Lower Net Primary Productivity Research Articles

Elevational gradients in species abundance, assemblage structure and energy use of rainforest birds in the Australian Wet Tropics bioregion

AbstractElevational patterns of species richness, local abundance and assemblage structure of rainforest birds of north‐eastern Australia were explored using data from extensive standardized surveys throughout the region. Eighty‐two species of birds were recorded with strong turnover in assemblage structure across the elevational gradient and high levels of regional endemism in the uplands. Both species richness and bird abundance exhibited a humped‐shaped pattern with elevation with the highest values being between 600 and 800 m elevation. While much of the variability in species richness could be explained by the species–area relationship, analyses of net primary productivity (NPP) and total daily energy consumption of the bird community (energy use) suggest that ecosystem energy flow or constraints may be a significant determinant of species richness. Species richness is positively correlated with local bird abundance which itself is closely related to total energy use of the bird community. We suggest the hypothesis that lower NPP limits bird abundance and energy use in the uplands (>500 m) and that low bird energy use and species richness in the lowlands is limited by a seasonal bottleneck in available primary productivity and/or a species pool previously truncated by an extinction filter imposed by the almost complete disappearance of rainforest in the lowlands during the glacial maxima. We suggest that some of the previously predicted impacts of global warming on biodiversity in the uplands may be partially ameliorated by increases in NPP because of increasing temperatures. However, these relationships are complex and require further data specifically in regard to direct estimates of primary productivity and detailed estimates of energy flow within the assemblage.

Global potential net primary production predicted from vegetation class, precipitation, and temperature: comment

[Extract] Terrestrial vegetation acts as a reservoir for significant amounts of carbon and is involved in annual processing of carbon through photosynthesis and respiration (Clark 2007). Empirical study of contemporary patterns of terrestrial net primary production (NPP) is therefore critical for understanding how the global carbon budget will be affected by ongoing climate change (Clark et al. 2003). Field-based measurements of NPP are necessary to calibrate process-based models and remote sensed surrogates of productivity (Chen et al. 2004, Nightingale et al. 2008) as well as to build algorithms that link NPP to climate at global scales (Lieth 1975, Schuur 2003). In a recent analysis, Del Grosso et al. (2008) present an updated model of the global relationship between NPP and climate. The Del Grosso et al. (2008) model was considered to be an improvement on previous algorithms in two main respects. First, the model allowed for separate fitting of climate–NPP relationships for tree- and non-tree-dominated systems. For a given level of precipitation, non-tree-dominated systems were shown to exhibit significantly lower NPP than tree-dominated systems so it would seem sensible to parameterize equations for the two systems separately. Second, the new model drew upon a greatly expanded data set of NPP compiled as part of the Ecosystem Model–Data Intercomparison (EMDI) project (Olson et al. 2001) supported by the National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara, California, USA. The worldwide synthesis of data is expected to improve coverage, representation and replication of data from vegetation types, climates, and soils used to parameterize models. Here we raise some concerns about the methodological approach used to derive the “improved” models and their application in the retrospective reconstruction of past global trends in NPP in response to 20th-century climate change.

Separating effects of changes in atmospheric composition, climate and land-use on carbon sequestration of U.S. Mid-Atlantic temperate forests

Terrestrial carbon dynamics have been vastly modified because of changes in atmospheric composition, climate, and land-use. However, few studies provide a complete analysis of the factors and interactions that affect carbon dynamics over a large landscape. This study examines how changes in atmospheric composition (CO 2, O 3 and N deposition), climate and land-use affected carbon dynamics and sequestration in Mid-Atlantic temperate forests during the 20th century. We modified and applied the PnET-CN model, a well established process-based ecosystem model with a strong foundation of ecosystem knowledge from experimental studies. We validated the model results using the U.S. Forest Inventory and Analysis (FIA) data. Our results suggest that chronic changes in atmospheric chemistry over the past century markedly affected carbon dynamics and sequestration in Mid-Atlantic temperate forests, while climate change only had a minor impact although inter-annual climatic variability had a far more substantial effect. The NPP response to a century of chronic change in atmospheric composition at the regional scale was an increase of 29%, of which, 14% was from elevated CO 2, 17% from N deposition, 6% from the interaction between CO 2 and N deposition, and minus 8% from tropospheric ozone. Climate change increased NPP by only 4%. Disturbed forests had 6% lower NPP than undisturbed forests after seven decades. Regrowing forests after harvesting and natural disturbances had much greater capacity for sequestering carbon than undisturbed old-growth forests even though the newer forests had slightly lower net primary production (NPP). The modeling results indicated that N deposition was a stronger force than elevated CO 2 for increasing NPP and fast turnover tissues, while elevated CO 2 favored more sustainable carbon storage and sequestration. The model results are consistent with various experiments and observations and demonstrate a powerful approach to integrate and expand our knowledge of complex interactive effects of multiple environmental changes on forest carbon dynamics.

Why is marsh productivity so high? New insights from eddy covariance and biomass measurements in a Typha marsh

Researchers have a poor understanding of the mechanisms that allow freshwater marshes to achieve rates of net primary production (NPP) that are higher than those reported for most other types of ecosystems. We used an 8-year record of the gross primary production (GPP) and NPP at the San Joaquin Freshwater Marsh (SJFM) in Southern California to determine the relative importance of GPP and carbon use efficiency (CUE; the ratio of total NPP to GPP calculated as NPP GPP −1 ) in determining marsh NPP. GPP was calculated from continuous eddy covariance measurements and NPP was calculated from annual harvests. The NPP at the SJFM was typical of highly productive freshwater marshes, while the GPP was similar to that reported for other ecosystem types, including some with comparatively low NPPs. NPP was weakly related to GPP in the same year, and was better correlated with the GPP summed from late in the previous year's growing season to early in the current growing season. This lag was attributed to carbohydrate reserves, which supplement carbon for new leaf growth in the early growing season of the current year. The CUE at the SJFM for the 8-year period was 0.61 ± 0.05. This CUE is larger than that reported for tropical, temperate, and boreal ecosystems, and indicates that high marsh NPP is attributable to a high CUE and not a high GPP. This study underscores the importance of autotrophic respiration and carbon allocation in determining marsh NPP.

GLOBAL POTENTIAL NET PRIMARY PRODUCTION PREDICTED FROM VEGETATION CLASS, PRECIPITATION, AND TEMPERATURE

Net primary production (NPP), the difference between CO2 fixed by photosynthesis and CO2 lost to autotrophic respiration, is one of the most important components of the carbon cycle. Our goal was to develop a simple regression model to estimate global NPP using climate and land cover data. Approximately 5600 global data points with observed mean annual NPP, land cover class, precipitation, and temperature were compiled. Precipitation was better correlated with NPP than temperature, and it explained much more of the variability in mean annual NPP for grass- or shrub-dominated systems (r2 = 0.68) than for tree-dominated systems (r2 = 0.39). For a given precipitation level, tree-dominated systems had significantly higher NPP (approximately 100-150 g C m(-2) yr(-1)) than non-tree-dominated systems. Consequently, previous empirical models developed to predict NPP based on precipitation and temperature (e.g., the Miami model) tended to overestimate NPP for non-tree-dominated systems. Our new model developed at the National Center for Ecological Analysis and Synthesis (the NCEAS model) predicts NPP for tree-dominated systems based on precipitation and temperature; but for non-tree-dominated systems NPP is solely a function of precipitation because including a temperature function increased model error for these systems. Lower NPP in non-tree-dominated systems is likely related to decreased water and nutrient use efficiency and higher nutrient loss rates from more frequent fire disturbances. Late 20th century aboveground and total NPP for global potential native vegetation using the NCEAS model are estimated to be approximately 28 Pg and approximately 46 Pg C/yr, respectively. The NCEAS model estimated an approximately 13% increase in global total NPP for potential vegetation from 1901 to 2000 based on changing precipitation and temperature patterns.

Spatial Patterns of Anthropogenic Carbon Emission and Terrestrial Net Productivity

This paper describes the current spatial patterns of the net primary productivity (NPP) of the terrestrial vegetation and carbon emission (C) in the world due to the burning of fossil fuels in order to clarify the amount of expansion of human activity. The C/NPP value varies spatially from almost zero to several tens of thousand times the local NPP. C/NPP is higher under the condition of extensive human activities due to a high human population density or when the local NPP is extremely low in severe climatic zones. In contrast, the low C/NPP areas are distributed mainly in sparsely populated districts, loading to a low impact of human activity. Although the area where C/NPP is less than 10% accounts for about 70% of the entire land area, one-third of these areas cannot contribute to carbon absorption because of low NPP with a shortage of climatic resources. Since more than half of the areas of the remaining areas are agricultural land and forest ecosystems with high NPP, the possible afforestation area was evaluated to be maximum of <TEX>$30{\times}10^{6}\;km^{2}$</TEX>; here only sequestrate carbons that correspond to 2% of the global total NPP are present. These analyses revealed that presently most of the areas where the NPP is high are those exclusively used by humans and that it is difficult for large-scale forest plantations to absorb a substantial amount of the carbon emitted annually by humans.

Simulated ecosystem threshold responses to co‐varying temperature, precipitation and atmospheric CO2 within a region of Amazonia

ABSTRACTAim Using a Dynamic Global Vegetation Model (DGVM), we assessed the independent and co‐varying effects of temperature, precipitation and atmospheric CO2 on nonlinear (threshold) responses in carbon‐based processes, and evaluated whether these underlying process thresholds translate to the ecosystem‐scale.Location Amazon Basin, South America.Methods The Lund‐Potsdam‐Jena model (LPJ) was employed to determine responses in net primary production (NPP), heterotrophic respiration (RH), vegetation carbon (CV), soil carbon (CS), and plant functional type (PFT) composition to variations in temperature (± 9 °C relative to the control), precipitation (up to 80% reduction in rainfall relative to the control) and atmospheric CO2 (± 100 p.p.m.v. relative to the control).Results Our modelling experiments show that increases in temperature result in lower and steeper NPP and RH curves, indicating a thermal threshold at current temperature conditions. Under a combination of temperature and precipitation change, CV responds more to precipitation, while CS closely follows temperature gradients. Ecosystem thresholds, measured in terms of PFT composition stability, are surprisingly few. Simulations indicate an ecosystem threshold occurring at 80% reduction in rainfall; however, due to modelling limitations, this threshold is likely to occur at earlier drought stress conditions. Further empirical research on abiotic stress tolerance levels in tropical ecosystems must be performed in order to refine PFT descriptions used in DGVMs.Main conclusion In evaluating simulation scenarios that promote major changes in PFT assemblage, we conclude that the ‘natural’ Amazonian rain forest is resilient to environmental change, particularly to decreases in temperature and precipitation. Determining to what extent anthropogenic pressures have altered this resiliency is of utmost importance in predicting the future fate of the Amazon Basin.

Simulated ecosystem threshold responses to co-varying temperature, precipitation and atmospheric CO2 within a region of Amazonia

Aim Using a Dynamic Global Vegetation Model (DGVM), we assessed the independent and co-varying effects of temperature, precipitation and atmospheric CO2 on nonlinear (threshold) responses in carbon-based processes, and evaluated whether these underlying process thresholds translate to the ecosystem-scale. Location Amazon Basin, South America. Methods The Lund-Potsdam-Jena model (LPJ) was employed to determine responses in net primary production (NPP), heterotrophic respiration (RH), vegetation carbon (CV), soil carbon (CS), and plant functional type (PFT) composition to variations in temperature (± 9 °C relative to the control), precipitation (up to 80% reduction in rainfall relative to the control) and atmospheric CO2 (± 100 p.p.m.v. relative to the control). Results Our modelling experiments show that increases in temperature result in lower and steeper NPP and RH curves, indicating a thermal threshold at current temperature conditions. Under a combination of temperature and precipitation change, CV responds more to precipitation, while CS closely follows temperature gradients. Ecosystem thresholds, measured in terms of PFT composition stability, are surprisingly few. Simulations indicate an ecosystem threshold occurring at 80% reduction in rainfall; however, due to modelling limitations, this threshold is likely to occur at earlier drought stress conditions. Further empirical research on abiotic stress tolerance levels in tropical ecosystems must be performed in order to refine PFT descriptions used in DGVMs. Main conclusion In evaluating simulation scenarios that promote major changes in PFT assemblage, we conclude that the ‘natural’ Amazonian rain forest is resilient to environmental change, particularly to decreases in temperature and precipitation. Determining to what extent anthropogenic pressures have altered this resiliency is of utmost importance in predicting the future fate of the Amazon Basin.

Buffelgrass ( Pennisetum ciliare) land conversion and productivity in the plains of Sonora, Mexico

Bufflelgrass ( Pennisetum ciliare syn. Cenchrus ciliaris) is an African grass that has been widely introduced in subtropical arid regions of the world to improve rangelands for cattle production. However, it can have a negative effect on the diversity of native plant communities. Buffelgrass was introduced to Sonora, Mexico in the 1970s as a means to bolster the cattle industry. “Desmonte,” the process by which native desert vegetation is removed in preparation for buffelgrass seeding, alters the land surface such that buffelgrass plots are easily detectable from aerial and Landsat satellite images. We estimated the extent of conversion to buffelgrass in a 1,850,000 ha area centered on Hermosillo, from MSS and TM images from 1973, 1983, 1990 and 2000. We then compared the relative above-ground productivity of buffelgrass to native vegetation using Normalized Difference Vegetation Index values (NDVI) from Landsat and Moderate Resolution Imaging Spectrometer (MODIS) satellite sensor systems. Buffelgrass pastures have increased from just 7700 ha in 1973 to over 140,000 ha in 2000. Buffelgrass pastures now cover 8% of the land surface in the study area. Buffelgrass pastures have lower net primary productivity, estimated by MODIS NDVI values, than unconverted desert land. The desmonte process removes trees and shrubs, while the buffelgrass plantings are often sparse, leading to an apparent net loss in net primary production from land conversion. We recommend that the desmonte process be discontinued until its efficacy and safety for native ecosystems can be established, and that a comprehensive plan for preserving biodiversity while accomodating economic development be established for this region of the Sonoran Desert in Mexico.

Contrasting responses of forest ecosystems to rising atmospheric CO2: Implications for the global C cycle

In two parallel but independent experiments, Free Air CO2 Enrichment (FACE) technology was used to expose plots within contrasting evergreen loblolly pine (Pinus taeda L.) and deciduous sweetgum (Liquidambar styraciflua L.) forests to the level of CO2 anticipated in 2050. Net primary production (NPP) and net ecosystem production (NEP) increased in both forests. In the year 2000, after exposing pine and sweetgum to elevated CO2 for approximately 5 and 3 years, a complete budget calculation revealed increases in net ecosystem production (NEP) of 41% and 44% in the pine forest and sweetgum forest, respectively, representing the storage of an additional 174 gC m−2 and 128 gC m−2 in these forests. The stimulation of NPP without corresponding increases in leaf area index or light absorption in either forest resulted in 23–27% stimulation in radiation‐use efficiency, defined as NPP per unit absorbed photosynthetically active radiation. Greater plant respiration contributed to lower NPP in the loblolly pine forest than in the sweetgum forest, and these forests responded differently to CO2 enrichment. Where the pine forest added C primarily to long‐lived woody tissues, exposure to elevated CO2 caused a large increase in the production of labile fine roots in the sweetgum forest. Greater allocation to more labile tissues may cause more rapid cycling of C back to the atmosphere in the sweetgum forest compared to the pine forest. Imbalances in the N cycle may reduce the response of these forests to experimental exposure to elevated CO2 in the future, but even at the current stimulation observed for these forests, the effect of changes in land use on C sequestration are likely to be larger than the effect of CO2‐induced growth stimulation.

Long-term effects of fire frequency on carbon storage and productivity of boreal forests: a modeling study.

Climate change is predicted to shorten the fire interval in boreal forests. Many studies have recorded positive effects of fire on forest growth over a few decades, but few have modeled the long-term effects of the loss of carbon and nitrogen to the atmosphere. We used a process-based, dynamic, forest ecosystem model, which couples the carbon, nitrogen and water cycles, to simulate the effects of fire frequency on coniferous forests in the climate of Prince Albert, Saskatchewan. The model was calibrated to simulate observed forest properties. The model predicted rapid short-term recovery of net primary productivity (NPP) after fire, but in the long term, supported the hypotheses that (1) current NPP and carbon content of boreal forests are lower than they would be without periodic fire, and (2) any increase in fire frequency in the future will tend to lower NPP and carbon storage. Lower long-term NPP and carbon storage were attributable to (1) loss of carbon on combustion, equal to about 20% of NPP over a 100-200 year fire cycle, (2) loss of nitrogen by volatilization in fire, equal to about 3-4 kg N ha(-1) year(-1) over a 100-200 year fire cycle, and (3) the fact that the normal fire cycle is much shorter than the time taken for the forest (especially the soil) to reach an equilibrium carbon and nitrogen content. It was estimated that a shift in fire frequency from 200 to 100 years over 1000 Mha of boreal forest would release an average of about 0.1 Gt C year(-1) over many centuries.

An analysis of some diverse approaches to modelling terrestrial net primary productivity

We inter-compare the mathematical formulation of ten models of terrestrial net primary productivity (NPP) and their functional responses to temperature ( T), carbon dioxide (CO 2), soil water ( W) and photosynthetically active radiation (PAR). The models span a broad spectrum of complexity of approaches from the original, empirical Miami model, through β-factor, and global-average box models, to the dynamic global vegetation models (DGVMs) TRIFFID and BIOME3. Five of the models separate photosynthesis and respiration, although only three directly consider biochemistry. Equations for all the models are given in a complete, compact, standardized format. NPP responses to temperature differ markedly: β-factor models show only a tiny increase with temperature, empirical models predict a modest S-shaped response saturating above 35 °C (a surrogate for drought effects), biochemical and quasi-biochemical models show a peaked response with an optimum in the range 15–25 °C except for DEMETER (<10 °C). Qualitative differences in whether respiration exhibits a peaked or exponential response to temperature have only a modest effect on NPP. The CO 2 response of NPP is qualitatively similar in all but one model that misrepresents it. Where a water stress response of NPP is represented, it is saturating in all but the same model, where NPP declines under high soil moisture. Where represented, the PAR response of NPP is saturating, but the saturation point differs considerably. When responses to two environmental variables are combined, the second variable typically just scales the response to the first. In biochemical models, the temperature optimum of NPP increases with CO 2, and it also increases noticeably with PAR in TRIFFID and a derivative of it. Where different plant functional types (PFTs) are represented, C 4 grasses have the greatest maximum NPP at a higher optimum temperature than any C 3 PFT. Boreal, needle-leaf trees generally have the lowest NPP. Additional uncertainties in modelling respiration, stomatal conductance, light-use efficiency, and scaling to the canopy level are discussed. We suggest that a concerted effort to standardize definitions, notation, units and PFTs would add a degree of transparency that is currently lacking in NPP modelling.

Estimating net primary productivity of grasslands from field biomass measurements in temperate northern China

Data on field biomass measurements in temperate grasslands of northern China (141 samples from 74 sites) were obtained from 23 Chinese journals, reports and books. Net primary productivity (NPP) of grasslands was estimated using three algorithms (peak live biomass, peak standing crop and maximum minus minimum live biomass), respectively, based on availability of biomass data in sites. 135 samples which have aboveground biomass (AGB) measurements, have peak AGB ranges from 20 to 2021 g m–2 (mean = 325.3) and the aboveground NPP (ANPP) ranges from 15 to 1647.1 g m–2 per year (mean = 295.7). 72 samples which have belowground biomass (BGB) measurements, have peak BGB ranges from 226.5 to 12827.5 g m–2 (mean = 3116) and the belowground NPP (BNPP) ranges from 15.8 to 12827.5 g m–2 per year (mean = 2425.6). In total 66 samples have the total NPP (TNPP), ranging from 55.3 to 13347.8 g m–2 per year (mean = 2980.3). Mean peak biomass and NPP varied from different geographical sampling locations, but they had a general rough regularity in ten grasslands. Meadow, mountain and alpine grasslands had high biomass and NPP (sometimes including saline grassland). Forested steppe, saline grassland and desert had median values. Meadowed and typical steppes had low biomass and NPP (sometimes including desert). The lowest biomass and NPP occurred in deserted steppe and stepped desert. Grassland ANPP has significant positive relationships with annual and summer precipitation as well as summer temperature (all p<0.01). However, grassland BNPP and TNPP have more significant negative relationships with summer temperature (p<0.01) than with annual temperature (p<0.05). The analysis of climate – productivity correlations implied that aboveground productivity is more controlled by rainfall, whereas belowground and total productivity is more influenced by temperature in the temperate grasslands of northern China. The present study might underestimate grassland NPP in northern China due to limitation of biomass measurements. Data on relative long-term aboveground and belowground biomass dynamics, as well as data of standing dead matter, litterfall, decomposition and turnover, are required if grassland NPP is to be more accurately estimated and the role of temperate grasslands in the regional to global carbon cycles is to be fully appreciated.

Ecosystem health assessment in five shallow tropical waterbodies

Ecosystem health of five shallow tropical perennial waterbodies differing in pollution and productivity histories was evaluated by examining chemical parameters of water, biological production, and the Shannon diversity index. Samples of surface water were monitored every two weeks for pH, total hardness, chemical oxygen demand, dissolved oxygen, dissolved organic carbon, different species of nitrogen (ammonium, nitrite and nitrate nitrogen) and phosphorus (soluble reactive phosphate and total phosphorus) during the period of investigation. Net primary production of phytoplankton and abundance of zooplankton were also determined at biweekly intervals. High organic input either through bathing and domestic activities of large number of pilgrims during a 15-day annual fair in pond 2 or discharge of effluents from a brewery in pond 4 resulted in a sharper rise of net primary production than in the moderately manured fish pond (pond 3) or oligotrophic water body (pond 1). Influx of alkali from a gas factory into pond 5 was responsible for low net primary production and zooplankton abundance. The variability of net primary production in these waterbodies was primarily attributable to the variations of different species of phosphorus and nitrogen of surface water. Spatial and seasonal variability of some chemical indices such as chemical oxygen demand, dissolved oxygen and organic carbon concentrations suggested that pond 4 was more stable due to a consistent effluent discharge and pond 2 was less stable because of seasonal anthropogenic activity. The mean values of Shannon index of general diversity of zooplankton remained between 2.1 and 2.41 in pond 2 and showed the diversity pattern as in pond 1 and pond 3, whereas it remained between 1.0 and 2.0 in ponds 4 and 5, which suggested that stable chemical load throughout the year in these ponds resulted in loss of ecosystem health.

Modeling the spatial and temporal variability in climate and primary productivity across the Luquillo Mountains, Puerto Rico

There are few studies that have examined the spatial variability of forest productivity over an entire tropical forested landscape. In this study, we used a spatially-explicit forest productivity model, TOPOPROD, which is based on the FOREST-BGC model, to simulate spatial patterns of gross primary productivity (GPP), net primary productivity (NPP), and respiration over the entire Luquillo Experimental Forest (LEF) in the mountains of northeastern Puerto Rico. We modeled climate variables (e.g. solar insolation, temperature, rainfall and transpiration) using a topography-based climate model, TOPOCLIM. The simulated GPP ranged from 8 to 92 t C/ha per year with a mean of 51 t C/ha per year. The simulated NPP ranged from 0.5 to 24 t C/ha per year with a mean of 9.4 t C/ha per year. The simulated plant respiration ranged from 31 to 68 with a mean of 42 t C/ha per year. Simulated GPP and respiration declined with increased elevation whereas simulated NPP increased from low to middle elevation but decreased from middle to high elevations. Statistical analyses indicate that variation in solar insolation, which decreases with increase in elevation, is the most important factor controlling the spatial variation of forest productivity in the LEF. Validation with the limited spatial empirical data indicated that our simulations overestimated GPP by 2% for a middle elevation test site, and by 43% for a mountain peak site. Our simulations also overestimated NPP in the middle elevation Colorado forest and higher elevation Dwarf forest by 32 and 36%, respectively, but underestimated NPP in the Tabonuco and Palm forests at low to middle elevations by 9–15% and 18%, respectively. Simulated GPP and NPP would decrease under CO 2 doubling as projected temperatures increase and precipitation decreases. Different forest types respond differently to potential climate change and CO 2 doubling. Comparison with other tropical forests suggests that the LEF as a whole has higher GPP (51 t C/ha per year versus 40 t C/ha per year) but lower NPP (9.4 t C/ha per year versus 11 t C/ha per year) than other tropical rain forests.

The effects of partial throughfall exclusion on canopy processes, aboveground production, and biogeochemistry of an Amazon forest

Moist tropical forests in Amazonia and elsewhere are subjected to increasingly severe drought episodes through the El Niño–Southern Oscillation (ENSO) and possibly through deforestation‐driven reductions in rainfall. The effects of this trend on tropical forest canopy dynamics, emissions of greenhouse gases, and other ecological functions are potentially large but poorly understood. We established a throughfall exclusion experiment in an east‐central Amazon forest (Tapajós National Forest, Brazil) to help understand these effects. After 1‐year intercalibration period of two 1‐ha forest plots, we installed plastic panels and wooden gutters in the understory of one of the plots, thereby excluding ∼890 mm of throughfall during the exclusion period of 2000 (late January to early August) and ∼680 mm thus far in the exclusion period of 2001 (early January to late May). Average daily throughfall reaching the soil during the exclusion period in 2000 was 4.9 and 8.3 mm in the treatment and control plots and was 4.8 and 8.1 mm in 2001, respectively. During the first exclusion period, surface soil water content (0–2 m) declined by ∼100 mm, while deep soil water (2–11 m) was unaffected. During the second exclusion period, which began shortly after the dry season when soil water content was low, surface and deep soil water content declined by ∼140 and 160 mm, respectively. Although this depletion of soil water provoked no detectable increase in leaf drought stress (i.e., no reduction in predawn leaf water potential), photosynthetic capacity declined for some species, the canopy thinned (greater canopy openness and lower leaf area index) during the second exclusion period, stem radial growth of trees &lt;15 m tall declined, and fine litterfall declined in the treatment plot, as did tree fruiting. Aboveground net primary productivity (NPP) (stemwood increment and fine litter production) declined by one fourth, from 15.1 to 11.4 Mg ha−1 yr−1, in the treatment plot and decreased slightly, from 11.9 to 11.5 Mg ha−1 yr−1, in the control plot. Stem respiration varied seasonally and was correlated with stem radial growth but showed no treatment response. The fastest response to the throughfall exclusion, and the surface soil moisture deficits that it provoked, was found in the soil itself. The treatment reduced N2O emissions and increased CH4 consumption relative to the control plot, presumably in response to the improved soil aeration that is associated with soil drying. Our hypothesis that NO emissions would increase following exclusion was not supported. The conductivity and alkalinity of water percolating through the litter layer and through the mineral soil to a depth of 200 cm was higher in the treatment plot, perhaps because of the lower volume of water that was moving through these soil layers in this plot. Decomposition of the litter showed no difference between plots. In sum, the small soil water reductions provoked during the first 2 years of partial throughfall exclusion were sufficient to lower aboveground NPP, including the stemwood increment that determines the amount of carbon stored in the forest. These results suggest that the net accumulation of carbon in mature Amazon forests indicated by recent permanent plot and eddy covariance studies may be very sensitive to small reductions in rainfall. The soil water reductions were also sufficient to increase soil emissions of N2O and to increase soil consumption of CH4—both radiatively important gases in the atmosphere. The possible reduction of tree reproductive activity points to potentially important effects of drought on the long‐term species composition of Amazon forests.

Net primary production, carbon storage and climate change in Chinese biomes

Net primary production (NPP) and leaf area index (LAI) of Chinese biomes were simulated by BIOME3 under the present climate, and their responses to climate change and doubled CO2under a future climatic scenario using output from Hadley Center coupled ocean‐atmosphere general circulation model with CO2modelled at 340 and 500 ppmv. The model estimated annual mean NPP of the biomes in China to be between 0 and 1270.7 gC m‐2yr‐1at present. The highest productivity was found in tropical seasonal and rain forests while temperate forests had an intermediate NPP, which is higher than a lower NPP of temperate savannas, grasslands and steppes. The lowest NPP occurred in desert, alpine tundra and ice/polar desert in cold or arid regions, especially on the Tibetan Plateau. The lowest monthly NPP of each biome occurred generally in February and the highest monthly NPP occurred during the summer (June to August). The annual mean NPP and LAI of most of biomes at changed climate with CO2at 340 and 500 ppmv (direct effects on physiology) would be greater than present. The direct effects of carbon dioxide on plant physiology result in significant increase of LAI and NPP. The carbon storage of Chinese biomes at present and changed climates was calculated by the carbon density and vegetation area method. The present estimates of carbon storage are totally 175.83 × 1012gC (57.57 × 1012gC in vegetation and 118.28 × 1012gC in soils). Changed climate without and with the CO2direct physiological effects will result in an increase of carbon storage of 5.1 and 16.33 × 1012, gC compared to present, respectively. The interaction between elevated CO2and climate change plays an important role in the overall responses of NPP and carbon to climate change.

Succession May Maintain High Leaf Area: Sapwood Ratios and Productivity in OldSubalpine Forests

Old forests are generally believed to exhibit low net primary productivity (NPP) and therefore to be insignificant carbon sinks. This relationship between age and NPP is based, in part, on the hypothesis that the biomass of respiratory tissues such as sapwood increases with age to a point where all photosynthate is required just to maintain existing tissue. However, this theoretical connection between respiration:assimilation ratios and forest productivity is based on age-dependent trends in the sapwood:leaf ratios of individual trees and even-aged stands; it does not take into account such processes in natural forests as disproportional increases in shade-tolerant species over time and multiple-age cohorts. Ignoring succession and structural complexity may lead to large underestimates of the productivity of old forests and inaccurate estimates of the ages at which forest productivity declines. To address this problem, we compared biomass allocation and productivity between whitebark pine, a shade-intolerant, early-successional tree species, and subalpine fir, a shade-tolerant, late-successional species, by harvesting 14 whitebark pines and nine subalpine firs that varied widely in dbh and calculating regression models for dbh vs annual productivity and biomass allocation to leaves, sapwood, and heartwood. Late-successional subalpine fir allocated almost twice as much biomass to leaves as early-successional whitebark pine. Subalpine firs also had a much lower allocation to sapwood and higher growth rates across all tree sizes. We then modeled biomass allocation and productivity for 12 natural stands in western Montana that were dominated by subalpine fir and whitebark pine varying in age from 67 to 458 years by applying the regressions to all trees in each stand. Whole-stand sapwood:leaf ratios and stand productivity increased asymptotically with age. Sapwood:leaf ratios and productivity of whitebark pine in these stands increased for approximately 200–300 years and then decreased slowly over the next 200 years. In contrast, sapwood:leaf ratios of all sizes of subalpine fir were lower than those of pine and productivity was higher. As stands shifted in dominance from pine to fir with age, subalpine fir appeared to maintain gradually increasing rates of whole-forest productivity until stands were approximately 400 years old. These results suggest that forests such as these may continue to sequester carbon for centuries. If shade-tolerant species that predominate late in succession maintain high assimilation-to-respiration ratios in other forests, we may be underestimating production in old forests, and current models may underestimate the importance of mature forests as carbon sinks for atmospheric CO2 in the global carbon cycle.

Biomass accumulation over the first 150 years in coastal Oregon Picea‐Tsuga forest

Abstract. Production and mortality are the component processes that together determine the biomass dynamics of forests. Due to the significant role of forests in the global carbon cycle, it is important to assess how these two processes affect the maximum biomass attained by forests, as well as the dynamics leading up to and following peak biomass. We address these questions for two sets of plots in Picea sitchensis‐Tsuga heterophylla forest on the northern Oregon coast that originated from a catastrophic wildfire in the 1840s, using new data on dynamics of live trees and stocks of coarse woody debris (CWD). The set of plots closest to the ocean and occupying steeper, more dissected terrain with areas of thin soils has lower biomass, lower net primary production (NPP) of bole wood and higher tree mortality as a fraction of standing biomass. The two sets of plots have similar CWD levels, most of which has accumulated in the last 25 yr. The present disparity in biomass between the two sets of plots appears to be the result of lower NPP on the low‐biomass plots for the entire 140+ yr history of the forest. Over the 58 yr that the high‐biomass plots have been measured (from stand age 85 to 143 yr), NPP of bole wood has declined by 41%. Only ca. 6% of this decline can be accounted for by an increase in maintenance respiration of woody tissues. For both sets of plots relative constancy of biomass in the long term appears likely, due to a short time lag in tree regeneration, asynchronous tree mortality and little overall decline in NPP of bole wood in recent decades. However, since tree mortality as a fraction of standing biomass is higher on the low‐biomass plots, and NPP of bole wood is slightly lower, the difference in biomass between the two sets of plots should increase if current rates of production and mortality persist.

Aboveground production and nutrient circulation along a flooding gradient in a South Carolina Coastal Plain forest

Relative to effects of flooding, little is known about the influence of hydrology-nutrient interactions on aboveground net primary production (NPP) in forested wetlands. We found that nutrient circulation and NPP were closely related along a complex physical, chemical, and hydrologic gradient in a bottomland hardwood forest with four distinct communities. Aboveground biomass, NPP, biomass partitioning to stem production, growth efficiency, and soil macronutrient availability were greatest in the flooded zone, possibly because of the stable hydrologic regime. In the wet transition zone, trees were least productive, nutrient use efficiency was highest, and N retranslocation from foliage before abscission was "complete." Wet and dry transition zones had the lowest litterfall quality. Soil organic matter was negatively correlated with extractable NH4-N plus NO3-N before in situ incubations and positively correlated with litterfall lignin/N ratios. Lignin/P and C/N ratios were positively correlated with exchangeable soil Ca and Mg, cation exchange capacity, and clay content and negatively correlated with extractable soil P. We concluded that periodic flooding and associated widely fluctuating soil chemistry resulted in disequilibrium between the plant community and environmental conditions, which led to nutrient deficiency and low NPP in the transition zones compared with the continuously flooded and mesic zones.