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Species Interactions, Stand Structure, and Productivity in Agroforestry Systems
... Conflicting and inconclusive evidence remains as to the long-term quantity of soil nutrients and SOC in agroforestry cocoa systems (Schroth et al., 2001). Available short-term data are often used to make predictions on SOC and nutrient cycling over the long-term (Kelty, 2000). This type of extrapolation requires further investigation to confirm predictions about changes in SOC stocks in the productivity of agroforestry cocoa systems. ...
... This decomposition process depends on the species present and the climate region and microclimate conditions (Hossain et al, 2011). As previously mentioned, the litter from N 2 -fixing shade tree species is of higher quality (Kelty, 2000), which improves nutrient cycling around the shade trees (Isaac et al, 2007), promoting nutrient transfer into the system for plant uptake. ...
Although a common plant response to environmental gradients, leaf trait plasticity is often uncharted in agroforestry systems. The objective of this study was to examine the effect of a i) local-scale gradient (light, nutrients) induced by shade tree diversity and ii) large-scale gradient (climato-edaphic) induced by altitude on coffee plant response on multiple agroforestry research farms in Costa Rica. Results show large variability of coffee traits: leaf photosynthetic rates, specific leaf area (SLA) and number of fruiting nodes deviate along both gradients. Mean SLA increased with increasing shade tree diversity. However, with increasing altitude, full sun coffee photosynthesized at higher rates than shaded coffee. Concurrently, other coffee leaf physiological and morphological traits differentiated between full sun and shaded coffee with increasing altitude. Results suggest soil moisture and light availability dominate environmental correlates to intraspecific coffee trait plasticity, providing insight to sources of coffee performance variability in monoculture and agroforestry systems.
... The buffer zone must maintain a balance between agriculture development and protection of nature. The primary goal of the agroforestry system is to minimize resource competition and maximize ecological and economic benefits [2]. Agroforestry systems can improve environmental quality and promote biodiversity conservation [3]. ...
As the protection layer of world natural heritage sites, the buffer zone should enable economic development while protecting the environment. To carry out agricultural activities in the buffer zone, it is necessary to balance agricultural development and environmental protection. In addition, the development of agroforestry has the benefits of developing the economy, maintaining biodiversity, and protecting the environment. In order to promote the coordination of environmental protection and community economic development, it is particularly important to scientifically select agroforestry planting patterns in the buffer zone of world natural heritage sites. This study utilized a mixed-methods research approach that included qualitative and quantitative research. Taking the buffer zone of Shibing Karst Heritage Site in southern China as an example, based on the seven agroforestry planting patterns surveyed in the buffer zone of the world natural heritage site, the four dimensions of net output value, carbon emission, environmental cost, and comprehensive livelihood score of different agroforestry planting patterns were calculated. The sorting scores of the values were calculated as Borda numbers. The sorting scores of the seven agroforestry planting patterns were B(A1) = 17, B(A2) = 18, B(A3) = 8, B(A4) = 8, B(A5) = 14, B(A6) = 12, and B(A7) = 7. The results showed that the priority sequence of seven agroforestry patterns was A2 > A1 > A5 > A6 > A3 = A4 > A7. A2 was the best among the seven agroforestry planting patterns, and A7 ranked last. The results can provide a quantitative evaluation basis for scientific optimization of agroforestry development planting patterns, and provide a reference for promoting the protection of world natural heritage.
... Different species require different site conditions so one design is unlikely to work for all indigenous species. The best design combines species with niche differences in a way that reduces competition intensity (Matthew J, 1999;Kelty, 2006), and recognises that the site requirements of the target species may change with plant development, for example because of changes in the degree of asymmetric competition between species . In the 5MHRP, failure to manage competition for light was probably the crucial factor that resulted in poor outcomes. ...
... Different species require different site conditions so one design is unlikely to work for all indigenous species. The best design combines species with niche differences in a way that reduces competition intensity (Matthew J, 1999;Kelty, 2006), and recognises that the site requirements of the target species may change with plant development, for example because of changes in the degree of asymmetric competition between species (Forrester et al., 2006). In the 5MHRP, failure to manage competition for light was probably the crucial factor that resulted in poor outcomes. ...
... Depletion of resources associated with the · presence and abundance of plants Changes in physiological and morphological · growth responses associated with changes in the resource environment Correlations between the presence or abun-· dance of neighbors, depression in resource availability, and physiological performance In contrast, according to Kelty (2000), facilitative interactions are those in which one species benefits another and occur under four mechanisms: ...
... Agroforestr y systems with multiple crops generally yield higher production than monocultures (Nair 1993, Gajaseni 1997, Kelty 2000, Nolte et al. 2003, Pearce & Mourato 2004) and, thus, may accumulate more C and nutrients. However, in cajuput plantations, intensive harvesting of leaves, small branches and twigs (hereafter leaf-twigs) for oil extraction and branches for fuelwood, in addition to crop harvesting, may lead to substantial losses of C and nutrients from the system. ...
We compared aboveground C accumulation and C cycling of three types of cajuput (Melaleuca leucadendron) agroforestry plantations producing cajuput oil in Java, Indonesia. In the study site in east Java, where cajuput trees were planted with cassava and maize in a multiple-crop agroforestry system, cassava was the largest component of aboveground total C accumulation in the plantation. In the site in west Java, where trees are planted with rice in a single-crop agroforestry system, both cajuput and rice contributed similar amounts to aboveground total C accumulation. Aboveground net C accumulation was highest in the multiple-crop system, where it averaged 18.5 and 7.1 Mg C ha-1 year-1 in the 7- and 25-year old stands respectively. In the plantations, relative amounts of harvested (leaf-twigs and branches of cajuput and edible crop biomass) and returned (non-edible crop biomass) C were similar and did not change with stand age. This indicated that although crop harvesting removed large amounts of C from the system, an almost equal amount of organic waste was returned, thus, establishing a dynamic C cycle. The C:N ratio of returned waste was high, suggesting that decomposition rate was slow and that C accumulated in the soil. Compared with more complex agroforestry systems, carbon sequestration of cajuput plantations was low. Our results, however, suggested that C accumulation may be increased and a sustainable C cycle established by returning more biomass waste and maintaining multiple crop systems.
... Stages of stand initiation, stem exclusion and understory re-initiation, although simplified, are an appropriate model for early growth in even-aged stands (Franklin et al., 2002). Over the 25 year chronosequence, this agroforestry system does not simply reflect a static stand structure and composition (Kelty, 2000), but an analogous sequence of partial stand development (Fig. 1). Two years after farm establishment represents a stand initiation phase, followed by a stem exclusion phase (15 years), when the cocoa canopy closes and understory vegetation is eliminated from the system through predominantly farmer managed practices. ...
In cocoa (Theobroma cacao) agroforestry systems, upper canopy trees and food crops are frequently planted for shade, diversification of farm products, and improved soil fertility. As diversification remains a top priority for farm economic and environmental sustainability, systemmanagement plays a substantial role in farm diversity, requiring long-term analysis. We measured species richness dynamics in a chronosequence (N = 4) representing farms aged 2 to 25 years old in the western cocoa-growing region of Ghana, West Africa. Subsequently, we conducted farmer interviews to establish farming practices with regards to integration of non-cocoa species. After farm establishment, increases were recorded in non-cocoa and Simpson diversity index (for 2 years = 0.17 ± 0.167; for 15 years = 0.68 ± 0.026; for 25 years = 0.68 ± 0.036), but no changes were observed in crop richness. All participants interviewed managed supplementary species on their cocoa farms, with 92% of farmers purposefully establishing an upper canopy stratum. We discuss the use of principles from natural stand development for sustaining such diversity. Farmer managed crop re-initiation during farm maturation may promote higher diversity within an existing agroforestry framework.
... A stratified foliage profile, with light-demanding, high-yielding species occupying the upper canopy, and lower-yielding but shade-tolerant species occupying the lower tiers, might therefore outyield simple canopies composed exclusively of one or the other, through complementary light use (Kelty 1989). Facilitation mechanisms are also conceivable: although many shade-tolerant species grow well in full sunlight once established, a partial or complete overstorey could confer some protection against photoinhibition (Naidu & De Lucia 1997;Mohammed & Parker 1999;Kelty 2000). ...
Question: Does overyielding of tree species mixtures in vertically stratified forests depend on complementary light use?
Location: Andes of south-central Chile.
Methods: Basal area data were obtained from 80 circular plots distributed regularly throughout old-growth stands with an emergent Nothofagus dombeyi tier over a canopy composed mainly of Laureliopsis philippiana and Saxegothaea conspicua. Radial growth was measured from cores obtained from trees at the centre of each plot. The effects of competition on growth were evaluated through a competition index (CI) based on distances to and diameters of the two nearest neighbours.
Results: Overall, basal area of the canopy species was only weakly affected by the number of N. dombeyi per plot, and with basal area of N. dombeyi. However, the two main canopy species responded differently: whereas basal area of S. conspicua was negatively correlated with that of N. dombeyi, that of L. philippiana showed no response. Radial growth of S. conspicua was negatively correlated with CI calculated from canopy trees and more weakly so from emergent N. dombeyi. In contrast, radial growth of L. philippiana was not affected by competition with either canopy or emergent neighbours.
Conclusions: Results indicate that emergent N. dombeyi tend to depress growth and basal area of S. conspicua, but not of the more shade-tolerant L. philippiana. This supports the proposal that enhancement of wood production in stratified mixtures will be greatest when component species have strongly contrasting light use traits.
... Conflicting and inconclusive evidence remains as to the long-term quantity of soil nutrients and soil organic matter (Schroth et al. 2001), although previous research on comparing tropical agroforestry systems to sole cropping systems has revealed short-term enhancement of soil organic matter and total organic nitrogen (Young 1989;Hagger et al. 1993). The available short-term data are often used to make predictions of organic matter and nutrient behavior over longer periods (Kelty 2000). This type of extrapolation requires further investigation to confirm predictions about the productivity of agroforestry systems. ...
The conversion of forests to agroecosystems or agroforests comes with many changes in biological and chemical processes. Agroforestry,
a tree based agroecosystem, has shown promise with respect to enhanced system nutrient accumulation after land conversion
as compared to sole cropping systems. Previous research on tropical agroforestry systems has revealed increases in soil organic
matter and total organic nitrogen in the short term. However, research is lacking on long-term system level sustainability
of nutrient cycles and storage, specifically in traditional multi-strata agroforestry systems, as data on both the scope and
duration of nutrient instability are inconclusive and often conflicting. This study, conducted in Ghana, West Africa, focused
on carbon and nitrogen dynamics in a twenty-five year chronosequence of cacao (Theobroma cacao Linn.) plantations. Three treatments were selected as on-farm research sites: 2, 15 and 25-year-old plantations. Soil carbon
(C, to a depth of 15cm) varied between treatments (2years: 22.6MgCha−1; 15years: 17.6MgCha−1; 25years: 18.2MgCha−1) with a significant difference between the 2- and 15- and the 2- and 25-year-old treatments (p<0.05). Total soil nitrogen in the top 15cm varied between 1.09 and 1.25MgNha−1 but no significant differences were noted between treatments. Soil nitrification rates and litter fall increased significantly
with treatment age. However, photosynthetically active radiation (PAR) and soil temperature showed a significant decrease
with age. No difference was found between decay rates of litter at each treatment age. By 25years, system carbon sequestration
rates were 3MgCha−1y−1, although results suggest that even by 15years, system-level attributes were progressing towards those of a natural system.
... Problems of limited land area require farmers to use agricultural land more efficiently to realize higher production per land area. One solution may be to advocate agroforestry plantation systems to utilize limited land area for social and economic objectives, as well as environmental conservation (Young 1989;Nair 1993;Kelty 2000). In agroforestry plantations, trees are planted to prevent erosion and improve poor soil, as well as to assist growth and productivity of intercropped crops (e.g., Nair 1993;Wild 1993;Farrel and Altieri 1997;Huxley 1998;Kaur et al. 2002;Pandey 2002). ...
We investigated age-related changes in biomass cycling and soil properties in a kayu putih (Melaleuca leucadendron LINN, Myrtaceae) with improved shifting cultivation system in East Java, Indonesia, to design better management plans for
sustainable kayu putih oil production and crop yield. Harvested biomass of kayu putih increased markedly from 7- to 15-year-old stands, but did not increase further in older stands averaging 2.41Mgha−1yr−1. Kayu putih contributed 1.9–31.0% of total biomass production. Cassava (Manihot esculenta Crantz) tuber was the largest component of biomass cycling. Harvested biomass of cassava decreased with increasing stand
age and there was a trade-off between above- and below-ground biomass productions. Harvested biomass of maize (Zea mays L.) grain was constant throughout stand growth, whereas returned biomass fluctuated. With increasing stand age, soil organic
matter (SOM) accumulated in soil indicating that returned biomass was decomposing slowly. Content of primary nutrients did
not change with stand age. The contribution of cassava to harvested biomass was considerably larger than that of kayu putih. For this reason, selection of cassava as a single companion crop may not be suitable for sustaining kayu putih oil production. Maize may be a good companion crop for kayu putih because it produces a constant grain yield throughout stand growth and its contribution to harvested biomass is comparable
to that of kayu putih. In addition, maize is shallow-rooted and causes less below-ground competition with deep-rooted kayu putih trees.
... At this point, we want to emphasize that agroforestry systems explicitly make use of resource complementarity and facilitation to increase and/or stabilize yields by deliberately selecting species with differing functional traits, and that many aspects of the biodiversity-ecosystem functioning relation have been discussed in that area (Ewel 1986;Huxley 1999;Ashton 2000;Kelty 2000). ...
Over the past 30 years, successional agroforestry systems (SAFS) have been increasingly promoted in Latin America as an approach for recovering soils and improving agro-ecosystems in degraded landscapes. Successional agroforestry systems (SAFS) are complex, multi-strata systems composed of species assemblages that resemble native forest structures. The concept of SAFS integrates indigenous knowledge of intercropping multipurpose subsistence species, modern agroforestry techniques, and applications of assisted natural regeneration to emphasize biodiversity, adaptive management, and the use of ecological succession to establish a productive system. Much like the management of assisted regeneration of forest stands, mimicking natural ecosystems in agroecosystems requires the knowledge of species survival, growth, functional traits, and niche resource requirements in order to appropriately select multifunctional species and to develop spatial arrangements for stratified stand structures. In recent years, conceptual theories have been proposed that support parallels drawn between natural succession models of forest stand development to management of SAFS. This chapter summarizes background theory to ground the reader in key principles of ecological regeneration and silvicultural management, provides examples that have tested biomimicry hypotheses in agroforestry systems in the tropics, and introduces three case studies from current SAFS in Brazil, Nicaragua, and Belize to examine their potential to promote agro-biodiversity, regenerate severely disturbed agricultural landscapes, diversify harvest yields, and reduce ecological and economic risks associated with conventional agricultural systems.
Over the past 30 years, successional agroforestry systems (SAFS) have been increasingly promoted in Latin America as an approach for recovering soils and improving agro-ecosystems in degraded landscapes. Successional agroforestry systems (SAFS) are complex, multi-strata systems composed of species assemblages that resemble native forest structures. The concept of SAFS integrates indigenous knowledge of intercropping multi-purpose subsistence species, modern agroforestry techniques, and applications of assisted natural regeneration to emphasize biodiversity, adaptive management, and the use of ecological succession to establish a productive system. Much like the management of assisted regeneration of forest stands, mimicking natural ecosystems in agroecosystems requires the knowledge of species survival, growth, functional traits, and niche resource requirements in order to appropriately select multi-functional species and to develop spatial arrangements for stratified stand structures. In recent years, conceptual theories have been proposed that support parallels drawn between natural succession models of forest stand development to management of SAFS. This chapter summarizes background theory to ground the reader in key principles of ecological regeneration and silvicultural management; provides examples that have tested biomimicry hypotheses in agroforestry systems in the tropics; and introduces three case studies from current SAFS in Brazil, Nicaragua, and Belize to examine their potential to promote agro-biodiversity, regenerate severely disturbed agricultural landscapes, diversify harvest yields, and reduce ecological and economic risks associated with conventional agricultural systems.
The agroforestry program of the AMISCONDE Initiative was implemented in 13 buffer zone communities of La Amistad Biosphere Reserve. This program introduced citrus (Citrus spp.) and promoted the widespread inclusion of poró (Erythrina poeppigiana) shade trees, ground story vegetation, and soil conservation techniques to the local cultivation of coffee (Coffea arabica var caturra). This program sought long-term socioeconomic and ecological health in these buffer zone communities through conservation and development projects such as coffee agroforestry systems. This paper examines the ecological and socioeconomic benefits of two introduced coffee agroforestry systems: coffee-poró and coffee-citrus. The project has decreased agrochemical inputs, integrated multi-strata vegetation, and implemented soil conservation techniques such as vetiver grass, cover crops, terraces, water channeling, and shade trees in an effort to sustainably manage coffee production on the steep buffer zone slopes. The agroforestry project of the AMISCONDE Initiative has likely improved the production of coffee ecologically and economically. However, new specialty markets should be explored to increase economic and ecological gains. Organic and fair trade coffee niche markets are suggested as alternatives for meeting the long-term AMISCONDE objectives of community development and conservation.
Predictive models were developed for Cordia alliodora branch and Theobroma cacao branch or leaf biomass,based on branch basal areas (r2 0.79) but the model of C. alliodora leaf biomass, although significant, was of very low accuracy (r2 = 0.09) due to annual leaf fall. At age 10 years, shade tree stem biomass accounted for 80% of the total above-ground biomass of either tree. However, between the ages of 6 and 10 years, the biomass increment of T. cacao branches (3–4t.ha–1.a–1) was similar to that of the shade tree stems. During the same period, the net primary productivity was 35 and 28 t.ha–1.a–1, for the Erythrina poepigiana and and C. alliodora systems, respectively.Cocoa production under either of the shade trees C. alliodora or E. poeppigiana was 1000 kg.ha–1.a–1 (oven-dry; ages 6–10 yr). During the same period, C. alliodora timber production was 9 m3.ha–1.a–1 whilst the leguminous shade tree E. poeppigiana does not produce timber. Litterfall over the same 5 years, including crop and/or shade tree pruning residues, averages 11 and 23 t.ha–1.a–1, respectively. The main difference was due to E. poeppigiana pruning residues (10t.ha–1.a–1).Soil organic material reserves (0–45 cm) increased over 10 years from 198 to 240 t.ha–1 in the E. poeppigiana plots and from 168–184 t.ha–1 in the C. alliodora plots. These values, together with the productivity indices presented, provide evidence that the systems are sustainable.For economic reasons, the use of C. alliodora is recommended under the experimental conditions. however, on less fertile soils without fertilization, the greater biomass and hence nutrient return to the soil surface under E. poeppigiana, might make this the preferable shade tree.
Coffee and cocoa are the main cash crops of Cte d'Ivoire. They are mainly produced by small farmers in a rather extensive way. The shade trees used are mostly wild forest species yielding many different products. In the Baoul region, an inventory of those trees and their, often multiple, uses was established. Of the 41 tree species, 22 are used as firewood and 16 as timber for local constructions. Nineteen furnish pharmaceutical products for traditional medicine and 15 have edible parts (fruits, leaves, flowers, palm wine). Those products are essential in daily life and play an important role in the local economy. The plantations can therefore be considered as agroforestry systems. Part of the world-wide research on coffee and cocoa should be reoriented to such systems, adapted to small farmer holdings, where few inputs are available and conditions of production are less favourable.Le caf et le cacao sont les biens d'exportation les plus importants de Cte d'Ivoire. Ils sont surtout produits de manire extensive dans des plantations villageoises. La majorit des arbres d'ombrage utiliss sont des espces forestires sauvages. En plus de I'ombre, ils fournissent une multitude de produits. Dans deux villages de la rgion Baoul, un inventaire de ces arbres et de leurs multiples utilisations a t tabli. De l'ensemble des 41 arbres recenss, 22 servent comme bois de chauffe et 16 comme bois d'oeuvre, 19 sont utiliss dans la mdecine traditionnelle et 15 fournissent des produits alimentaires (fruits, feuilles, fleurs, vin de palme). Ces produits sont indispensables dans la vie quotidienne et jouent un rle important dans l'conomie villageoise. Il est alors propos de considrer les plantations de cafiers et de cacaoyers comme des systmes agroforestiers et de rorienter une partie de la recherche effectue partout dans le monde sur des systmes de production adapts aux petits cultivateurs qui produisent avec peu de moyens dans des conditions sub-optimales.
This literature review presents information about the role of tree root systems for the functioning of agroforestry associations and rotations and attempts to identify root-related criteria for the selection of agroforestry tree species and the design of agroforestry systems. Tree roots are expected to enrich soil with organic matter, feed soil biomass, reduce nutrient leaching, recycle nutrients from the subsoil below the crop rooting zone and improve soil physical properties, among other functions. On the other hand, they can depress crop yields in tree-crop associations through root competition. After a brief review of favourable tree root effects in agroforestry, four strategies are discussed as potential solutions to the dilemma of the simultaneous occurrence of desirable and undesirable tree root functions: 1) the selection of tree species with low root competitiveness, eventually supplemented by shoot pruning; 2) the identification of trees with a root distribution complementary to that of the crops; 3) the reduction of tree root length density by trenching or tillage; and 4) the use of agroforestry rotations instead of tree-crop associations. The potential and limitations of these strategies are discussed, and deficits in current understanding of tree root ecology in agroforestry are identified. In addition to the selection of tree species and provenances according to root-related criteria, the development of management techniques that allow the manipulation of tree root systems to maximize benefit and minimize competition are proposed as important tasks for future agroforestry research.
A survey was conducted in 17 selectedthaluks (revenue sub-divisions) of Kerala state to elucidate the floristic structure, composition and the extent of similarities and diversities in the composition of homegardens of Kerala, with particular reference to small (below 0.4 ha), medium (0.5 to 2 ha) and large (>2.0 ha) holding size categories. Besides attempts wer also made to characterize the potential of homegardens to supply commercial timber and fuelwood. Two hundred and fifty two farmers were selected through a stratified random process. In addition to gathering general information on crop and livestock production enterprises, all scattered trees and shrubs (≥15 cm girth at breast height) on the homestead and border trees except palms and rubber were enumerated.
There was tremendous variability both in the number of trees and shrubs present and species diversity of the selected homesteads in different provinces. The small, medium and large sized holdings also exhibited profound variability in the number of woody taxa and individuals present. In total, 127 woody species (Girth at Breast Height (GBH) ≥15 cm) were encountered. The mean number of woody taxa found in the homegardens ranged from 11 for Pathanapuram to 39 in Perinthalmanna. Floristic diversity was higher in the smaller homesteads. It decreased with increasing the size of holdings. Mean Simpson's diversity index for the homesteads ranged from 0.251 (Kochi) to 0.739 (Kottarakkara) suggesting that floristic diversity of homegardens was moderate to low compared to a value over 0.90 for the species-rich evergreen forests of the Western Ghats. The Sorensen's similarity indexes suggested a moderately high degree of similarity for the different tree species encountered in the homesteads of Kerala.
No clear cut planting pattern was discernible in the homegardens of Kerala. The homegarden trees and shrubs were either scattered throughout the homestead or on farm boundaries. Farmers tend to prefer timber trees such as ailanthus (highest frequency) and teak besides fruit trees such as mango, jack, cashew and the like. Major homegarden species were represented in all diameter classes. The diameter structure, however, exhibited a slightly skewed (+) distribution pattern, having the highest frequency in the 20–30 cm classes ensuring adequate regeneration status and in that process making homegardens a sustainable as well as dynamic land use system.
Standing stock of timber and firewood in the homestead of differentthaluks are presented. Average commercial standing stock of homesteads ranged from 6.6 to 50.8 m3 ha−1 and fuel wood volume was of the order of 23 to 86 m3 ha−1. Implicit in the high commercial timber volume and fuel wood volume is that a substantial proportion of the society's wood demands are met from the homesteads. Palms, however, constituted the dominant component of standing commercial timber and fuel wood volumes accounting as much as 63% and 72%, respectively, of the total wood in these categories.
In this article we review the use of natural enemies in crop pest management and describe research needed to better meet information needs for practical applications. Endemic natural enemies (predators and parasites) offer a potential but understudied approach to controlling insect pests in agricultural systems. With the current high interest in environmental stewardship, such an approach has special appeal as a method to reduce the need for pesticides while maintaining agricultural profitability. Habitat for sustaining populations of natural enemies occurs primarily at field edges where crops and edge vegetation meet. Conservation and enhancement of natural enemies might include manipulation of plant species and plant arrangement, particularly at these edges; and consideration of optimum field sizes, number of edges, and management practices in and near edges. Blending the benefits of agricultural and forestry (windbreak) systems is one promising approach to field edge management that has additional benefits of wind protection and conservation of desirable wildlife species.
Hydraulic lift is the process by which some deep-rooted plants take in water from lower soil layers and exude that water into upper, drier soil layers. Hydraulic lift is beneficial to the plant transporting the water, and may be an important water source for neighboring plants. Recent evidence shows that hydraulically lifted water can promote greater plant growth, and could have important implications for net primary productivity, as well as ecosystem nutrient cycling and water balance.
ONE of the ecological tenets justifying conservation of biodiversity is that diversity begets stability. Impacts of biodiversity on population dynamics and ecosystem functioning have long been debated1-7, however, with many theoretical explorations2-6,8-11 but few field studies12-15. Here we describe a long-term study of grasslands16,17 which shows that primary productivity in more diverse plant communities is more resistant to, and recovers more fully from, a major drought. The curvilinear relationship we observe suggests that each additional species lost from our grasslands had a progressively greater impact on drought resistance. Our results support the diversity-stability hypothesis5,6,18,19, but not the alternative hypothesis that most species are functionally redundant19-21. This study implies that the preservation of biodiversity is essential for the maintenance of stable productivity in ecosystems.
The practice of growing two or more crops together is widespread throughout the tropics and is becoming increasingly practised in temperate agriculture. The benefits of nutrient exchange, reduced weed competition and pathogen control can generate substantial improvements in growth and yield. In this book John Vandermeer, a leading worker on the subject, shows how classical ecological principles, especially those relating to competition and population ecology, can be applied to intercropping. Despite the large amount of research activity directed towards the subject over the last 20 years, the practice of intercropping has, until now, received very little serious academic attention. The Ecology of Intercropping is unique in approaching the question of intercropping from a theoretical point of view. In addition the details of the approach will take as their starting point well-accepted ecological theory. Using this basis the author shows how the approach can be used to design and evaluate intercropping systems to improve agricultural yields.
Much of the world's forested land is dominated by mixed-species stands. Understanding the complex structure and dynamics of these mixtures is a necessary step in the process of formulating appropriate silvicultural systems for their management. David M. Smith, Professor Emeritus of Silviculture at Yale University, has devoted much of his career to the study of the structure, development, and silvicultural treatment of these kinds of stands. This volume is presented by Professor Smith's collegues to honor the contributions he has made to the field. It contains both reviews of past work and results of current studies of mixed stands: topics range from analysis of forest dynamics in unmanaged stands to studies of silvicultural systems applied to mixtures, with examples drawn from boreal, temperate, and tropical regions. Much of the work stresses the importance of understanding the characteristic growth patterns of individual species within mixed stands, and how species interactions shape developmental patterns.
A vegetation survey of 54 randomly-selected Pohnpeian land holdings quantified agroforest vegetation patterns in terms of horizontal distribution in the landscape and changes over time. Spatial distribution of species, expressed as distance from the main household, varied greatly with successional stages to produce the characteristic pattern of the Pohnpeian agroforest. Food crops, bananas, and Piper methysticum were planted around the house compound first, then gradually planted further away over time. Upland forest and secondary successional trees were removed by girdling, and gradually replaced by annual and perennial crops and breadfruit and other trees. A fallow or reduced management stage was discerned, characterized by low densities of early successional crops and higher densities of weedy secondary successional species; this stage resulted from a reduction in management intensity, mainly due to a variety of socioeconomic factors. Although the indigenous Pohnpeian agroforestry system is permanent and thought to be sustainable, it is dynamic in space and time.
This chapter focuses on biomass productivity of mixtures and provides a comparison of the biomass yields of mixtures with those of their components'. It discusses the types of interaction causing non-transgressive deviations of mixture yields from mid-monoculture values. The chapter discusses mechanisms causing transgressive deviation that ensure that the yield of the mixture is either less than that of the lower-yielding monoculture or more than that of the higher-yielding monoculture. Most binary mixtures have been recorded as yielding at a level between the yields of the components' monoculture. This non-transgressive yielding can be predicted on the assumption of competition among components for the same resources. Such competition leads to equal proportional increases and decreases of plant biomass compared with per-plant performance of the components in monocultures.
Preface. I: Introduction. 1. The History of Agroforestry. 2. Definition and Concepts of Agroforestry. II: Agroforestry Systems and Practices. 3. Classification of Agroforestry Systems. 4. Distribution of Agroforestry Systems in the Tropics. 5. Shifting Cultivation and Improved Fallows. 6. Taungya. 7. Homegardens. 8. Plantation Crop Combinations. 9. Alley Cropping. 10. Other Agroforestry Systems and Practices. III: Agroforestry Species. 11. General Principles of Plant Productivity. 12. Agroforestry Species: the Multipurpose Trees. 13. Component Interactions. IV: Soil Productivity and Protection. 14. Tropical Soils. 15. Effects of Trees on Soils. 16. Nutrient Cycling and Soil Organic Matter. 17. Nitrogen Fixation. 18. Soil Conservation. V: Design and Evaluation of Agroforestry Systems. 19. The Diagnosis and Design (D&D) Methodology. 20. Field Experiments in Agroforestry. 21. On-Farm Research. 22. Economic Considerations. 23. Sociocultural Considerations. 24. Evaluation of Agroforestry Systems. 25. Agroforestry in the Temperate Zone. Glossary. SI Units and Conversion Factors. List of Acronyms and Abbreviations. Subject Index.
Agroforestry models are needed to help define the biophysical boundary conditions where agroforestry may be beneficial. A generic, process-based model is outlined which couples light and water use in a tree–crop–soil system. The model predicts the net primary production (NPP) of a uniform overstorey of tropical broadleaved trees, using parts of the model Hybrid [Friend, A.D., et al., 1997. A process-based, biogeochemical, terrestrial biosphere model of ecosystem dynamics (Hybrid v3.0). Ecol. Model. 95, 249–287.] and the growth and grain yield of an understorey sorghum crop, using PARCH [Bradley, R.G., Crout, N.M.J., 1994. PARCH-User Guide. Tropical Crops Research Unit, University of Nottingham, Sutton Bonington, Leicestershire, UK, 122 pp.]. Belowground competition for water is simulated. The model is driven by daily climate, which can be generated for any half-degree pixel in the world. The performance of the model is illustrated for a site in Ghana. The model gives validated output of the NPP of natural forest/woodland vegetation and of potential sorghum grain yield. Tests of the sensitivity of the model to simplifying assumptions are presented and discussed. It is concluded that the model provides a useful tool to explore opportunities for complementarity of light and water use by trees and sorghum in a range of climates.
The diversity of plant life forms in tropical forests affords the opportunity for assembly of plant combinations, both natural and managed, that make full use of resources and sustain high productivity. The influence of combining life forms on productivity and resource use was evaluated using three fast-growing tree species (Hyeronima alchorneoides, Cedrela odorata, and Cordia alliodora); each species was grown alone and with two perennial, large-stature, self-supporting monocots (Euterpe oleracea and Heliconia imbricata). Aboveground net primary productivity was extremely high in all stands. The monocots did not contribute significantly to the productivity of the Hyeronima polycultures, which was 4.5-8.4 g.m(-2).d(-1) between 18 and 36 mo. In contrast, the monocots accounted for 57% of the productivity (9.7 g.m(-2).d(-1)) of Cedrela polycultures and 67% of the productivity (6.8 g.m(-2).d(-1)) of the Cordia polycultures, by age 3 yr. The leaf area and density of fine roots in the Cedrela and Cordia polycultures were also significantly increased by the presence of the monocots, reaching or surpassing the levels found in the Hyeronima stands. The high productivity of Hyeronima, coupled with poor growth of its interplanted monocots, indicated that Hyeronima was able to achieve nearly complete use of resources. Ecosystem productivity and resource capture were increased when the monocots were grown with the other two tree species, and this occurred because of the inability of the tree species to completely utilize available resources, which provided an opportunity for the monocots to flourish in the understory. Monocot productivity in the Cedrela stands was additive to that of the trees, indicating complementary resource use between the monocots and this tree species. In the case of Cordia, tree productivity was slowed by the monocots, but this decline was more than compensated for by the high productivity of the associated monocots. Whether in natural forests or human-constructed agroforestry systems, the presence of dominant species that do not fully exploit all available resources allows the coexistence of other species and creates the potential for complementary resource use. The resource use characteristics of such species should be a key consideration in forest restoration efforts and in the design of sustainable land use systems.
A generic, process-based model [Mobbs, D.C., Cannell, M.G.R., Crout, N.M.J., Lawson, G.L., Arah, J., Friend, A.D., 1997. Complementarity of light and water use in tropical agroforests: I. Model outline, performance and sensitivity. For. Ecol. Manage. 102, 275–282] was used to calculate 50-yr mean potential sorghum grain yields and overstory tree annual net primary productivities (NPP) in nine climates (with 348–2643 mm rainfall, ranging from Mali to southern Nigeria) with uniform overstory leaf area indices (LAIs) of zero to 1.5. It was concluded that in regions with less than about 800 mm rainfall, simultaneous agroforestry may enable more light and water resources (current rainfall) to be `captured' than sole cropping, giving complementarity of resource use. However, owing to the low water use efficiency of (C3) trees at dry sites and the sensitivity of (C4) crop yield to shading, it may be difficult to increase total site productivity by growing trees with crops in regions with less than 800 mm rainfall without jeopardising food security—unless tree roots tap the water table, trees improve soil fertility and/or they produce biomass of high value (fuel, gum, fodder, fruits).
Fertility of a volcanic-ash derived Inceptisol was monitored at a site in the humid tropics of Costa Rica. After forest felling and burning, the authors established four treatments in a randomized block design with six blocks: a sequence of monocultures (two crops of maize Zea mays followed by cassava Manihot esculenta, then the tree Cordia alliodora), successional vegetation, a mimic of successional vegetation, and a species-enriched version of successional vegetation. Species-rich successional vegetation was effective at maintaining soil fertility, although there were general trends of soil-nutrient decline beneath all treatments, presumably because of plant uptake. Successive peaks of nitrate-N in soil solution, extractable P and extractable K occurred during the 1st yr, perhaps driven by an early increment of organic matter from postburn debris and roots. Organic matter, total N and extractable S were remarkably stable during the 5-yr period. Depletions of cations, decreases in effective cation exchange capacity, and increases in acid saturation were related to treatment in the following order: bare soil > monocultures > the three diverse, successional communities. In the bare-soil plot, fertility decreased dramatically: there was a net loss of exchangeable cations and inorganic N, the P-fixation capacity increased, and acid saturation reached a potentially toxic 86%. The less fertile soil under monocultures proved exceptionally vulnerable to loss of fertility; after 5 yr under monocultures, for example, acid saturation reached 38% in the more fertile soil and 75% in the less fertile soil. In the species-rich communities, however, changes in soil fertility were far less marked. -from Authors
A review is made of the ecological interactions that occur between shade trees and the perennial crops: coffee (Coffee spp. L.), cacao (Theobroma cacao L.) and tea (Camellia sinensis L. Kuntze). These interactions are classified firstly as advantages or disadvantages, and secondly as: effects on crop management; effects on the hydrological cycle; effects on pathogens, insects and climatic conditions; and effects on soils. References are given for the 20 advantageous and 16 disadvantageous consequences of using shade trees, emphasizing publications that provide original data and useful methodologies. Finally a check list of desirable characteristics for perennial crop shade trees is presented.Se hace revisin de las interacciones ecolgicas que ocurren entre rboles de sombra y los cultivos perennes: caf (Coffee spp. L.), cacao (Theobroma cacao L.) y t (Camellia sinensis L. Kuntze). Estas interacciones fueron clasificadas en primer nivel como ventajas o desventajas, y en segundo nivel como: efectos sobre manejo de los cultivos; efectos sobre el ciclo hidrolgico; efectos sobre patgenos, insectos y condiciones climticas; y afectos sobre los suelos. Se dan referencias para 20 consecuencias ventajosas y 16 consecuencias desventajosas al utilizar rboles de sombra, dando nfasis a publicaciones que proveen datos originales y metodologias tiles. Finalmente se presenta una lista de las caractersticas deseables para rboles de sombra para cultivos perennes.
Agroforestry research is being transformed from a collection of largely descriptive studies into more scientific approaches, based on process-oriented research. The development of agroforestry as a science should be based on four key features:competition, complexity, profitability and sustainability. Managing thecompetition between trees and crops for light, water and nutrients to the farmers' benefit is the biophysical determinant of successful agroforestry systems. Simultaneous agroforestry systems are more susceptible to competition than sequential ones. A tree-crop interaction equation helps quantify competition vs. complementary effects on fertility. Alley cropping, a simultaneous agroforestry system, has limited applicability because the competition factor usually exceeds the beneficial fertility effects. TheFaidherbia albida parkland, another simultaneous system, is almost always beneficial since the reverse phenology ofF. albida minimizes competition while enhancing the fertility effect. Sequential systems such as relay intercropping and improved fallows also minimize competition but the processes responsible for crop yield increases are largely unquantified. New methodologies for reliably measuring complex below-ground interactions are being developed.
Socioeconomic and ecologicalcomplexity are typical of agroforestry systems. Participatory, analytical and multidisciplinary characterization at different spatial scales is the required first step in effective agroforestry research. Diversity of products and services should be manipulated in a way that puts money in farmers' pockets. Domestication of indigenous trees with high-value products enhancesprofitability, particularly those that can be marketed as ingredients of several finished products. Policy research interventions are often necessary to help farmers during the initial years before trees become productive and exert their positive ecological functions.Profitable agroforestry systems are potentiallysustainable, controlling erosion, enhancing biodiversity and conserving carbon, provided nutrient offtake is balanced by nutrient returns via litter and the strategic use of fertilizers, particularly phosphorus. A list of research gaps indicates where hard data are needed to provide a predictive understanding of the competition, complexity, profitability and sustainability aspects of agroforestry.
A major tenet of agroforestry, that trees maintain soil fertility, is based primarily on observations of higher crop yields near trees or where trees were previously grown. Recently objective analyses and controlled experiments have addressed this topic. This paper examines the issues of tree prunings containing sufficient nutrients to meet crop demands, the timing of nutrient transfer from decomposition to intercrops, the percent of nutrients released that are taken up by the crop, and the fate of nutrients not taken up by the crop.
The amount of nutrients provided by prunings are determined by the production rate and nutrient concentrations, both depending on climate, soil type, tree species, plant part, tree density and tree pruning regime. A large number of screening and alley cropping trials in different climate-soil environments indicate that prunings of several tree species contain sufficient nutrients to meet crop demand, with the notable exception of phosphorus. Specific recommendations for the appropriate trees in a given environment await synthesis of existing data, currently only general guidelines can be provided.
Tree biomass containing sufficient nutrients to meet crop demand is not enough, the nutrients must be supplied in synchrony to crop needs. Nutrient release patterns from organic materials are, in part, determined by their chemical composition, or quality. Leguminous materials release nitrogen immediately, unless they contain high levels of lignin or polyphenols. Nonlegumes and litter of both legumes and nonlegumes generally immobilize N initially. There is little data on release patterns of other nutrients. Indices that predict nutrient release patterns will assist in the selection of species for synchronizing with crop demand and improve nutrient use-efficiency.
Field trials with agroforestry species ranging in quality show that as much as 80% of the nutrients are released during the course of annual crop growth but less than 20% is captured by the crop, a low nutrient-use efficiency. There are insufficient data to determine how much of the N not captured by the crop is captured by the trees or is in the soil organic matter, the availability of that N to subsequent crops, or how much of that N is lost through leaching, volatilization or denitrification. Longer term trials are needed.
Forest-covered home gardens around the tropical world vary in their structure, but serve to supply food and other products for direct family consumption or marketing. Little quantitative data exist defining home garden structure. Thus, this study was undertaken to examine the variation in home garden structures in response to market or household needs and the subsequent variation in light interception and productivity.Four home gardens were studied in the Department of the Petn, in northern Guatemala. Areal extent and height of canopies were mapped in transects through four gardens representing a range of site water regimes and market orientation. Light intensities incident on the ground were measured along each transect to assess light use. Results showed structural complexity, with full canopy closure in the one or more layers within the canopy for most gardens. The garden architectures made efficient use of light and space, with intersive management for food and fuel production. Farmers grew the crops for both cash and family subsistence. One home garden on a comparatively dry site with shallow soil seemed less structured, with only a single broken canopy layer, but with diverse species of plants.The results indicate that development of gardens in this area utilized existing trees, thinning them to leave the most useful, and inserting other desirable trees and shrubs in the understory and in open spaces. This strategy seemed to maximize light use, regardless of market orientation.This look at the structure and composition of four home gardens, in a forested area of current immigration, demonstrated (1) variety of organization and plant components, (2) different architecture for different soil/site conditions and market orientations, and (3) efficient use of available light through the arrangement of plants.
Fine and coarse root mass and fine root surface area were studied during 5 yr following the felling and burning of a tropical
forest near Turrialba, Costa Rica. Five experimental ecosystems were established: 1) natural successional vegetation, 2) successional
vegetation enriched by seed applications, 3) imitation of succession (built by substituting investigator-selected species
for natural colonizers), 4) monocultures (two maize crops followed by cassava andCordia alliodora), and 5) a bare plot.
Fine roots grew rapidly in all treatments during the first 15 wk, at which time there were 75 gm−2 in the monoculture and 140 gm−2 in the enriched and natural successions. Subsequent growth was slower, and fine-root mass decreased during the first dry
season. After 5 yr coarse root mass to a depth of 85 cm was about 800, 1370, and 1530 gm−2 in the succession, enriched succession and imitation of succession, respectively. At the final harvest, the 3.5 yr-oldC. alliodora plantation had 1000 g m−2 of coarse-root biomass. Roots <1 mm in diameter were concentrated in the upper 5 cm of soil and accounted for most fine-root
surface area. Total fine-root surface area was greatest in the enriched successional vegetation and usually lowest in the
monoculture.
An analogue approach to analysing the traditional homegardens of Keralavis--vis natural climax ecosystems is adopted. The traditional homegarden is apparently a climax ecosystem, where ecological succession is consciously manipulated by human beings. High intensity of vertical and horizontal space use, the highly dynamic chronological structure and the capacity to perform essential ecological processes make this ecosystem relatively sustainable. Homegardens in Kerala effectively serve as human ecosystems with their low input demand, staggered supply of outputs, and enhancement of habitat quality. However, recent trends in agrarian structure and the high market orientation exert pressures on the homegarden, and its sustainability as a human ecosystem is in question.
Tropical home gardens are an agroforestry practice found worldwide. From an analytical perspective little is known about this practice. The universality of their complex structure would indicate the existence of common objectives. The objectives and resulting implementation strategies will dictate the form, layout, species composition and management style of each home garden. Understanding the interrelationships between these various elements is one step toward utilising the advantages of home gardens in a range of agroforestry practices.
Agroforestry is a general concept for a land management system combining trees and agricultural crops. For application, various specific techniques can be chosen. Each of these techniques is adjusted to a specific set of environmental as well as socio-economic factors. Agroforestry cultivators or managers belonging to varying social strata and institutional groupings may practice different forms of agroforestry, even within the same general region. This is demonstrated on the basis of two contrasting types of agroforestry which are found on the Indonesian island of Java. Tree gardening or the cultivation of a wide variety of crops in a multiple-storeyed agroforestry system is an indegenous practice on private lands, while taungya or the intercropping of young tree plantations with staple crops is practiced on state forest lands. Both systems are described as to their management characteristics, past development as well as possibilities and constraints for further development. These two practices are then compared as to various attributes, like producer group, production purpose, area of cultivation, land ownership situation, structural organization of crop combinations, possibilities for improved cultivation techniques, and suitability for application in rural development for specific target groups.
A field experiment was conducted for eight cropping seasons from 1988 to 1991 in semiarid Machakos, Kenya, to compare the productivity of shrubs and crops in intercropping (alleycropping) versus block (sole) planting systems. The study, conducted in a split-plot experiment with three replications, consisted of two tree species (Leucaena leucocephala andSenna siamea syn.Cassia siamea) in the main-plots and combinations of two planting systems (alleycropping and block planting of the shrubs and maize) with five different ratios of land allocation for the shrub and crop (1000, 1585, 2080, 2575, and 0100) in sub-plots. Thus, a sole maize and a sole tree were included in the subplot treatments. The spacing between hedgerows of shrubs in intercropping was 6.7, 5, and 4 m, respectively, corresponding to 15, 20, and 25% land allocation to shrubs. The trees were pruned to 0.5 m height four times a year. Intercropped hedgerows of senna and leucaena produced 10% and 24% more biomass than their respective block planting systems. On average, leucaena produced more biomass than senna. Maize alleycropped with leucaena yielded 16% less grain than sole-crop maize, whereas senna intercropping caused hardly any maize-yield reduction. Compared with the respective sole-crop systems, leucaena intercropping did not affect land equivalent ratio (LER), whereas LER increased by 28% with senna intercropping. The different tree:crop land occupancy ratios did not affect the production of either the component species or of the total system, except that LER declined with incrreased spacing between hedgerows. It is concluded that in semiarid highlands of Kenya, leucaena and crops should better be grown in sole blocks, not in alleycropping. In the case of slow-growing species such as senna, intercropping is worthwhile to consider only if the additional labour needed does not pose a serious problem for management, and the species has fodder value.
In the ‘Experimento Central’ of CATIE (Centro Agronómico Tropical de Investigación y Enseñanza, Turrialba, Costa Rica) studies on organic matter and nutrient cycling have been carried out in the following agroforestry systems, planted in 1977: •- Theobroma cacao with Cordia alliodora and Erythrina poeppigiana;•- Coffea arabica with the same shade trees;•- Cynodon plectostachyus (star grass) associated with the same trees.Results are presented and discussed using the T. cacao systems as an example. The accumulation of organic matter, measured for the different species (leaves, branches, trunks, roots, fruits and litter) at an age of 5 years (1982) and of 10 years is very large: 50.3 and 110.6 t ha−1 for T. cacao/C. alliodora.The average cacao bean harvest at an age of 6–10 years reached 1036 and 1057 kg ha−1 a−1 under shade of C. alliodora and E. poeppigiana, respectively. Total stem volume growth of C. alliodora is presently 9.6 m3 ha−1 a−1. Measurements of the natural leaf fall and of prunings were made over 5 years, reaching 8.11/3.29 t ha−1 a−1 for T. cacao/C. alliodora and 9.29/13.57 t ha−1 a−1 for T. cacao/ E. poeppigiana, respectively.With the values obtained, quinquennial models for organic matter and nutrients are presented and discussed.
COMMUNITIES of species and their associated biological, chemical and
physical processes, collectively known as ecosystems, drive the Earth's
biogeochemical processes1,2. Currently most ecosystems are
experiencing loss of biodiversity associated with the activities of
human expansion3-5, raising the issue of whether the
biogeochemical functioning of ecosystems will be impaired by this loss
of species6-8. Current ecological knowledge supports a wide
range of views on the subject9-13, but empirical tests are
few9,14-16. Here we provide evidence from direct experimental
manipulation of diversity by over an order of magnitude, using
multi-trophic level communities and simultaneous measures of several
ecosystem processes, that reduced biodiversity may indeed alter the
performance of ecosystems.
To investigate how plant diversity affects ecosystem-level processes such as primary production and nutrient cycling, I established an experimental plant diversity gradient in serpentine grassland using four functional groups of plants: early season annual forbs (E), late season annual forbs (L), perennial bunchgrasses (P), and nitrogen fixers (N). These groups differ in growth form, phenology, and other traits relevant to nutrient cycling (e.g., rooting depth, litter C:N ratio). Two or three species of each type were planted in single-group treatments, and in two-, three-, and four-way combinations, giving a range of richness from zero to nine species. I tested the hypothesis that, because of complementary resource use, increasing functional group diversity will lead to higher net primary production. At the scale of this experiment (α-diversity and yearly production), more diverse treatments were not necessarily the most productive. Live plant biomass varied more within than among levels of diversity. In most two-, three-, and four-way mixtures of functional groups, overall productivity did not differ significantly from the average of the yields of component one-group treatments. This pattern apparently resulted from competition: early season annuals and late season annuals reduced the biomass of perennial bunchgrasses (the most productive group in monoculture) below levels expected from monoculture yields. Relative Yield Totals (RYT) indicated complementary resource use in the EL and LP two-way and ELPN four-way mixtures. In the EL mixture, complementarity appeared to result from temporal rather than spatial partitioning of resources. Because of shifts in root: shoot ratio in mixtures, however, only the LP treatment had consistently significant RYT > 1 when assessing total (roots plus shoots) productivity and nitrogen yield. These results show that (1) composition (the identity of the species present) can be at least as important as richness (the number of species present) in effects on ecosystem processes; (2) competition during critical parts of the growing season may prevent absolute increases in net primary production with increasing diversity, despite complementary resource use at other times of the year; and (3) shifts in belowground allocation in species mixtures can have significant effects on estimates of productivity and resource use as species diversity changes.
Abstract Belowground competition occurs when plants decrease the growth, survival, or fecundity of neighbors by reducing available soil resources. Competition belowground can be stronger and involve many more neighbors than aboveground competition. Physiological ecologists and population or community ecologists have traditionally studied belowground competition from different perspectives. Physiologically based studies often measure resource uptake without determining the integrated consequences for plant performance, while population or community level studies examine plant performance but fail to identify the resource intermediary or mechanism. Belowground competitive ability is correlated with such attributes as root density, surface area, and plasticity either in root growth or in the properties of enzymes involved in nutrient uptake. Unlike competition for light, in which larger plants have a disproportionate advantage by shading smaller ones, competition for soil resources is apparently more symmetric. Belowground competition often decreases with increases in nutrient levels, but it is premature to generalize about the relative importance of above- and belowground competition across resource gradients. Although shoot and root competition are often assumed to have additive effects on plant growth, some studies provide evidence to the contrary, and potential interactions between the two forms of competition should be considered in future investigations. Other research recommendations include the simultaneous study of root and shoot gaps, since their closures may not occur simultaneously, and improved estimates of the belowground neighborhood. Only by combining the tools and perspectives from physiological ecology and population and community biology can we fully understand how soil characteristics, neighborhood structure, and global climate change influence or are influenced by plant competition belowground.