Towards a mechanistic understanding of the synergistic effects of harvesting timber and non-timber forest products

Abstract

Classic theories of resource harvest assume logistic growth and incorporate harvest through an additional loss term. This methodology has been applied successfully in forest products harvesting such as timber logging. However, modelling harvest through a loss term is not appropriate for non-timber forest products (NTFP) since harvesting in this case does not always require the complete removal of individual plants. Empirical evidence suggest that NTFP harvest affects plant population growth rates. Additionally, timber and NTFP harvest can have synergistic effects on population dynamics. We develop and analyse a simple model that incorporates non-lethal harvest implicitly through the population growth rate of plants and lethal harvest explicitly through permanent removal of whole plants, with additional synergistic effects on population growth rate. To capture transient dynamics, we model the growth rate of plants explicitly as a dynamic variable affected by harvesting. Transient dynamics results indicate that populations have delayed response to discrete harvesting. We demonstrate exactly how the sustainability of lethal harvest, non-lethal harvest or both types of harvests depends on the demographic effect of each type of harvest on the population growth rate. Finally, we identify a threshold parameter R, such that combined lethal and non-lethal harvest results in plant population sustainability when R>1 and extinction when R≤1.