Stand-replacing wildfires alter the community structure of wood-inhabiting fungi in southwestern ponderosa pine forests of the USA

Increases in stand-replacing wildfires in the western USA have widespread implications for ecosystem carbon (C) cycling, in part because the decomposition of trees killed by fire can be a long-term source of CO₂ to the atmosphere. Knowledge of the composition and function of decay fungi communities may be important to understanding how wildfire alters C cycles. We assessed the effects of stand-replacing wildfires on the community structure of wood-inhabiting fungi along a 32-yr wildfire chronosequence. Fire was associated with low species richness for up to 4 yr and altered species composition relative to unburned forest for the length of the chronosequence. A laboratory incubation demonstrated that species varied in their capacity to decompose wood; Hypocrea lixii, an indicator of the most recent burn, caused the lowest decomposition rate. Our results show that stand-replacing wildfires have long-term effects on fungal communities, which may have consequences for wood decomposition and C cycling.     

Contrasting soil respiration in young and old-growth ponderosa pine forests

Three years of fully automated and manual measurements of soil CO₂ efflux, soil moisture and temperature were used to explore the diel, seasonal and inter‐annual patterns of soil efflux in an old‐growth (250‐year‐old, O site) and recently regenerating (14‐year‐old, Y site) ponderosa pine forest in central Oregon. The data were used in conjunction with empirical models to determine which variables could be used to predict soil efflux in forests of contrasting ages and disturbance histories. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. Soil CO₂ efflux at both sites showed large inter‐annual variability that could be attributed to soil moisture availability in the deeper soil horizons (O site) and the quantity of summer rainfall (Y site). Seasonal patterns of soil CO₂ efflux at the O site showed a strong positive correlation between diel mean soil CO₂ efflux and soil temperature at 64 cm depth whereas diel mean soil efflux at the Y site declined before maximum soil temperature occurred during summer drought. The use of diel mean soil temperature and soil water potential inferred from predawn foliage water potential measurements could account for 80% of the variance of diel mean soil efflux across 3 years at both sites, however, the functional shape of the soil water potential constraint was site‐specific. Based on the similarity of the decomposition rates of litter and fine roots between sites, but greater productivity and amount of fine litter detritus available for decomposition at the O site, we would expect higher rates of soil CO₂ efflux at the O site. However, annual rates were only higher at the O site in one of the 3 years (597 ± 45 vs. 427 ± 80 g C m^?2). Seasonal patterns of soil efflux at both sites showed influences of soil water limitations that were also reflected in patterns of canopy stomatal conductance, suggesting strong linkages between above and below ground processes.     

Carbon storage and fluxes in ponderosa pine forests at different developmental stages

We compared carbon storage and fluxes in young and old ponderosa pine stands in Oregon, including plant and soil storage, net primary productivity, respiration fluxes, eddy flux estimates of net ecosystem exchange (NEE), and Biome‐BGC simulations of fluxes. The young forest (Y site) was previously an old‐growth ponderosa pine forest that had been clearcut in 1978, and the old forest (O site), which has never been logged, consists of two primary age classes (50 and 250 years old). Total ecosystem carbon content (vegetation, detritus and soil) of the O forest was about twice that of the Y site (21 vs. 10 kg C m^?2 ground), and significantly more of the total is stored in living vegetation at the O site (61% vs. 15%). Ecosystem respiration (R_e) was higher at the O site (1014 vs. 835 g C m^?2 year^?1), and it was largely from soils at both sites (77% of R_e). The biological data show that above‐ground net primary productivity (ANPP), NPP and net ecosystem production (NEP) were greater at the O site than the Y site. Monte Carlo estimates of NEP show that the young site is a source of CO₂ to the atmosphere, and is significantly lower than NEP(O) by c. 100 g C m^?2 year^?1. Eddy covariance measurements also show that the O site was a stronger sink for CO₂ than the Y site. Across a 15‐km swath in the region, ANPP ranged from 76 g C m^?2 year^?1 at the Y site to 236 g C m^?2 year^?1 (overall mean 158 ± 14 g C m^?2 year^?1). The lowest ANPP values were for the youngest and oldest stands, but there was a large range of ANPP for mature stands. Carbon, water and nitrogen cycle simulations with the Biome‐BGC model suggest that disturbance type and frequency, time since disturbance, age‐dependent changes in below‐ground allocation, and increasing atmospheric concentration of CO₂ all exert significant control on the net ecosystem exchange of carbon at the two sites. Model estimates of major carbon flux components agree with budget‐based observations to within ± 20%, with larger differences for NEP and for several storage terms. Simulations showed the period of regrowth required to replace carbon lost during and after a stand‐replacing fire (O) or a clearcut (Y) to be between 50 and 100 years. In both cases, simulations showed a shift from net carbon source to net sink (on an annual basis) 10–20 years after disturbance. These results suggest that the net ecosystem production of young stands may be low because heterotrophic respiration, particularly from soils, is higher than the NPP of the regrowth. The amount of carbon stored in long‐term pools (biomass and soils) in addition to short‐term fluxes has important implications for management of forests in the Pacific North‐west for carbon sequestration.     

Charcoal and shrubs modify soil processes in ponderosa pine forests of western Montana

In split-root systems of alfalfa (Medicago sativa L.), already existing nodules or arbuscular mycorrhizal roots suppress further establishment of symbiosis in other root parts, a phenomenon named autoregulation. Roots treated with rhizobial nodulation signals (Nod factors) induce a similar systemic suppression of symbiosis.In order to test the hypothesis that flavonoids play a role in this systemic suppression, split-root systems of alfalfa plants were inoculated on one side of the split-root system with Sinorhizobium meliloti or Glomus mosseae or were treated with Nod factor. HPLC-analysis of alfalfa root extracts from both sides of the split-root system revealed a persistent local and systemic accumulation pattern of some flavonoids associated with the different treatments. The two flavonoids, formononetin and ononin, could be identified to be similarily altered after rhizobial or mycorrhizal inoculation or when treated with Nod factor.Exogenous application of formononetin and ononin partially restored nodulation and mycorrhization pointing towards the involvement of these two secondary compounds in the autoregulation of both symbioses.     

The carbon costs of mitigating high‐severity wildfire in southwestern ponderosa pine

Forests provide climate change mitigation benefit by sequestering carbon during growth. This benefit can be reversed by both human and natural disturbances. While some disturbances such as hurricanes are beyond the control of humans, extensive research in dry, temperate forests indicates that wildfire severity can be altered as a function of forest fuels and stand structural manipulations. The purpose of this study was to determine if current aboveground forest carbon stocks in fire‐excluded southwestern ponderosa pine forest are higher than prefire exclusion carbon stocks reconstructed from 1876, quantify the carbon costs of thinning treatments to reduce high‐severity wildfire risk, and compare posttreatment (thinning and burning) carbon stocks with reconstructed 1876 carbon stocks. Our findings indicate that prefire exclusion forest carbon stocks ranged from 27.9 to 36.6 Mg C ha^?1 and that the current fire‐excluded forest structure contained on average 2.3 times as much live tree carbon. Posttreatment carbon stocks ranged from 37.9 to 50.6 Mg C ha^?1 as a function of thinning intensity. Previous work found that these thinning and burning treatments substantially increased the 6.1 m wind speed necessary for fire to move from the forest floor to the canopy (torching index) and the wind speed necessary for sustained crown fire (crowning index), thereby reducing potential fire severity. Given the projected drying and increase in fire prevalence in this region as a function of changing climatic conditions, the higher carbon stock in the fire‐excluded forest is unlikely to be sustainable. Treatments to reduce high‐severity wildfire risk require trade‐offs between carbon stock size and carbon stock stability.     

Wildfire reduces carbon dioxide efflux and increases methane uptake in ponderosa pine forest soils of the southwestern USA

Severe wildfire may cause long-term changes in the soil-atmosphere exchange of carbon dioxide and methane, two gases known to force atmospheric warming. We examined the effect of a severe wildfire 10?years after burning to determine decadal-scale changes in soil gas fluxes following fire, and explored mechanisms responsible for these dynamics. We compared soil carbon dioxide efflux, methane uptake, soil temperature, soil water content, soil O horizon mass, fine root mass, and microbial biomass between a burned site and an unburned site that had similar stand conditions to the burned site before the fire. Compared to the unburned site, soil carbon dioxide efflux was 40% lower and methane uptake was 49% higher at the burned site over the 427-day measurement period. Soil O horizon mass, microbial biomass, fine root mass, and surface soil water content were lower at the burned site than the unburned site, but soil temperature was higher. A regression model showed soil carbon dioxide efflux was more sensitive to changes in soil temperature at the burned site than the unburned site. The relative importance of methane uptake to carbon dioxide efflux was higher at the burned site than the unburned site, but methane uptake compensated for only 1.5% of the warming potential of soil carbon dioxide efflux at the burned site. Our results suggest there was less carbon available at the burned site for respiration by plants and microbes, and the loss of the soil O horizon increased methane uptake in soil at the burned site.     

Influence of induced water stress in ponderosa pine on pine sawflies

Summary Three levels of water stress were induced on pole-size ponderosa pine (Pinus ponderosa) to determine the influence of plant moisture stress on oviposition, survival, and growth of two species of pine sawfly (Neodiprion fulviceps and N. autumnalis). It was found that water stress affected oviposition and subsequent egg survival but not larval development or survival. Stress had a negative effect on early season oviposition (N. fulviceps) and a positive effect on late season oviposition (N. autumnalis). Egg hatch was different between species and years and among treatment levels. Larval development, feeding, and survival were not affected by water stress. Overall, the effect of stress was not sufficient to explain population outbreaks of sawflies. Several hypotheses are presented as possible explanations for the observed experimental results.     

Influence of temperature on spring flight initiation for southwestern ponderosa pine bark beetles (Coleoptera: Curculionidae, Scolytinae)

Determination of temperature requirements for many economically important insects is a cornerstone of pest management. For bark beetles (Coleoptera: Curculionidae, Scolytinae), this information can facilitate timing of management strategies. Our goals were to determine temperature predictors for flight initiation of three species of Ips bark beetles, five species of Dendroctonus bark beetles, and two genera of bark beetle predators, Enoclerus spp. (Coleoptera: Cleridae) and Temnochila chlorodia (Mannerheim) (Coleoptera: Ostomidae), in ponderosa pine forests of northcentral Arizona. We quantified beetle flight activity using data loggers and pheromone-baited funnel traps at 18 sites over 4 yr. Ambient air temperature was monitored using temperature data loggers located in close proximity to funnel traps. We analyzed degree-day accumulation and differences between minimum, average, and maximum ambient temperature for the week before and week of first beetle capture to calculate flight temperature thresholds. Degree-day accumulation was not a good predictor for initiation of beetle flight. For all species analyzed other than D. adjunctus Blandford, beetles were captured in traps only when springtime temperatures exceeded 15.0 degrees C. D. adjunctus was collected when maximum temperatures reached only 14.5 degrees C. Once initial flights had begun, beetles were often captured when maximum ambient air temperatures were below initial threshold temperatures. Maximum and average air temperatures were a better predictor for beetle flight initiation than minimum temperature. We establish a temperature range for effective monitoring of bark beetles and their predators, and we discuss the implications of our results under climate change scenarios.     

Components of tree resilience: effects of successive low‐growth episodes in old ponderosa pine forests

Recent world‐wide episodes of tree dieback have been attributed to increasing temperatures and associated drought. Because these events are likely to become more common, improved knowledge of their cumulative effects on resilience and the ability to recover pre‐disturbance conditions is important for forest management. Here we propose several indices to examine components of individual tree resilience based on tree ring growth: resistance (inverse of growth reduction during the episode), recovery (growth increase relative to the minimum growth during the episode), resilience (capacity to reach pre‐episode growth levels) and relative resilience (resilience weighted by the damage incurred during the episode). Based on tree ring analyses, we analyzed historical patterns of tree resilience to successive drought‐induced low growth periods in ponderosa pine trees growing in unmanaged, remote forests of the Rocky Mountains. Low‐growth periods registered in tree rings were related to anomalies in the Palmer drought severity index (PDSI) and were attributed to drought. Independently of the impact of a specific event, subsequent growth after a single low‐growth episode was related to the growth prior to the event. Growth performance differed with tree age: young trees were overall more resistant to low‐growth periods, but older trees recovered better from more recent events. Regardless of tree age, recently burned sites exhibited lower post‐episode growth and lower resistance and resilience than unburned ones. We found mixed evidence for the cumulative effect of past low‐growth episodes: overall, greater impacts of a prior event and greater cumulative effects of past low‐growth periods caused a decrease in resistance. However, we did not find a progressive decrease in resilience over time in old trees. Our results highlight the value of using a combination of estimators to evaluate the different components of resilience. Specifically, while tree responses to disturbance depend on past disturbance episodes, the response is context‐specific and depends on the impact the capacity to recover after disturbance. This suggests that recent increases in forest mortality under current climate trends could relate to thresholds on specific components of resilience (resistance, recovery, resilience itself) rather than to an overall loss of resilience over time. Identifying such thresholds and their underlying mechanisms is a promising area of research with important implications for forest management.     

Glyoxysomes in megagamethophyte of germinating ponderosa pine seeds

Decoated ponderosa pine (Pinus ponderosa Laws) seeds contained 40% lipids, which were mainly stored in megagametophytic tissue and were utilized or converted to sugars via the glyoxylate cycle during germination. Mitochondria and glyoxysomes were isolated from the tissue by sucrose density gradient centrifugation at different stages of germination. It was found that isocitrate lyase, malate synthase, and catalase were mainly bound in glyoxysomes. Aconitase and fumarase were chiefly localized in mitochondria, whereas citrate synthase was common for both. Both organelles increased in quantity and specific activity of their respective marker enzymes with the advancement of germination. When the megagametophyte was exhausted at the end of germination, the quantity of these organelles and the activity of their marker enzymes decreased abruptly. At the stage of highest lipolysis, the isolated mitochondria and glyoxysomes were able to synthesize protein from labeled amino acids. Both organellar fractions contained RNA and DNA. Some degree of autonomy in glyoxysomes is indicated.     

Process-centred restoration in a fire-adapted ponderosa pine forest

Accurate and ecologically meaningful characterisation of reference conditions is a fundamental premise of restoration ecology. Restoration practice and research commonly define reference conditions in terms of compositional and structural elements. We propose a “process-centred” framework that places central emphasis on ecological functions and ecosystem processes. A wide variety of processes is central to the functioning and dynamics of ecological systems, and can be placed at the foundation of restoration research and practice. A process-centred approach allows the definition of “reference dynamics”, where spatial and temporal variability and underlying mechanisms of change are primary. We illustrate this approach using a 303-yr reconstruction of the natural surface fire regime to guide restoration of a Pinus ponderosa forest in the Jemez Mountains of southwestern North America. Fire occurrence varied over space and time during the period of record, with ecologically significant variation in fire intervals (yr fire⁻¹) governed by process–structure interactions. We defined a variety of reference variables for reintroduction of fire as the keystone ecological process, along with related structural variation. A process-centered approach and the reference dynamics paradigm can replace a more static concept of reference conditions in defining restoration baselines and provide an improved standard of comparison for restoration ecology.     

Evaluation of funnel traps for characterizing the bark beetle (Coleoptera: Scolytidae) communities in ponderosa pine forests of north-central Arizona

Lindgren funnel traps baited with aggregation pheromones are widely used to monitor and manage populations of economically important bark beetles (Coleoptera: Scolytidae). This study was designed to advance our understanding of how funnel trap catches assess bark beetle communities and relative abundance of individual species. In the second year (2005) of a 3-yr study of the bark beetle community structure in north-central Arizona pine (Pinus spp.) forests, we collected data on stand structure, site conditions, and local bark beetle-induced tree mortality at each trap site. We also collected samples of bark from infested (brood) trees near trap sites to identify and determine the population density of bark beetles that were attacking ponderosa pine, Pinus ponderosa Douglas ex Lawson, in the area surrounding the traps. Multiple regression models indicated that the number of Dendroctonus and Ips beetles captured in 2005 was inversely related to elevation of the trap site, and positively associated with the amount of ponderosa pine in the stand surrounding the site. Traps located closer to brood trees also captured more beetles. The relationship between trap catches and host tree mortality was weak and inconsistent in forest stands surrounding the funnel traps, suggesting that trap catches do not provide a good estimate of local beetle-induced tree mortality. However, pheromone-baited funnel trap data and data from gallery identification in bark samples produced statistically similar relative abundance profiles for the five species of bark beetles that we examined, indicating that funnel trap data provided a good assessment of species presence and relative abundance.     

Ectomycorrhizal communities of ponderosa pine and lodgepole pine in the south-central Oregon pumice zone

Forest ecosystems of the Pacific Northwest of the USA are changing as a result of climate change. Specifically, rise of global temperatures, decline of winter precipitation, earlier loss of snowpack, and increased summer drought are altering the range of Pinus contorta. Simultaneously, flux in environmental conditions within the historic P. contorta range may facilitate the encroachment of P. ponderosa into P. contorta territory. Furthermore, successful pine species migration may be constrained by the distribution or co-migration of ectomycorrhizal fungi (EMF). Knowledge of the linkages among soil fungal diversity, community structure, and environmental factors is critical to understanding the organization and stability of pine ecosystems. The objectives of this study were to establish a foundational knowledge of the EMF communities of P. ponderosa and P. contorta in the Deschutes National Forest, OR, USA, and to examine soil characteristics associated with community composition. We examined EMF root tips of P. ponderosa and P. contorta in soil cores and conducted soil chemistry analysis for P. ponderosa cores. Results indicate that Cenococcum geophilum, Rhizopogon salebrosus, and Inocybe flocculosa were dominant in both P. contorta and P. ponderosa soil cores. Rhizopogon spp. were ubiquitous in P. ponderosa cores. There was no significant difference in the species composition of EMF communities of P. ponderosa and P. contorta. Ordination analysis of P. ponderosa soils suggested that soil pH, plant-available phosphorus (Bray), total phosphorus (P), carbon (C), mineralizable nitrogen (N), ammonium (NH₄), and nitrate (NO₃) are driving EMF community composition in P. ponderosa stands. We found a significant linear relationship between EMF species richness and mineralizable N. In conclusion, P. ponderosa and P. contorta, within the Deschutes National Forest, share the same dominant EMF species, which implies that P. ponderosa may be able to successfully establish within the historic P. contorta range and dominant EMF assemblages may be conserved.     

Mountain pine beetle attack associated with low levels of 4-allylanisole in ponderosa pine

Mountain pine beetle (Dendroctonus ponderosae) is the most important insect pest in southern Rocky Mountain ponderosa pine (Pinus ponderosa) forests. Tree mortality is hastened by the various fungal pathogens that are symbiotic with the beetles. The phenylpropanoid 4-allylanisole is an antifungal and semiochemical for some pine beetle species. We analyzed 4-allylanisole and monoterpene profiles in the xylem oleoresin from a total of 107 trees at six sites from two chemotypes of ponderosa pine found in Colorado and New Mexico using gas chromatography-mass spectroscopy (GC-MS). Although monoterpene profiles were essentially the same in attacked and nonattacked trees, significantly lower levels of 4-allylanisole were found in attacked trees compared with trees that showed no evidence of attack for both chemotypes.     

Long-term benefits to the growth of ponderosa pines from controlling southwestern pine tip moth (Lepidoptera: Tortricidae) and weeds

The southwestern pine tip moth, Rhyacionia neomexicana (Dyar) (Lepidoptera: Tortricidae), is a native forest pest that attacks seedlings and saplings of ponderosa pine, Pinus ponderosa Dougl. ex Laws, in the southwestern United States. Repeated attacks can cause severe deformation of host trees and significant long-term growth loss. Alternatively, effective control of R. neomexicana, vegetative competition, or both in young pine plantations may increase survival and growth of trees for many years after treatments are applied. We test the null hypothesis that 4 yr of R. neomexicana and weed control with insecticide, weeding, and insecticide plus weeding would not have any residual effect on survival and growth of trees in ponderosa pine plantation in northern Arizona 14 yr post-treatment, when the trees were 18 yr old. Both insecticide and weeding treatment increased tree growth and reduced the incidence of southwestern pine tip moth damage compared with the control. However, weeding alone also significantly increased tree survival, whereas insecticide alone did not. The insecticide plus weeding treatment had the greatest tree growth and survival, and the lowest rate of tip moth damage. Based on these results, we rejected our null hypothesis and concluded that there were detectable increases in the survival and growth of ponderosa pines 14 yr after treatments applied to control R. neomexicana and weeds.     

Testate amoebas of pine forests in Mexico

The population of testate amoebas in the soils of pine forests in Mexico has been studied. In total, 68 species, varieties, and types of testate amoebas with cosmopolite distribution were found. The species diversity of the testate population includes hygrophilous species that differ from hygrophilous species with Luvisols in higher Andosols. Comparative analysis using the results of one available study of soil testate amoebas from Mexico has been carried out (Bonnet, 1977).     

Primers designed to amplify a mitochondrial nad1 intron in ponderosa pine, Pinus ponderosa, limber pine, P. flexilis, and Scots pine, P. sylvestris

The b/c intron of the mitochondrial nad1 gene, was sequenced to characterize the indel region of ponderosa pine, Pinus ponderosa. The sequence in ponderosa pine was aligned with the sequence in Scots pine, Pinus sylvestris, to design seven primers that are useful for sequencing and for revealing size variation in amplified fragments in ponderosa pine, Scots pine, and limber pine, Pinus flexilis. These primers reveal variability in all three species, and the pattern of variability within ponderosa pine is described by a preliminary survey. The indel region of ponderosa pine contains three distinct elements with lengths of 31, 32, and 34 bp. Received: 1 March 2000 / Accepted: 14 April 2000<@head-com-p1a.lf>Communicated by P.M.A. Tigerstedt     

Seed supply of native and cultivated grasses in pine forests of the southwestern United States and the potential for vegetation recovery following wildfire

In this study we examined seed supply (seed banks and seed rain) and vegetation of seeded cultivated grasses and naturally occurring native grasses following a wildfire in northern New Mexico, USA. We specifically examined density of native and cultivated grass seeds and plants in areas of high, moderate, and low fire severity. We also examined the similarity between density of native and cultivated grass seeds to density of above ground plants. Density of native grass seed per square meter was higher in areas that burned under low fire severity (85.18 ± 44.83) compared to areas of moderate (18.52 ± 11.26) and high (7.41 ± 4.90) fire severity; however, differences were not statistically significant due, in part, to the high error associated with estimates. Density of cultivated grass seed per square meter was higher than that of native grass seed in areas of high (439.60 ± 117.98) and moderate (437.02 ± 146.50) fire severity, areas that were seeded with cultivated grasses after the wildfire for erosion control. Density of seeded grass plants per square meter was also higher than that of native grass plants in areas of high (18.78 ± 4.59 versus 0.33 ± 0.24) and moderate (8.22 ± 1.76 versus 0.22 ± 0.15) fire severity. There was a higher correspondence between the density of cultivated grass seeds and plants (highest value 0.32 ± 0.11) compared to density of native grass seeds and plants (highest value 0.05 ± 0.04). The high density of seeds, plants, and correspondence indicated that seeds from cultivated grasses are more likely to establish as post-fire vegetation than seeds from native grasses. Seeding with cultivated grasses following a wildfire may slow or inhibit recovery of native grasses in the short term. Longer-term implications for site occupancy deserve further study.     

Postfire carbon pools and fluxes in semiarid ponderosa pine in Central Oregon

Forest fire dramatically affects the carbon storage and underlying mechanisms that control the carbon balance of recovering ecosystems. In western North America where fire extent has increased in recent years, we measured carbon pools and fluxes in moderately and severely burned forest stands 2 years after a fire to determine the controls on net ecosystem productivity (NEP) and make comparisons with unburned stands in the same region. Total ecosystem carbon in soil and live and dead pools in the burned stands was on average 66% that of unburned stands (11.0 and 16.5 kg C m⁻², respectively, P<0.01). Soil carbon accounted for 56% and 43% of the carbon pools in burned and unburned stands. NEP was significantly lower in severely burned compared with unburned stands (P<0.01) with an increasing trend from −125±44 g C m⁻² yr⁻¹ (±1 SD) in severely burned stands (stand replacing fire), to −38±96 and +50±47 g C m⁻² yr⁻¹ in moderately burned and unburned stands, respectively. Fire of moderate severity killed 82% of trees <20 cm in diameter (diameter at 1.3 m height, DBH); however, this size class only contributed 22% of prefire estimates of bole wood production. Larger trees (> 20 cm DBH) suffered only 34% mortality under moderate severity fire and contributed to 91% of postfire bole wood production. Growth rates of trees that survived the fire were comparable with their prefire rates. Net primary production NPP (g C m⁻² yr⁻¹, ±1 SD) of severely burned stands was 47% of unburned stands (167±76, 346±148, respectively, P<0.05), with forb and grass aboveground NPP accounting for 74% and 4% of total aboveground NPP, respectively. Based on continuous seasonal measurements of soil respiration in a severely burned stand, in areas kept free of ground vegetation, soil heterotrophic respiration accounted for 56% of total soil CO₂ efflux, comparable with the values of 54% and 49% previously reported for two of the unburned forest stands. Estimates of total ecosystem heterotrophic respiration (R_h) were not significantly different between stand types 2 years after fire. The ratio NPP/R_h averaged 0.55, 0.85 and 1.21 in the severely burned, moderately burned and unburned stands, respectively. Annual soil CO₂ efflux was linearly related to aboveground net primary productivity (ANPP) with an increase in soil CO₂ efflux of 1.48 g C yr⁻¹ for every 1 g increase in ANPP (P<0.01, r²= 0.76). There was no significant difference in this relationship between the recently burned and unburned stands. Contrary to expectations that the magnitude of NEP 2 years postfire would be principally driven by the sudden increase in detrital pools and increased rates of R_h, the data suggest NPP was more important in determining postfire NEP.