Wildfires and harvesting are important disturbances to forest ecosystems, but their effects on soil microbial communities are not well characterized and have not previously been compared directly. This study was conducted at sites with similar soil, climatic, and other properties in a spruce-dominated boreal forest near Chisholm, Alberta, Canada. Soil microbial communities were assessed following four treatments: control, harvest, burn, and burn plus timber salvage (burn-salvage). Burn treatments were at sites affected by a large wildfire in May 2001, and the communities were sampled 1 year after the fire. Microbial biomass carbon decreased 18%, 74%, and 53% in the harvest, burn, and burn-salvage treatments, respectively. Microbial biomass nitrogen decreased 25% in the harvest treatment, but increased in the burn treatments, probably because of microbial assimilation of the increased amounts of available NH 4 ؉ and NO 3 ؊ due to burning. Bacterial community composition was analyzed by nonparametric ordination of molecular fingerprint data of 119 samples from both ribosomal intergenic spacer analysis (RISA) and rRNA gene denaturing gradient gel electrophoresis. On the basis of multiresponse permutation procedures, community composition was significantly different among all treatments, with the greatest differences between the two burned treatments versus the two unburned treatments. The sequencing of DNA bands from RISA fingerprints revealed distinct distributions of bacterial divisions among the treatments. Gamma-and Alphaproteobacteria were highly characteristic of the unburned treatments, while Betaproteobacteria and members of Bacillus were highly characteristic of the burned treatments. Wildfire had distinct and more pronounced effects on the soil microbial community than did harvesting.
Wildfires, insect outbreaks, and windstorms are increasingly common forest disturbances. Post-disturbance management often involves salvage logging, i.e., the felling and removal of the affected trees; however, this practice may represent an additional disturbance with effects on ecosystem processes and services. We developed a systematic map to provide an overview of the primary studies on this topic and created a database with information on the characteristics of the retrieved publications, including information on stands, disturbance, intervention, measured outcomes, and study design. Of 4341 retrieved publications, 90 were retained in the systematic map. These publications represented 49 studies, predominantly from North America and Europe. Salvage logging after wildfire was addressed more frequently than after insect outbreaks or windstorms. Most studies addressed logging after a single disturbance event, and replication of salvaged stands rarely exceeded 10. The most frequent response variables were tree regeneration, ground cover, and deadwood characteristics. This document aims to help managers find the most relevant primary studies on the ecological effects of salvage logging. It also aims to identify and discuss clusters and gaps in the body of evidence, relevant for scientists who aim to synthesize previous work or identify questions for future studies.
Human-induced changes in atmospheric composition are expected to affect primary productivity across terrestrial biomes. Recent changes in productivity have been observed in many forest ecosystems, but low-latitude upper tree line forests remain to be investigated. Here, we use dendrochronological methods and isotopic analysis to examine changes in productivity, and their physiological basis, in Abies religiosa (Ar) and Pinus hartwegii (Ph) trees growing in high-elevation forests of central Mexico. Six sites were selected across a longitudinal transect (Transverse Volcanic Axis), from the Pacific Ocean toward the Gulf of Mexico, where mature dominant trees were sampled at altitudes ranging from 3200 to 4000 m asl. A total of 60 Ar and 84 Ph trees were analyzed to describe changes in growth (annual-resolution) and isotopic composition (decadal-resolution) since the early 1900s. Our results show an initial widespread increase in basal area increment (BAI) during the first half of the past century. However, BAI has decreased significantly since the 1950s with accentuated decline after the 1980s in both species and across sites. We found a consistent reduction in atmosphere to wood (13) C discrimination, resulting from increasing water use efficiency (20-60%), coinciding with rising atmospheric CO2 . Changes in (13) C discrimination were not followed, however, by shifts in tree ring δ(18) O, indicating site- and species-specific differences in water source or uptake strategy. Our results indicate that CO2 stimulation has not been enough to counteract warming-induced drought stress, but other stressors, such as progressive nutrient limitation, could also have contributed to growth decline. Future studies should explore the distinct role of resource limitation (water vs. nutrients) in modulating the response of high-elevation ecosystems to atmospheric change.
Boreal mixedwood forests with varying proportions of coniferous and deciduous species are found throughout the North American continent. Maintenance of a deciduous component within boreal forests is currently favoured, as deciduous species are believed to promote faster nutrient turnover and higher nutrient availability. Results of comparisons of deciduous and coniferous forests are, however, inconsistent in supporting this generalization. We compared indices of soil nitrogen (N) availability in the forest floor and mineral soil of deciduous, mixed, and coniferous stands of boreal mixedwood forest in northwestern Alberta. Deciduous stands had higher N availability, reflected by higher pools of NH 4 -N and inorganic N in the forest floor. Forest floors of deciduous stands also tended to have higher concentrations of microbial N but did not have higher levels of NO 3 -N or higher rates of net nitrification. Mixed stands showed the highest rates of net N mineralization. Soil N availability was more closely related to litter N content than to litter decomposition rate. The variation among the forest types is likely attributable to vegetation, as topography is fairly uniform, stands do not differ in soil texture, and N-availability indices correlated directly with the proportion of deciduous trees. Résumé :Les forêts boréales mélangées, qui contiennent des proportions variables de conifères et de feuillus, sont pré-sentes partout sur le continent nord-américain. On encourage actuellement le maintien d'une composante feuillue dans les forêts boréales croyant que les espèces feuillues favorisent un recyclage plus rapide et une plus grande disponibilité des nutriments. Les résultats de comparaisons entre des forêts résineuses et feuillues sont cependant trop inconsistants pour supporter cette généralisation. Les auteurs ont comparé les indices de disponibilité de l'azote (N) du sol dans la couverture morte et le sol minéral de peuplements feuillus, mélangés et résineux de la forêt boréale mélangée du nordouest de l'Alberta. Les peuplements feuillus avaient une plus grande disponibilité de N reflétée par des pools de N-NH 4 et de N inorganique plus importants dans la couverture morte. La couverture morte des peuplements feuillus avait aussi tendance à avoir de plus fortes concentrations de N microbien mais n'avait pas de niveaux plus élevés de N-NO 3 ou de plus hauts taux de nitrification nette. Les peuplements mélangés avaient les taux de minéralisation nette de N les plus élevés. La disponibilité de N du sol était plus étroitement reliée au contenu en N de la litière qu'à son taux de dé-composition. Les différences entre les types de forêts sont probablement attribuables à la végétation étant donné que la topographie est assez uniforme, que les peuplements ont des sols dont la texture est semblable et que les indices de disponibilité de N sont directement corrélés avec la proportion de tiges feuillues.[Traduit par la Rédaction] Jerabkova et al. 122
1 Rising economic demands for boreal forest resources along with current and predicted increases in wildfire activity have increased salvage logging of burned forests. Currently, the ecological consequences of post-fire salvage logging are insufficiently understood to develop effective management guidelines or to adequately inform policy decision-makers. 2 We used both field and laboratory studies to examine the effects of post-fire salvage logging on populations of the white-spotted sawyer Monochamus scutellatus scutellatus (Say) (Coleoptera: Cerambycidae) and its ecological function in boreal forest. 3 Monochamus s. scutellatus adults were relatively abundant in both burned and clearcut logged sites but were absent from salvage logged sites. 4 An in situ mesocosm experiment showed that the abundance of M. s. scutellatus larvae in burned white spruce bolts was linked to changes in total organic nitrogen and carbon in mineral soil. 5 Organic nutrient inputs in the form of M. s. scutellatus frass increased mineral soil microbial respiration rates by more than three-fold and altered the availability of nitrogen. Changes in nitrogen availability corresponded with decreased germination and growth of Epilobium angustifolium and Populus spp. but not Calamagrostis canadensis. 6 Although the present study focused on local scale effects, the reported findings suggest that continued economic emphasis on post-fire salvage logging may have implications beyond the local scale for biodiversity conservation, nutrient cycling and plant community composition in forest ecosystems recovering from wildfire.
Four rates of N (0, 50, 100, and 150 kg·ha -1 ) with and without a fertilizer mix containing P, K, S, Ca, Mg, and micronutrients were applied to a stand of 8-year-old lodgepole pine (Pinus contorta var. latifolia Engelm.) in four applications during 7 years. Fourteen years after the first fertilization, mean tree basal area and volume were significantly increased in the treatments containing the two highest rates of N plus the fertilizer mix. Mean tree basal area was 38% greater and mean tree volume was 42% greater in the highest N (525 kg·ha -1 ) plus mix treatment than in the control treatment. Stand basal area increased 48% over the control in the highest N plus mix treatment. Mean tree basal area and volume were 27 and 25% greater than the control under the intermediate N rate (350 kg·ha -1 ) with mix, respectively. Height growth was not significantly increased by fertilization. Both higher rates of N (four applications of >100 kg·ha -1 ) and the fertilizer mix were required for significant growth responses to occur. Addition of N alone or mix alone had a negative effect on mean tree basal area and volume. Addition of higher N rates without the mix decreased foliar total S and SO 4 -S concentration and increased the N/S ratio. Addition of the higher rates of N without the mix aggravated slight K deficiencies. The responses to the higher N rates in combination with the fertilizer mix are mainly attributable to S.Résumé : Quatre taux de N (0, 50, 100 et 150 kg·ha -1 ) avec ou sans un mélange d'engrais contenant P, K, S, Ca, Mg et des micro-éléments ont été appliqués dans un peuplement de pin lodgepole (Pinus contorta var. latifolia Engelm.) quatre fois sur une période de sept ans. Quatorze ans après la première application, la surface terrière et le volume moyens étaient significativement plus élevés dans les traitements contenant les deux taux de N les plus élevés avec le mélange d'engrais. La surface terrière et le volume moyens des arbres étaient respectivement 38 et 42% plus élevés dans le traitement avec le plus haut taux de N (525 kg·ha -1 ) combiné au mélange d'engrais que dans le traitement té-moin. La surface terrière et le volume moyens des arbres étaient respectivement 27 et 25% plus élevés dans le traitement avec le taux intermédiaire de N (350 kg·ha -1 ) combiné au mélange de fertilisant que dans le traitement témoin. La fertilisation n'a pas significativement augmenté la croissance en hauteur. La croissance en hauteur n'a pas été significativement affectée par la fertilisation. Les deux plus hauts taux de N (quatre applications de plus 100 kg N·ha -1 ) combinés au mélange d'engrais étaient nécessaires pour obtenir une augmentation significative de croissance. L'ajout de N seul ou du mélange d'engrais seul avait un effet négatif sur la surface terrière et le volume moyens des arbres. L'ajout des plus hauts taux de N sans le mélange d'engrais a causé une diminution de la concentration de S total et de S sous forme de SO 4 et a augmenté le rapport N/S. L'ajout des plus hauts taux de N sans...
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