Andrew M. Gordon

Ecology of tree intercropping systems in the North temperate region: Experiences from southern Ontario, Canada

Agroforestry practices in northern latitudes, although less diverse than those in warmer regions, have unique advantages over conventional land-use systems in the region in terms of water-quality enhancement, carbon sequestration, and biodiversity conservation. Tree intercropping, especially, is a potentially promising agroforestry option in the region. Understanding the ecological interactions between trees and crops in such intercropped systems provides the basis for designing efficient systems with potential for wider applicability. With this objective, the experience from several years of research on this aspect at the University of Guelph, in southern Ontario, Canada are presented. Yields of C3 crops intercropped with trees, as well as growth of trees, did not differ from those in corresponding sole-stand (conventional) systems of crops and trees. But, soil organic carbon content and bird and insect diversity increased in the intercropped area. The abundance and distribution of earthworms were higher closer to the tree rows indicating improved soil health. The C sequestration potential in fast-growing tree (hybrid-poplar)-based intercropping systems was four times more than that reported for conventional agricultural fields in the region. Because of reduced fertilizer use and more efficient N-cycling, the tree-intercropping systems could also lead to the reduction of nitrous oxide emissions from agricultural fields by about 0.7 kg ha−1 yr−1. Marginal or degraded land that is suitable for agroforestry is estimated to be 57 million ha in Canada. Tree/crop intercropping is one agroforestry system that shows great potential for this region. We suggest that this land-management option can be placed above conventional agriculture in terms of long term-productivity and sustainability.

Poplar leaf biomass distribution and nitrogen dynamics in a poplar-barley intercropped system in southern Ontario, Canada

The effect of hybrid poplar (Populus spp. clone DN 177) leaf biomass distribution on soil nitrification was investigated in two experiments during the 1993, 1994 and 1995 growing seasons in a poplar-barley (Hordeum vulgare cv. OAC Kippen) intercropping experiment established at Guelph, Ontario, Canada. In experiment 1, poplar was intercropped with barley during all three years and the poplar leaves shed during the fall season were removed from the soil surface during 1993 and 1994. In experiment 2, poplar was intercropped with barley in 1993 and with corn (Zea mays cv. Pioneer 3917) in 1994 an 1995, respectively, and the shed poplar leaves were not removed. In experiment 1, the nitrification rates were lower during 1994 and 1995 when the dropped leaves were removed from the field. The total above-ground biomass of barley within 2.5 m of the tree row was 517, 500 and 450 g×m−2, respectively during the three years, whereas in the middle of the crop row (4–11 m), the corresponding figures were 491, 484 and 464 g×m–2. Mean nitrification rates, N availability and carbon content were higher in soils close to the poplar tree rows (2.5 m) compared to the corresponding values in the middle of the crop alley (4–11 m from the tree row). In experiment 2, where poplar leaves were not removed from the field, nitrification rates in soils within 2.5 m distance from the poplar row were fairly constant (range 100 to 128 μg 100 g−1 dry soil day−1) during the three years. Results suggest that soil nitrification rates, soil carbon content and plant N uptake adjacent to the poplar tree rows are influenced by poplar leaf biomass input in the preceding year.

Optical porosity and windspeed reduction by coniferous windbreaks in Southern Ontario

Relative windspeed reduction was measured behind nine relatively narrow, homogeneous windbreaks in southern Ontario, Canada to assess whether any characteristics of the windspeed reduction curve could be predicted from optical porosity. The latter was determined for each windbreak using high contrast black and white photographic silhouettes on a computer digitizing system. Minimum windspeeds behind the windbreaks ranged from 29 to 71% of open windspeed; these minima were located 2 to 6 multiples of windbreak height away from the windbreak. Optical porosities of the bottom half of the windbreak ranged from 0 to 31%. Multiple regression of the shelter parameters (location and value of minimum relative windspeed) on the independent variables (optical porosity, open windspeed, surface roughness, approaching wind direction relative to the windbreak, average tree diameter and average tree spacing) showed that the minimum relative windspeed could be predicted from the optical porosity of the bottom half of the windbreak. The results suggest that optical porosity can be used to predict minimum relative windspeeds and may therefore be useful as a guide in the field evaluation of windbreaks.

Leaf litter colonization by invertebrates in the littoral zone of a small oligotrophic lake

The colonization of deciduous leaf litter by aquatic invertebrates was studied at Scott Lake in Algonquin Park, Ontario, Canada. Deciduous leaf packs were colonized after only 2 days submergence. The invertebrate community was dominated by chironomids (25–94% depending on sampling period), and to a lesser extent by oligochaetes, turbellarians, and mayflies. Collectors, such as the chironomids Dicrotendipes, Pseudochironomus, Paratanytarsus and Parakiefferiella were the dominant functional-feeding group suggesting that leaf litter is being used as habitat rather than a direct food source. Deciduous leaf litter lost a substantial amount of weight, due to leaching, after only 48 h submergence. Fall-shed beech (Fagus grandifolia) leaves decomposed more rapidly than fall-shed sugar maple (Acer saccharum) leaves with daily processing coefficients (k), determined using an exponential decay model, of 0.0058 and 0.0039, respectively. Conversely, conditioned maple leaves, defined as leaves remaining on the ground over winter, were processed faster than conditioned beech leaves, with coefficients of 0.0042 and 0.0014, respectively. It is speculated that inhibitory compounds have been leached from the maple leaves, allowing for faster leaf processing.

The mycorrhizal status and colonization of 26 tree species growing in urban and rural environments

Urban environments are highly disturbed and fragmented ecosystems that commonly have lower mycorrhizal fungal species richness and diversity compared to rural or natural ecosystems. In this study, we assessed whether the mycorrhizal status and colonization of trees are influenced by the overall environment (rural vs. urban) they are growing in. Soil cores were collected from the rhizosphere of trees growing in urban and rural environments around southern Ontario. Roots were extracted from the soil cores to determine whether the trees were colonized by arbuscular mycorrhizal fungi, ectomycorrhizal fungi, or both, and to quantify the percent colonization of each type of mycorrhizal fungi. All 26 tree species were colonized by arbuscular mycorrhizal fungi, and seven tree species were dually colonized by arbuscular mycorrhizal and ectomycorrhizal fungi. Overall, arbuscular mycorrhizal and ectomycorrhizal fungal colonization was significantly (p < 0.001) lower in trees growing in urban compared to rural environments. It is not clear what ‘urban’ factors are responsible for the reduction in mycorrhizal fungal colonization; more research is needed to determine whether inoculating urban trees with mycorrhizal fungi would increase colonization levels and growth of the trees.

Growth response of crops to soil microbial communities from conventional monocropping and tree-based intercropping systems

Background and aimsRecent studies have shown that tree-based intercropping (TBI) systems support a more diverse soil microbial community compared to conventional agricultural systems. However, it is unclear whether differences in soil microbial diversity between these two agricultural systems have a functional effect on crop growth.MethodsIn this study, we used a series of greenhouse experiments to test whether crops respond differently to the total soil microbial community (Experiment 1) and to arbuscular mycorrhizal (AM) fungal communities alone (Experiment 2) from conventionally monocropped (CM) and TBI systems.ResultsThe crops had a similar growth response to the total soil microbial communities from both cropping systems. However, when compared to sterilized controls, barley (Hordeum vulgare) and canola (Brassica napus) exhibited a negative growth response to the total soil microbial communities, while soybean (Glycine max) was unaffected. During the AM fungal establishment phase of the second experiment, ‘nurse’ plants had a strong positive growth response to AM fungal inoculation, and significantly higher biomass when inoculated with AM fungi from the CM system compared to the TBI system. Soybean was the only crop species to exhibit a significant positive growth response to AM fungal inoculation. Similar to the total soil microbial communities, AM fungi from the two cropping systems did not differ in their effect on crop growth.ConclusionOverall, AM fungi from both cropping systems had a positive effect on the growth of plants that formed a functional symbiosis. However, the results from these experiments suggest that negative effects of non-AM fungal microbes are stronger than the beneficial effects of AM fungi from these cropping systems.

Biophysical and Ecological Interactions in a Temperate Tree-Based Intercropping System

SUMMARY Tree-based intercropping is considered an excellent farming system and can contribute much to our understanding of sustainable agriculture practices. Our current research goals are to address and quantify the numerous biophysical interactions that occur at the tree-crop interface in order to enhance our understanding of the ecology of tree-based intercropping (a form of agroforestry). In 1987, the University of Guelph established a large field experiment on 30 ha of prime agricultural land in Wellington county southern Ontario, Canada to investigate various aspects of intercropping trees with agricultural crops. A variety of spacing, crop compatibility and tree growth, and survival experiments were initiated at that time, utilizing 10 tree species within the genera Picea, Thuja, Pinus, Juglans, Quercus, Fraxinus, Acer, and Populus. Two between row-spacings (12.5 m or 15 m) and two within row-spacings (3 m, or 6 m) were utilized in conjunction with all possible combinations of three agricultural crops (soybean, corn, and either winter wheat or barley). Investigations over the last decade have documented several complementary biophysical interactions. Nitrogen (N) transfer from fall-shed leaves to adjacent crops with enhanced soil nitrification as the proposed mechanism was estimated to be 5 kg N ha−1. Soil organic carbon (C) adjacent to tree rows has increased by over 1%, largely as a result of tree litterfall inputs and fine root turnover. It is estimated that intercropping has reduced nitrate loading to adjacent waterways by 50%, a hypothesized function of deep percolate interception by tree roots. We have also noticed increased bird diversity and usage within the intercropped area as compared to mono-cropped adjacent agricultural areas, and have recorded increases in small mammal populations. Earthworm distribution and abundance was also found to be higher closer to the tree rows when compared to earthworm numbers in the crop alleys. We speculate that these are indicative of major changes in the flow of energy within the trophic structure identified with intercropping systems. In light of climate change mitigation processes, C sequestration and NO2 reduction potentials in tree-based intercropping systems were studied and compared to conventional agricultural systems. The results suggest that sequestration of C was 5 times more in the former system than in the latter. Competitive interactions between trees and crops for nutrients, moisture and light were also studied. The tangible benefits that are derived from properly designed and managed tree-based intercropping systems place this land management option above conventional agriculture in terms of long-term productivity and sustainability.

SPATIAL AND TEMPORAL DISTRIBUTION OF EARTHWORMS IN A TEMPERATE INTERCROPPING SYSTEM IN SOUTHERN ONTARIO, CANADA

Earthworms are known to increase soil bulk density, soil porosity, mixing of organic matter, and to strengthen aggregation of soil particles. They perform important functions in the maintenance and stabilization of the soil matrix. Historically, temperate intercropping research has focused on the above-ground benefits of adding trees into the agricultural landscape. Earthworm research in temperate intercropping systems has been non-existent to date. More emphasis on studying below-ground components, such as earthworms, is required in order to better understand the mechanisms of intercropping ecosystem function. The purpose of this study was to examine seasonal changes in distribution and abundance of earthworms under a temperate intercropping system in southwestern Ontario, Canada. Sampling occurred during the spring and summer of 1997 at the University of Guelphs Agroforestry Research Station, Guelph, Ontario. Earthworm samples were collected at various distances from the tree rows. Significant variation in both earthworm biomass and density were found between the three tree species sampled. Total mean earthworm density was 182 m-2 within the poplar rows, 71 m-2 within the silver maple rows, and 90 m-2 within the white ash rows. A marked difference was also observed in the distribution of earthworms within the tree rows and the field area. For example, total mean density within the tree rows for poplar was 182 individuals m-2, as compared to total mean densities of 117 and 95 individuals m-2, two metres and six metres into the field from the tree, respectively.

Landowner perceptions and the adoption of agroforestry practices in southern Ontario, Canada

A mail-out survey questionnaire was developed by the Agroforestry group at the University of Guelph to determine the level of awareness and interest in the adoption of agroforestry systems by landusers from four townships in Wellington County, Ontario. The questionnaire investigated: (1) the current level of knowledge regarding windbreaks, woodlots and plantations, intereropping, riparian plantations and silvipasture, (2) the present level of participation in each of these systems on-farm, (3) the perceived benefits and/or drawbacks of each of these initiatives with respect to total farm income, income diversity, land rehabilitation, land value/equity, soil/water conservation, labour intensity, overhead and return on the term of investment.The majority of respondents were familiar with conventional agroforestry systems such as windbreaks and woodlots/plantations (80%, 62% respectively), therefore the level of interest in the adoption of these practices was significant (74%, 66% respectively). Response rates were lower for silvipasture, riparian plantations and intercropping, most likely as result of the low level of familiarity with these practices (20%, 32%, 4% respectively). Respondents commented that agroforestry systems would have a neutral effect on farm income, and would increase land stewardship. In some cases, interested landusers indicated a willingness to participate in agroforestry systems even though they anticipated increases in overhead and labour intensity; however, this was only true if they held land stewardship as a priority. Landusers were more concerned with the economic aspects of agroforestry, as a determinant to the future adoptability of particular practices. Age, gender, farm operation and farm size were not correlated with the adoption of agroforestry systems.The success of agroforestry programs on farms in the study area is largely dependent on the attitudes and willingness of landusers to participate in non-traditional agricultural systems.

The effect of cattle foot traffic on soil compaction in a silvo-pastoral system

This paper reports on the extent of soil compaction due to cattle traffic around hardwood and softwood tree seedlings established in existing pasture, and subject to cattle pressure. A higher soil penetrometer resistance in the grazed areas pointed to a significant change in soil structure (i.e. dry bulk density) as a result of cattle traffic in the area.In a related experiment comparing the effect of three different levels of soil compaction on tree seedling growth and nitrogen cycling it was found that water infiltration and nitrogen uptake were reduced in soils treated with a medium and high level of compactive effort. This resulted in a slower rate of growth of the tree seedlings. The addition of an additional nitrogen source improved seedling growth in both the medium and high density compactive treatments.