Thirty years of stasis in the dynamics of the Knysna Afro-montane forest, South Africa

Abstract

We analysed nearly 31 years (1991–2022) of tree population dynamics in 108 permanent plots each 0.04 ha of the Lilyvlei Nature Reserve, a section of the Knysna Forest in South Africa which has not historically been disturbed by harvesting. The Knysna Forest is the only large piece of forest in South Africa and is marginal for tall forest, having low mean annual rainfall of only about 1000 mm and occurs on nutrient poor soils. In contrast to many studies of other forests globally, we found almost no change in overall biomass. Even at the scale of our small plots and on 10–30 year time scales, there has been little change. The forest has very low recruitment and mortality rates and thus low growth rates. Climatic changes too were minimal over this 30-year period. The proportion of stems that died per size class was concentrated in the larger size-classes per species, even for shorter species. This suggests that senescence rather than external disturbances determines the low mortality and thus the low dynamism and the high biomass (approx. 600 Mg ha⁻¹AGB or 41 m² ha⁻¹ basal area) of the forest. Mortality of some species exceeded conspecific recruitment, a trend accompanied by non-significant declines in biomass. However, these trends are not significant. In conclusion, neither composition nor biomass has changed significantly in these forests over a period of 30 years. This suggests that not all Southern Hemisphere forests are in a state of decline.