Assessing the relative fire proneness of different forest types in Portugal

Abstract

We have assessed the fire proneness of the main forest types in Portugal classified according to the main species, using three different approaches: the use of resource selection ratios applied to burned patches, the proportion of randomly located plots that were burned and the proportion of burned National Forest Inventory plots. The results allowed ranking fire proneness according to the following decreasing order: maritime pine forests, eucalyptus forests, unspecified broadleaf forests, unspecified conifer forests, cork oak forests, chestnut forests, holm oak forests and stone pine forests. In order to understand the obtained results we have assessed the structure of the different forest types using the percent cover of seven vegetation layers (C1–C7), a Tree Dominance Index, a Height Index and a Cover Index (IC). Structural variables and stand composition were used to predict fire probability according to binary logistic modelling. Only four structural variables and stand composition provided significant results, the latter being the most important variable for explaining fire probability. These models were used to predict fire probability for different stand types as a function of IC.     

Modelling Variable Fire Severity in Boreal Forests: Effects of Fire Intensity and Stand Structure

It is becoming clear that fires in boreal forests are not uniformly stand-replacing. On the contrary, marked variation in fire severity, measured as tree mortality, has been found both within and among individual fires. It is important to understand the conditions under which this variation can arise. We integrated forest sample plot data, tree allometries and historical forest fire records within a diameter class-structured model of 1.0 ha patches of mono-specific black spruce and jack pine stands in northern Québec, Canada. The model accounts for crown fire initiation and vertical spread into the canopy. It uses empirical relations between fire intensity, scorch height, the percent of crown scorched and tree mortality to simulate fire severity, specifically the percent reduction in patch basal area due to fire-caused mortality. A random forest and a regression tree analysis of a large random sample of simulated fires were used to test for an effect of fireline intensity, stand structure, species composition and pyrogeographic regions on resultant severity. Severity increased with intensity and was lower for jack pine stands. The proportion of simulated fires that burned at high severity (e.g. >75% reduction in patch basal area) was 0.80 for black spruce and 0.11 for jack pine. We identified thresholds in intensity below which there was a marked sensitivity of simulated fire severity to stand structure, and to interactions between intensity and structure. We found no evidence for a residual effect of pyrogeographic region on simulated severity, after the effects of stand structure and species composition were accounted for. The model presented here was able to produce variation in fire severity under a range of fire intensity conditions. This suggests that variation in stand structure is one of the factors causing the observed variation in boreal fire severity.     

Tree Species Traits Influence Soil Physical,Chemical, and Biological Properties in High Elevation Forests

Background

Previous studies have shown that plants often have species-specific effects on soil properties. In high elevation forests in the Southern Rocky Mountains, North America, areas that are dominated by a single tree species are often adjacent to areas dominated by another tree species. Here, we assessed soil properties beneath adjacent stands of trembling aspen, lodgepole pine, and Engelmann spruce, which are dominant tree species in this region and are distributed widely in North America. We hypothesized that soil properties would differ among stands dominated by different tree species and expected that aspen stands would have higher soil temperatures due to their open structure, which, combined with higher quality litter, would result in increased soil respiration rates, nitrogen availability, and microbial biomass, and differences in soil faunal community composition.

Methodology/Principal Findings

We assessed soil physical, chemical, and biological properties at four sites where stands of aspen, pine, and spruce occurred in close proximity to one-another in the San Juan Mountains, Colorado. Leaf litter quality differed among the tree species, with the highest nitrogen (N) concentration and lowest lignin∶N in aspen litter. Nitrogen concentration was similar in pine and spruce litter, but lignin∶N was highest in pine litter. Soil temperature and moisture were highest in aspen stands, which, in combination with higher litter quality, probably contributed to faster soil respiration rates from stands of aspen. Soil carbon and N content, ammonium concentration, and microbial biomass did not differ among tree species, but nitrate concentration was highest in aspen soil and lowest in spruce soil. In addition, soil fungal, bacterial, and nematode community composition and rotifer, collembolan, and mesostigmatid mite abundance differed among the tree species, while the total abundance of nematodes, tardigrades, oribatid mites, and prostigmatid mites did not.

Conclusions/Significance

Although some soil characteristics were unaffected by tree species identity, our results clearly demonstrate that these dominant tree species are associated with soils that differ in several physical, chemical, and biotic properties. Ongoing environmental changes in this region, e.g. changes in fire regime, frequency of insect outbreaks, changes in precipitation patterns and snowpack, and land-use change, may alter the relative abundance of these tree species over coming decades, which in turn will likely alter the soils.     

Effects of fire severity on plant nutrient uptake reinforce alternate pathways of succession in boreal forests

Fire activity in the North American boreal region is projected to increase under a warming climate and trigger changes in vegetation composition. In black spruce forests of interior Alaska, fire severity impacts residual organic layer depth which is strongly linked to the relative dominance of deciduous versus coniferous trees in early succession. These alternate successional pathways may be reinforced by biogeochemical processes that affect the relative ability of deciduous versus coniferous trees to acquire limiting nutrients. To test this hypothesis, we examined changes in soil inorganic nitrogen (N) supply and in situ ¹⁵N root uptake by aspen (Populus tremuloides) and black spruce (Picea mariana) saplings regenerating in lightly and severely burned sites, 16 years following fire. Fire severity did not impact the composition or magnitude of N supply, and nitrate represented nearly 40 % of total N supply. Both aspen and spruce took up more N in severely burned than in lightly burned sites. Spruce exhibited only a moderately lower rate of nitrate uptake, and a higher ammonium uptake rate than aspen in severely burned sites. At the stand level, differences in species nutrient uptake were magnified, with aspen taking up nearly an order-of-magnitude more N per m² in severely burned than in lightly burned sites. We suggest that differences in nutrient sinks (biomass) established early in succession and effects of post-fire organic layer depth on nutrient uptake, are key mechanisms reinforcing the opposing stand dominance patterns that have developed in response to variations in organic layer depth.     

Afforestation causes changes in post‐fire regeneration in native shrubland communities of northwestern Patagonia,Argentina

Question: What are the effects of fire in native shrubland communities and in pine plantations established in these shrublands? Location: Northern Patagonia, Argentina. Methods: We surveyed four sites in Chall‐Huaco valley, located in northwest Patagonia. Each site was a vegetation mosaic composed of an unburned Pinus ponderosa plantation, a plantation burned in 1996, and an unburned matorral and a matorral burned by the same fire. We recorded the cover of all vascular plant species. We also analysed species richness, total cover, proportion of exotic species, abundance of woody species and herb species, cover of exotic species, abundance of woody and herb species and differences in composition of species. For both shrubs and tree species we recorded the main strategy of regeneration (by resprouting or by seed). Results: We found that fire had different effects on native matorral and pine plantations. Five years after fire, plantations came to be dominated by herbs and exotic species, showing differences in floristic composition. In contrast, matorral communities remained very similar to unburned matorral in terms of species richness, proportion of woody species, and herb species and proportion of exotics. Also, pine plantations were primarily colonized by seedlings, while matorrals were primarily colonized by resprouting. Conclusions: Matorrals are highly fire resilient communities, and the practice of establishing plantations on matorrals produces a strong reduction in the capacity of matorral to return to its original state. The elimination of shrubs owing to the effect of plantations can hinder regeneration of native ecosystems. Burned plantations may slowly develop into ecosystems similar to the native ones, or they may produce a new ecosystem dominated by exotic herbs. This study shows that plantations of exotic conifers affect native vegetation even after they have been removed, as in this case by fire.     

Fire severity mediates climate‐driven shifts in understorey community composition of black spruce stands of interior Alaska

Question: How do pre‐fire conditions (community composition and environmental characteristics) and climate‐driven disturbance characteristics (fire severity) affect post‐fire community composition in black spruce stands? Location: Northern boreal forest, interior Alaska. Methods: We compared plant community composition and environmental stand characteristics in 14 black spruce stands before and after multiple, naturally occurring wildfires. We used a combination of vegetation table sorting, univariate (ANOVA, paired t‐tests), and multivariate (detrended correspondence analysis) statistics to determine the impact of fire severity and site moisture on community composition, dominant species and growth forms. Results: Severe wildfires caused a 50% reduction in number of plant species in our study sites. The largest species loss, and therefore the greatest change in species composition, occurred in severely burned sites. This was due mostly to loss of non‐vascular species (mosses and lichens) and evergreen shrubs. New species recruited most abundantly to severely burned sites, contributing to high species turnover on these sites. As well as the strong effect of fire severity, pre‐fire and post‐fire mineral soil pH had an effect on post‐fire vegetation patterns, suggesting a legacy effect of site acidity. In contrast, pre‐fire site moisture, which was a strong determinant of pre‐fire community composition, showed no relationship with post‐fire community composition. Site moisture was altered by fire, due to changes in permafrost, and therefore post‐fire site moisture overrode pre‐fire site moisture as a strong correlate. Conclusions: In the rapidly warming climate of interior Alaska, changes in fire severity had more effect on post‐fire community composition than did environmental factors (moisture and pH) that govern landscape patterns of unburned vegetation. This suggests that climate change effects on future community composition of black spruce forests may be mediated more strongly by fire severity than by current landscape patterns. Hence, models that represent the effects of climate change on boreal forests could improve their accuracy by including dynamic responses to fire disturbance.     

The trophic structure of bark-living oribatid mite communities analysed with stable isotopes (15N, 13C) indicates strong niche differentiation

The aim of the present study was to identify food sources of bark-living oribatid mites to investigate if trophic niche differentiation contributes to the diversity of bark living Oribatida. We measured the natural variation in stable isotope ratios (¹⁵N/¹⁴N, ¹³C/¹²C) in oribatid mites from the bark of oak (Quercus robur), beech (Fagus sylvatica), spruce (Picea abies) and pine (Pinus sylvestris) trees and their potential food sources, i.e., the covering vegetation of the bark (bryophytes, lichens, algae, fungi). As a baseline for calibration the stable isotope signatures of the bark of the four tree species were measured and set to zero. Oribatid mite stable isotope ratios spanned over a range of about 13 δ units for ¹⁵N and about 7 δ units for ¹³C suggesting that they span over about three trophic levels. Different stable isotope signatures indicate that bark living oribatid mites feed on different food sources, i.e., occupy distinct trophic niches. After calibration stable isotope signatures of respective oribatid mite species of the four tree species were similar indicating close association of oribatid mites with the corticolous cover as food source. Overall, the results support the hypothesis that trophic niche differentiation of bark living oribatid mites contributes to the high diversity of the group.     

Differences in bird communities in postfire silvicultural practices stands within pine forest of South Korea

We examined differences in bird communities in relation to characteristics of habitat structure in a pine forest, Samcheok, South Korea. An unburned stand, a stand burned 7 years earlier and then naturally restored, and a stand where Japanese red pine Pinus densiflora seedlings were planted after the fire were used for the survey. Habitat structure was dramatically changed by postfire silvicultural practices. Number of stand trees, shrubs, seedlings, snags, and vegetation coverage were significantly different among study stands. We made 1,421 detections of 46 bird species during 23 separate line transect surveys per stand between February 2007 and December 2008. The mean number of observed bird species and individuals, bird species diversity index (H′), and Simpson’s diversity index (D _s) were highest in the unburned stand and lowest in the pine seedling stand. There were more species and individuals of forest-dwelling birds in the unburned stand than both burned stands. Canopy and cavity nesters, foliage searchers, bark gleaners, and timber drillers were significantly higher in the unburned stand. In the pine seedling stand, densities of birds that prefer open field and shrub cover were higher. Stand structure was simplified in the pine seedling stand by postfire practices. Because of differences in habitat structure and bird communities, postfire practices in the burned stand should be re-evaluated. Also, management strategies for pine forest after forest fires are needed based on results of long-term experiments.     

Distribution of eriophyoid mites (Acari: Eriophyoidea) on coniferous trees

The aim of the paper was to determine the infestation parameters and species composition of eriophyoid mites for different parts of Norway spruce and Scots pine as well as for different age groups of the trees. The observations on the occurrence of the mites were conducted in 2004 and 2005 in 4 locations distributed in various regions of Poland, accounting for 11 environments (location x year). Three plant age groups were studied: (1) adult trees: 40–60 years old, additionally divided into three levels: top, middle and bottom; (2) young trees: 6–10 years old; and (3) seedlings: 2–3 years old. The same number of species (five) occurred on each coniferous tree, but only one, the rarest, was common on both tree species. Out of 500 samples for each species, mites were found on 279 pine (55.8%) and 252 spruce samples (50.2%). No tendency for the mites to choose any particular level on Scots pine and Norway spruce was observed. In addition, no tendency for the mites to choose trees from any of the age groups was observed for both Scots pine and Norway spruce, in the latter case the result obtained also for mite species subdivided into vagrant and refuge-seeking ones. Final conclusions were that in case of adult trees samples can be taken from the bottom part of a tree; however, sampling from young trees growing among adult trees may be seen as the most efficient sampling method.     

Recovery of forest soil fauna diversity and composition after repeated summer droughts

To examine the resilience of soil animal communities to large-scale disturbances. we studied the recovery of total abundance, diversity and community composition of forest soil mesofauna after a 6-year climatic disturbance. This was done in a pre-established experiment in a Norway spruce Picea abies stand in southern Sweden in which long-term summer droughts had been experimentally imposed and had caused large changes in soil fauna communities. We included both predators (mesostigmatid mites) and fungivores/detritivores (oribatid mites, collembolans) in the study because of the likelihood that they would differ in recovery ability due to differences in their feeding habits, dispersal ability and reproductive strategies. Total abundances of Collembola, Oribatida and Mesostigmata were similar in recovery and control plots after three years, but species richness, the Shannon-Wiener diversity index, and community composition recovered more slowly, particularly among the Oribatida. To only use total abundance of higher taxonomic groups was thus not sufficient when measuring community recovery. There was a tendency for more mobile groups to recover faster than the slow-moving oribatids, indicating the importance of dispersal ability for the resilience of soil communities.     

Soil arthropoda of post-fire successions in northern taiga of West Siberia

The soil arthropoda population of northern taiga was investigated in the primary pine forest which had not been subject to any fire for at least 100 years and at two burnt sites of different opost-fire successions. The population of the primary nonburnt forest is represented by 54 microarthropoda species. The mesofauna is composed almost exceptionally of spiders: 10 species of them were identified. The foliage underwood develops on young burnt sites; the soil is covered with thick mossy cushions, with spots of reedgrass. The density of collembolan and oribatid mite population there is about two times lower than in nonburnt pine forests; gamasid mites are absent. The microarthropoda population is represented by the species which are common in the surrounding primary pine forests. The mesofauna consists mainly of ground beetles, open-land predators, and myxophytophagans. Spiders are represented by some widespread mobile species of low density. A mixed forest with mossy-lichen cover is formed on old burnt sites (of an age of about 50 years). Diversity and density of microarthropoda increase reaching the values characteristic of the primary forest; however, the species composition and dominance are still different. The fraction of spiders in mesofauna increases. The ground beetle population is the most abundant and diverse.     

Variation in the impact of exotic grasses on native plant composition in relation to fire across an elevation gradient in Hawaii

The impact that an exotic species can have on the composition of the community it enters is a function of its abundance, its particular species traits and characteristics of the recipient community. In this study we examined species composition in 14 sites burned in fires fuelled by non‐indigenous C4 grasses in Hawaii Volcanoes National Park, Hawaii. We considered fire intensity, time since fire, climatic zone of site, unburned grass cover, unburned native cover and identity of the most abundant exotic grass in the adjacent unburned site as potential predictor variables of the impact of fire upon native species. We found that climatic zone was the single best variable for explaining variation in native cover among burned sites and between burned and unburned pairs. Fire in the eastern coastal lowlands had a very small effect on native plant cover and often stimulated native species regeneration, whereas fire in the seasonal submontane zone consistently caused a decline in native species cover and almost no species were fire tolerant. The dominant shrub, Styphelia tameiameia, in particular was fire intolerant. The number of years since fire, fire intensity and native cover in reference sites were not significantly correlated with native species cover in burned sites. The particular species of grass that carried the fire did however, have a significant effect on native species recovery. Where the African grass Melinis minutiflora was a dominant or codominant species, fire impacts were more severe than where it was absent regardless of climate zone. Overall, the impacts of exotic grass‐fuelled fires on native species composition and cover in seasonally dry Hawaiian ecosystems was context specific. This specificity is best explained by differences between the climatic zones in which fire occurred. Elevation was the main physical variable that differed among the climatic zones and it alone could explain a large percentage of the variation in native cover among sites. Rainfall, by contrast, did not vary systematically with elevation. Elevation is associated with differences in composition of the native species assemblages. In the coastal lowlands, the native grass Heteropogon contortus, was largely responsible for positive changes in native cover after fire although other native species also increased. Like the exotic grasses, this species is a perennial C4 grass. It is lacking in the submontane zone and there are no comparable native species there and almost all native species in the submontane zone were reduced by fire. The lack of fire tolerant species in the submontane zone thus clearly contributes to the devastating impact of fire upon native cover there.     

Predicting the composition of Canadian southern boreal forest in different fire cycles

Abstract. Post-fire succession was reconstructed for a sector located in the southern part of the Québec boreal forest. Forest composition for different periods since fire was evaluated using a stand initiation map together with ecological maps representing both site conditions and stand types. Nine fires covering at least 100 ha and representing a chronosequence of more than 230 yr were used. Although a relatively clear successional pattern from deciduous to coniferous composition relating to time-since-fire was observed, Pinus banksiana stands showed an erratic distribution not related to succession but possibly to the pre-fire stand composition. A comparison with forest cover maps produced after a recent spruce budworm outbreak, showed that succession toward coniferous dominance appeared to be interrupted by spruce budworm (Choristoneura fumiferana) outbreaks which, by killing Abies balsamea, lead to a mixed deciduous forest composition. A simple empirical model based on a negative exponential distribution of age classes was developed to evaluate how changes in the fire cycle would affect the composition of the forest mosaic. The transition between deciduous dominance and coniferous dominance occurs in a fire cycle > 200 yr. Although pure deciduous stands tend to disappear during long fire cycles, the proportion of mixed stands remains relatively constant. Prediction of the forest composition for longer fire cycles is complicated by the interaction between post-fire composition and stand vulnerability to spruce budworm outbreaks.     

Secondary succession, soil formation and development of a diverse community of oribatids and saprophagous soil macro-invertebrates

Investigations into different stages of secondary succession (from a wheat field to a beechwood on Threstone; Northern Germany) demonstrated the formation of a carbon rich top soil in later successional stages. Parallel to changes in plant species and soil formation, there were also changes in species composition and diversity of saprophagous macro-invertebrates (Lumbricidae, Diplopoda, Isopoda) and oribatid mites (Acari: Oribatida). Diversity of diplopod and isopod species increased after cessation of cultivation, but in a late successional stage (ca 50 y-old fallow, ash-dominated wood) species number of diplopods and isopods declined strongly. In comparison with the other soil invertebrate groups, species composition of earthworms among the sites was more similar. Accumulation of soil C was assumed to be related to wood formation and occurrence of woody debris and recalcitrant leaf litter of beech trees. Incorporation of recalcitrant litter materials by earthworm species living in the upper mineral soil presumably contributed significantly to accumulation of soil C. Accumulation of soil C was accompanied by the development of an oribatid mite community rich in species. In early successional stages oribatids predominantly colonized the litter layer, while most oribatid mites of the beechwood inhabited the upper mineral soil. Maximum diversity of oribatid mites in the beechwood is assumed to be related to instability of the mineral soil caused by earthworm activity. Changes in species composition and diversity are discussed considering succession theory. Even soil invertebrates of similar trophic groups appear to respond very differently to successional changes. It is concluded that conservation strategies to maintain high diversity of soil invertebrates are most likely to be successful if a wide range of habitats of different successional stages is included.     

Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forest

Predicting plant community responses to changing environmental conditions is a key element of forecasting and mitigating the effects of global change. Disturbance can play an important role in these dynamics, by initiating cycles of secondary succession and generating opportunities for communities of long‐lived organisms to reorganize in alternative configurations. This study used landscape‐scale variations in environmental conditions, stand structure, and disturbance from an extreme fire year in Alaska to examine how these factors affected successional trajectories in boreal forests dominated by black spruce. Because fire intervals in interior Alaska are typically too short to allow relay succession, the initial cohorts of seedlings that recruit after fire largely determine future canopy composition. Consequently, in a dynamically stable landscape, postfire tree seedling composition should resemble that of the prefire forest stands, with little net change in tree composition after fire. Seedling recruitment data from 90 burned stands indicated that postfire establishment of black spruce was strongly linked to environmental conditions and was highest at sites that were moist and had high densities of prefire spruce. Although deciduous broadleaf trees were absent from most prefire stands, deciduous trees recruited from seed at many sites and were most abundant at sites where the fires burned severely, consuming much of the surface organic layer. Comparison of pre‐ and postfire tree composition in the burned stands indicated that the expected trajectory of black spruce self‐replacement was typical only at moist sites that burned with low fire severity. At severely burned sites, deciduous trees dominated the postfire tree seedling community, suggesting these sites will follow alternative, deciduous‐dominated trajectories of succession. Increases in the severity of boreal fires with climate warming may catalyze shifts to an increasingly deciduous‐dominated landscape, substantially altering landscape dynamics and ecosystem services in this part of the boreal forest.     

Influence of wildfire on diversity of culturable soil microfungal communities in the Mount Carmel forest,Israel

The fire-related variations in culturable microfungal communities in the soil of the Mount Carmel forest, Israel, were examined by comparing the communities from burned and adjacent unburned soil plots under pine and oak trees – collected 6, 18, and 26 months after the fire. A total of 82 species representing 44 genera were isolated using the soil dilution plate method. The results showed that the fire had strongly influenced the composition and structure of microfungal communities. The fire remarkably changed physical and chemical properties of the environment, decreasing water holding capacity, organic matter and total nitrogen content in the burned soil. These changes supported abundant development of fast-growing mycoparasitic species (Clonostachys rosea and Trichoderma spp.) and caused significant decrease in species richness. The variations in community composition were much more expressed in the burned soils under oak vegetation as compared with the pine trees. In the oak burned soils, the contribution of the “mesic” component, Penicillium spp., was markedly lower, whereas the contribution of the “xeric”, stress-selected component, melanin-containing species, was higher than in the unburned communities. Such variations can be also considered as a community response to the fire-related decrease in water and nutrient content in the burned soils.     

Spatial Heterogeneity and Soil Nitrogen Dynamics in a Burned Black Spruce Forest Stand: Distinct Controls at Different Scales

We evaluated spatial patterns of soil N and C mineralization, microbial community composition (phospholipid fatty acids), and local site characteristics (plant/forest floor cover, soil pH, soil %C and %N) in a 0.25-ha burned black spruce forest stand in interior Alaska. Results indicated that factors governing soil N and C mineralization varied at two different scales. In situ net N mineralization was autocorrelated with microbial community composition at relatively broad scales (∼ ∼8 m) and with local site characteristics (`site' axis 1 of non-metric scaling ordination) at relatively fine scales (2–4 m). At the scale of the individual core, soil moisture was the best predictor of in situ net N mineralization and laboratory C mineralization, explaining between 47 and 67% of the variation (p < 0.001). Ordination of microbial lipid data showed that bacteria were more common in severely burned microsites, whereas fungi were more common in low fire severity microsites. We conclude that C and N mineralization rates in this burned black spruce stand were related to different variables depending on the scale of analysis, suggesting the importance of considering multiple scales of variability among key drivers of C and N transformations.     

Features of Species and Structural Diversity of Artificial Pine Stands on Soddy–Podzolic Loam Soils

The method of ecological transects was used for studying plant-species composition in an aged artificial pine stand created on a composite spruce forest on soddy–podzolic soil (underlain by blanket loams) in the broadleaved–spruce forest subzone (Moscow oblast). The change of edificatory tree species was followed by changes in the ecological conditions and, as a consequence, in the species composition, abundance, and cenotic significance of vegetation. Changes in the pattern of restoration of floristic diversity in different soil-geochemical facies of the transect were observed.     

Conversion of Andean montane forests into plantations: Effects on soil characteristics,microorganisms, and microarthropods

Tropical montane forests in the Andes are subjected to deforestation and subsequent transformation into pastures. Abandoned pastures are frequently reforested by planting monoculture timber plantations, resulting in reduced aboveground diversity and changes in soil characteristics compared to primary forests. In this study, we evaluated differences in soil properties (litter layer thickness, pH, water content, and C-to-N ratio) between degraded primary montane forest and monoculture pine (Pinus patula) and alder (Alnus acuminata) plantations and their effects on density, diversity, and community structure of litter and soil-living mesofauna, with focus on oribatid mites (Acariformes). The study was performed in a montane region in the southern Ecuadorian Andes (2,000–2,600 m a.s.l.). C-to-N ratios in the litter and upper 5 cm soil layer were higher in pine plantations, while other soil characteristics were similar between vegetation types. Surprisingly, microbial biomass and density of soil mesofauna in the litter layer did not differ between vegetation types, while density and species richness of oribatid mites were higher in pine plantations. Community structure of oribatid mites differed between vegetation types with only a few species overlapping. The results indicate that quality and diversity of litter were not the major factors regulating the mesofauna community. Instead, soil animals benefited from increased habitat structure in thicker litter layers and potentially increased availability of root-derived resources. Overall, the results suggest that from a soil animal perspective, monoculture plantations are less detrimental than commonly assumed and enrichment of abandoned plantations with native tree species may help to restore tropical montane forests.     

Changing growth response to wildfire in old‐growth ponderosa pine trees in montane forests of north central Idaho

North American fire‐adapted forests are experiencing changes in fire frequency and climate. These novel conditions may alter postwildfire responses of fire‐adapted trees that survive fires, a topic that has received little attention. Historical, frequent, low‐intensity wildfire in many fire‐adapted forests is generally thought to have a positive effect on the growth and vigor of trees that survive fires. Whether such positive effects can persist under current and future climate conditions is not known. Here, we evaluate long‐term responses to recurrent 20th‐century fires in ponderosa pine, a fire‐adapted tree species, in unlogged forests in north central Idaho. We also examine short‐term responses to individual 20th‐century fires and evaluate whether these responses have changed over time and whether potential variability relates to climate variables and time since last fire. Growth responses were assessed by comparing tree‐ring measurements from trees in stands burned repeatedly during the 20th century at roughly the historical fire frequency with trees in paired control stands that had not burned for at least 70 years. Contrary to expectations, only one site showed significant increases in long‐term growth responses in burned stands compared with control stands. Short‐term responses showed a trend of increasing negative effects of wildfire (reduced diameter growth in the burned stand compared with the control stand) in recent years that had drier winters and springs. There was no effect of time since the previous fire on growth responses to fire. The possible relationships of novel climate conditions with negative tree growth responses in trees that survive fire are discussed. A trend of negative growth responses to wildfire in old‐growth forests could have important ramifications for forest productivity and carbon balance under future climate scenarios.