Components Of Net Primary Productivity Research Articles

A modified framework for the regional assessment of climate and human impacts on net primary productivity

Improving models that depict the components of net primary production (NPP) in ecosystems will help us to better understand how climate change and human activities affect the biosphere. In this study, NPPgap was introduced into the present human appropriation of net primary production (HANPP) framework. We introduced NPPgap in this study as potential NPP (NPPpot) minus the sum of ecosystem NPP (NPPeco) and HANPP, which relates to the ability of models to depict NPP components. Using the Lhasa River region of the Tibetan Plateau, we examined temporal and spatial variations in the components of NPP over a 10-year period. Results showed that NPPpot, HANPP and NPPeco increased from 2000 to 2010, but at different rates and with different spatial patterns. NPPgap each year ranged from −9.2% to 13.1% for each site and on average composed 1.2% of the total NPPpot. NPPgap was significantly correlated with precipitation, plant biodiversity, plant height and soil properties. NPPgap increased if either of the previous 2 years had been wet years with relative high precipitation. An increase in the richness of palatable species would lead to a larger NPPgap through more compensatory growth. The large fluctuation level of NPPgap reflected the higher stability of vegetation productivity, which is caused by higher plant heights and soil maximum water capacity. This study showed the potential of the HANPP framework in regional assessment of climate and human impacts on net primary productivity. The use of the NPPgap measure reflects the gap in our knowledge and our ability to accurately estimate the components of NPP.

Drought impact on forest carbon dynamics and fluxes in Amazonia.

In 2005 and 2010 the Amazon basin experienced two strong droughts, driven by shifts in the tropical hydrological regime possibly associated with global climate change, as predicted by some global models. Tree mortality increased after the 2005 drought, and regional atmospheric inversion modelling showed basin-wide decreases in CO2 uptake in 2010 compared with 2011 (ref. 5). But the response of tropical forest carbon cycling to these droughts is not fully understood and there has been no detailed multi-site investigation in situ. Here we use several years of data from a network of thirteen 1-ha forest plots spread throughout South America, where each component of net primary production (NPP), autotrophic respiration and heterotrophic respiration is measured separately, to develop a better mechanistic understanding of the impact of the 2010 drought on the Amazon forest. We find that total NPP remained constant throughout the drought. However, towards the end of the drought, autotrophic respiration, especially in roots and stems, declined significantly compared with measurements in 2009 made in the absence of drought, with extended decreases in autotrophic respiration in the three driest plots. In the year after the drought, total NPP remained constant but the allocation of carbon shifted towards canopy NPP and away from fine-root NPP. Both leaf-level and plot-level measurements indicate that severe drought suppresses photosynthesis. Scaling these measurements to the entire Amazon basin with rainfall data, we estimate that drought suppressed Amazon-wide photosynthesis in 2010 by 0.38petagrams of carbon (0.23-0.53petagrams of carbon). Overall, we find that during this drought, instead of reducing total NPP, trees prioritized growth by reducing autotrophic respiration that was unrelated to growth. This suggests that trees decrease investment in tissue maintenance and defence, in line with eco-evolutionary theories that trees are competitively disadvantaged in the absence of growth. We propose that weakened maintenance and defence investment may, in turn, cause the increase in post-drought tree mortality observed at our plots.

The Carbon Cycle of a Maritime Ancient Temperate Broadleaved Woodland at Seasonal and Annual Scales

This study compares different approaches to quantifying the carbon cycle in a temperate deciduous forest at Wytham Woods in England, which is unusual in its maritime climate and mixed age structure, reflecting low levels of past management. We tested whether eddy covariance and biometric measurements gave consistent estimates of woodland productivity and ecosystem respiration at monthly and annual timescales. Biometric methods estimated gross primary productivity (GPP) as 22.0 ± 1.6 Mg C ha−1 y−1, close to the eddy covariance GPP value of 21.1 Mg C ha−1 y−1. Annual ecosystem respiration (R ECO) was similar, at 20.3 ± 1.5 Mg C ha−1 y−1 for biometric and 19.8 Mg C ha−1 y−1for eddy covariance. The seasonal cycle of monthly biometric and eddy covariance R ECO estimates also closely matched. Net primary productivity (NPP) was 7.0 ± 0.8 Mg C ha−1 y−1, 37% of which was allocated below ground. Leaf fluxes were the greatest component of NPP and R ECO. Ecosystem carbon-use efficiency (CUE = NPP/GPP) was 0.32 ± 0.04; low compared to many temperate broadleaved sites but close to values for old-growth sites. This may reflect the age of some trees, and/or the oceanic climate with relatively mild winters during which there can be substantial autotrophic maintenance respiration in winter but negligible growth. This study demonstrates that biometric measurements can provide robust estimates of site productivity and respiration and that eddy covariance and bottom-up measurements can be combined on seasonal and interannual timescales to enable a detailed understanding of the forest carbon cycle.

Asynchronism in leaf and wood production in tropical forests: a study combining satellite and ground-based measurements

Abstract. The fixation of carbon in tropical forests mainly occurs through the production of wood and leaves, both being the principal components of net primary production. Currently field and satellite observations are independently used to describe the forest carbon cycle, but the link between satellite-derived forest phenology and field-derived forest productivity remains opaque. We used a unique combination of a MODIS enhanced vegetation index (EVI) dataset, a wood production model based on climate data and direct litterfall observations at an intra-annual timescale in order to question the synchronism of leaf and wood production in tropical forests. Even though leaf and wood biomass fluxes had the same range (respectively 2.4 ± 1.4 and 2.2 ± 0.4 Mg C ha−1 yr−1), they occurred separately in time. EVI increased with leaf renewal at the beginning of the dry season, when solar irradiance was at its maximum. At this time, wood production stopped. At the onset of the rainy season, when new leaves were fully mature and water available again, wood production quickly increased to reach its maximum in less than a month, reflecting a change in carbon allocation from short-lived pools (leaves) to long-lived pools (wood). The time lag between peaks of EVI and wood production (109 days) revealed a substantial decoupling between the leaf renewal assumed to be driven by irradiance and the water-driven wood production. Our work is a first attempt to link EVI data, wood production and leaf phenology at a seasonal timescale in a tropical evergreen rainforest and pave the way to develop more sophisticated global carbon cycle models in tropical forests.

Ecosystem productivity and carbon cycling in intact and annually burnt forest at the dry southern limit of the Amazon rainforest (Mato Grosso, Brazil)

Background: The impact of fire on carbon cycling in tropical forests is potentially large, but remains poorly quantified, particularly in the locality of the transition forests that mark the boundaries between humid forests and savannas. Aims: To present the first comprehensive description of the impact of repeated low intensity, understorey fire on carbon cycling in a semi-deciduous, seasonally dry tropical forest on infertile soil in south-eastern Amazonia. Methods: We compared an annually burnt forest plot with a control plot over a three-year period (2009–2011). For each plot we quantified the components of net primary productivity (NPP), autotrophic (R a) and heterotrophic respiration (R h), and estimated total plant carbon expenditure (PCE, the sum of NPP and R a) and carbon-use efficiency (CUE, the quotient of NPP/PCE). Results: Total NPP and R a were 15 and 4% lower on the burnt plot than on the control, respectively. Both plots were characterised by a slightly higher CUE of 0.36–0.39, compared to evergreen lowland Amazon forests. Conclusions: These measurements provide the first evidence of a distinctive pattern of carbon cycling within this transitional forest. Overall, regular understorey fire is shown to have little impact on ecosystem-level carbon fluxes.

The productivity, allocation and cycling of carbon in forests at the dry margin of the Amazon forest in Bolivia

Background: The dry transitional forests of the southern Amazonia have received little attention from a carbon cycling and ecosystem function perspective, yet they represent ecosystems that may be impacted by global climate change in the future. Aims: To compare the full carbon cycle for two 1-ha forest plots that straddle the ecotone between humid forest and dry forest in Amazonia, ca. 100 km from Santa Cruz, Bolivia. Methods: 2.5 years of measurements of the components of net primary production (NPP) and autotrophic respiration were collected. Results: Total NPP was 15.5 ± 0.89 Mg C ha−1 year−1 at the humid site and 11.27 ± 0.68 Mg C ha−1 year−1 at the dry site; a total Gross Primary Production (GPP) of 34.14 ± 2.92 Mg C ha−1 year−1 and 26.88 ± 2.70 Mg C ha−1 year−1 at the two sites. Carbon use efficiency for both sites was higher than reported for other Amazonian forests (0.45 ± 0.05 and 0.42 ± 0.05). Conclusions: Drier soil conditions selected for the dry deciduous tree species which had higher leaf photosynthesis and total GPP. NPP allocation patterns were similar at the two sites, suggesting that in terms of carbon allocation, the dry forests of the southern Amazonia behave as a scaled-down version of wetter humid forests.

The seasonal cycle of productivity, metabolism and carbon dynamics in a wet aseasonal forest in north-west Amazonia (Iquitos, Peru)

Background: The forests of north-west Amazonia are characterised by the highest levels of tree diversity in the world, high rainfall and relatively fertile soils. Aims: Here we present a comprehensive description of the carbon cycle of two 1 ha forests plots in Allpahuayo, near Iquitos, Peru, one on an occasionally inundated alisol/gleysol landscape, the other on an arenosol (sandy soil). Methods: Data on the components of net primary productivity (NPP) and autotrophic respiration were collected over the period 2009–2011, and summed to estimate gross primary productivity (GPP) and carbon use efficiency (CUE). Results: Overall, these forests showed high values of GPP (39.05 ± 4.59 and 41.88 ± 4.60 Mg C ha−1 year−1). Despite the lack of a dry season, the forests showed distinct seasonality in tree growth, litterfall, flowering and fine root productivity. This showed that tropical forests with little seasonality in water supply can still exhibit distinct seasonality in NPP and carbon use, apparently synchronised to the solar radiation cycle. We found remarkably little difference in productivity between the alisol/gleysol plot and the arenosol plot. Conclusions: The GPP was higher than those reported for forests in Brazilian Amazonia on more infertile soils. The CUE was also higher, which may be related to the high forest dynamism and natural disturbance rate. These two factors combined to result in amongst the highest NPP values reported for Amazonia.

The productivity, metabolism and carbon cycle of two lowland tropical forest plots in south-western Amazonia, Peru

Background: The forests of western Amazonia are known to be more dynamic that the better-studied forests of eastern Amazonia, but there has been no comprehensive description of the carbon cycle of a western Amazonian forest. Aims: We present the carbon budget of two forest plots in Tambopata in south-eastern Peru, western Amazonia. In particular, we present, for the first time, the seasonal variation in the detailed carbon budget of a tropical forest. Methods: We measured the major components of net primary production (NPP) and total autotrophic respiration over 3–6 years. Results: The NPP for the two plots was 15.1 ± 0.8 and 14.2 ± 1.0 Mg C ha−1 year−1, the gross primary productivity (GPP) was 35.5 ± 3.6 and 34.5 ± 3.5 Mg C ha−1 year−1, and the carbon use efficiency (CUE) was 0.42 ± 0.05 and 0.41 ± 0.05. NPP and CUE showed a large degree of seasonality. Conclusions: The two plots were similar in carbon cycling characteristics despite the different soils, the most notable difference being high allocation of NPP to canopy and low allocation to fine roots in the Holocene floodplain plot. The timing of the minima in the wet–dry transition suggests they are driven by phenological rhythms rather than being driven directly by water stress. When compared with results from forests on infertile forests in humid lowland eastern Amazonia, the plots have slightly higher GPP, but similar patterns of CUE and carbon allocation.

Seasonal production, allocation and cycling of carbon in two mid-elevation tropical montane forest plots in the Peruvian Andes

Background: Tropical montane cloud forests (TMCF) are unique ecosystems with high biodiversity and large carbon reservoirs. To date there have been limited descriptions of the carbon cycle of TMCF. Aims: We present results on the production, allocation and cycling of carbon for two mid-elevation (1500–1750 m) tropical montane cloud forest plots in San Pedro, Kosñipata Valley, Peru. Methods: We repeatedly recorded the components of net primary productivity (NPP) using biometric measurements, and autotrophic (R a) and heterotrophic (R h) respiration, using gas exchange measurements. From these we estimated gross primary productivity (GPP) and carbon use efficiency (CUE) at the plot level. Results: The plot at 1500 m was found very productive, with our results comparable with the most productive lowland Amazonian forests. The plot at 1750 m had significantly lower productivity, possibly because of greater cloud immersion. Both plots had similar patterns of NPP allocation, a substantial seasonality in NPP components and little seasonality in Ra . Conclusions: These two plots lie within the ecotone between lower and upper montane forests, near the level of the cloud base. Climate change is likely to increase elevation of the cloud base, resulting in shifts in forest functioning. Longer-term surveillance of the carbon cycle at these sites would yield valuable insights into the response of TMCFs to a shifting cloud base.

Climate Responses of Aboveground Productivity and Allocation in Fagus sylvatica: A Transect Study in Mature Forests

According to recent climate change scenarios, temperate forests will be increasingly exposed to droughts in the 21st century which are thought to affect productivity. Although decreasing timber yield with reduced precipitation has frequently been reported from temperate forests, the dependence of forest net primary production (NPP) on precipitation is little understood. In a 3-year transect study (2009–2011) carried out in 12 mature beech forests (Fagus sylvatica) along a precipitation gradient (820–540 mm y−1) in Northern Germany, we measured all aboveground NPP components (NPPa; stem wood, leaf mass, flower and fruit production) and analyzed relationships with monthly weather data. Because we measured NPPa under a broad range of precipitation levels, drought lengths and mast fruiting intensities, the climatic controls of aboveground productivity and carbon allocation could be analyzed in detail. Despite a significant decrease in annual (and growing season) precipitation sums along the transect, NPPa remained largely invariant in each of the years, but varied remarkably between the years (means of 981, 702, 955 g DM m−2 y−1, respectively). Variation in NPPa was most closely related to current year’s early summer weather conditions (June–July), whereas the patterns of biomass allocation to wood, leaf, and fruit production responded to the previous summer’s weather. Wood production cannot predict NPPa in beech due to alternative allocation priorities of vegetative and reproductive growth. Our results show that apparent drought-induced reductions in beech timber yield often are the result of allocation shifts toward fruit production triggered by warm and dry weather in the previous summer.

Ecosystem respiration and net primary productivity after 8–10 years of experimental through-fall reduction in an eastern Amazon forest

Background: There is much interest in how the Amazon rainforest may respond to future rainfall reduction. However, there are relatively few ecosystem-scale studies to inform this debate. Aims: We described the carbon cycle in a 1 ha rainforest plot subjected to 8–10 consecutive years of ca. 50% through-fall reduction (TFR) and compare these results with those from a nearby, unmodified control plot in eastern Amazonia. Methods: We quantified the components of net primary productivity (NPP), autotrophic (R a) and heterotrophic respiration, and estimate gross primary productivity (GPP, the sum of NPP and R a) and carbon-use efficiency (CUE, the ratio of NPP/GPP). Results: The TFR forest exhibited slightly lower NPP but slightly higher R a, such that forest CUE was 0.29 ± 0.04 on the control plot but 0.25 ± 0.03 on the TFR plot. Compared with four years earlier, TFR plot leaf area index and small tree growth recovered and soil heterotrophic respiration had risen. Conclusions: This analysis tested and extended the key findings of a similar analysis 4 years earlier in the TFR treatment. The results indicated that, while the forest recovered from extended drought in some respects, it maintained higher overall R a relative to the undroughted control, potentially causing the droughted forest to act as a net source of CO2.

Annual budget and seasonal variation of aboveground and belowground net primary productivity in a lowland dipterocarp forest in Borneo

AbstractTropical forests are thought to have among the highest values of net primary productivity (NPP) on Earth, but comprehensive data on NPP have rarely been collected for tropical forests, especially outside of the Neotropics. In this study, we quantify aboveground and belowground NPP, along with additional environmental factors over a 1–2 year period in a lowland dipterocarp forest in Borneo. The site is characterized by high annual rainfall and typically no month of the year when the forest is under water stress. We estimated the total NPP to be 15.89 ± 0.90 Mg C ha−1 yr−1 (mean ± standard error) for a forest plot on clay soils and 12.79 ± 0.68 Mg C ha−1 yr−1 for a forest plot on sandy loam soils. Of this productivity, the allocation to aboveground NPP is 81.8 ± 6.0%/80.4 ± 6.0% and to belowground NPP is 18.2 ± 3.7%/19.6 ± 2.6% on clay and sandy loam, respectively. Fine root productivity (NPPfine roots) shows stronger seasonal variation relative to other components of NPP. There is evidence suggesting interannual variation in NPPfine roots, leaf flush, NPPlitterfall, and NPPACW. This is the first attempt to describe how the biomass of a Bornean tropical forest is allocated to the various components of NPP over the seasonal cycle. The study highlights the marked seasonality of a tropical forest even under largely aseasonal environmental conditions, as well as the important effect of contrasting seasonality on different soil types.

Net primary productivity, allocation pattern and carbon use efficiency in an apple orchard assessed by integrating eddy covariance, biometric and continuous soil chamber measurements

Abstract. Carbon use efficiency (CUE), the ratio of net primary production (NPP) over gross primary production (GPP), is a functional parameter that could possibly link the current increasingly accurate global GPP estimates with those of net ecosystem exchange, for which global predictors are still unavailable. Nevertheless, CUE estimates are actually available for only a few ecosystem types, while information regarding agro-ecosystems is scarce, in spite of the simplified spatial structure of these ecosystems that facilitates studies on allocation patterns and temporal growth dynamics. We combined three largely deployed methods, eddy covariance, soil respiration and biometric measurements, to assess monthly values of CUE, NPP and allocation patterns in different plant organs in an apple orchard during a complete year (2010). We applied a measurement protocol optimized for quantifying monthly values of carbon fluxes in this ecosystem type, which allows for a cross check between estimates obtained from different methods. We also attributed NPP components to standing biomass increments, detritus cycle feeding and lateral exports. We found that in the apple orchard, both net ecosystem production and gross primary production on a yearly basis, 380 ± 30 g C m−2 and 1263 ± 189 g C m−2 respectively, were of a magnitude comparable to those of natural forests growing in similar climate conditions. The largest differences with respect to forests are in the allocation pattern and in the fate of produced biomass. The carbon sequestered from the atmosphere was largely allocated to production of fruit: 49% of annual NPP was taken away from the ecosystem through apple production. Organic material (leaves, fine root litter, pruned wood and early fruit falls) contributing to the detritus cycle was 46% of the NPP. Only 5% was attributable to standing biomass increment, while this NPP component is generally the largest in forests. The CUE, with an annual average of 0.71 ± 0.12, was higher than the previously suggested constant values of 0.47–0.50. Low nitrogen investment in fruit, the limited root apparatus, and the optimal growth temperature and nutritional condition observed at the site are suggested to be explanatory variables for the high CUE observed.

Fertile forests produce biomass more efficiently

Trees with sufficient nutrition are known to allocate carbon preferentially to aboveground plant parts. Our global study of 49 forests revealed an even more fundamental carbon allocation response to nutrient availability: forests with high-nutrient availability use 58 ± 3% (mean ± SE; 17 forests) of their photosynthates for plant biomass production (BP), while forests with low-nutrient availability only convert 42 ± 2% (mean ± SE; 19 forests) of annual photosynthates to biomass. This nutrient effect largely overshadows previously observed differences in carbon allocation patterns among climate zones, forest types and age classes. If forests with low-nutrient availability use 16 ± 4% less of their photosynthates for plant growth, what are these used for? Current knowledge suggests that lower BP per unit photosynthesis in forests with low- versus forests with high-nutrient availability reflects not merely an increase in plant respiration, but likely results from reduced carbon allocation to unaccounted components of net primary production, particularly root symbionts.

The allocation of ecosystem net primary productivity in tropical forests

The allocation of the net primary productivity (NPP) of an ecosystem between canopy, woody tissue and fine roots is an important descriptor of the functioning of that ecosystem, and an important feature to correctly represent in terrestrial ecosystem models. Here, we collate and analyse a global dataset of NPP allocation in tropical forests, and compare this with the representation of NPP allocation in 13 terrestrial ecosystem models. On average, the data suggest an equal partitioning of allocation between all three main components (mean 34 ± 6% canopy, 39 ± 10% wood, 27 ± 11% fine roots), but there is substantial site-to-site variation in allocation to woody tissue versus allocation to fine roots. Allocation to canopy (leaves, flowers and fruit) shows much less variance. The mean allocation of the ecosystem models is close to the mean of the data, but the spread is much greater, with several models reporting allocation partitioning outside of the spread of the data. Where all main components of NPP cannot be measured, litterfall is a good predictor of overall NPP (r(2) = 0.83 for linear fit forced through origin), stem growth is a moderate predictor and fine root production a poor predictor. Across sites the major component of variation of allocation is a shifting allocation between wood and fine roots, with allocation to the canopy being a relatively invariant component of total NPP. This suggests the dominant allocation trade-off is a 'fine root versus wood' trade-off, as opposed to the expected 'root-shoot' trade-off; such a trade-off has recently been posited on theoretical grounds for old-growth forest stands. We conclude by discussing the systematic biases in estimates of allocation introduced by missing NPP components, including herbivory, large leaf litter and root exudates production. These biases have a moderate effect on overall carbon allocation estimates, but are smaller than the observed range in allocation values across sites.

Human Influence and Threat to Biodiversity and Sustainable Living

More than half the habitable surface of the planet has already been significantly altered by human activity. Scientists suggested that this planets species are on the verge of mass extinction while our knowledge of diversity and variability of plants, animals, microorganisms and the ecosystem in which they occurs incomplete. Hence, “Biological diversity is the variability among living organisms from all sources, including interalias, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part”. The values, deriving forces and human influences, as well as the measures for conservation and sustainable use of the biodiversity vary greatly with in and between different cultures. Recent estimates have shown that humans use or pre-empt 40% of the terrestrial component of net primary productivity (total photosynthetic production). No part of the world is considered truly undisturbed; indicating that the world's habitats have been significantly modified by human action. Humans' influences affect biodiversity by: (1) agriculture, fisheries and over harvesting of resources, (2) habitat destruction, conversion, fragmentation and degradation, (3) introduction of exotic or invasive organisms and diseases, (4) Pollution of soil water and atmosphere and, (5) global change. All these impacts call for the promotion of environmental friendly living with biodiversity. Environmental friendly living exists within the socio-economic value of biodiversity. Biodiversity conservation, cultural survival and the search for new products are intractably linked. Ethno ecology, the study of the management system of indigenous people, which is particularly important for its application to the action of sustainable, uses systems, must be prompted in communities. This needs to adapted many indigenous techniques and values because they are (1) provide a medium to link people to their ancestors (2) cultural ceremonies frequently make use of biodiversity. It is also good to note that community participation and benefit sharing are not the sole solution to conserve ever increasing biodiversity degradation. There is a need to have balance vision and supportive sectarian and cross- sartorial actions of the governments with local, national, regional and international consideration that can save the biodiversity. Any strategy to slow the loss of biodiversity and to enhance its contributions to development must integrate conservation of biodiversity, sustainable use of its components and equitable sharing of benefits at all levels Ethiopian Journal of Education and Sciences Vol. 3 (1) 2007: pp. 85-96

What is NPP? Inconsistent accounting of respiratory fluxes in the definition of net primary production

Primary production represents a globally important flux of carbon between the atmosphere and the biosphere. From an ecological perspective, it measures the rate at which solar energy is stored by plants as organic matter, and is therefore a measure of the rate at which solar energy is captured and made available to the rest of the food chain (Odum 1971). From a biogeochemical perspective, primary production provides links between the biosphere and the climate system through the global cycling of C, water and nutrients (Roy et al . 2001). Gross primary production (GPP) is the total amount of C assimilated by plants within a given area over a specified timeframe. (Here GPP refers to the total C fixed by photosynthesis, minus the C lost by photorespiration. We have adopted the convention of specifying productivity in units of C mass per unit ground area per unit time, although we could have alternatively used units of total biomass, or energy: Odum 1971.) Net primary production (NPP) is GPP less the flux of autotrophic respiration of assimilate used for the plant’s own metabolism ( R ), therefore:

COUPLING FINE ROOT DYNAMICS WITH ECOSYSTEM CARBON CYCLING IN BLACK SPRUCE FORESTS OF INTERIOR ALASKA

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs, and mosses were measured relative to soil CO2 fluxes.Fine root production, measured over a two‐year period using minirhizotrons, varied from 0.004 ± 0.001 mm·cm–2·d–1 over winter, to 0.051 ± 0.015 mm·cm–2·d–1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. Monthly rates of production and mortality were somewhat asynchronous because mortality tended to peak during fall and be minimal during periods of peak production. Production and mortality were, however, positively correlated across all tubes and time periods. Annual fine root production averaged 2.45 ± 0.31, 8.01 ± 1.39, and 2.53 ± 0.27 mm·cm–2·yr–1 (means ± 1 se) among the three sites, when averaged across years.Fine root survival and decomposition were measured by tracking and analyzing the fate of individual fine roots using mark–recapture techniques. Fine root survival was greatest during periods of peak root growth, and least over winter (φtime). Roots first appearing in the middle of the growing season had higher survival rates than those first appearing early or late in the growing season, or over winter (φcohort), and risk of mortality decreased with root age (φage). Survival estimates translate to mean life spans of 108 ± 4 d during the growing season. While these values are in striking contrast to needle longevity and rates of aboveground litter decomposition, they are similar to many values found for temperate systems, supporting the notion that there are basic morphological and physiological traits of first‐order roots that are common to most woody plant root systems. During the growing season, monthly fine root decomposition rates averaged 0.46 ± 0.01 per month, while decomposition rates over winter averaged 0.73 ± 0.01 per winter. These growing season estimates translate to 49 ± 2 d from the time a root was first observed as dead, to the time it disappeared. For roots that decomposed during the growing season, those with longer life spans decomposed more slowly after death. Comparing these results with other minirhizotron studies suggests that life‐history traits of black spruce first‐order roots are similar to those from temperate (and perhaps most) forest ecosystems.Annual production of fine roots averaged 228 ± 75 g biomass·m–2·yr–1, constituting ∼56% of total stand production. Aboveground production of trees (50 ± 14 g biomass·m–2·yr–1, 13%) and shrubs (40 ± 2 g biomass·m–2·yr–1, 11%) contributed similarly to total production, while mosses (73 ± 14 g biomass·m–2·yr–1, 20%) accounted for the largest component of aboveground production. Soil temperature had a strong control over both soil respiration (Q10 = 2.21 ± 0.31) and root respiration (Q10 = 2.30 ± 0.37). During the growing season (15 May to 15 September), ∼56% of soil CO2 efflux (580 ± 40 g C/m2) was derived from fine root respiration (329 ± 54 g C/m2). Although apparent rates of heterotrophic respiration (May through September) and total production did not differ, definitive estimates of net ecosystem production are impossible given the potentially large, unmeasured components of NPP (net primary production), such as root exudation and mycorrhizal production. Nevertheless, rates of fine root production, mortality, and decomposition indicate that in these black spruce ecosystems, fine roots are much more dynamic than would be predicted from patterns of aboveground processes, and that carbon, and presumably nutrients, are cycling through fine roots at rates several orders of magnitude faster than through aboveground tissues.Corresponding Editor: J. Yavitt

Spatial variability of aboveground net primary production for a forested landscape in northern Wisconsin

Quantifying forest net primary production (NPP) is critical to understanding the global carbon cycle because forests are responsible for a large portion of the total terrestrial NPP. The objectives of this study were to measure above ground NPP (NPPA) for a land surface in northern Wisconsin, examine the spatial patterns of NPPA and its components, and correlate NPPA with vegetation cover types and leaf area index. Mean NPPA for aspen, hardwoods, mixed forest, upland conifers, nonforested wetlands, and forested wetlands was 7.8, 7.2, 5.7, 4.9, 5.0, and 4.5 t dry mass·ha–1·year–1, respectively. There were significant (p = 0.01) spatial patterns in wood, foliage, and understory NPP components and NPPA (p = 0.03) when the vegetation cover type was included in the model. The spatial range estimates for the three NPP components and NPPA differed significantly from each other, suggesting that different factors are influencing the components of NPP. NPPA was significantly correlated with leaf area index (p = 0.01) for the major vegetation cover types. The mean NPPA for the 3 km × 2 km site was 5.8 t dry mass·ha–1·year–1.

Effects of stand age on net primary productivity of boreal black spruce forests in Ontario, Canada

Quantification of the effects of stand age on its net primary productivity (NPP) is critical for estimating forest NPP and carbon budget at regional to global scales. This paper reports a practical method for quantifying age–NPP relationships using existing normal yield tables, biomass equations, and measurements of fine-root turnover and litterfall. Applying this method, we developed mean age–NPP relationships for black spruce (Picea mariana (Mill.) BSP) stands in Ontario. We define "mean age–NPP relationship", as the changes in NPP that occur with age under long-term mean environmental conditions. These relationships indicate that NPP at more productive sites culminates to a higher value and at an earlier age and also declines more rapidly thereafter. A further component analysis indicates that the decrease in biomass growth of woody tissues is the main contributor to the decline with age. Finally, error assessment suggests that the uncertainty in NPP estimates can be substantially reduced with a better quantification of fine-root turnover and litterfall, which are the two dominant NPP components, particularly in the later stages of stand development. With new techniques now available, more accurate measurement of these components is possible, and thus strongly recommended.