Rehabilitation of forest-savannas in Ghana: The impacts of land use, shade, and invasive species on tree recruitment

Applied Geography 31 (2011) 181e190

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Rehabilitation of forest-savannas in Ghana: The impacts of land use, shade, and invasive species on tree recruitment Louis Awanyo a, *, Emmanuel Morgan Attuah b, Michelle McCarron c a

Luther College, University of Regina, Regina, Saskatchewan, Canada S4S 0A2 Department of Geography and Resource Development, University of Ghana, Legon, Ghana c Department of Psychology, University of Regina, Regina, Saskatchewan, Canada S4S 0A2 b

a b s t r a c t Keywords: Agricultural land use management Chromolaena odorata Forest-savanna Ghana Tree recruitment

More than 90% of the closed canopy forests and their immense biodiversity in the forest-savanna ecological region of Ghana have been cleared since the late 19th century. Insights into the opportunities and the challenges of recruiting trees for the rehabilitation of these forests are much needed. This article examines the potential for the natural recruitment of trees from the soil seed bank following various types of agricultural land uses and conditions associated with them in the SuhumeKraboaeCoaltar district of the Eastern Region. Seedling recruitment data from soil seed banks are interrogated with Repeated Measures Analyses of Variance, and these data show that, first, tree life forms are not significantly greater than other life forms, and that in fact tree life forms are the minority in the conditions of the examined agricultural land uses. Second, the analyses indicate that the natural recruitment of tree seedlings for tree rehabilitation confronts enormous competition from non-tree species. The herb/shrub species, Chromolaena odorata is identified as a primary factor for the difficulties of tree recruitment. Drawing on these findings and the detailed narratives of farmers, the article submits that the challenge for the natural recruitment of trees in the study region is for farmers to adopt land management practices that significantly increase the numbers of tree species while reducing the competition from non-tree species, such as C. odorata. Ó 2010 Elsevier Ltd. All rights reserved.

Introduction Ninety percent of the closed canopy forest in the southern forest-savanna transition ecological region of Ghana (Fig. 1) has been cleared for agriculture, timber, and other land uses since the beginning of the 20th century (Agyepong & Kufogbe, 1997; The Republic of Ghana, Ministry of Food and Agriculture, 1999).1 While governments of Ghana and external donors have funded programs for rehabilitating forests, recent research indicates that

tree species continue to be threatened and endangered by land uses in the southern forest-savanna region (Awanyo, 2007; Gyasi, 2002).2 An approach with potential for addressing this problem is the economic and ecologically-effective self-design approach to rehabilitation and restoration. Advocated by some restoration ecologists, this approach suggests that “species re-establishment is a fate determined by the environment and that succession ultimately determines species composition” (Middleton, 2003: 1026). But local ecological knowledge (LEK) conveyed to the authors by

* Corresponding author. E-mail address: [email protected] (L. Awanyo). 1 The 1994 Republic of Ghana Forest and Wildlife policy and its Forestry Development Master Plan (1996e2020), and the 10-year (1999e2008) National Resource Management Program (NRMP) all aim “to protect, rehabilitate and sustainably manage national land, forest and wildlife resources and to sustainably increase the income of rural communities who own these resources” across Ghana (World Bank, 1998: 1; The Republic of Ghana, Ministry of Lands and Forestry, 1994).

2 Three propositions are important for strategies aimed at reversing the trend of loss of tree species. The first proposition is the observation that “natural regeneration is generally much cheaper than planting trees” (Honu & Dang, 2002: 186). A second well-documented assertion worth noting is that the natural recruitment of trees has the potential to contribute to the biodiversity rehabilitation and restoration of degraded forest ecosystems, in contrast to the human-assisted single species (or limited species) approach which has significant ecological and socioeconomic shortcomings (see for instance Robbins, 2001; Hall, 1996). Third, the notion that local farmers have rich local ecological knowledge (LEK) about the processes of vegetation succession and about the agricultural land uses that naturally recruit a heavy density of trees, and those that do not, must be taken seriously (Amanor, 1996; Awanyo, 2007, 2009).

0143-6228/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeog.2010.04.002

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Fig. 1.

farmers in the forest-savanna transition region supports Middleton (2003: 1026) who emphasizes that the “self-design fails to acknowledge that pivotal species [or formerly dominant species] may be absent [or may be in insufficient numbers] in restoration sites due to a lack of seeds.”3 Informed by LEK, field research was initiated in farming communities in the southern forest-savanna ecological region of

3 Four insights from anecdotes by farmers in the forest-savanna region about the natural recruitment of trees in their three predominant agricultural land use/land cover of forest fallows, the perennial cacao farming, and annual food crop farming are illuminating. First, LEK indicates that the successful natural regeneration of a heavy density of trees is associated with land use practices which first and foremost enhance the natural availability and recruitment potential of tree seeds and seedlings compared to non-tree life forms. And while LEK acknowledges the variations in the natural recruitment of tree seedlings under the three predominant agricultural land uses/land cover, all three are acknowledged to have tree recruitment potential. Second, farmers insist that the greater availability of tree seedlings compared with other species in any agricultural land use is especially pivotal in the forest-savanna context where the dominance of invasive species, primarily the herb/shrub Chromolaena odorata, has increasingly undermined the recruitment of trees. Indeed, farmers in this region as well as researchers working in other parts of Ghana (Honu & Dang, 2000) indicate that the removal of the invasive species C. odorata and other weeds leads to more abundant tree seedling germination from seeds and tree seedling release. Farmers in this region point to the role of such efforts in their successful extension of forests. Third, some farmers are advocates of tree recruitment strategies that manage the forest fallow and cacao spacing strategies on cacao fields to provide more sunlight on soil seed banks, which they suggest enhances the germination of tree seedlings from seeds compared with non-tree species such as C. odorata. Fourth, some farmers, in opposition to the previous view, argue that the partial shading of soil seed banks by trees in the forest fallow and cacao farming land uses has little effect on the germination of tree seedlings and actually gives tree seedlings a competitive edge over non-tree species including C. odorata. Thus, they recommend the incorporation of trees on annual food cropping land uses to provide the shading conditions that will in turn increase the natural recruitment of trees.

Ghana to investigate the varying potential of soil seed banks of the predominant agricultural land uses for the recruitment of trees from seeds and seedlings (compared to non-tree species). This investigation occurred in a context where there is evidence that the invasive non-tree species, C. odorata, is increasingly becoming dominant. The following research questions were addressed: 1. What are the natural recruitment potentials for tree species from the soil seed bank, compared with non-tree species, in the three predominant agricultural land uses of forest fallows, the perennial cacao farming, and annual food crop farming? 2. With regard to the forest fields and the cacao fields with partial shading of the soil seed bank, would more sunlight break overall seed dormancy and lead to more germination of tree seeds relative to non-tree seeds, or do seeds, in general, germinate equally well in both shade and full sunlight? 3. With regard to the food crop fields with very little shading of the soil seed bank, would more shade break overall seed dormancy and lead to more germination of tree seeds relative to non-tree seeds, or do seeds, in general, germinate equally well in both shade and full sunlight? The sources for the natural rehabilitation of trees from these land uses are multiple and include, in addition to the soil seed bank, seed rain, dispersal from nearby trees and forests, and from the sprouting and vegetative reproduction of trees from roots and stem coppice shoots. However, the germination of tropical tree species seedlings from seeds in the soil seed bank has become an important focus given its important role in the “persistence of the species of populations” (Vásquez-Yanes & Orozco-Segovia, 1993: 70). The present study also acknowledges the wide variety of intertwined environmental influences that drive the germination and growth of tropical tree seedlings from seed sources for the natural recruitment of trees. These multiple conditioning environmental factors include soil moisture, sunlight irradiance and its influences on both soil moisture content and soil temperature and their fluctuations, and fire. Other factors include soil nutrients, soil fertility, and herbivory (Khurana & Singh, 2001; Kyereh et al., 1999; Swaine, 1996; Whitmore, 1996). This study’s focus on sunlight/shading conditions was driven by LEK and its contentions about the photoblasticity of seeds in the study area. The importance of this environmental factor has been widely noted in the ecological literature (Burslem, 1996; Hall & Swaine, 1980; Khurana & Singh, 2001; Kyereh et al., 1999; Press et al., 1996; Whitmore, 1996). The study region Three farming communitiesdWhanabenya, Amanase, and Trayodin the SuhumeKraboaeCoaltar district of the Eastern Region of Ghana, located in the southern forest-savanna ecological region (Fig. 1) and with a total population of 4296 (The Republic of Ghana Statistical Service, 2002), were the sites for addressing the research questions. The patterns of land use/cover in this ecological region are broadly similar and are conditioned by the biophysical environment. Mean annual precipitation is 1260 mm, and the rainfall is bimodal. Soils are predominantly forest ochrosols on which extensive semideciduous canopy forests once grew (Dickson & Benneh, 1977: 23e44). Prior to 1850, this ecological region had large tracts of “virtually uninhabited virgin high forest” (Gyasi, 1997: 84). Agricultural land use in this region was based on principles of agro-forestry set within the bush fallow system of alternating between short periods of cultivation and long fallows. Commercial production of (the tree crop) oil palm (Elaeis guineensis) for export during the colonial period started a trend of widespread alteration and exploitation of forests to make room for larger farms. The decline in world oil palm prices encouraged a shift to another tree crop, cacao (Theobroma

L. Awanyo et al. / Applied Geography 31 (2011) 181e190

cacao) (Amanor, 1994; Hill, 1963). An even more extensive commercial production of cacao for export from the early 1890s to the 1940s colonial period marked a phase of further disturbance of forests and their biodiversity. The decimation of cacao farms by disease led to another focus in this region but at a larger cost to tree species and biodiversity. After the 1940s, agricultural land use increasingly focused on annual food crops, mainly cassava (Manihot esculenta), maize (Zea mays), plantain (Musa spp.), and yam (Discoria spp.), and this region evolved to become part of the commercial domestic food crop belt of Ghana (Dickson & Benneh, 1977: 154e8). The cultivation of these mainly annual food crops as in the past continues to be primarily based on the use of simple tools and intensive labor (Gyasi, 1997, 2002). The shifting cultivation/bush fallow system (particularly of the slash-and-burn variety) associated with food crop farming is viewed as a key driver of the decline in tree species and their forest habitats that support the biodiversity of the region, because this land use involves the almost complete removal of trees (Conservation International, 2000; World Wildlife Fund, 2001). Currently, the three major/most extensive types of agricultural land use/cover in the study region (Agyepong & Kufogbe, 1997) are: cacao fields, fields associated with current/recent food crop cultivation (farms under current cultivation/fallows of recently abandoned food crop farms), and broken-canopy forest fallows (that were disturbed by agricultural activities in the distant past). The soil seed bank and the environmental factors which shape tree recruitment have been driven by these agricultural land uses/cover since the 19th century.

Materials and methods LandSat ETMþ (Enhanced Thematic Mapper Plus) satellite images for 1990 and 2000 were analyzed with geographic information systems (GIS) techniques to provide area statistics and to illuminate landcover/use changes. The supervised classification method of the Maximum Likelihood Classifier module of IDRISI (15.0) Andes GIS and image processing software program was used for land cover classifications. Classifications were checked with global positioning satellite (GPS)-assisted field surveys. Statistics were generated for the land cover classes using the same software. Land cover change detection involved a postclassification procedure of using the software’s AREA module for comparing the area statistics for each of the land cover classes in 1990 and 2000. Much of the focus of field work was to illuminate the LEK of farmers in the study region about the potential for the natural recruitment of trees from vulnerable newly-germinated tree seedlings by exploring the effects of two factors on tree seedling recruitmentdsoil seed bank conditions associated with three predominant land uses and sunlight (or shading conditions). Nine fields of farmers in each community (n ¼ 27) provided the setting for the experiment on tree recruitment potential. Three of these nine fields (in each community) were cacao fields planted in 1984 (and which provided partial shading of soils). Three fields were forests of at least 10 years of fallow following a 2-year cultivation period (and which also provided partial shading of soils). At the beginning of the field experiment, the other three fields were either just beginning their regular cycle of 3-year fallow phase following 3 years of cultivating maize and cassava or in the final year (third year) of food crop cultivation (with very little shading of soils). In the latter stages of the field experiment, the food crop field, in its final year of cultivation at the beginning of the experiment, transitioned to a field just beginning its fallow phase. Experimental data were thus based on three

183

replicates each of the categories of food crop/recent fallow field, cacao field, and forest field in each community.4 A sampling plot of 1 hectare (100 m  100 m) was established on each field. Three soil samples, to a depth of 20 cm, were collected with an auger (912 cm3 of soil) from each of twenty-five random 1 m2 quadrats on each sampling plot during each of four periods from November 2001 to October 2002. This staggering of sampling was meant to capture the potential different periods of seed dispersal, and dormancy and germination of seeds (Khurana & Singh, 2001: 40e42; Kyereh et al., 1999). Soil samples were collected during the dry season (late November), beginning of the rainy season (March and April), during the major rainy season (May and June), and during the minor rainy season (September and October). The collected soil samples from each repetition (a total of nine replicates) in a particular agricultural field category (the primary treatment) and for each period were composited. For each replicate in each category, the soils collected in each time period were thoroughly-mixed, air dried, and passed through a sieve to remove root fragments, large stones, and other “foreign” material, and placed in seed boxes/trays, each measuring 35 cm  28 cm  18 cm, to a depth of 16 cm. Eight soil seed boxes (two for each sampling time period) for each replicate of land use field were established. From the 216 soil seed banks in the 216 seed boxes, the study sought to explore the life form distribution of germinating plants, and in particular to uncover the potential for the germination of tree species relative to that of other life forms. Another eight trays (two for each sampling period) were used as controls. These control trays contained steam sterilized soil only. Secondary treatments were then imposed on the seed stocks and the control. To capture the relative effects of the conditions of shading and sunlight within the financial constraints of this study and to assess LEK about photoblasticity and how human-assisted strategies for shading/sunlight could accelerate tree recruitment, two secondary treatments (shaded versus open/un-shaded) were applied to the collected seed stocks and controls. Half of the soil seed boxes and controls from each sampling period and for each replicate of a land use field was placed under shade of trees (>70% overstorey aerial cover cast by trees/foliage cover, that is less than 30% irradiance) in a canopy forest about 400 meters from the home of a key respondent in Amanase, and the other half was left in an un-shaded area (<30% overstorey aerial cover/foliage cover, that is more than 70% irradiance) near the same respondent’s home. To control immigration and in-loco deposition of seeds, each seed box was covered with a transparent 1 mm mesh sieve. Soils in each box were kept moist by watering when necessary, and all soils were watered whenever watering was necessary for anyone. Germinating seedlings were identified as they emerged from the seed boxes, at 3-day intervals. They were then counted, and then removed. After removal of seedlings, soils were stirred every two weeks to stimulate the germination of seeds in the lower soil layers in the seed boxes. Observations of the germinating life forms continued from November 2001 till end of January 2003, when monitoring of germination ceased following two weeks of no further germination of seeds. Repeated Measures Analyses of Variances (ANOVAs) were used to analyse the effects of the conditions/factors of land use/cover type (forest field soil, cacao field soil, and the food crop field soil) and shading on germinating plant species. In these analyses, species (“subjects”) were classified as trees, herbs and shrubs, or “other” species life form. The analyses tested whether seedlings vary significantly across six repeated measures (conditions of land use/cover

4 These three categories of fields and replicates were carefully selected, after extensive interviewing of ninety farmers, to ensure that fields within each category shared a similar land use and land management history. Twenty-seven of these farmers (with fields of interest to the researchers) voluntarily offered their fields for the study.

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Table 1 Average species seedling counts by land use/cover and shading treatments. Species

Acacia kamerunensis Ageratum conyzoides Albizia sp. Alchornea cordifolia Antiaris toxicaria (Africana)* Baphia sp. Boerhavia diffusa Capsicum annuum Cardiospermum halicacabum Carica papaya* Ceiba pentandra* Centrosema pubescens Chromolaena odorata Clausena anisata Commelina capitula Cyperus rotundus Desmodium adscendens Digitaria diagonalis Elaeis guineensis* Elytraria marginata Euphorbia heterophylla Euphorbia prostrata Fleurya aestuans Fleurya ovalifolia Ficus exasperata* Funtumia sp.* Gongronema latifolium Griffonia simplicifolia Hypselodelphis violacea Ipomea sp. Lantana camara Lantana taraxacifolia Mallotus oppositifolius Mansonia altissima* Momordica charantia Panicum maximum Paspalum orbiculare Phyllanthus amarus Pouzolzia guineensis Pueraria phaseoloides Rauvolfia vomitoria* Setaria sp. Solanum torvum Sporobolus pyramidalis Talinum triangulare Trichilia sp.* Tridax procumbens Xanthosoma mafaffa Zanthoxylum gilletii*

Cacao farm

Cacao farm

Food crop farm

Open (36 soil seed boxes)

Shade (36 soil seed boxes)

Open (36 soil seed boxes)

Shade (36 soil seed boxes)

Open (36 soil seed boxes)

Shade (36 soil seed boxes)

2 1 0 1 11 0 0 4 5 7 5 0 74 0 0 0 0 0 0 0 0 0 11 19 0 0 2 2 7 0 2 0 0 0 0 0 0 3 0 0 0 0 4 0 0 2 0 0 2

0 2 0 0 8 0 0 1 2 6 8 0 68 1 0 0 0 0 0 0 0 0 13 16 0 0 0 0 6 0 0 0 2 0 0 0 0 0 0 0 0 0 4 0 0 1 0 0 3

3 0 1 2 8 1 0 2 0 3 5 2 42 0 27 0 12 4 0 2 0 0 17 15 0 0 0 1 0 1 2 0 0 0 0 4 2 0 2 2 3 0 6 24 12 1 0 21 0

2 2 3 4 6 0 0 6 0 3 6 4 33 0 20 0 18 2 0 2 0 0 19 10 0 1 0 1 0 2 0 0 1 2 0 1 0 0 1 2 4 0 10 17 16 0 0 16 0

0 29 0 0 0 0 16 0 0 0 7 0 36 0 12 17 0 6 2 0 107 8 13 7 6 0 0 0 0 0 0 9 0 0 3 4 6 0 12 2 1 16 0 0 15 0 22 11 0

0 37 0 0 0 0 12 0 0 0 4 0 28 0 9 6 0 4 0 0 92 11 15 13 2 0 0 0 0 0 0 6 0 0 4 2 0 0 14 0 2 6 0 0 12 0 19 13 0

*Denotes trees species. Source: Fieldwork.

and shading) and across life forms, and whether there are interactions among them.5 Hotelling’s Trace statistic was used for within-subjects variables (Gardner, 2001). When significant differences in species life forms were observed for main effects involving more than two factors/conditions, pairwise comparisons using the Bonferroni statistic (with necessary adjustments for multiple comparisons) were calculated. When significant differences were observed for

5 The between-subjects variables are species life forms. These variables subdivide the sample into discrete subgroups. Trees, herbs and shrubs, and “other” species life forms are the subjects. The within-subjects (repeated measures) variables are conditions/factors whose levels are all measured on the same subject. Each subject in the study was observed (measured) in six conditions: open forest field soils, shaded forest field soils, open cacao field soils, shaded cacao field soils, open food crop field soils, and shaded food crop field soils. Mauchley’s test of sphericity was violated for both Repeated Measures ANOVAs performed for this study; therefore, the more conservative HuyhneFeldt statistics were used.

interaction effects, Tukey’s Honestly Significant Difference (HSD) for unequal Ns was used to determine where the significant differences lay (Gravetter & Wallnau, 2002). Primary social research complemented ecological research. A referral/purposive sampling survey of 90 farmers and in depth interviews of 15 key respondents occurred from October 2001 to August 2004, with follow-up interviews in December 2009 to March 2010. This research provided detailed insights into LEK about vegetation succession following agricultural land uses and factors of tree recruitment. Results Land use/cover changes Satellite image processing and GIS analysis for the Suhume KraboaeCoaltar district as a whole showed that cacao farms and

L. Awanyo et al. / Applied Geography 31 (2011) 181e190 Table 2 Tree and “other” life form seedling counts by land use/cover and shading treatments. Life form

Forest

Cacao farm

Food crop farm

Open Shade Open Shade Open Shade Tree Others Total Of which: Chromolaena odorata (herb/shrub) Euphorbia hetrophylla (Herb)

27 137 164

26 115 141

20 207 227

22 192 214

16 351 367

8 303 311

74 0

68 0

42 0

33 0

36 107

28 92

Source: Fieldwork.

fallowing fields with trees and undergrowth of shrubs and herbaceous plants comprised 46% of total land cover/use in 1990 and 45% in 2000. GPS-assisted field surveys indicated the dominant presence of C. odorata herb/shrub in the undergrowth of this land cover class. Broken canopy forests/open forests were 17% and 23% of total land cover/use in 1990 and 2000, respectively. Food crop fields and recently abandoned food crop fields under fallow were 34% and 29% of land cover/use in 1990 and 2000, respectively. These data support farmers’ insistence that they actively assist in the recruitment of trees from seedlings and other sources by such management practices as removing the competition of C. odorata and other weeds, and thereby increasing the size of forests. GIS analysis suggests broken canopy forests increased by 5314 hectares from 1990 to 2000. LEK also indicates that the recruitment of trees from the soil seed bank associated with the major land uses is a critical component of the expansion of forests and the density of trees. The field experiment of the present study helps to illuminate the potential of the different land uses/cover for recruiting trees from the soil seed bank. Seedling life forms and frequencies No germinants were recorded in the control trays. Table 1 is the species seedlings count, and Table 2 summarizes seedling life form counts on the land use/cover treatments by their shading treatments. Tree seedlings range from 3% of total life forms (in shaded food crop farm soils) to 18% of total life forms (in shaded forest cover soils). To meet the goal of tree rehabilitation of the forest-savanna ecological region, trees should dominate and the numbers of trees should be significantly greater than other life forms. In mature forests, for instance, 90% of the species life forms are trees (Hopkins, 1965: 22e28; Lawson, 1966: 10e16). But as Table 2 shows, trees are the minority life forms in the study area. This seems to support the farmer narratives that human-assisted tree recruitment is critical. The repeated measures ANOVA “between-subjects” main effects (Table 3) suggest that the means of the seedling counts of individual species when grouped by life forms (trees and “other”) do not significantly vary across the life forms (p ¼ 0.418).6 However, an overview of Tables 1 and 2 does show that there are some species that stand out in terms of seedling counts, for instance the herb/ shrub Chromolaena odorata and the herb Euphorbia heterophylla. The former is prominent in all the treatments, whereas the latter is limited to the food crop farm soil treatments. This observation, supported by widespread farmers’ narratives that indicate the ubiquity of C. odorata and its impact on tree recruitment, underscores the importance of separating this species as a “subject” on its own in the analyses. The second repeated measures ANOVA “between-subjects” main effects in which C. odorata is not included

6

F(1, 47)¼0.668, p ¼ 0.418.

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Table 3 Repeated measures ANOVA, between-subjects effects (C. odorata included with herbs and shrubs). Source

Type III sum of squares

df

Mean square

F

Sig.

Intercept Life form Error

3253.700 279.890 19,691.579

1 1 47

3253.700 279.890 418.970

7.766 0.668

0.008 0.418

Table 4 Repeated measures ANOVA, between-subjects effects (C. odorata not included with herbs and shrubs). Source

Type III sum of squares

df

Mean square

F

Sig.

Intercept Life form Error

15,531.309 10,883.140 9088.329

1 2 46

15,531.309 5441.570 197.572

78.611 27.542

<0.001 <0.001

in herbs and shrubs (Table 4) reveals a significant variation across the life forms (p < 0.001).7 The significant effect that arises as a result of isolating C. odorata as a subject supports the LEK which points to the need to centrally place this species in any strategy of tree recruitment. And whatever the strategy is, it is clear that the recruitment of tree seedlings confronts enormous competition from non-tree species. The effects of land use/cover and shading on mean numbers of seedling life forms For further interrogation of the recruitment of tree seedling life forms and the effects of other life forms and the species C. odorata on their recruitment, the repeated measures of them in terms of their mean numbers within different soil and shading treatments are explored in repeated measures ANOVA within-subjects effects. With C. odorata included in herbs and shrubs, soil types, shading conditions, and the interactions between them and life forms did not lead to any significant differences in the mean numbers of seedlings among the treatments (Table 5). Thus, while nature can begin the process of recruiting trees from the soil seed bank, the process would have to be backed-up with interventions that significantly increase the mean number of tree seedlings compared to non-tree species. As C. odorata is an important species in the study area, the second repeated measures ANOVA within-subjects effects isolated this species (Table 6). The main effects of land use/cover condition show that there are significant differences in the land use/cover (forest field, cacao field, food crop field) conditions in their potential for the natural recruitment of plant species (p < 0.001).8 An initial interpretation would be that one land use type (or more than one land use type) would be significantly more favourable for the natural recruitment of some species life form relative to others (or relative to another species life form). The main effect of the shading treatment also indicates that shading conditions (shade/open) have significantly different potentials for naturally recruiting plant species (p < 0.001).9 This observation could also be initially interpreted to mean shading condition could be manipulated to enhance the natural recruitment of some species life form relative to others (or

7

F(2, 46)¼27.542, p < 0.001. Hotelling’s T2 (2, 45)¼11.406, p < 0.001. This statistic indicates that there significant differences in the average number of seedlings observed across different land use/cover (forest field, cacao field, food crop field) conditions. 9 Hotelling’s T2 (1, 46)¼26.651, p < 0.001. This statistic indicates that there significant differences in the average number of seedlings observed across shading conditions (shaded or open). 8

are the are the

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Table 5 Repeated measures ANOVA, within-subjects effects (C. odorata included with herbs and shrubs). Source Landuse Landuse  life form Shading Shading  life form Landuse  shading Landuse  shading  life form

Hotelling’s Hotelling’s Hotelling’s Hotelling’s Hotelling’s Hotelling’s

Trace Trace Trace Trace Trace Trace

Value

F

Hypothesis, df

Error df

Sig.

0.028 0.016 0.055 0.005 0.016 0.006

0.643 0.361 2.594 0.245 0.360 0.130

2.000 2.000 1.000 1.000 2.000 2.000

46.000 46.000 47.000 47.000 46.000 46.000

0.530 0.699 0.114 0.623 0.699 0.878

relative to another species life form). The interaction between land use type and life form, and between shading condition and life form are also significant (p < 0.001, respectively), while all other interactions are not significant. In instances of significant differences in the mean numbers of seedling life forms in the varied treatments, where the significant differences actually lie are revealed with Tukey’s Honestly Significant Difference (HSD). Table 7 summarizes the interaction between life form and land use type. There is a significant difference between the mean number of tree seedlings (2.40) and the mean number of C. odorata seedlings (71) in forest cover type. To achieve the goal of tree rehabilitation, tree recruitment must be dominant. However, the natural recruitment of trees into such dominance is being obstructed, and at least one reason for this obstruction is the competitiveness of C. odorata. C. odorata also out-competes other herbs and shrubs in forest cover types of soils. In all the other land use types there are no significant differences in the mean numbers of seedling life forms. This finding also does not bode well for tree recruitment. While non-tree species are not statistically significantly dominant, neither are tree species. The interactions between life form and shading condition are described in Table 8. C. odorata’s prominence is again revealed. Both in the open and in the shade, the mean numbers of C. odorata seedlings are significantly greater than that of tree seedlings, with the mean number of tree seedlings in the open reaching 3.10 seedlings, while C. odorata’s is 50.67 seedlings. The numbers of seedlings in the shade for both trees and C. odorata are fewer, but again C. odorata seedlings are significantly greater than tree seedlings. Local ecological knowledge about vegetation succession and farmerassisted tree recruitment in the context of C. odorata In lieu of a long term study of vegetation succession from seedlings’ establishment to the competitive interactions among species, and the multiple factors that induce vegetation changes following agricultural land uses, the study draws on in depth interviews with farmers about their LEK and hands-on experiences

Table 6 Repeated measures ANOVA, within-subjects effects (C. odorata not included with herbs and shrubs). Source

Value F

Landuse

0.507 11.406 2.000

45.000

<0.001

0.855

9.401 4.000

88.000

<0.001

0.579 26.651 1.000

46.000

<0.001

0.454 10.442 2.000

46.000

<0.001

0.027

0.608 2.000

45.000

0.549

0.040

0.445 4.000

88.000

0.776

Hotelling’s Trace Landuse  life form Hotelling’s Trace Shading Hotelling’s Trace Shading  life form Hotelling’s Trace Landuse  shading Hotelling’s Trace Landuse  shading Hotelling’s  life form Trace

Hypothesis, Error df Sig. df

to interpret the long term process of tree recruitment in the context of competition from invasive species such as C. odorata. Local ecological knowledge (LEK) views the recruitment of trees on farms as beginning with the wind dispersal of seeds from nearby trees and forests, in situ germination of seeds in the soil, and from the sprouting and vegetative reproduction of trees from roots and stem coppice shoots. The principal environmental factors listed by farmers as the greatest single factor (one response only) that drives the germination, establishment and growth of tree species are fire (24%), soil moisture (23%), soil fertility (22%), sunlight/shade (22%), and herbivory (8%). The environmental factors that farmers listed as the greatest single factor that drives tree recruitment (and over which they have the most control) are sunlight/shade (54%), soil fertility (28%), soil moisture (11%), and fire (7%). As indicated above, farmers view the availability of tree seedlings from a seed source in the soil as an important component of tree seedling establishment and tree recruitment. Competition from C. odorata in particular is stressed in farmers’ narratives as a major cause of the “demographic squeeze” and the mortality of tree seedlings, or their slow growth. One group of narratives insist that seeds will germinate and tree seedling release will occur if the shading effects of other trees and C. odorata are reduced, leading to greater recruitment of trees.10 Intensive weeding of C. odorata, transplanting of seedlings, pruning of branches, and thinning of trees are among the strategies for increasing sunlight. These strategies are claimed to have

10 Mr. Boah described the processes of vegetation succession and tree recruitment as follows: “My father cultivated food crops a few times on this land before he changed to cocoa [in 1984]. When I was gifted the cocoa farm [in 1986] there were not many trees. There was a lot of Acheampong [C. odorata] on the farm. I worked hard and now there are more trees on the farm. The trees and the cocoa go well together because I still use the old type of cocoa [amelonado variety]. I want more trees to grow on the farm so that I can use some of it as fuelwood. When the cocoa trees are too old to produce cocoa, I want the land to be kwae [forest]. As I work on the farm, I see many tree seeds. Many of them will stay [remain] in the soil [without germination] and some of them will germinate, but most of them die in a few months. When I started looking after [nurturing] trees [in 1986], at the beginning, there was a lot of Acheampong and some sare [Panicum maximum] and not a lot of tree seedlings. Acheampong fights [competes] with them and takes the soil food [nutrients].. Sunlight doesn’t touch the seeds and seedlings and so they can’t grow.. When I weed Acheampong and there is sunlight more trees can grow.. When I cut [prune] the branches of other trees and clear Acheampong and the other weeds, more sunlight touches the soil and the seeds of the trees can germinate. In the first year, I had to do a lot of weeding to take care of the growing trees because there was a lot of Acheampong. If I don’t cut them the weeds will grow faster than the young trees. At the end of the second year because the trees were growing quickly, they were stronger than Acheampong.. Onyina [Ceiba pentandra] and Kakapenpen [Rauvolfia vomitoria] grow very quickly.. There were fewer Acheampong and the trees could grow faster. You could not even find any sare on the farm. In the next, about 2 years [third and fourth years] my farm was like a small kwae [forest] with many young trees. When I cut the branches of the trees and there is more sunlight, more trees can grow and some of the young trees like Kyenkyen (Antiaris toxicaria), Okuro (Albizia zygia) grow very quickly. Sometimes I dig out and plant the growing tree somewhere else [transplant] where it can get more sunlight. When fast growing young trees and cocoa trees are too close together I have to cut some of them [thinning] so that the sunlight will help many different types of trees to grow on the farm. I do all of this hard work. This is why, today, you can see many trees on my cocoa farm. My cocoa farm is in a forest.. The forest is 16 years old” (Interview at Amanase with Mr. Boah, 21st October, 2002).

L. Awanyo et al. / Applied Geography 31 (2011) 181e190 Table 7 Tukey’s HSD for life form  landuse (C. odorata not included with herbs and shrubs).

Table 8 Tukey’s HSD for life form  shading (C. odorata not included with herbs and shrubs).

1

2

Trees, forest

Trees, cacao Trees, food crop Herbs/shrubs, forest C. odorata, forest

2.40 2.40 2.40 2.40

3.95 2.40 1.51 71.00

0.116 0.000 0.066 5.133**

Trees, cacao

Trees, food crop Herbs/shrubs, cacao C. odorata, cacao

3.95 3.95 3.95

2.40 3.78 37.50

0.116 0.013 2.511

Trees, food crop

Herbs/shrubs, food crop C. odorata, food crop

2.40 2.40

7.45 32.00

0.338 2.215

Herbs/shrubs, forest

Herbs/shrubs, cacao Herbs/shrubs, food crop C. odorata, forest

1.51 1.51 1.51

3.78 7.45 71.00

0.169 0.444 5.200*

Note: MSW ¼ 38.074, Nh ¼ 7.9915, df ¼ 46. *p < 0.05; **p < 0.01.

Herbs/shrubs, cacao

Herbs/Shrubs, food crop C. odorata, cacao

3.78 3.78

7.45 37.50

0.275 2.524

succession process. How shading assists in this recruitment is contested among the farmers.

7.45

32.00

1.837

71.00 71.00 37.50

37.50 32.00 32.00

2.520 2.918 0.412

Herbs/shrubs, food crop C. odorata, food crop C. odorata, forest C. odorata, cacao

C. odorata, cacao C. odorata, food crop C. odorata, food crop

Mean 1 Mean 2 q

187

Note: MSW ¼ 951.248, Nh ¼ 5.3267, df ¼ 88. *p < 0.05; **p < 0.01.

increased the numbers and densities of trees and extended forests in the study area. Another group of narratives focus on the challenge of C. odorata on food farms and suggest that increasing the shading on food crop farms will reduce the competition from C. odorata which would in turn increase the rate of establishment, survival and growth of tree species. The strategy employed for increasing shading is one that carefully nurtures the growth of trees from the seedling stage within food farms, so that trees have a head start during fallows. A pivot of the strategy is intensive weeding of C. odorata and other weeds for the survival and growth of tree species. As the young trees and the food crops grow, their shading effect further reduces the competition from weeds and enhances the growth of trees, which leads to a rapid development of forests.11 The common thread in both narratives is that human interventions and management of C. odorata and other weeds, like Euphorbia hetrophylla, are critical for recruiting trees in the vegetation

11 Mr. Afwireng’s testimony is as follows: “Acheampong is everywhere. When you see a lot of Acheampong on a farm, you have to do a lot of weeding.trees cannot grow well there.. We need trees for firewood/fuelwood and many things so you [we] need many trees on your [our] farm[s] at all times.. When I start the farm, I cut some of the trees to grow cocoyam, yams, plantain, cassava, vegetables, and oil palm. Acheampong, sare (Panicum maximum) grow very quickly in a few months and fight with the food I am growing. I weed carefully to remove Acheampong so that the crops I have planted will grow well. The shade from the big trees (left on the farm) and the growing plants is very good because it bothers [slows the growth of] Acheampong and allows the seeds of trees to germinate.. I advised my friend whose farm was spoilt (overworked soils/degraded soils] and so had a lot of juice [Euphorbia heterophylla] to work on his farm and help the trees to grow and provide shade. The shade will reduce the juice on his farm and the small trees will grow quickly.. I work very hard in the first year I make a farm so that the small trees can grow quickly. It is very hard because the weeds grow very quickly in the first year. If you don’t work hard the crops will not grow well. If you don’t cut the weeds no tree will grow. Things are better in the second year because the broad leaves of the cocoyam [Colocasia esculenta], plantain [Musa spp.], and cassava [Manihot esculenta] and the trees on the farm help the small trees to grow because the shade makes Acheampong and sare, and all the other weeds weak. By the time we are in the third year the young trees grow well and they are many. The trees bring a lot of shade over the plantain and cocoyam and the weeds during the major rains [particularly in May/June/July of the third year] and they [crops and weeds] begin to die. The trees are more than Acheampong because the shade kills Acheampong.. When I leave the trees on the land to fallow, the land will become kwae [forest] in seven years” (Interview at Whanabenya with Mr. Afwireng, 6th July 2004).

1

2

Mean 2

q

Trees, Open

Trees, shade Herbs/shrubs, open C. odorata, open

Mean 1 3.10 3.10 3.10

2.73 4.50 50.67

0.168 0.641 21.792**

Trees, shade

Herbs/shrubs, shade C. odorata, shade

2.73 2.73

3.99 43.00

0.569 18.448**

Herbs/shrubs, open

Herbs/shrubs, shade C. odorata, open

4.50 4.50

3.99 50.67

0.233 21.150**

Herbs/shrubs, shade C. odorata, open

C. odorata, shade C. odorata, shade

3.99 50.67

43.00 43.00

17.872** 3.513

Discussion Recruiting trees The repeated measures ANOVA between-subjects main effects’ response (Table 4) to the first research question about the natural rehabilitation/restoration potential for trees following various types of local agricultural land uses is that the potential for recruitment is low. And both seed bank analysis and farmer narratives suggest a significant threat to the natural recruitment of trees comes from C. odorata. Analyses of the germination of seedlings indicate that even without isolating the effects of C. odorata, tree seedlings do not stand out to warrant much optimism about the natural recruitment of trees (Table 3). The within-subjects analyses clarified with Tukey’s HSD provide a response to the second and third research questions. None of the local agricultural land uses and conditions (shading or not) provide much promise for the natural recruitment of tree species, as the average numbers of tree seedlings are not significantly greater than those of other species. Rather, C. odorata in soils under forest cover and in soils under all kinds of shading conditions is in significantly greater numbers than tree seedlings. Seed bank analysis and farmer narratives suggest a potential solution for the challenges of significant tree rehabilitation in the forest-savanna region to be one involving a reduction in the competition from C. odorata, together with human-assisted increases in tree seedling numbers in all land uses. The seemingly more tree recruitment-friendly land use fieldsdforest covered land uses and cacao land usesdwere not much different from the more intensively disturbed annual food crop fields in terms of the differences in the means of germinating seedlings. As Table 1 suggests, all local agricultural land uses are not promising for the natural recruitment and regeneration of trees. Tree seedlings are the minority across all the land uses, and this indicates that humanassisted tree recruitment efforts, particularly those that manage the ever-present challenge of C. odorata, are a much needed complement to the self design approach of natural tree rehabilitation. Recruiting trees in the context of C. odorata C. odorata is an herb that develops into a perennial shrub, which reaches 2e3 m in height and thrives in a variety of soils (Awanyo, 2001: 99; Hall, Kumar & Enti, 1972: 76; Honu & Dang, 2002: 186; McFadyen & Skarratt, 1996: 89). A native of tropical America, C. odorata is currently largely viewed as a menace to agriculture and silviculture in much of the humid tropics.12

12

McFadyen and Skarratt (1996, p. 87) suggest Jamaica as the likely source.

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Believed to have been introduced into West Africa in the 1930s, it has since spread throughout this region to Southern Africa (McFadyen & Skarratt, 1996: 90; Witkowski & Wilson, 2001: 13). The concern about this species in Ghana has been widely noted (Amanor, 1994, 1996; Awanyo, 2001, 2007; Hall et al., 1972; Honu & Dang, 2000, 2002). C. odorata’s proliferation in Ghana was reported as early as the late 1960s. The western Ghana spread of this species has been linked to its earlier establishment in Côte d’Ivoire and its spread from the 1950s via prevailing dominant winds and road transportation vehicles (Amanor, 1994: 201; Hall et al., 1972; Rouw, 1991: 14). The traits of this early-flowering perennial include a short juvenile stage, such that it produces seeds in its first year. Prolific seed production and recruitment from seeds is another characteristic (Hall et al., 1972: 76, 78; Honu & Dang, 2000: 80, 2002: 186; McFadyen & Skarratt, 1996: 89; Rouw, 1991: 17; Slaats et al., 1996: 180; Witkowski & Wilson, 2001: 26). The viable seed population was estimated at 66 million light, small mass, easily/wind dispersed seeds per hectare in a study area in Ghana, which was 1607 times more than tree seeds (Honu & Dang, 2000, 2002: 189). C. odorata is dominant, with a single plant capable of producing 860,000 seeds (Witkowski & Wilson, 2001: 14, 19). Slaats et al. (1996) observed in their study site that 56e71% of the seedling population at the start of a fallow, following agricultural land uses, was C. odorata. C. odorata’s strong capabilities of resprouting after burning in the slash-and-burn farming systems of the Tropics (Rouw, 1991: 14), and the lateral spread of its roots have also been documented (Rouw, 1991: 17; Slaats et al., 1996: 186). This invasive species is also characterized by the habits of vigorous growth, freely-branching stems, and sprawling as tickets, which obstruct forest fallow (McFadyen & Skarratt, 1996: 89; Rouw, 1991: 14). The dense canopy and long trailing branches of C.odorata makes it very difficult for tree seedlings to penetrate through. Under the dense interwoven canopy, below which only 7% of full sunlight may be received, trees face a major competitive disadvantage (Honu & Dang, 2000: 80, 2002: 186). Further exacerbating the competitive disadvantage of trees is that viable seeds and adults of C.odorata species are present together and thus this species has a constant presence. How do these characteristics of C. odorata threaten tree recruitment within the repeatedly measured conditions of this study? Rouw (1991) observes that fields disturbed by only a year of cultivation quickly recruit trees and develop into forest fallows. These forest fallow fields reduce the competition from C. odorata in three ways. First, the significant pool of tree seeds in the soil increases the competitiveness of tree seedlings and their height growth outstrips that of other life forms (Rouw, 1991: 17). Second, the shading effects of thicker tree canopies and taller trees reduce the density of C. odorata and its seed production (Slaats et al., 1996: 188). Third, high levels of plant cover slow down germination from the soil seed bank (Rouw, 1991: 16; Slaats et al., 1996, 1998). There are two kinds of forest fields in which tree species are clearly dominant and that create conditions which reduce the competitiveness of C. odorata. The first type is forest fields with a land use history of just 1 year of cultivation and about 60 years of no cultivation disturbance (fallow). Few herbs and shrubs are found in the soil of this land use/cover (Rouw, 1991: 16). The second type of forest fields is associated with 16e25 years of fallow following just 1 year of cultivation. Tree species are dominant although the proportion of herbs is much larger (Rouw, 1991: 16). While C. odorata seedlings are more numerous, tree saplings outgrow C. odorata. However, beyond the single year of the severe perturbation of cultivation, as was characteristic of all the land use/cover conditions in the study area, seedlings of trees decline precipitously and herbs and grasses become frequent and invasive (Ekeleme et al., 2004: 29; Rouw,

1991: 16; Slaats et al., 1996, 1998), with C. odorata in particular becoming the dominant species. According to Honu and Dang (2000) and some farmer narratives, there is a straightforward relation between trees and C. odorata in land uses with severe disruptions: once C. odorata is removed and tree seedlings are exposed to sunlight, height increment, increase in number of leaves, and survival of tree seedlings is significantly enhanced. Trees grow more vigorously and this is why some farmers insist that human assistance, which removes C. odorata and releases tree seedlings, enhances the recruitment of trees. Cacao land use also conditions tree recruitment in at least three ways. First, the permanent/long-term cultivation of cacaod beyond 1 year of perturbationdmeans seedlings of trees decline precipitously. Second, cacao as a tree crop has a shading effect that reduces C. odorata densities (Ekeleme et al., 2004; Honu & Dang, 2000; Slaats et al., 1996), which may increase the recruitment of trees if tree seedlings are released. Third, cacao stand senescence breaks up the canopies, as a result of which conditions are created for herbs and grasses to become invasive (McFadyen & Skarratt, 1996: 89; Rouw, 1991: 16e17; Witkowski & Wilson, 2001: 18), and C. odorata becomes prominent (Rouw, 1991: 22). The predominance of C. odorata seedlings from the soil seed banks of the mature cacao stands can thus be comprehended. However, the human assisted removal of C. odorata enhances the germination of tree seeds in the seed bank (according to farmers such as Mr. Boah) and assists in tree seedling release (according to Honu & Dang, 2000). Soil conditions of food crop farms shape tree recruitment. Rouw (1991, 1995) indicates that with prolonged cultivation and repeated weeding, the pool of fallow tree seeds becomes exhausted on food farms. C. odorata becomes more numerous, and becomes more firmly established (Slaats et al., 1996). Continued weeding not only on food crop farms but also on cacao farms “exhaust most of the tree seed pool” and “the more weeding is done, the more Chromolaena odorata is freed from competition with trees, [and] the longer it can maintain site tenure” (Rouw, 1991: 17). The C. odorata tickets prevent the establishment of other species, and become an “obstruction to the replanting of trees, coffee, cocoa, and timber” (Rouw, 1991: 17). Also, the continued cutting and burning of vegetation in the slash-andburn agriculture of food crop production entrenches the competitiveness of C. odorata as it is freed from competition with tree species. These are among the reasons for the present study’s data showing that tree seedlings are a minority and that they do face immense competition from other life forms, including herbs/ shrubs of which C. odorata is dominant. A focus on shading as a measured condition across all land uses/ cover shows that it is also an influence on the natural recruitment of tree seedlings and C. odorata. While tree seedling analysis could not resolve the contested LEK about the photoblasticity of tree seeds, there is a convergence between the seedling analysis and LEK about the significance of C. odorata for tree recruitment in all shading conditions. Rouw (1991) working in south-west Côte d’Ivoire and Norgrove et al. (2000) in southern Cameroon indicate that C. odorata degenerated rapidly under overstorey shade of trees. C. odorata rapidly declines in abundance in soils under a thick canopy and is shaded out within 2e3 years (Honu & Dang, 2000: 79; Rouw, 1991: 17), and after 5 years other woody species become abundant (Slaats et al., 1996). On the other hand, according to McFadyen and Skarratt (1996: 89), C. odorata’s growth “is optimal in the open or in partial shade.” This view is confirmed by Witkowski and Wilson (2001: 18) who also note that the number of seeds produced per plant and the number of germinable seeds per plant was significantly higher for C. odorata in the sun (Witkowski & Wilson, 2001: 19e22). The present study’s portrayal of the significant effect of the lack of shading

L. Awanyo et al. / Applied Geography 31 (2011) 181e190

conditions on the poor recruitment of tree seedlings can thus be comprehended. With C. odorata growing vigorously in the open, its canopy casts a shadow over competing species and obstructs their growth (Slaats et al., 1996: 188). While Honu and Dang (2000: 76) counted 11,700 tree seedlings per hectare under the shadow of C. odorata, they observed that the growth of the seedlings was poor. Furthermore, the present study also indicated that while shading may reduce the competitiveness of C. odorata, tree seedlings continue to have a competitive disadvantage over C. odorata even in the shade. This is explained by the severe perturbation of cultivation on all land uses/cover in the study area that has significantly reduced the tree seeds in the soil seed bank. On the other hand, the traits of C. odorata give this species a significant competitive advantage, which is further enhanced by the declining competition from tree life forms. Conclusion The natural recruitment/rehabilitation of trees in the once densely forested forest-savanna ecological region of Ghana faces tremendous competitive challenges from non-tree life forms. Tree rehabilitation in this ecological region requires that trees constitute the dominant life form. This study in the agricultural-based communities of Whanabenya, Amanase, and Trayo in this ecological region provides farmer insights about the potential for the natural recruitment of trees from the soil seed banks associated with three agricultural land uses, specifies the role of varying shading and sunlight of the land uses in such recruitment, and points to the critical role of human-assistance in the realization of the potential for tree recruitment. Both field experiments and LEK indicate that tree seedlings are the minority life forms, that the potential for natural tree recruitment is low in all land uses/cover and within all shading/sunlight conditions, and that while nature can initiate the process of tree rehabilitation, human-assisted rehabilitation practices that nurture more tree seedlings and that reduce the competition from other life forms are critical. With regard to the competition that threatens tree species, this article submits that at least one speciesdthe herb/shrub C. odoratadrequires attention. Within all types of land uses/cover and within all shading conditions investigated, C. odorata out-competed tree species, with the mean number of C. odorata seedlings actually significantly greater than tree species within forest fallow cover soils (71 seedlings and 2.40 seedlings, respectively). C. odorata seedlings were also significantly greater than tree species in the open/sunlight (50.67 seedlings and 3.10 seedlings, respectively) and in the shade (43 seedlings and 2.73 seedlings, respectively). Human-assisted tree recruitment that nurtures and increases tree seedlings, and that manages C. odorata appears to be key to successful tree rehabilitation in not only the study area but in much of Ghana, given the proliferation of C. odorata in this country. Acknowledgments Our deepest gratitude goes to the remarkably generous, knowledgeable, and patient people of Whanabenya, Amanase, and Trayo. The indispensable financial support by the Social Sciences and Humanities Research Council of Canada (Standard Research Grant Program) and Luther College, University of Regina is gratefully acknowledged. The comments and suggestions of the anonymous reviewers and the editor are much appreciated. References Agyepong, G., & Kufogbe, S. (1997). Land use and cover patterns. In E. Gyasi, & J. Uitto (Eds.), Environment, biodiversity and agricultural change in West Africa:

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