Forest Floor Carbon Research Articles

Forest floor carbon exchange of a boreal black spruce forest in eastern North America

Abstract. This study reports continuous automated measurements of forest floor carbon (C) exchange over feathermoss, lichen, and sphagnum micro-sites in a black spruce forest in eastern North America during snow-free periods over three years. The response of soil respiration (Rs-auto) and forest floor photosynthesis (Pff) to environmental factors was determined. The seasonal contributions of scaled up Rs-auto adjusted for spatial representativeness (Rs-adj) and Pff (Pff-eco) relative to that of total ecosystem respiration (Re) and photosynthesis (Peco), respectively, were also quantified. Shallow (5 cm) soil temperature explained 67–86% of the variation in Rs-auto for all ground cover types, while deeper (50 and 100 cm) soil temperatures were related to Rs-auto only for the feathermoss micro-sites. Base respiration was consistently lower under feathermoss, intermediate under sphagnum, and higher under lichen during all three years. The Rs-adj/Re ratio increased from spring through autumn and ranged from 0.85 to 0.87 annually for the snow-free period. The Rs-adj/Re ratio was negatively correlated with the difference between air and shallow soil temperature and this correlation was more pronounced in autumn than summer and spring. Maximum photosynthetic capacity of the forest floor (Pff-max) saturated at low irradiance levels (~200 μmol m−2 s−1) and decreased with increasing air temperature and vapor pressure deficit for all three ground cover types, suggesting that Pff was more limited by desiccation than by light availability. Pff-max was lowest for sphagnum, intermediate for feathermoss, and highest for lichen for two of the three years. Pff normalized for light peaked at air temperatures of 5–8°C, suggesting that this is the optimal temperature range for Pff. The Pff-eco/Peco ratio varied from 13 to 24% over the snow-free period and reached a minimum in mid-summer when both air temperature and Peco were at their maximum. On an annual basis, Pff-eco accounted for 17–18% of Peco depending on the year and the snow-free season totals of Pff-eco were 23–24% that of Rs-adj.

Humus conditions of stands with different proportion of Douglas fir in the Hůrky Training Forest District and Křtiny Training Forest Enterprise

The paper presented evaluates reserves and chemical composition of forest floor of three stands of Douglas fir, spruce and spruce with beech at acid sites (3K) in the Hůrky Training Forest District (TFD) and at a meso-trophic site (4H) in the Křtiny Training Forest Enterprise (TFE). The aim of the study was to evaluate: (<I>i</I>) reserves of forest floor, (<I>ii</I>) soil reaction, (<I>iii</I>) total content of carbon and nitrogen for the forest floor layers, <I>iv</I>) C/N ratio, and (<I>v</I>) the content of dissolved organic carbon (DOC). The lowest reserve occurs in the Douglas fir stand at a mesotrophic site (25.0 t/ha), the highest accumulation occurs in the spruce stand and in the spruce/beech stand at an acid site (79.4–79.6 t/ha). The soil reaction is strongly acid to acid. The most favourable values of pH for forest floor and soil at acid (4.6 ± 0.4) and mesotrophic sites (5.2 ± 0.4) occur in the Douglas fir stand. It also corresponds to C/N ratio (23–26). The highest reserve of carbon in forest floor occurs at the acid site 34.7 t/ha (1.3 t/ha nitrogen). The lowest reserve of carbon in forest floor at the mesotrophic site amounts to 8.5 t/ha (0.4 t/ha nitrogen). The higher content of DOC in stands at acid sites can result in a higher risk of soil acidification. Keywords: spe

Detrital carbon pools in temperate forests: magnitude and potential for landscape-scale assessment

Reliably estimating carbon storage and cycling in detrital biomass is an obstacle to carbon accounting. We examined carbon pools and fluxes in three small temperate forest landscapes to assess the magnitude of carbon stored in detrital biomass and determine whether detrital carbon storage is related to stand structural properties (leaf area, aboveground biomass, primary production) that can be estimated by remote sensing. We characterized these relationships with and without forest age as an additional predictive variable. Results depended on forest type. Carbon in dead woody debris was substantial at all sites, accounting for ∼17% of aboveground carbon, whereas carbon in forest floor was substantial in the subalpine Rocky Mountains (36% of aboveground carbon) and less important in northern hardwoods of New England and mixed forests of the upper Midwest (∼7%). Relationships to aboveground characteristics accounted for between 38% and 59% of the variability in carbon stored in forest floor and between 21% and 71% of the variability in carbon stored in dead woody material, indicating substantial differences among sites. Relating dead woody debris or forest floor carbon to other aboveground characteristics and (or) stand age may, in some forest types, provide a partial solution to the challenge of assessing fine-scale variability.

Forest floor fuel consumption and carbon emissions in Canadian boreal forest fires

In many forest types, over half of the total stand biomass is located in the forest floor. Carbon emissions during wildland fire are directly related to biomass (fuel) consumption. Consumption of forest floor fuel varies widely and is the greatest source of uncertainty in estimating total carbon emissions during fire. We used experimental burn data (59 burns, four fuel types) and wildfire data (69 plots, four fuel types) to develop a model of forest floor fuel consumption and carbon emissions in nonpeatland standing-timber fuel types. The experimental burn and wildfire data sets were analyzed separately and combined by regression to provide fuel consumption models. Model variables differed among fuel types, but preburn fuel load, duff depth, bulk density, and Canadian Forest Fire Weather Index System components at the time of burning were common significant variables. The regression R2 values ranged from 0.206 to 0.980 (P < 0.001). The log–log model for all data combined explained 79.5% of the regression variation and is now being used to estimate annual carbon emissions from wildland fire. Forest floor carbon content at the wildfires ranged from 40.9% to 53.9%, and the carbon emission rate ranged from 0.29 to 2.43 kg·m–2.

Forest floor carbon dioxide fluxes within an upland‐peatland complex in the Western Boreal Plain, Canada

AbstractThis study examined the midday (10:00 to 16:00) growing season (April to October) surface cover CO2 relationships with different canopy closures in a forested upland–peatland–pond complex in the Western Boreal Plain (WBP), north‐central Alberta, Canada. Large differences were observed among the forest floors of landscape units with different canopy covers with respect to midday total respiration [(Rtot) = vegetation respiration (Rveg) + soil respiration (Rsoil)] and gross ecosystem production (GEP), and the seasonal pattern of GEP and Rtot. Highest rates of Rtot followed the general progression of riparian > upland > open peatland > covered peatland, with high Rsoil contributions. Strong correlations were observed between C:N, soil temperature, moisture and Rtot. Photosynthetic active radiation (PAR) controlled the GEP, which was highest in the open and covered peatland. GEP and Rtot were highest in the middle of the growing season when soil and air temperatures were warmest; in addition Rveg contributed more to Rtot during this time. However, Rsoil dominated the flux.This study demonstrated that controlling environmental factors on CO2 exchange cannot necessarily be extrapolated among land cover units within a sub‐humid region, such as Canada's WBP. In addition, forest floors of different land cover units, and microtopography should be taken into account when discussing understory contributions to CO2 exchange. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of tree species on carbon stocks in forest floor and mineral soil and implications for soil carbon inventories

Forest soil organic carbon (SOC) and forest floor carbon (FFC) stocks are highly variable. The sampling effort required to assess SOC and FFC stocks is therefore large, resulting in limited sampling and poor estimates of the size, spatial distribution, and changes in SOC and FFC stocks in many countries. Forest SOC and FFC stocks are influenced by tree species. Therefore, quantification of the effect of tree species on carbon stocks combined with spatial information on tree species distribution could improve insight into the spatial distribution of forest carbon stocks. We present a study on the effect of tree species on FFC and SOC stock for a forest in the Netherlands and evaluate how this information could be used for inventory improvement. We assessed FFC and SOC stocks in stands of beech ( Fagus sylvatica), Douglas fir ( Pseudotsuga menziesii), Scots pine ( Pinus sylvestris), oak ( Quercus robur) and larch ( Larix kaempferi). FFC and SOC stocks differed between a number of species. FFC stocks varied between 11.1 Mg C ha −1 (beech) and 29.6 Mg C ha −1 (larch). SOC stocks varied between 53.3 Mg C ha −1 (beech) and 97.1 Mg C ha −1 (larch). At managed locations, carbon stocks were lower than at unmanaged locations. The Dutch carbon inventory currently overestimates FFC stocks. Differences in carbon stocks between conifer and broadleaf forests were significant enough to consider them relevant for the Dutch system for carbon inventory.

Long-Term Trends in Stream Nitrate Concentrations and Losses Across Watersheds Undergoing Recovery from Acidification in the Czech Republic

Stream nitrogen (N) export and nitrate \((\text {NO}^-_3) \) concentration were measured at 14 forested watersheds (GEOMON network) in the Czech Republic between 1994 and 2005. In the last several decades, emissions of sulfur (S) and N compounds have decreased throughout much of Europe. In the Czech Republic, atmospheric deposition of S has decreased substantially since the beginning of 1990s, whereas N deposition remains largely unchanged at most sites. The mean dissolved inorganic nitrogen (DIN) streamwater export ranged from 0.2 to 12.2 kg ha−1 y−1 at the GEOMON sites. Despite decades of elevated N deposition, 44–98% of DIN inputs to these watersheds were retained or denitrified, and many watersheds showed seasonal variation in nitrate concentrations. Dissolved organic N export was quantified in 1 year only and ranged from 0.05 to 3.5 kg ha−1 y−1. Spatial variability in DIN export among watersheds was best explained by spatial variability in average acidic deposition, particularly S deposition (R 2 = 0.81, P < 0.001); DIN input and forest floor carbon:nitrogen (C/N) also provided significant explanatory power. DIN export was strongly influenced by the forest floor C/N ratio and depth of the forest floor soils (R 2 = 0.72, P < 0.001). The only variable that predicted variations in forest floor C/N (R 2 = 0.32, P < 0.05) among watersheds was S deposition. Forest floor depth was also related to deposition variables, with S deposition providing the most explanatory power (R 2 = 0.50, P < 0.01). Variation in forest floor depth was also associated with climatic factors (precipitation and temperature). Temporal variability in DIN export was primarily associated with changes in acidic deposition over time; S deposition explained 41% of variability in DIN exports among all watersheds and years. Extensive acidification of forested watersheds was associated with the extraordinarily high S inputs to much of the Czech Republic during earlier decades. We hypothesize that recovery from acidification has led to improved tree health as well as enhanced microbial activity in the forest floor. As these watersheds move into a new regime with dramatically lower sulfur inputs, we expect continued declines in nitrate output.

Fire, thinning, and the carbon economy: Effects of fire and fire surrogate treatments on estimated carbon storage and sequestration rate

Changes in estimated standing stocks of carbon (C) in vegetation, forest floor, dead wood, and mineral soil for the fire and fire surrogate (FFS) network sites were evaluated in relation to the application of prescribed fire, mechanical treatments designed as surrogates for prescribed fire, and the combination of mechanical treatment and fire. Pre-treatment C stocks and changes in C stocks over two intervals (pre-treatment to first post-treatment year and first post-treatment to a 2nd, 3rd, or 4th post-treatment year, depending on site) were evaluated using meta-analytical methods. Total C storage across the network averaged 185 ± 8 (standard error) Mg C ha −1, of which 45% was in vegetation, 38% in soil organic matter, 10% in the forest floor and 7% in dead wood. C stored in vegetation was not significantly affected by prescribed fire, but decreased ∼30 Mg ha −1 as the result of mechanical or mechanical + fire treatment; in contrast, forest floor C storage was reduced by ∼1–7 Mg ha −1 by fire or mechanical + fire treatment, but unaffected by mechanical treatment alone. Neither dead wood C nor soil organic C was significantly affected by the treatments. At the network scale, total ecosystem C was not significantly affected by fire, though four individual sites did exhibit significant C losses to fire. Mechanical treatment, with or without fire, produced significant reductions of 16–32 Mg ha −1 during the first post-treatment year, but this was partially balanced by enhanced net C uptake of ∼12 Mg ha −1 during the subsequent 1–3 years. In terms of C storage and uptake, western coniferous forests responded differently to the FFS treatments than did eastern deciduous, coniferous, and mixed forests, suggesting that optimal management for fire, harvesting, and C sequestration may differ between regions.

Carbon and nitrogen in forest floor and mineral soil under six common European tree species

The knowledge of tree species effects on soil C and N pools is scarce, particularly for European deciduous tree species. We studied forest floor and mineral soil carbon and nitrogen under six common European tree species in a common garden design replicated at six sites in Denmark. Three decades after planting the six tree species had different profiles in terms of litterfall, forest floor and mineral soil C and N attributes. Three groups were identified: (1) ash, maple and lime, (2) beech and oak, and (3) spruce. There were significant differences in forest floor and soil C and N contents and C/N ratios, also among the five deciduous tree species. The influence of tree species was most pronounced in the forest floor, where C and N contents increased in the order ash = lime = maple < oak = beech ≪ spruce. Tree species influenced mineral soil only in some of the sampled soil layers within 30 cm depth. Species with low forest floor C and N content had more C and N in the mineral soil. This opposite trend probably offset the differences in forest floor C and N with no significant difference between tree species in C and N contents of the whole soil profile. The effect of tree species on forest floor C and N content was primarily attributed to large differences in turnover rates as indicated by fractional annual loss of forest floor C and N. The C/N ratio of foliar litterfall was a good indicator of forest floor C and N contents, fractional annual loss of forest floor C and N, and mineral soil N status. Forest floor and litterfall C/N ratios were not related, whereas the C/N ratio of mineral soil (0–30 cm) better indicated N status under deciduous species on rich soil. The results suggest that European deciduous tree species differ in C and N sequestration rates within forest floor and mineral soil, respectively, but there is little evidence of major differences in the combined forest floor and mineral soil after three decades.

Concentrations and fluxes of dissolved organic carbon in an age-sequence of white pine forests in Southern Ontario, Canada

We determined concentrations and fluxes of dissolved organic carbon (DOC) in precipitation, throughfall, forest floor and mineral soil leachates from June 2004 to May 2006 across an age-sequence (2-, 15-, 30-, and 65-year-old) of white pine (Pinus strobus L.) forests in southern Ontario, Canada. Mean DOC concentration in precipitation, throughfall, leachates of forest floor, Ah-horizon, and of mineral soil at 1 m depth ranged from ∼2 to 7, 9 to 18, 32 to 88, 20 to 66, and 2 to 3 mg DOC L−1, respectively, for all four stands from April (after snowmelt) through December. DOC concentration in forest floor leachates was highest in early summer and positively correlated to stand age, aboveground biomass and forest floor carbon pools. DOC fluxes via precipitation, throughfall, and leaching through forest floor and Ah-horizon between were in the range of ∼1 to 2, 2 to 4, 0.5 to 3.5, and 0.1 to 2 g DOC m−2, respectively. DOC export from the forest ecosystem during that period through infiltration and groundwater discharge was estimated as ∼7, 4, 3, and 2 g DOC m−2 for the 2-, 15-, 30-, and 65-year-old sites, respectively, indicating a decrease with increasing stand age. Laboratory DOC sorption studies showed that the null-point DOC concentration fell from values of 15 to 60 mg DOC L−1 at 0 to 5 cm to <15 mg DOC L−1 at 50 cm. Specific ultraviolet light absorption at 254 nm (SUVA254) increased from precipitation and throughfall to a maximum in forest floor and decreased with mineral soil depth. No age-related pattern was observed for SUVA254 values. DOC concentration in forest floor soil solutions showed a positive exponential relationship with soil temperature, and a negative exponential relationship with soil moisture at all four sites. Understanding the changes and controls of DOC concentrations, chemistry, and fluxes at various stages of forest stand development is necessary to estimate and predict DOC dynamics on a regional landscape level and to evaluate the effect of land-use change.

Effects of land use change on soil carbon cycling in the conterminous United States from 1900 to 2050

We developed matrices representing historical area transitions between forest and other land uses. We projected future transitions on the basis of historical transitions and econometric model results. These matrices were used to drive a model of changes in soil and forest floor carbon stocks. Our model predicted net carbon emission from 1900 until 1982, then sequestration until 2030, with little subsequent change. However, the northeast region showed substantial carbon sequestration from 1900 to the present. From 1990 to 2004, afforestation caused sequestration averaging 17 Tg C yr−1: 6 Tg C yr−1 in soil and 11 Tg C yr−1 in forest floor. Deforestation caused emission averaging 12 Tg C yr−1: 3 Tg C yr−1 from soil and 9 Tg C yr−1 from forest floor. However, these effects were only 5% of the total change in carbon stocks in all forestland.

Seasonal and spatial variability of soil respiration in four Sitka spruce stands

We investigated the causes for the seasonal and spatial variation of soil respiration in a first rotation Sitka spruce chronosequence composed of four age classes (10, 15, 31, and 47 year old) in Central Ireland. The study aimed at identifying easily determinable environmental parameters that explained the variation in soil respiration rates. The variation in temperature and soil water content influenced the seasonal trend observed in the spatial variability of soil respiration. The highest coefficients of variation in soil respiration were observed during autumn drought, while lower coefficients were generally observed during periods with highest soil respiration rates. On average, the sampling strategy of 30 sampling points per stand was adequate to obtain an average rate of soil respiration within 20% of its actual value at the 95% confidence level. Significantly higher soil respiration rates were observed at locations with high accumulation of organic matter and in collars established in close vicinity to tree stems. The organic layer thickness was the only variable that yielded significant regressions for explaining spatial variation in soil respiration in all the stands. Correlation analyses between the studied variables and soil respiration suggested the relative importance of heterotrophic and autotrophic components differed in their annual contribution to total soil respiration at each forest stand. Multiple regression analyses were used to assess the relative importance of primary temporal and spatial controls over soil respiration. Soil temperature and organic layer thickness explained most of the variance of soil respiration for the different sampling periods, while soil water content had a weaker effect as well as a different influence on soil respiration depending on the time of the year. The strong linear correlation between forest floor carbon and soil carbon stock further confirmed organic layer thickness as an integrative factor encompassing the effect of soil carbon pools on soil respiration. Moreover, its inclusion in the multiple regression analyses overrode the influence of both distance and fine root biomass. Overall, a multiple linear regression model driven by easily determinable environmental variables such as soil temperature, organic thickness, soil water content, soil bulk density, and soil organic carbon concentration allowed us to explain 54% of total variance of soil respiration over the different stand ages for the entire year (P < 0.05). Our results show that the adoption of an adequate sampling strategy, and the determination of some key environmental variables may help to explain a large proportion of total variation of soil respiration over the entire rotation length of afforested ecosystems.

Biomass and composition of understory vegetation and the forest floor carbon stock across Siberian larch and mountain birch chronosequences in Iceland

Changes in understory biomass, forest floor carbon (C) stock and vegetation composition were studied in six age-classes of Siberian larch (Larix sibirica) and two age-classes of native birch (Betula pubescens) in Iceland. The ground vegetation was less in the larch during the thicket stage and in the old-growth birch compared to a treeless pasture. Understory biomass was strongly related to canopy gap fraction across forest stands (P < 0.001), but not to soil pH or soil C/N ratio. Increased mass of dead wood and alterations in vegetation composition increased the forest floor C-stock of older forests. The forest floor had reached as high C-stock as the pasture's ground vegetation in ca. 50 years in the managed larch plantations and in ca. hundred years in the unmanaged birch forest. This study clearly shows the importance of which time-step is used when changes in forest floor C-stocks are computed for afforestation areas. (Less)

A model for the investigation of long-term carbon dynamics in boreal forests of western Canada: I. Model development and validation

To interpret field studies of forest floor dynamics in the continental boreal forest of western Canada and identify areas for future research, we developed a boreal forest carbon dynamics model based on the decay algorithms of the CENTURY model. Forest floor and mineral soil carbon pools were separately represented. We quantified detrital inputs for nine separate components of live vegetation using published data to relate litterfall to biomass and regression models to relate biomass of each component to stand age and other significant variables. The regression models were developed from observed C stocks of each component using data from 80 forest stands forming six chronosequences. The six chronosequences were for two of the principal species in this region, Pinus banksiana and Populus tremuloides, in each of three distinct climatic areas (approximately represented by the BOREAS SSA, BOREAS NSA and Wood Buffalo National Park). We carried out extensive sensitivity testing and validation tests against a wide range of data, including forest floor carbon stocks measured in the six chronosequences. The validation tests highlighted the need for better data in several areas, particularly root litter, woody detritus dynamics, coniferous foliage litter and nitrogen budgets. Nevertheless, the model performed well against the available data and was judged adequate for its purposes.

Annual soil respiration in broadleaf forests of northern Wisconsin: influence of moisture and site biological, chemical, and physical characteristics

Soil temperature and moisture influence soil respiration at a range of temporal and spatial scales. Although soil temperature and moisture may be seasonally correlated, intra and inter-annual variations in soil moisture do occur. There are few direct observations of the influence of local variation in species composition or other stand/site characteristics on seasonal and annual variations in soil moisture, and on cumulative annual soil carbon release. Soil climate and soil respiration from twelve sites in five different forest types were monitored over a 2-year period (1998–1999). Also measured were stand age, species composition, basal area, litter inputs, total above-ground wood production, leaf area index, forest floor mass, coarse and fine root mass, forest floor carbon and nitrogen concentration, root carbon and nitrogen concentration, soil carbon and nitrogen concentration, coarse fraction mass and volume, and soil texture. General soil respiration models were developed using soil temperature, daily soil moisture, and various site/soil characteristics. Of the site/soil characteristics, above-ground production, soil texture, roots + forest floor mass, roots + forest floor carbon:nitrogen, and soil carbon:nitrogen were significant predictors of soil respiration when used alone in respiration models; all of these site variables were weakly to moderately correlated with mean site soil moisture. Daily soil climate data were used to estimate the annual release of carbon (C) from soil respiration for the period 1998–1999. Mean annual soil temperature did not differ between the 2 years but mean annual soil moisture was approximately 9% lower in 1998 due to a summer drought. Soil C respired during 1998 ranged from 8.57 to 11.43 Mg C ha−1 yr−1 while the same sites released 10.13 and 13.57 Mg C ha−1 yr−1 in 1999; inter-annual differences of 15.41 and 15.73%, respectively. Among the 12 sites studied, we calculated that the depression of soil respiration linked to the drought caused annual differences of soil respiration from 11.00 to 15.78%. Annual estimates of respired soil C decreased with increasing site mean soil moisture. Similarly, the difference of respired carbon between the drought and the non-drought years generally decreased with increasing site mean soil moisture.

Stand restoration burning in oak–pine forests in the southern Appalachians: effects on aboveground biomass and carbon and nitrogen cycling

Understory prescribed burning is being suggested as a viable management tool for restoring degraded oak–pine forest communities in the southern Appalachians yet information is lacking on how this will affect ecosystem processes. Our objectives in this study were to evaluate the watershed scale effects of understory burning on total aboveground biomass, and the carbon and nitrogen pools in coarse woody debris (CWD), forest floor and soils. We also evaluated the effects of burning on three key biogeochemical fluxes; litterfall, soil CO 2 flux and soil net nitrogen mineralization. We found burning significantly reduced understory biomass as well as the carbon and nitrogen pools in CWD, small wood and litter. There was no significant loss of carbon and nitrogen from the fermentation, humus and soil layer probably as the result of low fire intensity. Burning resulted in a total net loss of 55 kg ha −1 nitrogen from the wood and litter layers, which should be easily replaced by future atmospheric deposition. We found a small reduction in soil CO 2 flux immediately following the burn but litterfall and net nitrogen mineralization were not significantly different from controls throughout the growing season following the burn. Overall, the effects of burning on the ecosystem processes we measured were small, suggesting that prescribed burning may be an effective management tool for restoring oak–pine ecosystems in the southern Appalachians.

Controls on the release of dissolved organic carbon and nitrogen from a deciduous forest floor investigated by manipulations of aboveground litter inputs and water flux

Although dissolved organic matter (DOM) released from the forest floor plays a crucial role in transporting carbon and major nutrients through the soil profile, its formation and responses to changing litter inputs are only partially understood. To gain insights into the controlling mechanisms of DOM release from the forest floor, we investigated responses of the concentrations and fluxes of dissolved organic carbon (DOC) and nitrogen (DON) in forest floor leachates to manipulations of throughfall (TF) flow and aboveground litter inputs (litter removal, litter addition, and glucose addition) at a hardwood stand in Bavaria, Germany. Over the two-year study period, litter manipulations resulted in significant changes in C and N stocks of the uppermost organic horizon (Oi). DOC and DON losses via forest floor leaching represented 8 and 11% of annual litterfall C and N inputs at the control, respectively. The exclusion of aboveground litter inputs caused a slight decrease in DOC release from the Oi horizon but no change in the overall leaching losses of DOC and DON in forest floor leachates. In contrast, the addition of litter or glucose increased the release of DOC and DON either from the Oi or from the lower horizons (Oe + Oa). Net releases of DOC from the Oe + Oa horizons over the entire manipulation period were not related to changes in microbial activity (measured as rates of basal and substrate-induced respiration) but to the original forest floor depths prior to manipulation, pointing to the flux control by the size of source pools rather than a straightforward relationship between microbial activity and DOM production. In response to doubled TF fluxes, net increases in DOM fluxes occurred in the lower forest floor, indicating the presence of substantial pools of potentially soluble organic matter in the Oe + Oa horizons. In contrast to the general assumption of DOM as a leaching product from recent litter, our results suggest that DOM in forest floor leachates is derived from both newly added litter and older organic horizons through complex interactions between microbial production and consumption and hydrologic transport.

Pine Forest Floor Carbon Accumulation in Response to N and PK Additions: Bomb 14 C Modelling and Respiration Studies

The addition of nitrogen via deposition alters the carbon balance of temperate forest ecosystems by affecting both production and decomposition rates. The effects of 20 years of nitrogen (N) and phosphorus and potassium (PK) additions were studied in a 40-year-old pine stand in northern Sweden. Carbon fluxes of the forest floor were reconstructed using a combination of data on soil 14C, tree growth, and litter decomposition. N-only additions caused an increase in needle litterfall, whereas both N and PK additions reduced long-term decomposition rates. Soil respiration measurements showed a 40% reduction in soil respiration for treated compared to control plots. The average age of forest floor carbon was 17 years. Predictions of future soil carbon storage indicate an increase of around 100% in the next 100 years for the N plots and 200% for the NPK plots. As much as 70% of the increase in soil carbon was attributed to the decreased decomposition rate, whereas only 20% was attributable to increased litter production. A reduction in decomposition was observed at a rate of N addition of 30 kg C ha−1 y−1, which is not an uncommon rate of N deposition in central Europe. A model based on the continuous-quality decomposition theory was applied to interpret decomposer and substrate parameters. The most likely explanations for the decreased decomposition rate were a fertilizer-induced increase in decomposer efficiency (production-to-assimilation ratio), a more rapid rate of decrease in litter quality, and a decrease in decomposer basic growth rate.

Detecting Change in Forest Floor Carbon

Changes over time in forest soils are important to global C balance and to local ecosystem function. Detecting change in C storage in the forest floor is hampered by high variability and the use of study designs that are not adequate to statistically detect change. Using estimates of variability from previous forest floor studies, mostly conducted in the northern USA and Canada, we conducted statistical power analyses to assess the ability of such studies to detect various magnitudes of change in forest floor C. The studies we surveyed were unable to detect statistically significant changes in forest floor C or mass smaller than 15 to 20%. Studies that remeasure plots or sites (i.e., paired designs) have greater statistical power to detect changes than those in which experimental units are independently located for the two sampling dates. The causal mechanisms of forest floor change influence the magnitude of the change, and accordingly our ability to detect such changes. The direct effects of climate change may be too small to be detectable by current designs, but larger changes in forest floor mass resulting from forest management, changes in tree species, changes in fire regime, or the introduction of earthworms are more likely to be detectable. With paired resampling and more efficient allocation of sampling effort, it should be possible for future studies to detect smaller changes.

The contribution of bryophytes to the carbon exchange for a temperate rainforest

AbstractBryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp‐broadleaved forest in New Zealand, dominated by 100–400‐year‐old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth &lt;30 mm), but dense, canopy that experienced elevated CO2 partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light‐saturated rates of net CO2 exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis – whole‐plant respiration) per unit ground area at saturating irradiance was 1.9 μmol m−2 s−1 and in one microcosm, the net rate of CO2 exchange was negative (respiration). CO2 exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air‐dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO2 uptake per unit ground area.The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m−2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of −1010 g m−2. To put this in perspective of the magnitude of the components of CO2 exchange for the forest floor, the bryophyte layer reclaimed an amount of CO2 equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ∼11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO2 exchange may be highly responsive to rapid changes in climate.