Defoliation by pastoralists affects savanna tree seedling dynamics by limiting the facilitative role of canopy cover

Abstract

Recurrent tree defoliation by pastoralists, akin to herbivory, can negatively affect plant reproduction and population dynamics. However, our understanding of the indirect role of defoliation in seedling recruitment and tree-grass dynamics in tropical savanna is limited. In West African savanna, Fulani pastoralists frequently defoliate several fodder tree species to feed livestock in the dry season. We investigated the direct and indirect effects of recurrent defoliation of African mahogany (Khaya senegalensis) by Fulani people on seedling (< 2 cm basal diameter) and sapling dynamics in West Africa using four years of demographic data on seedling and sapling density, growth, and survival, coupled with fruit production and microhabitat data over the same time period. Tree canopy cover facilitated seedlings but had negative effects on sapling growth possibly via intraspecific competition with adult plants. Interspecific competition with grasses strongly reduced seedling survival but had a weak effect on sapling growth. Fire reduced seedling survival and weakly reduced growth of seedlings and saplings, but did not affect sapling survival. These results indicate that the effect of fire on seedlings and saplings is distinct, a mechanism suitable for an episodic recruitment of seedlings into the sapling stage and consistent with predictions from the demographic bottleneck model. Defoliation affected seedling density and sapling growth through changes in canopy cover, but had no effect on seedling growth and sapling survival. In the moist region, sapling density was higher in sites with low-intensity defoliation, indicating that defoliation may strengthen the tree recruitment bottleneck. Our study suggests that large-scale defoliation can alter the facilitative role of nurse trees on seedling dynamics and tree-sapling competition. Given that tree defoliation by local people is a widespread activity throughout savanna-forest systems in West Africa, it has the potential to affect tree-grass coexistence. Incorporating the influence of large tree defoliation into existing models of savanna dynamics can further our understanding of tree-grass coexistence and improve management. A rotating harvest system, which allows seedlings to recruit episodically, or a patchwork harvest, which maintains some nursery trees in the mosaic, could help sustain seedling recruitment and minimize the indirect effects of harvest.

Multiple disturbance patterns and population structure of a tropical tree species,Afzelia africana(Leguminosae–Caesalpinioideae), in two contrasting bioclimatic zones of the Republic of Benin

“…However, overexploitation of species used for NTFPs disturbs the harvested individuals and can have a negative effect on the population structure and survival of the species. Harvesting affects the population structure in different ways (Benoît and Swain 2008;Eichhorn et al 2010;Maclean et al 2011;Jurisch et al 2012;Bufford and Gaoue 2015), including reproduction (Anten et al 2003;Gaoue and Ticktin 2008) and growth (Anten and Ackerly 2001;Anten et al 2003;Dharani et al 2008;Paine et al 2012). These effects may differ across soil and climatic gradients (Hawkes and Sullivan 2001;Staver et al 2009;Gaoue et al 2011).…”

Response of a tropical tree to non-timber forest products harvest and reduction in habitat size

“…butyracea which have dual sexual and asexual reproductive capacity, there is a possibility that they compensate for loss of seed sources, via fruit harvesting, by increasing clonal output which may buffer the negative effect of harvest on seeding recruitment and population structure. Such compensatory mechanisms can also be triggered or reinforced by other type of disturbance that often follow NTFP harvesting such as fire, trampling or random cutting of neighboring plants [ 57 ]. Consistent with this assumption, the highest regeneration was observed in the Setou (15040 stems ha -1 ) and Tchoundékou (14160 stems ha -1 ) populations which were severely affected by fire but were moderately harvested.…”

Plant Love Stories: Share Your Story and Grow a Movement

“…After ten years studying the demography of Khaya senegalensis , I observed how the tree is defended by weaver ant ( Oecophylla longinoda ) against shoot borers ( Hypsipyla robusta ). I continue to work on the population ecology of African Mahogany (Bufford and Gaoue , Gaoue ), and my students focus on the responses of multi‐trophic communities around the species to chronic anthropogenic disturbance. Turning my early fear of African Mahogany into a lifetime study on the consequences of chronic anthropogenic disturbance on plant population dynamics facilitated not just my immigration into the United States, but also my career in ecology.…”