Prescribed fires as ecological surrogates for wildfires: A stream and riparian perspective

Forest managers use prescribed fire to reduce wildfire risk and to provide resource benefits, yet little information is available on whether prescribed fires can function as ecological surrogates for wildfire in fire-prone landscapes. Information on impacts and benefits of this management tool on stream and riparian ecosystems is particularly lacking. We used a beyond-BACI (Before, After, Control, Impact) design to investigate the effects of a prescribed fire on a stream ecosystem and compared these findings to similar data collected after wildfire. For 3 years after prescribed fire treatment, we found no detectable changes in periphyton, macroinvertebrates, amphibians, fish, and riparian and stream habitats compared to data collected over the same time period in four unburned reference streams. Based on changes in fuels, plant and litter cover, and tree scorching, this prescribed fire was typical of those being implemented in ponderosa pine forests throughout the western U.S. However, we found that the extent and severity of riparian vegetation burned was substantially lower after prescribed fire compared to nearby wildfires. The early-season prescribed fire did not mimic the riparian or in-stream ecological effects observed following a nearby wildfire, even in catchments with burn extents similar to the prescribed fire. Little information exists on the effects of long-term fire exclusion from riparian forests, but a “prescribed fire regime” of repeatedly burning upland forests while excluding fire in adjacent riparian forests may eliminate an important natural disturbance from riparian and stream habitats.     

Positive fire–grass feedback in Mediterranean Basin woodlands

Fires can mediate switches between alternative vegetation types which may be more flammable and thus reinforce fire spread. We tested if there is a positive feedback between the expansion of the tussock grass Ampelodesmos mauritanica (hereafter Ampelodesmos) and fire hazard in Mediterranean Basin communities and its relation to tree cover decline. The effect of fire on Ampelodesmos population structure was analysed by surveying stands burned at different fire frequencies. The effect of vegetation dominated by Ampelodesmos on fire behaviour compared to other species coexisting in the community was predicted by the Rothermel fire propagation model BEHAVE. There was a negative correlation between pine cover and percentage of Ampelodesmos plants. Ampelodesmos mortality after fire is very low. Recently burned stands had a higher proportion of reproductive plants and higher seedling density than unburned stands. The high temperatures reached during fire may kill seeds, the higher seedling recruitment results from fast resprouting and higher seed production of burned plants compared to unburned plants 1 year after fire. Simulations with the BEHAVE fire model predict that Ampelodesmos increases fire intensity and spread because of its high accumulation of fuel load and standing dead material. The results suggest that there is a positive relationship between Ampelodesmos abundance and fire regime which increases the invasive potential of this grass and the fire risk of the community where it dominates.     

Herbaceous species diversity in relation to fire severity in Zagros oak forests,Iran

Zagros forests are mainly covered byQuercus brantii L. coppices and oak sprout clumps occupy the forest area like patches. We investigated post-fire herbaceous diversity in the first growing season after fire. For this purpose neighboring burned and unburned areas were selected with the same plant species and ecological conditions. The data were collected from areas subjected to different fire severities. Overall 6 treatments were considered with respect to fire severity and the mi-crosites of inside and outside of oak sprout clumps including： unburned inside and outside of sprout clumps （Ni and No）, inside of sprout clumps that burned with high fire severity （H）, inside of sprout clumps that burned with moderate fire severity （M）, outside of sprout clumps that burned with low fire severity （OH and OM）. Different herbaceous com-position was observed in the unburned inside and outside of oak sprout clumps. The species diversity and richness were increased in treatments burned with low and moderate fire severity. However, in treatment burned with high fire severity （H）, herbaceous cover was reduced, even-ness was increased, and richness and diversity were not significantly changed. We concluded that besides the microsites conditions in forest, fire severity is an inseparable part of the ecological effect of fire on her-baceous composition.     

Effect of fire disturbance on active organic carbon of <Emphasis Type="Italic">Larix gmelinii</Emphasis> forest soil in Northeastern China

Active organic carbon in soil has high biological activity and plays an important role in forest soil ecosystem structure and function. Fire is an important disturbance factor in many forest ecosystems and occurs frequently over forested soils. However, little is known about its impact on soil active organic carbon (SAOC), which is important to the global carbon cycle. To investigate this issue, we studied the active organic carbon in soils in the Larix gmelinii forests of the Da Xing’an Mountains (Greater Xing’an Mountains) in Northeastern China, which had been burned by high-intensity wildfire in two different years (2002 and 2008). Soil samples were collected monthly during the 2011 growing season from over 12 sample plots in burned and unburned soils and then analyzed to examine the dynamics of SAOC. Our results showed that active organic carbon content changed greatly after fire disturbance in relation to the amount of time elapsed since the fire. There were significant differences in microbial biomass carbon, dissolved organic carbon, light fraction organic carbon, particulate organic carbon between burned and unburned sample plots in 2002 and 2008 (p < 0.05). The correlations between active organic carbon and environmental factors such as water content, pH value and temperature of soils, and correlations between each carbon component changed after fire disturbance, also in relation to time since the fire. The seasonal dynamics of SAOC in all of the sample plots changed after fire disturbance; peak values appeared during the growing season. In plots burned in 2002 and 2008, the magnitude and occurrence time of peak values differed. Our findings provide basic data regarding the impact of fire disturbance on boreal forest soil-carbon cycling, carbon-balance mechanisms, and carbon contributions of forest ecosystem after wildfire disturbance.     

Effect of fire disturbance on active organic carbon of Larix gmelinii forest soil in Northeastern China

Active organic carbon in soil has high biological activity and plays an important role in forest soil ecosystem structure and function. Fire is an important disturbance factor in many forest ecosystems and occurs frequently over forested soils. However, little is known about its impact on soil active organic carbon(SAOC), which is important to the global carbon cycle. To investigate this issue, we studied the active organic carbon in soils in the Larix gmelinii forests of the Da Xing'an Mountains(Greater Xing'an Mountains) in Northeastern China, which had been burned by high-intensity wildfire in two different years(2002 and 2008). Soil samples were collected monthly during the 2011 growing season from over 12 sample plots in burned and unburned soils and then analyzed to examine the dynamics of SAOC. Our results showed that active organic carbon content changed greatly after fire disturbance in relation to the amount of time elapsed since the fire. There were significant differences in microbial biomass carbon, dissolved organic carbon, light fraction organic carbon, particulate organic carbon between burned and unburned sample plots in 2002 and 2008(p0.05). The correlations between active organic carbon and environmental factors such as water content, p H value and temperature of soils, and correlations between each carbon component changed after fire disturbance, also in relation to time since the fire. The seasonal dynamics of SAOC in all of the sample plots changed after fire disturbance; peak values appeared during the growing season. In plots burned in 2002 and 2008, the magnitude and occurrence time of peak values differed. Our findings provide basic data regarding the impact of fire disturbance on boreal forest soil-carbon cycling, carbon-balance mechanisms, and carbon contributions of forest ecosystem after wildfire disturbance.     

Distribution of saproxylic beetles in a recently burnt landscape of the northern boreal forest of Québec

This study evaluated the importance of burned habitat characteristics as well as the likely dispersal from specific habitats in the distribution of saproxylic beetles the same year as a fire occurred, in burned black spruce stands (Picea mariana [Mill] B.S.P.) in the northern boreal forest of Québec. The distribution of early post-fire saproxylic species was mainly driven by burned habitat attributes at the plot scale (0.04 ha), especially fire severity, suggesting that the effect of environment attributes can act at a relatively fine scale. Some xylophagous and most predaceous species were more abundant in severely burned stands whereas fire severity had the opposite effect on several common mycophagous species. The amount of newly fire-killed trees that could be used as breeding substrates in the burned stands had only a weak positive influence on these functional groups. The great majority of early saproxylic species were weakly associated with the distance from unburned forests or other recently burned patches that could act as potential “source habitats”. Indeed, these variables were of lesser importance than the attributes of the burned habitat. Woody debris that were already present in plots before the fire, potentially serving as local of source-populations for early colonizers, had virtually no influence on the local abundance of species. Many saproxylic species, including some true pyrophilous, clearly showed higher abundance as distance from unburned stands increased. This unexpected relation may reflect that dispersal of insects toward the burnt landscape very shortly after fire could be driven by the higher amount of volatiles released by severely burned forests, which are more likely as distance from unburned forest increased.     

文峪河上游河岸带及其毗邻高地植物群落优势种密度结构的研究

研究了文峪河上游3个河段6种不同生境条件下群落优势种组成及其密度结构,结果表明:1)青杄林河段植被处于演替顶级或近顶级阶段,受人为干扰极小,是一种典型的地带性植被类型;2)白桦林河段植被如能避免或减少人为干扰,河岸带有可能向白杄林方向演替,高地森林则会在较长时间内处于针阔混交林阶段;3)农田河段河岸带植被已基本退出自然演替序列,高地华北落叶松林将在较长时间内保持稳定状态。     

Controls on early post-fire woody plant colonization in riparian areas

Fire in riparian areas has the potential to influence the functions riparian vegetation provides to streams and aquatic biota. However, there is little information on the effects of fire on riparian areas. The objectives of the present study were to: (i) determine how fire severity interacts with riparian topographic setting, micro-environmental conditions, and pre-fire community composition to control post-fire regeneration; (ii) determine how riparian regeneration patterns and controls change during early succession; and (iii) determine how critical riparian functions are influenced by and recover after fire. Study locations included the Biscuit Fire in southwestern Oregon and the B&B Complex Fire in the Cascade Mountain Range of west-central Oregon, USA. We measured post-fire woody species regeneration, and measured factors such as fire severity, pre-fire species composition, and stream size as potential factors associated with post-fire regeneration patterns. At a relatively coarse spatial scale, patterns in post-fire colonization were influenced by elevation. At finer spatial scales, both conifer- and hardwood-dominated riparian plant communities were self-replacing, suggesting that each community type tends to occur in specific ecological settings. Abundant post-fire regeneration in riparian areas and the self-replacement of hardwood- and conifer-dominated communities indicate high resilience of these disturbance-adapted plant communities.     

Fine-scale heterogeneity in overstory composition contributes to heterogeneity of wildfire severity in southern boreal forest

Wildfire can create a mosaic of impacts of varying severity across the landscape. Although widely recognized, this feature and its causes are little understood or studied in ecology. We studied a 1,200-ha wildfire in the southern boreal forest of the Boundary Waters Canoe Area Wilderness (BWCAW) in northeastern Minnesota, USA, using 275 ground plots (stand-scale) and 1:7,000 scale aerial photographs for the entire burned area (landscape-scale). Fire severity was markedly heterogeneous. Overall, 50% of the burn extent was classified as high burn severity, but patches burned this severely were on average less than 70 m from patches of low severity. As expected, lowlands had lower average fire severity than uplands, but several lowland areas burned, and some upland areas remained unburned. At the landscape scale, pre-fire vegetation type—itself heterogeneous—and patch size of less flammable cover types influenced fire severity. Crown fire severity in upland areas was lowest in pure aspen–birch and red/white pine stands and highest in jack pine and spruce–fir stands. At the stand-scale, slope position and the density of certain tree species at adjacent plots influenced fire severity. Improved understanding of the severity patterns created by wildfire can help to guide the management of spatial patterns of forested systems. Based on our study, a larger range in disturbance severity at scales of 0.1 to several ha and increasing the average size, and range of sizes, of residual patches would in aggregate better mimic natural disturbance than typical harvests.     

Moving riparian management guidelines towards a natural disturbance model: An example using boreal riparian and shoreline forest bird communities

Forest harvesting strategies that approximate natural disturbances have been proposed as a means of maintaining natural species’ diversity and richness in the boreal forests of North America. Natural disturbances impact shoreline forests and upland areas at similar rates. However, shoreline forests are generally protected from harvest through the retention of treed buffer strips. We examined bird community responses to forest management guidelines intended to approximate shoreline forest fires by comparing bird community structure in early (1–4 years) post-burned and harvested boreal riparian habitats and the adjacent shoreline forest. We sampled riparian areas with adjacent: (1) burned merchantable shoreline forest (n = 21), (2) burned non-merchantable shoreline forest (n = 29), (3) 10 m treed buffer with 25% retention in the next 30 m (n = 18), and (4) 30 m treed buffer (n = 21). Only minor differences were detected in riparian species’ abundance and bird community composition between treatments with greater differences in these parameters occurring between post-fire and post-harvest upland bird communities. Indicators of all merchantable treatments were dominated by upland species with open-habitat species and habitat generalists being typical upland indicator species of burned merchantable habitats and forest specialists typical upland indicators of harvested treatments. Riparian species indicative of burned riparian habitats were Common Yellowthroat (Geothlypis trichas), Le Conte’s Sparrow (Ammodramus leconteii) and Eastern Kingbird (Tyrannus tyrannus) and indicators of 30 m buffers were Alder Flycatcher (Empidonax alnorum) and Wilson’s Warbler (Wilsonia pusilla). Multivariate Redundancy Analysis (RDA) of the overall (riparian and upland birds) community showed greater divergence than RDA with only riparian species suggesting less effect of fire and forestry on riparian birds than on upland birds. Higher natural range of variability (NRV) of overall post-fire bird communities compared to post-harvest communities emphasizes that harvesting guidelines currently do not achieve this level of variability. However, lack of a large negative effect on common riparian species in the first 4 years post-disturbance allows for the exploration of alternative shoreline forest management that better incorporates bird community composition of post-fire riparian areas and shoreline forests.     

Long term effects of fire on ectomycorrhizas and soil properties in Nothofagus pumilio forests in Argentina

The forests of Nothofagus pumilio have historically been affected by forest fires. The effects of fire on certain above and belowground, biotic and abiotic components of these ecosystems have been previously documented, albeit belowground components have received much less attention. It has been suggested that the effects observed in the short-term after a fire usually differ from the longer-term effects. The long-term effects of fire (i.e. >5 years after burning) on belowground components in Nothofagus forests are currently unknown. In the present study we evaluated the long-term effect of fire on ectomycorrhiza (ECM) colonization and morphotype composition in N. pumilio roots, as well as soil chemical properties in temperate forests in Patagonia. Sampling was conducted in three mature monospecific forests. In each, nearby burned and unburned sites were selected. The time since the occurrence of fires differed between areas (i.e. 6-10 years). Within each site, 3 transects of 40 m were established randomly along which 5 samples of roots and soil were collected in spring and autumn. The main results were: (1) in comparison with the unburned site, ECM colonization was lower in the burned site in the area with the shorter time length since fire occurrence and no effects in the other two areas were observed; (2) richness and diversity were not significantly affected by fire but there was a significant effect of season for both parameters, being higher in spring; (3) ECM dominance was significantly higher in the unburned than in the burned site in Tronador, while in Challhuaco the opposite was observed, mainly in autumn; (4) in general carbon, nitrogen and phosphorous decreased while pH increased in the burned sites; (5) ECM colonization positively correlated with NH⁴⁺ and phosphorus and negatively with pH but was not significantly correlated with organic matter or any other soil variable. Altogether the results suggest that effects of fire on ectomycorrhiza and soil properties in N. pumilio forests are probably related to the time elapsed since fire occurrence combined with site characteristics. In addition, the direct and indirect effects of fire in these forest systems may persist for more than 10 years.     

Toward reference conditions: wildfire effects on flora in an old-growth ponderosa pine forest

Remote ponderosa pine (Pinus ponderosa) forests on the North Rim of Grand Canyon National Park, Arizona, USA provide valuable examples of reference conditions due to their relatively uninterrupted fire regimes, limited grazing history, and protection from logging. Wildfire is an important disturbance agent in upland forests of the Interior West, yet repeated measurements taken before and after lightning-ignited fires are rare. In 1999, a low-severity Wildland Fire Use fire burned 156 ha on Fire Point, a peninsula dominated by old-growth ponderosa pines, which had not burned for at least 76 years. We measured understory plant community and forest floor characteristics in 1998 (1 year before the fire) and 2001 (2 years after the fire) at this site and at nearby reference sites that did not burn in 1999 but have had continuing fire regimes throughout the past century. After the wildfire, the plant community at Fire Point shifted toward higher compositional similarity with the reference sites. Analysis of functional group composition indicated that this change was due primarily to an increase in annual and biennial forbs. Gayophytum diffusum, Polygonum douglasii, Chenopodium spp., Solidago spp., Elymus elymoides, Calochortus nuttallii, Hesperostipa comata, and Lotus spp. were indicative of forests influenced by recent fires. Species richness, plant cover, plant layer density and plant diversity were significantly lower at Fire Point than at the reference sites, possibly due to long-term fire exclusion, but the fire did not increase the rate of change in these variables after 2 years. Few exotic species were present at any site. Forest floor depths at Fire Point were reduced to depths similar to the reference sites, primarily due to consumption of the duff layer. There was a significant inverse relationship between the ratio of duff:litter and species richness. Compared to fire-excluded forests, old-growth ponderosa pine forests influenced by low-intensity surface fires generally have greater plant species richness (especially annual forbs) and lighter fuel loads. This study supports the continued application of the Wildland Fire Use strategy in old-growth montane forests to maintain and improve forest health by altering understory species composition and reducing fuel loads.     

Microbial properties and litter and soil nutrients after two prescribed fires in developing savannas in an upland Missouri Ozark Forest

On some landscapes periodic fire may be necessary to develop and maintain oak-dominated savannas. We studied the effects of two annual prescribed burns to determine their effect on microbial activity and soil and litter nutrients 1 year after the last burn. Surface litter and soil from the upper 0–5 cm soil layer in three developing savannas (oak-hickory, Quercus-Carya), oak-hickory-pine (Quercus-Carya-Pinus), and pine (Pinus) were collected one year after the second of two annual prescribed burns. Surface litter was analyzed for nutrients and soil was analyzed for phospholipid fatty acids (PLFAs) and nutrients. Surface litter chemistry differed across the three savannas for potassium (K) and boron (B), being significantly (P < 0.05) higher for unburned forest than for burned forest. Among savannas, only sulfur (S) was higher for the pine savanna and B for the oak-hickory savanna, both were higher for unburned forest than for burned forest. For soil, calcium (Ca) and B differed across savannas, being higher for burned forest than for unburned forest. Among savannas, soil pH, Ca, and B concentrations were higher in soil from burned forest than from unburned forest. Total PLFA differed among savannas, but was not affected by burning treatments. However, the amounts of biomarkers for Gram-positive and Gram-negative bacteria were higher while the amount of biomarker for fungal PLFA was lower for burned forest than for unburned forest. Our results indicate that the two annual prescribed burns moderately affected PLFA microbial community structure and litter and soil nutrient concentrations. However, the long-term effects of fire on these study sites are not known and merit further study.     

Persistent effects of fire severity on early successional forests in interior Alaska

There has been a recent increase in the frequency and extent of wildfires in interior Alaska, and this trend is predicted to continue under a warming climate. Although less well documented, corresponding increases in fire severity are expected. Previous research from boreal forests in Alaska and western Canada indicate that severe fire promotes the recruitment of deciduous tree species and decreases the relative abundance of black spruce (Picea mariana) immediately after fire. Here we extend these observations by (1) examining changes in patterns of aspen and spruce density and biomass that occurred during the first two decades of post-fire succession, and (2) comparing patterns of tree composition in relation to variations in post-fire organic layer depth in four burned black spruce forests in interior Alaska after 10-20 years of succession. We found that initial effects of fire severity on recruitment and establishment of aspen and black spruce were maintained by subsequent effects of organic layer depth and initial plant biomass on plant growth during post-fire succession. The proportional contribution of aspen (Populus tremuloides) to total stand biomass remained above 90% during the first and second decades of succession in severely burned sites, while in lightly burned sites the proportional contribution of aspen was reduced due to a 40-fold increase in spruce biomass in these sites. Relationships between organic layer depth and stem density and biomass were consistently negative for aspen, and positive or neutral for black spruce in all four burns. Our results suggest that initial effects of post-fire organic layer depths on deciduous recruitment are likely to translate into a prolonged phase of deciduous dominance during post-fire succession in severely burned stands. This shift in vegetation distribution has important implications for climate-albedo feedbacks, future fire regime, wildlife habitat quality and natural resources for indigenous subsistence activities in interior Alaska.     

Tree type and forest management effects on the structure of stream wood following wildfires

Wildfires are an increasingly common disturbance influencing wood recruitment to streams, and thereby affecting their physical and biological condition. Mediterranean countries such as Portugal, where more than 25% of the land area has burned since 1990, are ideal areas to study impacts of wildfire effects on streams. We evaluated the physical structure of 2206 downed wood pieces (DWP) across 27 first- to third-order streams in central Portugal, all of which had experienced recent wildfires. The streams flowed through monospecific upland forests of Eucalyptus, Maritime pines, or Cork oaks and were fringed by a mixture of riparian tree species. DWP structure differed between tree types and between burned and unburned pieces. Post-fire timber-production forests (Maritime pines and Eucalyptus) contributed a higher quantity of thinner, longer and straighter DWP to streams than Cork oak stands. Pieces from Maritime pines had more rootwads and branches than DWP from the other tree types. Pieces from Cork oak and riparian species generally had a bent form, were shorter and had no rootwads. Burned DWP in streams were often from riparian trees. Relative to unburned DWP, the burned DWP occurred more frequently, were larger and straighter, had branches less often, and were more decayed. With more complex branches, rootwads, and a larger diameter, inputs from burned Maritime pine forests are more likely to change stream hydraulics and habitat complexity, relative to inputs from Eucalyptus forests with their simpler structure. This study shows that, less than a decade after wildfires, structure of downed wood in and near streams is strongly influenced by wildfire, but also still reflects intrinsic species characteristics and respective silviculture practices, even after the effects of fire have been accounted for. Under an anticipated shift in landscape cover with higher shrubland proportions and more mixing of Maritime pine and Eucalyptus forests, our results suggest that instream large wood will become scarcer and more structurally homogeneous.     

Bird communities following high-severity fire: Response to single and repeat fires in a mixed-evergreen forest,Oregon, USA

Fire is a widespread natural disturbance agent in most conifer-dominated forests. In light of climate change and the effects of fire exclusion, single and repeated high-severity (stand-replacement) fires have become prominent land management issues. We studied bird communities using point counting in the Klamath-Siskiyou ecoregion of Oregon, USA at various points in time after one or two high-severity fires. Time points included 2 and 3 years after a single fire, 17 and 18 years after a single fire, 2 and 3 years after a repeat fire (15 year interval between fires), and >100 years since stand-replacement fire (mature/old-growth forest). Avian species richness did not differ significantly among habitats. Bird density was highest 17 and 18 years after fire, lowest 2 years after fire, and intermediate in repeat burns and unburned forest. Bird community composition varied significantly with habitat type (A = 0.24, P < 0.0001) with two distinct gradients in species composition relating to tree structure (live to dead) and shrub stature. Using indicator species analysis, repeat burns were characterized by shrub-nesting and ground-foraging bird species while unburned mature forests were characterized by conifer-nesting and foliage-gleaning species. Bird density was not related to snag basal area but was positively related to shrub height. Contrary to expectations, repeated high-severity fire did not reduce species richness, and bird densities were greater in repeat burns than in once-burned habitats. Broad-leaved hardwoods and shrubs appear to play a major role in structuring avian communities in the Klamath-Siskiyou region. In light of these results, extended periods of early seral broadleaf dominance and short-interval high-severity fires may be important to the conservation of avian biodiversity.     

Forest fire effects in beech dominated mountain forest of Iran

Most of world's forests of different climates have a history of fire, but with different severities. Fire regimes for broadleaf deciduous forests have return intervals that vary from many decades (or less) to centuries (or more). Iran has a total of 1.2 million ha of temperate forest in the north, where fires burn about 300–400 ha annually. This study focused on the impact of fire on forest structure, tree species quality, and regeneration composition (specially beech) in the Chelir forest of northern Iran. The results showed that forest fires changed the structure and had different effects on tree species composition between burned and control areas. Thin barked species such as oriental beech (Fagus orientalis Lipsky) and coliseum maple (Acer cappadocicum Gled.) have been affected more than those with thick bark, like hornbeam (Carpinus betulus L.) and chestnut-leaved oak (Quercus castaneifolia C.A. Mey). The density of oriental beech regeneration in the unburned area was greater than in the burned area, while the quantity of regeneration of hornbeam, coliseum maple and velvet maple (Acer velutinum Boiss) was higher in burned area. Forest fire had a greater effect on oriental beech quality, and changed regeneration composition in the burned area. Fire prevention activities should be considered as a silvicultural treatment for preserving these valuable forests.     

Changes in forest vegetation and arbuscular mycorrhizae along a steep elevation gradient in Arizona

We assessed species composition, richness and abundance of understory vegetation, as well as arbuscular mycorrhizal (AM) inoculum potential on the San Francisco Peaks, tallest mountains in Arizona, crossing a steep, south-facing elevational gradient. These mountains have a high conservation value due to their rare habitats but previous vegetation studies have been limited. Because mature trees in the Pinaceae do not form associations with AM fungi, there may be more variation in plant community and AM fungal associations in coniferous forest than in ecosystems where all species associate with AM fungi. Differences in species composition between forest types reflected differences in the historical disturbance regimes. Species richness was highest in ponderosa pine forest (32.6 ± 1.4 per 1000 m² plot), although plant abundance was highest in aspen forest (49.4 ± 3.8%). Ponderosa pine and bristlecone pine forest were both high in species richness and contained species which were tolerant of frequent, low-intensity fire. Exotic species richness and abundance were highest in the lower elevations, which were also areas of high species richness and greater anthropogenic disturbance. Arbuscular mycorrhizal inoculum potential varied widely (1.2–80.1%), decreasing with increases in tree cover. We suggest indicator species that may be of use in monitoring these forests under changing climate and fire regimes.     

Short-term wildfire effects on the spatial pattern and scale of labile organic-N and inorganic-N and P pools

The spatial heterogeneity of essential plant resources plays a crucial role in the structure, composition and productivity of many terrestrial ecosystems. Fires may affect both the availability and spatial pattern of soil nutrients. However, little is known about the effect of fire on the spatial pattern of soil resources. We hypothesized that shortly after a wildfire, the spatial patterns of soil mineral-N, organic labile-N (microbial biomass-N and dissolved organic-N) and extractable-P pools would become more clumped because of ash accumulation and post-fire deposition of litter around individual adult trees. To test this hypothesis, we used plots within a Pinus canariensis forest (with both Pinus canariensis and Adenocarpus viscosus present) and sampled them one month before and one month after a wildfire. Using geostatistical analyses, we examined the spatial patterns of soil mineral-N (NH₄-N and NO₃-N), dissolved organic-N (DON), microbial biomass-N (MB-N) and soil extractable-P (PO₄-P). Burned plots of P. canariensis and A. viscosus both had values that were significantly greater than the unburned plots for all variables, except for DON in both cases, and the N:P ratio in the case of A. viscosus, which showed significantly lower values. Except for DON, we observed an increased spatial dependence and range after a fire for all studied variables in the P. canariensis plots (large individuals). However, in plots with A. viscosus (smaller individuals), we only found differences before and after the fire for the PO₄-P and DON spatial patterns. Our results confirm the changes in the spatial structure of soil variables with fire, and suggest that, on a short-term basis, the physical structure of the plant community may determine the new spatial structure after fire, with a more clumped distribution around large surviving trees and shrubs. The spatial patch size of limiting resources has important consequences for the success of restoration of forest communities on burned areas.     

宝天曼自然保护区林火干扰下不同恢复阶段栎林群落幼苗库动态特征

依据空间代替时间的方法,研究地表火干扰下不同恢复阶段栎类群落幼苗库的动态特征。结果表明:在地表火干扰后栎林群落内有木本幼苗25科37属49种,随着火烧迹地植被的恢复,乔木幼苗种类数量增多,灌木幼苗数量减少,但栎类幼苗始终占据优势;林下幼苗库的密度随恢复时间增加逐步变小,栎类幼苗密度则呈现出从高到低再升高然后变低的趋势,栎类幼苗密度在火后1年最高,平均达到(2.30±2.9)株·(10 m²)^-1;林下幼苗库恢复1年与恢复10年之间、恢复15年和对照林的密度差异显著,但恢复1年与恢复5年、恢复10年的幼苗库与对照林的幼苗密度之间没有显著差异;栎类幼苗密度变化出现2个过程,恢复1年与恢复5年、恢复10年与恢复15年幼苗密度之间有显著差异,但不同恢复阶段的栎类幼苗库与对照林相比没有显著差异;幼苗库不同恢复阶段的Shannon-Wiener指数(H')之间有极显著差异;相邻恢复阶段共有的幼苗物种较多,相似系数较大,以每2种不同恢复阶段的栎林群落进行比较,其共有物种数量为4～17种。随演替时间的增加,乔木实生苗比例逐渐增加,乔木根蘖苗的比例不断减少,到植被恢复后5年,栎类群落林下没有发现根蘖苗生存,而林下灌木幼苗也在恢复10年的群落内消失。林火干扰对栎类Ⅰ、Ⅱ和Ⅲ级的幼苗高度分配和苗木转化率有显著影响,但火烧干扰对Ⅳ级以上幼苗的高度分布和转化率影响不显著。