Patterns of harvesting foliage and bark from the multipurpose tree Khaya senegalensis in Benin: Variation across ecological regions and its impacts on population structure

Abstract

Non-timber forest products (NTFPs) represent important resources for millions of communities worldwide. Concerns over NTFP overexploitation has led to a growing number of studies on the ecological impacts of harvest. Few studies however, have addressed species harvested for multiple parts or investigated how spatial variation affects harvest patterns and their impacts. We documented rates and patterns of pruning and debarking and their impacts on density and population structure, for 12 populations of the multiuse tree, Khaya senegalensis (Meliaceae) in two ecological regions (Sudano-Guinean versus Sudanian) of Benin, West Africa. Half of the populations had low or no harvest and half were highly harvested. Patterns of pruning and debarking were size-specific, with harvesters tending to prefer larger trees. Foliage harvest pressures were very high across both regions, with >70% of trees harvested for 100% of their crowns. A significantly greater proportion of trees were harvested for foliage in the wetter Sudano-Guinean region than in Sudanian region. The reverse was true for the proportion of foliage and bark-harvested per tree. In the Sudano-Guinean region, high harvest populations had significantly lower densities of seedlings and saplings than low harvest populations. The size-class distribution coefficient of skewness was significantly correlated with rainfall, habitat, and soil type. Variation in harvesting patterns and their ecological impacts can be explained in large part by differences in water availability between the two regions. Effective conservation plans for K. senegalensis require close consideration of the environmental and land-use context in which populations occur.

Introduction

Non-timber forest products (NTFPs) play important roles in the livelihoods of hundreds of millions of rural and urban peoples across the globe (e.g., Ganesan, 1993, Bernal, 1998, Shackleton et al., 2002, Griffiths et al., 2003, Emanuel et al., 2005). For instance, many NTFP species with medicinal value are harvested for local healthcare needs as well as for sale in national and international industries (Stewart, 2003, Hamilton, 2004). However, the growing demand for NTFPs used for both subsistence uses and commercial trade (Hamilton, 2004, Botha et al., 2004a) has, in many cases, led to unsustainable management of forest resources (e.g., Cunningham and Mbenkum, 1993, Peres et al., 2003, Botha et al., 2004b).

The most direct ecological consequence of NTFP extraction is alteration of the rates of survival, growth and reproduction of harvested individuals. These changes can in turn affect the structure and dynamics of harvested populations (Peres et al., 2003, Ticktin, 2004). For example, repeated harvesting of Waburgia salutaris individuals in South Africa results in decreased basal diameter and height, and higher rates of fungal attack and mortality (Botha et al., 2004b). The impacts of NTFP harvest are also highly dependent on harvest patterns, and on size-specific harvest preferences (Nantel et al., 1996, Ticktin et al., 2002, Shackleton et al., 2005).

Although a growing number of studies have assessed the demographic impacts of NTFPs harvest (e.g., Pinard, 1993, Nantel et al., 1996, Bernal, 1998, Soehartono and Newton, 2001, Ticktin et al., 2002, Peres et al., 2003, Emanuel et al., 2005), very few have assessed spatial variation in harvest patterns or their impacts. This is important because NTFPs are often harvested in the context of other kinds of disturbances such as fire, logging, or grazing, and their responses to harvest may be affected by these factors (Sinha and Brault, 2005, Ticktin, 2005). Similarly, harvest and its impacts may also be affected by variation in environmental variables (Siebert, 2000, Ticktin and Nantel, 2004).

In addition, few studies have assessed patterns of harvest or their demographic impacts for species that are harvested for multiple plant parts. The multiple use of single species is however, widespread and common. For example, 51.35% of plant species harvested in western Burkina Faso are harvested for multiple parts and the situation is similar elsewhere (Taita, 2003, Kristensen and Balslev, 2003, Harris and Mohammed, 2003). The ecological impacts of harvesting multipurpose species may be greater than those of other species due to the combined effects of harvesting multiple parts. These species may therefore be at higher risk of overexploitation. Multipurpose NTFP species also often represent important resources for multiple user groups (Shackleton et al., 2002, Lykke et al., 2004), Gaining an understanding of patterns of harvest by different user groups and their impacts is essential for designing plans for sustainable management for these species.

We assessed patterns of NTFP harvest from the multipurpose tree, Khaya senegalensis (Desr.) A. Juss (Meliaceae), and its impacts on 12 populations spread across two contrasting ecological regions in Benin. K. senegalensis is heavily harvested for both its bark as well as its foliage by local people in Benin. During the dry season, the indigenous Fulani people heavily prune K. senegalensis trees to feed their livestock (Sinsin et al., 1989, Petit, 2003). The Fulani specialize in raising cattle, which are a vital source of income for them and an integral part of their culture. K. senegalensis stem bark is also a very important medicine used by traditional healers and local populations to cure several human diseases (Sokpon and Ouinsavi, 2002) as well as livestock diseases (Atawodi et al., 2002). It is an important medicine for malaria, one of the most dangerous and common diseases in Tropical Africa.K. senegalensis is also one of the best timber species in Benin, and was the first timber species exported from Africa early during the colonial era (Dalziel, 1937). Commercial companies as well as local populations illegally harvest the timber to depletion.

K. senegalensis is found in various vegetation types, including gallery forest, dry dense forest, woodland forest, savannah and in both the Sudano-Guinean and the Sudanian ecological regions of Benin (Sokpon and Ouinsavi, 2002). The Sudanian region is drier than Sudano-Guinean region, which has higher rates of annual rainfall, a longer rainy season, lower potential evapotranspiration and higher diversity of habitat (Natta, 2003).

We address the following questions:

• What are the rates and patterns of bark and foliage harvest for K. senegalensis and do these vary between the Sudanian and Sudano-Guinean ecological regions? We quantify rates and patterns of harvest at the level of individuals (% of each tree harvested) and populations (% of trees harvested), and according to size-classes.

• Does bark and fodder harvest affect K. senegalensis density and population structure (size-class distribution) and do the effects vary between the Sudanian and Sudano-Guinean ecological regions?

• Do other environmental factors and human activities also explain patterns of variation in harvest rates, population size-class distribution and density?

• What features of K. senegalensis harvest practices may best allow for sustained use and what additional management strategies may foster conservation?

Despite the great importance of NTFP to local people’s livelihoods in Africa, this study is one of very few to quantify harvest patterns and their impacts for an African NTFP outside of South Africa.