Carbon and nitrogen dynamics during forest stand development: a global synthesis

Our knowledge of carbon (C) and nitrogen (N) dynamics during stand development is not only essential for evaluating the role of secondary forests in the global terrestrial C cycle, but also crucial for understanding long-term C-N interactions in terrestrial ecosystems. However, a comprehensive understanding of forest C and N dynamics over age sequence remains elusive due to the diverse results obtained across individual studies. Here, we synthesized the results of more than 100 studies to examine C and N dynamics during forest stand development. Our results showed that C accumulated in aboveground vegetation, litter and forest floor pools, while the mineral soil C pool did not exhibit significant changes in most studies. The rate of C changes declined with stand age and approached equilibrium during the later stage of stand development. The rate of N changes exhibited linear increases with that of C changes, indicating that N also accrued in various ecosystem components except mineral soil. These results demonstrate that substantial increases in C pools over age sequence are accompanied by N accretion in forest ecosystems. The concurrent C and N dynamics suggest that forest ecosystems may have an intrinsic ability to preclude progressive N limitation during stand development.     

Optimal co-allocation of carbon and nitrogen in a forest stand at steady state

Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.     

Carbon and nitrogen dynamics in a Pinus densiflora forest with low and high stand densities

Aims Understanding carbon (C) and nitrogen (N) dynamics and their dependence on the stand density of an even-aged, mature forest provides knowledge that is important for forest management. This study investigated the differences in ecosystem total C and N storage and flux between a low-density stand (LD) and a high-density stand (HD) and examined the effects of stand density on aboveground net primary productivity (ANPP), total belowground C allocation (TBCA) and net ecosystem production (NEP) in a naturally regenerated, 65- to 75-year-old Pinus densiflora S. et Z. forest.Methods LD (450 trees ha^-1) and HD (842 trees ha^-1) were established in an even-aged, mature P. densiflora forest in September 2006. The forest had been naturally regenerated following harvesting, and the stand density was naturally maintained without any artificial management such as thinning. The diameter at breast height (DBH ≥ 5.0cm) of all live stems within the stands was measured yearly from 2007 to 2011. To compare C and N storage and fluxes in LD and HD, C and N pools in aboveground and belowground biomass, the forest floor, coarse woody debris (CWD) and soil; soil CO₂ efflux (R S); autotrophic respiration (R A); litter production; and soil N availability were measured. Further, ANPP, TBCA and NEP were estimated from plot-based measurement data.Important findings Ecosystem C (Mg C ha^-1) and N (Mg N ha^-1) storage was, respectively, 173.0±7.3 (mean ± SE) and 4.69±0.30 for LD and 162±11.8 and 4.08±0.18 for HD. There were no significant differences in C and N storage in the ecosystem components, except for soils, between the two stands. In contrast, there were significant differences in aboveground ANPP and TBCA between the two stands (P < 0.05). Litterfall, biomass increment and R S were major C flux components with values of, respectively, 3.89, 3.74 and 9.07 Mg C ha^-1 year^-1 in LD and 3.15, 2.94 and 7.06 Mg C ha^-1 year^-1 in HD. Biometric-based NEP (Mg C ha^-1 year^-1) was 4.18 in LD and 5.50 in HD. Although the even-aged, mature P. densiflora forest had similar C and N allocation patterns, it showed different C and N dynamics depending on stand density. The results of the current study will be useful for elucidating the effects of stand density on C and N storage and fluxes, which are important issues in managing natural mature forest ecosystems.     

Nitrogen fixation and the mass balances of carbon and nitrogen in ecosystems

Ecosystems with high rates of nitrogen fixation often have high loss rates through leaching or possibly denitrification. However, there is no formal theoretical context to examine why this should be the case nor of how nitrogen accumulates in such open systems. Here, we propose a simple model coupling nitrogen inputs and losses to carbon inputs and losses. The nitrogen balance of this model system depends on plant (nitrogen fixer) growth rate, its carrying capacity, N fixed/C fixed, residence time of nitrogen and carbon in biomass, litter decay rate, litter N/C, and fractional loss rate of mineralized nitrogen. The model predicts the requirements for equilibrium in a nitrogen-fixing system, and the conditions on nitrogen fixation and losses in order for the system to accumulate nitrogen and carbon. In particular, the accumulation of nitrogen and carbon in a nitrogen-fixing system depend on an interaction between residence time in vegetation and litter decay rate in soil. To reflect a possible increased uptake of soil nitrogen and decreased respiratory cost of symbiotic nitrogen fixers, the model was then modified so that fixation rate decreased and growth rate increased as nitrogen capital accumulated. These modifications had only small effects on carbon and nitrogen accumulation. This suggests that switching from uptake of atmospheric nitrogen to mineral soil nitrogen as nitrogen capital accumulates simply results in a trade-off between energetic limitations and soil nitrogen limitations to carbon and nitrogen accumulation. Experimental tests of the model are suggested.     

Rainfall and labile carbon availability control litter nitrogen dynamics in a tropical dry forest

N cycling in tropical dry forests is driven by rainfall seasonality but the mechanisms involved are not well understood. We studied the seasonal variation in N dynamics and microbial biomass in the surface litter of a tropical dry forest ecosystem in Mexico over a 2-year period. Litter was collected at 4 different times of the year to determine changes in total, soluble, and microbial C and N concentrations. Additionally, litter from each sampling date was incubated under laboratory conditions to determine potential C mineralization rate, net N mineralization, net C and N microbial immobilization, and net nitrification. Litter C concentrations were highest in the early-dry season and lowest in the rainy season, while the seasonal changes in N concentrations varied between years. Litter P was higher in the rainy than in the early-dry season. Water-soluble organic C (WSOC) and water-soluble N concentrations were highest during the early- and late-dry seasons and represented up to 4.1 and 5.9% of the total C and N, respectively. NH₄⁺ and NO₃⁻ showed different seasonal and annual variations. They represented an average 23% of soluble N. Microbial C was generally higher in the dry than in the wet seasons, while microbial N was lowest in the late-dry and highest in the early-rainy seasons. Incubations showed that lowest potential C mineralization rates and C and N microbial immobilization occurred in rainy season litter, and were positively correlated to WSOC. Net nitrification was highest in rainy season litter. Our results showed that the seasonal pattern in N dynamics was influenced by rainfall seasonality and labile C availability, and not by microbial biomass. We propose a conceptual model to hypothesize how N dynamics in the litter layer of the Chamela tropical dry forest respond to the seasonal variation in rainfall.     

Quantifying uncertainty from large-scale model predictions of forest carbon dynamics

Linking environmental computer simulation models and geographic information systems (GIS) is now a common practice to scale up simulations of complex ecosystem processes for decision support. Unfortunately, several important issues of upscaling using GIS are rarely considered; in particular scale dependency of models, availability of input data, support of input and validation data, and uncertainty in prediction including error propagation from the GIS. We linked the biogeochemical Forest‐DNDC model to a GIS database to predict growth of Eucalyptus globulus plantations at two different scales (～0.045 ha plot^?1 scale and ～100 ha grid^?1 scale) across Victoria, in south‐eastern Australia. Results showed that Forest‐DNDC was not scale dependent across the range of scales investigated. Reduced availability of input data at the larger scale may introduce severe prediction errors, but did not require adjustment of the model in this study. Differences in the support of input and validation data led to an underestimation of predictive precision but an overestimation of prediction accuracy. Increasing data support, produced a high level of prediction accuracy (^?e%), but a medium level of predictive precision (r²=0.474, ME=0.318) after statistical validation. GIS error contribution could be detected but was not readily or reliably quantified. In a regional case study for 2653 ha of E. globulus plantations, the linked model GIS system estimated a total standing biomass of 95 260 t C for mid‐2003 and a net CO₂ balance of ?45 671 t CO₂‐C yr^?1 for the entire year of 2002. This study showed that regional predictions of forest growth and carbon sequestration can be produced with greater confidence after a comprehensive assessment of upscaling issues.     

Modeling dynamics of stable carbon isotopic exchange between a boreal forest ecosystem and the atmosphere

Stable isotopes of CO₂ contain unique information on the biological and physical processes that exchange CO₂ between terrestrial ecosystems and the atmosphere. In this study, we developed an integrated modeling system to simulate dynamics of stable carbon isotope of CO₂, as well as moisture, energy, and momentum, between a boreal forest ecosystem and the atmosphere, as well as their transport/mixing processes through the convective boundary layer (CBL), using remotely sensed surface parameters to characterize the surface heterogeneity. It has the following characteristics: (i) it accounts for the influences of the CBL turbulent mixing and entrainment of the air aloft; (ii) it scales individual leaf‐level photosynthetic discrimination up to the whole canopy (Δ_canopy) through the separation of sunlit and shaded leaf groups; (iii) it has the capacity to examine the detailed interrelationships among plant water‐use efficiency, isotope discrimination, and vapor pressure deficit; and (iv) it has the potential to investigate how an ecosystem discriminates against ¹³C at various time and spatial scales. The monthly mean isotopic signatures of ecosystem respiration (i.e. δ¹³C_R) used for isotope flux calculation are retrieved from the nighttime flask data from the intensive campaigns (1998–2000) at 20 m level on Fraserdale tower, and the data from the growing season in 1999 are used for model validation. Both the simulated CO₂ mixing ratio and δ¹³C of CO₂ at the 20 m level agreed with the measurements well in different phases of the growing season. On a diurnal basis, the greatest photosynthetic discrimination at canopy level (i.e. Δ_canopy) occurred early morning and late afternoon with a varying range of 10–26‰. The diurnal variability of Δ_canopy was also associated with the phases of growing season and meteorological variables. The annual mean Δ_canopy in 1999 was computed to be 19.58‰. The monthly averages of Δ_canopy varied between 18.55‰ and 20.84‰ with a seasonal peak during the middle growing season. Because of the strong opposing influences of respired and photosynthetic fluxes on forest air (both CO₂ and ¹³CO₂) on both the diurnal and seasonal time scales, CO₂ was consistently enriched with the heavier ¹³C isotope (less negative δ¹³C) from July to October and depleted during the remaining months, whereas on a diurnal basis, CO₂ was enriched with the heavier ¹³C in the late afternoon and depleted in early morning. For the year 1999, the model results reveal that the boreal ecosystem in the vicinity of Fraserdale tower was a small sink with net uptake of 29.07 g ¹²C m^?2 yr^?1 and 0.34 g ¹³C m^?2 yr^?1.     

Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest

Overwinter and snowmelt processes are thought to be critical to controllersof nitrogen (N) cycling and retention in northern forests. However, therehave been few measurements of basic N cycle processes (e.g.mineralization, nitrification, denitrification) during winter and littleanalysis of the influence of winter climate on growing season N dynamics.In this study, we manipulated snow cover to assess the effects of soilfreezing on in situ rates of N mineralization, nitrification and soilrespiration, denitrification (intact core, C₂H₂ – based method),microbial biomass C and N content and potential net N mineralization andnitrification in two sugar maple and two yellow birch stands with referenceand snow manipulation treatment plots over a two year period at theHubbard Brook Experimental Forest, New Hampshire, U.S.A. The snowmanipulation treatment, which simulated the late development of snowpackas may occur in a warmer climate, induced mild (temperatures >–5 °C) soil freezing that lasted until snowmelt. The treatmentcaused significant increases in soil nitrate (NO₃ ^–)concentrations in sugar maple stands, but did not affect mineralization,nitrification, denitrification or microbial biomass, and had no significanteffects in yellow birch stands. Annual N mineralization and nitrificationrates varied significantly from year to year. Net mineralization increasedfrom 12.0 g N m^–2 y^–1 in 1998 to 22 g N m^–2 y^–1 in 1999 and nitrification increased from 8 g N m^–2 y^–1 in 1998 to 13 g N m^–2 y^–1 in 1999.Denitrification rates ranged from 0 to 0.65 g N m^–2 y^–1. Ourresults suggest that mild soil freezing must increase soil NO₃ ^– levels by physical disruption of the soil ecosystem and not by direct stimulation of mineralization and nitrification. Physical disruption canincrease fine root mortality, reduce plant N uptake and reduce competitionfor inorganic N, allowing soil NO₃ ^– levels to increase evenwith no increase in net mineralization or nitrification.     

Frost and forest stand effects on the population dynamics of Asplenium scolopendrium

Our objective was to analyze which factors are critical for the dynamics of terrestrial Asplenium scolopendrium populations at the northern edge of its distribution. Therefore, a long-term study (1978–1999) on the performance and demography of this fern species has been carried out in three different forest stands (Picea sitchensis with Fagus sylvatica, P. sitchensis with thinning, and Fraxinus excelsior) in the Netherlands. We used the recorded demographic data to parameterize 37 transition matrices. The number of frost days in severe winters correlated closely with frond damage and resulted in increased mortality and retrogression. Landslip on the trench banks and intraspecific competition were also found to increase mortality. In the F. excelsior plot, plants grew faster and bigger, produced more fronds and formed a more closed fern cover than in the P. sitchensis stands, likely due to higher light levels. Life-table response experiments revealed that reproduction contributed greatly to the differences in projected population growth rates: reproduction was importantly higher in the F. excelsior and in the thinned P. sitchensis plots than in the P. sitchensis–F. sylvatica plot. These differences can be attributed to an initial difference in light climate and to the accumulation of F. sylvatica litter which reduced recruitment. Recruitment occurred on bare soil but also in open moss carpets. We expect that the fern Asplenium scolopendrium will profit at its northern distribution edge when severe winters will occur less frequently, which is one of the expectations for global climate change.     

Controls on the dynamics of dissolved organic carbon and nitrogen in a Central European deciduous forest

Despite growing attention concerning therole of dissolved organic matter (DOM) inelement cycling of forest ecosystems, thecontrols of concentrations and fluxes of bothdissolved organic carbon (DOC) and nitrogen(DON) under field conditions in forest soilsremain only poorly understood. The goal ofthis project is to measure the concentrations and fluxes of DON, NH₄ ⁺, NO₃ ^–and DOC in bulkprecipitation, throughfall, forest floorleachates and soil solutions of a deciduousstand in the Steigerwald region (northernBavaria, Germany). The DOC and DONconcentrations and fluxes were highest inleachates originating from the Oa layer of theforest floor (73 mg C L^–1, 2.3 mg NL^–1 and about 200–350 kg C, 8–10 kg Nha^–1 yr^–1). They were observed to behighly variable over time and decreased in themineral topsoil (17 mg C L^–1, 0.6 mg NL^–1 and about 50–90 kg C, 2.0 to 2.4 kg Nha^–1 yr^–1). The annual variability ofDOC and DON concentrations and subsequentialDOC/DON ratios was substantial in allsolutions. The DOC and DON concentrations inthroughfall were positively correlated withtemperature. The DOC and DON concentrationsdid not show seasonality in the forest floorand mineral soil. Concentrations were notrelated to litterfall dynamics but didcorrespond in part to the input of DOC and DONfrom throughfall. The throughfall contributionto the overall element fluxes was higher forDON than for DOC. Concentrations and fluxes ofDON were significantly correlated to DOC inthroughfall and the Oi layer. However, thecorrelation was weak in Oa leachates. Inaddition, seasonal and annual variation ofDOC/DON ratios indicated different mechanismsand release rates from the forest floor forboth components. The concentrations of DOC andDON in forest floor leachates were in mostcases dependent neither on the pH value orionic strength of the solution, nor on thewater flux or temperature changes. As aconsequence, the DOC and DON fluxes from theforest floor into the mineral soil werelargely dependent on the water flux if annualand biweekly time scales are considered.     

Error analysis of metabolic-rate measurements in mammalian-cell culture by carbon and nitrogen balances

The analysis of metabolic fluxes of large stoichiometric systems is sensitive to measurement errors in metabolic uptake and production rates. It is therefore desirable to independently test the consistency of measurement data, which is possible if at least two elemental balances can be closed. For mammalian-cell culture, closing the C balance has been hampered by problems in measuring the carbon-dioxide production rate. Here, it is shown for various sets of measurement data that the C balance can be closed by applying a method to correct for the bicarbonate buffer in the culture medium. The measurement data are subsequently subject to measurement-error analysis on the basis of the C and N balances. It is shown at 90% reliability that no gross measurement errors are present, neither in the measured production- and consumption rates, nor in the estimated in- and outgoing metabolic rates of te subnetwork, that contains the glycolysis, the pentose-phosphate, and the glutaminolysis pathways. This revised version was published online in July 2006 with corrections to the Cover Date.     

Invasive insects impact forest carbon dynamics

Invasive 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 (NEE_yr) 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 NEE_yr in 2005 for the oak‐, mixed‐, and pine‐dominated stands, respectively. Gypsy moth defoliation strongly reduced fluxes in 2006 and 2007 compared with 2005; NEE_yr 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 NEE_yr 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.     

全球森林土壤微生物生物量碳氮磷化学计量的季节动态

土壤微生物生物量在森林生态系统中充当具有生物活性的养分积累和储存库。土壤微生物转化有机质为植物提供可利用养分, 与植物的相互作用维系着陆地生态系统的生态功能。同时, 土壤微生物也与植物争夺营养元素, 在季节交替过程和植物的生长周期中呈现出复杂的互利-竞争关系。综合全球数据对温带、亚热带和热带森林土壤微生物生物量碳(C)、氮(N)、磷(P)含量及其化学计量比值的季节动态进行分析, 发现温带和亚热带森林的土壤微生物生物量C、N、P含量均呈现夏季低、冬季高的格局。热带森林四季的土壤微生物生物量C、N、P含量都低于温带和亚热带森林, 且热带森林土壤微生物生物量C含量、N含量在秋季相对最低, 土壤微生物生物量P含量四季都相对恒定。温带森林的土壤微生物生物量C:N在春季显著高于其他两个森林类型; 热带森林的土壤微生物生物量C:N在秋季显著高于其他2个森林类型。温带森林土壤微生物生物量N:P和C:P在四季都保持相对恒定, 而热带森林土壤微生物生物量N:P和C:P在夏季高于其他3个季节。阔叶树的土壤微生物生物量C含量、N含量、N:P、C:P在四季都显著高于针叶树; 而针叶树的土壤微生物生物量P含量在四季都显著高于阔叶树。在春季和冬季时, 土壤微生物生物量C:N在阔叶树和针叶树之间都没有显著差异; 但是在夏季和秋季, 针叶树的土壤微生物生物量C:N显著高于阔叶树。对于土壤微生物生物量的变化来说, 森林类型是主要的显著影响因子, 季节不是显著影响因子, 暗示土壤微生物生物量的季节波动是随着植物其内在固有的周期变化而变化。植物和土壤微生物密切作用表现出来的对养分的不同步吸收是保留养分和维持生态功能的一种权衡机制。     

中亚热带次生林和人工林凋落枝水溶性碳氮磷动态特征

凋落枝是森林地上部分凋落物的重要组分,揭示其水溶性碳氮磷的动态规律对于认识森林物质循环过程具有重要意义,但目前研究集中于凋落叶,而对凋落枝缺乏必要关注。因此,以中亚热带典型马尾松（Pinus massoniana）人工林、杉木（Cunninghamia lanceolata）人工林和米槠（Castanopsis carlesii）次生林为研究对象,在一个自然年内调查了凋落枝水溶性碳、氮、磷含量及其芳香化指数以及化学计量比的动态变化过程。结果显示：（1）米槠次生林凋落枝水溶性碳、氮、磷含量及芳香化指数明显大于马尾松和杉木人工林;（2）水溶性碳和磷、水溶性碳比磷、水溶性氮比磷和芳香化指数有明显的季节变化;（3）水溶性碳、水溶性磷、水溶性氮比磷和芳香化指数在不同林分和季节间有交互作用。（4）马尾松和杉木人工林、米槠次生林凋落枝水溶性物质含量的季节变化多数与气温和降水呈显著负相关。这些结果表明亚热带次生林可能相对于人工林具有更为高效的以凋落枝为载体的物质循环过程,在未来气候变暖背景下亚热带森林由凋落枝归还给土壤的养分浓度可能降低。     

Dissolved organic carbon affects soil microbial activity and nitrogen dynamics in a Mexican tropical deciduous forest

Seasonal variation of dissolved organic C (DOC) and its effects on microbial activity and N dynamics were studied during two consecutive years in soils with different organic C concentrations (hilltop and hillslope) in a tropical deciduous forest of Mexico. We found that DOC concentrations were higher at the hilltop than at the hillslope soils, and in both soils generally decreased from the dry to the rainy season during the two study years. Microbial biomass and potential C mineralization rates, as well as dissolved organic N (DON) and NH₄⁺ concentrations and net N immobilization were higher in soils with higher DOC than in soils with lower DOC. In contrast, net N immobilization and NH₄⁺ concentration were depleted in the soil with lowest DOC, whereas NO₃⁻ concentrations and net nitrification increased. Negative correlations between net nitrification and DOC concentration suggested that NH₄⁺ was transformed to NO₃⁻ by nitrifiers when the C availability was depleted. Taken together, our results suggest that available C appears to control soil microbial activity and N dynamics, and that microbial N immobilization is facilitated by active heterotrophic microorganisms stimulated by high C availability. Soil autotrophic nitrification is magnified by decreases in C availability for heterotrophic microbial activity. This study provides an experimental data set that supports the conceptual model to show and highlight that microbial dynamics and N transformations could be functionally coupled with DOC availability in the tropical deciduous forest soils. Responsible Editor: Chris Neill     

Modeling nitrogen flux by larval insect herbivores from a temperate hardwood forest

Herbivorous insects flux considerable amounts of nitrogen from the forest canopy to the soil in the form of frass. The amount of nitrogen fluxed varies depending on the characteristics of the herbivores, their food resources, and their physical environment. We used concepts from metabolic ecology and ecological stoichiometry to develop a general model of individual nitrogen flux via frass fall for moth and sawfly larvae from a temperate hardwood forest in northern Wisconsin, USA. We found that individual nitrogen flux (Q _N, mg N/day) was related to larval body mass (M _B, mg dry), short-term variation in environmental temperature (T, K), and larval nitrogen concentration (N _B, proportion dry mass) as Q _N = e^25.75 M _B^0.77 e^−0.83/kT N _B^−1.56, where k is Boltzmann’s constant (8.62 × 10⁻⁵ eV/K). We also found that larval nitrogen flux did not vary with the nitrogen concentration of food, and suggest that this was due to compensatory feeding by larvae living on low-quality leaves. With further work, models of individual N flux could be used to scale individual fluxes to population and community levels, and thus link the characteristics of insect herbivore communities with the flow of nitrogen through forested ecosystems.     

Mid-successional stand dynamics in a cool-temperate conifer-hardwood forest in northern Japan

Stand dynamics was studied over 13 years in a cool-temperate conifer-hardwood forest, northern Japan. A total 30 hardwood species and one conifer, Abies sachalinensis, larger than 1.5 cm DBH were recorded. The total stand density was 1677 trees ha⁻¹ at the beginning, decreasing to 1184 trees ha⁻¹ (30% reduction) over the study period, but the total stand basal area was almost unchanged (about 49 m² ha⁻¹). This large reduction in total density was mainly due to the death of saplings and infrequent recruitment. Number of recruits gradually decreased with time, while that of dead trees was constant. Cause of death of small trees was mainly due to suppression by tall trees. Skewness of the DBH frequency distribution varied among the species. A less skewed frequency distribution (i.e., few number of saplings) was shown by shade-intolerant species such as Populus maximowiczii and Betula maximowicziana, and a more skewed frequency distribution (i.e., large number of saplings) by shade-tolerant species such as Acer mono and Tilia japonica. DBH frequency distribution changed to less skewed patterns with reduction of density in most species during the census period. Rank of shade tolerance positively correlated with tree density and skewness, and negatively correlated with mean DBH. Skewness also positively correlated with recruitment rates. Furthermore, rank of shade tolerance positively correlated with seed size. These results suggest that shade-intolerant species regenerated immediately after disturbances by wide dispersal of small seeds, but their recruitment was interrupted after that. By contrast, shade-tolerant species were able to recruit even after the ceasation of recruitment of shade-intolerant species, but suffered severe mortality due to the increasing shading with the progress of stand development. This study suggests that the stand is still developing, with changes in species composition and size structure, and that species differences in shade tolerance and seed size are important for the stand structural changes.     

Soil nitrogen dynamics in a holm oak forest

Soil nitrogen (N) dynamics were studied in a dense, holm oak (Quercus ilex ssp. ilex) stand in the Montseny mountains to determine annual and seasonal patterns of N availability and uptake in an undisturbed Mediterranean forest on acidic soil. Soil mineral N content, net N mineralization (NNM), and net nitrification (NN) were determined by monthly sampling at two soil depths followed by in situ incubation in polyethylene bags. NNM per unit of soil mass was much higher at 0–5 cm than at 5–20 cm (annual means 24 and 2.5 mg N/kg, respectively) but on an area basis NNM was similar at both depths. A total of 80 kg N/ha/yr were mineralized from the first 20 cm of soil. NN amounted to only 9% of the annual NNM (7.5 kg N/ha/yr) and it occurred only in the upper 5 cm. NNM was maximum in June and July, while the NN peaked in May. Despite favourable soil temperature and moisture, NNM was negative in autumn because of microbial immobilization. Seasonal and depth variations of NNM appeared to be controlled more by substrate quality than by organic matter quantity, temperature or moisture. NN was not limited by ammonium availability. Calculated N uptake amounted to 91 kg/ha yr, peaking in June and July. The investigated stand showed a moderately high N availability, but ammonium was the major form of mineral N supply for holm oak.