Dissolved organic carbon and nitrogen leaching from Scots pine, Norway spruce and silver birch stands in southern Sweden

The effects of three common tree species - Scots pine, Norway spruce and silver birch - on leaching of dissolved organic carbon and dissolved nitrogen were studied in an experimental forest with podzolised soils in southern Sweden. We analyzed soil water collected with lysimeters and modeled water fluxes to estimate dissolved C and N fluxes. Specific UV absorbance (SUVA) was analyzed to get information about the quality of dissolved organic matter leached from the different stands. Under the O horizon, DOC concentrations and fluxes in the birch stands were lower than in the spruce and pine stands; annual fluxes were 21 g m⁻² y⁻¹ for birch and 38 g m⁻² y⁻¹ and 37 g C m⁻² y⁻¹ for spruce and pine, respectively. Under the B horizon, annual fluxes for all tree species ranged between 3 and 5 g C m⁻² y⁻¹, implying greater loss of DOC in the mineral soil in the coniferous stands than in the birch stands. We did not find any effect of tree species on the quality of the dissolved organic matter, as measured by SUVA, indicating that the chemical composition of the organic matter was similar in leachates from all three tree species. Substantial amounts of nitrogen was leached out of the soil profile at the bottom of the B horizon from the pine and birch stands, whereas the spruce stands seemed to retain most of the nitrogen in the soil. These differences in N leaching have implications for soil N budgets.     

Differences in soil properties in adjacent stands of Scots pine, Norway spruce and silver birch in SW Sweden

Soil properties were compared in adjacent 50-year-old Norway spruce, Scots pine and silver birch stands growing on similar soils in south-west Sweden. The effects of tree species were most apparent in the humus layer and decreased with soil depth. At 20-30 cm depth in the mineral soil, species differences in soil properties were small and mostly not significant. Soil C, N, K, Ca, Mg, and Na content, pH, base saturation and fine root biomass all significantly differed between humus layers of different species. Since the climate, parent material, land use history and soil type were similar, the differences can be ascribed to tree species. Spruce stands had the largest amounts of carbon stored down to 30 cm depth in mineral soil (7.3 kg C m⁻²), whereas birch stands, with the lowest production, smallest amount of litterfall and lowest C:N ratio in litter and humus, had the smallest carbon pool (4.1 kg C m⁻²), with pine intermediate (4.9 kg C m⁻²). Similarly, soil nitrogen pools amounted to 349, 269, and 240 g N m⁻² for spruce, pine, and birch stands, respectively. The humus layer in birch stands was thin and mixed with mineral soil, and soil pH was highest in the birch stands. Spruce had the thickest humus layer with the lowest pH.     

Impact of timber harvesting on litterfall inputs and benthic coarse particulate organic matter (CPOM) storage in a small stream draining a eucalyptus plantation

Eucalyptus plantations have a short rotation cycle and harvesting occurs every 12-15 years, with the potential to modify the ecological integrity of the small streams draining the harvested areas through the reduction of litterfall inputs. We studied litterfall inputs and benthic coarse particulate organic matter (CPOM) storage in a small headwater stream draining a eucalyptus (Eucalyptus globulus Labill.) plantation before and after clear felling of the plantation. We hypothesized that wood harvesting will result in a reduction of CPOM inputs and storage in the stream. Litterfall inputs ranged 530-700 g m⁻² y⁻¹ and were approximately halved (200-320 g m⁻² y⁻¹) after the harvesting of the eucalyptus trees. Bark and woody materials showed the largest reduction. Leaf inputs were initially reduced sharply, but, during the second year after the harvest, they recovered to about 90% of the values observed before the harvesting. Harvesting of the eucalyptus plantation caused an increase of benthic CPOM storage to 535 g m⁻², but this was a temporary effect and these materials were washed downstream of the study reach. One year after the harvesting, benthic CPOM was reduced below 15 g m⁻². Bark, twigs and other woody residues generated during the preparation of the logs for transportation were retained within the study site and represented the main component (>90%) of the benthic CPOM after timber harvesting. However, 2 years after the harvesting, low inputs of these materials caused an overall reduction of in-stream retention and residence time of benthic CPOM. Amount and composition of benthic CPOM changed quickly in response to alterations of the riparian forest, so we propose the use of CPOM as an indicator of the impact of forestry activities on the ecological functioning of small streams.     

Controls of litter quality on the carbon sink in soils through partitioning the products of decomposing litter in a forest succession series in South China

Through the long-term measurement and development of a method for partitioning the products of decomposing litter, the impact of chemical components of forest debris on soil organic carbon (SOC) accumulation was studied in a forest succession series in South China. We quantified how litter quality is strongly correlated with the partitioning of respiration, dissolved organic carbon (DOC) and fragments of decomposing litter. In the succession sequence of 60-year-old pine forest (PF), to 80-year-old mixed pine and evergreen broadleaved forest (MF) to more than 400-year-old monsoon evergreen broadleaved forest (MEBF), the litter C/N ratios and lignin contents were gradually decreasing, which in turn were correlated with increasing litter decomposition constants (k-values), gradually shortening residence times of standing litter pool. And, 53.5%, 65.6% and 76.2% of the gravimetric litter mass losses were going belowground through both DOC and fragmentation. Correspondingly, the SOC accumulation rates in the top 20 cm of mineral soils for the three forests from 1978 to 2008 were 26 ± 4, 33 ± 5 and 67 ± 5 g C m⁻² yr⁻¹, respectively. Results of the study support the idea that in order to increase carbon sequestration in soils and long-term functional ability of forest ecosystems to act as carbon sinks, “Kyoto Forests” should be designed and reconstructed with a high diversity of broadleaved species, especially containing nitrogen-fixing trees.     

Does tree species composition control soil organic carbon pools in Mediterranean mountain forests?

We compared soil organic carbon (SOC) stocks and stability under two widely distributed tree species in the Mediterranean region: Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus pyrenaica Willd.) at their ecotone. We hypothesised that soils under Scots pine store more SOC and that tree species composition controls the amount and biochemical composition of organic matter inputs, but does not influence physico-chemical stabilization of SOC. At three locations in Central Spain, we assessed SOC stocks in the forest floor and down to 50 cm in the mineral in pure and mixed stands of Pyrenean oak and Scots pine, as well as litterfall inputs over approximately 3 years at two sites. The relative SOC stability in the topsoil (0-10 cm) was determined through size-fractionation (53 μm) into mineral-associated and particulate organic matter and through KMnO₄-reactive C and soil C:N ratio.Scots pine soils stored 95-140 Mg ha⁻¹ of C (forest floor plus 50 cm mineral soil), roughly the double than Pyrenean oak soils (40-80 Mg ha⁻¹ of C), with stocks closely correlated to litterfall rates. Differences were most pronounced in the forest floor and uppermost 10 cm of the mineral soil, but remained evident in the deeper layers. Biochemical indicators of soil organic matter suggested that biochemical recalcitrance of soil organic matter was higher under pine than under oak, contributing as well to a greater SOC storage under pine. Differences in SOC stocks between tree species were mainly due to the particulate organic matter (not associated to mineral particles). Forest conversion from Pyrenean oak to Scots pine may contribute to enhance soil C sequestration, but only in form of mineral-unprotected soil organic matter.     

Influences of forest tending works on carbon distribution and cycling in a Pinus densiflora S. et Z. stand in Korea

This study was conducted to determine carbon (C) dynamics following forest tending works (FTW) which are one of the most important forest management activities conducted by Korean forest police and managers. We measured organic C storage (above- and below-ground biomass C, forest floor C, and soil C at 50 cm depth), soil environmental factors (soil CO₂ efflux, soil temperature, soil water content, soil pH, and soil organic C concentration), and organic C input and output (litterfall and litter decomposition rates) for one year in FTW and non-FTW (control) stands of approximately 40-year-old red pine (Pinus densiflora S. et Z.) forests in the Hwangmaesan Soopkakkugi model forest in Sancheonggun, Gyeongsangnam-do, Korea. This forest was thinned in 2005 as a representative FTW practice. The total C stored in tree biomass was significantly lower (P < 0.05) in the FTW stand (40.17 Mg C ha⁻¹) than in the control stand (64.52 Mg C ha⁻¹). However, C storage of forest floor and soil layers measured at four different depths was not changed by FTW, except for that at the surface soil depth (0–10 cm). The organic C input due to litterfall and output due to needle litter decomposition were both significantly lower in the FTW stand than in the control stand (2.02 Mg C ha⁻¹ year⁻¹ vs. 2.80 Mg C ha⁻¹ year⁻¹ and 308 g C kg⁻¹ year⁻¹ vs. 364 g C kg⁻¹ year⁻¹, respectively, both P < 0.05). Soil environmental factors were significantly affected (P < 0.05) by FTW, except for soil CO₂ efflux rates and organic C concentration at soil depth of 0–20 cm. The mean annual soil CO₂ efflux rates were the same in the FTW (0.24 g CO₂ m⁻² h⁻¹) and control (0.24 g CO₂ m⁻² h⁻¹) stands despite monthly variations of soil CO₂ efflux over the one-year study period. The mean soil organic C concentration at a soil depth of 0–20 cm was lower in the FTW stand (81.3 g kg⁻¹) than in the control stand (86.4 g kg⁻¹) but the difference was not significant (P > 0.05). In contrast, the mean soil temperature was significantly higher, the mean soil water content was significantly lower, and the soil pH was significantly higher in the FTW stand than in the control stand (10.34 °C vs. 8.98 °C, 48.2% vs. 56.4%, and pH 4.83 vs. pH 4.60, respectively, all P < 0.05). These results indicated that FTW can influence tree biomass C dynamics, organic C input and output, and soil environmental factors such as soil temperature, soil water content and soil pH, while soil C dynamics such as soil CO₂ efflux rates and soil organic C concentration were little affected by FTW in a red pine stand.     

Seasonal and spatial variation of nitrogen dynamics in the litter and surface soil layers on a tropical dry evergreen forest slope

Seasonal and spatial variability of litterfall and NO₃⁻ and NH₄⁺ leaching from the litter layer and 5-cm soil depth were investigated along a slope in a tropical dry evergreen forest in northeastern Thailand. Using ion exchange resin and buried bag methods, the vertical flux and transformation of inorganic nitrogen (N) were observed during four periods (dry, early wet, middle wet, and late wet seasons) at 15 subplots in a 180-m × 40-m rectangular plot on the slope. Annual N input via litterfall and inorganic N leached from the litter layer and from 5-cm depth soil were 12.5, 6.9, and 3.7 g N m⁻² year⁻¹, respectively, whereas net mineralization and the inorganic N pool in 0–5-cm soil were 7.1 g N m⁻² year⁻¹ and 1.4 g N m⁻², respectively. During the early wet season (90 days), we observed 82% and 74% of annual NO₃⁻ leaching from the litter layer and 5-cm soil depth, respectively. Higher N input via leaf litterfall in the dry season and via precipitation in the early wet season may have led to higher NO₃⁻ leaching rate from litter and surface soil layers during the early wet season. Large spatial variability in both NO₃⁻ vertical flux and litterfall was also observed within stands. Small-scale spatial patterns of total N input via litterfall were significantly correlated with NO₃⁻ leaching rate from the surface soil layer. In tropical dry evergreen forests, litterfall variability may be crucial to the remarkable seasonal changes and spatial variation in annual NO₃⁻ vertical flux in surface soil layers.     

Factors causing variation in fine root biomass in forest ecosystems

Fine roots form one of the most significant components contributing to carbon cycling in forest ecosystems. We study here the effect of variation in root diameter classes, sampling depth and the inclusion of understorey vegetation root biomass in fine root biomass (FRB) estimates. The FRB estimates for different forest biomes are updated using a database of 512 forest stands compiled from the literature. We also investigate the relationships between environmental or forest stand variables and fine root biomass (≤2 mm in diameter) at the stand (g m⁻²) and tree level (g tree⁻¹). The FRB estimates extrapolated for the whole rooting depth were 526 ± 321 g m⁻², 775 ± 474 g m⁻² and 776 ± 518 g m⁻² for boreal, temperate and tropical forests, respectively, and were 26-67% higher than those based on the original sampling depths used. We found significant positive correlations between ≤1 and ≤2 mm diameter roots and between ≤2 and ≤5 mm roots. The FRB estimates, standardized to the ≤2 mm diameter class, were 34-60% higher and 25-29% smaller than those standardized to the ≤1 mm and ≤5 mm diameter classes, respectively. The FRB of the understorey vegetation accounted for 31% of the total FRB in boreal forests and 20% in temperate forests. The results indicate that environmental factors (latitude, mean annual precipitation, elevation, temperature) or forest stand factors (life form, age, basal area, density) can not explain a significant amount of the variation in the total FRB and a maximum of 30% that in the FRB of trees at the stand level, whereas the mean basal area of the forest stand can explain 49% of the total FRB and 79% of the FRB of trees at the tree level.     

Modelling soil organic carbon turnover in improved fallows in eastern Zambia using the RothC-26.3 model

Scarcity of simple and reliable methods of estimating soil organic carbon (SOC) turnover and lack of data from long-term experiments make it difficult to estimate attainable soil C sequestration in tropical improved fallows. Testing and validating existing and widely used SOC models would help to determine attainable C storage in fallows. The Rothamsted C (RothC) model, therefore, was tested using empirical data from improved fallows at Msekera in eastern Zambia. This study (i) determined the effects of nitrogen fixing tree (NFT) species on aboveground organic C inputs to the soil and SOC stocks, (ii) estimated annual net organic C inputs to the soil using the RothC, and (iii) tested the performance of RothC model using empirical data from improved fallows. Soil samples (0–20 cm) were collected from coppicing and non-coppicing fallow experiments in October 2002 for determination of SOC by LECO CHN-1000 analyser. Data on surface litter, maize and weed biomasses, and on weather, were supplied by the Zambia/ICRAF Agroforestry Project. Measured SOC stocks to 20 cm depth ranged from 32.2 to 37.8 t ha⁻¹ in coppicing fallows and 29.5 to 30.1 t ha⁻¹ in non-coppicing fallows compared to 22.2–26.2 t ha⁻¹ in maize monoculture systems. Coppicing fallows accumulated more SOC (680–1150 g m⁻² year⁻¹) than non-coppicing fallows (410–789 g m⁻² year⁻¹). While treatments with NFTs accumulated more SOC than NFT-free systems, SOC stocks increased with increasing tree biomass production and tree rotation. For food security and C sequestration, coppicing fallows are a potentially viable option.     

Optimizing the joint production of timber and bilberries

Berries and mushrooms are increasingly appreciated products of Finnish forests. Therefore, there is a need to integrate them in silvicultural planning. Bilberry (Vaccinium myrtillus L.) is an economically important wild berry that is widely collected for household consumption and sale in North Karelia, Finland. In this study, bilberry yield models developed recently were included in a stand growth simulator and the joint production of timber and bilberry was optimized by maximizing soil expectation value (SEV) with 3% discounting rate, assuming that 75% of the bilberry yield is harvested. The effect of bilberry production on the optimal stand management increased with increasing bilberry price. With high bilberry prices (4–8 € kg⁻¹) it was optimal to manage the mixed stand of Scots pine, Norway spruce and birch, and the pure stand of Norway spruce so as to promote bilberry production. In the Scots pine stand, where bilberry yields are higher, bilberry production affected optimal stand management already with a price of 2 € kg⁻¹. Compared to timber production, joint production led to longer rotation lengths, higher thinning intensities, more frequent thinnings, and higher share of Scots pine in the mixed stand. The contribution of bilberries to the total SEV increased with increasing bilberry price and discounting rate. In the mixed stand and pine stand the SEV of bilberry production, calculated with 3% discounting rate, exceeded the SEV of timber production when bilberry price was 4 € kg⁻¹.With 4% discounting rate this happened already with bilberry price of 2 € kg⁻¹. It was concluded that forest management which promotes bilberry yields is the most profitable in pine stands where the potential bilberry yields are high.     

Mixed litter decomposition in a managed Missouri Ozark forest ecosystem

We monitored the decomposition of mixed leaf litter (Quercus spp., Carya spp., and Pinusechinata) in a Missouri Ozark forest eight years after experimental harvest. Leaf litter mass losses and changes in carbon chemistry (extractive, acid soluble, and acid insoluble fractions) were measured over 32 months in field incubations to determine the effects of litter composition and stand manipulation on decomposition and nitrogen (N) concentration in the remaining litter. The decay (k) rate over this period ranged between 0.39 (±0.010) and 0.51 (±0.002) year⁻¹ for oak, oak–hickory, and oak–pine litter. There were significant main effects of stand manipulation (p = 0.03) and litter type (p < 0.01) on decay. Mass losses of oak and oak–hickory litter were 7% (p = 0.02) and 4% (p = 0.04) higher on harvested stands than controls, respectively. Mass loss of oak–hickory litter was 3% faster than oak–pine (p = 0.03) and 6% faster than oak (p = 0.02) litter on control stands, whereas the oak–hickory litter mass loss was 5% higher than oak litter on harvested stands (p = 0.01). The decay (k) rate had a linear relationship with initial leaf litter nitrogen content and lignin-to-N ratio. The nitrogen concentration in remaining litter had a nonlinear relationship to cumulative mass loss suggesting an exogenous source of N. In summary, this study demonstrated significant effects of timber harvest and litter mixtures on decomposition and N dynamics in a managed Missouri Ozark forest.     

Effects of [CO2] and nitrogen on morphological and biomass traits of white birch (Betula papyrifera) seedlings

To investigate the interactive effects of CO₂ concentration ([CO₂]) and nitrogen supply on the growth and biomass of boreal trees, white birch seedlings (Betula papyrifera) were grown under ambient (360 μmol mol⁻¹) and elevated [CO₂] (720 μmol mol⁻¹) with five nitrogen supply regimes (10, 80, 150, 220, and 290 μmol mol⁻¹) in greenhouses. After 90 days of treatment, seedling height, root-collar diameter, biomass of different organs, leaf N concentration, and specific leaf area (SLA) were measured. Significant interactive effects of [CO₂] and N supply were found on height, root-collar diameter, leaf biomass, stem biomass and total biomass, stem mass ratio (SMR), and root mass ratio (RMR), but not on root mass, leaf mass ratio (LMR), leaf to root ratio (LRR), or leaf N concentration. The CO₂ elevation generally increased all the growth and biomass parameters and the increases were generally greater at higher levels of N supply or higher leaf N concentration. However, the CO₂ elevation significantly reduced SLA (13.4%) and mass-based leaf N concentration but did not affect area-based leaf N concentration. Increases in N supply generally increased the growth and biomass parameters, but the relationships were generally curvilinear. Based on a second order polynomial model, the optimal leaf N concentration was 1.33 g m⁻² for height growth under ambient [CO₂] and 1.52 g m⁻² under doubled [CO₂]; 1.48 g m⁻² for diameter under ambient [CO₂] and 1.64 g m⁻² under doubled [CO₂]; 1.29 g m⁻² for stem biomass under ambient [CO₂] and 1.43 g m⁻² under doubled [CO₂]. The general trend is that the optimal leaf N was higher at doubled than ambient [CO₂]. However, [CO₂] did not affect the optimal leaf N for leaf and total biomass. The CO₂ elevation significantly increased RMR and SMR but decreased LMR and LRR. LMR increased and RMR decreased with the increasing N supply. SMR increased with increase N supply up to 80 μmol mol⁻¹ and then leveled off (under elevated [CO₂]) or stated to decline (under ambient [CO₂]) with further increases in N supply. The results suggest that the CO₂ elevation increased biomass accumulation, particularly stem biomass and at higher N supply. The results also suggest that while modest N fertilization will increase seedling growth and biomass accumulation, excessive application of N may not stimulate further growth or even result in growth decline.     

Fine root production and turnover in forest ecosystems in relation to stand and environmental characteristics

The production and turnover of fine roots (diameter ?2 mm) contributes significantly to carbon cycling in forest ecosystems. We compiled an up-to-date global database covering 186 stands from the literature and estimated fine root production (FRP) and fine root turnover (FRT) for boreal, temperate and tropical forests in order to study the relationships between FRP or FRT and environmental and stand variables. FRP for all plants (trees + understorey) was 311 ± 259 (n = 39), 428 ± 375 (n = 71) and 596 ± 478 g m⁻² a⁻¹ (n = 32) in the boreal, temperate and tropical forests, respectively, and the corresponding annual FRT rates were 0.77 ± 0.70, 1.21 ± 1.04 and 1.44 ± 0.76, respectively. When the FRP and FRT of trees were estimated separately for boreal and temperate forests the differences between the two biomes were insignificant. The mean FRP of trees for the two biomes combined was 306 ± 240 g m⁻² a⁻¹ (n = 86) and the annual FRT was 1.31 ± 1.43. Fine root biomass (FRB) was the most significant factor explaining the variation in FRP, and more so at the tree level than at the stand level, explaining 53% of the variation in FRP for trees at the tree level. The corresponding proportions at the stand level were 21% for all plants and 12% for trees. Latitude, mean annual temperature and annual precipitation each explained <20% of the variation in FRP or FRT. Fine root production and FRT estimates are highly dependent on the species included in the sampling, the sampling depth and the methods used for estimating FRP or calculating FRT. The results indicate that the variation in FRP on a global scale can be explained to a higher degree if we focus on tree roots separately from the roots of the understorey vegetation and on FRP at the tree level instead of FRP at the stand level or on FRT.     

Establishing grazing and grazing-excluded patches increases plant and invertebrate diversity in a Mediterranean oak woodland

Grazing is a global, dominant land use affecting biodiversity and ecosystem processes. In Mediterranean ecosystems grazing is a major ecological and evolutionary driver but, surprisingly, there is little information on the use of grazing as a tool to manage biodiversity in these ecosystems. We conducted an experiment to assess if establishing grazing and small scale grazing-excluded areas would increase plant and invertebrate diversity in a Mediterranean evergreen oak woodland. Plant community traits were different between treatments. Biomass of herbs (176.7 ± 18.3 g m⁻²vs 100.4 ± 10.6 g m⁻²) and litter (291.0 ± 38.3 g m⁻²vs 186.8 ± 26.4 g m⁻²), as well as the total cover of legumes (0.83 ± 0.05 vs 0.91 ± 0.03) were higher, and the proportion of bare ground (0.83 ± 0.05 vs 0.91 ± 0.03) was lower, in ungrazed plots. There were no differences in the number of plant species between treatments. Some plant species and invertebrate taxa were recorded exclusively in grazed or ungrazed plots. Invertebrate detritivores and sap sucking insects were more abundant in ungrazed plots. Ant assemblages were functionally different between treatments: Honeydew-gatherer ants were associated with ungrazed and higher plant biomass plots, and seed-eaters as well as aggressive predator ant species were associated with grazed, open habitat, plots. Management practices that maintain grazing and small scale grazing-excluded areas can increase habitat heterogeneity and promote herbaceous plant and invertebrate diversity at the local level.     

Aboveground biomass and nutrient accumulation dynamics in young black alder, silver birch and Scots pine plantations on reclaimed oil shale mining areas in Estonia

The growth, aboveground biomass production and nutrient accumulation in black alder (Alnus glutinosa (L.) Gaertn.), silver birch (Betula pendula Roth.) and Scots pine (Pinus sylvestris L.) plantations during 7 years after planting were investigated on reclaimed oil shale mining areas in Northeast Estonia with the aim to assess the suitability of the studied species for the reclamation of post-mining areas. The present study revealed changes in soil properties with increasing stand age. Soil pH and P concentration decreased and soil N concentration increased with stand age. The largest height and diameter of trees, aboveground biomass and current annual production occurred in the black alder stands. In the 7-year-old stands the aboveground biomass of black alder (2100 trees ha⁻¹) was 2563 kg ha⁻¹, in silver birch (1017 trees ha⁻¹) and Scots pine (3042 trees ha⁻¹) stands respective figures were 161 and 1899 kg ha⁻¹. The largest amounts of N, P, K accumulated in the aboveground part were in black alder stands. In the 7th year, the amount of N accumulated in the aboveground biomass of black alder stand was 36.1 kg ha⁻¹, the amounts of P and K were 3.0 and 8.8 kg ha⁻¹, respectively. The larger amounts of nutrients in black alder plantations are related to the larger biomass of stands. The studied species used N and P with different efficiency for the production of a unit of biomass. Black alder and silver birch needed more N and P for biomass production, and Scots pine used nutrients most efficiently. The present study showed that during 7 years after planting, the survival and productivity of black alder were high. Therefore black alder is a promising tree species for the reclamation of oil shale post-mining areas.     

Fuel deposition rates of montane and subalpine conifers in the central Sierra Nevada,California, USA

Fire managers and researchers need information on fuel deposition rates to estimate future changes in fuel bed characteristics, determine when forests transition to another fire behavior fuel model, estimate future changes in fuel bed characteristics, and parameterize and validate ecosystem process models. This information is lacking for many ecosystems including the Sierra Nevada in California, USA. We investigated fuel deposition rates and stand characteristics of seven montane and four subalpine conifers in the Sierra Nevada. We collected foliage, miscellaneous bark and crown fragments, cones, and woody fuel classes from four replicate plots each in four stem diameter size classes for each species, for a total of 176 sampling sites. We used these data to develop predictive equations for each fuel class and diameter size class of each species based on stem and crown characteristics. There were consistent species and diameter class differences in the annual amount of foliage and fragments deposited. Foliage deposition rates ranged from just over 50 g m⁻² year⁻¹ in small diameter mountain hemlock stands to ∼300 g m⁻² year⁻¹ for the three largest diameter classes of giant sequoia. The deposition rate for most woody fuel classes increased from the smallest diameter class stands to the largest diameter class stands. Woody fuel deposition rates varied among species as well. The rates for the smallest woody fuels ranged from 0.8 g m⁻² year⁻¹ for small diameter stands of Jeffrey pine to 126.9 g m⁻² year⁻¹ for very large diameter stands of mountain hemlock. Crown height and live crown ratio were the best predictors of fuel deposition rates for most fuel classes and species. Both characteristics reflect the amount of crown biomass including foliage and woody fuels. Relationships established in this study allow predictions of fuel loads to be made on a stand basis for each of these species under current and possible future conditions. These predictions can be used to estimate fuel treatment longevity, assist in determining fuel model transitions, and predict future changes in fuel bed characteristics.     

Future quantities and spatial distribution of harvesting residue and dead wood from natural disturbances in Canada

Interest in the use of bioenergy is increasing because of the need to mitigate climate change, the increasing costs and finite supply of fossil fuels, and the declining price of lumber and paper. Sound bioenergy policies must be informed by accurate estimates of potential feedstock production, rights to the production, social values and economics. Two of the main sources of bioenergy feedstock from forests are (i) harvesting residue and (ii) dead wood resulting from natural disturbances (i.e. standing dead timber). We modeled the production of bioenergy feedstock from these two sources from 2005 to 2020 for Canada's managed forest south of 60° N so that this information can be used in provincial and national strategic planning. Published estimates of harvesting residue vary widely, and our objective was to provide more precise estimates based on new forest inventory data and regional modeling. Natural disturbances result in very large quantities of dead wood on the landscape, but estimates of future stocks and annual production have not previously been made. Our estimates included a 50% discount factor to net-down theoretically available quantities to a more realistic estimate of potential ecologically sustainable bioenergy feedstock. The total future annual production averaged 51 ± 17 Tg year⁻¹ from natural disturbances and 20 ± 0.6 Tg year⁻¹ from clearcut harvesting residues. Harvesting residue for the area logged varied spatially from a low of 1.0 ± 0.77 kg m⁻² year⁻¹ to a high of 6.7 ± 0.1 kg m⁻² year⁻¹. Dead wood production due to insects was forecast to peak in the Montane Cordillera of British Columbia (BC) at 16.7 Tg year⁻¹ due to the current mountain pine beetle outbreak. Total dead wood production due to fire was highest in the western portion of the boreal forest (3.6 Tg year⁻¹ in the Boreal Shield of Saskatchewan), in part due to the high frequency of fires in these ecosystems and the large area of western boreal forest, but the highest density production was in BC: >9 kg m⁻² year⁻¹ in the burned area. Our results showed that the dead wood stocks of 331 Tg oven-dry matter potentially available for bioenergy in 2020 are much smaller than the 3100 ± 84 Tg of dead wood stocks estimated based on ecosystem dynamics. While bioenergy use will accelerate the release of greenhouse gases compared to on-site decay, the energy is renewable and can be used as a substitute for fossil fuels. The net benefit to the atmosphere of forest bioenergy use is affected by many factors, and future research should further assess which sustainable wood-based bioenergy strategies yield the greatest net greenhouse gas benefits over the different time scales needed for post-disturbance forest recovery.     

Key nitrogen cycling processes in pine plantations along a short urban–rural gradient in Nanchang,China

We used pine (Pinus elliottii Engelm.) forests located along a short urban–rural gradient in Nanchang, China to study nitrogen (N) cycling responses to urbanization. Annual average rates of nitrification and net N-mineralization in soils (0–15 cm depth) measured from February 2007 to January 2009 increased from rural (8 and 37 kg ha⁻¹ year⁻¹) to suburban (69 and 79 kg ha⁻¹ year⁻¹) and urban sites (114 and 116 kg ha⁻¹ year⁻¹) (P < 0.05). Soil nitrate and mineral N pools exhibited the same spatial patterns in response to urban location. In comparison to rural sites, urban and suburban sites experienced soil microbial biomass N that increased by 98% and 38%, sucrase activity that increased by 40% and 26%, and urease activity that decreased by 35% and 25%, respectively. Soil microbial biomass C:N and free amino acids varied little along the urban–rural gradient. Foliar N concentrations and N resorption proficiencies were higher in urban (12.3 and 4.8 g kg⁻¹) and suburban (12.3 and 6.2 g kg⁻¹) than in rural (9.9 and 3.6 g kg⁻¹) sites, while N resorption efficiencies (from 58% to 72%) were not statistically different. These results indicate that forests in suburban and especially in urban areas are moving rapidly towards a state of “N saturation” and increased potential N loss most likely attributable to higher N deposition to these sites.     

Early growth and nutrition of hybrid poplars fertilized at planting in the boreal forest of western Quebec

In order to maximize growth and diagnose nutritional requirements of hybrid poplars (Populus spp.) grown in the boreal forest of western Quebec, the Diagnosis and Recommendation Integrated System (DRIS) was evaluated in conjunction with N:P ratios of trees fertilized at planting. Three hybrid poplar clones (747210; P. balsamifera × trichocarpa, 915005; P. balsamifera × maximowiczii, and 915319; P. maximowiczii × balsamifera) were fertilized with 18 combinations of nitrogen (N), phosphorus (P) and potassium (K). Fertilizers used were granules of ammonium nitrate (34.5-0-0) at 3 levels (0, 20 and 40 g tree⁻¹ of N), triple-superphosphate (0-45-0) at 3 levels (0, 25 and 50 g tree⁻¹ of P), and potassium sulfate (0-0-50) at 2 levels (0, 20 g tree⁻¹ of K). After two growing seasons, P fertilization was the most effective in promoting growth and 25 g tree⁻¹ increased mean stem volume by 41% compared to unfertilized trees. The predictive accuracy of the N:P and DRIS diagnosis methods was generally reliable, however they failed to predict some co-limitations of N and P.     

Logging residue removal after thinning in Nordic boreal forests: Long-term impact on tree growth

The aim of this study was to determine the effect of whole-tree harvesting (WTH) on the growth of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) as compared to conventional stem harvesting (CH) over 10 and 20 years. Compensatory (WTH + CoF) and normal nitrogen-based (CH + F or WTH + F) fertilisation were also studied. A series of 22 field experiments were established during 1977-1987, representing a range of site types and climatic conditions in Finland, Norway and Sweden. The treatments were performed at the time of establishment and were repeated after 10-13 years at 11 experimental sites. Seven experiments were followed for 25 years.Volume increment was on average significantly lower after WTH than after CH in both 10-year periods in the spruce stands. In the pine stands thinned only once, the WTH induced growth reduction was significant during the second 10-year period, indicating a long-term response.Volume increment of pine stands was 4 and 8% and that of spruce stands 5 and 13% lower on the WTH plots than on CH during the first and the second 10-year period, respectively. For the second 10-year period the relative volume increment of the whole-tree harvested plots tended to be negatively correlated with the amount of logging residue. Accordingly, the relative volume increment decreased more, the more logging residue was harvested, stressing the importance of developing methods for leaving the nutrient-rich needles on site.If nutrient (N, P, K) losses with the removed logging residues were compensated with fertiliser (WTH + CoF), the volume increment was equal to that in the CH plots. Nitrogen (150-180 kg ha⁻¹) or N + P fertilisation increased tree growth in all experiments except in one very productive spruce stand. Pine stands fertilised only once had a normal positive growth response during the first 10-year period, on average 13 m³ ha⁻¹, followed by a negative response of 5 m³ ha⁻¹ during the second 10-year period. The fertilisation effect of WTH + F and WTH + CoF on basal area increment was both smaller and shorter than with CH + F.