Cattle browsing impacts on stunted Avicennia marina mangrove trees

Abstract

Mangroves in South Africa are threatened by over-utilization through harvesting for firewood and building materials as well as excessive browsing and trampling by livestock. At Nxaxo Estuary (32°S; 28°E) the response of Avicennia marina to cattle browsing and trampling was investigated by using exclusion plots. These were established by fencing in five 25m2 quadrats and adjacent to each experimental quadrat a browsed quadrat. Trees were tagged and measured annually from 2010 to 2012. Sediment salinity, pH, moisture, organic content, compaction as well as sediment particle size did not differ between browsed and non-browsed quadrats. Significant increases in mean tree height (5.41±0.53cm), crown volume (0.54±0.01m3) and crown diameter (7.09±0.60cm) from 2010 to 2012 were recorded for the non-browsed plots. Trees in the browsed plots had significantly lower growth (p<0.05). The browsed trees were stunted with horizontal spreading of branches while the trees in the non-browsed plots showed an increase in vertical growth and expansion. There was a greater percentage of flowering (54%) and fruiting (19%) for trees in non-browsed plots compared to the browsed sites where 34% of the trees were flowering and 6% of the trees carried immature propagules. The study concluded that browsing changes the morphological structure of mangrove trees and reduces growth and seedling establishment. This is the first study to document the impact of browsing on mangroves.