The authors have declared that no competing interests exist.
Conceived and designed the experiments: JS SN. Performed the experiments: JS. Analyzed the data: JS. Contributed reagents/materials/analysis tools: JS. Wrote the paper: JS SN.
Recent studies indicate that species richness can enhance the ability of plant assemblages to support multiple ecosystem functions. To understand how and when ecosystem services depend on biodiversity, it is valuable to expand beyond experimental grasslands. We examined whether plant diversity improves the capacity of agroecosystems to sustain multiple ecosystem services—production of wood and forage, and two elements of soil formation—in two types of smallholder fallows in western Kenya. In 18 grazed and 21 improved fallows, we estimated biomass and quantified soil organic carbon, soil base cations, sand content, and soil infiltration capacity. For four ecosystem functions (wood biomass, forage biomass, soil base cations, steady infiltration rates) linked to the focal ecosystem services, we quantified ecosystem service multi-functionality as (1) the proportion of functions above half-maximum, and (2) mean percentage excess above mean function values, and assessed whether plant diversity or environmental favorability better predicted multi-functionality. In grazed fallows, positive effects of plant diversity best explained the proportion above half-maximum and mean percentage excess, the former also declining with grazing intensity. In improved fallows, the proportion above half-maximum was not associated with soil carbon or plant diversity, while soil carbon predicted mean percentage excess better than diversity. Grazed fallows yielded stronger evidence for diversity effects on multi-functionality, while environmental conditions appeared more influential in improved fallows. The contrast in diversity-multi-functionality relationships among fallow types appears related to differences in management and associated factors including disturbance and species composition. Complementary effects of species with contrasting functional traits on different functions and multi-functional species may have contributed to diversity effects in grazed fallows. Biodiversity and environmental favorability may enhance the capacity of smallholder fallows to simultaneously provide multiple ecosystem services, yet their effects are likely to vary with fallow management.
Biodiversity in its broad sense is responsible for the ecosystem services relied on by human societies
In recent decades a growing body of evidence from manipulative experiments has demonstrated that diversity in ecological communities can significantly influence ecosystem functioning
Biodiversity effects on ecosystem function are generally attributed to complementarity effects and sampling (or selection) effects. Complementarity effects reflect the net outcome of all biological processes affecting the functioning of an assemblage
Biodiversity-ecosystem function research is currently expanding to address multiple ecosystem functions and a broader range of ecosystems. From a more theoretical perspective, the focus on diversity effects on individual ecosystem functions does not embrace the likelihood that species affect different ecosystem functions differently
While relatively few studies have addressed plant diversity effects on terrestrial ecosystem function outside of grasslands, a handful of studies have investigated whether plant diversity enhances the capacity to provide multiple functions simultaneously, i.e., ecosystem multi-functionality.
Evidence for a greater ability of diverse plant assemblages to support multiple ecosystem functions has emerged from grassland diversity manipulation studies. These studies tested distinct yet related hypotheses: (1) considering a greater number of functions increases the number of species important for functioning
Complementarity among plant species, both within as well as among ecosystem functions, may significantly underlie plant diversity effects on ecosystem multi-functionality. That is, plant diversity may improve multi-functionality in part through complementarity effects specific to individual ecosystem functions. Meanwhile, different species providing complementary benefits to different ecosystem functions should further enhance multi-functionality. ‘Multi-functional’ species able to support more than one function may also promote multi-functionality. Some agroforestry trees in our study area are multi-functional: Sesbania sesban (L.) Merr., for instance, is productive, can fix over 70% of its nitrogen content from the atmosphere, and can prevent leaching of soil NO3−
In agroecosystems, plant diversity might influence multiple ecosystem functions more strongly than in experiments. Management practices such as harvesting, grazing, and land use rotations cause significant disturbance. Stronger effects of diversity on ecosystem function in the presence of disturbance
The potential of biodiversity to buffer ecosystem functioning against environmental change is known as the insurance hypothesis
When a biotic community experiences a perturbation, the importance of functional trait diversity depends on whether the traits engendering vulnerability (response traits) also determine species effects on ecosystem function (effect traits). When species with strong effects are more vulnerable, ecosystem function declines more severely than with random species loss
Plant responses to aboveground biomass removal are mediated by traits associated with two main strategies: tolerance and avoidance. The first suite of traits enables plants to withstand disturbance, such as resprouting ability
At global scales, large herbivore grazing tends to favor annuals (indicating avoidance) and short, prostrate, or stoloniferous plants (indicating tolerance)
In addition to disturbance, tropical smallholder agroecosystems have further characteristics that may increase the likelihood of plant diversity influencing ecosystem multi-functionality. Smallholder farms tend to be more multi-functional
The goal of the current study is to assess whether plant diversity and environmental conditions influence the capacity of tropical smallholder fallows to provide multiple ecosystem services. A set of four ecosystem functions were selected as indicators for the ecosystem services of wood production, livestock forage production, and soil formation, the latter of which affects post-fallow crop production. Ecosystem service multi-functionality was quantified as (1) the proportion of ecosystem functions above half-maximum (henceforth, “proportion above half-maximum”), and (2) mean percentage excess above mean ecosystem function values (“mean percentage excess”). Two specific hypotheses were tested: first, plant diversity increases the ecosystem multi-functionality of fallows; and second, multi-functionality increases with environmental favorability, in terms of lower grazing intensity (in grazed fallows only) and higher soil fertility, since favorable environmental conditions may benefit each ecosystem function. Here we present the first assessment of plant diversity influences on multiple ecosystem functions in the humid tropics, and the first test of biodiversity influences on multiple ecosystem functions directly linked with ecosystem services relevant to smallholder farmer livelihoods.
Correlations among the selected set of four ecosystem functions were weak in both grazed and improved fallows, with the exception of the negative correlation between wood biomass and forage biomass in improved fallows. Wood and forage biomass were positively and marginally significantly correlated (
Ecosystem multi-functionality, in terms of the proportion of ecosystem functions above half-maximum (“proportion above half-maximum”) and mean percentage excess above mean function values (“mean percentage excess”), varied more widely in grazed than in improved fallows (
Indicator | Indicator of: | Units (Transformation) | Fallow type | n | Mean | SD | SEM | Max |
Wood biomass | Wood production ES | t ha−1 | Grazed | 18 | 3.0 | 3.3 | 0.8 | 8.0 |
(natural log) | Improved | 21 | 24.2 | 5.9 | 1.3 | 31.1 | ||
Forage biomass | Forage production ES | t ha−1 | Grazed | 18 | 2.4 | 1.2 | 0.3 | 4.4 |
(square root) | Improved | 21 | 1.7 | 1.1 | 0.2 | 3.5 | ||
Soil base cations | Soil formation ES | meq 100 g−1 | Grazed | 18 | 4.8 | 2.2 | 0.5 | 8.8 |
(arcsine square root) | Improved | 21 | 3.7 | 1.2 | 0.3 | 5.6 | ||
Steady infiltration rate | Soil formation ES | mm hr−1 | Grazed | 18 | 261.8 | 198.1 | 46.7 | 571.7 |
(square root) | Improved | 21 | 371.0 | 148.1 | 32.3 | 617.3 | ||
Proportion of ecosystem functions above half-maximum | Ecosystem multi-functionality | % | Grazed | 18 | 31.9 | 25.5 | 6.0 | na |
(arcsine square root) | Improved | 21 | 56.0 | 17.5 | 3.8 | na | ||
Mean percentage excess above mean ecosystem function values | Ecosystem multi-functionality | Mean % | Grazed | 18 | 0.0 | 33.0 | 7.8 | na |
(none) | Improved | 21 | 0.0 | 8.6 | 1.9 | na |
The maximum of each ecosystem function is the mean of the 3 highest values in a given fallow type.
Grazed fallows had substantially higher plant diversity than improved fallows, both in terms of the species richness (ANOVA, d.f. = 1,37;
In grazed fallows, the proportion of functions above half-maximum and mean percentage excess exhibited similar relationships with plant diversity and environmental variables (
Model specification | d.f. |
Intercept | Predictor | Est. |
|
|
|
Functional diversity | 1,16 | −0.10 | Functional diversity | 0.04 | 2.62 | 0.019 | 0.256 |
Species richness | 1,16 | −0.11 | Species richness | 0.05 | 2.71 | 0.016 | 0.271 |
Soil carbon | 1,16 | −0.07 | Soil carbon | 4.77 | 0.74 | 0.467 | −0.027 |
Grazing intensity | 1,16 | 1.18 | Grazing intensity | −0.73 | −2.15 | 0.047 | 0.176 |
Functional diversity+Soil carbon | 2,15 | −0.12 | Functional diversity | 0.04 | 2.39 | 0.030 | 0.207 |
Soil carbon | 0.15 | 0.03 | 0.980 | ||||
Species richness+Soil carbon | 2,15 | −0.02 | Species richness | 0.06 | 2.48 | 0.025 | 0.224 |
Soil carbon | −0.81 | −0.13 | 0.895 | ||||
Functional diversity+Grazing intensity | 2,15 | 0.46 | Functional diversity | 0.03 | 1.91 |
|
0.293 |
Grazing intensity | −0.46 | −1.35 | 0.196 | ||||
Species richness+Grazing | 2,15 | 0.43 | Species richness | 0.04 | 1.97 |
|
0.301 |
Grazing intensity | −0.45 | −1.30 | 0.214 |
d.f. = model, error degrees of freedom.
Model specification | d.f. |
Intercept | Predictor | Est. |
|
|
|
Functional diversity | 1,16 | −61.08 | Functional diversity | 3.89 | 3.03 | 0.008 | 0.324 |
Species richness | 1,16 | −58.88 | Species richness | 5.00 | 2.88 | 0.011 | 0.301 |
Soil carbon | 1,16 | −71.40 | Soil carbon | 578.14 | 1.03 | 0.320 | 0.003 |
Grazing intensity | 1,16 | 48.03 | Grazing intensity | −53.42 | −1.70 | 0.109 | 0.100 |
Functional diversity+Soil carbon | 2,15 | −76.13 | Functional diversity | 3.77 | 2.69 | 0.017 | 0.283 |
Soil carbon | 137.56 | 0.27 | 0.789 | ||||
Species richness+Soil carbon | 2,15 | −67.61 | Species richness | 4.89 | 2.53 | 0.023 | 0.255 |
SOC | 81.35 | 0.16 | 0.879 | ||||
Functional diversity+Grazing intensity | 2,15 | −32.24 | Functional diversity | 3.43 | 2.42 | 0.029 | 0.309 |
Grazing intensity | −24.08 | −0.80 | 0.436 | ||||
Species richness+Grazing | 2,15 | −29.62 | Species richness | 4.37 | 2.26 | 0.039 | 0.283 |
Grazing intensity | −24.24 | −0.78 | 0.445 |
d.f. = model, error degrees of freedom.
In linear models combining grazing intensity with plant diversity variables, functional diversity and species richness had marginally significant effects on the proportion above half-maximum, and remained significant predictors of mean percentage excess, while grazing intensity was not a significant predictor of either multi-functionality indicator (
Functional diversity was most evenly distributed at moderate grazing intensity (and was slightly higher), while the sample was small at low grazing intensity (n = 4), and diversity varied little at high grazing intensity. As such, the moderately grazed group (i.e., ∼50% of area with evidence of grazing) provided the most meaningful test of diversity effects on multi-functionality within a single grazing group. In moderately grazed fallows, the proportion above half-maximum (d.f. = 1,4;
Improved fallows supported between two and three functions above 50% of function maxima, and between three and four functions above 25% of function maxima (
In improved fallows, ecosystem multi-functionality exhibited considerably different relationships with plant diversity and environmental conditions depending on the multi-functionality indicator considered. The proportion above half-maximum was unassociated with fallow plant diversity or soil carbon (
Model specification | d.f. |
Intercept | Predictor | Est. |
|
|
|
Functional diversity | 1,19 | 0.90 | Functional diversity | 0.003 | 0.47 | 0.642 | −0.040 |
Species richness | 1,19 | 0.89 | Species richness | 0.007 | 0.78 | 0.446 | −0.020 |
Soil carbon | 1,19 | 0.50 | Soil carbon | 3.612 | 1.62 | 0.121 | 0.076 |
Functional diversity+Soil carbon | 2,18 | 0.49 | Functional diversity | 0.001 | 0.17 | 0.865 | 0.026 |
Soil carbon | 3.531 | 1.51 | 0.147 | ||||
Species richness+Soil carbon | 2,18 | 0.49 | Species richness | 0.005 | 0.48 | 0.634 | 0.037 |
Soil carbon | 3.380 | 1.46 | 0.162 |
d.f. = model, error degrees of freedom.
Model specification | d.f. |
Intercept | Predictor | Est. |
|
|
|
Functional diversity | 1,19 | −9.73 | Functional diversity | 0.92 | 2.16 | 0.044 | 0.155 |
Species richness | 1,19 | −8.45 | Species richness | 1.28 | 2.29 | 0.034 | 0.175 |
Soil carbon | 1,19 | −43.36 | Soil carbon | 356.38 | 2.75 | 0.013 | 0.247 |
Functional diversity+Soil carbon | 2,18 | −45.27 | Functional diversity | 0.73 | 1.88 |
|
0.336 |
Soil carbon | 308.95 | 2.48 | 0.023 | ||||
Species richness+Soil carbon | 2,18 | −43.87 | Species richness | 1.02 | 2.01 |
|
0.351 |
Soil carbon | 305.23 | 2.48 | 0.023 |
d.f. = model, error degrees of freedom.
Patterns of ecosystem multi-functionality can be decomposed by assessing pair-wise jointness among ecosystem functions (see
In improved fallows, the negative correlation between wood and forage biomass (
Soil base cations and infiltration rates jointly exceeded 50% of maxima or mean values in only a few grazed fallows. The probability distributions of base cations and infiltration were skewed left-ward and the incidence of jointness was lower than in improved fallows (
In improved fallows, approximately half the sample exhibited jointness for soil base cations and infiltration rates (
In grazed fallows, wood biomass increased with the abundance-weighted mean for green tissue lignin content of trees and shrubs (d.f. = 1,16;
In both grazed and improved fallows, plant diversity displayed significant positive effects on ecosystem service multi-functionality. The strength of evidence for plant diversity effects on multi-functionality depended on fallow type. In grazed fallows there was stronger evidence for positive relationships of plant diversity with ecosystem multi-functionality, especially since environmental conditions could not provide a superior explanation. While significant effects of plant diversity on mean percentage excess were observed in improved fallows, environmental favorability appeared more influential.
In grazed fallows, both the proportion of ecosystem functions above half-maximum (“proportion above half-maximum”) and mean percentage excess above mean ecosystem function values (“mean percentage excess”) increased with functional diversity and species richness. The decline in the proportion above half-maximum with grazing intensity explained less of the variance than did plant diversity variables, and no such decline was observed for mean percentage excess. Positive effects of fallow plant diversity on ecosystem multi-functionality were stronger than negative effects of grazing, yet it was not unequivocally clear that diversity effects were independent of variation in grazing intensity. It remains possible that more intense grazing, perhaps in conjunction with variation in soil conditions, could have influenced plant diversity or altered diversity effects. Significant effects of functional diversity on multi-functionality in moderately grazed fallows indicated that within this group of fallows, effects of functional diversity on both the proportion above half-maximum and mean percentage excess were independent of grazing.
In improved fallows, there was mixed evidence for effects of plant diversity and environmental conditions on ecosystem multi-functionality. Improved fallows typically sustained at least two of the four ecosystem functions above half-maximum, irrespective of both diversity and soil carbon. In contrast, mean percentage excess increased with both diversity and soil carbon, with soil carbon out-performing diversity variables as a predictor. Positive effects of soil carbon on mean percentage excess indicated that multi-functionality may have been more easily attained under environmental conditions favorable for the production of biomass, development of soil structure and improvement of infiltration, and the deposition of base cations in litter and their retention in soils. Environmental favorability appeared more influential than plant diversity, and the potential of diversity to benefit multiple ecosystem functions in improved fallows was uncertain. In improved fallows soil carbon was positively though weakly associated with plant functional diversity (d.f. = 1,19;
Pair-wise jointness among ecosystem functions is a component of ecosystem multi-functionality as quantified here (
The incidence of jointness was at times attributable to the probability distributions of ecosystem functions (
The results for grazed fallows indicate possible means by which plant diversity may have enhanced the capacity of fallow plant assemblages to sustain multiple ecosystem functions. Diversity may have increased multi-functionality through complementary benefits of species with different functional traits to different functions, and through effects of multi-functional species on more than one function. In addition, by elevating the importance of disturbance response traits or altering species interactions, grazing disturbance may have promoted plant diversity effects on multi-functionality.
While diversity may have positively affected ecosystem functioning through complementarity and sampling effects specific to individual ecosystem functions, multi-functionality should increase with representation of species that contribute to different ecosystem functions. Otherwise, individual functions could benefit from diversity, but diversity might not improve multi-functionality.
Complementary effects of species on different ecosystem functions should exert additive effects on multi-functionality—that is, functions would exhibit trade-offs in accordance with the occurrence of species specialized in providing each function. If diversity did not influence multi-functionality through any other means (e.g., complementarity and/or sampling effects on individual functions, multi-functional species, disturbance-related effects), multi-functionality could be predicted from the values of individual functions alone. However, the presence of species providing complementary benefits to different functions may contribute, in part, to diversity effects on multi-functionality. Complementary effects of species on different functions may occur alongside complementarity effects specific to individual functions, and multi-functionality was expected to increase when species are complementary both within and among ecosystem functions.
In grazed fallows, wood originates from trees and shrubs, and forage primarily from herbaceous species. Different functional traits were associated with and may have benefitted these two components of fallow biomass. Wood biomass increased with green tissue content of lignins, and since wood declined with grazing intensity, the correlation between wood and tissue lignin suggests that less palatable woody plants were more successful in the presence of grazing. In contrast, forage biomass increased with green tissue N, and also declined with grazing intensity, suggesting that less palatable forage species prevailed under intense grazing. Thus, not only are wood and forage produced by different subsets of the assemblage, they appeared to increase with divergent functional traits, and therefore appear to benefit from plant species with complementary influences on wood versus forage production. As such, high levels of both wood and forage should co-occur where species have high content of either lignins or N. As calculated here, greater functional diversity would meet this condition, since high lignin values were uncommon, and herbaceous lignin content was particularly low.
While wood biomass increased with lignin content, polyphenol content was negatively though insignificantly associated with wood biomass. Since infiltration and soil carbon (which correlated with soil base cations) responded positively to vegetation with higher tissue content of recalcitrant C compounds—lignins+polyphenols in particular (J. Sircely,
Species capable of effectively supporting more than one ecosystem function may have enhanced multi-functionality in grazed fallows. It is possible, if perhaps unlikely, that a few multi-functional species could provide sufficient levels of all ecosystem services demanded from a managed system, at least over finer temporal and spatial scales. However, such a system could be unrealistic in practice due to the probability of incomplete knowledge on species effects on each ecosystem service under particular abiotic and management regimes and combinations of species.
The increase in wood biomass with lignin content paralleled the positive influence of recalcitrant C compounds on soil conditions. In addition, some woody species have deep rooting systems, to 2.5 m in depth in the study area
Since both wood biomass and the proportion of functions above half-maximum declined with grazing intensity, the loss of potentially multi-functional woody species may have negatively influenced multi-functionality. The positive correlation between functional diversity and wood biomass indicated that plant diversity in grazed fallows increased with the representation of large woody plants, many of which can be considered multi-functional. The representation of multi-functional species in biotic communities may increase probabilistically with species richness, effectively generating a sampling effect on multiple functions. The presence of multi-functional species possessing unique combinations of functional traits should disproportionately increase functional diversity.
Echoing observations of diversity effects on individual ecosystem functions strengthening under disturbance
The diversity in plant functional response traits—traits mediating responses to environmental change—influences ecosystem functioning by determining species composition
Species interactions significantly determine ecosystem effects of biodiversity
The negative relationships of grazing intensity with tree and shrub biomass (d.f. = 1,16;
Grazing may have also increased facilitation, since species less palatable to grazers often facilitate those more palatable
In improved fallows, the fact that soil carbon best predicted mean percentage excess indicates that multi-functionality may be sensitive to the favorability of environmental conditions. Higher soil carbon is indicative of soil conditions that support biomass production as well as improvement in the structure and fertility of soils. Although the proportion of functions above half-maximum was unaffected by soil carbon, it is probably justifiable to conclude that environmental favorability enhanced multi-functionality, in part because mean percentage excess appeared to be a more robust indicator of multi-functionality (
Beyond the evidence for environmental influences on multi-functionality, there are additional explanations for the weak and inconsistent diversity effects in improved fallows. A primary rationale for the use of improved fallows is their high productivity
However, the species composition of improved fallows was not conducive to detecting diversity effects. The dominance of woody legumes and the commonness of
It is unknown whether species augmentation would improve the ability of improved fallows to sustain multiple ecosystem functions. Ndufa et al. (2009) conducted an experiment with improved fallow monocultures and two-species mixtures. While monocultures and mixtures produced similar amounts of biomass, mixed fallows out-performed monocultures on average in terms of N recycled to soils and post-fallow maize yields, although a monoculture performed best for both functions. Thus, while complementarity may enhance multiple ecosystem services in improved fallows, the potential may be limited and dependent on species composition.
The observed relationships among plant diversity, environmental conditions, and ecosystem service multi-functionality differed substantially between grazed and improved fallows. In grazed fallows, significant effects of plant diversity on multi-functionality indicators provided evidence in support of plant diversity enhancing the capacity of fallows to provide multiple ecosystem services. Diversity effects on multi-functionality were stronger and more consistent than negative effects of grazing intensity. In contrast, soil carbon appeared more influential than diversity in improved fallows, and evidence for diversity effects on multi-functionality was scant. The ecological factors influencing multi-functionality thus appeared to differ appreciably among fallow types, with plant diversity more influential in grazed fallows and environmental favorability more important in improved fallows.
Differences in management between grazed and improved fallows created contrasting ecological conditions that may explain much of the apparent divergence in ecological controls over ecosystem multi-functionality. The improved fallows here were ungrazed and experienced little disturbance, while grazed fallows were subjected to consistent disturbance from livestock grazing. The other major management difference was that improved fallows were intentionally planted with leguminous trees and large-statured shrubs, while the vegetation in grazed fallows reflected largely unguided regeneration. These management dissimilarities were likely responsible for generating the prevailing differences in vegetation structure and species composition among fallow types.
The results indicate that the management of smallholder fallows influences the relationship between plant diversity and ecosystem service multi-functionality. The productive, multi-functional species planted in improved fallows were highly dominant, generating competition and limiting variation in plant diversity, functional traits, and the abundance of multi-functional species. As a result, fallow management to create dense stands of woody legumes appeared to reduce or obscure the potential for plant diversity to affect multi-functionality. In contrast, grazing disturbance and unguided regeneration in grazed fallows led to variation in plant diversity, functional traits, and multi-functional species abundance, and reduced dominance and competition (and possibly increased facilitation). Effects of plant diversity on multi-functionality in grazed fallows therefore appear related to management of fallows with livestock grazing and without planting woody legumes. The results from grazed fallows are consistent with complementary benefits of different plant species to different ecosystem functions and multi-functional species being among the sources of positive influences of plant diversity on ecosystem service multi-functionality.
To improve understanding of the relationship between biodiversity and ecosystem service multi-functionality, research investigating sources of variation in the relationship is a sensible extension of the findings presented here. Disturbance and perturbations, environmental conditions, dominance, and species interactions—all recognized as significant in previous biodiversity-ecosystem function research—are promising avenues toward more fully elucidating the potential significance of plant diversity in sustaining the ability of agroecosystems to provide multiple ecosystem services, and the possible roles of biodiversity in maintaining the overall functioning of ecosystems managed and natural alike.
The Millennium Villages Project (MVP) is an evidence-based approach to alleviating extreme rural poverty in sites throughout Sub-Saharan Africa, in which agricultural development plays a strategic role
‘Improved’ fallows of fast-growing leguminous trees and shrubs are promoted by MVP to improve soil fertility and produce fuelwood. Improved fallows in the area are primarily
Fallow fields identified for sampling ranged in size from 0.02 to 0.25 ha, with a mean of approximately 0.1 ha. Data were collected from 18 grazed and 21 improved fallows using a down-scaled version of the Land Degradation Surveillance Framework (LDSF) protocol
For all woody stems ≥2.5 cm DBH, aboveground standing biomass (t ha−1) was estimated by using DBH to calculate whole-tree biomass (
For all woody plants <2.5 cm DBH and all non-woody species biovolume (m3) was multiplied by a conversion factor to estimate aboveground biomass (
Soil organic carbon, soil base cations, and soil sand content were quantified using near infrared reflectance spectroscopy (NIRS), utilizing extensive soil libraries from the study site and the region to predict values of soil properties
Samples of leaves and young green twigs (leaves only for herbaceous species) were collected from a subset of study plots, and NIRS was used to predict content of N, lignins and polyphenols for most common species. Predictions were made using partial least squares regression to model N, lignins, and total soluble polyphenols from PCs of first-derivative NIR spectral reflectance in the 1250 to 2500 nm range. Independent sample cross-validation yielded model fit
For the ecosystem service of wood production, the ecosystem service providers, or ESPs
For the ecosystem service of forage production, the ESPs were taken to be the forage species producing higher quality forage in terms of the green tissue crude protein (CP; i.e., the standard tissue N ×6.25) to lignin ratio
Species richness was calculated as the number of species comprising ≥1% of standing green biomass in a fallow. The influence of species with low plot-scale abundance on ecosystem functioning is likely to be minor; these species disproportionately influence evenness and are likely to bias estimation of any influences of plant diversity on ecosystem functioning. Although species richness and evenness were negatively correlated, when species comprising <1% of green biomass were removed, the correlation was effectively eliminated.
Functional diversity (FD) was used as a proxy for niche complementarity among fallow plant species
Ecosystem functions directly related to specific ecosystem services served as indicators of the focal ecosystem services, and each is relevant to the livelihoods of smallholder farmers in the study area. Because total standing green biomass and soil carbon were correlated to varying degrees with other ecosystem functions, collinearity among functions could have biased analysis of ecosystem multi-functionality. Instead, a conservative approach to ecosystem multi-functionality was adopted. The ecosystem functions selected for analysis (
In these agroecosystems, each ecosystem function reflects specific stocks or fluxes of energy, materials, or both. Wood biomass specifically indicates the stock of non-living aboveground biomass, and the potential flux of C, energy, and nutrients in wood to households for fuelwood, poles, etc. Forage biomass is the stock of biomass available for consumption by livestock, and reflects the potential flux of C, energy, and nutrients to livestock biomass and waste. Soil base cation content integrates the cycling, movements, and soil functions provided by Mg, Ca, and K, including cation retention and exchange, plant uptake, leaching and prevention thereof, and retrieval from the subsoil by deep-rooted plants. Soil infiltration capacity is indicative of robust soil structure, avoidance of runoff and soil erosion, and retention and availability of soil water to plants.
Fallow ecosystem service multi-functionality among the 4 selected ecosystem functions was quantified by means of 2 indicators: (1) the proportion of ecosystem functions above half-maximum (“proportion above half-maximum”), and (2) the mean percentage excess above mean function values (“mean percentage excess”). That is, “proportion above half-maximum” is the proportion of the 4 ecosystem functions that were higher than 50% of the respective maximum value of each function, and “mean percentage excess” is the mean of the percentage by which ecosystem functions exceeded their mean values. The maximum for each function in the grazed and improved fallow types was defined as the mean of the 3 highest values in each fallow type. One mean and one maximum was established for each function in each fallow type among the two sampling years, as forage biomass did not differ between 2008 and 2009 (not presented), and is the only function likely to vary across years with rainfall and other climatic conditions. The proportion of functions above 25 and 75% of maxima were also calculated. Ecosystem functions were transformed (
All analyses were conducted with R software
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We thank Patrick Mutuo and the Millennium Villages Project for facilitating completion of the research, Victor Omollo, Chris Ekise, Herine Okoth, Steve Okulo, Carlos Onyalo, and George Baison for field assistance, and Keith Shepherd and Elvis Weullow of ICRAF-Nairobi for assistance with soil analysis. Thanks are also due to Cheryl Palm, Markus Walsh, Maria Uriarte, and two reviewers for helpful suggestions and comments.