On-farm trees and aboveground carbon stocks
A total of 8712 trees belonging to 83 tree and shrub species were inventoried in 26 farms with a total area of 34.94 ha; average farm size was 1.34 ± 1.09 ha (Table 1). An additional 48 individuals belonging to Carica papaya and 170 individuals of unknown identity were also documented. Individuals of Carica papaya and unknown species were not included in diversity analysis or estimation of biomass. About 51% of trees inventoried were native; the rest were exotic. In terms of abundance, Eucalyptus spp., Euphorbia tirucalli, Acacia spp., G. robusta and Balanites aegyptiaca accounted for 21.2., 9.6, 7.6, 6.3 and 5.0%, respectively of the total number of trees found in Lower Nyando (“Appendix 1”). These five most dominant species accounted for about half of the trees documented. Thirty-eight rare species (those with less than 10 individuals per species) formed 1.5% of the trees recorded. Most of the rare species (25 species) were native to Africa while 13 were of exotic origin. Eucalyptus spp. was mainly planted in woodlots; Acacia spp. was mainly found in grazing fields while E. tirucalli and G. robusta were mainly found on boundary.
The land use type with the richest species diversity were homestead (61 species) and boundary (59 species) (Table 1). The lowest species richness was found in shrub land and woodlots, which were characterised by monospecific stands. A converse trend was observed for tree density, being highest for trees planted in boundaries and in woodlots (2211–2835 trees ha−1) compared to other land use types, where tree density ranged from 57 to 518 tree ha−1 (Table 1). Shannon diversity index for the entire study area was 3.06 (Table 1) with an average H′ of 1.65 ± 0.09 per farm. The value of H′ varied across different land uses, being highest for tree populations in homestead and cropland, and lowest in woodlots (Table 1). According to the Bootstrap, Chao and Jackknife I and II species richness prediction method, the tree inventory captured between 61 and 84% of the tree species richness in the study area.
Trees in Lower Nyando were estimated to store an average of 4.07 ± 0.68 Mg C ha−1 per farm (Table 1). Total carbon stored in aboveground biomass within the 26 farms (34.94-ha) sampled was 105 Mg C ha−1. This amount is indicative of the carbon that can be lost if the farmland trees are cleared or die and decompose. Large trees (with DBH above 30 cm) were few in Lower Nyando, representing only 5% of the trees inventoried but over half (51%) of the carbon stocks. Larger trees were mainly concentrated in homesteads and boundaries. Eighty percent (80%) of all trees inventoried had a DBH less than 10 cm and held about 16% of the total carbon. The mean carbon stocks per farm was 2.2 ± 2.2 Mg C ha−1 for exotic trees and 3.2 ± 1.6 Mg C ha−1 for native tree species. Carbon stocks within the farm differed among the land use types, being largest in homestead and lowest in shrub land and cropland (Table 1). Summing the average contributions of each land use, homestead (37.6%), woodlots (28.5%) and boundaries (22.5%) accounted for over 88.6% of the total carbon found in the farms.
Mean carbon stocks per farm had a significant positive correlation with farm size (Fig. 2). There was a moderate and non-significant positive correlation between carbon stocks and the Shannon Index (r = 0.451, p = 0.080). However, the relationship between farm carbon and species richness was weak, negative and not significant (r = − 0.044, p = 0.833). A weak negative correlation was found between farm level carbon stocks and the number of retained tree species (r = − 0.261, p = 0.198), the number of land use types (r = − 0.002, p = 0.993), and the number of benefits or uses of trees (r = − 0.001, p = 0. 632). There was, however, a moderate negative and significant association between farm level carbon stocks and tree density (− 0.582, p < 0.05) as well as average size of trees on the farm (r = − 0.420, p = 0.05). No significant correlations were evident between carbon stocks and the measured socio-economic variables: off-farm income, household size, and access to off-farm products. A negative (non-significant) correlation was evident between the total number of ruminant livestock and carbon stocks (r = − 0.344, p = 0.085). However, ruminant number correlated significantly with tree use (r = − 0.495, p < 0.05), number of exotic species (r = 0.439, p < 0.05) and the number of timber trees (r = 0.432, p < 0.05). There was a significant relationship between farms with a woodlot and their firewood self-sufficiency (Fisher’s extract p = 0.009, n = 26). Seventy-eight percent of the farms with a woodlot were also self-sufficient in firewood while 82% of the farms without a woodlot were not self-sufficient in firewood.
Household uses of trees
A total of 18 uses and benefits of on-farm trees were documented for 68 tree species mentioned by farmers in Lower Nyando (Fig. 3). Trees provided benefits for household consumption and for sale. The most frequently mentioned use of trees was firewood, followed by construction material, shade, fruit and timber (Fig. 3). The bulk of these benefits were provided by Eucalyptus spp., G. robusta, Vepris nobilis, and Terminalia brownii (Table 2). Respondents ranked timber, firewood, construction material, fruits and shade as the foremost important benefits. Ten percent of the households mentioned other uses, such as aesthetic value, scent of trees, boundary demarcation, seedlings and food. On average each household extracted five (range between 1 and 9) unique benefits from all on-farm trees. A complete list of most frequently mentioned uses and benefits of trees is presented in “Appendix 3”. There was a significant positive correlation between the mean uses and the absolute number of fruit trees per farm (r = 0.401, p < 0.05), and between uses and the number of timber trees (r = 0.425, p < 0.05).
Trees were planted by the farmer, grown without farmer intervention, or deliberately retained in the farms. Fifty-four percent of all trees surveyed were planted by the farmer, 39% were retained, and data were not available for the seven percent. The majority of retained on-farm trees were native (77%), while the majority of planted on-farm trees were exotic (76%). There was a significant positive correlation (r = 0.71, p < 0.01) between the number of planted species and the number of exotic trees per farm, however no correlation was found between the number of planted species and the number of native trees. Eucalyptus (E. camaldulensis, E. Saligna and E. grandis) and G. robusta are the two most frequently planted tree species and also the two most frequently mentioned species in the entire study area. Fruit tree species (M. indica, Persea americana) are mainly planted, except P. guajava, which was mostly retained, as it tends to grow wildly in bush- and shrub land. G. robusta, Eucalyptus spp., Cupressus lusitanica, M. indica, Citrus sinensis and P. americana were cited as the six most preferred additional tree species that farmers would like to have on the farm. These species are desired because they grow fast and are tolerant to pests and diseases (G. robusta and Eucalyptus), have good quality of timber (C. lusitanica), and can improve soil fertility (G. robusta). The main purpose of timber production is to generate income and/or provide construction material for the household. Calliandra calothyrsus and Leucaena leucocephala were preferred for fodder while Terminalia mantaly was preferred for aesthetics and shade.
There was a significant difference in farm elevation between farmers that stated different uses as most important for the 60 survey households (p = 0.008, n = 60). Households on higher elevation farms reported the sale of whole trees (mean = 1643 m) and timber (mean = 1486 m) as most important, while households in lower elevation areas stated windbreak (mean = 1251 m) and construction material (mean = 1320 m) as most important. Farms that stated firewood as most important had a mean farm elevation of 1475 m. Moreover, there was a difference in the number of retained species on farm (p = 0.042, n = 60). Farms that stated firewood as the most important use have on average seven retained species. Farmers stating timber as most important have on average two retained species.
Tree products that cannot be obtained from the own farm were sourced off-farm. These are either bought in formal or in informal markets (e.g., from neighbours) or collected freely from nearby bush lands. Over one-third (38.3%) of all households access two products off-farm, more than half (52%) of all households access one product and 3.3% of all households access three products. Fruits were the major product accessed off-farm and were both collected and bought from a market. Other products that were purchased include construction material, timber, seedlings, leaves of sisal (Agave sisalana) plant, furniture and wood for charcoal. Medicine was mainly collected from the wild. Women were mainly responsible for acquiring fruits off-farm while men sourced construction material and timber. The number of use and off-farm products were negatively correlated (r = − 0.256, p = 0.048, n = 60). Forty percent of households were self-sufficient in firewood, which means they meet household fuelwood consumption from their own farm and do not access firewood off-farm. Forty-three percent of the households collected firewood and 17% buy from off-farm. There were significant differences between means of uses of trees between households that were firewood self-sufficient and those that were not (p < 0.01, n = 60). Firewood self-sufficient households average seven uses of trees while those that are not had only five. Most of the farms that were firewood self-sufficient were located on higher elevations (mean = 1509 m) while farms without firewood self-sufficiency were located in lower areas (mean = 1358 m). In total, 68 different tree species were cited as a source of firewood. Yet, there was a low abundance of fruit trees; three fruit tree species (M. indica, P. americana, P. guajava) were represented among the 15 most frequently mentioned species.