Michael J. Gundale

Charcoal effects on soil solution chemistry and growth of Koeleria macrantha in the ponderosa pine/Douglas-fir ecosystem

We conducted laboratory and greenhouse experiments to determine whether charcoal derived from the ponderosa pine/Douglas-fir ecosystem may influence soil solution chemistry and growth of Koeleria macrantha, a perennial grass that thrives after fire. In our first experiment, we incubated forest soils with a factorial combination of Douglas-fir wood charcoal generated at 350°C and extracts of Arctostaphylos uva-ursi with and without the addition of glycine as a labile N source. These results showed that charcoal increased N mineralization and nitrification when glycine was added, but reduced N mineralization and nitrification without the addition of glycine. Charcoal significantly reduced the solution concentration of soluble phenols from litter extracts, but may have contributed bioavailable C to the soil that resulted in N immobilization in the no-glycine trial. In our second experiment, we grew K. macrantha in soil amended with charcoal made at 350°C from ponderosa pine and Douglas-fir bark. Growth of K. macrantha was significantly diminished by both of these charcoal types relative to the control. In our third experiment, we grew K. macrantha in soil amended with six concentrations (0, 0.5, 1, 2, 5, and 10%) of charcoal collected from a wildfire. The data showed increasing growth of K. macrantha with charcoal addition, suggesting some fundamental differences between laboratory-generated charcoal and wildfire-produced charcoal. Furthermore, they suggest a need for a better understanding of how temperature and substrate influence the chemical properties of charcoal.

Ecosystem Feedbacks and Nitrogen Fixation in Boreal Forests

Biological feedback mechanisms regulate fundamental ecosystem processes and potentially regulate ecosystem productivity. To date, no studies have documented the down-regulation of terrestrial nitrogen (N) fixation via an ecosystem-level feedback mechanism. Herein, we demonstrate such a feedback in boreal forests. Rapid cycling of N in early secondary succession forests yielded greater throughfall N deposition, which in turn decreased N fixation by cyanobacterial associates in feather moss carpets that reside on the forest floor. The forest canopy exerts a tight control on biotic N input at a period of high productivity.

Bryophyte-cyanobacteria associations as regulators of the northern latitude carbon balance in response to global change

Ecosystems in the far north, including arctic and boreal biomes, are a globally significant pool of carbon (C). Global change is proposed to influence both C uptake and release in these ecosystems, thereby potentially affecting whether they act as C sources or sinks. Bryophytes (i.e., mosses) serve a variety of key functions in these systems, including their association with nitrogen (N2 )-fixing cyanobacteria, as thermal insulators of the soil, and producers of recalcitrant litter, which have implications for both net primary productivity (NPP) and heterotrophic respiration. While ground-cover bryophytes typically make up a small proportion of the total biomass in northern systems, their combined physical structure and N2 -fixing capabilities facilitate a disproportionally large impact on key processes that control ecosystem C and N cycles. As such, the response of bryophyte-cyanobacteria associations to global change may influence whether and how ecosystem C balances are influenced by global change. Here, we review what is known about their occurrence and N2 -fixing activity, and how bryophyte systems will respond to several key global change factors. We explore the implications these responses may have in determining how global change influences C balances in high northern latitudes.

Nitrogen Spatial Heterogeneity Influences Diversity Following Restoration in a Ponderosa Pine Forest, Montana

The resource heterogeneity hypothesis (RHH) is frequently cited in the ecological literature as an important mechanism for maintaining species diversity. The RHH has rarely been evaluated in the context of restoration ecology in which a commonly cited goal is to restore diversity. In this study we focused on the spatial heterogeneity of total inorganic nitrogen (TIN) following restoration treatments in a ponderosa pine (Pinus ponderosa)/Douglas-fir (Pseudotsuga menziesii) forest in western Montana, USA. Our objective was to evaluate relationships between understory species richness and TIN heterogeneity following mechanical thinning (thin-only), prescribed burning (burn-only), and mechanical thinning with prescribed burning (thin/burn) to discern the ecological and management implications of these restoration approaches. We employed a randomized block design, with three 9-ha replicates of each treatment and an untreated control. Within each treatment, we randomly established a 20 x 50 m (1000 m2) plot in which we measured species richness across the entire plot and in 12 1-m(2) quadrats randomly placed within each larger plot. Additionally, we measured TIN from a grid consisting of 112 soil samples (0-5 cm) in each plot and computed standard deviations as a measure of heterogeneity. We found a correlation between the net increase in species richness and the TIN standard deviations one and two years following restoration treatments, supporting RHH. Using nonmetric multidimensional scaling ordination and chi-squared analysis, we found that high and low TIN quadrats contained different understory communities in 2003 and 2004, further supporting RHH. A comparison of restoration treatments demonstrated that thin/burn and burn-only treatments created higher N heterogeneity relative to the control. We also found that within prescribed burn treatments, TIN heterogeneity was positively correlated with fine-fuel consumption, a variable reflecting burn severity. These findings may lead to more informed restoration decisions that consider treatment effects on understory diversity in ponderosa pine/Douglas-fir ecosystems.

The interactive effects of temperature and light on biological nitrogen fixation in boreal forests

Plant productivity is predicted to increase in northern latitudes as a result of climate warming; however, this may depend on whether biological nitrogen (N)-fixation also increases. We evaluated how the variation in temperature and light affects N-fixation by two boreal feather mosses, Pleurozium schreberi and Hylocomium splendens, which are the primary source of N-fixation in most boreal environments. We measured N-fixation rates 2 and 4 wk after exposure to a factorial combination of environments of normal, intermediate and high temperature (16.3, 22.0 and 30.3°C) and light (148.0, 295.7 and 517.3 μmol m(-2) s(-1)). Our results showed that P. schreberi achieved higher N-fixation rates relative to H. splendens in response to warming treatments, but that the highest warming treatment eventually caused N-fixation to decline for both species. Light strongly interacted with warming treatments, having positive effects at low or intermediate temperatures and damaging effects at high temperatures. These results suggest that climate warming may increase N-fixation in boreal forests, but that increased shading by the forest canopy or the occurrence of extreme temperature events could limit increases. They also suggest that P. schreberi may become a larger source of N in boreal forests relative to H. splendens as climate warming progresses.

Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils

It is proposed that carbon (C) sequestration in response to reactive nitrogen (Nr ) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While studies have helped clarify the magnitude by which Nr deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha(-1) yr(-1) ) in the boreal zone of northern Sweden to understand how atmospheric Nr deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg(-1) N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg(-1) N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.

The sensitivity of nitrogen fixation by a feathermoss-cyanobacteria association to litter and moisture variability in young and old boreal forests

We conducted a pair of experiments to assess whether nitrogen (N) fixation by a feathermoss–cyanobacteria association was sensitive to moisture availability and quality of litter inputs, and whether sensitivity to these factors differed between young and old forests. In our first greenhouse experiment, we experimentally varied the frequency of water addition to Pleurozium schreberi (Brid.) Mitt. collected from young and old forest sites. This experiment revealed that the extreme drought treatment reduced N fixation capacity (measured via acetylene reduction), whereas daily watering increased N fixation capacity. The experiment also demonstrated that sensitivity to moisture variability was greater in old forests than in young forests. In a second greenhouse experiment, we repeatedly applied litter extracts from six common boreal species, Pinus sylvestris L., Picea abies (L.) Karst., Betula pubescens Ehrh., Vaccinium myrtillus L., Vaccinium vitis-idaea L., and Empetrum hermaphroditum Lange ex Hagerup. After...

Vascular plant removal effects on biological N fixation vary across a boreal forest island gradient

There is currently much interest in understanding how biodiversity loss affects the functioning of ecosystems, but few studies have evaluated how ecosystem processes change in response to one another following biodiversity loss. We focused on a well-described gradient of 30 forested lake islands in northern Sweden, where island size determines the occurrence of lightning-ignited wildfire, which in turn determines successional stage, plant species composition, and productivity. We investigated the effect of biodiversity loss on biological nitrogen fixation by feathermosses through an experiment consisting of factorial removals of three understory shrub species (Vaccinium myrtillis, Vaccinium vitis-idaea, and Empetrum hermaphroditum) and two plant functional groups (shrubs and tree roots). We tested the hypothesis that, following vascular plant species loss, N fixation rates would be impaired by changes in pools or processes that increase extractable soil N, because changes in the supply rate of N to feathermosses should influence their demand for newly fixed N. Further, we hypothesized that the effects of removals on N fixation would depend on environmental context (i.e., island size), because it has been previously demonstrated that the effect of vascular plant species removal on N recycling pools and processes was strongest on productive islands. The data demonstrated that removal of two shrub species (V. vitis-idaea and E. hermaphroditum) negatively aflected the N fixation of Hylocomium splendens, but positively affected Pleurozium schreberi, resulting in unchanged areal N fixation rates. In the functional removal experiment, tree root removal resulted in a significant negative effect on N fixation. The effects of shrub and root removals on N fixation occurred only on small islands and thus were context dependent. This pattern did not correspond to the effect of shrub and root removal treatments on N-recycling pools or processes, which only occurred in response to specific vascular plant removals on large or medium islands. The data thus did not support our hypothesis that N fixation was directly responsive to changes in N-recycling pools or processes caused by vascular plant species removals, but instead highlighted the importance of species-specific interactions and environmental context in determining the manner in which biodiversity loss alters ecosystem processes.

Decoupled long‐term effects of nutrient enrichment on aboveground and belowground properties in subalpine tundra

Fertilizer experiments aid understanding of the role of nutrient limitation in tundra ecosystems, and of the effects that global change may exert on them through enhancing nutrient availability. However, little is known about whether fertilizer additions influence tundra ecosystem properties directly, or indirectly through altering plant community composition. To separate these direct and indirect effects, we used an ongoing fertilization experiment in subalpine tundra in northern Sweden initiated in 1989. Here, the slow-growing dwarf shrub Empetrum hermaphroditum (hereafter Empetrum) dominates, but fertilization causes its replacement by the fast-growing grass Deschampsia flexuosa (hereafter Deschampsia). Therefore, in 1999, we set up three subplots in every plot of the fertilizer experiment subjected to different treatments, i.e., regular removal of Empetrum or of Deschampsia, or no removal. An interactive effect between fertilizer and removal treatments means that fertilization indirectly influences ecological properties through its effects on the dominant species whereas no interactive effect means that fertilization effects are more likely direct. We measured plant community properties four times between 1999 and 2010, and belowground properties in 2010. Fertilization exerted large effects on nearly all variables aboveground (i.e., vascular plant community properties, bryophyte and lichen cover) and belowground (i.e., measures of nutrient availability, mass of humus and major elements per area, microbial community properties). Meanwhile, loss of either Deschampsia or Empetrum reduced total vascular plant cover, with greatest effects in those fertilization treatments where they otherwise had the most mass. Deschampsia removal affected several other plant community properties, but mostly independently of fertilizer treatment; Empetrum removal had little effect on any other plant community variable. Belowground, both removal treatments reduced humus mass per area but had no effect on any other variable. Our finding of few interactive effects of fertilizer and removal treatments aboveground and none belowground means that fertilizer effects on most community and ecosystem properties operate independently of even large shifts in the plant community. These results also point to a decoupling between aboveground and belowground communities in tundra, and therefore offer insights about the mechanistic basis through which global change impacts on soil nutrient availability may transform tundra ecosystems.

The effect of altered macroclimate on N-fixation by boreal feather mosses

Plant productivity is predicted to increase in boreal forests owing to climate change, but this may depend on whether N inputs from biological N-fixation also increases. We evaluated how alteration of climatic factors affects N input from a widespread boreal N-fixer, i.e. cyanobacteria associated with the feather moss Pleurozium schreberi. In each of 10 forest stands in northern Sweden, we established climate-change plots, including a control (ambient climate) plot and three plots experiencing a +2°C temperature increase, an approximately threefold reduction in precipitation frequency, and either 0.07, 0.29 or 1.16 times normal summer precipitation. We monitored N-fixation in these plots five times between 2007 and 2009, and three times in 2010 after climate treatments ended to assess their recovery. Warmer temperatures combined with less frequent precipitation reduced feather moss moisture content and N-fixation rates regardless of total precipitation. After climate treatments ended, recovery of N-fixation rates occurred on the scale of weeks to months, suggesting resilience of N-fixation to changes in climatic conditions. These results suggest that modelling of biological N-inputs in boreal forests should emphasize precipitation frequency and evaporative water loss in conjunction with elevated temperature rather than absolute changes in mean precipitation.