Jose D. Fuentes

FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S. FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite. Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO 2 exchange of temperate broadleaved forests increases by about 5.7 g C m −2 day −1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO 2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO 2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO 2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Biogenic hydrocarbons in the atmospheric boundary layer: A review

Nonmethane hydrocarbons are ubiquitous trace atmospheric constituents yet they control the oxidation capacity of the atmosphere. Both anthropogenic and biogenic processes contribute to the release of hydrocarbons to the atmosphere. In this manuscript, the state of the science concerning biosynthesis, transport, and chemical transformation of hydrocarbons emitted by the terrestrial biosphere is reviewed. In particular, the focus is on isoprene, monoterpenes, and oxygen-ated hydrocarbons. The generated science during the last 10 years is reviewed to explain and quantify hydrocarbon emissions from vegetation and to discern impacts of biogenic hydrocarbons on local and regional atmospheric chemistry. Furthermore, the physiological and environmental processes controlling biosynthesis and production of hydrocarbon compounds are reported on. Many advances have been made on measurement and modeling approaches developed to quantify hydrocarbon emissions from leaves and forest ecosystems. A synthesis of the atmospheric chemistry of biogenic hydrocarbons and their role in the formation of oxidants and aerosols is presented. The integration of biogenic hydrocarbon kinetics and atmospheric physics into mathematical modeling systems is examined to assess the contribution of biogenic hydrocarbons to the formation of oxidants and aerosols, thereby allowing us to study their impacts on the earths climate system and to develop strategies to reduce oxidant precursors in affected regions.

Snowpack photochemical production of HONO : a major source of OH in the Arctic boundary layer in springtime

Both snow manipulation experiments and ambient measurements during the Polar Sunrise Experiment 2000 at Alert (Alert2000) indicate intensive photochemical production of nitrous acid (HONO) in the snowpack. This process constitutes a major HONO source for the overlying atmospheric boundary layer in the Arctic during the springtime, and sustained concentrations of HONO high enough that upon photolysis they became the dominant hydroxyl radical (OH) source. This implies a much greater role for OH radicals in Arctic polar sunrise chemistry than previously believed. Although the observations were made in the high Arctic, this finding has a significant implication for the boundary layer atmospheric chemistry in Antarctica during sunlit seasons and in the mid to high latitudes of the Northern Hemisphere during the winter and spring seasons when approximately 50% of the land mass may be covered by snow.

Responses of net ecosystem exchanges of carbon dioxide to changes in cloudiness: Results from two North American deciduous forests

We analyzed half-hourly tower-based flux measurements of carbon dioxide (CO 2 ) from a boreal aspen forest and a temperate mixed deciduous forest in Canada to examine the influences of clouds on forest carbon uptake. We showed that the presence of clouds consistently and significantly increased the net ecosystem exchanges (NEE) of CO 2 of both forests from the level under clear skies. The enhancement varied with cloudiness, solar elevation angles, and differed between the two forests. For the aspen forest the enhancement at the peak ranged from about 30% for the 20°-25° interval of solar elevation angles to about 55% for the 55°-60° interval. For the mixed forest the enhancement at the peak ranged from more than 60% for the 30°-35° interval of solar elevation angles to about 30% for the 65°-70° interval. Averaged over solar elevation angles >20°, the aspen and mixed forests had the maximal NEE at the irradiance equivalent to 78 and 71% of the clear-sky radiation, respectively. The general patterns of current sky conditions at both sites permit further increases in cloudiness to enhance their carbon uptake. We found that both forests can tolerate exceedingly large reductions of solar radiation (53% for the aspen forest and 46% for the mixed forest) caused by increases in cloudiness without lowering their capacities of carbon uptake. We suggest that the enhancement of carbon uptake under cloudy conditions results from the interactions of multiple environmental factors associated with the presence of clouds.

Ozone in the Arctic lower troposphere during winter and spring 2000 (ALERT2000)

Abstract A summary of the temporal and vertical characteristics of ozone in the Arctic boundary layer as observed during winter and spring 2000 near Alert, Nunavut, Canada (82°N, 62°W) is presented. The measurements were made during the Polar Sunrise Experiments ALERT2000. Particular attention is given to identifying chemical and atmospheric characteristics of short-lived (

Ozone deposition onto a deciduous forest during dry and wet conditions

Abstract Results of an experiment conducted to quantify the ozone deposition onto a deciduous forest stand in an acid-precipitation-impacted area of Canada are presented and discussed. The ozone deposition data were obtained above and within the forest canopy. The deposition process was affected by solar radiation, wind speed and ambient ozone concentration. Solar radiation was likely acting through its influence on stomatal opening and wind speed through its effects on bulk boundary layer resistance. Ozone deposition deep in the canopy was negligibly small compared with that in the upper canopy. The difference is ascribed to a larger biological sink for ozone in the upper canopy and to a lack of efficient transport in the lower canopy. Substantial ozone deposition was measured while the forest canopy remained wet with either dew or rain water, during night-time and daytime conditions. This is contrary to assumptions made in some deposition models that ozone uptake is reduced when foliage is wet.

Controls on mangrove forest‐atmosphere carbon dioxide exchanges in western Everglades National Park

August 2005. Maximum daytime NEE ranged from −20 to −25 mmol (CO2 )m −2 s −1 between March and May. Respiration (Rd) was highly variable (2.81 ± 2.41 mmol (CO2) m −2 s −1 ), reaching peak values during the summer wet season. During the winter dry season, forest CO2 assimilation increased with the proportion of diffuse solar irradiance in response to greater radiative transfer in the forest canopy. Surface water salinity and tidal activity were also important controls on NEE. Daily light use efficiency was reduced at high (>34 parts per thousand (ppt)) compared to low (<17 ppt) salinity by 46%. Tidal inundation lowered daytime Rd by ∼0.9 mmol (CO2 )m −2 s −1 and nighttime Rd by ∼0.5 mmol (CO2 )m −2 s −1 . The forest was a sink for atmospheric CO2, with an annual NEP of 1170 ± 127 g C m −2 during 2004. This unusually high NEP was attributed to year‐round productivity and low ecosystem respiration which reached a maximum of only 3 g C m −2 d −1 . Tidal export of dissolved inorganic carbon derived from belowground respiration likely lowered the estimates of mangrove forest respiration. These results suggest that carbon balance in mangrove coastal systems will change in response to variable salinity and inundation patterns, possibly resulting from secular sea level rise and climate change.

Ambient biogenic hydrocarbons and isoprene emissions from a mixed deciduous forest

Experiments were conducted during the growing season of 1993 at a mixed deciduous forest in southern Ontario, Canada to investigate the atmospheric abundance of hydrocarbons from phytogenic origins, and to measure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes α-pinene and β-pinene. Prior to leaf-bud break during spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (∼20 parts per trillion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billion (ppbv) during mid-growing season while maximum monoterpene mixing ratios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios were highest during mid-afternoon and were lowest during nighttime. Peak isoprene mixing ratios coincided with maximum canopy temperature. The diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foliage were measured by enclosing branches of trees inside environment-controlled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depended on tree species, foliage ontogeny, and environmental factors such as foliage temperature and intercepted photosynthetically active radiation (PAR). For instance, young (<1 month old) aspen leaves released approximately 80 times less isoprene than mature (>3 months old) leaves. During the latter part of the growing season the amount of carbon released back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed between the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approximately 30% higher compared to measured fluxes. Further comparisons between measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.

ON THE SEASONALITY OF ISOPRENE EMISSIONS FROM A MIXED TEMPERATE FOREST

Measurements of isoprene concentration and flux were made at a mixed deciduous forest in southern Canada during 1995 to characterize diel and seasonal emissions and thus deduce annual inventories. Isoprene inventories are necessary for inputs to modeling systems to study atmospheric chemistry and carbon budgets. Despite adequate environmental conditions to promote emissions, the onset of isoprene emission occurred two weeks after full leaf expansion, and two additional weeks were required for plants to emit isoprene at the maximum capacity. Such maximum isoprene emission was measured during July when canopy isoprene fluxes reached 40–60 nmol (isoprene)·m−2 (ground area)·s−1. Isoprene emission precipitously declined in concert with autumnal leaf senescence, with fluxes reaching the detection limit before the forest became leafless. In addition to plant development controls on emissions, temperatures below 10°C strongly modulated isoprene emission. After plants were exposed to low temperatures, isoprene emission remained suppressed and did not correspondingly increase in the manner that temperature is known to influence isoprene biosynthesis. Using a one-dimensional model to vertically adjust temperature and visible solar radiation with depth in the canopy, coupled with a seasonally adjusted emission rate, we estimated that the forest produced 71 mmol isoprene/m2 during 1995. For a deciduous forest with final leaf-area index of 4.1 and active isoprene biomass of 75 g (dry mass)/m2, on average such isoprene source accounted for 2% of the carbon fixed through photosynthesis. The percentage of carbon entering the atmosphere in the form of isoprene became as high as 10% during warm (>30°C) and dry conditions. The data set reported here demonstrates that constant emission rates are inadequate to characterize emission rates for the entire growing season. Improved isoprene-emission inventories can be achieved if emission factors are seasonally adjusted. In this study we adopted a method to express the emission rates as a function of degree days.