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- Materials and methods
One of the ways in which NTFP harvest can impact plant populations is by altering reproductive performance, including the number and size of fruits and seeds produced and the timing, frequency and probability of reproduction. This is especially true for NTFP harvest, that involves the removal of parts that are photosynthetically active or nutrient-rich or expensive to replace; or whose removal may damage the plant and leave it more vulnerable to subsequent infection, disease or attack. For example, leaf harvest of palms can significantly reduce the number of inflorescences and infruitescences produced per palm (Ratsirason, Silander & Richard 1996; Flores & Ashton 2000; Endress, Gorchov & Noble 2004), as well as the number of fruiting individuals in a population (Endress, Gorchov & Noble 2004). Similarly, in African savanna, successive pruning of Adansonia digitata L. branches leads to a decrease in mean fruit production (Dhillion & Gustad 2004). These effects may be caused by decreased photosynthetic capacity and/or reallocation of resources or stored reserves from reproduction to regrowth (Whitham et al. 1991; Fong 1992; Kigomo, Woodell & Savill 1994).
Although few studies have assessed the impacts of bark harvest on vital rates (Ticktin 2004), bark harvest can also be hypothesized to decrease reproductive performance. Bark is produced by a thin layer of cambium cells which surround the xylem and phloem tissues that transport water and nutrients to and from the roots and leaves. Bark also protects plants against fire, fungal and insect attack (Cunningham 2001). Removal of bark can therefore damage phloem or expose it to desiccation and fungal or parasite attack. This may disrupt the conduction of nutrients and hormones involved in flower bud production (Primack 1987; Mohr & Schopfer 1995), decreasing flower induction and therefore fruit and seed set. The need for resources to repair damage to bark could also result in lower resource allocation to reproduction.
The effects of foliage or bark harvest on reproductive performance may vary according to environmental gradients, with the negative impacts increasing with decreasing resource availability. For example, some NTFP species recuperate significantly faster from harvest in forest patches with higher light availability (Siebert 2000; Ticktin & Nantel 2004). Similarly, the impacts of harvest can also be expected to depend on the intensity and/or frequency of harvest. Thus multipurpose species, in which multiple plant parts are harvested from the same individuals, may, in some cases, be at higher risk of over-exploitation. Although multipurpose species make up a large proportion of NTFP world-wide (e.g. Harris & Mohammed 2003; Kristensen & Balslev 2003; Taita 2003), few studies have quantified the effects of harvest on them.
We assessed the impacts of foliage and bark harvest on reproductive performance of the multipurpose tree, Khaya senegalensis Desr (A.Juss), in two contrasting ecological regions in Benin, West Africa. K. senegalensis is harvested heavily for its leaves, which are pruned by the indigenous Fulani tribe to feed their livestock (Sinsin, Oloulotan & Oumorou 1989; Petit 2003). It is also harvested by local people for its bark (Gaoue & Ticktin 2007), which is used to treat various diseases including malaria, gastrointestinal diseases and anaemia (Arbonnier 2002; Kone et al. 2004). In addition to its value as an important source of foliage and bark, K. senegalensis is also highly prized for its timber. In Benin, K. senegalensis has a wide distribution, spanning both the Sudanian and the Sudano–Guinean ecological regions. The Sudanian region has lower rainfall, with a longer dry reason and higher rates of evapotranspiration than the Sudano–Guinean region. We hypothesized that high pruning and debarking pressure on K. senegalensis decreases reproductive output and that these effects are stronger in the Sudanian region than in the Sudano–Guinean region. Specifically, our objectives were to (1) assess the impacts of combined foliage and bark harvest of K. senegalensis on reproductive performance including seed mass, and number of fruits and seeds produced per tree, and (2) assess if and how the above impacts vary between ecological regions (Sudanian vs. Sudano–Guinean) of Benin.
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- Materials and methods
In the Sudanian region, the number of fruits per tree (ancova, P < 0·001) and the number of seeds per tree (ancova, P < 0·001) were significantly lower in high harvest populations than in low harvest populations (Fig. 1a). Trees in high harvest populations produced fruits at a significantly smaller size than those in low harvest populations (Kruskal–Wallis, P = 0·050; Fig. 2a). There was no significant difference in number of seeds produced per fruit between high and low harvest populations (Fig. 1b). However, seed produced by trees in high harvest populations had higher mass than those in low harvest populations (ancova, P < 0·001; Fig. 1c).
Figure 1. Measures of reproductive output of Khaya senegalensis trees in high harvest vs. low harvest populations, and in two ecological regions in Benin: (a) number of fruits per tree, (b) number of seeds per fruit and (c) seed mass (g). S: Sudanian region, SG: Sudano–Guinean region. Values are mean ± 1 SE. Asterisks on the figure show significant differences between high vs. low harvest within each ecological region *P = 0·05; **P = 0·01; ***P = 0·001.
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Figure 2. Reproductive performance of Khaya senegalensis populations in high harvest vs. low harvest populations, and in two ecological regions in Benin: (a) minimum fruiting diameter (cm) and (b) percentage of trees fruiting (%). Boxplots show interquartile ranges and expected minimum and maximum values.
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In contrast to the Sudanian region, there was no significant difference in fruit production per tree (ancova, P = 0·334; Fig. 1a) between high and low harvest populations in the wetter Sudano–Guinean region. There was also no significant difference in minimum fruiting diameter between populations subject to the two harvesting intensities (Fig. 2a). However, a significantly smaller proportion of trees produced fruits in high harvest than low harvest populations (Kruskal–Wallis, P = 0·046; Fig. 2b). Trees in high harvest populations produced significantly more seeds per fruit than trees in low harvest populations (ancova, P = 0·019; Fig. 1b), but these seeds had lower mass than those produced in low harvest populations (ancova, P = 0·031; Fig. 1c).
Both the ancova to test for differences in reproductive output between regions while controlling for differences in d.b.h., pruning intensity and debarking intensity and the multiple regression analysis illustrated that pruning had a significant impact on fruit production in the Sudanian region but not in the Sudano–Guinean region (region × pruning, P = 0·0157; Fig. 1a, Tables 3 and 4). The increase in fruit production per tree with increasing d.b.h. was greater in the Sudanian region than in the Sudano–Guinean region (region × d.b.h., P = 0·0017), since trees in the Sudano–Guinean region began producing fruit at a significantly smaller size than trees in the Sudanian region (Fig. 2a). In addition, pruning caused a greater decrease in fruit production in larger trees than it did in smaller trees (pruning × d.b.h., P = 0·0191). The interaction between pruning intensity and debarking intensity was not significant (P = 0·4295).
Table 3. Analysis of covariance to test the effects of ecological region on Khaya senegalensis reproductive output with diameter at breast height (d.b.h.), tree pruning (percentage of tree crown pruned) and debarking (percentage of trunk bark removed) intensities as covariates; d.f.: degree of freedom; Asterisks represent the significance level for each term of the model: *P = 0·05; **P = 0·01; ***P = 0·001
|Source of variation||d.f.||Log10(fruit)||Seeds/fruit||Seed mass|
|d.b.h.|| 1||27·4231||0·0000012***||0.1874||0·66628|| 2·4051||0·1248907|
|Pruning|| 1||14·5489||0·0002675***||0·3173||0·57481|| 2·0464||0·1564610|
|Debarking|| 1|| 0·0486||0·8260016||0·9425||0·33456|| 0·9957||0·3213618|
|Region × d.b.h.|| 1||10·5452||0·0017032**||0·0122||0·91223|| 1·0807||0·3016786|
|Region × pruning|| 1|| 6·0858||0·0157634*||2·9647||0·08896||12·3873||0·0007164***|
|Dbh × pruning|| 1|| 5·7125||0·0191961*||0·5479||0·46133|| 0·0044||0·9472546|
|Region × debarking|| 1|| 0·3782||0·5403064||0·0004||0·98351|| 0·0499||0·8238086|
|Dbh × debarking|| 1|| 0·1613||0·6890526||1·2091||0·27481|| 2·2205||0·1401209|
|Pruning × debarking|| 1|| 0·6305||0·4295368||0·7238||0·39744|| 0·0726||0·7882350|
|Region × d.b.h. × pruning|| 1|| 0·1102||0·7407509||2·2160||0·14052|| 2·3146||0·1321039|
|Region × d.b.h. × debarking|| 1|| 0·2550||0·6149798||0·1640||0·68657|| 0·6664||0·4167398|
|d.b.h. × pruning × debarking|| 1|| 1·0348||0·3121046||0·1007||0·75182|| || |
|Residuals||80|| || || || || || |
Table 4. Linear regression model for Khaya senegalensis fruit production between two ecological regions. The independent variables are pruning intensity, debarking intensity, tree d.b.h., ecological region (used as indicator variable: region = 1 if Sudanian, otherwise, region = 0). The model tested was: log10(fruits/tree) = β0 + β1(pruning) + β2(debarking) + β3(d.b.h.) + β4(region) + β5(region × pruning) + ɛ; adj. R2: adjusted R2
|Sudanian: log10(fruits/tree) = 4·749 + 0·218 (d.b.h.) – 0·0178 (pruning)||0·367||< 0·0001|
|Sudano–Guinean: log10(fruits/tree) = 2·399 + 0·218 (d.b.h.).|| || |
Seed mass was significantly lower in the Sudanian region than in the Sudano–Guinean region (P = 0·000284). However, the pruning × region interaction was significant (P = 0·0012), as pruning increased seed mass in the Sudanian region but decreased it in the Sudano–Guinean region (Fig. 1b). There were no significant differences between regions in the number of seeds per fruit or any significant interactions between pruning or debarking intensity and region for this variable.
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- Materials and methods
K. senegalensis foliage and bark are harvested heavily by indigenous and local peoples across the Sudanian and Sudano–Guinean regions of Benin. Our results illustrate that harvest of foliage can have significant impacts on K. senegalensis reproductive performance, but that responses to harvest vary significantly between the two ecological regions.
In contrast, there were no significant effects of harvest on fruit production observed in the Sudano–Guinean region. Even when differences between regions in d.b.h., pruning and debarking intensity were controlled for (covariates) in the ancova, pruning still had a significantly greater effect on fruit production in the Sudanian region than in the Sudano–Guinean region. This suggests that the differential effect of pruning between the regions is explained at least in part by ecological differences. The longer dry season and higher rates of evapotranspiration in the Sudanian region make it a much more stressful environment for plants than the Sudano–Guinean region during the dry season. The added stress of high rates of foliage removal, which takes place specifically during the dry season, may therefore be expected to have a greater impact on trees in the Sudanian region than in the Sudano–Guinean region. The more stressful conditions of the Sudanian region may also explain why trees in this region begin to fruit at a significantly larger size than those in the Sudano–Guinean region. Other research has demonstrated that a plant's ability to compensate loss can depend on the resources available to it for recuperation (Obeso 1993), and that allocation of plant resources to reproduction may decrease under stressful conditions (Bazzaz et al. 1987; Primack 1987; He, Wolfe-Bellin & Bazzaz 2005).
However, the observed differences in response to harvest between the two regions may also be compounded by existing differences in harvesting rates and patterns between the two regions. High harvest populations in the Sudanian region have significantly higher levels of foliage harvested per tree (91·35 ± 1·83% of crown pruned) than in the Sudano–Guinean region (83·41 ± 3·92% of crown pruned) (Gaoue & Ticktin 2007). However, these differences in pruning intensity result from the migratory patterns of the Fulani, which are driven in turn by the ecological differences between the regions. During the dry season Fulani harvesters, in pursuit of both water and fodder for their herds, move southward from the drier Sudanian toward the Sudano–Guinean region, where there are better water resources available. As Fulani head south their herds become increasingly hungry, and they therefore harvest almost all the foliage of any K. senegalensis tree available along their migration corridor. This probably explains the high pruning pressures on trees in Sudanian region (Gaoue & Ticktin 2007).
In contrast, because most of the Fulani herds spend the peak and remaining part of the dry season in the Sudano–Guinan region, a greater proportion of trees are pruned in that region than in the Sudanian region (Gaoue & Ticktin 2007). This may help to explain the significantly lower proportion of fruiting trees in high harvest populations in the Sudano–Guinean region. It is also possible that differences in K. senegalensis habitat within the Sudano–Guinean region could confound some of the effects of harvest in this region.
Our results indicate that bark harvest does not contribute significantly to the observed effects of pruning on reproduction, and that there are no additive effects of combined pruning and debarking. This is probably because bark harvest intensity was fairly low across all populations (17·69 ± 7·32% of trees were debarked), and only 13·20 ± 5·45% of trees were subject to both debarking and pruning (Gaoue & Ticktin 2007).
Plants may increase the probability of successful germination and establishment by investing resources in producing a large number of seeds or by increasing the size of seeds produced, as larger seeds tend to have a better chance of germinating and producing larger viable seedlings with a better probability of establishment than smaller seeds (Kidson & Westoby 2000; Baraloto, Forget & Goldberg 2005; Moles & Westoby 2006). For K. senegalensis this trade-off between seed size and seed number appears to vary between the two ecological regions. In low harvest populations, trees in the Sudanian region produced more seeds than those in the Sudano–Guinean region, but these seeds had lower mass. In addition, the decreased rates of seed production in high harvest populations in the Sudanian region were accompanied by an increase in seed mass, while the reverse was true for the trees in high harvested populations in the Sudano–Guinean region. Further investigation will be necessary to assess whether seed mass does indeed affect germination and establishment of K. senegalensis in both regions.
implications for conservation
This study illustrates that heavy rates of foliage harvest can decrease rates and patterns of reproduction in K. senegalensis, and that the impacts may vary significantly across differing environmental and/or harvesting contexts. Lowered rates of reproduction may lead to decreases in K. senegalensis population size over the long term. However, this is difficult to test in K. senegalensis and other NTFP species like it that are subject to multiple sources of disturbance. For instance, other research has illustrated that, in some areas, high harvest populations of K senegalensis have lower densities of seedlings and saplings (Gaoue & Ticktin 2007), but these results are difficult to interpret as K. senegalensis populations are subject to many other confounding disturbances that also affect seedling germination and survival, including grazing and dredging.
None the less, reports from local Fulani harvesters indicate that populations are decreasing, due most probably to the logging pressure to which this species is also subject. Logging would contribute to a decrease in fruit production at the population level, as it decreases the number of reproductive individuals. In addition, the increasing populations of Fulani harvesters migrating into Benin from other neighbouring countries, and the increasing drought in the region, are leading to increasing harvesting pressure of K. senegalensis. This can be expected to increase the negative effects of harvest. There is therefore a need to develop better ex situ conservation strategies for K. senegalensis in order to provide other sources of fodder to Fulani cattle. Our results indicate that priority should be placed clearly in the Sudanian region, where the impacts of pruning on K. senegalensis, regardless of harvest intensity, are more severe. This may be achieved most effectively through the promotion of K. senegalensis plantation programmes involving Fulani harvesters. K. senegalensis plantations (although for timber) have been established successfully in other parts of the country.
To date, most studies of the effects of NTFP harvest have reported results from less than three populations (Ticktin 2004). Our work underscores the great importance of assessing the impacts of NTFP harvest in a range of locations and environmental conditions in order to design effective conservation and management plans.