Forest Floor Mass Research Articles

Seasonal changes in multi-scale spatial structure of soil pH and related parameters along a tropical dry evergreen forest slope

Seasonal changes in multi-scale spatial variation in soil chemical properties, which may be controlled simultaneously by biotic and abiotic factors, have not been studied in tropical dry forests. We evaluated the spatial variation of physico-chemical soil properties, plant litter and terrain attributes at multiple scales in a tropical dry evergreen forest using multivariate geostatistics. Soil samples were collected at different depths using nested interval sampling during 1- and 10-m intervals in both the wet and dry seasons. We measured pH, exchangeable cations (Ex-K + and Ex-Ca 2+), acidity (Ex-H + and Ex-Al 3+), particle size (clay and sand contents), and forest floor mass (O i and O a). Pronounced spatial variation in pH was observed in surface soil (0–5 cm) but not in deeper soil (5–55 cm). Multi-scale spatial structures with short (20 m) and long (86 m) ranges were observed in the auto- and cross-variograms of soil, litter and slope gradient. Pronounced multi-scale structures were observed simultaneously in pH and Ex-Ca 2+ both in the wet and dry seasons. Only a short-range structure was observed in Ex-K + and O a, whereas a long-range structure was pronounced in sand contents and slope gradients. Although the variograms had similar shapes between wet and dry seasons for almost all variables, the short-range structure of the cross-variogram between O a with pH and base cations was more pronouncedly developed in the wet season than in the dry season. Scale-dependent correlation coefficients suggest that a small-scale spatial variation in pH was connected to heterogeneous litter accumulation via base-cation input, whereas long-range spatial variation was simultaneously linked to particle size and slope gradient. This multivariate geostatistical approach applied within a stand detected biotic and abiotic factors controlling spatial variation in soil properties at both short and long distances.

Tree species and soil nutrient profiles in old-growth forests of the Oregon Coast Range

Old-growth forests of the Pacific Northwest provide a unique opportunity to examine tree species – soil relationships in ecosystems that have developed without significant human disturbance. We characterized foliage, forest floor, and mineral soil nutrients associated with four canopy tree species (Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco), western hemlock (Tsuga heterophylla (Raf.) Sarg.), western redcedar (Thuja plicata Donn ex D. Don), and bigleaf maple (Acer macrophyllum Pursh)) in eight old-growth forests of the Oregon Coast Range. The greatest forest floor accumulations of C, N, P, Ca, Mg, and K occurred under Douglas-fir, primarily due to greater forest floor mass. In mineral soil, western hemlock exhibited significantly lower Ca concentration and sum of cations (Ca + Mg + K) than bigleaf maple, with intermediate values for Douglas-fir and western redcedar. Bigleaf maple explained most species-based differences in foliar nutrients, displaying high concentrations of N, P, Ca, Mg, and K. Foliar P and N:P variations largely reflected soil P variation across sites. The four tree species that we examined exhibited a number of individualistic effects on soil nutrient levels that contribute to biogeochemical heterogeneity in these ecosystems. Where fire suppression and long-term succession favor dominance by highly shade-tolerant western hemlock, our results suggest a potential for declines in both soil Ca availability and soil biogeochemical heterogeneity in old-growth forests.

Complementary Models of Tree Species–Soil Relationships in Old-Growth Temperate Forests

Ecosystem-level studies identify plant–soil feedbacks as important controls on soil nutrient availability, particularly for nitrogen and phosphorus. Although site- and species-specific studies of tree species–soil relationships are relatively common, comparatively fewer studies consider multiple co-existing species in old-growth forests across a range of sites that vary in underlying soil fertility. We characterized patterns in forest floor and mineral soil nutrients associated with four common tree species across eight undisturbed old-growth forests in Oregon, USA, and used two complementary conceptual models to assess tree species–soil relationships. Plant–soil feedbacks that could reinforce site-level differences in nutrient availability were assessed using the context-dependent relationships model, whereby relative species-based differences in each soil nutrient diverged or converged as nutrient status changed across sites. Tree species–soil relationships that did not reflect strong feedbacks were evaluated using a site-independent relationships model, whereby forest floor and surface mineral soil nutrient pools differed consistently by tree species across sites, without variation in deeper mineral soils. We found that the organically cycled elements carbon, nitrogen, and phosphorus exhibited context-dependent differences among species in both forest floor and mineral soil, and most often followed a divergence model, whereby species differences were greatest at high-nutrient sites. These patterns are consistent with theory emphasizing biotic control of these elements through plant–soil feedback mechanisms. Site-independent species differences were strongest for pools of the weatherable cations calcium, magnesium, potassium, as well as phosphorus, in mineral soils. Site-independent species differences in forest floor nutrients were attributable to one species that displayed significantly greater forest floor mass accumulation. Our findings confirm that site-independent and context-dependent tree species-soil relationships occur simultaneously in old-growth temperate forests, with context-dependent relationships strongest for organically cycled elements, and site-independent relationships strongest for weatherable elements with inorganic cycling phases. These models provide complementary explanations for patterns of nutrient accumulation and cycling in mixed-species old-growth temperate forests.

Deer density effects on vegetation in aspen forest understories over site productivity and stand age gradients

Studies examining the interacting effects of ungulate herbivore pressure and site productivity on vegetation are mostly on grassland–grazing systems and have shown conflicting patterns. Here we examine the effects of deer density (>30 years differences in density between two landowners), site productivity (site index, SI) and stand age on subcanopy vegetation characteristics in 60 closed canopy, clear-cut origin Populus tremuloides dominated stands, Michigan, USA. Stand age effects were included because age varied among stands and can affect subcanopy vegetation patterns. Compared with fewer deer, stands with more deer had greater total forest floor vegetation mass, and its major components bracken fern (Pteridium aquilinum), sedge (mostly Carex pensylvanica) and trees/shrubs <0.25m tall, but lower forb mass and lower forest floor vegetation species richness and diversity. Deer density and SI had strong interacting effects on total forest floor mass, forb mass, and species richness. Forb mass increased with SI, but only in stands with fewer deer, whilst total vegetation mass was greater in stands with more deer at lower SI and declined with SI more sharply than for stands with fewer deer. Species richness increased with SI but more so at lower than higher deer density. Deer density and age had interacting effects on mass of trees/shrubs <0.25m tall and sedge. Compared with fewer deer, stands with more deer had greater sedge and tree/shrub mass, and sedge mass decreased and tree/shrub mass increased more sharply with age. In lower deer stands there was a dense subcanopy tree and shrub strata within and beyond the reach of deer 0.9–10m tall whereas in higher deer stands this vegetation layer was nearly absent. We conclude that higher deer browse pressure in early successional Populus stands (1) strongly limits the recruitment of woody stems to larger (>0.9m tall) size classes, which could affect long-term successional trajectories, and (2) diminishes forb density and species richness, especially at higher site productivity, but increases total forest floor vegetation mass (mostly bracken fern and sedge), especially at lower site productivity. Given associations of bracken fern and sedge with poorer and/or more open sites and assuming high palatability of forbs, this pattern may result from the combination of selective herbivory and higher light availability caused by limited recruitment of trees and shrubs to taller strata.

Soil Carbon After Midrotation Phosphorus Fertilization of Loblolly Pine (Pinus Taeda L.) Stands in East Texas

Abstract Pine plantation management impacts forests' role as a carbon (C) sink. We evaluated the status of forest floor mass and soil C content 8 years after phosphorus (P) fertilization of midrotation loblolly pine (Pinus taeda L.) plantations at seven sites of different drainage characteristics (excessively, well, and poorly drained) in East Texas. The impacts of stand age, site drainage class, and P fertilization were evaluated. Mean C accumulation in the forest floor near the end of a 30-year rotation was 10 Mg C ha−1. Phosphorus fertilization resulted in a mean increase of 3 Mg C ha−1 in the forest floor across drainage conditions. However, the direction and magnitude were markedly influenced by site drainage characteristics. Forest floor mass was inversely related to surface soil water content, with the highest C accumulation recorded on the excessively drained site (19 Mg ha−1 vs. mean of 8 ± 2 Mg ha−1 in all other sites). Yet, the poorly drained sites showed the highest forest floor C accumulation after fertilization (up to 77% increase). No significant differences in forest floor C and N composition or in soil C content were observed among sites or treatments. Carbon sequestration in the mineral soil was not affected by P fertilization and seems to be limited by the mineralogy and the coarse texture of the forest soils. Our results suggest that a site's inherent moisture regime dominated soil C accumulation processes. Because the relative P fertilization effect on forest floor mass accumulation followed the order poorly drained sites > well drained > excessively well drained, moisture limitations likely suppressed decomposition rates more than it did stand growth, resulting in overall decreases in forest floor C accumulation near the end of a 30-year rotation in the order excessively well-drained > well-drained > poorly drained sites.

Carbon resources, soil organisms, and nitrogen availability: Landscape patterns in a northern hardwood forest

The interactions among different components of detrital food webs are likely to influence carbon processing and linkages between C and nutrient cycling. Our objective was to identify potentially important interactions in detrital food webs at the Hubbard Brook Experimental Forest, NH, USA, by exploring relationships among C resources, microbial biomass, microarthropod abundance and nitrogen availability as they vary across the landscape. We found significant spatial variation in microarthropod abundance in the forest floor across elevation zones in two watersheds, with consistently higher abundance in low-elevation hardwoods and upper conifer zones and lower abundance in mid- and high-elevation hardwoods. The same pattern was observed in the Oe horizon for microbial biomass C and respiration but not for N transformations; however, no patterns were observed in the Oa horizon. Microarthropod abundance and microbial biomass C were significantly positively correlated, but neither were related to forest floor mass or to annual aboveground fine litterfall flux. Instead, a positive correlation with fine root biomass suggests that C supply from roots plays a key role in the fungal channel of the detrital food web of these forests. The lack of relationship between patterns of microarthropod abundance and net N mineralization leads us to hypothesize that spatial patterns of nitrogen availability are not closely linked to variation in carbon flow through the detrital food web, within this forested landscape. In contrast, microarthropod abundance and net N mineralization did exhibit similar interannual variation and may respond to the same temporal controls.

Spatial variability of an invasive earthworm (Amynthas agrestis) population and potential impacts on soil characteristics and millipedes in the Great Smoky Mountains National Park, USA

European and Asian earthworm invasions are widespread in North America. European earthworms especially are well-known to cause dramatic changes in ecosystems in northern, formerly glaciated portions of the continent, but less is known about the impacts of earthworm invasions in unglaciated areas inhabited by indigenous earthworms. We monitored fluctuations in the spatial extent of an Amynthas agrestis (Megascolecidae) population in the Great Smoky Mountains National Park in eastern Tennessee, USA. Two years of monthly growing-season observations revealed that the distribution of the earthworm population was dynamic, but overall distribution was closely linked to temperature and moisture with dramatic reductions of earthworm numbers associated with very dry conditions. In plots where A. agrestis were more often detected, we measured increased A-horizon soil aggregation and decreased thickness of the Oe/Oa-horizon. However, A. agrestis was not related to A-horizon microbial biomass, A-horizon C:N, Oi-horizon (litter) thickness, or mass of forest floor (O-horizon). Reductions in millipede species richness and density were associated with frequency of A. agrestis presence, possibly due to direct competition for food resources (Oe/Oa material). This evidence for potentially negative interactions between millipedes and A. agrestis suggests that expansion of the non-native earthworm into new habitats in the Park may alter soil physical properties and could pose a threat to native millipede diversity.

Comparison of soil respiration among three temperate forests in Changbai Mountains, China

CO2 efflux from forest soils is an important process in the global carbon cycle; however, effects of stand age and successional status remain uncertain. We compared soil respiration and its relationship to soil carbon content, forest floor mass, root biomass, soil temperature, and soil moisture content among three temperate forest ecosystems in Changbai Mountains, northeastern China, from 2003 to 2005. Forest types included an old-growth, mixed coniferous and broad-leaved primary forest (MN), a middle-aged, broad-leaved secondary forest (BL), and a young coniferous plantation forest (CP). Average annual soil CO2 efflux at BL (1477.9 ± 61.8 g C·m–2·year–1) was significantly higher than at CP (830.7 ± 48.7 g C·m–2·year–1) and MN (935.4 ± 53.3 g C·m–2·year–1). Differences in soil temperature among those sites were not statistically significant but contributed to the differences in annual CO2 efflux. In addition, the temperature response of soil CO2 efflux was higher at MN (Q10 = 2.78) than that at BL (Q10 = 2.17) and CP (Q10 = 2.02). Our results suggest that successional stage affects soil respiration by the differences in substrate quantity and quality, environmental conditions, and root respiration.

Arthropod Abundance and Diversity in a Lowland Tropical Forest Floor in Panama: The Role of Habitat Space vs. Nutrient Concentrations

ABSTRACTTropical forest floor characteristics such as depth and nutrient concentrations are highly heterogeneous even over small spatial scales and it is unclear how these differences contribute to patchiness in forest floor arthropod abundance and diversity. In a lowland tropical forest in Panama we experimentally increased litter standing crop by removing litter from five plots (L−) and adding it to five other plots (L+); we had five control plots. After 32 mo of treatments we investigated how arthropod abundance and diversity were related to differences in forest floor physical (mass, depth, water content) and chemical properties (pH, nutrient concentrations). Forest floor mass and total arthropod abundance were greater in L+ plots compared with controls. There were no treatment differences in nutrient concentrations, pH or water content of the organic horizons. Over all plots, the mass of the fermentation horizon (Oe) was greater than the litter horizon (Oi); arthropod diversity and biomass were also greater in the Oe horizon but nutrient concentrations tended to be higher in the Oi horizon. Arthropod abundance was best explained by forest floor mass, while arthropod diversity was best explained by phosphorus, calcium and sodium concentrations in the Oi horizon and by phosphorus concentrations in the Oe horizon. Differences in arthropod community composition between treatments and horizons correlated with phosphorus concentration and dry mass of the forest floor. We conclude that at a local scale, arthropod abundance is related to forest floor mass (habitat space), while arthropod diversity is related to forest floor nutrient concentrations (habitat quality).

Influence of variable organic matter retention on nutrient availability in a 10-year-old loblolly pine plantation

The effects of varying forest floor and slash retention at time of regeneration were evaluated 10 years after the establishment of a loblolly pine plantation near Millport, Alabama. Treatments included removing, leaving unaltered, or doubling the forest floor and slash material. Forest floor and litterfall mass and nutrient concentrations, available soil N, foliar nutrient concentrations and stand yield were all impacted by the treatments. Forest floor mass and nutrient contents in the doubled treatment were significantly greater than the other two treatments. The doubled treatment accumulated 25, 45 and 350% more forest floor mass and 56, 56, and 310% more N than the control treatment in the Oi, Oe, and Oa layers, respectively. The other nutrients followed similar patterns. Potentially mineralized NO 3 −-N in the mineral soil was also significantly higher in the doubled treatment. The positive effect of doubling the forest floor on soil N availability was reflected in greater foliage production, 30% more litterfall and 25% more stand yield for this treatment. This study shows that increasing the forest floor retention has resulted in increased nutrient availability and improved tree growth.

Can structural and functional characteristics be used to identify riparian zone width in southern Appalachian headwater catchments?

We characterized structural and functional attributes along hillslope gradients in headwater catchments. We endeavored to identify parameters that described significant transitions along the hillslope. On each of four catchments, we installed eight 50 m transects perpendicular to the stream. Structural attributes included woody and herbaceous vegetation; woody debris and forest floor mass, nitrogen (N) and carbon (C); total soil C and N; litterfall amount and quality by species; and microclimatic conditions. Functional attributes included litter decomposition, soil microarthropods, soil CO2evolution, soil solution chemistry, and soil extractable N. Forest floor mass, N and C, and soil depth increased with distance from the stream and transitioned between 10 and 20 m. In contrast, litterfall N rate (kilograms of nitrogen per hectare per day), downed woody debris, soil A-horizon C and N, and soil solution NO3concentration all decreased with distance, and exhibited significant transitions. Certain overstory species were more abundant in the uplands than near the stream. Herbaceous diversity and richness were similar across the hillslope, but species distributions varied in response to hillslope moisture content. Taken together, these results suggest that at 10–20 m from the stream, transitions occur that separate riparian from upland conditions and may provide valuable insight into riparian zone definition.

Invasive insects impact forest carbon dynamics

AbstractInvasive insects can impact ecosystem functioning by altering carbon, nutrient, and hydrologic cycles. In this study, we used eddy covariance to measure net CO2 exchange with the atmosphere (NEE), and biometric measurements to characterize net ecosystem productivity (NEP) in oak‐ and pine‐dominated forests that were defoliated by Gypsy moth (Lymantria dispar L.) in the New Jersey Pine Barrens. Three years of data were used to compare C dynamics; 2005 with minimal defoliation, 2006 with partial defoliation of the canopy and understory in a mixed stand, and 2007 with complete defoliation of an oak‐dominated stand, and partial defoliation of the mixed and pine‐dominated stands. Previous to defoliation in 2005, annual net CO2 exchange (NEEyr) was estimated at −187, −137 and −204 g C m−2 yr−1 at the oak‐, mixed‐, and pine‐dominated stands, respectively. Annual NEP estimated from biometric measurements was 108%, 100%, and 98% of NEEyr in 2005 for the oak‐, mixed‐, and pine‐dominated stands, respectively. Gypsy moth defoliation strongly reduced fluxes in 2006 and 2007 compared with 2005; NEEyr was −122, +103, and −161 g C m−2 yr−1 in 2006, and +293, +129, and −17 g C m−2 yr−1 in 2007 at the oak‐, mixed‐, and pine‐dominated stands, respectively. At the landscape scale, Gypsy moths defoliated 20.2% of upland forests in 2007. We calculated that defoliation in these upland forests reduced NEEyr by 41%, with a 55% reduction in the heavily impacted oak‐dominated stands. ‘Transient’ disturbances such as insect defoliation, nonstand replacing wildfires, and prescribed burns are major factors controlling NEE across this landscape, and when integrated over time, may explain much of the patterning of aboveground biomass and forest floor mass in these upland forests.

Decomposition of pure and mixed foliage litter in a young lodgepole pine – Sitka alder stand in the central interior of British Columbia

A 7-year litterbag experiment examined mass loss dynamics of Sitka alder ( Alnus viridis (Chaix) DC. subsp. sinuata (Regel) A. Löve &amp; D. Löve) and lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Watson) foliage litter in a young stand dominated by these species in the Sub-Boreal Spruce biogeoclimatic zone of central British Columbia. Although mass loss was initially faster for alder litter, the amounts of mass remaining for alder, pine, and mixed pine–alder (50:50 by mass) litter began to converge after 2 years. Mass loss by the mixed litter differed significantly from that predicted from the behaviour of its pure components, but the mechanisms responsible could not be determined from this experiment. Limit values for the projected maximum extent of mass loss ranged from 63.5% (alder) to 76.9% (pine), with the latter value considerably below estimates reported for lodgepole pine in Scandinavia. Despite these differing limit values, forest floor mass measurements prior to the experiment did not show significantly higher organic matter accumulations under long-established alder clumps on this site.

Structure and composition of streamside management zones following reproduction cutting in shortleaf pine stands

Streamside management zones (SMZs) in the Ouachita Mountains of Arkansas and Oklahoma are frequently established along headwater ephemeral and intermittent streams to protect water quality, provide wildlife habitat, and increase landscape diversity. To better understand the function of these riparian forest corridors, we characterized the tree density and composition, forest floor mass, and downed woody debris volume within SMZs located in undisturbed, mature, upper mid-slope shortleaf pine stands and then compared these attributes to those in upland portions of these stands. In addition to evaluate the impact of upland forest harvesting on these riparian corridors, we compared the amounts and distribution of forest floor, downed woody debris (DWD), snags, and windthrows in SMZs within shortleaf pine stands that had been clearcut, had a shelterwood harvest, and had no recent management activity (uncut stands). Total tree and hardwood basal area was significantly higher (4.4 and 4.2 m 2 ha −1) while forest floor mass was significantly lower (0.5 kg m −2) in the SMZs than in the upland portion of the undisturbed stands. Five years following the reproduction cuttings tree basal area, DWD volume, and forest floor mass within SMZs did not significantly differ among stands that had or had not been harvested. Snag density was significantly lower within SMZs that occurred in clearcut stands compared to those in the uncut or shelterwood stands. Harvesting activities that retain few or no residual trees appear to increase the degradation of snags. This study provided evidence that clearcutting may also increase the risk of windthrow in SMZs as well. There was little difference in the distribution of forest floor within SMZs regardless of whether the stand was harvested or the type of harvesting that occurred in the stand. However, DWD amounts were higher near the SMZs edge than in the interior of the SMZs with the greatest differences in distributions in stands that were clearcut.

Effects of prescribed fire in mixed oak forests of the southern Appalachians: forest floor, soil, and soil solution nitrogen responses

Abstract We examined nutrient cycling responses to prescribed fire on three sub-mesic, mixed-oak sites located in the Blue Ridge Physiographic province of the southern Appalachian Mountains: Alarka Laurel Branch (AL), Robin Branch (RB), and Roach Mill Branch (RM). Each study site was located within a sub-watershed that drained a first order stream. Our objective was to quantify the effects of prescribed burning on forest floor mass, nitrogen and carbon pools; and soil and soil water available nitrogen. Each site included a burned and unburned control area; both burned and control areas were sampled before and after burning. Within each plot, we sampled forest floor mass, carbon and nitrogen, soil and soil solution nitrate (NO3-N) and ammonium (NH4-N) concentrations before and after the prescribed burns. All prescribed fires were conducted in the dormant season and were low to moderate intensity. All sites lost a significant amount of forest floor mass due to burning; 82 to 91% of the Oi layer and 26 to 46...

Effects of tree species and clear-cut forestry on forest-floor characteristics in adjacent temperate forests in northern Spain

The litter layer (L) and fermented humified layer (FH) in forest floor under European beech ( Fagus sylvatica L.), pedunculate oak ( Quercus robur L.), and radiata pine ( Pinus radiata D. Don) were studied in northern Spain. Recovery from heavy mechanization was also assessed in a chronosequence from two adjacent pine plantations (3 and 16 years old). Interspecific differences in forest-floor mass and mass distribution were related to litter quality and decomposition rates. In the L layer, acid-insoluble residue/N, C/N, and C/P ratios were lower, and pH and Ca contents higher in the deciduous stands than in the pine stand. The microbial respiration rate was higher and functional diversity lower in the pine stand than in the deciduous stands, without differences in microbial biomass. Cross-polarization, magic angle spinning 13C nuclear magnetic resonance and proximate analysis revealed similar C functional groups in beech and pine stands. Forest floor was still absent 3 years after heavy mechanization, and after 16 years, it was 50% less abundant than in the mature stand. Microbial respiration rate, biomass, and diversity were similar in the L layer in 16-year-old and mature pine stands, but in the FH layer, microbial-community diversity remained low after 16 years. The results underline the effects of forest management on C transformations in the forest-floor layers. These effects may be evident even after 16 years of heavy mechanization.

The national Fire and Fire Surrogate study: effects of fuel reduction methods on forest vegetation structure and fuels

Changes in vegetation and fuels were evaluated from measurements taken before and after fuel reduction treatments (prescribed fire, mechanical treatments, and the combination of the two) at 12 Fire and Fire Surrogate (FFS) sites located in forests with a surface fire regime across the conterminous United States. To test the relative effectiveness of fuel reduction treatments and their effect on ecological parameters we used an information-theoretic approach on a suite of 12 variables representing the overstory (basal area and live tree, sapling, and snag density), the understory (seedling density, shrub cover, and native and alien herbaceous species richness), and the most relevant fuel parameters for wildfire damage (height to live crown, total fuel bed mass, forest floor mass, and woody fuel mass). In the short term (one year after treatment), mechanical treatments were more effective at reducing overstory tree density and basal area and at increasing quadratic mean tree diameter. Prescribed fire treatments were more effective at creating snags, killing seedlings, elevating height to live crown, and reducing surface woody fuels. Overall, the response to fuel reduction treatments of the ecological variables presented in this paper was generally maximized by the combined mechanical plus burning treatment. If the management goal is to quickly produce stands with fewer and larger diameter trees, less surface fuel mass, and greater herbaceous species richness, the combined treatment gave the most desirable results. However, because mechanical plus burning treatments also favored alien species invasion at some sites, monitoring and control need to be part of the prescription when using this treatment.

Variation of soil respiration at three spatial scales: Components within measurements, intra-site variation and patterns on the landscape

Soil respiration is an important component of terrestrial carbon cycling and can be influenced by many factors that vary spatially. This research aims to determine the extent and causes of spatial variation of soil respiration, and to quantify the importance of scale on measuring and modeling soil respiration within and among common forests of Northern Wisconsin. The potential sources of variation were examined at three scales: [1] variation among the litter, root, and bulk soil respiration components within individual 0.1 m measurement collars, [2] variation between individual soil respiration measurements within a site (<1 m to 10 m), and [3] variation on the landscape caused by topographic influence (100 m to 1000 m). Soil respiration was measured over a two-year period at 12 plots that included four forest types. Root exclusion collars were installed at a subset of the sites, and periodic removal of the litter layer allowed litter and bulk soil contributions to be estimated by subtraction. Soil respiration was also measured at fixed locations in six northern hardwood sites and two aspen sites to examine the stability of variation between individual measurements. These study sites were added to an existing data set where soil respiration was measured in a random, rotating, systematic clustering which allowed the examination of spatial variability from scales of <1 m to 100+ m. The combined data set for this area was also used to examine the influence of topography on soil respiration at scales of over 1000 m by using a temperature and moisture driven soil respiration model and a 4 km 2 digital elevation model (DEM) to model soil moisture. Results indicate that, although variation of soil respiration and soil moisture is greatest at scales of 100 m or more, variation from locations 1 m or less can be large (standard deviation during summer period of 1.58 and 1.28 μmol CO 2 m −2 s −1, respectively). At the smallest of scales, the individual contributions of the bulk soil, the roots, and the litter mat changed greatly throughout the season and between forest types, although the data were highly variable within any given site. For scales of 1–10 m, variation between individual measurements could be explained by positive relationships between forest floor mass, root mass, carbon and nitrogen pools, or root nitrogen concentration. Lastly, topography strongly influenced soil moisture and soil properties, and created spatial patterns of soil respiration which changed greatly during a drought event. Integrating soil fluxes over a 4 km 2 region using an elevation dependent soil respiration model resulted in a drought induced reduction of peak summer flux rates by 37.5%, versus a 31.3% when only plot level data was used. The trends at these important scales may help explain some inter-annual and spatial variability of the net ecosystem exchange of carbon.

SIMULATED ATMOSPHERIC NO3− DEPOSITION INCREASES SOIL ORGANIC MATTER BY SLOWING DECOMPOSITION

Presently, there is uncertainty regarding the degree to which anthropogenic N deposition will foster C storage in the N-limited forests of the Northern Hemisphere, ecosystems which are globally important sinks for anthropogenic CO2. We constructed organic matter and N budgets for replicate northern hardwood stands (n = 4) that have received ambient (0.7-1.2 g N x m(-2) x yr(-1) and experimental NO3- deposition (ambient plus 3 g NO3(-)-N x m(-2) x yr(-1)) for a decade; we also traced the flow of a 15NO3- pulse over a six-year period. Experimental NO3- deposition had no effect on organic matter or N stored in the standing forest overstory, but it did significantly increase the N concentration (+19%) and N content (+24%) of canopy leaves. In contrast, a decade of experimental NO3- deposition significantly increased amounts of organic matter (+12%) and N (+9%) in forest floor and mineral soil, despite no increase in detritus production. A greater forest floor (Oe/a) mass under experimental NO3- deposition resulted from slower decomposition, which is consistent with previously reported declines in lignolytic activity by microbial communities exposed to experimental NO3- deposition. Tracing 15NO3- revealed that N accumulated in soil organic matter by first flowing through soil microorganisms and plants, and that the shedding of 15N-labeled leaf litter enriched soil organic matter over a six-year duration. Our results demonstrate that atmospheric NO3- deposition exerts a direct and negative effect on microbial activity in this forest ecosystem, slowing the decomposition of aboveground litter and leading to the accumulation of forest floor and soil organic matter. To the best of our knowledge, this mechanism is not represented in the majority of simulation models predicting the influence of anthropogenic N deposition on ecosystem C storage in northern forests.

Soil disturbance following four wheel rubber skidder logging on the steep trail in the north mountainous forest of Iran

Reduced soil porosity, water content, forest floor mass and rutting are four common disturbances during timber harvesting and skidding. The present study was conducted to evaluate effects of skid trail slope and traffic frequency on soil disturbance in a temperate mountainous forest. Four levels of traffic (3, 7, 14 and 20 passes of a rubber skidder HSM 904), and three levels of slopes (gentle <10%, moderate 10–20%, steep >20%) were applied in three replicates consequently, 36 plots with 10 m long by 4 m wide were utilized in the study. All samples were taken from four randomized 4 m-transects in each plot across the wheel track perpendicular to the direction of travel with 2 m buffer zone between lines to avoid interactions. Results showed that during skidding total porosity, soil water and Forest floor decreased significantly ( P ≤ 0.05). Average total porosity on the treatments of >20% slope reduced significantly ( P ≤ 0.05) in comparison with those of <20% slope through all level of traffics. The lowest total porosity was identified in treatment with slope >20% and 14 passes by soil compaction. Soil water content was decreased up to 45% after skidding in comparison with undisturbed area. There was a negative correlation between water content and skid trail slope. The forest floor mass decreased on steep trail during skidding in all traffics. The amount of forest floor mass on the treatments under the 7 passes and slopes of >20% treatments (437.6 kg ha −1) was significantly ( P ≤ 0.05) lower than those under the 14 passes and slopes of <10% (841.4 kg ha −1). Rut depth were recorded in 14 passes and increased with slope (18 cm < 10%, 27.5 cm in 10–20% and 34.5 cm in >20%). In 3 passes, increase of skid trail slope from 10% to 20% corresponded to a decrease of 15%, 22%, and 67% in total porosity, water content and forest floor mass, respectively. The results indicated that slope steepness had a strong effect on the soil physical properties. In the present study, the skidder extracted different volume of logs during skidding operation. In order to obtain better interpretation the skidder must carry a fixed load during operation.