Primary Productivity In Terrestrial Ecosystems Research Articles

A MODIS-Based Energy Balance to Estimate Evapotranspiration for Clear-Sky Days in Brazilian Tropical Savannas

Evapotranspiration (ET) plays an important role in global climate dynamics and in primary production of terrestrial ecosystems; it represents the mass and energy transfer from the land to atmosphere. Limitations to measuring ET at large scales using ground-based methods have motivated the development of satellite remote sensing techniques. The purpose of this work is to evaluate the accuracy of the SEBAL algorithm for estimating surface turbulent heat fluxes at regional scale, using 28 images from MODIS. SEBAL estimates are compared with eddy-covariance (EC) measurements and results from the hydrological model MGB-IPH. SEBAL instantaneous estimates of latent heat flux (LE) yielded r 2= 0.64 and r2 = 0.62 over sugarcane croplands and savannas when compared against in situ EC estimates. At the same sites, daily aggregated estimates of LE were r 2 = 0.76 and r2 = 0.66, respectively. Energy balance closure showed that turbulent fluxes over sugarcane croplands were underestimated by 7% and 9% over savannas. Average daily ET from SEBAL is in close agreement with estimates from the hydrological model for an overlay of 38,100 km2 (r2 = 0.88). Inputs to which the algorithm is most sensitive are vegetation index (NDVI), gradient of temperature (dT) to compute sensible heat flux (H) and net radiation (Rn). It was verified that SEBAL has a tendency to overestimate results both at local and regional scales probably because of low sensitivity to soil moisture and water stress. Nevertheless the results confirm the potential of the SEBAL algorithm, when used with MODIS images for estimating instantaneous LE and daily ET from large areas.

Changes in the Abundance of Vascular Plants under the Impact of Industrial Air Pollution: A Meta-analysis

We applied a meta-analytic approach to improve an understanding of the effects of pollution on the primary productivity of terrestrial ecosystems. We identified 73 data sources reporting changes in the abundance of vascular plants (measured as the percentage of ground cover, biomass per unit area, canopy closure and stem basal area) around 75 polluters located in 18 countries (137 effect sizes). The overall effect was negative but varied among the vegetation layers and types of polluters. The strongest detrimental impact on productivity was due to non-ferrous smelters. The abundance of trees was decreased around both acidifying and alkalifying polluters, whereas the abundance of shrubs and field-layer vegetation decreased only near acidifying polluters. Overall, a decline of vegetation was observed in boreal and temperate forests but not in treeless biomes (deserts, tundra or grasslands). The magnitude of the detrimental effects on trees increased with the duration of the polluters’ impact around acidifying polluters but decreased around alkalifying polluters. Adverse effects on the field layer became more severe with increasing community productivity; in combination with the conclusions of previous meta-analyses, this result indicates that diverse and highly productive communities are likely to be the most susceptible to pollution impacts. In general, the pollution effects on the abundance of vascular plants were less detrimental than the effects on their growth and diversity, indicating that primary productivity in polluted ecosystems is maintained by the preferential growth of several pollution-tolerant species.

Effects of Nitrogen Deposition on Ecosystem Carbon Fluxes in Sanjiang Plain Marsh of Northeastern China

It has widely been demonstrated that nitrogen (N) addition enhances plant growth and net primary productivity of terrestrial ecosystems. Moreover, N enrichment could have a profound impact on ecosystem carbon fluxes, especially in the regions where N is deficient. However, there is still debate on how N affects net ecosystem CO2 exchange (NEE). A field experiment manipulating N has been conducted in Calamagrostis anagustifolia community of Sanjiang Plain marsh of northeastern China from 2009 to 2010. N was added at a rate of 5 and 10gN m -2 yr -1with NH4NO3. The results for the 2 yr showed that gross ecosystem productivity (GEP) was higher than ecosystem respiration, leading to net carbon(C)sink (measured by NEE) over the growing season in the study site. Furthermore, low nitrogen (LN) and high nitrogen (HN) addition all significantly stimulated growing-season NEE, on average, by 22 and 36%, respectively. It’s indicated that air temperature plays a major role in regulating ecosystem net C exchange and their responses to climatic change in Sanjiang Plain of northern China. we need long-term field studies to predict the long-term effects of N deposition on ecosystem processes.

BIOMASSA RADICULAR FINA EM FRAGMENTOS DA MATA ATLÂNTICA FLUMINENSE

A escassez de estudos empíricos dos sistemas radiculares limita a precisão das predições relativas à economia de carbono, pois eles representam até 55% da biomassa florestal. Somente as raízes finas (diâmetro <2 mm) respondem por cerca de um terço da produção primária líquida dos ecossistemas terrestres. A hipótese de que a maior disponibilidade de água e nutrientes no solo reduz a alocação de biomassa nas raízes tem sido contestada na literatura cientifica tanto em relação às raízes finas (BRF) como a toda biomassa radicular. Avaliou-se aqui a correlação (Pearson) entre BRF e variáveis edáficas até 10 cm de profundidade em seis fragmentos florestais (48 amostras.fragmento.estação-1) na Baixada Litorânea Fluminense, nas estações de seca e de chuvas. Os solos apresentaram diferentes níveis de fertilidade. A BRF média geral foi de 641 g.m-2 (florestas similares = 140–1040 g.m-2). As médias de BRF obtidas no período chuvoso ou na várzea foram significativamente mais elevadas. Foram detectadas correlações positivas e significativas de BRF com Ca + Mg, K, P e N. Os resultados não respaldam a hipótese de redução da BRF em resposta ao aumento de água e nutrientes do solo.Palavras-chave: Fragmentação florestal; florestas tropicais; solos florestais e trópico úmido. AbstractFine roots biomass in fragments of Atlantic Forest from Rio de Janeiro’s State. Despite the relative importance of roots, which may reach up to 55% of forests biomass, there is a lack of empirical data from tropical regions. Just fine roots biomass (FRB; < 2 mm) represent around one third of the net primary production of terrestrial ecosystems. The general acceptance that higher levels of water and nutrients decrease biomass allocation in roots has been contested in the literature. Six forest fragments along the Coastal Plains of Rio de Janeiro’ State were taken to assess correlations (Pearson’s) between FRB and soil variables within a depth of 10 cm. Soil and root samples were collected (N=48.fragment-1.season-1) in wet and dry seasons. Forest soils showed different fertility levels. Overall means of FRB was 641 g.m-2 (other tropical forests = 140 - 1040 g.m-2). Humid season presented the highest average of FRB and also the hydromorphic soil of the sample. Positive and significant correlationn between Ca + Mg, K, P, N with FRB were found. The hypothesis of FRB decrease in response to availability decrease of resources is not supported by obtained results.Keywords: Forest soils; forest fragmentation; humid tropics; forest soils.

Estimation of Light-use Efficiency of Terrestrial Ecosystems from Space: A Status Report

A critical variable in the estimation of gross primary production of terrestrial ecosystems is light-use efficiency (LUE), a value that represents the actual efficiency of a plant’s use of absorbed radiation energy to produce biomass. Light-use efficiency is driven by the most limiting of a number of environmental stress factors that reduce plants’ photosynthetic capacity; these include short-term stressors, such as photoinhibition, as well as longer-term stressors, such as soil water and temperature. Modeling LUE from remote sensing is governed largely by the biochemical composition of plant foliage, with the past decade seeing important theoretical and modeling advances for understanding the role of these stresses on LUE. In this article we provide a summary of the tower-, aircraft-, and satellite-based research undertaken to date, and discuss the broader scalability of these methods, concluding with recommendations for ongoing research possibilities.

Turning the table: plants consume microbes as a source of nutrients.

Interactions between plants and microbes in soil, the final frontier of ecology, determine the availability of nutrients to plants and thereby primary production of terrestrial ecosystems. Nutrient cycling in soils is considered a battle between autotrophs and heterotrophs in which the latter usually outcompete the former, although recent studies have questioned the unconditional reign of microbes on nutrient cycles and the plants' dependence on microbes for breakdown of organic matter. Here we present evidence indicative of a more active role of plants in nutrient cycling than currently considered. Using fluorescent-labeled non-pathogenic and non-symbiotic strains of a bacterium and a fungus (Escherichia coli and Saccharomyces cerevisiae, respectively), we demonstrate that microbes enter root cells and are subsequently digested to release nitrogen that is used in shoots. Extensive modifications of root cell walls, as substantiated by cell wall outgrowth and induction of genes encoding cell wall synthesizing, loosening and degrading enzymes, may facilitate the uptake of microbes into root cells. Our study provides further evidence that the autotrophy of plants has a heterotrophic constituent which could explain the presence of root-inhabiting microbes of unknown ecological function. Our discovery has implications for soil ecology and applications including future sustainable agriculture with efficient nutrient cycles.

Changes in soil microbial biomass and community structure with addition of contrasting types of plant litter in a semiarid grassland ecosystem

Aims Elevated atmospheric CO2 has the potential to enhance the net primary productivity of terrestrial ecosystems. However, the role of soil microorganisms on soil C cycling following this increased available C remains ambiguous. This study was conducted to determine how quality and quantity of plant litter inputs would affect soil microorganisms and consequently C turnover. Methods Soil microbial biomass and community structure, bacterial community-level physiological profile, and CO2 emission caused by different substrate C decomposition were investigated using techniques of biological measurements, chemical and stable C isotope analysis, and BIOLOG-ECO microplates in a semiarid grassland ecosystem of northern China in 2006 and 2007 by mixing three contrasting types of plant materials, C3 shoot litter (SC3), C3 root litter (RC3), and C4 shoot litter (SC4), into the 10- to 20-cm soil layer at rates equivalent to 0 (C0), 60 (C60), 120 (C120 )a nd 240 g C m � 2 (C240). Important Findings Litter addition significantly enriched soil microbial biomass C and N and resulted in changes in microbial structure. Principal component analysis of microbial structure clearly differentiated among zero addition, C3-plant-derived litter, and C4-plant-derived litter and among shoot- and root-derived litter of C3 plants; soil microorganisms mainly utilized carbohydrates without litter addition, carboxylic acids with C3-plant-derived litter addition and amino acids with C4-plant-derived litter addition. We also detected stimulated decomposition of older substrate with C4-plant-derived litter inputs. Our results show that both quality and quantity of belowground litter are involved in affecting soil microbial community structure in semiarid grassland ecosystem.

Is the net new carbon increment of coppice forest stands of Quercus ilex ssp. ballota affected by post-fire thinning treatments and recurrent fires?

Coppice forest stands of Quercus ilex have been one of the forest types most impacted by fire in Spain. After fire, their capability to resprout produces a high density of stems that requires thinning in order to avoid stagnation within the stands. In August 1993 and July 2001, two consecutive fires affected a Quercus ilex coppice stand in SE Spain. This study investigated the effects of different post‐fire thinning treatments and recurrent fires on stock and net new carbon increment (NNCI) in a 6‐year‐old coppice stand. Four degrees of thinning were applied: medium thinning (to a final density of 5000 trees ha–1), drastic thinning (to 1800 trees ha–1), full felling (all trees removed) and no thinning (control). Results showed NNCI was within the lower limit of the average global values reported for net primary productivity of terrestrial ecosystems. The best thinning treatment to maximise both current annual NNCI and mean annual NNCI stimulation, and keep new resprouting within reasonable levels was medium thinning. However, recurrent fires caused the observed net primary productivity to decrease, which allowed us to conclude that stump vitality is affected by successive fires, at least the first year after a new fire.

Modelling net primary productivity of terrestrial ecosystems in East Asia based on an improved CASA ecosystem model

By using a land cover map, normalized difference vegetation index (NDVI) data sets, monthly meteorological data and observed net primary productivity (NPP) data, we have improved the method of estimating light use efficiency (LUE) for different biomes and soil moisture coefficients in the Carnegie–Ames–Stanford Approach (CASA) ecosystem model. Based on this improved model we produced an annual NPP map (in 1999) for the East Asia region located at 10–70° N, 70–170° E (about 19.66% of the terrestrial surface of the Earth). The results show that the mean NPP for the study area in 1999 was 374.12 g carbon (C) m−2 year−1 and the total NPP was 1.096 × 1014 kg C year−1, making up 17.51–18.39% of the global NPP. Comparison between the estimated NPP obtained from this improved CASA ecosystem model and the observed NPP obtained from two NPP databases indicates that the estimated NPP is close to the observed NPP, with an average error of 5.15% for the study region. We used two different land cover maps of China to drive the improved CASA model by keeping other inputs unchanged to determine how the classification accuracy of the land cover map affects the estimated NPP, and the results indicate that an accurate land cover map is important for obtaining an accurate and reliable estimate of NPP for some regions, especially for a particular biome.

Divergent patterns of photosynthetic phosphorus‐use efficiency versus nitrogen‐use efficiency of tree leaves along nutrient‐availability gradients

Summary 1. Nitrogen (N) and phosphorus (P) are essential nutrients for photosynthetic carbon assimilation and most frequently limit primary productivity in terrestrial ecosystems. Efficient use of those nutrients is important for plants growing in nutrient‐poor environments. 2. We investigated the pattern of photosynthetic phosphorus‐use efficiency (PPUE) in comparison with photosynthetic nitrogen‐use efficiency (PNUE) along gradients of P and N availability across biomes with 340 tree and shrub species. We used both total soil N and P concentration and foliar N/P ratios for indicating nutrient‐availability gradients. 3. Photosynthetic phosphorus‐use efficiency increased with greater leaf mass per area (LMA) toward decreasing P availability. By contrast, PNUE decreased with greater LMA towards decreasing N and P availability. 4. The increase in PPUE with decreasing P availability was caused by the net effects of a relatively greater reduction in foliar P concentration and a relatively constant photosynthetic carbon assimilation rate. The decrease in PNUE with decreasing N availability was caused by the effects of a reduction in photosynthetic carbon assimilation rate with greater LMA. 5. Synthesis. Our results suggest that higher PPUE may be an effective leaf‐level adaptation to P‐poor soils, especially in tropical tree species. Future research should focus on the difference between PPUE and PNUE in relation to leaf economics, physiology and strategy.

Responses of net ecosystem CO 2 exchange to nitrogen fertilization in experimentally manipulated grassland ecosystems

Nitrogen (N) addition enhances primary productivity of terrestrial ecosystems. However, the effects of N fertilization and/or deposition on net ecosystem CO 2 exchange (NEE) are not fully understood. The effects of N on NEE were investigated in two experimental cheatgrass ecosystems in Ecologically Controlled Enclosed Lysimeter Laboratories (EcoCELLs), Reno, Nevada. In this experiment, no N fertilization was added to the two EcoCELLs in the first year and two different N fertilization regimes were applied in the second year. N fertilizer was applied once to one EcoCELL (pulse fertilization, PF), and the same total amount of N in biweekly increments to the other EcoCell (gradual fertilization, GF). NEE, photosynthetically active radiation (PAR) and canopy green leaf area index (LAI) were continuously measured in the two EcoCELLs during the pretreatment and N-fertilized years. Plant N content and biomass were measured at the end of the growing season in each year. Radiation-use efficiency (RUE CO2) was calculated as the ratio of gross ecosystem photosynthesis (GEP) to the intercepted photosynthetically active radiation (IPAR). The responses of NEE to IPAR were used to estimate the maximum ecosystem photosynthetic capacity ( F max). N fertilization stimulated canopy LAI, plant N content, F max, RUE CO2, NEE and biomass in both methods of N supply applications. PF led to higher LAI, F max and NEE than GF, but both had a similar RUE CO2 during the early growing season. GF maintained higher LAI, F max, RUE CO2 and NEE than PF during the late growing season. At the ecosystem level, N fertilization stimulated daily NEE directly by increasing canopy LAI, plant N content, shoot/root ratio and the maximum ecosystem photosynthetic capacity, and increased the seasonally accumulated NEE indirectly by extending the growing season. PF differed significantly from GF in its effects on NEE and RUE CO2, possibly due to differential rates and timing of N availability. Our study suggested that these changes in the canopy RUE CO2 and growing season under N fertilization or N deposition regimes should be considered in modeling studies of ecosystem C sequestration.

Effects of macroalgal species identity and richness on primary production in benthic marine communities

Plant biodiversity can enhance primary production in terrestrial ecosystems, but biodiversity effects are largely unstudied in the ocean. We conducted a series of field and mesocosm experiments to measure the relative effects of macroalgal identity and richness on primary productivity (net photosynthetic rate) and biomass accumulation in hard substratum subtidal communities in North Carolina, USA. Algal identity consistently and strongly affected production; species richness effects, although often significent, were subtle. Partitioning of the net biodiversity effect indicated that complementarity effects were always positive and species were usually more productive in mixtures than in monoculture. Surprisingly, slow growing species performed relatively better in the most diverse treatments than the most productive species, thus selection effects were consistently negative. Our results suggest that several basic mechanisms underlying terrestrial plant biodiversity effects also operate in algal-based marine ecosystems, and thus may be general.

Modeling of values of the net primary production for the zonal vegetation in European Russia

Several modern approaches to the problem of estimating the net primary production of terrestrial ecosystems are discussed. A method for predicting the dynamics of this parameter as a function of radiation balance and annual evapotranspiration is described. The values of annual carbon by the zonal vegetation of European Russia are calculated in two ways: by methods based on the empirically determined relationships between the annual average values of climatic parameters and net primary production and on the basis of models describing carbon flows between the compartments of ecosystems. The model estimations of net primary production are compared with experimental data.

Time Course of Radiation Use Efficiency in a Shortgrass Ecosystem: Consequences for Remotely Sensed Estimation of Primary Production

A reliable estimation of primary production of terrestrial ecosystems is often a prerequisite for land survey and management, while being important also in ecological and climatological studies. At a regional scale, grassland primary production estimates are increasingly being made with the use of satellite data. In a currently used approach, regional gross, net, and aboveground net primary productivity (GPP, NPP, and ANPP) are derived from the parametric model of Monteith and are calculated as the product of the fraction of incident photosynthetically active radiation absorbed by the canopy ( f APAR) and gross, net, and aboveground net production (radiation-use) efficiencies (ϵ g, ϵ n, and ϵ an); f APAR being derived from indices calculated from satellite-measured reflectances in the red and near infrared. The accuracy and realism of the primary production values estimated by this approach therefore largely depend on an accurate estimation of ϵ g, ϵ n, and ϵ an. However, data are scarce for production efficiencies of semiarid grasslands, and their time and spatial variations are poorly documented, often leading to large errors for the estimates. In this paper, a modeling approach taking into account relevant ecosystem processes and based on extensive field data was used to estimate time variations of ϵ g, ϵ n and ϵ an of a shortgrass site in Arizona. These variations were explained by variations in plant water stress, temperature, leaf aging, and processes such as respiration and changes in allocation pattern between above- and below-ground compartments. Over the 3 study years, averaged values of ϵ g, ϵ n, and ϵ an were found to be 1.92, 0.74, and 0.29 g DM (MJ IPAR) −1, respectively. ϵ g and ϵ n exhibited large interannual and seasonal variations mainly due to changes in water limitations during the growing season. Interannual variations of ϵ an were much less important. However, for shorter periods, ϵ an exhibited very contrasting values from regrowth to senescence. The calculation of ANPP seems less prone to errors due to environmental effects when computed on an annual basis. When estimating GPP and NPP, better results are expected if water limitations are taken into account. This could be possible through the estimation of a water-stress factor by using surface temperature or other indices derived from thermal infrared remote sensing data. The limitations due to temporally varying efficiencies, shown here for shortgrass ecosystems, are also relevant to all drought-exposed ecosystems, particularly those with abundant evergreen or perennial species.

Direct and Indirect Estimation of Leaf Area Index, fAPAR, and Net Primary Production of Terrestrial Ecosystems

A primary objective of the Earth Observing System (EOS) is to develop and validate algorithms to estimate leaf area index ( L), fraction of absorbed photosynthetically active radiation ( f APAR), and net primary production (NPP) from remotely sensed products. These three products are important because they relate to or are components of the metabolism of the biosphere and can be determined for terrestrial ecosystems from satellite-borne sensors. The importance of these products in the EOS program necessitates the need to use standard methods to obtain accurate ground truth estimates of L, f APAR, and NPP that are correlated to satellite-derived estimates. The objective of this article is to review direct and indirect methods used to estimate L, f APAR, and NPP in terrestrial ecosystems. Direct estimates of L, biomass, and NPP can be obtained by harvesting individual plants, developing allometric equations, and applying these equations to all individuals in the stand. Using non-site-specific allometric equations to estimate L and foliage production can cause large errors because carbon allocation to foliage is influenced by numerous environmental and ecological factors. All of the optical instruments that indirectly estimate L actually estimate “effective” leaf area index ( L E ) and underestimate L when foliage in the canopy is nonrandomly distributed (i.e., clumped). We discuss several methods, ranging from simple to complex in terms of data needs, that can be used to correct estimates of L when foliage is clumped. Direct estimates of above-ground and below-ground net primary production (NPP A and NPP B, respectively) are laborious, expensive and can only be carried out for small plots, yet there is a great need to obtain global estimates of NPP. Process models, driven by remotely sensed input parameters, are useful tools to examine the influence of global change on the metabolism of terrestrial ecosystems, but an incomplete understanding of carbon allocation continues to hamper development of more accurate NPP models. We summarize carbon allocation patterns for major terrestrial biomes and discuss emerging allocation patterns that can be incorporated into global NPP models. One common process model, light use efficiency or epsilon model, uses remotely sensed f APAR, light use efficiency (LUE) and carbon allocation coefficients, and other meteorological data to estimates NPP. Such models require reliable estimates of LUE. We summarize the literature and provide LUE coefficients for the major biomes, being careful to correct for inconsistencies in radiation, dry matter and carbon allocation units.

Predicting Gross Primary Productivity in Terrestrial Ecosystems

Predicting Gross Primary Productivity in Terrestrial Ecosystems

Foliage litter quality and annual net N mineralization: comparison across North American forest sites.

The feedback between plant litterfall and nutrient cycling processes plays a major role in the regulation of nutrient availability and net primary production in terrestrial ecosystems. While several studies have examined site-specific feedbacks between litter chemistry and nitrogen (N) availability, little is known about the interaction between climate, litter chemistry, and N availability across different ecosystems. We assembled data from several studies spanning a wide range of vegetation, soils, and climatic regimes to examine the relationship between aboveground litter chemistry and annual net N mineralization. Net N mineralization declined strongly and non-linearly as the litter lignin:N ratio increased in forest ecosystems (r 2 = 0.74, P < 0.01). Net N mineralization decreased linearly as litter lignin concentration increased, but the relationship was significant (r 2 = 0.63, P < 0.01) only for tree species. Litterfall quantity, N concentration, and N content correlated poorly with net N mineralization across this range of sites (r 2 < 0.03, P = 0.17-0.26). The relationship between the litter lignin:N ratio and net N mineralization from forest floor and mineral soil was similar. The litter lignin:N ratio explained more of the variation in net N mineralization than climatic factors over a wide range of forest age classes, suggesting that litter quality (lignin:N ratio) may exert more than a proximal control over net N mineralization by influencing soil organic matter quality throughout the soil profile independent of climate.

Soils as biotic constructs favouring net primary productivity

Many, if not most, physical and chemical properties of soils required for plant growth are affected strongly by biotic processes. Feedback processes involving primary producers and decomposers may be involved in the development of properties that favour net primary productivity in terrestrial ecosystems. However, both constructive and destructive effects of biota on soils can be observed. Apparently, effects favouring net primary production have accumulated and presently prevail in the various terrestrial ecosystems of the world. In some ecosystems, however, the dominant vegetation gains competitive advantage by making soils un “favourable” for most other plants. Ombrotrophic peat bogs and heathlands are cases in point. On the global scale, biotic processes can be seen as responsible for the persistence of water on the earth, through control of the earth's surface temperature under the influence of greenhouse gases. As a result, therefore, the large-scale geochemical and hydrological cycles, which are essential for chemical rejuvenation of the earth's surface, also depend on life processes. This is an aspect of Lovelock's Gaia hypothesis, which states that the earth has evolved over geologic time by feedback processes keeping the earth in a state comfortable for life by the action of living organisms. While the development of soils with “favourable” properties may be explained in evolutionary terms, such an explanation apparently does not suffice for the development of Gaia as a whole.

A Comparison of Seasonal Primary Production of Mojave Desert Shrubs During Wet and Dry Years

Estimates of net primary productivity were made for five major shrub species in the northern Mojave Desert during 2 years of contrasting moisture regimes using harvest and gas exchange techniques. Production increased by 224% (harvest) to 260% (gas exchange) in 1973 over 1972. Production gains varied among species depending on whether the species was favored by the cool, moist spring of 1973. Lycium andersonii and L. pallidum, which have high photosynthetic rates and start growth in the early spring, showed the greatest increases. Larrea tridentata and Krameria parvifolia, species adapted to growing under warmer, drier conditions, exhibited the smallest changes in production. Ambrosia dumosa, which has high photosynthetic rates but responds more favorably to warmer spring temperatures than the Lycium species, had moderate production gains. Reasons for the different estimates obtained by these two methods are discussed. INTRODUCTION The determination of primary productivity in terrestrial ecosystems is a necessary but difficult step in providing information on the basic. energy or carbon input into biological systems. Productivity estimates are important for predictive models of a general nature in ecosystem analysis. In general, two methods have been used for net primary production estimates. The first method is a direct harvest technique in which whole plants or samples are clipped throughout the growing season, dried and weighed, and productivity calculated on dry weight per unit area. The second method involves measuring the total CO2 or carbon exchange per weight of photosynthetic organs, and calculating productivity as carbon or biomass fixed minus respiratory losses. There have been several measurements of net primary production and biomass in deserts made by the harvest and other methods (Burk and Dick-Peddie, 1973; Chew and Chew, 1965; Pearson, 1965; Rodin and Bazilevich, 1967). Measurements of CO2 exchange rates in desert plants have been made in the Negev Desert (Lange et al., 1969) and in the North American deserts (Bamberg et al., 1975; Caldwell, 1972; Patten and Dinger, 1969; Strain, 1969, 1970; Strain and Chase, 1966). Attempts at relating some of these gas exchange rates to production are only now being done. The US/IBP Desert Biome is currently obtaining production estimates from several sites in the western United States. 1 Present address: EG&G Environmental Consultants, 9025 E. Kenyon Avenue, Denver, Colorado 80237. 2 Present address: University of Idaho, Research and Extension Center, Route 1, Kimberly, Idaho 83341.

Primary Production in Terrestrial Ecosystems

“Primary production” refers to energy fixed by plants. The total amount of energy fixed is usually called “gross production.” A certain fraction of gross production is used in respiration by the plants; the remainder appears as new biomass or “net primary production.” Thus for a single plant or a community of green plants: Net Primary Production = Gross Production − Respiration (of Autotrophs) Similar relationships occur in ecosystems except that the organic matter and respiration of heterotrophs must be included. The increase in total organic matter is “net ecosystem production”; respiration is the total respiration of the green plants (autotrophs) and the animal community and decay organisms (heterotrophs). Gross production is of course identical to that of the plant community. Thus for an ecosystem: Net Ecosystem Production = Gross Production − Respiration (of Autotrophs and Heterotrophs) Study of these attributes of terrestrial ecosystems is difficult, both because of the complex interrelations of the processes involved, and because of the problems of working with systems as large as whole forests. Three approaches are in use: (1) Harvest techniques measure weight increase (and caloric equivalent and chemical composition) of net production. A refinement ot this approach based on “dimension analysis” has made possible important recent advances in the study of forests. Other techniques approach gross production and respiration through measurement of exchange of gases, especially CO2. These include: (2) Enclosure studies, involving measurements of CO2 exchange in plastic enclosures of parts of ecosystems and (3) Flux techniques based on measurement of CO2 levels in the environment. All three approaches are being applied to a forest at Brookhaven National Laboratory to determine the production equation of this ecosystem. Results to date have established general ranges of such parameters of ecosystems as total biomass, total surface area of leaves and of stems and branches, rates of decay of organic matter in soils, rates of production of roots, and rates of photosynthesis and respiration under different environmental conditions. In the Brookhaven forest net primary production is 1124 dry g/m2/yr (with an energy equivalent of 492 cal/cm2/yr), and gross production is about 2550 dry g/m2/yr; the producers or green plants thus respire 56% of their gross production. Net ecosystem production is 422 dry g/m2/yr in this young forest. The ratio of total respiration to gross production is a convenient expression of successional status; a value of 0.82 for the Brookhaven forest indicates that this is a late successional community, but not in steady-state or climax condition (1.0). A leaf surface area of 3.8 m2 per m2 of ground surface intercepts sunlight energy, and the ratio of net primary production to incident visible sunlight energy gives a net efficiency of primary production of 0.0088. These and other functional characteristics of ecosystems are currently important topics of research—involving understanding of communities as biological systems, evaluation of the potential of environments to support life and man's harvest; and understanding of the fundamental meaning and consequences of man's alteration, exploitation, and pollution of ecosystems.