Cameron N. Carlyle

Worldwide evidence of a unimodal relationship between productivity and plant species richness

Grassland diversity and ecosystem productivity The relationship between plant species diversity and ecosystem productivity is controversial. The debate concerns whether diversity peaks at intermediate levels of productivity—the so-called humped-back model—or whether there is no clear predictable relationship. Fraser et al. used a large, standardized, and geographically diverse sample of grasslands from six continents to confirm the validity and generality of the humped-back model. Their findings pave the way for a more mechanistic understanding of the factors controlling species diversity. Science, this issue p. 302 The humped-back model of plant species diversity is confirmed by a global grassland survey. The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

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 270u2009km 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.

Disentangling herbivore impacts on Populus tremuloides: a comparison of native ungulates and cattle in Canada’s Aspen Parkland

Ungulates impact woody species’ growth and abundance but little is understood about the comparative impacts of different ungulate species on forest expansion in savanna environments. Replacement of native herbivore guilds with livestock [i.e., beef cattle (Bos taurus)] has been hypothesized as a factor facilitating trembling aspen (Populus tremuloides Michx.) encroachment into grasslands of the Northern Great Plains. We used a controlled herbivory study in the Parklands of western Canada to compare the impact of native ungulates and cattle on aspen saplings. Native ungulate treatments included a mixed species guild and sequences of herbivory by different ungulates [bison (Bison bison subsp. bison), elk (Cervus elaphus) then deer (Odocoileus hemionus); or deer, elk, then bison]. Herbivory treatments were replicated in three pastures, within which sets of 40 marked aspen saplings (<1.8xa0m) were tracked along permanent transects at 2-week intervals, and compared to a non-grazed aspen stand. Stems were assessed for mortality and incremental damage (herbivory, leader breakage, stem abrasion and trampling). Final mortality was greater with exposure to any type of herbivore, but remained similar between ungulate treatments. However, among all treatments, the growth of aspen was highest with exposure only to cattle. Herbivory of aspen was attributed primarily to elk within the native ungulate treatments, with other forms of physical damage, and ultimately sapling mortality, associated with exposure to bison. Overall, these results indicate that native ungulates, specifically elk and bison, have more negative impacts on aspen saplings and provide evidence that native and domestic ungulates can have different functional effects on woody plant dynamics in savanna ecosystems.

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

Western Canadas 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.

A Review of Sustainability Enhancements in the Beef Value Chain: State-of-the-Art and Recommendations for Future Improvements

Simple Summary To better address consumer concerns, the beef sector is working on strategies to enhance the sustainability of all aspects of the beef supply chain. Among these strategies are (1) the development of science-based frameworks and indicators capable of measuring progress at all stages of beef production; (2) the engagement of different stakeholders along the beef supply chain at regional and global levels; and (3) the improvement of communication among stakeholders and transparency towards consumers. Progress on these three fronts was presented during the 2nd Global Conference on Sustainable Beef, hosted by the Global and Canadian Roundtables for Sustainable Beef. During the event, there was a clear understanding that the beef industry is substantially advancing efforts to continuously improve its sustainability, both at regional and global levels, by developing assessment frameworks and indicators to measure progress. However, it is also clear that the beef sector has a need to more clearly define the concept of beef sustainability, strengthen cooperation and exchange of information among national roundtables for sustainable beef, as well as improve the flow of information along the supply chain. An improved transparency in the beef sector will help consumers make more informed decisions about food products. Abstract The beef sector is working towards continually improving its sustainability in order to achieve environmentally, socially and economically desirable outcomes, all of which are of increasing concern to consumers. In this context, the Global Roundtable for Sustainable Beef (GRSB) provides guidance to advance the sustainability of the beef industry, through increased stakeholder engagement and the formation of national roundtables. Recently, the 2nd Global Conference on Sustainable Beef took place in Banff, Alberta, Canada, hosted by the GRSB and the Canadian Roundtable for Sustainable Beef. Conference attendees discussed the various initiatives that are being developed to address aspects of beef sustainability. This paper reviews the main discussions that occurred during this event, along with the key lessons learned, messages, and strategies that were proposed to improve the sustainability of the global beef industry.

Predicting plant trait similarity along environmental gradients

Plant traits affect the success or failure of plants to establish, grow, and reproduce. Although we have an increased understanding of certain individual plant traits and their relative effects on performance and fitness, it is a challenge to predict relative similarity of traits between neighbouring plants. Assembly rules suggest that abiotic filters will restrict the range of viable strategies, thus creating a community of plants that share a similar suite of traits. In contrast, limiting similarity predicts that segregation of species’ resource use will lead to character displacement. What is the relative strength of these two processes and do they differ depending on site condition? We know that trait similarity of plants can vary with site productivity and disturbance. In this study, we investigate the interaction of these two ecological factors and how they affect plant trait similarity. We find support for the hypothesis that trait convergence occurs at low productivity/high disturbance and high productivity/low disturbance, and trait dispersion is most likely at intermediate levels of disturbance and productivity. The relationships among evolution, plant traits, and ecology are multivariate, hierarchical, and complex making plant traits at the ecosystem level an exciting and challenging agenda for the future.

Long-Term Grazing Accelerated Litter Decomposition in Northern Temperate Grasslands

Livestock grazing affects plant community composition, diversity, and carbon (C) and nutrient cycling in grasslands. Grazing leads to plant communities that have higher relative abundance of grazing-tolerant species, which in turn may alter the chemical composition of biomass and subsequent litter decomposition rates. To better understand the effects of long-term grazing and associated vegetation shifts on biogeochemical cycling in northern temperate grasslands of western Canada, we studied litter decomposition over 18 months at 15 locations, stratified across the Mixed-grass Prairie, Central Parkland, and Foothills Fescue natural subregions. At each location, we examined decomposition in an area exposed to grazing and an area where cattle were excluded. We used litterbags containing leaf litter from seven major grass species representing different grazing tolerances and included a local source of community litter from each study site and cellulose paper as standards. Decomposition was affected by litter types, with litter from grazing-tolerant species such as Poa pratensis and Bouteloua gracilis having faster decomposition rates compared to grazing-intolerant species, supporting the hypothesis that changes in vegetation composition due to grazing influences biogeochemical cycling by modifying litter decomposition in grasslands. Litter decomposition was also overall most rapid in the cool–wet Foothills Fescue, followed by the temperate mesic Central Parkland, and slowest in the warmer–drier Mixed-grass Prairie. Combined with known grazing-induced changes in grassland composition, these findings indicate that livestock grazing may accelerate litter decomposition rates in the more mesic Foothills Fescue and parkland regions, but not the more arid Mixed-grass Prairie. Overall, this study elucidates the role of livestock grazing and its associated effects on litter decomposition and ecosystem processes in northern grassland ecosystems.

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 (Pu202f<u202f0.001) and CH4 uptake (Pu202f<u202f0.05), but lower N2O emission (Pu202f<u202f0.01) 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 (Pu202f<u202f0.001) and CH4 uptake (Pu202f<u202f0.01), and lower N2O emissions (Pu202f<u202f0.001) 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 (Pu202f<u202f0.05) in the silvopasture than in the hedgerow or shelterbelt systems over the two spring thaw seasons examined, and greater (Pu202f<u202f0.05) 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 (15.4u202fgu202fCO2-equ202fm-2u202fday-1), the GWP of these land uses during spring thaw was small (<5% of the annual GWP) due to the short spring period (6u202fweeks) and the small GHG emission (2.5u202fg CO2-equ202fm-2u202fday-1). Although GHG emissions during spring thaw were small compared to those in the growing season, they should not be ignored.

Grazing and climate effects on soil organic carbon concentration and particle-size association in northern grasslands

Grasslands cover more than 40% of the terrestrial surface of Earth and provide a range of ecological goods and services, including serving as one of the largest reservoirs for terrestrial carbon. An understanding of how livestock grazing, influences grassland soil organic carbon (SOC), including its concentration, vertical distribution and association among soil-particle sizes is unclear. We quantified SOC concentrations in the upper 30u2009cm of mineral soil, together with SOC particle-size association, within 108 pairs of long-term grazed and non-grazed grassland study sites spanning six distinct climate subregions across a 5.7u2009Mu2009ha area of Alberta, Canada. Moderate grazing enhanced SOC concentration by 12% in the upper 15u2009cm of soil. Moreover, SOC concentrations in mineral layers were associated with regional climate, such that SOC increased from dry to mesic subregions. Our results also indicate that C concentrations in each of 2000–250, 250–53,u2009<u200953 μm soil particle-size fractions were consistent with total SOC concentrations, increasing from semi-arid to more mesic subregions. We conclude that long-term livestock grazing may enhance SOC concentrations in shallow mineral soil and affirm that climate rather than grazing is the key modulator of soil C storage across northern grasslands.

Long-term grazing impacts on vegetation diversity, composition, and exotic species presence across an aridity gradient in northern temperate grasslands

Little is known about the specific role of exotic species on measures of grassland plant diversity, including how this may vary with climatic conditions or large mammal herbivory. This study examined vegetation responses to long-term livestock grazing, including plant richness and diversity, as well as the contribution of exotic species to these metrics, across a network of 107 northern temperate grasslands in Alberta, Canada, spanning a broad aridity gradient. Exposure to grazing modestly increased plant richness, but did not alter Shannon’s diversity, Simpson’s diversity, or evenness, suggesting stability in floral diversity relative to grazing. However, grazing did increase grass cover while reducing shrub cover, the latter of which was only apparent in mesic grasslands. Unlike total plant diversity, exotic species richness and cover, together with exotic plant contributions to diversity, varied jointly with grazing and aridity. While long-term grazing increased exotic species, this response was most apparent in wetter areas, and non-grazed grasslands remained more resistant to the presence of exotics. Several exotic species were positive indicators of grazing in wetter grasslands, and coincided with lower native species cover, indicating grazing may be facilitating a shift from native to exotic vegetation under these conditions. Overall, our results indicate that while long-term grazing has altered the composition and cover of certain functional groups, including favoring exotics and minimizing woody vegetation in mesic areas, overall changes to plant diversity were limited. Additionally, these findings suggest that semi-arid northern temperate grasslands remain relatively resistant to grazing effects, including their susceptibility to exotic plant encroachment. These results improve our understanding of how ongoing grazing exposure may impact grassland diversity, including efforts to conserve native vegetation, as well as the important role of climate in altering fundamental grassland responses to grazing.