Increasing precipitation event size increases aboveground net primary productivity in a semi-arid grassland

Water availability is the primary constraint to aboveground net primary productivity (ANPP) in many terrestrial biomes, and it is an ecosystem driver that will be strongly altered by future climate change. Global circulation models predict a shift in precipitation patterns to growing season rainfall events that are larger in size but fewer in number. This “repackaging” of rainfall into large events with long intervening dry intervals could be particularly important in semi-arid grasslands because it is in marked contrast to the frequent but small events that have historically defined this ecosystem. We investigated the effect of more extreme rainfall patterns on ANPP via the use of rainout shelters and paired this experimental manipulation with an investigation of long-term data for ANPP and precipitation. Experimental plots (n = 15) received the long-term (30-year) mean growing season precipitation quantity; however, this amount was distributed as 12, six, or four events applied manually according to seasonal patterns for May–September. The long-term mean (1940–2005) number of rain events in this shortgrass steppe was 14 events, with a minimum of nine events in years of average precipitation. Thus, our experimental treatments pushed this system beyond its recent historical range of variability. Plots receiving fewer, but larger rain events had the highest rates of ANPP (184 ± 38 g m⁻²), compared to plots receiving more frequent rainfall (105 ± 24 g m⁻²). ANPP in all experimental plots was greater than long-term mean ANPP for this system (97 g m⁻²), which may be explained in part by the more even distribution of applied rain events. Soil moisture data indicated that larger events led to greater soil water content and likely permitted moisture penetration to deeper in the soil profile. These results indicate that semi-arid grasslands are capable of responding immediately and substantially to forecast shifts to more extreme precipitation patterns.     

Climate change alters interannual variation of grassland aboveground productivity: evidence from a 22-year measurement series in the Inner Mongolian grassland

Climate change is known to influence interannual variation in grassland aboveground net primary productivity (ANPP), or seasonal biomass, but direct, long-term ground observations are rare. We present a 22-year (1982–2003) measurement series from the Inner Mongolia grassland, China, to examine the effect of climate change on interannual variations in ANPP and monthly aboveground biomass (MAB). ANPP exhibited no increase over 1982–2003 but there was an association with previous-year precipitation. MAB in May increased by 21.8% from 47.8 g m⁻² (averaged for 1982–1984) to 58.2 g m⁻² (2001–2003), whereas there was no significant variation in June, July and August, and a decrease of 29.7% in September. The MAB increase in May was correlated with increases in precipitation and temperature in the preceding months. These findings suggest that the effects of climate change on grassland production vary throughout the growing season, with warmer and wetter springs resulting in increased biomass early in the growing season, and drier falls causing a decrease in biomass late in the growing season.     

The Role of Surface and Subsurface Processes in Keeping Pace with Sea Level Rise in Intertidal Wetlands of Moreton Bay,Queensland, Australia

Increases in the elevation of the soil surfaces of mangroves and salt marshes are key to the maintenance of these habitats with accelerating sea level rise. Understanding the processes that give rise to increases in soil surface elevation provides science for management of landscapes for sustainable coastal wetlands. Here, we tested whether the soil surface elevation of mangroves and salt marshes in Moreton Bay is keeping up with local rates of sea level rise (2.358 mm y⁻¹) and whether accretion on the soil surface was the most important process for keeping up with sea level rise. We found variability in surface elevation gains, with sandy areas in the eastern bay having the highest surface elevation gains in both mangrove and salt marsh (5.9 and 1.9 mm y⁻¹) whereas in the muddier western bay rates of surface elevation gain were lower (1.4 and −0.3 mm y⁻¹ in mangrove and salt marsh, respectively). Both sides of the bay had similar rates of surface accretion (~7–9 mm y⁻¹ in the mangrove and 1–3 mm y⁻¹ in the salt marsh), but mangrove soils in the western bay were subsiding at a rate of approximately 8 mm y⁻¹, possibly due to compaction of organic sediments. Over the study surface elevation increments were sensitive to position in the intertidal zone (higher when lower in the intertidal) and also to variation in mean sea level (higher at high sea level). Although surface accretion was the most important process for keeping up with sea level rise in the eastern bay, subsidence largely negated gains made through surface accretion in the western bay indicating a high vulnerability to sea level rise in these forests.     

Holocene Carbon Stocks and Carbon Accumulation Rates Altered in Soils Undergoing Permafrost Thaw

Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere. Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated ∆¹⁴C, C:N, δ¹³C, and δ¹⁵N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial, over 50 kg C m⁻² in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ¹³C, and δ¹⁵N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper, frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m⁻² y⁻¹ and the rate for the deep soil accumulation was 2.3 g C m⁻² y⁻¹, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m⁻² y⁻¹. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten thousand years.     

Resource manipulation effects on net primary production,biomass allocation and rain-use efficiency of two semiarid grassland sites in Inner Mongolia,China

Productivity of semiarid grasslands is affected by soil water and nutrient availability, with water controlling net primary production under dry conditions and soil nutrients constraining biomass production under wet conditions. In order to investigate limitations on plants by the response of root–shoot biomass allocation to water and nitrogen (N) availability, a field experiment, on restoration plots with rainfed, unfertilized control plots, fertilized plots receiving N (25 kg urea-N ha⁻¹) and water (irrigation simulating a wet season), was conducted at two sites with different grazing histories: moderate (MG) and heavy (HG) grazing. Irrigation and N addition had no effect on belowground biomass. Irrigation increased aboveground (ANPP) and belowground net primary production (BNPP) and rain-use efficiency based on ANPP (RUE_ANPP), whereas N addition on rainfed plots had no effect on any of the measured parameters. N fertilizer application on irrigated plots increased ANPP and RUE_ANPP and reduced the root fraction (RF: root dry matter/total dry matter), resulting in smaller N effects on total net primary production (NPP) and rain-use efficiency based on NPP. This suggests that BNPP should be included in evaluating ecosystem responses to resource availability from the whole-plant perspective. N effects on all measured parameters were similar on both sites. However, site HG responded to irrigation with higher ANPP and a lower RF when compared to site MG, indicating that species composition had a pronounced effect on carbon allocation pattern due to below- and aboveground niche complementarity.     

Aboveground Net Primary Production and Leaf-Area Index in Early Postfire Vegetation in Yellowstone National Park

Aboveground net primary production (ANPP) and leaf-area index (LAI) of lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) saplings and aboveground productivity of herbaceous vegetation components were determined 9 years after the 1988 fires in Yellowstone National Park (YNP). Measurements were made in four sites representing a wide range of early postfire vegetation present in YNP, including high-density lodgepole pine, low-density lodgepole pine, and two nonforest stands. LAI of the pine saplings and total ANPP (trees plus herbs) generally increased with increasing sapling density, from 0.002 m² m^{− 2} and 0.25 Mg ha^{− 1} year^{− 1} in the infertile nonforest stand (100 pine saplings ha^{− 1}) to 1.8 m² m^{− 2} and 4.01 Mg ha^{− 1} year^{− 1} in the high-density pine stand (62,800 saplings ha^{− 1}). Aboveground herbaceous productivity was not strongly correlated with sapling density, but appeared to be influenced by soil fertility. In the high-density pine stand, tree ANPP and LAI were within the lower range of values reported for similar mature coniferous forests. This finding suggests that at least some ecosystem processes (related to ANPP and LAI) may have nearly recovered after only 9 years of postfire succession, in at least some of the young forests developing after the 1988 Yellowstone fires. Received 7 April 1998; accepted 1 December 1998.     

Soil Nutrients Limit Fine Litter Production and Tree Growth in Mature Lowland Forest of Southwestern Borneo

Efforts to improve models of terrestrial productivity and to understand the function of tropical forests in global carbon cycles require a mechanistic understanding of spatial variation in aboveground net primary productivity (ANPP) across tropical landscapes. To help derive such an understanding for Borneo, we monitored aboveground fine litterfall, woody biomass increment and ANPP (their sum) in mature forest over 29 months across a soil nutrient gradient in southwestern Kalimantan. In 30 (0.07 ha) plots stratified throughout the watershed (∼340 ha, 8–190 m a.s.l.), we measured productivity and tested its relationship with 27 soil parameters. ANPP across the study area was among the highest reported for mature lowland tropical forests. Aboveground fine litterfall ranged from 5.1 to 11.0 Mg ha⁻¹ year⁻¹ and averaged 7.7 ± 0.4 (mean ± 95 C.I.). Woody biomass increment ranged from 5.8 to 23.6 Mg ha⁻¹ year⁻¹ and averaged 12.0 ± 2.0. Growth of large trees (≥60 cm dbh) contributed 38–82% of plot-wide biomass increment and explained 92% of variation among plots. ANPP, the sum of these parameters, ranged from 11.1 to 32.3 Mg ha⁻¹ year⁻¹ and averaged 19.7 ± 2.2. ANPP was weakly related to fine litterfall (r ² = 0.176), but strongly related to growth of large trees at least 60 cm dbh (r ² = 0.848). Adjusted ANPP after accounting for apparent “mature forest bias” in our sampling method was 17.5 ± 1.2 Mg ha⁻¹ year⁻¹.Relating productivity measures to soil parameters showed that spatial patterning in productivity was significantly related to soil nutrients, especially phosphorus (P). Fine litterfall increased strongly with extractable P (r ² = 0.646), but reached an asymptote at moderate P levels, whereas biomass increment (r ² = 0.473) and ANPP (r ² = 0.603) increased linearly across the gradient. Biomass increment of large trees was more frequently and strongly related to nutrients than small trees, suggesting size dependency of tree growth on nutrients. Multiple linear regression confirmed the leading importance of soil P, and identified Ca as a potential co-limiting factor. Our findings strongly suggest that (1) soil nutrients, especially P, limit aboveground productivity in lowland Bornean forests, and (2) these forests play an important, but changing role in carbon cycles, as canopy tree logging alters these terrestrial carbon sinks. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Nutrient transfer by leaf litterfall during a sugar maple decline episode at Lake Clair watershed,Québec,Canada

In a declining sugar maple (SM) stand, we tested the hypothesis that an increasing relative abundance of American beech (AB) and yellow birch (YB) would improve litter quality by providing a higher proportion of litterfall richer in base cations and lower in acidity. From 1989 to 2006, SM leaf fall diminished from 59% (1,718 kg ha⁻¹ year⁻¹) to 36% (915 kg ha⁻¹ year⁻¹) of the total leaf fall biomass. Overall, the increase in AB and YB litterfall compensated for the SM decrease, resulting in constant annual leaf litterfall fluxes (2,803 kg ha⁻¹ year⁻¹) over the period studied. However, because the leaf litter for AB and YB had Ca and Mg concentrations 2–3 times higher than did SM, Ca and Mg concentrations and fluxes in leaf litterfall significantly increased between 1989 and 2006. Leaf litterfall of AB and YB also has a higher base/acid ratio than SM. Consequently, changes in forest composition following SM decline led to a clear improvement in litterfall quality in terms of base cations content and fluxes and acid–base properties.     

草地退化对三江源国家公园高寒草甸表层土壤有机碳、全氮、全磷的空间驱动

草地退化显著削弱了三江源高寒草甸的土壤肥力及生态承载功能,但空间尺度上的驱动强度和环境调控尚不清晰。在2020年7—8月,基于三江源国家公园高寒草甸典型分布区原生植被和退化植被的60个配对采样,研究表层(0—30 cm)土壤有机碳(SOC)、全氮(TN)和全磷(TP)含量对草地退化的空间响应特征。三江源国家公园高寒草甸原生植被SOC和TN含量分别为(2.45±2.05)%(平均值±标准差,下同)和(0.25±0.20)%,配对样本t-检验的结果表明草地退化导致SOC和TN分别极显著(P<0.001)下降了44.0%和35.6%。TP对草地退化无显著响应(P=0.22)。原生植被的土壤C∶N∶P平均为59.6∶6.2∶1.0,草地退化导致化学计量值平均下降28.3%。一般线性模型的结果表明草地退化对SOC和TN及土壤生态化学计量特征的空间降低强度主要取决于纬度和海拔(P<0.01),与经度和土壤深度关系较弱(P>0.30),即低纬度高海拔的高寒草甸响应相对强烈。草地退化导致三江源国家公园高寒草甸土壤碳氮损失严重,降低了土壤生态化学计量。研究结果可为三江源退化高寒草甸土壤...     

Belowground responses of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings to warming and nitrogen fertilization in the eastern Tibetan Plateau

The impacts of global climatic change on belowground ecological processes of terrestrial ecosystems are still not clear. We therefore conducted an experiment in the subalpine coniferous forest ecosystem of the eastern edges of the Tibetan Plateau to study roots of Picea asperata seedlings and rhizosphere soil responses to soil warming and nitrogen availability from April 2007 to December 2008. The seedlings were subjected to two levels of temperature (ambient; infrared heater warming) and two nitrogen levels (0 or 25 g m⁻²year⁻¹ N). We used a free air temperature increase from an overhead infrared heater to raise both air and soil temperature by 2.1 and 2.6°C, respectively. The results showed that warming alone significantly increased total biomass, coarse root biomass and fine root biomass of P. asperata seedlings. Both total biomass and fine root biomass were increased, but coarse root biomass was significantly decreased by nitrogen fertilization and warming combined with nitrogen fertilization. Warming induced a prominent increase in soil organic carbon (SOC) and NO₃ ⁻-N of rhizosphere soil, while nitrogen fertilization significantly decreased SOC and NH₄ ⁺-N of rhizosphere soil. The warming, fertilization and warming × N fertilization interaction decreased soil microbial C significantly, but substantially increased soil microbial N. These results suggest that nitrogen deposition combined with warmer temperatures under future climatic change possibly will have no effect on fine root production of P. asperata seedlings, but could enhance the nitrification process of their rhizosphere soils in subalpine coniferous forests.     

Responses of sawgrass and spikerush to variation in hydrologic drivers and salinity in Southern Everglades marshes

Aboveground net primary production (ANPP) by the dominant macrophyte and plant community composition are related to the changing hydrologic environment and to salinity in the southern Everglades, FL, USA. We present a new non-destructive ANPP technique that is applicable to any continuously growing herbaceous system. Data from 16 sites, collected from 1998 to 2004, were used to investigate how hydrology and salinity controlled sawgrass (Cladium jamaicense Crantz.) ANPP. Sawgrass live biomass showed little seasonal variation and annual means ranged from 89 to 639 gdw m⁻². Mortality rates were 20–35% of live biomass per 2 month sampling interval, for biomass turnover rates of 1.3–2.5 per year. Production by C. jamaicense was manifest primarily as biomass turnover, not as biomass accumulation. Rates typically ranged from 300 to 750 gdw m⁻² year⁻¹, but exceeded 1000 gdw m⁻² year⁻¹ at one site and were as high as 750 gdw m⁻² year⁻¹ at estuarine ecotone sites. Production was negatively related to mean annual water depth, hydroperiod, and to a variable combining the two (depth-days). As water depths and hydroperiods increased in our southern Everglades study area, sawgrass ANPP declined. Because a primary restoration goal is to increase water depths and hydroperiods for some regions of the Everglades, we investigated how the plant community responded to this decline in sawgrass ANPP. Spikerush (Eleocharis sp.) was the next most prominent component of this community at our sites, and 39% of the variability in sawgrass ANPP was explained by a negative relationship with mean annual water depth, hydroperiod, and Eleocharis sp. density the following year. Sawgrass ANPP at estuarine ecotone sites responded negatively to salinity, and rates of production were slow to recover after high salinity years. Our results suggest that ecologists, managers, and the public should not necessarily interpret a decline in sawgrass that may result from hydrologic restoration as a negative phenomenon.     

Winter as a nutritional bottleneck for North American porcupines (Erethizon dorsatum)

North American porcupines are distributed across a wide variety of habitats where they consume many different species of plants. Winter is a nutritional bottleneck for northern populations, because porcupines remain active when environmental demands are high and food quality is low. We used captive porcupines to examine physiological responses to low-quality diets at high energy demands during winter at ambient temperatures as low as −39°C. We did not observe an endogenous pattern of body mass gain or loss when porcupines were fed a low nitrogen diet (1.1% dry matter) ad libitum through winter. Dry matter intake declined from 43.6 to 14.6 g kg^−0.75 d⁻¹ even though ambient temperatures declined from −3 to −30°C, which indicates a seasonal decrease in metabolic rate. Porcupines consuming white spruce needles maintained digestive efficiency for energy (61%) and neutral detergent fiber (NDF) (50%). However, low requirements for energy (398 kJ kg^−0.75 d⁻¹) and nitrogen (209 mg kg^−0.75 d⁻¹) minimized the loss of body mass when intakes were low and plant toxins increased urinary losses of energy and nitrogen. Porcupines were also able to tolerate low intakes of sodium, even when dietary potassium loads were high. Porcupines use a flexible strategy to survive winter: low requirements are combined with a high tolerance for dietary imbalances that minimize the use of body stores when demands exceed supply. However, body stores are rapidly restored when conditions allow. Porcupines posses many physiological abilities similar to specialist herbivores, but retain the ability of a generalist to survive extreme conditions by using a variety of foods.     

Response of Litter Decomposition to Simulated N Deposition in Disturbed, Rehabilitated and Mature Forests in Subtropical China

The response of decomposition of litter for the dominant tree species in disturbed (pine), rehabilitated (pine and broadleaf mixed) and mature (monsoon evergreen broadleaf) forests in subtropical China to simulated N deposition was studied to address the following hypothesis: (1) litter decomposition is faster in mature forest (high soil N availability) than in rehabilitated/disturbed forests (low soil N availability); (2) litter decomposition is stimulated by N addition in rehabilitated and disturbed forests due to their low soil N availability; (3) N addition has little effect on litter decomposition in mature forest due to its high soil N availability. The litterbag method (a total of 2880 litterbags) and N treatments: Control-no N addition, Low-N: −5 g N m⁻² y⁻¹, Medium-N: −10 g N m⁻² y⁻¹, and High-N: −15 g N m⁻² y⁻¹, were employed to evaluate decomposition_. Results indicated that mature forest, which has likely been N saturated due to both long-term high N deposition in the region and the age of the ecosystem, had the highest litter decomposition rate, and exhibited no significant positive and even some negative response to nitrogen additions. However, both disturbed and rehabilitated forests, which are still N limited due to previous land use history, exhibited slower litter decomposition rates with significant positive effects from nitrogen additions. These results suggest that litter decomposition and its responses to N addition in subtropical forests of China vary depending on the nitrogen status of the ecosystem.     

Abiotic immobilization of nitrate in two soils of relic <Emphasis Type="Italic">Abies pinsapo</Emphasis>-fir forests under Mediterranean climate

Evidence for abiotic immobilization of nitrogen (N) in soil is accumulating, but remains controversial. Identifying the fate of N from atmospheric deposition is important for understanding the N cycle of forest ecosystems. We studied soils of two Abies pinsapo fir forests under Mediterranean climate seasonality in southern Spain—one with low N availability and the other with symptoms of N saturation. We hypothesized that biotic and abiotic immobilization of nitrate (NO₃ ⁻) would be lower in soils under these forests compared to more mesic temperate forests, and that the N saturated stand would have the lowest rates of NO₃ ⁻ immobilization. Live and autoclaved soils were incubated with added ¹⁵NO₃ ⁻ (10 μg N g⁻¹ dry soil; 99% enriched) for 24 h, and the label was recovered as total dissolved-N, NO₃ ⁻, ammonium (NH₄ ⁺), or dissolved organic-N (DON). To evaluate concerns about possible iron interference in analysis of NO₃ ⁻ concentrations, both flow injection analysis (FIA) and ion chromatography (IC) were applied to water extracts, soluble iron was measured in both water and salt extracts, and standard additions of NO₃ ⁻ to salt extracts were analyzed. Good agreement between FIA and IC analysis, low concentrations of soluble Fe, and 100% (±3%) recovery of NO₃ ⁻ standard additions all pointed to absence of an interference problem for NO₃ ⁻ quantification. On average, 85% of the added ¹⁵NO₃ ⁻ label was recovered as ¹⁵NO₃ ⁻, which supports our hypothesis that rates of immobilization were generally low in these soils. A small amount (mean = 0.06 μg N g⁻¹ dry soil) was recovered as ¹⁵NH₄ ⁺ in live soils and none in sterilized soils. Mean recovery as DO¹⁵N ranged from 0.6 to 1.5 μg N g⁻¹ dry soil, with no statistically significant effect of sterilization or soil type, indicating that this was an abiotic process that occurred at similar rates in both soils. These results demonstrate a detectable, but modest rate of abiotic immobilization of NO₃ ⁻ to DON, supporting our first hypothesis. These mineral soils may not have adequate carbon availability to support the regeneration of reducing microsites needed for high rates of NO₃ ⁻ reduction. Our second hypothesis regarding lower expected abiotic immobilization in soils from the N-saturated site was not supported. The rates of N deposition in this region may not be high enough to have swamped the capacity for soil NO₃ ⁻ immobilization, even in the stand showing some symptoms of N saturation. A growing body of evidence suggests that soil abiotic NO₃ ⁻ immobilization is common, but that rates are influenced by a combination of factors, including the presence of plentiful available carbon, reduced minerals in anaerobic microsites and adequate NO₃ ⁻ supply.     

Greenhouse Gas Budget of a Cool-Temperate Deciduous Broad-Leaved Forest in Japan Estimated Using a Process-Based Model

A terrestrial ecosystem model, called the Vegetation Integrative Simulator for Trace gases model (VISIT), which fully integrates biogeochemical carbon and nitrogen cycles, was developed to simulate atmosphere–ecosystem exchanges of greenhouse gases (CO₂, CH₄, and N₂O), and to determine the global warming potential (GWP) taking into account the radiative forcing effect of each gas. The model was then applied to a cool-temperate deciduous broad-leaved forest in Takayama, central Japan (36°08′N, 137°25′E, 1420 m above sea level). Simulations were conducted at a daily time step from 1948 to 2008, using time-series meteorological and nitrogen deposition data. VISIT accurately captured the carbon and nitrogen cycles of this typical Japanese forest, as validated by tower and chamber flux measurements. During the last 10 years of the simulation, the model estimated that the forest was a net greenhouse gas sink, having a GWP equivalent of 1025.7 g CO₂ m⁻² y⁻¹, most of which (1016.9 g CO₂ m⁻² y⁻¹) was accounted for by net CO₂ sequestration into forest biomass regrowth. CH₄ oxidation by the forest soil made a small contribution to the net sink (11.9 g CO₂-eq. m⁻² y⁻¹), whereas N₂O emissions were a very small source (3.2 g CO₂-eq. m⁻² y⁻¹), as expected for a volcanic soil in a humid climate. Analysis of the sensitivity of GWP to changes in temperature, precipitation, and nitrogen deposition indicated that warming temperatures would decrease the size of the sink, mainly as a result of increased CO₂ release due to increased ecosystem respiration.     

Extracellular Enzyme Activities and Soil Organic Matter Dynamics for Northern Hardwood Forests receiving Simulated Nitrogen Deposition

Anthropogenic nitrogen enrichment alters decomposition processes that control the flux of carbon (C) and nitrogen (N) from soil organic matter (SOM) pools. To link N-driven changes in SOM to microbial responses, we measured the potential activity of several extracellular enzymes involved in SOM degradation at nine experimental sites located in northern Michigan. Each site has three treatment plots (ambient, +30 and +80 kg N ha⁻¹ y⁻¹). Litter and soil samples were collected on five dates over the third growing season of N treatment. Phenol oxidase, peroxidase and cellobiohydrolase activities showed significant responses to N additions. In the Acer saccharum–Tilia americana ecosystem, oxidative activity was 38% higher in the litter horizon of high N treatment plots, relative to ambient plots, while oxidative activity in mineral soil showed little change. In the A. saccharum–Quercus rubra and Q. velutina–Q. alba ecosystems, oxidative activities declined in both litter (15 and 23%, respectively) and soil (29 and 38%, respectively) in response to high N treatment while cellobiohydrolase activity increased (6 and 39% for litter, 29 and 18% for soil, respectively). Over 3 years, SOM content in the high N plots has decreased in the Acer–Tilia ecosystem and increased in the two Quercus ecosystems, relative to ambient plots. For all three ecosystems, differences in SOM content in relation to N treatment were directly related (r² = 0.92) to an enzyme activity factor that included both oxidative and hydrolytic enzyme responses.     

Climate Variation and Soil Carbon and Nitrogen Cycling Processes in a Northern Hardwood Forest

We exploited the natural climate gradient in the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF) to evaluate the effects of climate variation similar to what is predicted to occur with global warming over the next 50–100 years for northeastern North America on soil carbon (C) and nitrogen (N) cycle processes. Our objectives were to (1) characterize differences in soil temperature, moisture and frost associated with elevation at the HBEF and (2) evaluate variation in total soil (TSR) and microbial respiration, N mineralization, nitrification, denitrification, nitrous oxide (N₂O) flux, and methane (CH₄) uptake along this gradient. Low elevation sites were consistently warmer (1.5–2.5°C) and drier than high elevation sites. Despite higher temperatures, low elevation plots had less snow and more soil frost than high elevation plots. Net N mineralization and nitrification were slower in warmer, low elevation plots, in both summer and winter. In summer, this pattern was driven by lower soil moisture in warmer soils and in winter the pattern was linked to less snow and more soil freezing in warmer soils. These data suggest that N cycling and supply to plants in northern hardwood ecosystems will be reduced in a warmer climate due to changes in both winter and summer conditions. TSR was consistently faster in the warmer, low elevation plots. N cycling processes appeared to be more sensitive to variation in soil moisture induced by climate variation, whereas C cycling processes appeared to be more strongly influenced by temperature.     

Biomass and aboveground net primary production in a subtropical mangrove stand of Kandelia obovata (S., L.) Yong at Manko Wetland, Okinawa, Japan

Biomass and aboveground net primary production (ANPP) in a monospecific pioneer stand of a mangrove Kandelia obovata (S., L.) Yong were quantified. The estimated biomasses in leaves, branches, stems, roots, aboveground and total were 5.61 (3.68%), 28.8 (18.9%), 46.1 (30.2%), 71.8 (47.2%), 80.5 (52.8%) and 152 Mg ha⁻¹ (100%), respectively. Stem phytomass increment per tree was estimated using allometric relationships and stem analysis. Stem volume without bark of harvested trees showed a strong allometric relationship with D _0.1² H (D _0.1, diameter at a height of one-tenth of tree height H) (R ² = 0.924). Annual stem volume increment per tree showed a strong allometric relationship with D _0.1² H (R ² = 0.860). Litterfall rate ranges from 3.87 to 56.1 kg ha⁻¹ day⁻¹ for leaves and 0.177 to 46.2 kg ha⁻¹ day⁻¹ for branches. Seasonal changes of litterfall rate were observed, which showed a peak during wet season (August–September). Total annual litterfall was estimated as 10.6 Mg ha⁻¹ year⁻¹, in which 68.2% was contributed by the leaves. The ANPP in the K. obovata stand was 29.9–32.1 Mg ha⁻¹ year⁻¹, which is ca. 2.8–3.0 times of annual litterfall. The growth efficiency (aboveground biomass increment/LAI) was 5.35–5.98 Mg ha⁻¹ year⁻¹. The low leaf longevity (9.3 months) and high growth efficiency of K. obovata makes it a highly productive mangrove species.     

To bloom or not to bloom: contrasting responses of cyanobacteria to recent heat waves explained by critical thresholds of abiotic drivers

Past heat waves are considered harbingers of future climate change. In this study, we have evaluated the effects of two recent Central European summer heat waves (2003 and 2006) on cyanobacterial blooms in a eutrophic, shallow lake. While a bloom of cyanobacteria developed in 2006, consistent with our expectations, cyanobacterial biomass surprisingly remained at a record-low during the entire summer of 2003. Critical thresholds of abiotic drivers extracted from the long-term (1993–2007) data set of the studied lake using classification tree analysis (CTA) proved suitable to explain these observations. We found that cyanobacterial blooms were especially favoured in 2006 because thermal stratification was critically intense (Schmidt stability >44 g cm cm⁻²) and long-lasting (>3 weeks). Our results also suggest that some cyanobacterial species (Anabaena sp.) benefitted directly from the stable water column, whereas other species (Planktothrix sp.) took advantage of stratification-induced internal nutrient loading. In 2003, conditions were less favourable for cyanobacteria due to a spell of lower temperatures and stronger winds in mid-summer; as a result, the identified thresholds of thermal stratification were hardly ever reached. Overall, our study shows that extracting critical thresholds of environmental drivers from long-term records is a promising avenue for predicting ecosystem responses to future climate warming. Specifically, our results emphasize that not average temperature increase but changes in short-term meteorological variability will determine whether cyanobacteria will bloom more often in a warmer world.