Competition among lodgepole pine seedlings and plant species in a Sitka-alder dominated shrub community in the southern interior of British Columbia

Sitka alder (Alnus sinuata (Regel) Rydb.) dominates many lodgepole pine (Pinus contorta ssp. latifolia Dougl.) sites following clearcutting in the Montane Spruce zone of the southern interior of British Columbia. The objectives of this study were to examine the effects of the sitka alder-dominated shrub community on the performance of lodgepole pine and levels of environmental resources and conditions. Competitive interactions were examined in two studies: (1) among two year-old planted seedlings and plant species in various experimentally created shrub densities (0 to 2514 clumps/ha) and herb covers (0 to 100%), and (2) among eight year-old naturally regenerated saplings and plant species in an undisturbed community. In the first study, survival of seedlings among the experimentally created competition levels was 86% two years after planting. The main causes of mortality were drought and browsing by hares (Lepus sp.). Survival rate was not significantly affected by shrub and herb densities; however, survival was lowest where all vegetation had been removed. Seedling mortality in the total removal treatment may have been the result of high radiation loads and low moisture availability immediately following planting. Mean seedling size in the plantation was negatively affected by shrub and herb density. Stem diameter was the most responsive performance measure, smaller on average by 25% when seedlings were growing among maximum shrub and herb densities as compared with those growing free of competition. Height, in contrast, increased as shrub and herb densities increased. The decrease in diameter and increase in height in response to increasing vegetative competition reflected patterns in resource (particularly carbon) allocation. Several environmental factors were important to the enhancement of seedling water uptake and growth when competing vegetation was removed. Significant increases in seedling water uptake did not coincide with increases in soil water potential, but rather with increases in soil temperatures. Increases in seedling diameter corresponded with increased soil and air temperatures, light availability and mineralizable NO3-N. Individual seedling size in the plantation decreased with increasing amounts of neighboring plants. Visual estimates of percent cover of neighboring plants (extensive interspecific competition indices) explained more variation in pine size than did the more detailed measurements of alder size and proximity (intensive indices). Percent cover of all shrubs and herbs accounted for 16% of the variation in height:diameter ratio while angular dispersion and distance to neighboring sitka alder accounted for only 9%. A competition threshold, i.e. the amount of neighboring vegetation at which competition began and growth was limited, was not identified. Seedlings with the largest stem diameters, however, occurred in neighborhoods with less than 10% cover of herbs and shrubs each. The best multiple regression models developed explained 22% of the variation in pine diameter and 43% of the variation in height. The independent variables were initial height, seedling vigour, browsing damage and percent cover of all shrubs. Light and, to a lesser degree, soil water available to seedlings were reduced by neighboring vegetation. Within the experimentally created competition levels, sitka alder clumps sprouted to a mean height of 70 cm and mean diameter of 73 cm two growing seasons after manual cutting. The tallest stems (125 cm) reached 42% of the pre-treatment height (3 m). The density of sprouting alder clumps had a significant effect on the development of most neighboring shrub and herb species. Percent cover of alder, thimbleberry (Rubus parviflorus), fireweed (Epilobium angustifoliuin) and pinegrass (Calamagrostis rubescens) was greatest in the intermediate density range of 1258 to 1886 clumps/ha. Within this density range, threshold levels of environmental resources and conditions may have been reached which resulted in the greatest vegetative cover. Two growing seasons after planting, all neighboring species except grouseberry (Vacciniwn scoparium) were overtopping pine seedlings. In the second study of a 10 year-old undisturbed sitka alder dominated community, two vegetation types were identified. Type I was dominated by lodgepole pine and pinegrass while type II was dominated by sitka alder, thimbleberry and black huckleberry (Vaccinium membranaceum). The size of individual pine saplings was more negatively affected by neighboring plants in type II than I. Sitka alder, of all the dominant species in the undisturbed community, had the greatest competitive effect on pine size. The extensive competition index, percent cover of sitka alder, explained 45% of the variation in stem diameter. In contrast, the intensive indices, height of and distance to neighboring sitka alders, together explained 40% of the variation in stem diameter. A clearly defined competition threshold was not identified. Rather, pine size increased linearly as sitka alder densities decreased. Sitka alder had a negative effect on light availability to pine, particularly in type II

Long-term warming alters the composition of Arctic soil microbial communities

Despite the importance of Arctic soils in the global carbon cycle, we know very little of the impacts of warming on the soil microbial communities that drive carbon and nutrient cycling in these ecosystems. Over a 2-year period, we monitored the structure of soil fungal and bacterial communities in organic and mineral soil horizons in plots warmed by greenhouses for 18 years and in control plots. We found that microbial communities were stable over time but strongly structured by warming. Warming led to significant reductions in the evenness of bacterial communities, while the evenness of fungal communities increased significantly. These patterns were strongest in the organic horizon, where temperature change was greatest and were associated with a significant increase in the dominance of the Actinobacteria and significant reductions in the Gemmatimonadaceae and the Proteobacteria. Greater evenness of the fungal community with warming was associated with significant increases in the ectomycorrhizal fungi, Russula spp., Cortinarius spp., and members of the Helotiales suggesting that increased growth of the shrub Betula nana was an important mechanism driving this change. The shifts in soil microbial community structure appear sufficient to account for warming-induced changes in nutrient cycling in Arctic tundra as climate warm

Below-ground biotic interactions moderated the postglacial range dynamics of trees

Tree range shifts during geohistorical global change events provide a useful real-world model for how future changes in forest biomes may proceed. In North America, during the last deglaciation, the distributions of tree taxa varied significantly in the rate and direction of their responses for reasons that remain unclear. Local-scale processes such as establishment, growth, and resilience to environmental stress ultimately influence range dynamics. Despite the fact that interactions between trees and soil biota are known to influence local-scale processes profoundly, evidence linking belowground interactions to distribution dynamics remains scarce. • We evaluated climate velocity and plant traits related to dispersal, environmental tolerance, and belowground symbioses, as potential predictors of the geohistorical rates of expansion and contraction of the core distributions of tree genera between 16-7kaBP. • The receptivity of host genera towards ectomycorrhizal fungi was strongly supported as a positive predictor of poleward rates of distribution expansion, and seed mass was supported as a negative predictor. Climate velocity gained support as a positive predictor of rates of distribution contraction, but not expansion. • Our findings indicate that understanding how tree distributions, and thus forest ecosystems, respond to climate change requires the simultaneous consideration of traits, biotic interactions, and abiotic forcing

Transfer of 13C between paired Douglas-fir seedlings reveals plant kinship effects and uptake of exudates by ectomycorrhizas

• Processes governing the fixation, partitioning, and mineralization of carbon in soils are under increasing scrutiny as we develop a more comprehensive understanding of global carbon cycling. Here we examined fixation by Douglas-fir seedlings and transfer to associated ectomycorrhizal fungi, soil microbes, and full-sibling or non-sibling neighbouring seedlings. • Stable isotope probing with 99% 13C-CO2 was applied to trace 13C-labelled photosynthate throughout plants, fungi, and soil microbes in an experiment designed to assess the effect of relatedness on 13C-transfer between plant pairs. The fixation and transfer of 13C-label to plant, fungal, and soil microbial tissue was examined in biomass and PLFAs. • After a 6-day chase period, approximately 26.8% of the 13C remaining in the system was translocated belowground. Enrichment was proportionally greatest in ectomycorrhizal biomass. The presence of mesh barriers (0.5 or 35 µm) between seedlings did not restrict 13C-transfer. • Fungi were the primary recipients of 13C-labelled photosynthate throughout the system, representing 60–70% of total 13C-enriched phospholipids. Full-sibling pairs exhibited significantly greater 13C-transfer to recipient roots in two of four Douglas-fir families, representing 3- and 4-fold increases (+ approx. 4 µg excess 13C) compared to non-sibling pairs. The existence of a root/mycorrhizal exudation – hyphal uptake pathway was supported

Harvesting Intensity and Aridity Are More Important Than Climate Change in Affecting Future Carbon Stocks of Douglas-Fir Forests

Improved forest management may offer climate mitigation needed to hold warming to below 2°C. However, uncertainties persist about the effects of harvesting intensity on forest carbon sequestration, especially when considering interactions with regional climate and climate change. Here, we investigated the combined effects of harvesting intensity, climatic aridity, and climate change on carbon stocks in Douglas-fir [Pseudotsuga menziesii Mirb. (Franco)] stands. We used the Carbon Budget Model of the Canadian Forest Sector to simulate the harvest and regrowth of seven Douglas-fir stand types covering a 900 km-long climate gradient across British Columbia, Canada. In particular, we simulated stand growth under three regimes (+17%, −17% and historical growth increment) and used three temperature regimes [historical, representative concentration pathways (RCP) 2.6 and RCP 8.5]. Increasing harvesting intensity led to significant losses in total ecosystem carbon stocks 50 years post-harvest. Specifically, forests that underwent clearcutting were projected to stock about 36% less carbon by 2,069 than forests that were left untouched. Belowground carbon stocks 50 years into the future were less sensitive to harvesting intensity than aboveground carbon stocks and carbon losses were greater in arid interior Douglas-fir forests than in humid, more productive forests. In addition, growth multipliers and decay due to the RCP’s had little effect on total ecosystem carbon, but aboveground carbon declined by 7% (95% confidence interval [−10.98, −1.81]) in the high emissions (RCP8.5) scenario. We call attention to the implementation of low intensity harvesting systems to preserve aboveground forest carbon stocks until we have a more complete understanding of the impacts of climate change on British Columbia’s forests

Harvest intensity effects on carbon stocks and biodiversity are dependent on regional climate in Douglas-fir forests of British Columbia

Temperate forests provide crucial ecosystems services as living sinks for atmospheric carbon (C) and repositories of biodiversity. Applying harvesting at intensities that minimize losses offers one means for mitigating global change. However, little is known of overstory retention levels that best conserve ecosystem services in different regional climates, and likewise as climate changes. To quantify the effect of harvest intensity on C stocks and biodiversity, we compared five harvesting intensities (clearcutting,seedtree retention, 30% patch retention, 60% patch retention, and uncut controls) across a climatic aridity gradient that ranged from humid to semi-arid in the Douglas-fir (Pseudostuga menziesii) forests of British Columbia. We found that increased harvesting intensity reduced total ecosystem, aboveground, and live tree C stocks one year post-harvest, and the magnitude of these losses were negatively correlated with climatic aridity. In humid forests, total ecosystem C ranged from 50% loss following clearcut harvest, to 30% loss following large patch retention harvest. In arid forests this range was 60% to 8% loss, respectively. Where lower retention harvests are sought, the small patch retention treatment protected both C stocks and biodiversity in the arid forests, whereas the seedtree method performed as well or better in the humid forests. Belowground C stocks declined by an average of 29% after harvesting, with almost all of the loss from the forest floor and none from the mineral soil. Of the secondary pools, standing and coarse deadwood declined in all harvesting treatments regardless of cutting intensity or aridity, while C stocks in fine fuels and stumps increased. The understory plant C pool declined across all harvesting intensities in the humid forests, but increased in arid forests. Shannon’s diversity and richness of tree and bryoid species declined with harvesting intensity, where tree species losses were greatest in the humid forests and bryoid losses greatest in arid forests. Shrub and herb species were unaffected. This study showed that the highest retention level was best at reducing losses in C stocks and biodiversity, and clearcutting the poorest, and while partial retention of canopy trees can reduce losses in these ecosystem services, outcomes will vary with climatic aridity

Decline of an Ecotone Forest: 50 Years of Demography in the Southern Boreal Forest

Variation in tree recruitment, mortality, and growth can alter forest community composition and structure. Because tree recruitment and mortality events are generally infrequent, long‐time scales are needed to confirm trends in forests. We performed a 50‐yr demographic census of a forest plot located on the southern edge of the Canadian boreal forest, a region currently experiencing forest die‐back in response to direct and indirect effects of recent severe droughts. Here, we show that over the last 30 yr biomass, basal area, growth, and recruitment have decreased along with a precipitous rise in mortality across the dominant tree species. The stand experienced periods of drought in combination with multiple outbreaks of forest tent caterpillar (Malacosoma disstria) and bark beetles. These insect disturbances interacted to increase mortality rates within the stand and decrease stand density. The interaction of endogenous and exogenous factors may shift forests in this region onto novel successional trajectories with the possibility of changes in regional vegetation type

Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities

Adaptive behaviour of plants, including rapid changes in physiology, gene regulation and defence response, can be altered when linked to neighbouring plants by a mycorrhizal network (MN). Mechanisms underlying the behavioural changes include mycorrhizal fungal colonization by the MN or interplant communication via transfer of nutrients, defence signals or allelochemicals. We focus this review on our new findings in ectomycorrhizal ecosystems, and also review recent advances in arbuscular mycorrhizal systems. We have found that the behavioural changes in ectomycorrhizal plants depend on environmental cues, the identity of the plant neighbour and the characteristics of the MN. The hierarchical integration of this phenomenon with other biological networks at broader scales in forest ecosystems, and the consequences we have observed when it is interrupted, indicate that underground ‘tree talk’ is a foundational process in the complex adaptive nature of forest ecosystems

Shared compatibility of ectomycorrhizae on Pseudotsuga menziesii and Betula papyrifera seedlings grown in mixture in soils from southern British Columbia

Seedlings of Pseudotsuga menziesii (Mirb.) Franco and Betula papyrifera Marsh. were grown in the greenhouse in monoculture and dual culture in soils collected from a young mixed species plantation in the southern interior of British Columbia. The objectives of the study were (i) to evaluate the ability of P. menziesii and B. papyrifera to share compatible ectomycorrhizal fungi in order to assess their potential for hyphal linkages and (ii) to study the influence of neighboring seedlings on ectomycorrhizae occurrence. Eleven ectomycorrhizal morphotypes were recognized, seven of which P. menziesii and B. papyrifera seedlings shared in common over 90% of their root tips. The abundance and frequency of Rhizopogon, E-strain I, and Tuber on P. menziesii, and the frequency of Lactarius, Hebeloma, and Cenococcum on B. papyrifera, were affected by the presence of a neighboring seedling. The number of ectomycorrhizal morphotypes shared in common and colonization of root tips by common types were slightly greater when P. menziesii and B. papyrifera were grown in dual culture rather than in monoculture

Effects of observed and experimental climate change on terrestrial ecosystems in northern Canada: results from the Canadian IPY program

Published VersionTundra and taiga ecosystems comprise nearly 40 % of the terrestrial landscapes of Canada. These permafrost ecosystems have supported humans for more than 4500 years, and are currently home to ca. 115,000 people, the majority of whom are First Nations, Inuit and Métis. The responses of these ecosystems to the regional warming over the past 30–50 years were the focus of four Canadian IPY projects. Northern residents and researchers reported changes in climate and weather patterns and noted shifts in vegetation and other environmental variables. In forest-tundra areas tree growth and reproductive effort correlated with temperature, but seedling establishment was often hindered by other factors resulting in sitespecific responses. Increased shrub cover has occurred in sites across the Arctic at the plot and landscape scale, and this was supported by results from experimental warming. Experimental warming increased vegetation cover and nutrient availability in most tundra soils; however, resistance to warming was also found. Soil microbial diversity in tundra was no different than in other biomes, although there were shifts in mycorrhizal diversity in warming experiments. All sites measured were sinks for carbon during the growing season with expected seasonal and latitudinal patterns. Modeled responses of a mesic tundra system to climate change showed that the sink status will likely continue for the next 50–100 years, after which these tundra systems will likely become a net source of carbon dioxide to the atmosphere. These IPY studies were the first comprehensive assessment of the state and change in Canadian northern terrestrial ecosystems and showed that the inherent variability in these systems is reflected in their site-specific responses to changes in climate. They also showed the importance of using local traditional knowledge and science, and provided extensive data sets, sites and researchers needed to study and manage the inevitable changes in the Canadian North