Impacts of Cattle on the Vegetation Structure of Mangroves

Across the globe coastal wetlands have been lost and degraded due to agriculture. Here we test hypotheses that there are differences in vegetation structure of the mangrove Avicennia marina at locations with or without cattle in the lower Shoalhaven River Estuary in New South Wales, Australia. We sampled the pneumatophores, seedlings, saplings, and trees within the mangrove forest and landward of the forest where cattle are most active. Areas with cattle had fewer trees, and their lowest branches were more than 2 m above the ground, giving trees an umbrella-shaped morphology. Although abundances of saplings and seedlings were highly variable among locations, plants at both stages were shortest along the landward side of the forest in the presence of cattle and seedlings were bushier, suggesting consumption of the apical shoots. A reduction in pneumatophore density and the highest proportion of branched pneumatophores occurred along the landward side of the forest in the presence of cattle, indicating impacts of trampling. Prospects for regeneration of the mangroves in the presence of cattle appear limited due to grazing, physical disturbance and trampling across multiple life history stages. Livestock paddocks should be fenced to exclude cattle and prevent degradation of these coastal intertidal habitats.