Читать книгу North American Agroforestry - Группа авторов - Страница 80

Ecological goods and services are defined as logical benefits resulting from the “normal” functioning of an ecosystem. Maximum production of such goods and services is associated with unstressed agroecosystems, which within the context of agroforestry would constitute a variety of temporal and spatial configurations of trees on the farming landscape. Humans benefit from the maintenance of these goods (e.g., fresh water) within the ecosystem, and the “flow” of these services (e.g., greenhouse gas mitigation) to other systems.

Agroforestry systems, regardless of type, are capable of providing numerous ecological goods and services, of a range of complexities, over long periods of time (Hunt, 2005; Jose, 2009; Nair, Gordon, & Mosquera‐Losada, 2008). Indeed, agroforestry systems can be designed and engineered to provide specific quantities of particular goods and services. Nonetheless, the universal application of ecological principles to agroforestry system design and management is nearly impossible as a result of the many varied types of systems in existence—from riparian management systems that link terrestrial and aquatic systems to more traditional systems that integrate perennial plants with annual crops, with or without animals. The broad geographical range across which agroforestry systems may be successfully implemented and the scale at which interactions occur—from landscape to individual plant—also complicates the development of a universal understanding of nutrient and energy flows and the relationship of these to system productivity.

Although systems will differ in the nature and types of environmental services provided, some generalizations can be stated. Most agroforestry systems will tend to improve soils, including productivity, largely through the incorporation of organic matter and C into upper soil profiles from the production of annual litterfall from the tree component. As a result of the presence of perennial root systems, soil erosion relative to monocropped agroecosystems is often minimized.

With respect to the maintenance and proliferation of biodiversity, agroforestry systems often enhance the components of biodiversity, at scales ranging from the stand (farm level) to regional landscapes (Jose, 2009; Nair et al., 2008). Jose (2009) categorized the contributions of agroforestry systems toward biodiversity conservation into five major roles, which include: (a) species habitat provisioning; (b) germplasm preservation for sensitive species; (c) reduction of the rates of natural habitat loss through providing a more productive and sustainable alternative to traditional agricultural systems that may involve clearing natural habitats; (d) creation of corridors to connect habitat remnants for floral and faunal species; and (e) prevention of land degradation and habitat loss through provisioning additional ecosystem services such as erosion control and soil health enhancements. The potential for agroforestry systems to contribute to biodiversity conservation is especially high in the fragmented agricultural landscapes of North America, where widespread conventional agricultural practices are undoubtedly contributing to species decline across the continent. Gibbs et al. (2016) found that in a mature temperate tree‐based intercropping system, avian species richness was nearly 1.5 times greater than in a conventionally managed sole‐crop agricultural system (32 vs. 23 unique bird species). Additionally, avian diversity was more than twice as great in the tree‐based intercropping system than in the sole‐crop agricultural system (Shannon–Wiener diversity index values of 2.9 vs. 1.2). Tree‐based intercropping systems may therefore be one method for slowing or even reversing the decline of migratory bird populations in North America, as seen during the last several decades.

Agroforestry systems are being promoted as a means of mitigating climate change as a result of their C sequestration potentials. In all systems, the storage of C is enhanced (Jose, 2009, 2019; Nair et al., 2008; Thevathasan & Gordon, 2004), not only through the perennial nature of the trees, but also through increased soil C storage. The C sequestration potential for agroforestry systems is dependent on the type of agroforestry system, in addition to species composition and age, geographic location, environmental factors, as well as system management practices (Jose, 2009). A 2006 study examining the C sequestration potentials in a 13‐yr‐old temperate tree‐based intercropping system found that the carbon sequestration potential of systems incorporating barley (Hordeum vulgare L. ‘OAC Kippen’) and hybrid poplar (Populus deltoids × Populus nigra clone DN‐177) were four times greater than in a barley and Norway spruce (Picea abies L.) system and five times greater than the examined sole‐cropped barley system, with net C fluxes of 13.2, 1.1, and −2.9 Mg C ha⁻¹ annually (Peichl, Thevathasan, Gordon, Huss, & Abohassan, 2006). Wotherspoon, Thevathasan, Gordon, and Voroney (2014), utilizing the same research site, also quantified the C sequestration potential of the then 25‐yr‐old temperate tree‐based intercropping system and found net C fluxes for the soybean and hybrid poplar, soybean and Norway spruce, and sole‐crop soybean systems of 2.1, 1.6, and −1.2 Mg C ha⁻¹ yr⁻¹.

In addition to enhancing system‐level C sequestration, agroforestry systems may also contribute to reduced greenhouse gas emissions (Thevathasan & Gordon, 2004). Through reduced fertilizer requirements and more efficient N cycling in tree‐based intercropping systems, N₂O emissions reductions of nearly 1 kg ha⁻¹ yr⁻¹ compared with conventionally managed agricultural fields have been reported (Evers et al., 2010). Graungaard (2015) found that both tree species and proximity to trees influenced soil microbial communities. This study utilized a modified denitrification enzyme assay, which indicated a greater potential for N₂O production within tree‐based intercropping systems comprised of hybrid poplar versus red oak (Quercus rubra L.). Tree species themselves are associated with unique microbial communities within agroforestry systems, which may play a role in ecosystem functioning, including N₂O and other greenhouse gas emissions.

Agroforestry systems also have the potential to reduce agricultural runoff, reducing sedimentation, nutrient runoff, and the leaching of pesticides into nearby waterbodies and beyond, contributing to eutrophication in the Great Lakes, the Gulf of Mexico, and elsewhere across the continent (Jose, 2009). Improving the quality of surface water that is adversely affected by runoff from heavily fertilized row‐crop and pasture systems is an environmental benefit of agroforestry systems that is just beginning to be realized in quantitative terms (Michel, Nair, & Nair, 2007). Integrated riparian management systems address the interaction of terrestrial and aquatic environments in farming landscapes and can make major contributions to water quality at local scales and provide connectivity in agricultural landscapes at much larger scales (Schultz et al., 2000). Riparian buffers are able to reduce non‐point‐source pollution from agricultural fields through reduced runoff velocity and promotion of infiltration, increased nutrient retention through trees utilizing excess nutrients transported in runoff, and increased sediment deposition on land (Jose, 2009).

In intercropping systems, microclimate modification is common, and although the competition for water, light, and nutrient resources between the tree and crop components is complicated, improved and sustained crop yields have been noted (Thevathasan & Gordon, 2004). Due to enhanced structural diversity within agroforestry systems, microclimatic modifications and therefore plant growing conditions are not homogeneous as they are within conventional agricultural systems. A recent study by Coleman et al. (2020) found that both abiotic (light, soil moisture) and biotic (available soil nutrients) gradients within a 26‐yr‐old tree‐based intercropping system intercropped with concentrated short‐rotation willow (SV1; Salix dasyclados Wimm.) had significant influences on intraspecific variation in crop leaf traits, including increased specific leaf area and crop leaf N concentrations closest to the tree rows. These results contribute to an enhanced understanding of nutrient cycling within agroforestry systems and indicate that tree litter inputs may reduce the need for crop amendments, especially near the tree rows.

The presence of trees within agroforestry systems can also have more indirect influences on nutrient cycling and soil fertility. Price and Gordon (1998) examined the spatial and temporal distribution of earthworms in an 11‐yr‐old tree‐based intercropping system planted with silver maple, white ash (Fraxinus americana L.), and hybrid poplar, in combination with soybean. The researchers found the greatest density of earthworms within the tree rows, with typically decreasing earthworm density towards the middle of the cropping alley. Earthworm density was drastically reduced in the summer, potentially tracking with reduced food availability (litterfall) and reduced soil moisture compared to the spring, and earthworm distribution tended to become more uniform during the summer. The authors found that earthworm density was highest near poplars, providing further evidence of the importance of tree species selection when considering soil fertility and other ecosystem functions.

Many additional goods and services can be provided by the suite of recognized agroforestry practices, including odor control (Tyndall & Grala, 2009), opportunities to embrace integrated pest management systems with reduced pesticide input (Diaz‐Forestier, Gomez, & Montenegro, 2009), and the control of Escherichia coli outbreaks associated with manure application (Dougherty, 2007). Agroforestry systems can also enhance nutrient cycling and nutrient use efficiency with subsequent improvements in downstream water quality and reduced requirements for crop amendments (Jose, 2009). Thevathasan and Gordon (1997) utilized a 7–9‐yr‐old hybrid poplar tree based intercropping system planted with barley and found that mean nitrification rates, N availability, and C content were higher in soils closest to the poplar tree rows compared with the middle of the crop alley. It was also found that soil nitrification rates, soil C, and plant N uptake adjacent to the tree rows were influenced by the leaf biomass inputs of the preceding year, potentially contributing to increased aboveground biomass and greater grain N concentration in the barley intercrop.

In natural systems, a long‐term ecological approach has proven useful to understanding the importance of (a) slow processes that occur on the scale of decades to centuries, (b) processes with high annual variability, (c) rare and unique events, (d) subtle processes, and (e) complex processes with many interacting factors. A long‐term ecological research perspective also holds much potential for helping us understand agroforestry systems. The temporal context provided by engaging in such research can aid us greatly in understanding large‐scale changes in ecosystem processes and thereby reveal the secrecy inherent in what has been termed “the invisible present” (Magnuson, 1990).

Such an approach to understanding the structure and function of agroforestry systems and the relationship of these parameters to net primary productivity is a strong foundation upon which to evaluate the production of ecological goods and services over long periods of time (Gordon & Jose, 2008).