Effects of defoliation on regrowth and carbon budgets of three semi-arid Karoo shrubs

• Main Author: Van der Heyden, Francois
• Other Authors: Stock, WD
• Format: Doctoral Thesis
• Language: English
• Published: University of Cape Town 2015
• Subjects: Botany
• Online Access: http://hdl.handle.net/11427/14355

Includes bibliographies. === Plant regrowth, nonstructural carbohydrate utilization patterns, photosynthesis and the partitioning of photosynthetic products in response to foliage removal were studied for the following karoo shrubs: Osteospermum sinuatum, a dwarf deciduous shrub with fleshy leaves, Pteronia pallens, a dwarf evergreen shrub with sclerophyllous leaves and Ruschia spinosa, a dwarf evergreen shrub with succulent leaves. Defoliation adversely affected both vegetative growth and reproductive output for periods up to 26 weeks following foliage removal. A gradient of increasing regrowth capacity with decreasing defoliation intensity and frequency was observed in all species. In terms of biomass production, defoliation was the least detrimental to the deciduous shrub, O. sinuatum, and the evergreen shrub, P. pallens, and the most injurious to the succulent shrub, R. spinosa. All species regrew better during spring and autumn, and no regrowth was recorded in the moderate (40%) or intensely (80%) defoliated plants during summer and winter over the 6-week monitoring periods. Spatial patterns of carbohydrate accumulation were the same for all species, with most of the total nonstructural carbohydrates (TNC) being stored in the twigs and stems. Karoo shrubs can be divided into two distinct groups based on the primary nonstructural polysaccharides accumulated in their plant parts. The Asteraceous plants, O. sinuatum and P. pallens, accumulate predominantly fructans. In contrast, the succulent species, R. spinosa, accumulate starch and fructans in equal proportions. Differences among species in terms of seasonal changes in TNC levels of undefoliated plants reflect the extent to which different species are dependent on stored carbohydrates or photosynthesis for normal vegetative growth processes. Repeated defoliations at a moderate frequency (26-week interval) resulted in the elevation of TNC concentrations of O. sinuatum and P. pallens. In contrast, defoliations at heavy or at leniant frequencies caused decreases in TNC concentrations in all plant parts of Ruschia spinosa. Restoration of plant storage TNC levels in excess of undefoliated plant TNC levels occurred prior to complete vegetative regrowth in the two Asteraceous shrubs which suggests that some factor(s) other than the carbon resource was limiting vegetative regrowth in karoo shrubs. Analyses of short-term changes (2-weekly) in TNC levels in response to defoliation demonstrated the elevation in TNC concentrations of Pteronia pallens plant parts only during the periods when no regrowth was recorded. This phenomenon illustrates that on a short-terms basis, regrowth and over-replenishment of reserves represent two alternate responses to defoliation. However, during periods when regrowth was recorded for P. pallens (autumn and spring), and during all seasons of the year for O. sinuatum and R. spinosa, depressions in TNC concentrations were observed in most plant parts up to six weeks following defoliation. This illustrates the large dependence these shrubs have on stored carbohydrates following defoliation. Defoliation had no effect on the photosynthetic rates of karoo shrubs for at least 11 days following defoliation. Foliage removal resulted in the redistribution of photoassimilates in all plant species. These changes in the allocation of newly produced photosynthates appear to be associated largely with the replenishment of carbohydrate reserves following the initial TNC utilization caused by foliage removal. Comparison of TNC utilization patterns, following defoliation of O. sinuatum in the dark (no photosynthesis) and in the light resolved the question of the relative importance of reserve carbohydrates and photosynthates following defoliation. Reserve carbohydrates were used only for the first 2 weeks following defoliation for respiratory functions while photoassimilates were used for the production of new foliage. The magnitude of nonstructural carbohydrate utilization in the absence of photosynthesis emphasized the importance of continuing photosynthesis to the survival of defoliated karoo shrubs. The differences among species in terms of the timing and the extent of changes (elevations or decreases) in TNC levels in response to defoliation are interpreted as being the result of alterations in plant chemistry which in turn are governed by species specific physiological adaptations to environmental constraints. Rangeland management guidelines are recommended within the framework of the observed short-term and long-term defoliation effects on karoo shrub plant production.