The mangrove forest dynamics model mesoFON

摘要

This study presents mesoFON, an individual-based mangrove forest dynamics model that advances beyond current models by describing crown plasticity of mangrove trees. The crown plasticity routines take advantage of the fields-of-neighborhood (FON) approach and account for the trunk bending and the differential side branch growth mechanism. Competition for above-/below-ground resources is dealt with separately in this model. Offspring production depends on tree growth and rises with tree ontogeny. An extensive sensitivity analysis revealed that mesoFON resembles the behavior of known mangrove forest dynamics and is ready for application. In this study we exposed two plant functional types (PFTs) of the red mangrove (Rhizophora mangle L.) either in monoculture or as a community to two disturbance regimes, namely (1) without disturbances and (2) with hurricane impacts returning every 5 years. While one functional type possesses plastic crowns, the other PFT has rigid crowns. For the first time, long-term interaction of lateral crown displacement and disturbance was examined using a comprehensive comparative analysis including point patterns and canopy coverage. In the monoculture experiments disturbance strongly promoted the beneficial effects of crown plasticity. Without disturbance crown movements merely increased stand-based stem volume by 6.7% despite considerable displacement distances. We attribute this to the overall high competitive strength that constrained the effects of plasticity in the dense stands. Yet, in disturbed stands the plastic behavior raised stem volume and tree density by 12.5% and 7.5%, respectively, as a result of substantially reduced local competition (by 20.1%). In this treatment crown shifts are particularly advantageous because of their contribution to gap closure. Generally, the Clark Evans aggregation index of crown centers tended to be higher than that of stem bases indicating a more regular distribution of crown centers. The same was true for the canopy coverage of crowns located at their centers implying better space usage by shifted crowns. Pair-correlation functions revealed a plasticity-induced trend toward more regular distribution at low tree-to-tree distances and less aggregation at intermediate distances. The trend was stronger in disturbed communities. The plastic PFT was finally able to out-compete the rigid PFT in all community experiments. Hurricane impacts, however, accelerated the time to the extinction of the rigid PFT by a factor of 2.4.