Forest Flux Research Articles

Increasing trends of soil greenhouse gas fluxes in Japanese forests from 1980 to 2009

Forest soils are a source/sink of greenhouse gases, and have significant impacts on the budget of these terrestrial greenhouse gases. Here, we show climate-driven changes in soil GHG fluxes (CO2 emission, CH4 uptake, and N2O emission) in Japanese forests from 1980 to 2009, which were estimated using a regional soil GHG model that is data-oriented. Our study reveals that the soil GHG fluxes in Japanese forests have been increasing over the past 30 years at the rate of 0.31 Tg C yr−2 for CO2 (0.23 % yr−1, relative to the average from 1980 to 2009), 0.40 Gg C yr−2 for CH4 (0.44 % yr−1), and 0.0052 Gg N yr−2 for N2O (0.27 % yr−1). Our estimates also show large interannual variations in soil GHG fluxes. The increasing trends and large interannual variations in soil GHG fluxes seem to substantially affect Japan's Kyoto accounting and future GHG mitigation strategies.

GNSS Precipitable Water Vapor from an Amazonian Rain Forest Flux Tower

Abstract Understanding the complex interactions between water vapor fields and deep convection on the mesoscale requires observational networks with high spatial (kilometers) and temporal (minutes) resolution. In the equatorial tropics, where deep convection dominates the vertical distribution of the most important greenhouse substance—water—these mesoscale networks are nonexistent. Global Navigational Satellite System (GNSS) meteorological networks offer high temporal/spatial resolution precipitable water vapor, but infrastructure exigencies are great. The authors report here on very accurate precipitable water vapor (PWV) values calculated from a GNSS receiver installed on a highly nonideal Amazon rain forest flux tower. Further experiments with a mechanically oscillating platform demonstrate that errors and biases of approximately 1 mm (2%–3% of PWV) can be expected when compared with a stable reference GNSS receiver for two different geodetic grade receivers/antennas and processing methods [GPS-Inferred Positioning System (GIPSY) and GAMIT]. The implication is that stable fixed antennas are unnecessary for accurate calculation of precipitable water vapor regardless of processing techniques or geodetic grade receiver.

A meta-analysis of the effects of clearcut and variable-retention harvesting on soil nitrogen fluxes in boreal and temperate forests

One of the assumed advantages of variable-retention (VR) harvesting over clearcut harvesting is reduced postharvest leaching losses of nitrogen. We test this assumption by synthesizing results from long-term field trials in a meta-analysis. Overall, clearcutting significantly increased soil NO3-N concentration, NO3-N as a proportion of soluble inorganic nitrogen (SIN), N concentration in leachates, N flux, nitrification rates, and pH, but not total N, NH4-N, SIN concentration, ammonification, or N mineralization rate. The proportion of soil NO3-N in deciduous forests increased immediately and returned to preharvest levels within five years; the effect was delayed in coniferous forests, but levels remained elevated for several years. Deciduous leaf litter decomposed faster and needle litter decomposed more slowly on clearcut sites than in uncut forests. Single-tree selection caused smaller changes in NO3-N than removal of groups of trees (i.e., gap creation) and led to smaller increases in NO3-N as a proportion of SIN than clearcut harvesting. High levels of retention (>70%) were required to maintain uncut stand N-cycling characteristics. Postharvest NO3-N levels could be predicted from NO3-N availability in the uncut forests.

Responses of Soil CO2 Efflux to Precipitation Pulses in Two Subtropical Forests in Southern China

This study was designed to examine the responses of soil CO(2) efflux to precipitation pulses of varying intensities using precipitation simulations in two subtropical forests [i.e., mixed and broadleaf forests (MF and BF)] in southern China. The artificial precipitation event was achieved by spraying a known amount of water evenly in a plot (50×50cm(2)) over a 30min period, with intensities ranging from 10, 20, 50 and 100mm within the 30min. The various intensities were simulated in both dry season (in December 2007) and wet (in May 2008) season. We characterized the dynamic patterns of soil CO(2) efflux rate and environmental factors over the 5h experimental period. Results showed that both soil moisture and soil CO(2) efflux rate increased to peak values for most of the simulated precipitation treatments, and gradually returned to the pre-irrigation levels after irrigation in two forests. The maximum peak of soil CO(2) efflux rate occurred at the 10mm precipitation event in the dry season in BF and was about 3.5 times that of the pre-irrigation value. The change in cumulative soil CO(2) efflux following precipitation pulses ranged from -0.68 to 1.72g CO(2)m(-2) over 5h compared to the pre-irrigation levels and was generally larger in the dry season than in the wet season. The positive responses of soil CO(2) efflux to precipitation pulses declined with the increases in precipitation intensity, and surprisingly turned to negative when precipitation intensity reached 50 and 100mm in the wet season. These findings indicated that soil CO(2) efflux could be changed via pulse-like fluxes in subtropical forests in southern China as fewer but extreme precipitation events occur in the future.

Effects of manipulated herbivore inputs on nutrient flux and decomposition in a tropical rainforest in Puerto Rico

Forest canopy herbivores are known to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few studies have measured effects of herbivore-enhanced nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments), frassfall (insect feces), and throughfall (precipitation enriched with foliar leachates) in replicated experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10× greenfall or 10× frassfall to designated plots. Throughfall fluxes of NH(4), NO(3) and PO(4) (but not water) were doubled in treatment plots, based on published rates of fluxes of these nutrients in throughfall. Control plots received only background flux rates for these compounds but the same minimum amount of distilled water. We evaluated treatment effects as changes in flux rates for NO(3), NH(4) and PO(4), measured as decomposition rate of leaf litter in litterbags and as adsorption in ion-exchange resin bags at the litter-soil interface. Frass addition significantly increased NO(3) and NH(4) fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low-moderate herbivore outbreaks and demonstrated that herbivores, at these outbreak levels, increase ecosystem-level N and P fluxes by >30% in this tropical rainforest.

Seasonal variations in stem CO2 efflux in the Neotropical rainforest of French Guiana

Stem CO2 efflux (E s) is a significant component of total ecosystem respiration, but there is only scant information on seasonal variations in E s in tropical rainforests and on the main factors explaining these variations. We conducted a comprehensive 18-month study in French Guiana to try to better understand which environmental factors contribute to seasonal variations in E s in two habitats differing in soil water conditions. In both habitats, large seasonal variations in E s were observed for most trees. The main variations occurred during climatic transition periods and were consistent with seasonal variations in soil and total ecosystem respiration. Seasonal variations in atmospheric conditions, including air temperature, did not explain seasonal variations in E s. In contrast, seasonal variations in surface soil water content clearly induced seasonal variations in E s in each habitat. Both soil drought and flooded conditions induced a decrease in E s. Our results emphasize the need to integrate the influence of soil water content on E s into global models that simulate the response of forest ecosystem fluxes to climate changes.

Seasonal patterns of CO2fluxes in Amazon forests: Fusion of eddy covariance data and the ORCHIDEE model

[1] In some regions of the Amazon, global biogeophysical models have difficulties in reproducing measured seasonal patterns of net ecosystem exchange (NEE) of carbon dioxide. The global process-based biosphere model Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) used in this study showed that a standard model parameterization produces seasonal NEE patterns that are opposite in phase to the eddy flux data of the tropical evergreen forest at the Tapajos km 67 site (Brazil), like many other global models. However, we optimized several key parameters of ORCHIDEE using eddy covariance data of the Tapajos km 67 site in order to identify the driving factors of the seasonal variations in CO2 flux in this tropical forest ecosystem. The validity of the retrieved parameter values was evaluated for two other flux tower sites in the Amazon. The different tested optimization scenarios showed that only a few parameters substantially improve the fit to NEE and latent heat data. Our results confirm that these forests have the ability to maintain high transpiration and photosynthesis during the dry season in association with a large soil depth (Dsoil = 10 m) and a rooting system density that decreases almost linearly with depth (Hroot = 0.1). Previous analyses of seasonal variations in eddy covariance fluxes indicated that higher GPP levels were reached in the dry season compared to the wet season. Our optimization analysis suggests that this pattern could be caused by a leaf flush at the start of the dry season increasing the photosynthetic capacity of the canopy. Nevertheless, the current model structure is not yet able to simulate such a leaf flush, and we therefore suggest improving the ORCHIDEE model by including a specific phenology module that is driven by light availability for the tropical evergreen plant functional types. In addition, our results highlight both the potential and the limitations of flux data to improve global terrestrial models. Several parameters were not identifiable, and the risk of overfitting of the model was illustrated. Nevertheless, we conclude that these models can be improved substantially by assimilating site level flux data over the tropics.

A new model of net ecosystem carbon exchange for the deciduous-dominated forest by integrating MODIS and flux data

The eddy covariance technique provides continuous measurements of plot-level net ecosystem carbon exchange (NEE) across a wide range of vegetation types. However, these NEE estimates only represent fluxes at the tower footprint scale. To quantify the NEE over regions or continents, flux tower measurements need to be up-scaled to large areas. In the present study, we propose a new NEE model solely based on Moderate Resolution Imaging Spectroradiometer data, including enhanced vegetation index (EVI), land surface water index (LWSI), land surface temperature (LST), and Terra nighttime LST′. Site-specific data from the deciduous-dominated Harvard Forest flux site were used. Analysis covered six years (2001–2006) of CO 2 flux data. The data of the first four years were used for model building and the rest as validation set. Compared with the model based solely on EVI, we also introduced LST and LSWI into the new model. The results showed that this method could further improve the precision ( R 2 and RMSE reached 0.857 and 1.273, respectively) and generally capture the expected seasonal patterns of NEE.

Stocks and fluxes of carbon associated with land use change in Southeast Asian tropical peatlands: A review

[1] The increasing and alarming trend of degradation and deforestation of tropical peat swamp forests may contribute greatly to climate change. Estimates of carbon (C) losses associated with land use change in tropical peatlands are needed. To assess these losses we examined C stocks and peat C fluxes in virgin peat swamp forests and tropical peatlands affected by six common types of land use. Phytomass C loss from the conversion of virgin peat swamp forest to logged forest, fire-damaged forest, mixed croplands and shrublands, rice field, oil palm plantation, and Acacia plantation were calculated using the stock difference method and estimated at 116.9 ± 39.8, 151.6 ± 36.0, 204.1 ± 28.6, 214.9 ± 28.4, 188.1 ± 29.8, and 191.7 ± 28.5 Mg C ha−1, respectively. Total C loss from uncontrolled fires ranged from 289.5 ± 68.1 Mg C ha−1 in rice fields to 436.2 ± 77.0 Mg C ha−1 in virgin peat swamp forest. We assessed the effects of land use change on C stocks in the peat by looking at how the change in vegetation cover altered the main C inputs (litterfall and root mortality) and outputs (heterotrophic respiration, CH4 flux, fires, and soluble and physical removal) before and after conversion. The difference between the soil input-output balances in the virgin peat swamp forest and in the oil palm plantation gave an estimate of peat C loss of 10.8 ± 3.5 Mg C ha−1 yr−1. Peat C loss from other land use conversions could not be assessed due to lack of data, principally on soil heterotrophic respiration rates. Over 25 years, the conversion of tropical virgin peat swamp forest into oil palm plantation represents a total C loss from both biomass and peat of 427.2 ± 90.7 Mg C ha−1 or 17.1 ± 3.6 Mg C ha−1 yr−1. In all situations, peat C loss contributed more than 63% to total C loss, demonstrating the urgent need in terms of the atmospheric greenhouse gas burden to protect tropical virgin peat swamp forests from land use change and fires.

Biogeneous carbon fluxes in the boreal forests of Central Siberia

The assessments of the carbon pool and rate of plant biomass production, phytodetritus destruction, new formations of humic matters, and removal of water-soluble decomposition products for the forest ecosystems of the forest tundra and the northern and southern parts of the Central Siberian taiga were given. The rates of the main processes (organic-matter production and degradation) were demonstrated to be balanced in the ecosystems of the forest tundra. The larch forests of the northern taiga serve as a stock for a C atmosphere, which are equivalent to 32-34% of net primary production (NPP). The secondary birch growth where the understory needle-leaved trees have been formed and the primary old-growth fir forests are characterized by the balance of the main carbon fluxes in the southern taiga. The birch forests where the understory trees are just being formed and the fir forests at the age of 50-90 years serve as a stock for an average of 26% of carbon extracted as dioxide to make NPP.

Insect herbivory, organic matter deposition and effects on belowground organic matter fluxes in a central European oak forest

Apart from the forest floor, the canopy of forested ecosystems functions as the second most important source for dissolved and particulate fractions of organic and inorganic C and N compounds. However, under mass outbreak situations of insect herbivores this flux path of organic matter is considerably intensified clearly exceeding C and N fluxes from the forest floor. In this paper we report on herbivore-altered C and N fluxes from the canopy to the forest floor and effects on forest floor nutrient fluxes during severe defoliating herbivory of the winter moth (Operophtera brumata) and the mottled umber moth (Eranis defoliaria) in an oak forest in Germany. Over the course of 6.5 months we followed the C and N fluxes with bulk deposition, throughfall solution, insect frass deposits (green-fall together with insect faeces) and with forest floor solution in an 117-yr-old oak (Quercus petraea) forest. Compared to the control, herbivore defoliation significantly enhanced throughfall inputs of total and dissolved organic carbon and nitrogen by a factor of 3 and 2.5 (for TOC and DOC), and by 1.4 and 1.3 times (for TNb and DNb), respectively. Frass plus green-fall C and N fluxes peaked in May with 592 kg C ha−1 and 33.5 kg N ha−1 representing 79.6% (for C) and 78.3% (for N) of the total C and N input over 2.5 months. The quantitative and qualitative C and N input via faeces and litter deposition significantly differ between the insect affected and non-affected site. However, the C and N fluxes with throughfall did not significantly correlate with forest floor leachates. In this context, forest floor fluxes of TOC, DOC and NO3-N were significantly lower at the infested site compared to the control, whereas fluxes of NH4-N together with DON were significantly higher. The study demonstrates the importance of linking the population and associated frass dynamics of herbivorous insects with the cycling of nutrients and organic matter in forest ecosystems, highlighting the remarkable alterations in the timing, amounts and nature of organic matter dynamics on the ecosystem level. Consequently, the ecology of phytophagous insects allows partly to explain temporal-spatial alterations in nutrient cycling and thus ecosystem functioning.

Integration of ground and satellite data to model Mediterranean forest processes

The current work presents the testing of a modeling strategy that has been recently developed to simulate the gross and net carbon fluxes of Mediterranean forest ecosystems. The strategy is based on the use of a NDVI-driven parametric model, C-Fix, and of a biogeochemical model, BIOME-BGC, whose outputs are combined to simulate the behavior of forest ecosystems at different development stages. The performances of the modeling strategy are evaluated in three Italian study sites (San Rossore, Lecceto and Pianosa), where carbon fluxes are being measured through the eddy correlation technique. These sites are characterized by variable Mediterranean climates and are covered by different types of forest vegetation (pine wood, Holm oak forest and Macchia, respectively). The results of the tests indicate that the modeling strategy is generally capable of reproducing monthly GPP and NEE patterns in all three study sites. The highest accuracy is obtained in the most mature, homogenous pine wood of San Rossore, while the worst results are found in the Lecceto forest, where there are the most heterogeneous terrain, soil and vegetation conditions. The main error sources are identified in the inaccurate definition of the model inputs, particularly those regulating the site water budgets, which exert a strong control on forest productivity during the Mediterranean summer dry season. In general, the incorporation of NDVI-derived fAPAR estimates corrects for most of these errors and renders the forest flux simulations more stable and accurate.

Environmental factors controlling forest evapotranspiration and surface conductance on a multi-temporal scale in growing seasons of a Siberian larch forest

The water and energy fluxes in forests fluctuate on different temporal scales, reflecting the impact of environmental factors. We examined the temporal fluctuation of the turbulent fluxes, surface conductance (Gs), and four environmental factors (photosynthetic photon flux density [Q], vapour pressure deficit [D], air temperature [T], and volumetric soil water content [θ]) in a Siberian larch forest, using wavelet power spectra. The responses of the latent heat flux (λE) and Gs to the environmental factors were analysed using the wavelet scale-wise correlation coefficient (SWCC) on multiple temporal scales.

Satellite-Based Modeling of the Carbon Fluxes in Mature Black Spruce Forests in Alaska: A Synthesis of the Eddy Covariance Data and Satellite Remote Sensing Data

AbstractScaling up of observed point data to estimate regional carbon fluxes is an important issue in the context of the global terrestrial carbon cycle. In this study, the authors proposed a new model to scale up the eddy covariance data to estimate regional carbon fluxes using satellite-derived data. Gross primary productivity (GPP) and ecosystem respiration (RE) were empirically calculated using the normalized difference vegetation index (NDVI) and land surface temperature (LST) from the Moderate Resolution Imaging Spectroradiometer (MODIS). First, the model input is evaluated by comparing with the field data, then established and tested the model at the point scale, and then extended it into a regional scale. At the point scale, the empirical model could reproduce the seasonal and interannual variations in the carbon budget of the mature black spruce forests in Alaska and Canada sites, suggesting that seasonality of the NDVI and LST could explain the carbon fluxes and that the model is robust within mature black spruce forests in North America. Regional-scale analysis showed that the total GPP and RE between 2003 and 2006 were 1.76 ± 0.28 and 1.86 ± 0.26 kg CO2 m−2 yr−1, respectively, in mature black spruce forests in Alaska, indicating that these forests were almost carbon neutral. The authors’ model analysis shows that the proposed method is effective in scaling up point observations to estimate the regional-scale carbon budget and that the mature black spruce forests increased in sink strength during spring warming and decreased in sink strength during summer and autumn warming.

Measuring and modeling the spectrum of fine‐root turnover times in three forests using isotopes, minirhizotrons, and the Radix model

Fine root (<2 mm) cycling rates are important for understanding plant ecology and carbon fluxes in forests, but they are difficult to determine and remain uncertain. This paper synthesizes minirhizotron and isotopic data and a root model and concludes that (1) fine roots have a spectrum of turnover times ranging from months to many years and (2) the mean age of live root biomass (A) and the mean age of roots when they die (i.e., their turnover time (τ)) are not equal. We estimated A and τ of fine roots in three forests using the root model Radix. For short‐lived roots, we constrained τ with existing minirhizotron data; for long‐lived roots, we used new radiocarbon measurements of roots sampled by diameter size class and root branch order. Long‐lived root pools had site mean τ of 8–13 y and 5–9 y when sampled by diameter and branch order, respectively. Mean turnover times across sites were in general not significantly different as a function of branch‐order, size class, or depth. Our modeling results indicate that ∼20% of fine root biomass has turnover times of about a year, and ∼80% has decadal turnover times. This partitioning is reflected in our predicted mean ages of ∼9 y and turnover times of ∼3 y. We estimate that fine root mortality contributes between 38 and 104 g C m−2 y−1 to soil in these forests. These estimates are 20 to 80% of previous estimates in these and similar forests, in part because we explicitly account for the large portion of fine‐root biomass with decadal cycling rates. Our work shows that both fast and slow cycling roots must be modeled jointly to account for the heterogeneous nature of fine‐root dynamics.

The impacts of changes in federal timber harvest on forest carbon sequestration in western Oregon

This study examines the potential impacts of changes in federal timber harvest, acting through regional log markets, on the sequestration of carbon in forests and forest products in western Oregon. We construct a dynamic model of the region’s log markets in which market prices, log consumption at mills, and timber harvests and timber inventories on private, federal, and state forests are endogenous. Absent any policies regulating forest carbon sequestration, simulations show that regional carbon flux in forests and forest products would gradually decline as federal harvest rises from recent historical levels. If regional forest carbon flux were constrained to meet some minimum target, however, projections indicate that there would be opportunities for substituting carbon sequestration between federal and nonfederal lands through coordination of harvests across ownerships. We find that relatively small reductions in average private harvest could offset substantial losses of carbon flux on federal timberlands caused by increased federal harvest. One mechanism for achieving the changes needed in private harvest to meet a regional carbon flux target would be a carbon tax/subsidy program or a carbon offset market. For example, if federal owners offered timber for sale equal to the maximum sustainable level under the Northwest Forest Plan, our analysis indicates that a carbon price of roughly $US 19 per tonne of carbon would be sufficient to induce private owners to undertake the harvest and management modifications necessary to maintain regional forest carbon flux at its level in the early 2000s.

Detecting the critical periods that underpin interannual fluctuations in the carbon balance of European forests

The interannual variability of CO2 exchange by forest ecosystems in Europe was analyzed at site and regional scales by identifying critical periods that contributed to interannual flux anomalies. Critical periods were defined as periods in which monthly and annual flux anomalies were correlated. The analysis was first conducted at seven European forest flux tower sites with contrasting species and climatic conditions. Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE), a generic process‐based model, represented fairly well most features of the critical period patterns and their climate drivers at the site scale. Simulations at the scale of European forests were performed with ORCHIDEE integrated at a 0.25° spatial resolution. The spatial and temporal distributions of critical periods for canopy photosynthesis, ecosystem respiration, and net ecosystem exchange (NEE) as well as their underlying climate drivers were analyzed. The interannual variability in gross primary productivity (GPP) was explained by critical periods during spring and summer months. In contrast, the interannual variability in total ecosystem respiration (TER) was explained by critical periods occurring throughout the year. A latitudinal contrast between southern and northern Europe was observed in the distributions of critical periods for GPP and TER. The critical periods were positively controlled by temperature in northern Europe and by soil water availability in southern Europe. More importantly, the latitudinal transition between temperature‐driven and water‐driven critical periods for GPP varied from early spring to late summer. Such a distinct seasonal regime of critical periods was less clearly defined for TER and NEE. Overall, the critical periods associated with NEE variations and their meteorological drivers followed those associated with GPP.

Dynamics of gaseous nitrogen and carbon fluxes in riparian alder forests

We studied greenhouse gas (GHG) fluxes in two differently loaded riparian Alnus incana-dominated forests in agricultural landscapes of southern Estonia: a 33-year-old stand in Porijõgi, in which the uphill agricultural activities had been abandoned since the middle of the 1990s, and a 50-year-old stand in Viiratsi, which still receives polluted lateral flow from uphill fields fertilized with pig slurry. In Porijõgi, closed-chamber based sampling lasted from October 2001 to October 2009, whereas in Viiratsi the sampling period was from November 2003 to October 2009. Both temporal and spatial variations in all GHG gas fluxes were remarkable. Local differences in GHG fluxes between micro-sites (“Edge”, “Dry” and “Wet” in Porijõgi, and “Wet”, “Slope” and “Dry” in Viiratsi) were sometimes greater than those between sites. Median values of GHG fluxes from both sites over the whole study period and all microsites did not differ significantly, being 45 and 42 mg CO 2–C m −2 h −1, 8 and 0.5 μg CH 4–C m −2 h −1, 1.0 and 2.1 mg N 2–N m −2 h −1, and 5 and 9 μg N 2O–N m −2 h −1, in Porijõgi and Viiratsi, respectively. The N 2:N 2O ratio in Viiratsi (40–1200) was lower than in Porijõgi (10–7600). The median values-based estimation of the Global Warming Potential of CH 4 and N 2O was 19 and 185 kg CO 2 equivalents (eq) ha −1 yr −1 in Porijõgi and −14 and 336 kg CO 2 eq ha −1 yr −1 in Viiratsi, respectively. A significant Spearman rank correlation was found between the mean monthly air temperature and CO 2, CH 4 and N 2 fluxes in Porijõgi, and N 2O flux in Viiratsi, and between the monthly precipitation and CH 4 fluxes in both study sites. Higher groundwater level significantly increases CH 4 emission and decreases CO 2 and N 2O emission, whereas higher soil temperature significantly increases N 2O, CH 4 and N 2 emission values. In Porijõgi, GHG emissions did not display any discernable trend, whereas in Viiratsi a significant increase in CO 2, N 2, and N 2O emissions has been found. This may be a result of the age of the grey alder stand, but may also be caused by the long-term nutrient load of this riparian alder stand, which indicates a need for the management of similar heavily loaded riparian alder stands.

Age effects on carbon fluxes in temperate pine forests

This study evaluates age-related patterns of ecosystem carbon (C) fluxes in a chronosequence (5-, 18-, 33-, and 68-year old in 2007) of planted pine ( Pinus strobus L.) forests in southern Ontario, Canada, from 2003 to 2007. Carbon fluxes determined with the eddy covariance technique were normalized by differences in site index (SI; a measure of site quality based on a species-specific relationship between the height of the dominant trees and base age), to identify the effect of stand age on gross ecosystem productivity (GEP), ecosystem respiration (RE), and net ecosystem productivity (NEP). Observed GEP, RE, and NEP were highest in the 18-year-old stand, which differed from the two older stands by having a higher SI due to sustained availability of soil water in deeper soil layers (at ∼1 m depth). Typical age-related patterns in C fluxes were observed only when normalized for differences in SI. During periods without climatic constraints, SI-normalized GEP and RE increased with stand age, whereas SI-normalized NEP peaked at the 33-year-old site. In contrast, during periods with climatic constraints (e.g. drought, reduced radiation), monthly GEP and NEP at the 18-year-old site were higher compared to the 33-year-old site because deep soil water availability may have allowed this stand to maintain high productivity during these periods. Our analysis shows that differences in site quality may affect the interpretation of age-related C flux dynamics in chronosequence and synthesis studies. We therefore suggest that SI-normalization may offer a simple and efficient way to identify age-dependent processes in C fluxes in forest chronosequence studies, which may lead to improved large-scale estimates of C exchanges in forest ecosystems.

Trends in entropy production during ecosystem development in the Amazon Basin

Understanding successional trends in energy and matter exchange across the ecosystem-atmosphere boundary layer is an essential focus in ecological research; however, a general theory describing the observed pattern remains elusive. This paper examines whether the principle of maximum entropy production could provide the solution. A general framework is developed for calculating entropy production using data from terrestrial eddy covariance and micrometeorological studies. We apply this framework to data from eight tropical forest and pasture flux sites in the Amazon Basin and show that forest sites had consistently higher entropy production rates than pasture sites (0.461 versus 0.422 W m(-2) K(-1), respectively). It is suggested that during development, changes in canopy structure minimize surface albedo, and development of deeper root systems optimizes access to soil water and thus potential transpiration, resulting in lower surface temperatures and increased entropy production. We discuss our results in the context of a theoretical model of entropy production versus ecosystem developmental stage. We conclude that, although further work is required, entropy production could potentially provide a much-needed theoretical basis for understanding the effects of deforestation and land-use change on the land-surface energy balance.