Responses in plant, soil inorganic and microbial nutrient pools to experimental fire, ash and biomass addition in a woodland savanna

In order to investigate the effects of savanna fires on nutrient cycling a field experiment was carried out in an open woodland savanna of southwest Ethiopia. This involved manipulations of fire, fuel load and ash fertilisation in a fully factorial design, and recording of responses in plants, soil inorganic and microbial nutrient pools up to 1 year after the disturbances. As plant biomass nitrogen (N) was only 3.5% of that in topsoil the N loss in a single fire event was relatively small. The microbial N pool size in the topsoil was similar to the N pool size in the aboveground part of the plants. Soil microbial biomass carbon increased slightly 12 days after the low severity fire, but the effect was transient and was not accompanied by an increase in microbial N. Instead, the soil inorganic N concentration increased strongly 1 day after the fire, remained higher up to 3 months after the fire and probably caused the 40% higher grass biomass in burned than unburned plots, and the similar sized increase in grass nitrogen, phosphorus and potassium pools in the following rainy season. In contrast, broad-leaved herbs showed less strong increments in biomass and nutrient pool sizes. Fire interacted with fuel load, as burning of plots with double plant biomass led to reduced microbial biomass, plant nutrient pools and herb (but not grass) biomass. Low-severity-fire nutrient losses appear to be moderate and may be replenished from natural sources. However, in areas with frequent fires and high grass biomass (fuel) loads, or with late fires, nutrient losses could be much larger and non-sustainable to the persistence of the woodland savanna ecosystem.     

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

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.     

Controls of nitrogen limitation in tallgrass prairie

Summary The relationship between fire frequency and N limitation to foliage production in tallgrass prairie was studied with a series of fire and N addition experiments. Results indicated that fire history affected the magnitude of the vegetation response to fire and to N additions. Sites not burned for over 15 years averaged only a 9% increase in foliage biomass in response to N enrichment. In contrast, foliage production increased an average of 68% in response to N additions on annually burned sites, while infrequently burned sites, burned in the year of the study, averaged a 45% increase. These findings are consistent with reports indicating that reduced plant growth on unburned prairie is due to shading and lower soil temperatures, while foliage production on frequently burned areas is constrained by N availability. Infrequent burning of unfertilized prairie therefore results in a maximum production response in the year of burning relative to either annually burned or long-term unburned sites.Foliage biomass of tallgrass prairie is dominated by C₄ grasses; however, forb species exhibited stronger production responses to nitrogen additions than did the grasses. After four years of annual N additions, forb biomass exceeded that of grass biomass on unburned plots, and grasses exhibited a negative response to fertilizer, probably due to competition from the forbs. The dominant C₄ grasses may out-compete forbs under frequent fire conditions not only because they are better adapted to direct effects of burning, but because they can grow better under low available N regimes created by frequent fire.     

Retention of Nitrogen Following Wildfire in a Chaparral Ecosystem

Wildfires alter nitrogen (N) cycling in Mediterranean-type ecosystems, resetting plant and soil microbial growth, combusting plant biomass to ash, and enhancing N availability in the upper soil layer. This ash and soil N pool (that is, wildfire N) is susceptible to loss from watersheds via runoff and leaching during post-fire rains. Plant and soil microbial recovery may mitigate these losses by sequestering N compounds in new biomass, thereby promoting landscape N retention in N-limited chaparral ecosystems. We investigated the relative balance between wildfire N loss, and plant and soil microbial N uptake and stream N export for an upland chaparral watershed in southern California that burned (61%) in a high-intensity wildfire in 2009 by using a combination of stream, vegetation, soil microbial, and remote sensing analyses. Soil N in the burn scar was 440% higher than unburned soil N in the beginning of the first post-fire wet season and returned within 66 days to pre-fire levels. Stream N export was 1480% higher than pre-fire export during the first post-fire rain and returned within 106 days over the course of the following three rainstorms to pre-fire levels. A watershed-scale N mass balance revealed that 52% of wildfire N could be accounted for in plant and soil microbial growth, whereas 1% could be accounted for in stream export of dissolved nitrogen.     

Vegetation responses to different spatial patterns of soil disturbance in burned and unburned tallgrass prairie

Pocket gopher (Geomyidae) disturbances are created in spatiallypredictable patterns. This may influence resource heterogeneity and affectgrassland vegetation in a unique manner. We attempt to determine the extent towhich density and spatial pattern of soil disturbances influence tallgrassprairie plant community structure and determine how these disturbances interactwith fire. To investigate the effects of explicit disturbance patterns we createdsimulated pocket gopher burrows and mounds in various spatial patterns.Simulated burrows were drilled into the soil at different densities inreplicated plots of burned and unburned prairie. Separate plots of simulatedmounds were created in burned and unburned prairie at low, medium, or high mounddensities in clumped, uniform, or random spatial dispersions. In both burned and unburned plots, increased burrow density decreasedgraminoid biomass and increased forb biomass. Total-plant and graminoid biomasswere higher in burned than unburned plots while forb biomass was higher inunburned plots. Total-plant species richness was not significantly affected byburrow density or burning treatments, but graminoid species richness increasedin unburned plots and forb species richness increased in burned plots. Plant species richness was temporarily reduced directly on mounddisturbances compared to undisturbed prairie. Over time and at larger samplingscales, the interaction of fire and mound disturbance patterns significantlyaffected total-plant and graminoid species richness. The principal effect inburned and unburned prairie was decreased total-plant and graminoid speciesrichness with increased mound disturbance intensity. Although species richness at small patch scales was not increased by anyintensity of disturbance and species composition was not altered by theestablishment of a unique guild of disturbance colonizing plants, our studyrevealed that interactions between soil disturbances and fire alter the plantcommunity dominance structure of North American tallgrass prairie primarily viachanges to graminoids. Moreover, these effects become increasingly pronouncedover time and at larger spatial sampling scales.     

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

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. Received: 29 June 1999 / Accepted: 24 October 1999     

Fire and soil-plant nutrient relations in a pine-wiregrass savanna on the coastal plain of North Carolina

Summary Changes in soil and plant nutrient conditions were evaluated following various burn and clip treatments in a longleaf pine-wiregrass savanna in Bladen Co., N.C., USA. Ground fires were found to add substantial quantities of N, P, K, Ca, and Mg to the soil, though not necessarily in forms immediately available to plants. Less than 1% of the total nitrogen in the charred residue (ash) is present as nitrate or ammonium. Considerable quantities of all nutrients examined were lost to the atmosphere during burning. Green leaf tissue in recently burned areas was consistently higher in N, P, K, Ca, and Mg compared to unburned areas. Howerver, when compared to similar tissues from clipped plots, burned area tissues were significantly higher in N, Ca, and Mg only. Data presented here suggest that tissue age significantly affects nutrient content and must be considered in any analysis of tissue nutrient content following burning. Within 4–6 months following fire, burned-area tissue nutrient content decreases to concentrations found in the unburned area. Burning resulted in initial enrichment of available soil nutrients including PO₄, K⁺, Ca⁺⁺, and Mg⁺⁺, however, NO₃ ^-, and NH₄ ⁺ concentrations in burned soil were not significantly different from unbruned soil. Soil and plant nutrient changes in an area burned two years in succession indicate that repeated burning may diminish nutrient availability. Plant response to various nutrient enrichment treatments of the soil indicated that nitrogen is limiting growth in both burned and unburned soils and that burning may alter some factors other than nutrients which may retard plant growth in unburned areas.     

Burning in a New Zealand snow-tussock grassland: Effects on soil microbial biomass and nitrogen and phosphorus availability

Fire has been an important management tool in the pastoral use of New Zealand tussock grasslands. The effects of a farm-scale pastoral fire and subsequent grazing by sheep on soil biochemical properties in tussock grasslands dominated by the narrow-leaved snow tussock (Chionochloa rigida ssp. rigida) were investigated, 1.5 and 2.5 years after the fire event, in 0-2 cm depth mineral soil at a site at 975 m altitude in Central Otago, New Zealand. The nitrogen (N) and phosphorus (P) concentrations of C. rigida leaves were also measured. Comparisons were made with soil and tussock leaves from an adjacent unburned site. At both samplings, values of total soil organic carbon (C), extractable C, microbial biomass C, and basal respiratory activity were, on average, 14%, 18%, 23%, and 40%, respectively, lower at the burned than at the unburned site. In contrast, microbial N values were roughly similar at both sites, while microbial P values were 42% higher at the burned site after 1.5 years. Phosphomonoesterase and phosphodiesterase activities were then also similar at both sites, whereas invertase activity was higher at the burned site. The greater availability of N and P at the burned site was confirmed by the higher concentrations of N and P in C. rigida leaves sampled 2 years after the fire. Ratios of microbial C:microbial N and microbial C:microbial P were significantly lower at both samplings at the burned site, and emphasise the importance of the soil microbial biomass in conserving N and P after pastoral burning in a grassland ecosystem.     

Barriers to shrub reestablishment following fire in the seasonal submontane zone of Hawai'i

Introduced grass species have invaded extensive areas of Hawaii Volcanoes National Park and increased the size and frequency of fire. Following fire, grass cover is enhanced while native shrub cover is reduced; the reduction in most shrubs persists for at least 20 years even in the absence of fire. Shrub seedlings were planted in burned and unburned plots with and without grass cover. Biomass of 14 month old shrub seedlings was generally highest in recently burned/grass removed plots, intermediate in old burn/grass removed plots, and lowest in unburned/grass removed plots. In contrast, shrub biomass in plots with grass cover was low and did not differ significantly among burn treatments. Light competition is likely to be responsible for differences in shrub growth rates; grass cover reduced light to 1–10% of background levels. In addition, pool sizes of available soil N were highest in recently burned, intermediate in old burn, and lowest in unburned areas.     

火烧对内蒙古草原中坚韧胶衣固氮活性的影响

 坚韧胶衣(Collema tenax)是干旱和半干旱草原中常见的一种固氮地衣, 是草原生态系统中生物土壤结皮(Biological soil crust)的 主要组成部分, 对生态系统氮循环具有重要的影响。火烧作为一种干扰因子, 是草原生态系统结构和功能维持的重要因素之一。该文采用乙炔 还原法(Acetylene reduction assay), 研究了火烧对内蒙古草原生态系统中坚韧胶衣固氮活性的短期影响。结果表明, 在个体尺度上, 与对照 相比, 火烧区中地衣体烧损的坚韧胶衣固氮活性降低了42.3%, 而无烧损的个体固氮活性则升高了28.4%。这表明火烧对坚韧胶衣的固氮功能在 个体尺度上具有双重影响: 1)通过烧损地衣体、恶化地表温度和水分条件, 而抑制个体的固氮活性; 2)通过改善光照条件, 使表土养分呈现脉 冲式增高, 而促进未烧损个体的固氮活性。在种群尺度上, 火烧与对照之间固氮活性并无显著差异, 这可能是由于火烧在个体尺度上对坚韧胶 衣的固氮活性的双重影响相互抵消所致。     

Fire and site type effects on the long-term carbon and nitrogen balance in pristine Siberian Scots pine forests

Effects of fire and site type on carbon (C) and nitrogen (N) balances were determined by following the change of total and component C and N pools along four chronosequences of fire-prone Siberian Scots pine ecosystems. These differed in the mean return interval of surface fires (unburned – moderately burned, 40 years – heavily burned, 25 years) and site quality (lichen versus Vaccinium site type). Of the Vaccinium site type (higher site quality) only a moderately burned chronosequence was studied. A total of 22 even-aged stands were investigated with stand ages ranging from 2 to 383 years. The C balance was dominated by the opposing dynamics of coarse woody debris (CWD) and biomass and could be divided into three phases: (1) Young stands (up to 40 years)acted as a net source for C of 6-10 mol C m-2 year-1 because the previous generation CWD pool originating from stand-replacing crown fires decayed much faster than biomass increased. During this period the C pool in the unburned lichen type chronosequence decreased from 807 to 480 mol C m-2. (2) Middle aged stands (40-100 years) being in a stage of maximum biomass accumulation were a net sink of 8-10 mol C m-2 year-1. (3)Maturestands (100 to > 350 years) continued to sequester C at a lower rate (0.8-2.5mol C m-2 year-1). Differences in the rates of C sequestration during the two later phases could be explained by the complex interaction between surface fire regime and site type. Recurrent surface fires resulted in enhanced mortality and regularly redistributed C from the living to the CWD pool thereby lowering the rate of C sequestration. Site quality determined the potential to recover from disturbance by fire events. Differences in site type did not correlate with soil and total ecosystem N pool size. However, the N status of needles as well as the N pool of physiologically active tissue was highest in the stands of the Vaccinium type. The woody C pool (biomass + CWD) was sensitive to differences in surface fire regime and site type. It was lowest in the heavily burned lichen type chronosequence (297 ± 108 mol C m-2), intermediate in the unburned and moderately burned lichen type chronosequence (571 ± 179 mol C m-2) and highest in the moderately burned Vaccinium type chronosequence (810 ± 334 mol C m-2). In contrast, the total soil C pool (organic plus mineral layer down to a depth of 25 cm) was independent of stand age, surface fire regimeand site type and fluctuated around a value of 250 mol C m-2. The organic layer C pool oscillated in response to recurring surface fires and its C pool was dependent on time since fire increasing at a rate of about 1.5 mol C m-2 year-during the first 40 years and then reaching a plateau of 170 mol C m-2. The total ecosystem N pool was 7.4 ± 1.5 mol N m-2 on average of which only 25 % were stored in biomass or coarse woody debris. Total ecosystem N was independent of stand age, surface fire regime and site type. No correlation was found between total ecosystem C and N pools. Average total ecosystem C:N ratio was 114 ± 35 mol C mol N-1. A conceptual model illustrating how changes in the regime of stand-replacing crown fires and recurrent surface fires and changes in site quality interact in determining the long-term C balance in Siberian Scots pine forests is presented.     

Short-term effects of wildfire on microbial biomass and abundance in black pine plantation soils in Turkey

Measurement of soil microbial biomass and abundance offers a means of assessing the response of all microbial populations to changes in the soil environment after a fire. We examined the effects of wildfire on microbial biomass C and N, and abundance of bacteria and fungi 2 months after a fire in a pine plantation. Soil organic carbon (C_org), total nitrogen (N_tot), and electrical conductivity (EC) increased following the fire. In terms of microbial abundance, the overall results showed that burned forest soils had the most bacteria and fungi. Microbial biomass C and N from soil in the burned forest were not significantly different from their unburned forest counterparts. However, microbial indices indicated that fire affects soil microbial community structure by modifying the environmental conditions. The results also suggested that low-intensity fire promotes microorganism functional activity and improves the chemical characteristics of soils under humid climatic conditions.     

Fire affects the taxonomic and functional composition of soil microbial communities,with cascading effects on grassland ecosystem functioning

Fire is a crucial event regulating the structure and functioning of many ecosystems. Yet few studies have focused on how fire affects taxonomic and functional diversities of soil microbial communities, along with changes in plant communities and soil carbon (C) and nitrogen (N) dynamics. Here, we analyze these effects in a grassland ecosystem 9 months after an experimental fire at the Jasper Ridge Global Change Experiment site in California, USA. Fire altered soil microbial communities considerably, with community assembly process analysis showing that environmental selection pressure was higher in burned sites. However, a small subset of highly connected taxa was able to withstand the disturbance. In addition, fire decreased the relative abundances of most functional genes associated with C degradation and N cycling, implicating a slowdown of microbial processes linked to soil C and N dynamics. In contrast, fire stimulated above‐ and belowground plant growth, likely enhancing plant–microbe competition for soil inorganic N, which was reduced by a factor of about 2. To synthesize those findings, we performed structural equation modeling, which showed that plants but not microbial communities were responsible for significantly higher soil respiration rates in burned sites. Together, our results demonstrate that fire ‘reboots’ the grassland ecosystem by differentially regulating plant and soil microbial communities, leading to significant changes in soil C and N dynamics.     

Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe

The frequency and magnitude of drought is expected to increase in the US Great Plains under future climate regimes. Although semiarid systems are considered highly resistant to water limitation, novel drought events could alter linkages among biogeochemical processes, and result in new feedbacks that influence the timescale of ecosystem recovery. We examined changes in carbon and nitrogen cycling in the last 2 years of an 11-year drought manipulation in the shortgrass steppe, and under the first 2 years of recovery from drought. We measured plant production, plant tissue chemistry, soil trace gas flux, and soil inorganic nitrogen dynamics to test the extent that this magnitude of drought altered carbon and nitrogen fluxes and how these changes affected post-drought dynamics. We found that soil inorganic nitrogen was up to five times higher under severe drought than under control conditions, but that this nitrogen may not have been accessible to plants and microbial communities during drought due to diffusion limitations. Drought plots had higher N₂O flux when they received equal rainfall pulses, showing that this accumulated N may be vulnerable to loss. In addition, plants in drought plots had higher tissue nitrogen for 2 years following drought. These results show that decadal-length droughts that may occur under future precipitation regimes are likely to alter ecosystem properties through interactions among precipitation, vegetation, and N cycling. Shifts in plant N, vulnerability of nitrogen to loss, and rainfall use efficiency that we observed are likely to affect the recovery time of semiarid systems subject to droughts of this magnitude.     

Changes in leaf nutrient traits in a wildfire chronosequence

The effect of wildfire on ecosystem function is gaining interest since climate change is expected to increase fire frequency and intensity in many forest systems. Fire alters the nutritional status of forest ecosystems, affecting ecosystem function and productivity, but further studies evaluating changes in leaf nutrient traits induced by forest wildfires are still needed. We used a 17-year-old Pinus canariensis wildfire chronosequence to elucidate the nature of nutrient limitations in natural and unmanaged pine forest in the Canary Islands. Pine needles were sampled in winter and spring and analysed for N and P concentrations. As expected, we found the lowest leaf N and leaf P in recently burned plots. However, the leaf N:P ratio was higher in burned versus unburned plots, suggesting that the decrease in P availability due to the fire is larger than that of N. For all leaf traits and sampling dates, leaf trait values in burned plots matched those observed in unburned plots 17 years after a fire. The N:P ratio found in P. canariensis needles was one of the lowest values reported in the literature for woody species, and suggests that all pine trees in the chronosequence are unambiguously limited by low N availability. Our results show that these N-limited pine forests retained N more efficiently than P 4 years after a wildfire; however, leaf N recovery is slower than P recovery, suggesting that the mechanisms responsible for pine N limitation operate continuously in these forests.     

Press–pulse interactions: effects of warming,N deposition,altered winter precipitation,and fire on desert grassland community structure and dynamics

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long‐term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production (ANPP) in a northern Chihuahuan Desert grassland. In 2009, a lightning‐caused wildfire burned through the experiment. Here, we report on the interactive effects of these global change drivers on pre‐ and postfire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared with control plots. Average soil volumetric water content (VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared with control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced the cover of the dominant grasses by more than 75%. Following the fire, forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change.     

Long- and short-term effects of fire on nitrogen cycling in tallgrass prairie

Fires in the tallgrass prairie are frequent and significantly alter nutrient cycling processes. We evaluated the short-term changes in plant production and microbial activity due to fire and the long-term consequences of annual burning on soil organic matter (SOM), plant production, and nutrient cycling using a combination of field, laboratory, and modeling studies. In the short-term, fire in the tallgrass prairie enhances microbial activity, increases both above-and belowground plant production, and increases nitrogen use efficiency (NUE). However, repeated annual burning results in greater inputs of lower quality plant residues causing a significant reduction in soil organic N, lower microbial biomass, lower N availability, and higher C:N ratios in SOM. Changes in amount and quality of below-ground inputs increased N immobilization and resulted in no net increases in N availability with burning. This response occurred rapidly (e.g., within two years) and persisted during 50 years of annual burning. Plant production at a long-term burned site was not adversely affected due to shifts in plant NUE and carbon allocation. Modeling results indicate that the tallgrass ecosystem responds to the combined changes in plant resource allocation and NUE. No single factor dominates the impact of fire on tallgrass plant production.     

Effects of experimental season of prescribed fire and nutrient addition on structure and function of previously grazed grassland

Aims Understanding the drivers of grassland structure and function following livestock removal will inform grassland restoration and management. Here, we investigated the effects of fire and nutrient addition on structure and function in a subtropical semi-native grassland recently released from grazing in south-central Florida. We examined responses of soil nutrients, plant tissue nutrients, biomass of live, standing dead and litter, and plant species composition to experimental annual prescribed fire applied during different seasons (wet season vs. dry season), and nutrient additions (N, P and N + P) over 9 years.Methods Experimental plots were set up in a randomized block split-plot design, with season of prescribed fire as the main treatment and nutrient addition as the subplot treatment. Species cover data were collected annually from 2002 to 2011 and plant tissue and plant biomass data were collected in 2002–2006 and 2011. Soil nutrients were analyzed in 2004, 2006 and 2011.Important findings Soil total phosphorus (P) levels increased substantially with P addition but were not influenced by prescribed fire. Addition of P and N led to increased P and N concentrations in live plant tissues, but prescribed fire reduced N in live tissue. Levels of tissue N were higher in all plots at the beginning of the experiment, an effect that was likely due to grazing activity prior to removal of livestock. Plant tissue N steadily declined over time in all plots, with annually burned plots declining faster than unburned plots. Prescribed fire was an important driver of standing dead and litter biomass and was important for maintaining grass biomass and percent cover. Nutrient addition was also important: the addition of both N and P was associated with greater live biomass and woody forbs. Removal of grazing, lack of prescribed fire, and addition of N + P led to a reduction of grass biomass and a large increase in biomass of a woody forb. Annual prescribed fire promoted N loss from the system by reducing standing dead and litter, but maintained desirable biomass of grasses.     

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

Post-fire changes in desert vegetation patterns are known, but the mechanisms are poorly understood. Theory suggests that pulse dynamics of resource availability confer advantages to invasive annual species, and that pulse timing can influence survival and competition among species. Precipitation patterns in the American Southwest are predicted to shift toward a drier climate, potentially altering post-fire resource availability and consequent vegetation dynamics. We quantified post-fire inorganic N dynamics and determined how annual plants respond to soil inorganic nitrogen variability following experimental fires in a Mojave Desert shrub community. Soil inorganic N, soil net N mineralization, and production of annual plants were measured beneath shrubs and in interspaces during 6 months following fire. Soil inorganic N pools in burned plots were up to 1 g m⁻² greater than unburned plots for several weeks and increased under shrubs (0.5–1.0 g m⁻²) more than interspaces (0.1–0.2 g m⁻²). Soil NO₃ ⁻−N (nitrate−N) increased more and persisted longer than soil NH₄ ⁺−N (ammonium−N). Laboratory incubations simulating low soil moisture conditions, and consistent with field moisture during the study, suggest that soil net ammonification and net nitrification were low and mostly unaffected by shrub canopy or burning. After late season rains, and where soil inorganic N pools were elevated after fire, productivity of the predominant invasive Schismus spp. increased and native annuals declined. Results suggest that increased N availability following wildfire can favor invasive annuals over natives. Whether the short-term success of invasive species following fire will direct long-term species composition changes remains to be seen, yet predicted changes in precipitation variability will likely interact with N cycling to affect invasive annual plant dominance following wildfire.     

EFFECTS OF A WINTER FIRE ON SARRACENIA ALATA AND S. PSITTACINA

The effects of a prescribed winter burn on two species of pitcher plant, Sarracenia alata and S. psittacina, were investigated by comparing changes in variables measured before and after the fire in randomly selected plots in a Louisiana savanna. Burned plots showed an increase in foliage and unburned plots showed a decrease in foliage, as measured in total number of leaves (>25 cm) for S. alata and in total cover for S. psittacina. For S. alata, the gain in foliage in the burned plots was less than the loss in foliage in the unburned plots, but for S. psittacina the gain in burned plots was greater than the loss in unburned plots. Seedling recruitment of S. alata after the fire was exponentially related to the number of floral scapes produced in the year before the fire and was greater in burned plots than in unburned plots. Number of floral scapes of both pitcher plant species decreased in the year after the fire, but the decrease occurred equally in burned and unburned plots.