Paul Grogan

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Soil bacterial diversity in the Arctic is not fundamentally different from that found in other biomes

The severe environmental stresses of the Arctic may have promoted unique soil bacterial communities compared with those found in lower latitude environments. Here, we present a comprehensive analysis of the biogeography of soil bacterial communities in the Arctic using a high resolution bar-coded pyrosequencing technique. We also compared arctic soils with soils from a wide range of more temperate biomes to characterize variability in soil bacterial communities across the globe. We show that arctic soil bacterial community composition and diversity are structured according to local variation in soil pH rather than geographical proximity to neighboring sites, suggesting that local environmental heterogeneity is far more important than dispersal limitation in determining community-level differences. Furthermore, bacterial community composition had similar levels of variability, richness and phylogenetic diversity within arctic soils as across soils from a wide range of lower latitudes, strongly suggesting a common diversity structure within soil bacterial communities around the globe. These results contrast with the well-established latitudinal gradients in animal and plant diversity, suggesting that the controls on bacterial community distributions are fundamentally different from those observed for macro-organisms and that our biome definitions are not useful for predicting variability in soil bacterial communities across the globe.

Fire effects on ecosystem nitrogen cycling in a Californian bishop pine forest

Abstract Fire can cause severe nitrogen (N) losses from grassland, chaparral, and temperate and boreal forest ecosystems. Paradoxically, soil ammonium levels are markedly increased by fire, resulting in high rates of primary production in re-establishing plant communities. In a manipulative experiment, we examined the influence of wild-fire ash residues on soil, microbial and plant N pools in a recently burned Californian bishop pine (Pinus muricata D. Don) forest. Ash stimulated post-fire primary production and ecosystem N retention through direct N inputs from ash to soils, as well as indirect ash effects on soil N availability to plants. These results suggest that redistribution of surface ash after fire by wind or water may cause substantial heterogeneity in soil N availability to plants, and could be an important mechanism contributing to vegetation patchiness in fire-prone ecosystems. In addition, we investigated the impact of fire on ecosystem N cycling by comparing 15N natural abundance values from recently burned and nearby unburned P. muricata forest communities. At the burned site, 15N natural abundance in recolonising species was similar to that in bulk soil organic matter. By contrast, there was a marked 15N depletion in the same species relative to the total soil N pool at the unburned site. These results suggest that plant uptake of nitrate (which tends to be strongly depleted in 15N because of fractionation during nitrification) is low in recently burned forest communities but could be an important component of eco- system N cycling in mature conifer stands.

Detection of forest stand-level spatial structure in ectomycorrhizal fungal communities

Ectomycorrhizal fungal (EMF) communities are highly diverse at the stand level. To begin to understand what might lead to such diversity, and to improve sampling designs, we investigated the spatial structure of these communities. We used EMF community data from a number of studies carried out in seven mature and one recently fire-initiated forest stand. We applied various measures of spatial pattern to characterize distributions at EMF community and species levels: Mantel tests, Mantel correlograms, variance/mean and standardized variograms. Mantel tests indicated that in four of eight sites community similarity decreased with distance, whereas Mantel correlograms also found spatial autocorrelation in those four plus two additional sites. In all but one of these sites elevated similarity was evident only at relatively small spatial scales (< 2.6 m), whereas one exhibited a larger scale pattern ( approximately 25 m). Evenness of biomass distribution among cores varied widely among taxa. Standardized variograms indicated that most of the dominant taxa showed patchiness at a scale of less than 3 m, with a range from 0 to < or =17 m. These results have implications for both sampling scale and intensity to achieve maximum efficiency of community sampling. In the systems we examined, cores should be at least 3 m apart to achieve the greatest sampling efficiency for stand-level community analysis. In some cases even this spacing may result in reduced sampling efficiency arising from patterns of spatial autocorrelation. Interpretation of the causes and significance of these patterns requires information on the genetic identity of individuals in the communities.

Deeper Snow Enhances Winter Respiration from Both Plant-associated and Bulk Soil Carbon Pools in Birch Hummock Tundra

It has only recently become apparent that biological activity during winter in seasonally snow-covered ecosystems may exert a significant influence on biogeochemical cycling and ecosystem function. One-seventh of the global soil carbon pool is stored in the bulk soil component of arctic ecosystems. Consistent climate change predictions of substantial increases in winter air temperatures and snow depths for the Arctic indicate that this region may become a significant net annual source of CO2 to the atmosphere if its bulk soil carbon is decomposed. We used snow fences to investigate the influence of a moderate increase in snow depth from approximately 0.3 m (ambient) to approximately 1 m on winter carbon dioxide fluxes from mesic birch hummock tundra in northern Canada. We differentiated fluxes derived from the bulk soil and plant-associated carbon pools using an experimental ‘weeding’ manipulation. Increased snow depth enhanced the wintertime carbon flux from both pools, strongly suggesting that respiration from each was sensitive to warmer soil temperatures. Furthermore, deepened snow resulted in cooler and relatively stable soil temperatures during the spring-thaw period, as well as delayed and fewer freeze–thaw cycles. The snow fence treatment increased mean total winter efflux from 27 to 43 g CO2-C m−2. Because total 2004 growing season net ecosystem exchange for this site is estimated at 29–37 g CO2-C m−2, our results strongly suggest that a moderate increase in snow depth can enhance winter respiration sufficiently to switch the ecosystem annual net carbon exchange from a sink to source, resulting in net CO2 release to the atmosphere.

Arctic Soil Respiration: Effects of Climate and Vegetation Depend on Season

ABSTRACT Arctic ecosystems are important in the context of global climate change because the most rapid rises in air temperature are expected at high northern latitudes during winter. The presence of extensive soil carbon reserves in the Arctic suggests that substantial feedbacks to CO2-induced climate change could occur if warming alters carbon cycling belowground. Characterization of the controls on regional patterns of belowground CO2 release through the annual cycle is an important step towards evaluating potential feedbacks from arctic ecosystems to climate change. In this study, we assess seasonal control over the influences by climate and vegetation-type on CO2 efflux from belowground in the Alaskan tundra. Our results indicate that climate had strong effects on belowground CO2 release in both seasons. By contrast, vegetation-type had little impact on CO2 efflux from belowground in winter but was the principal control in summer. Together, these results demonstrate that seasonality is a critical factor regulating climate and vegetation-type effects on belowground CO2 release, which should be included in regional models of net carbon balance in arctic ecosystems.

Respiration and Microbial Dynamics in Two Subarctic Ecosystems during Winter and Spring Thaw: Effects of Increased Snow Depth

ABSTRACT Recent evidence suggests that biogeochemical processes in the Arctic during late winter and spring-thaw strongly affect the annual cycling of carbon and nutrients, indicating high susceptibility to climate change. We therefore examined the carbon and nutrient dynamics in a sub-arctic heath and a birch forest with high temporal resolution from March until snowmelt at both ambient and experimentally increased snow depths. Ecosystem respiration (ER) from mid-March to snowmelt at ambient snow was high, reaching 99 ± 19 (birch) and 67 ± 1.4 g C m−2 (heath). Enhanced snow depth by about 20–30 cm increased ER by 77–157% during late winter but had no effects during spring-thaw. ER rates at the birch site were poorly described by classic first-order exponential models (R2 = 0.06–0.10) with temperature as a single variable, but model fit improved considerably by including the supply of dissolved organic carbon (DOC) or nitrogen (DON) in the model (R2 = 0.40–0.47). At the heath, model fit with temperature as the single variable was better (R2 = 0.38–0.52), yet it improved when the supply of DOC or DON was included (R2 = 0.65–0.72). Microbial carbon decreased by 43% within a few days after the first soil freeze-thaw event, while microbial nitrogen and phosphorus decreased more slowly. Because soil inorganic nitrogen and phosphorus concentrations were low, nutrients released from lysed microbial cells may have been sequestered by surviving microbes or by plants resuming growth. The fast change in microbial biomass and the dependence of ER on substrate availability stress the need for high temporal resolution in future research on ecosystem carbon and nutrient dynamics at snowmelt in order to make robust models of their turnover.

Respiration of recently-fixed plant carbon dominates mid-winter ecosystem CO2 production in sub-arctic heath tundra

Arctic ecosystems could provide a substantial positive feedback to global climate change if warming stimulates below-ground CO2 release by enhancing decomposition of bulk soil organic matter reserves.Ecosystem respiration during winter is important in this context because CO2 release from snow-covered tundra soils is a substantial component of annual net carbon (C) balance, and because global climate models predict that the most rapid rises in regional air temperature will occur in the Arctic during winter. In this manipulative field study, the relative contributions of plant and bulk soil organic matter C pools to ecosystem CO2 production in mid-winter were investigated. We measured CO2 efflux rates in Swedish sub-arctic heath tundra from control plots and from plots that had been clipped in the previous growing season to disrupt plant activity. Respiration derived from recently-fixed plant C (i.e., plant respiration, and respiration associated with rhizosphere exudates and decomposition of fresh litter) was the principal source of CO2 efflux, while respiration associated with decomposition of bulk soil organic matter was low, and appeared relatively insensitive to temperature. These results suggest that warmer mid-winter temperatures in the Arctic may have a much greater impact on the cycling of recently-fixed, plant-associated C pools than on the depletion of tundra bulk soil C reserves, and consequently that there is a low potential for significant initial feedbacks from arctic ecosystems to climate change during mid-winter.

Initial effects of experimental warming on above- and belowground components of net ecosystem CO2 exchange in arctic tundra

Abstract. The Arctic contains extensive soil carbon reserves that could provide a substantial positive feedback to atmospheric CO2 concentrations and global warming. Evaluation of this hypothesis requires a mechanistic understanding of the in situ responses of individual components of tundra net ecosystem CO2 exchange (NEE) to warming. In this study, we measured NEE, total ecosystem respiration and respiration from below ground in experimentally warmed plots within Alaskan acidic tussock tundra. Soil warming of 2–4°C during a single growing season caused strong increases in total ecosystem respiration and belowground respiration from moss-dominated inter-tussock areas, and similar trends from sedge-dominated tussocks. Consequently, the overall effect of the manipulation was to substantially enhance net ecosystem carbon loss during mid-summer. Components of vascular plant biomass were closely correlated with total ecosystem respiration and belowground respiration in control plots of both microsites, but not in warmed plots. By contrast, in the warmed inter-tussock areas, belowground respiration was most closely correlated with organic-layer depth. Warming in tussock areas was associated with increased leaf nutrient pools, indicating enhanced rates of soil nutrient mineralisation. Together, these results suggest that warming enhanced net ecosystem CO2 efflux primarily by stimulating decomposition of soil organic matter, rather than by increasing plant-associated respiration. Our short-term experiment provides field evidence to support previous growth chamber and modelling studies indicating that arctic soil C reserves are relatively sensitive to warming and could supply an initial positive feedback to rising atmospheric CO2 concentrations/changing climate.

Soil nitrogen cycling rates in low arctic shrub tundra are enhanced by litter feedbacks

Shrub growth has increased across the Arctic in recent decades and is strongly limited by soil nitrogen (N) availability. In order to understand the role of N in controlling shrub growth, we compared N-cycling in tall birch (Betula glandulosa) and surrounding dwarf birch hummock vegetation on similar soils in a Canadian low arctic site. Stable isotope tracer analysis revealed N pools and cycling rates were ∼3 times larger and faster in the tall birch ecosystem in the late growing season, just prior to leaf senescence. Gross NH4+-N production rates in these ecosystems correlated positively with larger pools and production rates of dissolved soil C and N, higher quality litter inputs and lower soil C. Analyses of the soil microbial community in both ecosystems indicated similar fungal dominance (epifluorescence microscopy) and similar compositions of the principal fungal or bacterial phylotypes (denaturing gradient gel electrophoresis). Together, these results strongly suggest that vegetation feedbacks associated with larger inputs of higher quality litter promote rapid soil N-cycling and enhanced shrub growth in tall birch tundra. We conclude that these litter-related feedbacks during summer may be as important as snow-shrub feedbacks in maintaining and promoting differences in shrub growth across the arctic landscape.