Silvicultural options for the transformation of even-aged Rhizophora apiculata stands to irregular and diverse mangroves – A simulation experiment

Abstract

Mangrove rehabilitation efforts regularly intend to re-establish the former canopy cover quickly by establishing even-aged monocultures. However, there are concerns that these stands are less resilient to disturbances than structurally diverse forests and cannot provide ecosystem services as effectively. The objective of this study was to identify silvicultural systems for the transformation of even-aged Rhizophora apiculata stands to more irregular and species-diverse stands.The individual-based forest model KiWi was used to simulate the dynamics of a 1-ha sized Rhizophora apiculata stand under (1) passive management without intervention, (2) stand clearcutting, (3) group clearcutting, (4) expanding-gap shelterwood, and (5) crown thinning. As observed in the field, seedlings of the less shade-tolerant tree species Avicennia marina regenerated naturally under the canopy of R. apiculata. The transformation process was considered complete at the onset of the steady state characterized by a multi-layered forest structure as well as a stable basal area and stem size distribution.Without intervention, stands entered a steady state, on average, after 226 years under poor site conditions, or 179 years under good conditions. Silvicultural systems that required multiple entries, specifically expanding-gap shelterwood and crown thinning, halved this time by creating multi-layered stand structures while retaining a partial canopy cover. Moreover, continuous crown thinnings were necessary to maintain a species-diverse upper canopy. In contrast, small group clearcuts developed into two-layered stands and were ineffective. The removal of the upperstory through a stand clearcut or a large group clearcut reversed the initial planting arrangement but could develop into irregular stand structure only if propagules were supplied uninterruptedly. Intense forest operations were necessary to accelerate the transformation of even-aged mangrove stands. Forest managers should consider the potential future costs of transformation when planting even-aged stands instead of assisting the natural regeneration of former mangrove sites. Silvicultural field trials are proposed for the development of detailed guidelines.