Introducing trees to agricultural lands increases greenhouse gas emission during spring thaw in Canadian agroforestry systems.

The role of agroforestry systems in mitigating greenhouse gas (GHG) emission from agricultural soils during spring thaw (early April to mid-May) has been poorly studied. Soil CO2, CH4 and N2O fluxes were measured from treed areas and adjacent herblands (areas without trees) during spring thaw in 2014 and 2015 at 36 agroforestry sites (12 hedgerow, 12 shelterbelt and 12 silvopasture) in central Alberta, Canada. Fluxes of those GHGs varied with agroforestry systems and land-cover types. We found greater CO2 emission (P < 0.001) and CH4 uptake (P < 0.05), but lower N2O emission (P < 0.01) in the silvopasture than in the hedgerow and shelterbelt systems, with no difference between the last two systems. Treed areas in general had greater CO2 emissions (P < 0.001) and CH4 uptake (P < 0.01), and lower N2O emissions (P < 0.001) than the herblands. Soil temperature, moisture content, organic C content and soil available N concentration affected GHG fluxes. The global warming potential (GWP) was greater (P < 0.05) in the silvopasture than in the hedgerow or shelterbelt systems over the two spring thaw seasons examined, and greater (P < 0.05) in the treed areas than in the herblands during the cool spring in 2015. However, the GWP per unit soil organic C was lower in the treed areas (0.004-0.101%) than in the herblands (0.005-0.225%). As compared to previously reported mean growing season GHG emission (15.4 g CO2-eq m-2 day-1), the GWP of these land uses during spring thaw was small (<5% of the annual GWP) due to the short spring period (6 weeks) and the small GHG emission (2.5 g CO2-eq m-2 day-1). Although GHG emissions during spring thaw were small compared to those in the growing season, they should not be ignored.

Incorporating interspecific competition into species-distribution mapping by upward scaling of small-scale model projections to the landscape.

There are a number of overarching questions and debate in the scientific community concerning the importance of biotic interactions in species distribution models at large spatial scales. In this paper, we present a framework for revising the potential distribution of tree species native to the Western Ecoregion of Nova Scotia, Canada, by integrating the long-term effects of interspecific competition into an existing abiotic-factor-based definition of potential species distribution (PSD). The PSD model is developed by combining spatially explicit data of individualistic species' response to normalized incident photosynthetically active radiation, soil water content, and growing degree days. A revised PSD model adds biomass output simulated over a 100-year timeframe with a robust forest gap model and scaled up to the landscape using a forestland classification technique. To demonstrate the method, we applied the calculation to the natural range of 16 target tree species as found in 1,240 provincial forest-inventory plots. The revised PSD model, with the long-term effects of interspecific competition accounted for, predicted that eastern hemlock (Tsuga canadensis), American beech (Fagus grandifolia), white birch (Betula papyrifera), red oak (Quercus rubra), sugar maple (Acer saccharum), and trembling aspen (Populus tremuloides) would experience a significant decline in their original distribution compared with balsam fir (Abies balsamea), black spruce (Picea mariana), red spruce (Picea rubens), red maple (Acer rubrum L.), and yellow birch (Betula alleghaniensis). True model accuracy improved from 64.2% with original PSD evaluations to 81.7% with revised PSD. Kappa statistics slightly increased from 0.26 (fair) to 0.41 (moderate) for original and revised PSDs, respectively.

Forest and grassland cover types reduce net greenhouse gas emissions from agricultural soils.

Western Canada's prairie region is extensively cultivated for agricultural production, which is a large source of greenhouse gas emissions. Agroforestry systems are common land uses across Canada, which integrate trees into the agricultural landscape and could play a substantial role in sequestering carbon and mitigating increases in atmospheric GHG concentrations. We measured soil CO2, CH4 and N2O fluxes and the global warming potential of microbe-mediated net greenhouse gas emissions (GWPm) in forest and herbland (areas without trees) soils of three agroforestry systems (hedgerow, shelterbelt and silvopasture) over two growing seasons (May through September in 2013 and 2014). We measured greenhouse gas fluxes and environmental conditions at 36 agroforestry sites (12 sites for each system) located along a south-north oriented soil/climate gradient of increasing moisture availability in central Alberta, Canada. The temperature sensitivity of soil CO2 emissions was greater in herbland (4.4) than in forest (3.1), but was not different among agroforestry systems. Over the two seasons, forest soils had 3.4% greater CO2 emission, 36% higher CH4 uptake, and 66% lower N2O emission than adjacent herbland soils. Combining the CO2 equivalents of soil CH4 and N2O fluxes with the CO2 emitted via heterotrophic (microbial) respiration, forest soils had a smaller GWPm than herbland soils (68 and 89kgCO2ha(-1), respectively). While emissions of total CO2 were silvopasture>hedgerow>shelterbelt, soils under silvopasture had 5% lower heterotrophic respiration, 15% greater CH4 uptake, and 44% lower N2O emission as compared with the other two agroforestry systems. Overall, the GWPm of greenhouse gas emissions was greater in hedgerow (88) and shelterbelt (85) than in the silvopasture system (76kgCO2ha(-1)). High GWPm in the hedgerow and shelterbelt systems reflects the greater contribution from the monoculture annual crops within these systems. Opportunities exist for reducing soil greenhouse gas emissions and mitigating climate change by promoting the establishment of perennial vegetation in the agricultural landscape.

Determinants of bacterial communities in Canadian agroforestry systems.

Land-use change is one of the most important factors influencing soil microbial communities, which play a pivotal role in most biogeochemical and ecological processes. Using agroforestry systems as a model, this study examined the effects of land uses and edaphic properties on bacterial communities in three agroforestry types covering a 270 km soil-climate gradient in Alberta, Canada. Our results demonstrate that land-use patterns exert stronger effects on soil bacterial communities than soil zones in these agroforestry systems. Plots with trees in agroforestry systems promoted greater bacterial abundance and to some extent species richness, which was associated with more nutrient-rich soil resources. While Acidobacteria, Actinobacteria and Alphaproteobacteria were the dominant bacterial phyla and subphyla across land uses, Arthrobacter, Acidobacteria_Gp16, Burkholderia, Rhodanobacter and Rhizobium were the keystone taxa in these agroforestry systems. Soil pH and carbon contents emerged as the major determinants of bacterial community characteristics. We found non-random co-occurrence and modular patterns of soil bacterial communities, and these patterns were controlled by edaphic factors and not their taxonomy. Overall, this study highlights the drivers and co-occurrence patterns of soil microbial communities in agroforestry systems.