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 located in coastal subtropical urban KwaZulu-Natal (KZN), South Africa (SA). Materials and methods: Closed canopy (n = 15 quadrats per forest ) and four gaps (n = 12 quadrats per forest) from three size classes (‘small’ 45 m2) were surveyed for the primary least disturbed (PLD), primary highly disturbed (PHD) and transitional highly disturbed (THD) forests using classical vegetation sampling techniques. Soil moisture content and air temperature were measured within each gap. Main results: Cumulatively all forest gaps hosted 198 species. Equivalent number of species (Hill numbers from H = 0 to H = 2) were constantly higher in the PHD forest followed by the PLD forest and lowest in the THD forest. Alien and indigenous plant density were negatively correlated. 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. Floristic, abiotic and physical characteristics of gaps should be monitored within urban forests as these can influence gap infilling in terms of rate, species composition, and alien-indigenous plant interactions across the forest. This can inform management interventions such as species reintroduction and alien clearing. Keywords: conservation; disturbance; gaps; management; Northern Coastal Forest; subtropical; species richness. Abbreviations: SA: South Africa; KZN: KwaZulu-Natal; PLD: Primary Least Disturbed forest; PHD: Primary Highly Disturbed forest; THD: Transitional Highly Disturbed forest; IOCB: Indian Ocean Coastal Belt; New POSA: New Plants of southern Africa website; NEMBA: National Environmental Management: Biodiversity Act.