Harvesting plant parts from wild populations affects life history parameters of a tropical tree

Background/Question/Methods

Two decades of studies on harvest of non-timber forest products (NTFP) from wild populations, a widespread activity worldwide, reveal that harvest of products that are detachable from individuals can impact annual demographic rates and population dynamics. Expected lifespan of individuals and time to reproduction could be important parameters in wild populations that are being managed.  While the effect on such traits of selective harvest of whole individuals from populations has been explored recently (e.g. in fisheries, hunting), the effect of harvesting parts from standing individuals on life history traits has been ignored. Here, we examine how the level of foliage and bark harvest by local people affects expected lifespan and age of first reproduction for a tropical tree, Khaya senegalensis across two climatic regions (moist and dry) of Benin, West Africa. We also compare the sensitivity of life expectancy to transitions across the life-cycle under different harvest and climate regimes, employing recently introduced concepts and methods.

Results/Conclusions

The effect of harvest on life history traits differed between regions. Individual trees are expected to reproduce earlier but live longer in the moist than in the dry region. In the moist region, age at first reproduction is not affected very much by harvest, while life expectancy is affected considerably with a positive association between level of harvest and length of life.  In the dry region, harvest affects age at first reproduction but not life expectancy and high levels of harvest are associated with early reproduction. Life expectancy is more sensitive to survival than to growth or retrogression.  With the notable exception of populations in the moist region that are heavily harvested, life expectancy is more sensitive to survival of early rather than late life stages.  This pattern  contrasts with our previous results; we found that population growth rate, a parameter that is distinct from expected life span, was more sensitive to survival of late rather than early life stages. Models in which harvest level varies stochastically over time show an interaction effect between the initial harvest level and the frequency of harvest during the lifetime of the cohort. The nature of the interaction differed by region as well as between life history traits. Our findings suggest that the processes driving plant life expectancy, at least for long-lived species, may differ from those driving population dynamics.  Our results also underscore that climatic differences may override the effects of NTFP harvest on life history traits.