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

Interspecific carbon transfer in ectomycorrhizal tree species mixtures

The overall goal of this study was to investigate influences of ectomycorrhizae (EM) and interspecific carbon transfer on seedling performance in species mixtures. The objectives were to: (I) determine the potential for EM to link paper birch and Douglas-fir, (ii) quantify gross and net interspecific carbon transfer, and (iii) evaluate effect of transfer on seedling performance. A soil bioassay showed that paper birch and Douglas-fir shared seven EM morphotypes in common over 90% of their root tips, indicating potential for hyphal connections. The number and percent colonization of shared morphotypes were greater when species were grown in dual- than monoculture. Reciprocal labelling of paper birch and Douglas-fir with ¹³CO₂[subscript(gas)] and ¹⁴CO₂[subscript(gas)] in laboratory rootboxes and the field resulted in bi-directional transfer, with net gain by Douglas-fir. In rootboxes, gross and net transfer represented 29% and 4% of total isotope assimilated by both species. Net transfer was three times greater and one-way gross transfer to Douglas-fir 50% greater where interconnecting hyphae were left intact than where severed, but high p-values (p>0.05) leave in question whether hyphal connections facilitated transfer. In the field, gross and net transfer between paper birch and Douglas-fir represented 4% and 2%, respectively, of total isotope assimilated in 1993, and 7% and 6%, respectively, in 1994. Net transfer to Douglas-fir occurred where Douglas-fir grew full sun in 1993, and in all light intensities in 1994. The change in amount transferred and shading effect between years coincided with greater root development and improved seedling vigor in 1994 than 1993. Net and gross transfer were two times greater in 5% than 50% or 100% sun treatments in 1994, suggesting transfer was affected by changes in photosynthate sink strength of Douglas-fir. Isotope transferred to western redcedar represented <1-18% of gross transfer between paper birch and Douglas-fir, indicating most carbon was transferred between EM species via interconnecting hyphae. Douglas-fir seedlings were grown in untrenched and trenched treatments to evaluate the ability of overstory paper birch and Douglas-fir to influence seedling EM inoculation patterns and performance. Greater diversity of EM coincided with higher photosynthesis among seedlings in the untrenched than trenched treatment. The effect on seedling performance was attributed to differences in EM colonization, because trenching had no effect on soil water, soil nutrients, or light availability

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

Legacy of salmon-derived nutrients on riparian soil chemistry and soil fertility on the Central Coast of British Columbia, Canada

Every year, salmon return to their natal streams to spawn. Their return represents an ecosystem subsidy of nutrients and energy from the sea to the land; these materials feed into terrestrial food webs, plant communities, and forest soils. Here we test the long term effects of salmon inputs on soil fertility by sampling soils from Haíɫzaqv (Heiltsuk) Nation territory on the Central Coast of British Columbia, Canada. A total of 20 soil chemical properties were assessed via two sampling methodologies: first, across 23 watersheds representing a regional gradient of salmon density (kg of salmon per meter of stream reach per year); and second, at four sites above and below waterfalls that blocked salmon migration. At each sampling point, soil material at two depths was collected. Multivariate analysis regional gradient showed salmon density, the moss community, and the shrub community to be significant factors related to soil chemistry. Similarly, being above or below the waterfall, the moss community, and the tree community were significant in the waterfall comparison. Generalized linear mixed models along regional salmon density gradient showed an increase in nitrate (NO3−) correlated with salmon inputs (p &lt; 0.05), and moderately significant (p &lt; 0.1) increases in ammonium (NH4+), phosphorus (P), aluminum (Al), and copper (Cu). Net cation exchange capacity (CEC) did not change; however, magnesium (Mg) significantly decreased along these gradients (p &lt; 0.05), while sodium (Na) had a declining tendency and calcium (Ca) had an increasing tendency. Being below salmon-blocking waterfalls or the salmon density below falls was a factor in higher total nitrogen, nitrate, ammonium, phosphorus, total sulfur (S), magnesium, and sodium concentrations; below falls sites also had lower pH and aluminum. Exploratory analysis of the regional gradient data using a random forest model apportioned high importance to soil depth, the moss community, the shrub community, salmon density and distance from the stream. These results show that salmon inputs are correlated with a number of changes in recipient soils, and these are consistent with an interpretation of improved fertility at these sites

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

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