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Agroforestry Benefits in Canada

Agroforestry Benefits in Canada

, by Nina Mgrdichian, 37 min reading time

Agroforestry Benefits in Canada

Introduction

“Put simply, agroforestry is using trees on farms. ICRAF defines agroforestry as a dynamic, ecologically based, natural resources management system that, through the integration of trees on farms and in the agricultural landscape, diversifies and sustains production for increased social, economic and environmental benefits for land users at all levels.” (International Centre for Research in Agroforestry, 2002). Garrett et al. (2000) outline four key criteria that characterize agroforestry practices and distinguish them from other practices. In order for a practice to be considered agroforestry the following criteria need to be clearly identified in the practice:

 

Intentional - combinations of trees, crops and/or livestock intentionally designed, established and managed to work together and yield multiple products and benefits

Intensive - practices are intensively managed to maintain productive and protective functions

Integrated - components are combined into a single integrated management unit tailored to meet objectives of the landowner

Interactive - manipulates and utilizes the biophysical interactions among components to yield multiple harvestable products, while concurrently providing numerous conservation and ecological benefits

 

Agroforestry in Agro-Ecosystems

 In Canada, the need for agroforestry is being driven by the need to lessen environmental impacts of modern agriculture and balance productivity, profitability, and environmental stewardship; as well as the desire to stabilize and diversify rural economies.

For agroforestry to be successful it must offer viable options that are compatible with production agriculture and involve minimal tradeoffs to producers. Agroforestry must be economically viable and add value to the land. The bottom line for agroforestry is to be able to locally apply practices that generate predictable and positive interactions and optimize them for the benefit of the farmer and society as a whole.

Common features of agricultural landscapes in Canada are monocultured fields, woodlots, marsh areas and pasture. At a landscape level this is not agroforestry because there is no intentional integration and there is minimal interaction. Agroforestry at a landscape level would contain strips of woody vegetation either planted or retained that were strategically placed to maximize conservation benefits and create biophysical interactions with other components of the agricultural system.

 

Agroforestry Practices in Canada

Shelterbelts and Hedgerows

Shelterbelts and Hedgerows are linear features of agro-ecosystems comprised of trees and/or shrubs that form part of an agriculture production system. This includes trees and/or shrubs planted or retained as a barrier to reduce wind speed and to protect crops, livestock, buildings, work areas and roads from wind and snow as well as enhance biodiversity. Shelterbelts can be located around farmsteads, adjacent to roadsides, on the boundaries or within fields or around livestock facilities.


Riparian Buffers

Strips of permanent vegetation consisting of trees, shrubs and grasses planted or managed between agricultural land and water bodies to reduce runoff, and non-point source pollution, stabilize streambanks, improve aquatic and  terrestrial habitats (Schultz et al. 2000)

Alley Cropping

This practice involves the growing of an annual or perennial crop between rows of high value trees. The agricultural crop generates annual income while the longer term tree crop matures

Silvopasture

This practice combines growing trees for wood products with pasture and livestock production. These systems either have trees added to existing pasture or involve thinning an existing forest stand and adding or improving a pasture. The trees are managed for their timber value and at the same time provide an enhanced microclimate for livestock.

 

Agroforestry Benefits

A well-designed agroforestry system can include one or all of the above practices and serves multiple purposes but it usually includes both environmental benefits and economic benefits. This paper reviews some of the benefits associated with the most common agroforestry practices in Canada specifically shelterbelts and riparian buffers.

 

Wind

Many shelterbelt and hedgerow benefits are because of the ability of their to reduce windspeed. As the wind approaches the shelterbelt, some wind moves through the shelterbelt by the force of the wind. However, the resistance by the trees creates back pressure which causes some of the wind to be forced over the shelterbelt. The horizontal windspeed for a short distance upwind of the shelterbelt is less because of this back pressure. The air going through the shelterbelt moves more slowly and becomes turbulent because of the interference by the trees. The pressure in this “quiet zone” is less than that of the air moving over the shelterbelt so that the higher wind above the shelterbelt is gradually drawn back toward the ground. Shelterbelts give very good protection in the “quiet zone” which is normally 6 to 8 times the height of the shelterbelt. Scale models have been used to show that this is true regardless of the shelterbelt’s height (Woodruff and Zingg, 1952). For this reason, the extent of shelterbelt protection is usually expressed in units of “h” (ie. 6h is 6 times the height of the shelterbelt). Beyond the “quiet zone” there is a zone in which the windspeed gradually recovers to its unsheltered value. This distance, which McNaughton (1988) calls the “wake zone”, can extend as far as 20h. In this zone, turbulence is less than in the “quiet zone” but horizontal windspeed is greater.

 

The porosity of the shelterbelt has an effect on the extent of the “quiet zone” and the “wake zone” and so does the angle at which the wind hits the shelterbelt. More dense shelterbelts are more resistant to the wind and reduce the windspeed more but the protected zone is less. The zone of protection for a shelterbelt is greatest when the wind is perpendicular. T

 

Soil Erosion

Erosion by wind

Field shelterbelts have been planted on the prairies since the early 1900’s to protect the soil from wind erosion. However, it was not until later, that this protective function was well understood. Bagnold (1943) showed that the amount of soil movement due to wind erosion was approximately related to the cube of the wind velocity. Chepil (1945) found that there was a threshold windspeed which was required to initiate the movement of soil and that bouncing soil particles dislodged further particles so that there was an avalanching effect. Hagen (1976) used this relationship to show the reduction of wind erosion by shelterbelts and concluded that shelterbelt effects could extend as far as 30h.  However, in the development of the Wind Erosion Equation (Woodruff and Siddoway, 1965), this relationship was simplified so that shelterbelt protection of soil was assumed to be complete to a distance of 10h and that no shelterbelt effect extended beyond 10h. Shelterbelts can also trap significant amounts of soil blown from adjacent fields.  Soil cesium was found to be much higher near a Saskatchewan shelterbelt  than in the middle of the field (PFRA Shelterbelt Centre, 1987).

Erosion by water

Trees and shrubs may reduce soil erosion by water in several ways. Riparian zone plantations will be discussed in more detail in a later section but bio-engineering of streambanks often involves the use of trees or shrubs. Poplars and willows are adapted to such an environment and live poles planted in well-designed patterns will result in rapidly-growing trees and shrubs that can be important in stabilizing streambanks (Isebrands and Karnosky, 2001; National Agroforestry Center, 2002). In the same way, physical stabilization of soil by tree roots can be effective in reducing the spread of active gullies and have been used for this purpose in Russia (Schroeder and Kort, 1989)

By trapping snow, well-placed shelterbelts can direct spring meltwater into catchments  or other areas where it does not present an erosion risk. Snowmelt in forest plantations is less because of the shading by the trees so that peak flows in streams may be reduced (Kort et al., 1998). Good snow cover in forested areas insulates the underlying soil and, since this soil generally has increased soil permeability, meltwater infiltration into the soil is often better (Kort et al., 1998). By reducing overland flow, erosion is reduced. This also true for summer storm events. Agronomists have long advocated the planting of shrubs along land contour in rolling countryside (van Eimern et al. 1964). When they form a physical barrier, they physically trap sediment from sheet erosion above the tree row and, over time can form terraces which further reduce erosion.

The snow trapped by shelterbelts on slopes can result in water erosion if it melts quickly in the spring or if the soil below it is wet and frozen. Each condition results in an increase of overland flow with an increase in water erosion if the soil is not adequately protected by grass or crop residue. 

Tree buffers can also reduce water erosion by trapping sediment, especially when protecting riparian sites (Dosskey, 2001). Trees or shrubs have also been used to anchor soil, reducing streambank erosion or gully formation. In such cases, their effectiveness depends on proper design.

 
Microclimate and Crop Growth

The reduction of windspeed and the increase in turbulence near a shelterbelt has several effects on microclimate, thereby affecting the growth of plants in the protected zone. Especially in the “quiet zone”, temperature and humidity are increased during the day while nighttime temperature is often a little less (Rosenberg, 1976). The temperature increase, in the sheltered area, on a day of calm winds (less than 5 m/s) can be as much as 4oC (Rosenberg, 1974).

The improved microclimate in the sheltered zone improves crop growth rate and its Water Use Efficiency (Rosenberg, 1974). Higher daytime temperature increases the rate of photosynthesis while lower nighttime temperature reduces the amount of loss caused by respiration. High humidity means that plants can open their stomata more in sheltered area so that they use water more efficiently. According to Grace (1988), there is no evidence that the total water use by sheltered crops is less than used by unsheltered crops since plants open their stomata more and leaf surface areas are greater in sheltered areas. The increased turbulence in the “quiet zone” also transfers water vapour away from the crop canopy more efficiently than under conditions of laminar flow as would be experienced under open field conditions.

In addition, high windspeed causes direct damage to plants by causing leaves to rub against each other and to tear in the wind so that photosynthates must be used for the repair of wounds while damaged leaves lose water unnecessarily (Grace, 1988).  In high wind conditions, photosynthates are also needed to create structural strength in the plant. These effects cause the plant’s photosynthetic resources to be diverted away from growth.

Crops that depend on insects for pollination receive an additional benefit from wind protection since insects are more effective under protected conditions and warmer temperatures (Norton, 1988). Crops such as sensitive fruits also have better quality in shelter because there is less bruising and less dehydration.

The differential effects of shelter on crops results in some crops being much more sensitive to the protection provided by shelterbelts than other crops (Kort, 1988). Drought-adapted field crops such as cereal grains are not as responsive to shelter as leafy vegetable crops or fruiting crops, especially those that depend on insect pollination. Kort and Brandle (1991) incorporated these differences into an economic model, “WBECON”, in which crop yield benefits were translated into Net Present Value (NPV) over the life of a shelterbelt.

Effects on Water

Snow

In winter, shelterbelts control the movement of snow. Field shelterbelts can provide extra moisture for the following year’s crops. Most obviously, trapped snow may provide soil moisture when it melts in the spring. This is not necessarily the case though, since trapped snow above a wet, frozen soil will result in much of the water running off rather than infiltrating the soil (PFRA Shelterbelt Centre, 1990).  Snow meltwater that did infiltrate the soil in another Saskatchewan study was found to cause local groundwater “mounding” proving that snow trapping represents a significant source of groundwater recharge (PFRA Tree Nursery. 1986).

When snow is allowed to move freely across open fields, much of the moisture is lost to the atmosphere through sublimation (Gurevich, 1952; PFRA Shelterbelt Centre, 1992; Tabler, 1975). In a Saskatchewan study, the amount of snow on a landscape that was well protected by shelterbelts was 29% greater than the amount on nearby land that had no shelterbelts (PFRA Shelterbelt Centre, 1989).  The cause and mechanics of the reduction of snow erosion by shelterbelts is similar to that of soil erosion reduction. Prevention of snow erosion prevents significant loss of moisture.

Shelterbelts can effectively trap snow for harvesting water. Jairell and Schmidt (1990) showed that a well-placed snow fence would trap over five times as much snow in a pasture dugout in Wyoming as a dugout without a snow fence. By trapping snow where it is needed, shelterbelts can provide extra water for cattle or domestic uses, for groundwater recharge or for replenishing water supplies in natural riparian areas.

Water

It was established in the earlier section on “crop growth”, that shelterbelts do not significantly reduce the amount of water transpired by crops. However, shelterbelt trees generally use more water in the summer than the same area of crop so that groundwater can be diminished and crop yields in the root zone of the trees are less.

When shelterbelts are used to protect open water bodies such as streams and dugouts, the amount of water used by the trees can be offset by the reduced evaporation from the open water body. Despite the fact that turbulence and air temperature are greater in sheltered areas, Rosenberg (1974) showed that water vapour pressure is also higher and evaporation is decreased by more than 50% in well-sheltered sites. This has implications for the management of riparian zones in which it is important to maintain water levels.

Water protection by agroforestry buffers is closely related to their effects on soil erosion but there are other aspects of water protection that need to be considered. Agroforestry affect the hydrological cycle in numerous ways, both in winter through snow distribution and in summer by affecting transpiration rates. The use of trees for phytoremediation – the removal of dissolved nutrients or other pollutants from groundwater or the prevention of pollutant transport by the groundwater – has also been studied (O’Neill and Gordon, 1994; Dosskey, 2001) 


Riparian Buffers

Riparian zones are environmentally sensitive areas that require special thought and attention and have been shown to benefit from agroforestry practices. Special challenges occur in a riparian zone because 1) they are susceptible to water erosion, 2) water quality has become an important issue in Canada and sediments, pesticides or nutrients from adjacent farmland may pollute the water, 3) they are critical habitat for mammals, fish, birds, plants, fungi, insects, etc. 

Schultz et al (2000) describe the main functions of forested riparian buffers. Filtering sediment from surface flow and nutrients and chemicals from both surface flows and groundwater is most important in the smaller streams. This is because of a combination of physical trapping of sediments and root and microbial interactions. The effect of trees on nutrients and dissolved chemicals is greatest during the growing season. In addition to plant uptake, there are microbial processes that attenuate pollutants in forested riparian buffers. These processes include immobilization of nutrients, denitrification of NO3 and degradation of organic pollutants such as pesticides (Palone and Todd 1998, Arora et al. 1996).

Providing aquatic habitat and influencing stream morphology is most important for mid-sized streams (Schultz et al 2000). Trees do this by 1) physically reducing bank erosion, 2) contributing woody debris which provides shelter and slows the water flow 3) reducing stream temperature by shading it. A well-treed watershed results in more water infiltration into the soil during floods so that flow rates are reduced at the flood time but streamflow is maintained after the flood has passed.

Grass buffers alone have been found to be less effective than Riparian Forests Buffer Systems (Hubbard and Lowrance 1997) and the potential role of utilizing trees in different vegetative combinations has received support for use in riparian buffer designs (Welsch 1991, Shultz et al. 1995). Many studies have shown that riparian forests are effective in containing N, P, and sediment in runoff from adjacent cropland (Lowrance et al. 1984, Peterjohn and Correll 1984, Haycock and Pinay 1993, Jordan et al. 1993). Compared to other non-point source pollution control measures, riparian forest buffers, like other agroforestry systems,  represent a long-term investment which can lead to changes in the structure and function of agricultural landscapes (Lowrance et al. 1997).  In addition, additional nutrient uptake by riparian forest vegetation should increase forest productivity and the potential for short- and long-term sequestering of nutrients and carbon.

All of the functions of well-buffered riparian zones can be seen most importantly as benefits to society because they protect the water resource, the soil resource, the biodiversity resource and they provide an aesthetically pleasing area for recreational opportunities such as fishing, canoeing and hiking. Probably the most critical aspect of riparian buffers is that society generally recognizes that land management practices have a great impact on the quality of their water.


Benefits of Shelterbelts for Biodiversity and Integrated Pest Management

Biodiversity is protected by agroforestry including plants (de Blois et al. 2002), songbirds (Jobin et al., 2001; Mah, 2002), insects (Ouin and Burel, 2002), mammals. The agroforestry system’s characteristics (age, species, connectivity, location, etc) are expected to be well-linked with many aspects of biodiversity. For example, birds nest, perch and feed on specific tree or shrub species.

Numerous studies from a diversity of agricultural systems have demonstrated the link between shelterbelts, hedgerows or riparian buffers with greater biodiversity (Marshall and Moonen 2002, Varchola, 2001).  Research studies have also found that floral composition (Jobin et al. 2001), spatial arrangement (Croxton et al. 2002) and number of linear elements (Dover et al. 2000) affect species-specific abundance and diversity within these features as well as the broader agricultural landscape. Connectivity of shelterbelt elements with natural habitats (eg. woodlands) provides corridors for the movement of species among habitat fragments (Croxton et al. 2002). 

Shelterbelts and hedgerows have been proven important habitats from which beneficial insect species move onto surrounding cropland providing full or partial control of pest species (Baute et al. 2002). In general, there are a greater number of arthropod species inhabiting field margins as compared to the cultivated fields (Lewis 1969, Doane 1981).  This difference was attributed to a greater availability of microhabitats in the field margin, making them important in the source-sink dynamics of arthropod populations inhabiting the agro-ecosystem (Fry 1994). Since field boundary habitats represent an interface between farm practices and the ecosystem, it is desirable that the agriculture industry develop practices that enhance, rather than replace, these natural processes (Leaver 1994).

Studies have demonstrated that fields with a high diversity of plants tend to also have a higher diversity of parasitoids and predators because (1) pollen and nectar from weeds serve as supplementary food sources and (2) they are often host plants for alternative prey (Altieri 1994). So it is understood that conservation of biodiversity (ie. especially natural enemies) through habitat management, plant structure, and diversity can positively impact on our ability to manage the pest species. Management strategies for control of insect pests have broadened into the concept of ecological pest management and are no longer focused only on the pest species complex (Pimentel et al. 1992).  Field boundary areas including shelterbelts and hedgerows harbour many arthropods, including insects, spiders and mites, which are integral to crop loss and to soil health because they include both pest and beneficial species (Powell 1986). Sustainable management strategies, crop loss prevention and maintenance of soil health are central to our capacity to maintain the biological productivity of agricultural systems.

Wade et al. (1999) observed that natural treed habitats surrounding wheat fields served as permanent habitats for predatory ground beetles, many of which moved into the wheat and fed on economic pests of wheat. Baute et al. (2002) identified specific insect pests of field crops in Ontario that also use shelterbelts at field margins as part of their life history strategy.  Lygus plant bug species (Hemiptera: Miridae) overwinter as adults in shelterbelts and then re-invade adjacent agricultural crops (eg. strawberries, alfalfa and canola) in the spring (Broadbent et al. 2002) and it is suspected that beneficial parasitoids of Lygus also build up populations in these field margin habitats.

On the Canadian prairies, wildlife habitat has decreased because of farming activities. Field and farmyard shelterbelts provide significant benefits for wildlife, mainly for birds because of requirements for nesting habitat, food sources, and protection in both summer and winter that are readily provided by shelterbelts (Mah 2002). Abandoned farmstead shelterbelts often retain an array of fruiting species, coniferous trees and deciduous trees to create a complex habitat in which many different species can thrive.

 

Benefits for Livestock

Shelterbelts provide important wind protection for livestock especially in cold weather. The benefits include better animal health, better weight gain and feed efficiency, greater milk production and better survival, especially for young animals (Williams et al, 1997; Brandle et al, 2000). Optimum temperatures for cattle and sheep are 0 - 20oC (Brandle et al, 2000). Young animals, calves and lambs, prefer higher temperatures than older animals. When there is a wind, this value decreases so that a 50 km/hr wind causes a difference between actual and effective temperatures of 8.1C at an actual temperature of 0oC and 22.7oC at an actual temperature of –40oC. An effective shelterbelt that can reduce windspeed by 70% would increase the effective temperature by 3.7oC in the first example and 8.9oC in the second example. Since feed efficiency for cattle can increase by 1 to 2% for every oC (Williams et al, 1997), this effect represents a possible saving in feed of 3.7% to 17.8%.

 

Odour Mitigation

For intensive livestock operations, shelterbelts can be effective windbreaks and visual screens. Theobold and Milburn (2002) suggested that the planting of trees around farm buildings may be an effective and low-cost method to recapture gaseous ammonia near the farm and reduce long-range atmospheric transport. A thorough review of these benefits was recently done by Tyndall and Colletti (2000). Odours from such operations are considered a problem because they cause mental and physical health concerns for humans and animals, economic results for real estate values, tourism and recreation activities, and stress relationships within communities.  They reported that most odour complaints resulted from hog operations and land application of the effluent. In hog operations the liquid manure decomposes anaerobically, resulting in increased odour-related problems. They cited a study that intense and persistent odours travel as aerosols and concluded that shelterbelts can reduce the spread of odours mainly by causing their dilution in the atmosphere or by intercepting or causing the deposition of the particles. Increased dilution effects would be mainly due to the increased turbulence leeward of the shelterbelts and lower windspeed over storage lagoons would reduce the rate at which the aerosols entered the airstream.

Windspeed reductions leeward of shelterbelts were predicted by modeling to increase deposition of aerosols of up to 56%. Conifers were suggested as a possibly effective way of physically intercepting aerosols on their leaf surfaces and it was pointed out that, for evergreen conifers, the benefits would continue year-round.  However, in western Canada, odours in winter are not considered as serious as in summertime. Tyndall and Colletti (2000) also suggested that, to the extent that some volatile organic compounds could be directly adsorbed on the leaf surface or metabolized on entering the leaves through the stomata. Whether these effects are enough to reduce odours to a satisfactory level was not established since aerosol reduction has not been accurately quantified. Since Tyndall and Colletti (2000) reported that reductions in odours are logarithmically related to subjective odour perception (i.e. emissions reductions of 94-98% were needed to reduce perceived odours from “extremely strong” to “faint”). Pending more accurate quantification of odour reductions by shelterbelts, it would seem that shelterbelts designed for hog barns should be extensive and include multiple rows of conifers. Multiple-row shelterbelts should give the greatest reduction in wind, the greatest turbulence and the best visual screen.

 

Farmyard Shelterbelts

Shelterbelts for the protection of farmyards are important for farm families. In the winter, their functions are to control snow and to protect the farm buildings from cold winds. In the summer they protect against hot, dry winds. This gives the farm family a sense of home as the shelterbelt provides a yard in which they can relax outdoors or grow a garden.

 

In a study done at Indian Head, Saskatchewan (Moyer, 1990), it was found that a good shelterbelt could reduce the use of home-heating fuel by as much as 25%. Moyer showed  that heat loss from insulated test trailers proceeded by infiltration (physical movement of air from the inside to the outside) and through conduction (transmission of heat through walls, windows and roof). Wind protection reduced the heat loss through infiltration but not through conduction. The rate of conduction was controlled by the insulative value of the surface involved. For well-sealed, modern buildings, the infiltrative heat loss would be relatively less and the savings in heat cost would be expected to decrease accordingly.

 

Shelterbelts for Carbon and Biomass

Plants, through photosynthesis, convert carbon dioxide into oxygen by fixing the carbon into sugars and then into organic molecules such as cellulose. It is therefore quite easy to know the amount of carbon dioxide removed from the atmosphere by measuring the amount of carbon existing in the trees. By knowing the growth rate of different shelterbelt species under different climatic and soil conditions, the Shelterbelt Centre has been able to calculate the amount of carbon dioxide removed from the atmosphere by the shelterbelt program (Kort and Turnock, 1999; Kort and Turnock, 2001).  Turnock (2001) used these results to calculate that the trees sent out under the exisiting shelterbelt program (assuming 5.3  million trees and shrubs per year) since 1990 would sequester 1.7 Mt (CO2 equivalent) aboveground,  in the first Kyoto verification period (2008-2012).  Although this would not play a large role in meeting Canada’s Kyoto commitments, it is important to recognize the environmental role the shelterbelts play in this capacity. 

Tuskan et al (2000) reporting on work from the US Department of Energy at Oak Ridge, Tennessee, pointed out that short-rotation woody crops could be used for production of bio-fuels as well as serving as a carbon sink and that they would be an important component of the US fuel-switching strategy.

 

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