Long-term experimental warming and nutrient additions increase productivity in tall deciduous shrub tundra

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 5 (2014): art72, doi:10.1890/ES13-00281.1.Warming Arctic temperatures can drive changes in vegetation structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic biome is currently increasing in deciduous shrub cover and this increase is expected to continue with climate warming. However, little is known how current deciduous shrub communities will respond to future climate induced warming and nutrient increase. We examined the plant and ecosystem response to a long-term (18 years) nutrient addition and warming experiment in an Alaskan arctic tall deciduous shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems. In addition, we used a meta-analysis approach to compare the treatment effect size for aboveground biomass among seven long-term studies conducted across multiple plant community types within the Arctic. We found that biomass, productivity, and aboveground N pools increased with nutrient additions and warming, while species diversity decreased. Both nutrient additions and warming caused the dominant functional group, deciduous shrubs, to increase biomass and proportional C and N allocation to aboveground stems but decreased allocation to belowground stems. For all response variables except soil C and N pools, effects of nutrients plus warming were largest. Soil C and N pools were highly variable and we could not detect any response to the treatments. The biomass response to warming and fertilization in tall deciduous shrub tundra was greater than moist acidic and moist non-acidic tundra and more similar to the biomass response of wet sedge tundra. Our data suggest that in a warmer and more nutrient-rich Arctic, tall deciduous shrub tundra will have greater total deciduous shrub biomass and a higher proportion of woody tissue that has a longer residence time, with a lower proportion of C and N allocated to belowground stems.This research was supported by NSF grants DEB-0516041, DEB-0516509 and the Arctic LTER (DEB-0423385)

Effects of tillage and poultry manure application rates on Salmonella and fecal indicator bacteria concentrations in tiles draining Des Moines Lobe soils

Application of poultry manure (PM) to cropland as fertilizer is a common practice in artificially drained regions of the Upper Midwest United States. Tile-waters have the potential to contribute pathogenic bacteria to downstream waters. This 3-year study (2010–2012) was designed to evaluate the impacts of manure management and tillage practices on bacteria losses to drainage tiles under a wide range of field conditions. PM was applied annually in spring, prior to planting corn, at application rates ranging from 5 to 40 kg/ha to achieve target rates of 112 and 224 kg/ha nitrogen (PM1 and PM2). Control plots received no manure (PM0). Each treatment was replicated on three chisel-plowed (CP) plots and one no-till (NT) plot. Tile-water grab samples were collected weekly when tiles were flowing beginning 30 days before manure application to 100 days post application, and additional grab samples were obtained to target the full spectrum of flow conditions. Manure and tile-water samples were analyzed for the pathogen,Salmonella spp. (SALM), and fecal indicator bacteria (FIB), Escherichia coli (EC), and enterococci (ENT). All three bacterial genera were detected more frequently, and at significantly higher concentrations, in tile-waters draining NT plots compared to CP plots. Transport of bacteria to NT tiles was most likely facilitated by macropores, which were significantly more numerous above tiles in NT plots in 2012 as determined by smoke-testing. While post-manure samples contained higher concentrations of bacteria than pre-manure samples, significant differences were not seen between low (PM1) and high (PM2) rates of PM application. The highest concentrations were observed under the NT PM2 plot in 2010 (6.6 × 103 cfu/100 mL EC, 6.6 × 105 cfu/100 mL ENT, and 2.8 × 103 cfu/100 mL SALM). Individual and 30-day geometric mean ENT concentrations correlated more strongly to SALM than EC; however, SALM were present in samples with little or no FIB

The response of Arctic vegetation and soils following an unusually severe tundra fire

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Philosophical Transactions of the Royal Society B Biological Sciences 368 (2013): 20120490, doi:10.1098/rstb.2012.0490.Fire causes dramatic short-term changes in vegetation and ecosystem function, and may promote rapid vegetation change by creating recruitment opportunities. Climate warming likely will increase the frequency of wildfire in the Arctic, where it is not common now. In 2007, the unusually severe Anaktuvuk River fire burned 1039 km2 of tundra on Alaska's North Slope. Four years later, we harvested plant biomass and soils across a gradient of burn severity, to assess recovery. In burned areas, above-ground net primary productivity of vascular plants equalled that in unburned areas, though total live biomass was less. Graminoid biomass had recovered to unburned levels, but shrubs had not. Virtually all vascular plant biomass had resprouted from surviving underground parts; no non-native species were seen. However, bryophytes were mostly disturbance-adapted species, and non-vascular biomass had recovered less than vascular plant biomass. Soil nitrogen availability did not differ between burned and unburned sites. Graminoids showed allocation changes consistent with nitrogen stress. These patterns are similar to those seen following other, smaller tundra fires. Soil nitrogen limitation and the persistence of resprouters will likely lead to recovery of mixed shrub–sedge tussock tundra, unless permafrost thaws, as climate warms, more extensively than has yet occurred.This work was supported by NSF (no. OPP-0632264) and NSF (no. OPP-1107892) to M. S. Bret-Harte, NSF (no. OPP-0856853) to G. R. Shaver and NSF (no. OPP-6737545) to M. C. Mack

Bottom-up drivers of future fire regimes in western boreal North America

Forest characteristics, structure, and dynamics within the North American boreal region are heavily influenced by wildfire intensity, severity, and frequency. Increasing temperatures are likely to result in drier conditions and longer fire seasons, potentially leading to more intense and frequent fires. However, an increase in deciduous forest cover is also predicted across the region, potentially decreasing flammability. In this study, we use an individual tree-based forest model to test bottom-up (i.e. fuels) vs top-down (i.e. climate) controls on fire activity and project future forest and wildfire dynamics. The University of Virginia Forest Model Enhanced is an individual tree-based forest model that has been successfully updated and validated within the North American boreal zone. We updated the model to better characterize fire ignition and behavior in relation to litter and fire weather conditions, allowing for further interactions between vegetation, soils, fire, and climate. Model output following updates showed good agreement with combustion observations at individual sites within boreal Alaska and western Canada. We then applied the updated model at sites within interior Alaska and the Northwest Territories to simulate wildfire and forest response to climate change under moderate (RCP 4.5) and extreme (RCP 8.5) scenarios. Results suggest that changing climate will act to decrease biomass and increase deciduous fraction in many regions of boreal North America. These changes are accompanied by decreases in fire probability and average fire intensity, despite fuel drying, indicating a negative feedback of fuel loading on wildfire. These simulations demonstrate the importance of dynamic fuels and dynamic vegetation in predicting future forest and wildfire conditions. The vegetation and wildfire changes predicted here have implications for large-scale changes in vegetation composition, biomass, and wildfire severity across boreal North America, potentially resulting in further feedbacks to regional and even global climate and carbon cycling

Forest Composition Change and Biophysical Climate Feedbacks Across Boreal North America

Deciduous tree cover is expected to increase in North American boreal forests with climate warming and wildfire. This shift in composition has the potential to generate biophysical cooling via increased land surface albedo. Here we use Landsat-derived maps of continuous tree canopy cover and deciduous fractional composition to assess albedo change over recent decades. We find, on average, a small net decrease in deciduous fraction from 2000 to 2015 across boreal North America and from 1992 to 2015 across Canada, despite extensive fire disturbance that locally increased deciduous vegetation. We further find near-neutral net biophysical change in radiative forcing associated with albedo when aggregated across the domain. Thus, while there have been widespread changes in forest composition over the past several decades, the net changes in composition and associated post-fire radiative forcing have not induced systematic negative feedbacks to climate warming over the spatial and temporal scope of our study

Bottom-Up Drivers of Future Fire Regimes in Western Boreal North America

Forest characteristics, structure, and dynamics within the North American boreal region are heavily influenced by wildfire intensity, severity, and frequency. Increasing temperatures are likely to result in drier conditions and longer fire seasons, potentially leading to more intense and frequent fires. However, an increase in deciduous forest cover is also predicted across the region, potentially decreasing flammability. In this study, we use an individual tree-based forest model to test bottom-up (i.e. fuels) vs top-down (i.e. climate) controls on fire activity and project future forest and wildfire dynamics. The University of Virginia Forest Model Enhanced is an individual tree-based forest model that has been successfully updated and validated within the North American boreal zone. We updated the model to better characterize fire ignition and behavior in relation to litter and fire weather conditions, allowing for further interactions between vegetation, soils, fire, and climate. Model output following updates showed good agreement with combustion observations at individual sites within boreal Alaska and western Canada. We then applied the updated model at sites within interior Alaska and the Northwest Territories to simulate wildfire and forest response to climate change under moderate (RCP 4.5) and extreme (RCP 8.5) scenarios. Results suggest that changing climate will act to decrease biomass and increase deciduous fraction in many regions of boreal North America. These changes are accompanied by decreases in fire probability and average fire intensity, despite fuel drying, indicating a negative feedback of fuel loading on wildfire. These simulations demonstrate the importance of dynamic fuels and dynamic vegetation in predicting future forest and wildfire conditions. The vegetation and wildfire changes predicted here have implications for large-scale changes in vegetation composition, biomass, and wildfire severity across boreal North America, potentially resulting in further feedbacks to regional and even global climate and carbon cycling

Importance of Tree-and Species-Level Interactions with Wildfire, Climate, and Soils in Interior Alaska: Implications for Forest Change Under a Warming Climate

The boreal zone of Alaska is dominated by interactions between disturbances, vegetation, and soils. These interactions are likely to change in the future through increasing permafrost thaw, more frequent and intense wildfires, and vegetation change from drought and competition. We utilize an individual tree-based vegetation model, the University of Virginia Forest Model Enhanced (UVAFME), to estimate current and future forest conditions across sites within interior Alaska. We updated UVAFME for application within interior Alaska, including improved simulation of permafrost dynamics, litter decay, nutrient dynamics, fire mortality, and postfire regrowth. Following these updates, UVAFME output on species-specific biomass and stem density was comparable to inventory measurements at various forest types within interior Alaska. We then simulated forest response to climate change at specific inventory locations and across the Tanana Valley River Basin on a 2 × 2 km2 grid. We derived projected temperature and precipitation from a five-model average taken from the CMIP5 archive under the RCP 4.5 and 8.5 scenarios. Results suggest that climate change and the concomitant impacts on wildfire and permafrost dynamics will result in overall decreases in biomass (particularly for spruce (Picea spp.)) within the interior Tanana Valley, despite increases in quaking aspen (Populus tremuloides) biomass, and a resulting shift towards higher deciduous fraction. Simulation results also predict increases in biomass at cold, wet locations and at high elevations, and decreases in biomass in dry locations, under both moderate (RCP 4.5) and extreme (RCP 8.5) climate change scenarios. These simulations demonstrate that a highly detailed, species interactive model can be used across a large region within Alaska to investigate interactions between vegetation, climate, wildfire, and permafrost. The vegetation changes predicted here have the capacity to feed back to broader scale climate-forest interactions in the North American boreal forest, a region which contributes significantly to the global carbon and energy budgets

Impacts of Climate and Insect Herbivory on Productivity and Physiology of Trembling Aspen (Populus tremuloides) in Alaskan Boreal Forests

Climate change is impacting forested ecosystems worldwide, particularly in the Northern Hemisphere where warming has increased at a faster rate than the rest of the globe. As climate warms, trembling aspen (Populus tremuloides) is expected to become more successful in northern boreal forests because of its current presence in drier areas of North America. However, large-scale productivity decline of aspen has recently been documented throughout the United States and Canada as a result of drought and insect outbreaks. We used tree ring measurements (basal area increment (BAI) and stable carbon isotopes (δ 13C)) and remote sensing indices of vegetation productivity (NDVI) to study the impact of climate and damage by the aspen epidermal leaf miner (Phyllocnistis populiella) on aspen productivity and physiology in interior Alaska. We found that productivity decreased with greater leaf mining and was not sensitive to growing season (GS) moisture availability. Although productivity decreased during high leaf mining years, it recovered to pre-outbreak levels during years of low insect damage, suggesting a degree of resilience to P. populiella mining. Climate and leaf mining interacted to influence tree ring δ 13C, with greater leaf mining resulting in decreased δ 13C when GS moisture availability was low. We also found that NDVI was negatively associated with leaf mining, and positively correlated with BAI and the δ 13C decrease corresponding to mining. This suggests that NDVI is capturing not only variations in productivity, but also changes in physiology associated with P. populiella. Overall, these findings indicate that the indirect effects of P. populiella mining have a larger impact on aspen productivity and physiology than climate under current conditions, and is essential to consider when assessing growth, physiology and NDVI trends in interior Alaska

Plant functional types do not predict biomass responses to removal and fertilization in Alaskan tussock tundra

© 2008 The Authors. This is an open-access article distributed under the terms of the Creative Commons Attribution License 2.5. The definitive version was published in Journal of Ecology 96 (2008): 713-726, doi:10.1111/j.1365-2745.2008.01378.x.Plant communities in natural ecosystems are changing and species are being lost due to anthropogenic impacts including global warming and increasing nitrogen (N) deposition. We removed dominant species, combinations of species and entire functional types from Alaskan tussock tundra, in the presence and absence of fertilization, to examine the effects of non-random species loss on plant interactions and ecosystem functioning. After 6 years, growth of remaining species had compensated for biomass loss due to removal in all treatments except the combined removal of moss, Betula nana and Ledum palustre (MBL), which removed the most biomass. Total vascular plant production returned to control levels in all removal treatments, including MBL. Inorganic soil nutrient availability, as indexed by resins, returned to control levels in all unfertilized removal treatments, except MBL. Although biomass compensation occurred, the species that provided most of the compensating biomass in any given treatment were not from the same functional type (growth form) as the removed species. This provides empirical evidence that functional types based on effect traits are not the same as functional types based on response to perturbation. Calculations based on redistributing N from the removed species to the remaining species suggested that dominant species from other functional types contributed most of the compensatory biomass. Fertilization did not increase total plant community biomass, because increases in graminoid and deciduous shrub biomass were offset by decreases in evergreen shrub, moss and lichen biomass. Fertilization greatly increased inorganic soil nutrient availability. In fertilized removal treatments, deciduous shrubs and graminoids grew more than expected based on their performance in the fertilized intact community, while evergreen shrubs, mosses and lichens all grew less than expected. Deciduous shrubs performed better than graminoids when B. nana was present, but not when it had been removed. Synthesis. Terrestrial ecosystem response to warmer temperatures and greater nutrient availability in the Arctic may result in vegetative stable-states dominated by either deciduous shrubs or graminoids. The current relative abundance of these dominant growth forms may serve as a predictor for future vegetation composition.This work was supported by NSF grants DEB-0213130, DEB-0516509, OPP-0623364, DEB-981022 and DEB-0423385, and by the Inter-American Institute for Global Change Research (IAI) CRN 2015 which is supported by the US National Science Foundation (GEO-0452325). Open access to this publication was partially supported by the Berkeley Research Impact Initiative Program

Patterns of Ecosystem Structure and Wildfire Carbon Combustion Across Six Ecoregions of the North American Boreal Forest

Increases in fire frequency, extent, and severity are expected to strongly impact the structure and function of boreal forest ecosystems. An important function of the boreal forest is its ability to sequester and store carbon (C). Increasing disturbance from wildfires, emitting large amounts of C to the atmosphere, may create a positive feedback to climate warming. Variation in ecosystem structure and function throughout the boreal forest is important for predicting the effects of climate warming and changing fire regimes on C dynamics. In this study, we compiled data on soil characteristics, stand structure, pre-fire C pools, C loss from fire, and the potential drivers of these C metrics from 527 sites distributed across six ecoregions of North America’s western boreal forests. We assessed structural and functional differences between these fire-prone ecoregions using data from 417 recently burned sites (2004–2015) and estimated ecoregion-specific relationships between soil characteristics and depth from 167 of these sites plus an additional 110 sites (27 burned, 83 unburned). We found that northern boreal ecoregions were generally older, stored and emitted proportionally more belowground than aboveground C, and exhibited lower rates of C accumulation over time than southern ecoregions. We present ecoregion-specific estimates of depth-wise soil characteristics that are important for predicting C combustion from fire. As climate continues to warm and disturbance from wildfires increases, the C dynamics of these fire-prone ecoregions are likely to change with significant implications for the global C cycle and its feedbacks to climate change