A new approach to spatially explicit modelling of forest dynamics: spacing, ageing and neighbourhood competition of mangrove trees

This paper presents a new approach to spatially explicit modelling that enables the influence of neighbourhood effects on the dynamics of forests and plant communities to be analysed. We refer to this approach as ‘field of neighbourhood’ (FON). It combines the ‘neighbourhood philosophy’ of grid-based models with the description of individual spacing in the ‘zone of influence’ (ZOI) approach. The novel feature of FON is that modelling of local competition between neighbouring trees is based on the notion of a field of neighbourhood exerted by each tree. This field is defined only on the ZOI of a tree and depends on the distance to the stemming point. For the demonstration of FONʹs power, a simulation model (KiWi) was implemented that focuses on the dynamic of mangrove forests. The realistic self-thinning behaviour of modelled stands of Avicennia germinans and Rhizophora mangle confirms the suitability of the FON approach for the description of intra- and inter-specific competition. In KiWi, mortality is modelled in terms of a ‘memory function’, i.e. the yearly stem increment of each tree is stored in its ‘memory’ over a certain time period and determines — as a sign of vitality — tree mortality. The results of KiWi demonstrate that this description is sufficient to keep the maximum age of the trees within a reasonable limit. The model thus manages without a description of individual tree age. This is an important feature considering the fact that a direct relationship between tree age and mortality is questioned and there is no established method as yet for determining the age of mangrove trees.