Forest canopy gaps offer a window into the future The case of subtropical coastal forests within an urban matrix in South Africa

Abstract

Aim of the study: Alien and indigenous species emergence patterns within canopy gaps in urban subtropical forests are poorly understood. This study compared canopy gap floristics in relation to abiotic and physical characteristics across three subtropical urban forests differing in disturbance history.Area of study: Three Northern Coastal Forests of varying disturbance histories found in coastal subtropical urban KwaZulu-Natal (KZN), South Africa (SA).Materials and methods: Closed canopy ( n = 15 quadrats per forest ) and four gaps from three size classes (‘small’ < 25 m2, ‘medium’ 25 - 45 m2; ‘large’ > 45 m2) were surveyed for each forest using classical vegetation sampling techniques. Soil moisture content and air temperature were measured within each gap.Main results: Cumulatively all forests hosted 198 species. Species richness was highest in the primary highly disturbed forest followed by the primary least disturbed forest and lowest in the transitional highly disturbed forest. Alien taxa cover within gaps was correlated with higher indigenous seedling emergence in the primary forests. Species richness was positively correlated with gap size and soil moisture content, and negatively correlated with air temperature.Research highlights: Gap floristic patterns are influenced by size, abiotic factors, disturbance and forest successional status. Alien taxa may facilitate indigenous seedling emergence within gaps in primary forests. Floristic, abiotic and physical characteristics of canopy gaps should be monitored within urban forests as they can influence gap infilling regarding species composition and rate. This can inform management strategies including species reintroduction and enrichment planting.