Showing papers by "Sean C. Thomas published in 2017"

Diversity and carbon storage across the tropical forest biome

Abstract: Tropical forests are global centres of biodiversity and carbon storage. Many tropical countries aspire to protect forest to fulfil biodiversity and climate mitigation policy targets, but the conservation strategies needed to achieve these two functions depend critically on the tropical forest tree diversity-carbon storage relationship. Assessing this relationship is challenging due to the scarcity of inventories where carbon stocks in aboveground biomass and species identifications have been simultaneously and robustly quantified. Here, we compile a unique pan-tropical dataset of 360 plots located in structurally intact old-growth closed-canopy forest, surveyed using standardised methods, allowing a multi-scale evaluation of diversity-carbon relationships in tropical forests. Diversity-carbon relationships among all plots at 1 ha scale across the tropics are absent, and within continents are either weak (Asia) or absent (Amazonia, Africa). A weak positive relationship is detectable within 1 ha plots, indicating that diversity effects in tropical forests may be scale dependent. The absence of clear diversity-carbon relationships at scales relevant to conservation planning means that carbon-centred conservation strategies will inevitably miss many high diversity ecosystems. As tropical forests can have any combination of tree diversity and carbon stocks both require explicit consideration when optimising policies to manage tropical carbon and biodiversity.

Is There a Positive Synergistic Effect of Biochar and Compost Soil Amendments on Plant Growth and Physiological Performance

Abstract: The combination of biochar (BC) with compost has been suggested to be a promising strategy to promote plant growth and performance, but although “synergistic” effects have been stated to occur, full-factorial experiments are few, and explicit tests for synergism are lacking. We tested the hypothesis that a combination of BC and spent mushroom substrate (SMS) has a positive synergistic effect on plant growth and physiological performance in a nutrient-limited growing media. A greenhouse experiment with a full factorial design was conducted using mixed-wood BC (3.0 kg·m−2) and SMS (1.5 kg·m−2) (the combination was not co-composted) as organic soil amendments for the annual Abutilon theophrasti and the perennial Salix purpurea. Several measurements related to plant growth and physiological performance were taken throughout the experiment. Contrary to the hypothesis, we found that the combination of BC + SMS had neutral or antagonistic interactive effects on many plant growth traits. Antagonistic effects were found on maximum leaf area, above- and belowground biomass, reproductive allocation, maximum plant height, chlorophyll fluorescence, and stomatal conductance of A. theophrasti. The effect on S. purpurea was mostly neutral. We conclude that the generalization that BC and compost have synergistic effects on plant performance is not supported.

Interspecific variation of tree root architecture in a temperate agroforestry system characterized using ground-penetrating radar

Abstract: Root system architecture regulates belowground access to soil resources. Variation in root architecture is important in agroforestry systems given management objectives to optimize resource acquisition between trees and crops. However, the distributions of live tree roots in agroforestry systems remain understudied due to methodological constraints. In this study, we used ground-penetrating radar (GPR) to describe variation in whole-plant root architecture among tree species at the same agricultural site, with a specific focus on vertical coarse and fine root distributions within the zone of competition with neighbouring crops. Using GPR, we detected coarse roots of five trees species (Quercus rubra L., Juglans nigra L., Populus deltoides × nigra DN177, Picea abies (L.) Karst, and Thuja occidentalis L.) at a tree-based intercropping system in southern Ontario, Canada. A subset of soil profiles were assessed for GPR accuracy. A cumulative root distribution function was used to estimate the rooting depth (D95) of coarse roots. We also measured tree coarse root distributions and fine root density distributions 2 m into the crop rows, in the zone of competition. GPR accurately detected approximately 58 % of coarse roots for each study tree. Coarse root architecture varied among species, with differences in D95 and rooting patterns. Fine root length density distribution also varied among species, but was consistently high at 0.10 and 0.20 m depths regardless of species. Our results suggest differential tree suitability for minimizing belowground competition with crops. Additionally, we illustrate the viability of GPR to characterize vertical profiles of live tree root systems, which is critical for improving our understanding of whole-plant functional traits and belowground plant interactions.

Comparative responses of early-successional plants to charcoal soil amendments.

Abstract: Charcoal used as a soil amendment, or “biochar,” has received considerable recent research attention as a means to increase plant productivity while mitigating climate change through enhanced carbon sequestration. Interest in biochar for use in the restoration of disturbed sites is growing; however, biochar effects on wild plant species of the early phase of post-disturbance succession have received almost no prior research attention. Physiological adaptations that enable rapid growth in early-successional pioneers (e.g., high rates of photosynthesis) should be advantageous in soils with fresh charcoal since plants with a capacity for expeditious resource capture can capitalize on resource pulses from leachable mineral elements. In a glasshouse study, we tested the effects of biochar applied at two doses (10 and 20 t/ha) to brunisol/juvenile podzol soils, collected from a managed temperate mixed-wood forest, on the growth and physiology of 13 herbaceous old-field pioneers. We measured leaf-level physiology and nutrient supply rates throughout the experiment, and biomass and reproductive performance at experiment completion. Overall, biochar treatments resulted in 30–37% increases in final average aboveground biomass, 13–17% increases in photosynthesis, and an average ~44% increase in leaf-level water-use efficiency (at 10 t/ha), but with a high species-specific variation that included negative responses. We detected weak negative relationships between intrinsic photosynthetic rates (of non-biochar controls) and some biomass responses: Species with high photosynthetic capacities tended to have low or negative biomass responses to biochar. Plants in biochar treatments flowered earlier and on average had double the reproductive biomass overall. Pulses of PO4− and K+ were supplied by biochar in the first four weeks of the experiment, while NO3− was significantly immobilized by biochar. These results suggest that by providing a pulse of P and base cations, biochar can improve the restoration of disturbed landscapes by enhancing the physiological and reproductive performance of a subset of pioneers that have moderate photosynthetic rates and nitrogen demand. Biochar has important potential applications to restoration; however, biochar is likely to affect community composition strongly, and careful consideration of the physiological rates and nitrogen requirements of target species will be necessary to maximize the success of biochar-based restoration projects.

Effects of coarse woody debris on plant and lichen species composition in boreal forests

Abstract: Question Although the importance of coarse woody debris (CWD) for understorey species diversity has been recognized, the relative effects of coarse woody debris decay class and substrate species on understorey species composition have received little attention. We examined how the species composition of understorey vegetation change with CWD decay class and substrate species. Location Boreal mixed-wood forests, Ontario, Canada. Methods To cover a wide range of CWD decay classes and substrate species, we sampled fire-origin boreal forest stands that varied in stand age and canopy tree species composition. Vegetation on CWD was sampled by visually estimating percentage cover of each species within a 0.1 m × 0.5 m quadrat, randomly laid lengthwise on top of each sampled CWD log. We also recorded the forest floor vegetation by establishing an adjacent plot of the same size at a distance of 1.0 m in a random direction from the CWD vegetation sample. Results Multivariate analysis showed that understorey species composition differed among decay classes and substrate species. A NMDS ordination of understorey species composition revealed a clear separation of decay classes 1 and 2 from higher decay classes, and that decay classes 4 and 5 shared several species with the forest floor. The species composition on the forest floor was completely different from the species composition on CWD decay classes 1, 2 and 3. Two distinct groupings of substrates according to CWD species composition were found: conifer species (Pinus banksiana and Picea spp.) and broad-leaf species (Betula papyrifera and Populus spp.), with Abies balsamea taking an intermediate position. Indicator species analysis showed distinct understorey species affiliations to substrate species at advanced decay classes. Understorey species composition on the CWD of P. banksiana showed particularly pronounced changes from the dominance of lichens on decay classes 2 and 3 to dominance by mosses and vascular species on decay classes 4 and 5. Conclusions Understorey species composition on CWD not only differed with decay class, but also with CWD substrate species. Conservation strategies should aim at retaining diversity of CWD in terms of both decay classes and species composition in boreal forests.

Interactive effects of biochar and an organic dust suppressant for revegetation and erosion control with herbaceous seed mixtures and willow cuttings

Abstract: Land reclamation efforts on “non-soil” substrates frequently involve additions of organic material coupled with soil-stabilizing dust suppressants to control erosion, but formal experiments, particularly of interactive effects, are lacking. We examined the effects of Entac™, a tall oil pitch emulsion, and pyrolyzed wood waste-derived biochar on early plant establishment, growth, and survivorship, in 2 factorial greenhouse experiments: 1 using a seed mix widely used for roadside erosion control (the grasses Festuca rubra, Poa pratensis, Lolium perenne, and white clover [Trifolium repens]) and 1 using rooted cuttings of sand bar willow (Salix exigua). Biochar additions of 5, 10, and 20 t/ha enhanced biomass of clover by 14–250%, but had no significant effects on grasses. Entac additions inhibited early establishment and growth of both clover and grasses at all biochar treatment levels. Entac plus biochar treatments significantly enhanced survivorship of willows following a drought treatment in a greenhouse trial. In a field trial, Entac treatments had positive effects on willow diameter growth. Collectively, these results suggest that better soil moisture retention is the principal mechanism responsible for enhanced survivorship and growth of planted willows treated with Entac and biochar on sand substrates. Given Entac's inhibitive effects on early establishment of herbaceous species, and the abilities of Entac and biochar in combination to mitigate drought stress, these treatments may be particularly well suited for ecological restoration efforts using planted tree stock on coarse-textured substrates and where exposure and ingress of non-native herbaceous species are of concern.

Reproductive costs in Acer saccharum: exploring size-dependent relations between seed production and branch extension

Abstract: Branch extension of Acer saccharum is negatively correlated with seed production, with no independent effect of tree size, consistent with reproductive allocation as the main driver of the age-related decline in tree canopy growth. Life-history theory predicts that reproductive allocation should increase with age and size once plants reach reproductive maturity. This suggests that there may also be a subsequent decline in somatic growth as plants become larger or older. However, few studies have examined how the relationship between branch extension growth and reproduction varies with size or age in the longest-lived plants: trees. Using a mobile lift for canopy access, we retrospectively measured branch extension growth before, during and after two (between 2011 and 2013) Acer saccharum mast events (the synchronous production of many seeds at long intervals), quantifying seed production per internode and internode length. Branch extension was reduced by 24 and 36%, respectively, in 2011 and 2013 relative to non-mast years, consistent with the expectation that increased reproductive allocation comes at the cost of allocation to growth. Internode length decreased from 8 to 3 cm year−1 as seed production increased from zero to 17 seeds year−1; a similar decrease was observed at the whole-tree level using average internode extension rates and seed production per tree. Seed production alone was the most parsimonious predictor of branch extension growth, with no independent effect of tree size, suggesting that it is the increase in reproductive allocation, rather than an increase in tree size per se, that drives the decline in branch extension rates. The slope of the relationship between branch extension and reproduction did not vary with tree size, suggesting that there was no increase in the somatic cost of reproduction with tree size. We also found no evidence for lag effects of reproduction on extension growth in subsequent years. Overall, these results suggest that reproductive allocation assessed at the shoot level increases with tree size and is a major driver of the ontogenetic decline in branch extension growth.

Soil responses to non-nitrogenous amendments in a nitrogen-saturated temperate forest: an unexpected decrease in methane oxidation after phosphorus and lime addition

Abstract: We measured soil greenhouse gas fluxes and nutrient pools in a nitrogen (N)-polluted north-temperate forest and report relatively small N2O effluxes and high CH4 uptake. Six years after application, non-N fertilizers (P + K, lime) did not significantly reduce soil respiration or increase N2O efflux; however, rates of CH4 oxidation were significantly suppressed.

Satellite Observations of Leaf Area Index Decline Following a Spring 2010 Heatwave in Ontario's Northern Temperate Forests

Abstract: Increases in mean air temperature are thought to improve productivity in northern forests but site-specific observations have shown that high temperatures can have deleterious effects. This study reconstructs leaf area index (LAI) time series for the Great Lakes–St. Lawrence forest region of Ontario using SPOT vegetation optical satellite data and the University of Toronto v2 LAI algorithm to study the impact of a spring heatwave in 2010. The results indicate a decline in 2010 LAI values as compared to prior normal years (2006–2009), coincident with the onset of high spring temperatures, which lasts throughout the growing season. Fitting a linear mixed model shows a significant link between high temperatures and LAI declines throughout the study area. These results provide the first evidence of sustained declines in LAI, and by proxy productivity, following high spring temperatures at the landscape scale.