Heterogeneity in harvest intensity and plant population response to non-timber forest products harvesting
Harvesting wild plants for non-timber forest products (NTFP) serves as a valuable source of food and medicine for local communities and potentially contributes to poverty alleviation. However, frequent harvest at high intensity can lead to plant population decline, and jeopardize the ability of future generations to benefit from these ecosystem services. To assess the sustainability of NTFP harvest, a growing number of studies have used matrix projection models to estimate the effects of harvest on the long-term population growth rate, and its elasticity to changes in population vital rates. These models assume that harvesting is a population-level treatment. However, harvest takes place at the individual plant level where products such foliage, bark or fruits are extracted. As a result, in a harvested population, some individuals may be unharvested. If unharvested individuals are overachievers (e.g., faster growth, better survival, more reproductive output than the average), they may compensate for the negative effects of harvest on the demography of harvested individuals. The rate of biomass removed by harvest at the individual level is a random variable. I investigate to what extent greater or less heterogeneity in NTFP-harvest intensity affects plant population dynamics and response to biomass loss.
I used four years of demographic data on African mahogany to measure individual tree level harvest heterogeneity, and to parameterize an integral projection model which accounts for such heterogeneity. In West Africa, Khaya senegalensis trees are frequently defoliated by pastoralists to feed cattle and debarked for medicinal purposes. At the individual tree level, I found greater heterogeneity in within year harvest intensity for foliage than for bark. Foliage and bark harvest has negative effects on the mahogany population dynamics. Changes in individual tree level harvest heterogeneity alter the overall effect of harvest on population dynamics and this has implications for foliage sustainable harvesting limits. This study highlights the importance of accounting for individual level heterogeneity in biomass loss due to harvesting in designing sustainable harvest strategies.