Effects of stand age, wildfire and clearcut harvesting on forest floor in boreal mixedwood forests

Carbon (C) in the forest floor (FF) of the boreal region is an important reservoir of terrestrial C. We examined the effects of stand age and disturbance type (clearcutting vs. wildfire) on quantity and quality of organic C of FF in a boreal mixedwood forest of central Canada. Forest floor samples were collected from 6 post-fire (2- to 203-year-old) and 3 post-harvest age classes (2- to 28-year-old) on mesic sites, each randomly replicated three times. Samples were analyzed to determine the physical and chemical properties and the C quality was assessed by quantifying C fractions as easily labile, moderately labile and recalcitrant. Bulk density, total organic C concentration, N concentration and the cation exchange capacity increased with stand age and peaked at 85-year-old sites. Soil pH and concentration of P and K decreased with stand age. In post-fire stands, the depth of FF, total organic C, and labile C fractions increased with stand age in the 2- to 85-year-old stands, while recalcitrant C was lower in 2-year-old stands than older stands. In stands ≤28 years old, post-harvest sites had significantly higher concentration of total organic C and the three C fractions than post-fire sites in 2-year-old stands. No or marginal difference occurred between the two stand origins in 10- and 28-year-old stands. The relative proportions of C fractions did not differ with stand age or stand origin. Our results showed that the quantity of organic C in FF of boreal mixedwoods increased with stand development till 85 years and then slightly decreased in older stands, and post-harvest stands had a higher amount of organic C than post-fire stands immediately after disturbance, but the effect of two disturbances on C in FF converged shortly (within 10 years). The quality of organic C remains the same through stand development and between the two studied stand origins.     

Effects of fire on regional evapotranspiration in the central Canadian boreal forest

Changes in fire regimes are driving the carbon balance of much of the North American boreal forest, but few studies have examined fire‐driven changes in evapotranspiration (ET) at a regional scale. This study used a version of the Biome‐BGC process model with dynamic and competing vegetation types, and explicit spatial representation of a large (10⁶ km²) region, to simulate the effects of wildfire on ET and its components from 1948 to 2005 by comparing the fire dynamics of the 1948–1967 period with those of 1968–2005. Simulated ET averaged, over the entire temporal and spatial modeling domain, 323 mm yr⁻¹; simulation results indicated that changes in fire in recent decades decreased regional ET by 1.4% over the entire simulation, and by 3.9% in the last 10 years (1996–2005). Conifers dominated the transpiration (E_C) flux (120 mm yr⁻¹) but decreased by 18% relative to deciduous broadleaf trees in the last part of the 20th century, when increased fire resulted in increased soil evaporation, lower canopy evaporation, lower E_C, and a younger and more deciduous forest. Well‐ and poorly drained areas had similar rates of evaporation from the canopy and soil, but E_C was twice as high in the well‐drained areas. Mosses comprised a significant part of the evaporative flux to the atmosphere (22 mm yr⁻¹). Modeled annual ET was correlated with net primary production, but not with temperature or precipitation; ET and its components were consistent with previous field and modeling studies. Wildfire is driving significant changes in hydrological processes by affecting mean stand age, forest species, and energy balance. These changes, particularly in poorly drained areas, may control the future carbon balance of the boreal forest.     

Decomposing logs increase oribatid mite assemblage diversity in mixedwood boreal forest

The removal of timber during harvesting substantially reduces important invertebrate habitat, most noticeably microhabitats associated with fallen trees. Oribatid mite diversity in downed woody material (DWM) using species-level data has not been well studied. We investigated the influence of decaying logs on the spatial distribution of oribatid mites on the forest floor at the sylviculture et aménagement forestiers écosystémique (SAFE) research station in the Abitibi region in NW Québec. In June 2006, six aspen logs were selected for study, and samples were taken at three distances for each log: directly on top of the log (ON), directly beside the log (ADJ) and at least one metre away from the log and any other fallen wood (AWAY). Samples ON logs consisted of a litter layer sample, an upper wood sample and an inner wood sample. Samples at the ADJ and AWAY distances consisted of litter samples and soil cores. The highest species richness was collected ON logs, and logs harboured a distinct oribatid species composition compared to nearby forest floor. There were species-specific changes in abundance with increasing distance away from DWM, which indicates an influence of DWM in structuring oribatid assemblages on the forest floor. Additionally, each layer (litter, wood and soil) exhibited a unique species composition and hosted a different diversity of oribatid mites. This study further highlights the importance of DWM to forest biodiversity by creating habitat for unique assemblages of oribatid mites.     

Photosynthetic strategies of summergreen and evergreen understory herbs of the boreal mixedwood forest

Seasonal differences in photosynthesis and stomatal conductance of four herbaceous perennials from beneath a deciduous canopy was assessed at two light levels (60 and 400 μmol m⁻² s⁻¹ photosynthetic photon flux density, PPFD) and two leaf temperatures (7 and 20°C). Leaves of an evergreen, Pyrola asarifolia Michx., a wintergreen, Cornus canadensis L., and two summergreen species, Rubus pubescens Raf. and Aralia nudicaulis L., were collected at four times during the growing season. In addition, midsummer light response curves were obtained for one summergreen (A. nudicaulis) and one evergreen species (P. asarifolia) at both 7 and 20°C. Gas exchange measurements were made in the laboratory under controlled environmental conditions. For leaves collected in April, when insolation was high due to the leafless overstory, only P. asarifolia had green leaves, and there was no effect of temperature or light on this species' photosynthesis. P. asarifolia's net assimilation rate (NA) in April was about 30% of it's maximum in late summer. In early summer (June), A. nudicaulis and R. pubescens had higher NA at the higher temperature; at this time, these summergreen species also reached their maximum NA. Midsummer photosynthetic light response curves showed that the light-saturation point was higher and more responsive to leaf temperature in the summergreen A. nudicaulis than in the evergreen P. asarifolia. The summergreen species appear to have a photosystem which performs at high rates during early- and mid-summer, as well as a taller stature which allows them to intercept more light. The photosynthetic system of the ever/wintergreen species is adapted to the low ground-level light conditions in the summer and there does not appear to be an adjustment to take further advantage of the higher light in the spring and fall period. The adaptation of the evergreen and wintergreen understory species is tolerance to low temperatures, enabling them to photosynthesize into the fall till the first continuous frosts occur in the understory and also permitting the evergreen species to begin photosynthesis early in the spring. Received: 17 October 1996 / Accepted: 2 May 1997     

Changes in the forest floor of Canadian southern boreal forest after disturbance

Abstract. The concentrations and contents of organic matter and nutrients in organic deposits on the forest floor were estimated along a 231-yr chronosequence following fire at the southern limit of the boreal forest in eastern Canada. The sampling design was stratified to take into account the variability related to the presence of the principal tree species as well as to the presence of large gaps created by a recent spruce budworm (Choristoneura fumiferana) outbreak. The forest floor showed a steady accumulation of organic matter and total nutrients with time-since-fire and a 50 % decrease in the concentrations of available P and K, but not N (as determined by aerobic incubation). The increase in forest-floor weight was accompanied by an increased storage of available N, Ca and Mg. The availability of N and Ca was more strongly affected by tree species and gaps than by time-since-fire. A high N-availability was observed under Betula papyrifera and in gaps, while high a Ca-availability was found near Populus tremuloides and Thuja occidentalis. In old sites, the forest floor of gaps, created by a recent spruce budworm outbreak, had a necromass similar to that of a young forest, but the low concentrations of available P and K of an old forest.     

Succession in the southern part of the Canadian boreal forest

Forest succession following fire in a forest mosaic of northwestern Quebec has been studied in order to: (1) describe the successional pathways using communities of different ages and (2) evaluate convergence of successional pathways and possible effect of fire suppression on the establishment of steady-state communities. As a first step, ordination and classification techniques were used in order to remove changes in forest composition which are related to abiotic conditions. Then, ordinations based on tree diameter distributions were used to study shifts in species composition in relation to time since the last fire.Even under similar abiotic conditions, successional pathways are numerous. However, regardless of forest composition after fire, most stands show convergence toward dominance of Thuja occidentalis and Picea mariana on xeric sites and dominance of Abies balsamea and Thuja occidentalis on more mesic sites. Stable communities of >300 yr occur on xeric sites while on mesic sites directional succession still occurs after 224 yr. Nearly all species involved in succession are present in the first 50 yr following fire. Only Abies balsamea and Thuja occidentalis increase significantly in frequency during succession. Following initial establishment, successional processes can generally be explained by species longevity and shade tolerance. Early successional species may be abundant in the canopy for more than 200 yr while the rapid decrease of Picea glauca, a late successional species could be related to spruce budworm outbreaks. Considering the short fire rotation observed (about 150 yr), a steady-state forest is unlikely to occur under natural conditions, though it may be possible if fire is controlled.     

Effect of forestry on the abundance and diversity of arboreal spiders in the boreal spruce forest

Spiders were sampled from spruce branches during late winter in northern Sweden, to investigate the effects of forestry on the community structure of arboreal spiders. Five lichen-rich, natural spruce Picea abies forests and adjacent mature, selectively-logged lichen-poor forests were selected as sample sites. Lichen-rich forests had over three times more spiders on the branches than the lichen-poor forests. The spider community was dominated by web spinners, i.e. the families Linyphiidae. Araneidae, Tetragnathidae and Theridiidae. Hunters, i.e. Philodromidae and Clubionidae. comprised < 3% of the individuals and > 82% of all spiders were juveniles.
Among the dominant species, only the orb-weaver Araneus nordmanni was found in all sites whereas the sheet-web spider Lepthyphantes suffusus was found in all five lichen-rich forests but only in two of the lichen-poor. Both species composition and dominance differed from spider communities in southern boreal spruce canopies. Small prey items, severe abiotic conditions and high predation pressure from birds are possible reasons why web spiders dominate the arboreal community in northern Sweden. These factors could also explain the observed shift in dominance from sheet-web spiders with large body size to those with a small body size, compared with arboreal spider communities in southern boreal forests.
Diversity indices (jack-knifing of Simpson index and Q statistic) showed a higher diversity of spiders in lichen-rich than in lichen-poor forests. The lichen-rich forest had more species and less dominance, in both rank abundance and the Berger-Parker index of dominance. However, rarefaction plots indicated no differences besides lower abundance of spiders on sampled branches in lichen-poor forests. It is suggested, that habitat structure (branch size and epiphytic lichen abundance) could be an explanation for the greater number of spiders in old, lichen-rich spruce forests.     

Changes in the understory of Canadian southern boreal forest after fire

Abstract. We investigated changes in the composition and abundance of understory species after fire in the southern boreal forest around Lake Duparquet, Québec. Ten plots of 100 m² were sampled in each of eight sites varying in post-fire age from 26 to 230 yr, with 20 1-m² quadrats in each of these 80 plots. Variation in the understory was described by DCA ordination and interpreted as a regeneration succession series. Thickness of the organic layers, stand age and canopy composition were all correlated with vegetational change. This change was not constant throughout succession; some old sites showed an increase in the diversity and abundance of certain pioneer species. This was partly related to openings in the canopy resulting from a major outbreak of spruce budworm, which affected sites dominated by Abies balsamea. The ordinations were performed on both the 100-m² plots and the 1-m² quadrats. Heterogeneity within sites was larger at the 1-m² scale and there was a great deal of overlap in the position of the quadrats in ordination space. At the smaller scale of analysis, stand age and thickness of the organic layers were not correlated with the changes observed in the understory.     

Effects of upland clearcutting and riparian partial harvesting on leaf pack breakdown and aquatic invertebrates in boreal forest streams

1. Leaf litter breakdown and associated invertebrates were compared among three logged and three reference stream reaches 2–3 years before and 3–4 years after logging to assess the environmental impacts of partial‐harvest logging as a novel riparian management strategy for boreal forest streams. 2. Partial‐harvest logging at three sites resulted in 10, 21 and 28% average basal area removal from riparian buffers adjacent to upland clear‐cut areas. 3. Leaf litter breakdown rates were not significantly different between reference and logged sites after logging, but litter breakdown was significantly different from year to year at all sites. 4. Significant post‐logging differences in aquatic invertebrate communities were detected at only one of the three logged sites. These differences were largely the result of increases in some leaf‐shredding stoneflies and a detritivorous mayfly and a decrease in a chironomid group 2–4 years after logging. This site where significant change was detected had the lowest intensity of riparian logging (average 10% removal) but the highest proportion of the catchment area that was clear cut (85%). 5.The post‐logging differences in invertebrate communities at this site were more related to catchment‐wide influences (e.g. weather patterns, water yield, possibly upland clearcutting) than to reach‐level disturbances from riparian logging. 6.The study indicates that partial‐harvest logging in riparian buffers at up to 50% removal should pose little risk of harm to leaf litter breakdown processes or aquatic invertebrate communities beyond any impacts that might arise from upland logging disturbance or catchment‐wide influences. However, the results should be viewed in the context of the natural disturbance (summer drought conditions) through the post‐logging assessment period of this study. Post‐logging summer drought conditions may have masked or confounded logging impacts on streams.     

Chemical composition of forest floor and consequences for nutrient availability after wildfire and harvesting in the boreal forest

In boreal forests of eastern Canada, wildfire has gradually been replaced by clearcut harvesting as the most extensive form of disturbance. Such a shift in disturbance may influence the chemical properties of the forest floor and its capacity to cycle and supply nutrients, with possible implications for forest productivity. We compared the effects of stem-only harvesting (SOH), whole-tree harvesting (WTH) and wildfire on the chemical composition of forest floor organic matter and nutrient availability for plants, 15–20 years after disturbance in boreal coniferous stands in Quebec (Canada). The forest floor on plots of wildfire origin was significantly enriched in aromatic forms of C with low solubility, whereas the forest floor from SOH and WTH plots was enriched with more soluble and labile C compounds. The forest floor of wildfire plots was also characterized by higher N concentration, but its high C:N and high concentration of ¹⁵N suggest that its N content could be recalcitrant and have a slow turnover rate. Total and exchangeable K were associated with easily degradable organic structures, whereas total and exchangeable Ca and Mg were positively correlated with the more recalcitrant forms of C. We suggest that the bulk of Ca and Mg cycling in the soil–plant system is inherited from the influx of exchangeable cations in the forest floor following disturbance. The buildup of Ca and Mg exchangeable reserves should be greater with wildfire than with harvesting, due to the sudden pulse of cation-rich ash and to the deposition of charred materials with high exchange capacity. This raises uncertainties about the long-term availability of Ca and Mg for plant uptake on harvested sites. In contrast, K availability should not be compromised by either harvesting or wildfire since it could be recycled rapidly through vegetation, litter and labile organic compounds.     

Effects of prescribed burning on leaves and flowering of Quercus garryana

Many woodland understories are managed with prescribed fire. While prescribed burns intended to manipulate understory vegetation and fuels usually do not cause excessive tree mortality, sublethal canopy damage may occur and can affect tree vigor and reproductive output. We monitored Quercus garryana trees in western Washington, USA with multiple canopy thermocouples during three prescribed burns. Peak temperatures recorded in tree canopies ranged from 36 to 649°C. We assessed leaf damage immediately after burning, and flower, leaf and acorn production in the following year in the vicinity of each thermocouple. Leaf scorch first occurred with peak thermocouple temperatures around 45°C, was variable up to 75°C, but above 75°C all leaves were killed. Buds, including their reproductive and leaf organs were more resistant to heat damage than leaves, but leaf scorch had predictive value in forecasting bud organ damage. Staminate and pistillate inflorescences and acorn production per bud decreased and bud mortality increased with maximum thermocouple temperature. In two burns where the highest peak temperatures reached 137°C, there was no difference in leaf production between burned and control plots in the spring following burning. However, no staminate or pistillate inflorescences were produced when thermocouple peak temperatures went above 55 or 68°C, respectively. While heat damage to bud organs was detected, production of reproductive organs was also curtailed at temperatures lower than could reasonably be attributed to heat damage. Thus, it is probable that some other fire-related factor, possibly smoke, was also involved.     

Effects of global climatic warming on the boreal forest

On the basis of the predictions of the global climatic warming induced by anthropogenic activities, as provided by climatologists, current state of knowledge regarding possible ecological consequences of the warming on the boreal biome was discussed. A 600 to 700 km northward advance of the biome along with the warming was predicted. Such a shift could take place for half a century or so, which would be an unprecedentedly fast rate of progression. This might cause a serious disorder in species composition of the biome, particularly in the boundary regions. As to the carbon sink or source issues, considerable uncertainties and knowledge gaps existed. Elevated temperature and CO₂ levels would stimulate photosynthesis to result in an increase of CO₂ uptake, while the temperature increase would promote decomposition of organic matter especially that stored in the soils to release CO₂ to the atmosphere. Behaviors of northern peat bogs, whereca. 700 Gt of organic matter was thought to be accumulated, would seriously affect the balance. However, overall ecosystematic carbon balance was yet to be fully studied. It was realized that multifunctional approaches needed to be developed so as to integrate pieces of various information into a holistic picture. Need for international collaboration research efforts was also addressed.     

Spider assemblages in the overstory, understory, and ground layers of managed stands in the western boreal mixedwood forest of Canada

Logging is the main human disturbance in the boreal forest; thus, understanding the effects of harvesting practices on biodiversity is essential for a more sustainable forestry. To assess changes in spider composition because of harvesting, samples were collected from three forest layers (overstory, understory, and ground) of deciduous and conifer dominated stands in the northwestern Canadian boreal mixedwood forest. Spider assemblages and feeding guild composition were compared between uncut controls and stands harvested to 20% retention. In total, 143 spider species were collected, 74 from the ground, 60 from the understory, and 71 from the overstory, and species composition of these three pools differed considerably among layers. Distinctive spider assemblages were collected from the canopy of each forest cover type but these were only slightly affected by harvesting. However, logging had a greater impact on the species composition in the understory and ground layers when compared with unharvested controls. Guild structure differed among layers, with wandering and sheet-weaving spiders dominant on the ground while orb-weaving and ambush spiders were better represented in the understory and overstory, respectively. Given the ecological importance of spiders and the expectation of faunal changes with increased harvesting, further efforts toward the understanding of species composition in higher strata of the boreal forest are needed.     

Effects of prescribed burning on plant available nutrients in dry heathland ecosystems

Heathland management is an important tool with which to modify ecosystem impacts caused by atmospheric nutrient deposition. Since changes in nutrient availability as a result of management measures affect the outcomes of heathland succession and species competition, studies on this issue are important from both a nature conservation and management point of view. This study reports the effects of prescribed burning on nutrient availability in dry heathland soils and the nutrient content of the two competing heathland species Calluna vulgaris and Deschampsia flexuosa, with particular reference to N and P. We hypothesise that winter prescribed burning leads to additional N availability, which enhances the importance of P in the context of nutrient limitation in heathland ecosystems. In the nature reserve “Lueneburg Heath” (NW Germany) we examined the availability of nutrients in the humus horizons and in the leachate as well as the relevant C:element ratios in Calluna and Deschampsia before and after a burning experiment. Our results show that prescribed burning resulted in drastically increased NH₄⁺ availability in the O-horizon. We observed only short-term effects (for NO₃⁻, PO₄³⁻, Mg) and insignificant effects on the availability of other nutrients (K, Ca). As a consequence of an increased nutrient availability in the humus horizons and a limited nutrient uptake by plants after burning, leaching increased significantly for N, Ca, K, and Mg after burning treatment. No significant changes were found in the foliar C:N ratios for either species after prescribed burning, although Deschampsia showed an increased deficiency for all the other nutrients, particularly for P, as expressed by increased foliar C:P and N:P ratios. By contrast, the nutrient content of Calluna did not change significantly, suggesting that prescribed burning favours the competitive capacity of Calluna as against Deschampsia. We assume that water shortage as a result of changes in the microclimate was mainly responsible for the deterioration of the nutrient content of Deschampsia. This gives Calluna a competitive advantage, enabling it to out-compete Deschampsia on burned heathlands, with respect to the key factor P-limitation.     

荒漠草原区柠条林平茬和牧草补播对地面节肢动物群落的影响

以荒漠草原区25年龄柠条林为研究对象,通过调查未平茬未补播、补播、平茬和既平茬又补播牧草的柠条林地中地面节肢动物群落特征,分析了补播和平茬及其交互作用对柠条林间和林下地面节肢动物个体数和类群数分布的影响.结果表明:在未平茬未补播牧草的林地中,林间地面节肢动物个体数和类群数均显著低于林下.与未平茬未补播牧草林地相比,补播、平茬和既平茬又补播牧草均显著增加柠条林间地面节肢动物个体数和类群数,而对柠条林下地面节肢动物个体数和类群数分布无显著影响,经过不同管理措施处理后,柠条林下与林间的地面节肢动物分布无显著差异.平茬和补播对柠条灌丛内外地面节肢动物分布影响具有类同效应,而且相互之间存在缓冲作用,补播与平茬处理、平茬与既平茬又补播处理、补播与既平茬又补播处理之间,林间和林下地面节肢动物的分布均无显著差异.在荒漠草原区柠条人工林中,平茬、补播和既平茬又补播牧草均可以显著提高柠条林地特别是林间地面节肢动物多样性,有利于退化草地生物多样性保护、生态系统恢复和柠条人工林的有效管理.     

Long term repeated prescribed burning increases evenness in the basidiomycete laccase gene pool in forest soils

Repeated prescribed burning alters the biologically labile fraction of nutrients and carbon of soil organic matter (SOM). Using a long-term (30 years) repeated burning experiment where burning has been carried out at a 2- or 4-year frequency, we analysed the effect of prescribed burning on gross potential C turnover rates and phenol oxidase activity in relation to shifts in SOM composition as observed using Fourier-transform infrared spectroscopy. In tandem, we assessed the genetic diversity of basidiomycete laccases. While the overall effect of burning was a decline in phenol oxidase activity, Shannon diversity and evenness of laccases was significantly higher in burned sites. Co-correspondence analysis of SOM composition and laccase operational taxonomic unit frequency data also suggested a strong correlation. While this correlation could indicate that the observed increase in laccase genetic diversity due to burning is due to increased resource diversity, a temporal replacement of the most abundant members of the assembly by an otherwise dormant pool of fungi cannot be excluded. As such, our results fit the intermediate disturbance hypothesis. Effects were stronger in plots burned in 2-year rotations, suggesting that the 4-year burn frequency may be a more sustainable practice to ensure the long-term stability of C cycling in such ecosystems.     

Decomposition rates of bryophytes in managed boreal forests: influence of bryophyte species and forest harvesting

The slow decomposition rate of boreal forest floor bryophytes contributes both to maintaining high soil C reserves as well as affecting conditions for tree growth by maintaining excessively high soil water content, cooling the soil and slowing nutrient cycles. In this study, mass loss of three bryophyte species (Pleurozium schreberi, Sphagnum capillifolium, S. fuscum) was measured in unharvested, partial cut and low-retention cut forest blocks. Mesh decomposition bags containing the three species and wood sticks were placed at two depths in colonies of either P. schreberi or S. capillifolium (environment) in the three harvest treatments and retrieved after two growing seasons. Mass loss was primarily related to substrate type (P. schreberi > S. capillifolium > wood sticks > S. fuscum) and secondarily to depth. Harvest treatment and environment (P. schreberi or S. capillifolium) only weakly affected sphagna mass loss. The weak effect of harvest treatment suggests that conditions created by low retention cuts do not to stimulate decomposition in this system and are not important enough to stimulate carbon loss, or to counteract paludification. On the other hand, the strong effect of bryophyte type indicates that conditions affecting bryophyte colonization and succession are of great importance in driving carbon and nutrient cycles.