Mean Annual Net Primary Productivity Research Articles

Net primary productivity of forest stands in New Hampshire estimated from Landsat and MODIS satellite data

BackgroundA simulation model that relies on satellite observations of vegetation cover from the Landsat 7 sensor and from the Moderate Resolution Imaging Spectroradiometer (MODIS) was used to estimate net primary productivity (NPP) of forest stands at the Bartlett Experiment Forest (BEF) in the White Mountains of New Hampshire.ResultsNet primary production (NPP) predicted from the NASA-CASA model using 30-meter resolution Landsat inputs showed variations related to both vegetation cover type and elevational effects on mean air temperatures. Overall, the highest predicted NPP from the NASA-CASA model was for deciduous forest cover at low to mid-elevation locations over the landscape. Comparison of the model-predicted annual NPP to the plot-estimated values showed a significant correlation of R2 = 0.5. Stepwise addition of 30-meter resolution elevation data values explained no more than 20% of the residual variation in measured NPP patterns at BEF. Both the Landsat 7 and the 250-meter resolution MODIS derived mean annual NPP predictions for the BEF plot locations were within ± 2.5% of the mean of plot estimates for annual NPP.ConclusionAlthough MODIS imagery cannot capture the spatial details of NPP across the network of closely spaced plot locations as well as Landsat, the MODIS satellite data as inputs to the NASA-CASA model does accurately predict the average annual productivity of a site like the BEF.

The interactive climate and vegetation along the Pole-Equator belts simulated by a global coupled model

The interaction between climate and vegetation along four Pole-Equator-Pole (PEP) belts were explored using a global two-way coupled model, AVIM-GOALS, which links the ecophysiological processes at the land surface with the general circulation model (GCM). The PEP belts are important in linking the climate change with the variation of sea and land, including terrestrial ecosystems. Previous PEP belts studies have mainly focused on the paleoclimate variation and its reconstruction. This study analyzes and discusses the interaction between modern climate and vegetation represented by leaf area index (LAI) and net primary production (NPP). The results show that the simulated LAI variation, corresponding to the observed LAI variation, agrees with the peak-valley variation of precipitation in these belts. The annual mean NPP simulated by the coupled model is also consistent with PIK NPP data in its overall variation trend along the four belts, which is a good example to promote global ecological studies by coupling the climate and vegetation models. A large discrepancy between the simulated and estimated LAI emerges to the south of 15°N along PEP 3 and to the south of 18°S in PEP 1S, and the discrepancy for the simulated NPP and PIK data in the two regions is relatively smaller in contrast to the LAI difference. Precipitation is a key factor affecting vegetation variation, and the overall trend of LAI and NPP corresponds more obviously to precipitation variation than temperature change along most parts of these PEP belts.

Satellite Remote Sensing of Terrestrial Net Primary Production for the Pan-Arctic Basin and Alaska

We applied a terrestrial net primary production (NPP) model driven by satellite remote sensing observations of vegetation properties and daily surface meteorology from a regional weather forecast model to assess NPP spatial and temporal variability for the pan-Arctic basin and Alaska from 1982 to 2000. Our results show a general decadal trend of increasing NPP for the region of approximately 2.7%, with respective higher (3.4%) and lower (2.2%) rates for North America and Eurasia. NPP is both spatially and temporally dynamic for the region, driven largely by differences in productivity rates among major biomes and temporal changes in photosynthetic canopy structure and spring and summer air temperatures. Mean annual NPP for boreal forests was approximately 3 times greater than for Arctic tundra on a unit area basis and accounted for approximately 55% of total annual carbon sequestration for the region. The timing of growing season onset inferred from regional network measurements of atmospheric CO2 drawdown in spring was inversely proportional to annual NPP calculations. Our findings indicate that recent regional warming trends in spring and summer and associated advances in the growing season are stimulating net photosynthesis and annual carbon sequestration by vegetation at high latitudes, partially mitigating anthropogenic increases in atmospheric CO2. These results also imply that regional sequestration and storage of atmospheric CO2 is being altered, with potentially greater instability and acceleration of the carbon cycle at high latitudes.

Climatic and biological simulations in a two-way coupled atmosphere–biosphere model (CABM)

Today, most land surface process models have prescribed seasonal change of vegetation with regard to the exchange processes between land and the atmosphere. However, in order to consider the real interaction between vegetation and atmosphere and represent it best in a climate model, the vegetation growth process should be included. In other words, “life” should be brought into climate models. In this study, we have coupled the physical and biological components of AVIM (Atmosphere–Vegetation Interaction Model), a land surface model including plant ecophysiological processes, into the IAP/LASG L9 R15 GOALS GCM. To exhibit terrestrial vegetation information, the vegetation is given a high resolution of 1.5° by 1.5° to nest and couple the fine grid cells of land with the coarse grid cells of atmosphere, which is 7.5° longitude and 4.5° latitude. The simulated monthly mean surface air temperature and precipitation is close to the observations. The monthly mean Leaf Area Index (LAI) is consistent with the observed data. The global annual mean net primary production (NPP) simulation is also reasonable. The coupled model is stable, providing a good platform for research on two-way interaction between land and atmosphere, and the global terrestrial ecosystem carbon cycle.

Above- and belowground biomass and net primary production in a 73-year-old Scots pine forest.

We estimated above- and belowground biomass and net primary production (NPP) of a 73-year-old Scots pine (Pinus sylvestris L.) forest stand in the Belgian Campine region. Total biomass for the stand was 176 Mg ha(-1), of which 74.4% was found in stems. The root system contained 12.6% of total biomass, most of it in coarse roots (> 5 mm). Fine roots (< 5 mm) comprised only about 1.7% of total biomass, and more than 50% of fine root biomass was retrieved in the litter layer and the upper 15 cm of the mineral soil. The ratio of belowground biomass to aboveground biomass was 0.14, which is lower than that of other Scots pine forests and other coniferous forests. Between 1995 and 2001, mean annual NPP was 11.2 Mg ha(-1) year(-1), of which 68.7% was allocated to aboveground compartments. Stems, needles and cones made relatively high contributions to total NPP compared with branches. However, branch NPP was possibly underestimated because litterfall of big branches was neglected. The proportion of total NPP in belowground components was 31.3%. Coarse root NPP (2% of total) was low compared with its biomass. Fine root NPP was 3.3 Mg ha(-1) year(-1), representing about 29.5% of total NPP; however, the estimate of fine root NPP is much more uncertain than NPP of aboveground compartments. The ratio NPP/GPP (gross primary production) was 0.32, which was low compared with other coniferous forests.

Estimating regional terrestrial carbon fluxes for the Australian continent using a multiple-constraint approach. I. Using remotely sensed data and ecological observations of net primary production

We have developed a modelling framework that synthesizes various types of field measurements at different spatial and temporal scales. We used this modelling framework to estimate monthly means and their standard deviations of gross photosynthesis, total ecosystem production, net primary production (NPP) and net ecosystem production (NEP) for eight regions of the Australian continent between 1990 and 1998. Annual mean NPP of the Australian continent varied between 800 and 1100 Mt C yr−1 between 1990 and 1998, with a coefficient of variation that is defined as the ratio of standard deviation and mean between 0.24 and 0.34. The seasonal variation of NPP for the whole continent varied between 50 and 110 Mt C month−1 with two maxima, one in the autumn and another in the spring. NEP was most negative in the winter (a carbon sink) and was most positive (a carbon source) in the summer. However, the coefficient of variation of monthly mean NEP was very large (> 4), and consequently confidence in the predicted net carbon fluxes for any month in the period 1990–1998 for the whole continent was very low. A companion paper will apply atmospheric inverse technique to measurements of CO2 concentration to further constrain the continental carbon cycle and reduce uncertainty in estimated mean monthly carbon fluxes.

Impact of Disturbance of Planting Amomum villosum in Tropical Seasonal Rainforest on Forest net Primary Productivity in Xishuangbanna

Xishuangbanna, located in south Yunnan, southwest China, is the northern border of the tropical zone. It maintains large areas of tropical rainforest called tropical seasonal rainforest. Like most tropical rainforests all over the world, these tropical seasonal rainforests are under high pressure of long_term disturbance caused by forest utilization. Amomum villosum, a shade_tolerant perennial herb, prefers to grow in forest gaps. It is one of the main cash crops of local people as its fruit is widely used in Chinese traditional medicine. It has been widely planted in tropical seasonal rainforest understorey in Xishuangbanna. To promote its growth and fruit yield by raising the light level of habitat, the forest shrub and herb layers are cleaned and about 60% 70% of trees are thinned. As a result, A. villosum planting has become the most serious disturbance to the tropical seasonal rainforest. Net primary productivity ( NPP ) is the primary element of nutrient cycling and energy flow within an ecosystem. It reflects the ability of a plant community to use natural resources. Most of the studies on forest NPP change after disturbance have focused on secondary forest, and few studies report the impact on NPP of forest canopy damage. This paper aims to determine the effect of A. villosum planting on forest NPP , and to study if that disturbance is the current most important limiting factor on NPP of the rainforest in Xishuangbanna. The study was carried out at Menglun, Xishuangbanna. Three primary seasonal rainforest sites and three disturbed rainforest sites with A. villosum plantation were chosen for the study and one 0.25 hm 2 plot was established at each site. All six study sites were distributed along ravines, within 21°55′ 21°59′N,101°08′ 101°13′E and at altitude of 650 800 m. Pometia tomentosa is the dominant tree species of the top tree layers at all research sites. At each plot, all trees and lianas with diameter at breast height ( DBH ) ≥5 cm were numbered and marked at breast height, and their DBH were measured annually. The biomass ( B ) and its annual increment ( ΔB ) of the research plots was estimated using the allometric regression equation relating tree mass to DBH of Xishuangbanna tropical rainforest. The amount of dead wood on all plots was recorded annually. At the same time, 20 litter fall traps were placed at each plot and litter was collected every half month. Leaf herbivory was measured by leaf samples and the total leaf herbivory ( G ) was estimated through annual leaf litterfall. The primary net productivity was calculated using the NPP equation NPP = ΔB+L+G, in which L is annual litterfall together with dead wood. The shrub and herb biomass were determined by the harvest method and their biomass increment was estimated by biomass divided by age. The biomass increment for shrub and herb was approximate to NPP . The results show that the annual mean NPP (mean±SE) of the primary rainforest is 23.47±2.12 t·hm -2 ·a -1 , with 22.04±2.09 t·hm -2 ·a -1 in the tree layer, 0.75±0.08 t·hm -2 ·a -1 in the shrub layer, 0.43±0.05 t·hm -2 ·a -1 in woody liana and 0.25±0.03 t·hm -2 ·a -1 in the herb layer. The allocation of NPP of the tree layer was: for litterfall, 11.75±0.54 t·hm -2 ·a -1 ; for tree fall, 0.62±0.21 t·hm -2 ·a -1 ; for leaf herbivory, 0.66±0.05 t·hm -2 ·a -1 ; and for biomass accumulation, 9.01±2.70 t·hm -2 ·a -1 . Compared to the primary rainforest, the NPP of the tree layer, shrub layer, woody liana and total community of the disturbed rainforest decreased 26.1%, 65.7%, 86.1% and 22.5% respectively, but the herb NPP increased 536% because of the dominance of A. villosum. Similarly, litterfall and biomass accumulation of the tree layer decreased 25.5% and 53.4% respectively. The tree fall increased 356% relative to the primary forest due to the change of microenvironment after disturb

Estimating regional terrestrial carbon fluxes for the Australian continent using a multiple-constraint approach I. Using remotely sensed data and ecological observations of net primary production

We have developed a modelling framework that synthesizes various types of field measurements at different spatial and temporal scales. We used this modelling framework to estimate monthly means and their standard deviations of gross photosynthesis, total ecosystem production, net primary production (NPP) and net ecosystem production (NEP) for eight regions of the Australian continent between 1990 and 1998. Annual mean NPP of the Australian continent varied between 800 and 1100 Mt C yr−1 between 1990 and 1998, with a coefficient of variation that is defined as the ratio of standard deviation and mean between 0.24 and 0.34. The seasonal variation of NPP for the whole continent varied between 50 and 110 Mt C month−1 with two maxima, one in the autumn and another in the spring. NEP was most negative in the winter (a carbon sink) and was most positive (a carbon source) in the summer. However, the coefficient of variation of monthly mean NEP was very large (> 4), and consequently confidence in the predicted net carbon fluxes for any month in the period 1990–1998 for the whole continent was very low. A companion paper will apply atmospheric inverse technique to measurements of CO2 concentration to further constrain the continental carbon cycle and reduce uncertainty in estimated mean monthly carbon fluxes.

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.

Interannual variability of global terrestrial primary production: Results of a model driven with satellite observations

Interannual variation in terrestrial net primary production (NPP) was modeled using the global production efficiency model (GLO‐PEM), a semimechanistic plant photosynthesis and respiration model driven entirely with satellite advanced very high resolution radiometer (AVHRR) observations. The model also estimated a wide range of biophysical variables at 10‐day intervals for the period 1982–1989, including air temperature, vapor pressure deficit, soil moisture, biomass, autotrophic respiration, canopy‐absorbed photosynthetically active radiation, gross primary production, and light use efficiency. The accuracy of the simulated variables has previously been shown to be within 10–30% of field measurements, depending on the specific variable. We analyze here interannual changes in NPP, which showed large spatial variability (0–1500 gC m−2 yr−1) and trends that differed regionally over the 8‐year period. Annually integrated global NPP was found to vary as much as 12% between years and was very sensitive to air temperature. The coefficient of variation in NPP of sparsely vegetated areas (mostly semiarid) on an interannual basis was as much as 80%, whereas densely vegetated areas (broadleaf evergreen and seasonally deciduous forests) varied comparatively little (0–10%). Mean annual NPP of the latter decreased 36 gC m−2 yr−1 over the time series examined. There was extreme seasonal and moderate interannual variation (10–60%) in NPP of middle‐ to high‐latitude regions (temperate and boreal forests) with evidence for a slight trend toward increased values through time (+3 to 12 gC m−2 yr−1). The results indicate significant interannual and regional differences in responses to climate variability, with boreal regions increasing 39 gC m−2 yr−1 compared to a decrease of 116 gC m−2 yr−1 in tropical regions for each 1°C rise in air temperature. We explore a few of the possible reasons for these observations and discuss some of the issues and limitations to the use of the current global AVHRR observational record.

Comparison of alternative spatial resolutions in the application of a spatially distributed biogeochemical model over complex terrain

Spatially distributed biogeochemical models may be applied over grids at a range of spatial resolutions, however, evaluation of potential errors and loss of information at relatively coarse resolutions is rare. In this study, a georeferenced database at the 1-km spatial resolution was developed to initialize and drive a process-based model (Forest-BGC) of water and carbon balance over a gridded 54976 km 2 area covering two river basins in mountainous western Oregon. Corresponding data sets were also prepared at 10-km and 50-km spatial resolutions using commonly employed aggregation schemes. Estimates were made at each grid cell for climate variables including daily solar radiation, air temperature, humidity, and precipitation. The topographic structure, water holding capacity, vegetation type and leaf area index were likewise estimated for initial conditions. The daily time series for the climatic drivers was developed from interpolations of meteorological station data for the water year 1990 (1 October 1989–30 September 1990). Model outputs at the 1-km resolution showed good agreement with observed patterns in runoff and productivity. The ranges for model inputs at the 10-km and 50-km resolutions tended to contract because of the smoothed topography. Estimates for mean evapotranspiration and runoff were relatively insensitive to changing the spatial resolution of the grid whereas estimates of mean annual net primary production varied by 11%. The designation of a vegetation type and leaf area at the 50-km resolution often subsumed significant heterogeneity in vegetation, and this factor accounted for much of the difference in the mean values for the carbon flux variables. Although area-wide means for model outputs were generally similar across resolutions, difference maps often revealed large areas of disagreement. Relatively high spatial resolution analyses of biogeochemical cycling are desirable from several perspectives and may be particularly important in the study of the potential impacts of climate change.

Possible Impacts of Global Warming on Tundra and Boreal Forest Ecosystems: Comparison of Some Biogeochemical Models

Global warming affects the magnitude of carbon, water and nitrogen fluxes between biosphere and atmosphere as well as the distribution of vegetation types. Biogeochemical models, global as well as patch models, can be used to estimate the differences between the mean values of annual net primary production (NPP) for present and for future climate scenarios. Both approaches rely on the prescribed pattern of vegetation types. Structural, rule-based models can predict such patterns, provided that vegetation and climate are in equilibrium. The coupling of biogeochemical and structural models gives the opportunity of testing the sensitivity of biogeochemical processes not only to climatic change but also to biome shifts. Whether the annual mean NPP of a vegetation type increases or decreases depends strongly on the assumptions about a CO2 fertilization effect and nitrogen cycling. Results from our coupled model show that, given that direct CO2 effects are uncertain, (i) average NPP of these northern biomes might decrease under global warming, but (ii) total NPP of the region would increase, due to the northward shift of the taiga biome.

The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate

We review measured rates of soil respiration from terrestrial and wetland ecosystems to define the annual global CO 2 flux from soils, to identify uncertainties in the global flux estimate, and to investigate the influences of temperature, precipitation, and vegetation on soil respiration rates. The annual global CO 2 flux from soils is estimated to average (± S.D.) 68 ± 4 PgC/ yr, based on extrapolations from biome land areas. Relatively few measurements of soil respiration exist from arid, semi-arid, and tropical regions; these regions should be priorities for additional research. On a global scale, soil respiration rates are positively correlated with mean annual air temperatures and mean annual precipitation. There is a close correlation between mean annual net primary productivity (NPP) of different vegetation biomes and their mean annual soil respiration rates, with soil respiration averaging 24% higher than mean annual NPP. This difference represents a minimum estimate of the contribution of root respiration to the total soil CO 2 efflux. Estimates of soil C turnover rates range from 500 years in tundra and peaty wetlands to 10 years in tropical savannas. We also evaluate the potential impacts of human activities on soil respiration rates, with particular focus on land use changes, soil fertilization, irrigation and drainage, and climate changes. The impacts of human activities on soil respiration rates are poorly documented, and vary among sites. Of particular importance are potential changes in temperatures and precipitation. Based on a review of in situ measurements, the Q 10 value for total soil respiration has a median value of 2.4. Increased soil respiration with global warming is likely to provide a positive feedback to the greenhouse effect. DOI: 10.1034/j.1600-0889.1992.t01-1-00001.x

Potential Net Primary Productivity in South America: Application of a Global Model.

We use a mechanistically based ecosystem simulation model to describe and analyze the spatial and temporal patterns of terrestrial net primary productivity (NPP) in South America. The Terrestrial Ecosystem Model (TEM) is designed to predict major carbon and nitrogen fluxes and pool sizes in terrestrial ecosystems at continental to global scales. Information from intensively studies field sites is used in combination with continental-scale information on climate, soils, and vegetation to estimate NPP in each of 5888 non-wetland, 0.5° latitude °0.5° longitude grid cells in South America, at monthly time steps. Preliminary analyses are presented for the scenario of natural vegetation throughout the continent, as a prelude to evaluating human impacts on terrestrial NPP. The potential annual NPP of South America is estimated to be 12.5 Pg/yr of carbon (26.3 Pg/yr of organic matter) in a non-wetland area of 17.0 ° 106 km2 . More than 50% of this production occurs in the tropical and subtropical evergreen forest region. Six independent model runs, each based on an independently derived set of model parameters, generated mean annual NPP estimates for the tropical evergreen forest region ranging from 900 to 1510 g°m-2 °yr-1 of carbon, with an overall mean of 1170 g°m-2 °yr-1 . Coefficients of variation in estimated annual NPP averaged 20% for any specific location in the evergreen forests, which is probably within the confidence limits of extant NPP measurements. Predicted rates of mean annual NPP in other types of vegetation ranged from 95 g°m-2 °yr-1 in arid shrublands to 930 g°m@ ?yr-1 in savannas, and were within the ranges measured in empirical studies. The spatial distribution of predicted NPP was directly compared with estimates made using the Miami mode of Lieth (1975). Overall, TEM predictions were °10% lower than those of the Miami model, but the two models agreed closely on the spatial patterns of NPP in south America. Unlike previous models, however, TEM estimates NPP monthly, allowing for the evaluation of seasonal phenomena. This is an important step toward integration of ecosystem models with remotely sensed information, global climate models, and atmospheric transport models, all of which are evaluated at comparable spatial and temporal scales. Seasonal patterns of NPP in South America are correlated with moisture availability in most vegetation types, but are strongly influenced by seasonal differences in cloudiness in the tropical evergreen forests. On an annual basis, moisture availability was the factor that was correlated most strongly with annual NPP in South America, but differences were again observed among vegetation types. These results allow for the investigation and analysis of climatic controls over NPP at continental scales, within and among vegetation types, and within years. Further model validation is needed. Nevertheless, the ability to investigate NPP-environment interactions with a high spatial and temporal resolution at continental scales should prove useful if not essential for rigorous analysis of the potential effects of global climate changes on terrestrial ecosystems.

Net Primary Production of a Perennial Grass Ley (Festuca pratensis) Assessed with Different Methods and Compared with a Lucerne Ley (Medicago sativa)

(1) The above-ground production of an established perennial grass (meadow fescue) ley in central Sweden was analysed by repeated clipping during 1982 and 1983. Standing vegetation was separated into living and dead material and surface litter was collected. Three methods were used to calculate net above-ground primary production (NAPP). Two of them accounted for death and shedding between samplings. (2) Depending on the calculation method, annual NAPP of ash-free dry mass varied between 903-985 and 810-1039 g m-2 in 1982 and 1983, respectively. The two methods which included biomass turnover gave the highest production values. (3) The underestimation in NAPP due to decomposition of litter between samplings was evaluated by applying a decomposition constant, estimated in a separate litter-bag experiment, to the mean amount of litter present during a sampling period. It was found to be <10%. (4) The mean daily growth rates of above-ground crop were 4 7 and 6 0 g m-2 in 1982 and 1983, respectively. The maximum recorded daily crop growth (22 0 g m2) occurred in early June 1983. (5) The study was part of a multi-disciplinary ecosystem project, and estimates of below-ground production and herbivore consumption were available, allowing total net primary production (NPP) to be estimated. Mean annual NPP was 1468 g m-2. (6) NPP was underestimated as rhizodeposition was not measured. Its underestimation from neglecting death and decomposition was less severe above than below ground. The amount of organic matter input to the soil system via litterfall was a minor part of the NPP but a significant part of the total organic matter supplied to the soil organic matter pool during a year. (7) Growth pattern and total NPP were compared with those of a lucerne ley grown in parallel with the meadow fescue ley. A severe drought in 1983 affected the grass ley more than the lucerne ley. Average annual NPP was similar in both crops, but the amount of standing dead was higher in the grass ley than in the lucerne ley. The allocation below ground was similar, about 30%, but root death was higher in the grass ley.

Aerial Production, Mortality, and Mineral Accumulation‐Export Dynamics in Spartina Alterniflora and Juncus Roemerianus Plant Stands in a Georgia Salt Marsh

Biomass and disappearance of dead material were measured in stands of tall creekbank Spartina alterniflora, short high marsh S. alterniflora, and Juncus roemerianus in Georgia, USA at 4—wk intervals for 1 yr and at 8—wk intervals for a 2nd yr. Growth and mortality were calculated from these data. Net primary production estimates, using changes in biomass only, ranged from 10 to 75% lower than estimates which included the disappearance of dead material. Agreement between the methods was closest when the interval between harvest was shortest and the rate of dead material disappearance the slowest. Estimates of mean annual net primary production, computed from changes in biomass and disappearance of dead plant material, were: creekbank S. alterniflora 3700g/m2, high marsh S. alterniflora 1300 g/m2, and J. roemerianus 2200g/m2. The seasonal amplitude in the amount of N, P, K, Ca, and Mg in the living tissue was greatest in the creekbank S. alterniflora. The maximum accumulation of most elements was in late summer. In the tissue of S. alterniflora, N and P were highest in concentration in late winter and early spring. In summer, growth occurred faster than nutrient accumulation therefore, tissue concentrations decreased. Seasonal p patterns of element disappearance from the "dead plant community" showed that maximum export depended on community type and the element under consideration.

Phytoplankton in a temperate-zone salt marsh: Net production and exchanges with coastal waters

Phytoplankton production and associated variables were measured in Flax Pond, a 52 ha salt marsh on the north shore of Long Island, New York, from July 1972 to October 1973. Measurements made up to five times per day, once per week, yielded a mean annual net primary production, determined by the 14C technique, of 20.5 mg C/m3/h; daily means were as high as 60.0 mg C/m3/h. However, when productivity was calculated for the entire marsh ecosystem, the shallow water in the salt marsh produced only 11.7 g C/m2 of marsh/year. There was a net flux of phytoplankton from the coastal waters into the marsh; during the summer up to 0.2 g chlorophy 11/m2 of marsh was carried in with the tides daily and remained in the marsh. Analysis of the productivity data, as well as variables associated with productivity (pH, standing crop, nutrients, extinction coefficient), indicated that the aquatic portion of the marsh behaved more as a net consumer rather than a net producer of phytoplankton.