Quantitative classification and carbon density of the forest vegetation in Lüliang Mountains of China

Forests play a major role in global carbon (C) cycle, and the carbon density (CD) could reflect its ecological function of C sequestration. Study on the CD of different forest types on a community scale is crucial to characterize in depth the capacity of forest C sequestration. In this study, based on the forest inventory data of 168 field plots in the study area (E 111°30′–113°50′, N 37°30′–39°40′), the forest vegetation was classified by using quantitative method (TWINSPAN); the living biomass of trees was estimated using the volume-derived method; the CD of different forest types was estimated from the biomass of their tree species; and the effects of biotic and abiotic factors on CD were studied using a multiple linear regression analysis. The results show that the forest vegetation in this region could be classified into 9 forest formations. The average CD of the 9 forest formations was 32.09 Mg ha⁻¹ in 2000 and 33.86 Mg ha⁻¹ in 2005. Form. Picea meyeri had the highest CD (56.48 Mg ha⁻¹), and Form. Quercus liaotungensis + Acer mono had the lowest CD (16.14 Mg ha⁻¹). Pre-mature forests and mature forests were very important stages in C sequestration among four age classes in these formations. Forest densities, average age of forest stand, and elevation had positive relationships with forest CD, while slope location had negative correlation with forest CD.     

Absence of snow cover reduces understory plant cover and alters plant community composition in boreal forests

Snow regimes affect biogeochemistry of boreal ecosystems and are altered by climate change. The effects on plant communities, however, are largely unexplored despite their influence on relevant processes. Here, the impact of snow cover on understory community composition and below-ground production in a boreal Picea abies forest was investigated using a long-term (8-year) snow cover manipulation experiment consisting of the treatments: snow removal, increased insulation (styrofoam pellets), and control. The snow removal treatment caused longer (118 vs. 57 days) and deeper soil frost (mean minimum temperature −5.5 vs. −2.2°C) at 10 cm soil depth in comparison to control. Understory species composition was strongly altered by the snow cover manipulations; vegetation cover declined by more than 50% in the snow removal treatment. In particular, the dominant dwarf shrub Vaccinium myrtillus (−82%) and the most abundant mosses Pleurozium schreberi (−74%) and Dicranum scoparium (−60%) declined strongly. The C:N ratio in V. myrtillus leaves and plant available N in the soil indicated no altered nitrogen nutrition. Fine-root biomass in summer, however, was negatively affected by the reduced snow cover (−50%). Observed effects are attributed to direct frost damage of roots and/ or shoots. Besides the obvious relevance of winter processes on plant ecology and distribution, we propose that shifts in the vegetation caused by frost damage may be an important driver of the reported alterations in biogeochemistry in response to altered snow cover. Understory plant performance clearly needs to be considered in the biogeochemistry of boreal systems in the face of climate change.     

Altitudinal changes in carbon storage of temperate forests on Mt Changbai, Northeast China

A number of studies have investigated regional and continental scale patterns of carbon (C) stocks in forest ecosystems; however, the altitudinal changes in C storage in different components (vegetation, detritus, and soil) of forest ecosystems remain poorly understood. In this study, we measured C stocks of vegetation, detritus, and soil of 22 forest plots along an altitudinal gradient of 700–2,000 m to quantify altitudinal changes in carbon storage of major forest ecosystems (Pinus koraiensis and broadleaf mixed forest, 700–1,100 m; Picea and Abies forest, 1,100–1,800 m; and Betula ermanii forest, 1,800–2,000 m) on Mt Changbai, Northeast China. Total ecosystem C density (carbon stock per hectare) averaged 237 t C ha⁻¹ (ranging from 112 to 338 t C ha⁻¹) across all the forest stands, of which 153 t C ha⁻¹ (52–245 t C ha⁻¹) was stored in vegetation biomass, 14 t C ha⁻¹ (2.2–48 t C ha⁻¹) in forest detritus (including standing dead trees, fallen trees, and floor material), and 70 t C ha⁻¹ (35–113 t C ha⁻¹) in soil organic matter (1-m depth). Among all the forest types, the lowest vegetation and total C density but the highest soil organic carbon (SOC) density occurred in Betula ermanii forest, whereas the highest detritus C density was observed in Picea and Abies forest. The C density of the three ecosystem components showed distinct altitudinal patterns: with increasing altitude, vegetation C density decreased significantly, detritus C density first increased and then decreased, and SOC density exhibited increasing but insignificant trends. The allocation of total ecosystem C to each component exhibited similar but more significant trends along the altitudinal gradient. Our results suggest that carbon storage and partitioning among different components in temperate forests on Mt Changbai vary greatly with forest type and altitude.     

Deposition and decomposition of cattle dung in forest grazing in southern Kyushu, Japan

This study monitored deposition and decomposition of cattle dung in a grazed young Chamaecyparis obtusa (an evergreen conifer) plantation in southwestern Japan, as a part of exploring the impacts of livestock in the forest grazing system. Animals defecated 10–19 times hd⁻¹ day⁻¹, producing feces of 2.2–3.5 kg DM and 33–73 g N per animal per day. The DM and N concentrations of feces ranged from 157–207 g DM kg⁻¹ and 14.8−23.1 g (kg DM)⁻¹, respectively. Occurrence of defecation was spatially heterogeneous, with feces being concentrated mainly on areas for resting (forest roads, ridges and valleys) and moving (forest roads and along fence lines). Decomposition of dung pats was considerably slow, showing the rates of 1.37–3.05 mg DM (g DM)⁻¹ day⁻¹ as DM loss. Decomposition was further slower on the basis of N release, 0.51–1.63 mg N (g N)⁻¹ day⁻¹, resulting in steadily increased N concentrations of dung pats with time after deposition. The results show that introduction of livestock into a forest (i.e., forest grazing) may limit nutrient availability to plants, by redistributing nutrients into areas with no vegetation (bare land and streams) and by establishing a large N pool as feces due to an imbalance between deposition and slow release, though further studies are necessary for investigating the occurrence of slow dung decomposition in other forest situations.     

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

Post-fire changes in desert vegetation patterns are known, but the mechanisms are poorly understood. Theory suggests that pulse dynamics of resource availability confer advantages to invasive annual species, and that pulse timing can influence survival and competition among species. Precipitation patterns in the American Southwest are predicted to shift toward a drier climate, potentially altering post-fire resource availability and consequent vegetation dynamics. We quantified post-fire inorganic N dynamics and determined how annual plants respond to soil inorganic nitrogen variability following experimental fires in a Mojave Desert shrub community. Soil inorganic N, soil net N mineralization, and production of annual plants were measured beneath shrubs and in interspaces during 6 months following fire. Soil inorganic N pools in burned plots were up to 1 g m⁻² greater than unburned plots for several weeks and increased under shrubs (0.5–1.0 g m⁻²) more than interspaces (0.1–0.2 g m⁻²). Soil NO₃ ⁻−N (nitrate−N) increased more and persisted longer than soil NH₄ ⁺−N (ammonium−N). Laboratory incubations simulating low soil moisture conditions, and consistent with field moisture during the study, suggest that soil net ammonification and net nitrification were low and mostly unaffected by shrub canopy or burning. After late season rains, and where soil inorganic N pools were elevated after fire, productivity of the predominant invasive Schismus spp. increased and native annuals declined. Results suggest that increased N availability following wildfire can favor invasive annuals over natives. Whether the short-term success of invasive species following fire will direct long-term species composition changes remains to be seen, yet predicted changes in precipitation variability will likely interact with N cycling to affect invasive annual plant dominance following wildfire.     

Soil moisture condition and soil nitrogen dynamics in a pure Alnus japonica forest in Korea

This study was conducted to examine the influences of soil-moisture conditions on soil nitrogen (N) dynamics, including in situ soil N mineralization, N availability, and denitrification in a pure Alnus japonica forest located in Seoul, central Korea. The soil N mineralization, N availability, and denitrification were determined using the buried bag incubation method, ion exchange resin bag method, and acetylene block method, respectively. The annual net N mineralization rate (kg N ha⁻¹ year⁻¹) and annual N availability (mg N bag⁻¹) were 40.26 and 80.65 in the relatively dry site, −5.43 and 45.39 in the moist site, and 7.09 and 39.17 in the wet site, respectively. The annual net N mineralization rate and annual N availability in the dry site were significantly higher than those in the moist and wet sites, whereas there was no significant difference between the moist and wet sites. The annual mean denitrification rate (kg N ha⁻¹ year⁻¹) in the dry, moist, and wet sites was 2.37, 2.76, and 1.59, respectively. However, there was no significant difference among sites due to the high spatial and temporal variations. Our results indicate that soil-moisture condition influenced the in situ N mineralization and resin bag N availability in an A. japonica forest, and treatments of proper drainage for poorly drained sites would increase soil N mineralization and N availability and consequently be useful to conserve and manage the A. japonica forest.     

Future Spruce Budworm Outbreak May Create a Carbon Source in Eastern Canadian Forests

Spruce budworm (Choristoneura fumiferana Clem.) is an important and recurrent disturbance throughout spruce (Picea sp.) and balsam fir (Abies balsamea L.) dominated forests of North America. Forest carbon (C) dynamics in these ecosystems are affected during insect outbreaks because millions of square kilometers of forest suffer growth loss and mortality. We tested the hypothesis that a spruce budworm outbreak similar to those in the past could switch a forest from a C sink to a source in the near future. We used a model of ecosystem C to integrate past spruce budworm impact sequences with current forest management data on 106,000 km² of forest in eastern Québec. Spruce budworm-caused mortality decreased stand-level merchantable C stocks by 11–90% and decreased ecosystem C stocks by 2–10% by the end of the simulation. For the first 13 years (2011–2024), adding spruce budworm significantly reduced ecosystem C stock change for the landscape from a sink (4.6 ± 2.7 g C m⁻² y⁻¹ in 2018) to a source (−16.8 ± 3.0 g C m⁻² y⁻¹ in 2018). This result was mostly due to reduced net primary production. The ecosystem stock change was reduced on average by 2 Tg C y⁻¹ for the entire simulated area. This study provides the first estimate that spruce budworm can significantly affect the C sink or source status of a large landscape. These results indicate that reducing spruce budworm impacts on timber may also provide an opportunity to mitigate a C source.     

Decline of soil nitrogen mineralization and nitrification during forest conversion of evergreen broad-leaved forest to plantations in the subtropical area of Eastern China

We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest: evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation (CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg⁻¹ yr⁻¹) and lowest in CFP (0.4 ± 0.1 mg-N kg⁻¹ yr⁻¹). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg⁻¹ yr⁻¹), but lowest in SS (0.02 ± 0.01 mg-N kg⁻¹ yr⁻¹). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively), soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%, respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass. In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and total C concentration and C:N ratio of forest floor.     

Net Primary Production and Carbon Stocks for Subarctic Mesic–Dry Tundras with Contrasting Microtopography, Altitude, and Dominant Species

Mesic–dry tundras are widespread in the Arctic but detailed assessments of net primary production (NPP) and ecosystem carbon (C) stocks are lacking. We addressed this lack of knowledge by determining the seasonal dynamics of aboveground vascular NPP, annual NPP, and whole-ecosystem C stocks in five mesic–dry tundras in Northern Sweden with contrasting microtopography, altitude, and dominant species. Those measurements were paralleled by the stock assessments of nitrogen (N), the limiting nutrient. The vascular production was determined by harvest or in situ growing units, whereas the nonvascular production was obtained from average species growth rates, previously assessed at the sites. Results showed that aboveground vascular NPP (15–270 g m⁻²), annual NPP (214–282 g m⁻² or 102–137 g C m⁻²) and vegetation biomass (330–2450 g m⁻²) varied greatly among communities. Vegetation dominated by Empetrum hermaphroditum is more productive than Cassiope tetragona vegetation. Although the large majority of the apical NPP occurred in early-mid season (85%), production of stems and evergreen leaves proceeded until about 2 weeks before senescence. Most of the vascular vegetation was belowground (80%), whereas most of the vegetation production occurred aboveground (85%). Ecosystem C and N stocks were 2100–8200 g C m⁻² and 80–330 g N m⁻², respectively, stored mainly in the soil turf and in the fine organic soil. Such stocks are comparable to the C and N stocks of moister tundra types, such as tussock tundra. Author Contributions  Matteo Campioli, Anders Michelsen, Roeland Samson, Raoul Lemeur—conceived and designed study, Matteo Campioli, Anders Michelsen, Andreas Demey, Annemie Vermeulen—performed research, Matteo Campioli—analyzed data, and Matteo Campioli—wrote the paper.     

Isotopic signatures and nutrient relations of plants inhabiting brackish wetlands in the northeastern coastal plain of Venezuela

The semi-diurnal tidal regime (≥2 m) in the Paria Gulf on the Atlantic coast of Venezuela, and the flat landscape of the region, allow the penetration for tens of km of marine waters into the rivers draining the northeastern coastal plain of the country. The levels of salinity, tidal flooding, and sedimentation decrease perpendicularly from the river channel toward the back swamps. The vegetation varies sequentially from fringe mangroves along the river margins, to back swamps containing forests dominated by Pterocarpus officinalis, herbaceous communities of Lagenocarpus guianensis, and palm swamps with Mauritia flexuosa, Chrysobalanus icaco, and Tabebuia spp. This environmental structure was used to test the hypotheses that: (a) mangrove distribution is strongly associated with salinity of interstitial water, and (b) they occupy areas where tidal influence and sediment dynamics determine a relatively open N cycle. Analyses of soil, water, and plants along a 1.5 km transect located near the confluence of the Guanoco and San Juan Rivers (Sucre and Monagas States, Venezuela) revealed that: (a) conductivity decreased from 11 to 0.2 mmhos cm⁻¹ from the river fringe to the internal swamp, whereas Na in the same stretch decreased from 100 to 2 μM; (b) average leaf tissue concentrations of Na, P, and N decreased significantly along the transect; (c) P. officinalis showed a large Na-exclusion capacity indicated by positive K/Na ratios from 8 to 200, and Crinum erubescens counteracted Na by accumulating K above 1,000 mmol kg⁻¹; (d) leaves varied widely in δ ¹³C (−25.5 to −32‰) and δ ¹⁵N (4 to −10.5‰) values. Samples were aggregated according to soil carbon content corresponding to those of the mangrove forest belt (5–28 mol C kg⁻¹; 0–650 from river fringe) and those of the back swamps (40–44 mol C kg⁻¹; 700–1,500 m from river fringe). The concentrations of Na, P, and N (in mmol kg⁻¹) and δ ¹⁵N values (in ‰) were significantly higher in the mangrove forest compared to the back swamp (Na 213 vs. 88; P 41 vs. 16; N 1,535 vs. 727; δ ¹⁵N 1.5 vs. −3.7), indicating that the fringe forest was not nutrient limited. These results support the hypotheses that mangroves are restricted to the more-saline sections of the transect, and that the fringe forest has a more open N cycle, favoring ¹⁵N accumulation within the system.     

Effects of Carbon Concentrations and Carbon to Nitrogen ratios on Sporulation of Two Biological Control Fungi as Determined by Different Culture Methods

The nematophagous fungus Pochonia chlamydosporia (Clavicipitaceae) and entomopathogenic fungus Beauveria bassiana (Cordycipitaceae) have great potential for biological control. However, a significant barrier to their commercial development as mycopesticides is the high costs associated with production. Carbon (C) concentration and C to nitrogen ratio (C:N ratio) greatly affect fungal growth and sporulation. Effects of C concentration and C:N ratio differed when the fungi were cultivated using two different methods: the conventional (continuous cultivation) method and a novel “two-stage” method. Sporulation of P. chlamydosporia (HSY-12-14) was the highest when the media contained 6 g l⁻¹ C and a C:N ratio of 40:1 or 8 g l⁻¹ C and C:N ratios of 20:1 or 40:1 for the conventional method but 8 g l⁻¹ C and a C:N ratio of only 10:1 with the novel “two-stage” method. Sporulation of B. bassiana (IBC1201) was the highest when the media contained 12 g l⁻¹ C and a C:N ratio of 40:1 with the conventional method but only 4 g l⁻¹ C and a C:N ratio of 5:1 with the novel “two-stage” method. In addition, the nutritional requirements as determined by the conventional method differed for mycelial growth and sporulation. Understanding the effects of nutrition on sporulation can help programs seeking to use these organisms as biological control agents and is essential for their mass production and commercialization.     

Mechanisms of Araucaria (Atlantic) Forest Expansion into Southern Brazilian Grasslands

Recent studies have shown that tropical and subtropical forests expanded during the late Holocene, but rates and mechanisms of expansion are still unknown. Here, we investigate how a forest–grassland mosaic changed over the past 10,000 years at the southernmost limit of the Brazilian Atlantic forest. We used soil organic matter carbon isotopes (δ¹³C and ¹⁴C) to quantify and date changes in vegetation, examining soil properties and leaf traits of tree species (nutrient content, δ¹³C, δ¹⁵N, and specific leaf area—SLA) to describe potential mechanisms of expansion. Our results show that after several millennia of stability, forests have been expanding over grasslands through continuous, but very slow, border dynamics and patch formation (<100 m since ~4,000 YBP). This process of expansion coincided with past changes in climate, but biotic feedback mechanisms also appear to be important for the long-term persistence and expansion of forests. Soil fertility and microbial biomass match current rather than past vegetation distribution, increasing progressively across the gradient: grasslands < isolated trees < forest patches < forests. Foliar δ¹⁵N values of trees that are able to colonize the grassland are consistently lower across this vegetation gradient, suggesting an increasingly greater reliance on symbiotic nutrient uptake from grasslands to forests. No significant relationships were found between soil and leaf nutrients, but SLA explained variation in leaf N, P, and K (positive relationships) and in leaf δ¹³C (negative relationship). These findings suggest that a tradeoff between tree growth and water use efficiency is an important regulator of forest–grassland dynamics in the study region.     

Nitrogen Transformations in Flowpaths Leading from Soils to Streams in Amazon Forest and Pasture

The modification of large areas of tropical forest to agricultural uses has consequences for the movement of inorganic nitrogen (N) from land to water. Various biogeochemical pathways in soils and riparian zones can influence the movement and retention of N within watersheds and affect the quantity exported in streams. We used the concentrations of NO₃ ⁻ and NH₄ ⁺ in different hydrological flowpaths leading from upland soils to streams to investigate inorganic N transformations in adjacent watersheds containing tropical forest and established cattle pasture in the southwestern Brazilian Amazon Basin. High NO₃ ⁻ concentrations in forest soil solution relative to groundwater indicated a large removal of N mostly as NO₃ ⁻ in flowpaths leading from soil to groundwater. Forest groundwater NO₃ ⁻ concentrations were lower than in other Amazon sites where riparian zones have been implicated as important N sinks. Based on water budgets for these watersheds, we estimated that 7.3–10.3 kg N ha⁻¹ y⁻¹ was removed from flowpaths between 20 and 100 cm, and 7.1–10.2 kg N ha⁻¹ y⁻¹ was removed below 100 cm and the top of the groundwater. N removal from vertical flowpaths in forest exceeded previously measured N₂O emissions of 3.0 kg N ha⁻¹ y⁻¹ and estimated emissions of NO of 1.4 kg N ha⁻¹ y⁻¹. Potential fates for this large amount of nitrate removal in forest soils include plant uptake, denitrification, and abiotic N retention. Conversion to pasture shifted the system from dominance by processes producing and consuming NO₃ ⁻ to one dominated by NH₄ ⁺, presumably the product of lower rates of net N mineralization and net nitrification in pasture compared with forest. In pasture, no hydrological flowpaths contained substantial amounts of NO₃ ⁻ and estimated N removal from soil vertical flowpaths was 0.2 kg N ha⁻¹ y⁻¹ below the depth of 100 cm. This contrasts with the extent to which agricultural sources dominate N inputs to groundwater and stream water in many temperate regions. This could change, however, if pasture agriculture in the tropics shifts toward intensive crop cultivation.     

Soil Inorganic N Leaching in Edges of Different Forest Types Subject to High N Deposition Loads

We report on soil leaching of dissolved inorganic nitrogen (DIN) along transects across exposed edges of four coniferous and four deciduous forest stands. In a 64-m edge zone, DIN leaching below the main rooting zone was enhanced relative to the interior (at 128 m from the edge) by 21 and 14 kg N ha⁻¹ y⁻¹ in the coniferous and deciduous forest stands, respectively. However, the patterns of DIN leaching did not univocally reflect those of DIN throughfall deposition. DIN leaching in the first 20 m of the edges was lower than at 32–64 m from the edge (17 vs. 36 kg N ha⁻¹ y⁻¹ and 15 vs. 24 kg N ha⁻¹ y⁻¹ in the coniferous and deciduous forests, respectively). Nitrogen stocks in the mineral topsoil (0–30 cm) were, on average, 943 kg N ha⁻¹ higher at the outer edges than in the interior, indicating that N retention in the soil is probably one of the processes involved in the relatively low DIN leaching in the outer edges. We suggest that a complex of edge effects on biogeochemical processes occurs at the forest edges as a result of the interaction between microclimate, tree dynamics (growth and litterfall), and atmospheric deposition of N and base cations.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO₃⁻) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha⁻¹ y⁻¹ as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha⁻¹ y⁻¹). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha⁻¹ y⁻¹. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO₃⁻ leaching in all three forests. We hypothesize that abiotic incorporation of excess NO₃⁻ (through chemically reactive NO₂⁻) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. Dr. YT Fang performed research, analyzed data, and wrote the paper; Prof. WX Zhu participated in the initial experimental design, analyzed data, and took part in writing the paper; Prof. P Gundersen conceived the study and took part in writing; Prof. JM Mo and Prof. GY Zhou conceived study; Prof. M Yoh analyzed part of the data and contributed to the development of DON model.     

Dynamics of fine roots in five Chinese temperate forests

We used a minirhizotron method to investigate spatial and temporal dynamics of fine roots (diameter ≤2 mm) in five Chinese temperate forests: Mongolian oak forest, aspen-birch forest, hardwood forest, Korean pine plantation and Dahurian larch plantation. Fine root dynamics were significantly influenced by forest type, soil layer, and sampling time. The grand mean values varied from 1.99 to 3.21 mm cm⁻² (root length per minirhizotron viewing area) for the fine root standing crop; from 6.7 to 11.6 μm cm⁻² day⁻¹ for the production; and from 3.2 to 6.1 μm cm⁻² day⁻¹ for the mortality. All forests had a similar seasonal “sinusoidal” pattern of standing crop, and a “unimodal” pattern of production. However, the seasonal dynamics of the mortality were largely unsynchronized with those of the production. The minimum values of standing crop, production and mortality occurred in March for all forests, whereas the maximum values and occurrence time differed among forest types. The standing crop, production and mortality tended to decrease with soil depth. The different spatiotemporal patterns of fine roots among the forests highlight the need for forest-specific measurements and modeling of fine root dynamics and forest carbon allocation.     

An Unexpected Nitrate Decline in New Hampshire Streams

Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO₃ ⁻ concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376–381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973–74 and 1996–97, mean NO₃ ⁻ concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 μmol/L), Ca²⁺ decreased by 24% (8 μmol/L), and Mg²⁺ decreased by 22% (5 μmol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among streams persisted: Streams draining old-growth stands maintained higher NO₃ ⁻ concentrations than those draining successional stands. The cause of the NO₃ ⁻ decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO₃ ⁻ decline across the region. Although the role of climate remains uncertain, interannual climate variation and its effects on biotic N retention may be responsible for the synchronous decrease in NO₃ ⁻ across all streams, overriding expected increases due to chronic N deposition and forest aging. Received 4 December 2001; accepted 30 May 2002.     

Responses of Vegetation and Ecosystem CO2 Exchange to 9 Years of Nutrient Addition at Mer Bleue Bog

Anthropogenic nitrogen (N) loading has the potential to affect plant community structure and function, and the carbon dioxide (CO₂) sink of peatlands. Our aim is to study how vegetation changes, induced by nutrient input, affect the CO₂ exchange of a nutrient-limited bog. We conducted 9- and 4-year fertilization experiments at Mer Bleue bog, where we applied N addition levels of 1.6, 3.2, and 6.4 g N m⁻² a⁻¹, upon a background deposition of about 0.8 g N m⁻² a⁻¹, with or without phosphorus and potassium (PK). Only the treatments 3.2 and 6.4 g N m⁻² a⁻¹ with PK significantly affected CO₂ fluxes. These treatments shifted the Sphagnum moss and dwarf shrub community to taller dwarf shrub thickets without moss, and the CO₂ responses depended on the phase of vegetation transition. Overall, compared to the large observed changes in the vegetation, the changes in CO₂ fluxes were small. Following Sphagnum loss after 5 years, maximum ecosystem photosynthesis (Pg_max) and net CO₂ exchange (NEE_max) were lowered (−19 and −46%, respectively) in the highest NPK treatment. In the following years, while shrub height increased, the vascular foliar biomass did not fully compensate for the loss of moss biomass; yet, by year 8 there were no significant differences in Pg_max and NEE_max between the nutrient and the control treatments. At the same time, an increase (24–32%) in ecosystem respiration (ER) became evident. Trends in the N-only experiment resembled those in the older NPK experiment by the fourth year. The increasing ER with increasing vascular plant and decreasing Sphagnum moss biomass across the experimental plots suggest that high N deposition may lessen the CO₂ sink of a bog.     

Variability in glacial and Holocene marine pollen records offshore from west southern Africa

The distribution of pollen in marine sediments is used to record vegetation changes over the past 30,000 years on the adjacent continent. A transect of marine pollen sequences from the mouth of the river Congo (∼5°S) to Walvis Bay and Lüderitz (∼25°S) shows vegetation changes in Congo, Angola and Namibia from the last glacial period into the Holocene. The comparison of pollen records from different latitudes provides information about the latitudinal shift of open forest and savannahs (Poaceae pollen), the extension of lowland forest (rain forest pollen) and Afromontane forest (Podocarpus pollen), and the position of the desert fringe (pollen of Caryophyllaceae, Chenopodiaceae and Amaranthaceae). High Cyperaceae pollen percentages in sediments from the last glacial period off the mouth of the river Congo suggest the presence of open swamps rather than savannah vegetation in the Congo Basin. Pollen from Restionaceae in combination with Stoebe-type pollen (probably from Elytropappus) indicates a possible northwards extension of winter rain vegetation during the last glacial period. The record of Rhizophora (mangrove) pollen is linked to erosion of the continental shelf and sea-level rise. Pollen influx is highest off river mouths (10–2000 grains year⁻¹ cm⁻²), close to the coast (300–6000 grains year⁻¹ cm⁻²), but is an order of magnitude lower at sites situated far from the continent (<10 grains year⁻¹ cm⁻²).