested that modification to the hormonal balance within the plant following defoliation can result in compensatory growth through stimulated growth and photosynthate production and the reallocation of reserves from elsewhere in the plant (McNaughton 1979). In woody plants, carbohydrate reserves are, generally, reduced following defoliation, especially after severe defoliation, and subsequent regrowth tends to be positively correlated with the carbohydrate status of the plant (see Teague 1989). In Acacia karroo plants, lowest levels of regrowth were observed in the early-flush phenophase when carbohydrate reserves were at their lowest but as the growing season progressed, reserves accumulated and defoliation resulted in higher production (Teague 1989).…”