Continuous eddy convariance measurements of carbon dioxide, water vapor and heat were measured continuously between an oak savanna and an annual grassland in California over a 4 year period. These systems serve as representative sites for biomes in Mediterranean climates and experience much seasonal and inter-annual variability in temperature and precipitation. These sites hence serve as natural laboratories for how whole ecosystem will respond to warmer and drier conditions. The savanna proved to be a moderate sink of carbon, taking up about 150 gC m-2y-1 compared to the annual grassland, which tended to be carbon neutral and often a source during drier years. But this carbon sink by the savanna came at a cost. This ecosystem used about 100 mm more water per year than the grassland. And because the savanna was darker and rougher its air temperature was about 0.5 C warmer. In addition to our flux measurements, we collected vast amounts of ancillary data to interpret the site and fluxes, making this site a key site for model validation and parameterization. Datasets consist of terrestrial and airborne lidar for determining canopy structure, ground penetrating radar data on root distribution, phenology cameras monitoring leaf area index and its seasonality, predawn water potential, soil moisture, stem diameter and physiological capacity of photosynthesis.
In the 2010 Global Carbon Project Report it was estimated that the terrestrial biosphere sequesters 2.4 Pg of anthropogenic carbon emissions per year although the uncertainty on that value is undoubtedly high as no error bars were given. As man-made emissions of CO<sub>2</sub> continue to increase unabated in the atmosphere, quantifying the fate of these anthropogenic sources requires both knowing the magnitude and uncertainty of background, natural CO<sub>2</sub> fluxes. Thus, reducing uncertainty in biospheric CO<sub>2</sub> flux measurements will not only give us direct, accurate observations for future carbon accounting and climate treaties but also indirectly help us quantify anthropogenic emissions for the same purposes. In conjuncture with ecosystem models or remote-sensing techniques, these measurements also provide accurate constraints on global and continental terrestrial CO<sub>2</sub> budgets. With the motivation stated above, we designed and executed a three-year Laboratory Directed Research and Development (LDRD) project with the aim of reducing the uncertainty in eddy-covariance biospheric CO<sub>2</sub> flux. Reducing uncertainty was approached in two ways. The first utilized nontraditional eddy covariance instrumentation to identify and characterize atmospheric flows above and within the plant canopy. Here, atmospheric laser detection and ranging (lidar) instrumentation were used to capture unique flow features at night which may explain erroneous or anomalous carbon fluxes. The second approach utilized a multi-layer, 3rd order closure canopy-atmosphere model to simulate fluxes at each field site. Three sites were chosen for field instrument deployment and modeling. These included the Wind River AmeriFlux tower in Washington State and the Tonzi AmeriFlux and Diablo AmeriFlux towers in northern California. These sites represent some of the extremes in the biological and meteorological conditions over which eddy covariance techniques are used. Wind River is a multi-layered, 60-m tall seasonal rainforest, Diablo is a 1-m tall grassland with a very short growing season, and Tonzi is a 2- layered savannah canopy with complex ecohydrology. All three are surrounded by complex terrain in varying degrees. The towers provided very different test sites for validating the UC Davis Advanced Canopy Atmosphere Soil Algorithm (ACASA) model. Such validation gives promise that the model can be used to independently verify and gap-fill biospheric CO<sub>2</sub> measurements from the network of ~ 550 global flux towers for future greenhouse gas emissions monitoring and verification studies.
Policies for climate mitigation on land rarely acknowledge biophysical factors, such as reflectivity, evaporation, and surface roughness. Yet such factors can alter temperatures much more than carbon sequestration does, and often in a conflicting way. We outline a framework for examining biophysical factors in mitigation policies and provide some best-practice recommendations based on that framework. Tropical projects—avoided deforestation, forest restoration, and afforestation—provide the greatest climate value, because carbon storage and biophysics align to cool the Earth. In contrast, the climate benefits of carbon storage are often counteracted in boreal and other snow-covered regions, where darker trees trap more heat than snow does. Managers can increase the climate benefit of some forest projects by using more reflective and deciduous species and through urban forestry projects that reduce energy use. Ignoring biophysical interactions could result in millions of dollars being invested in some mitigation projects that provide little climate benefit or, worse, are counter-productive.
The rate at which isoprene is emitted by a forest depends on an array of environmental variables, the forest's biomass, and its species composition. At present it is unclear whether errors in canopy-scale and process-level isoprene emission models are due to inadequacies in leaf-to-canopy integration theory or the imperfect assessment of the isoprene-emitting biomass in the flux footprint. To address this issue, an isoprene emission model (CANVEG) was tested over a uniform aspen stand and a mixed-species, broad-leaved forest. The isoprene emission model consists of coupled micrometeorological and physiological modules. The micrometeorological module computes leaf and soil energy exchange, turbulent diffusion, scalar concentration profiles, and radiative transfer through the canopy. Environmental variables that are computed by the micrometeorological module, in turn, drive physiological modules that calculate leaf photosynthesis, stomatal conductance, transpiration and leaf, bole and soil/root respiration, and rates of isoprene emission. The isoprene emission model accurately predicted the diurnal variation of isoprene emission rates over the boreal aspen stand, as compared with micrometeorological flux measurements. The model's ability to simulate isoprene emission rates over the mixed temperate forest, on the other hand, depended strongly upon the amount of isoprene-emitting biomass, which, in a mixed-species forest, is a function of the wind direction and the horizontal dimensions of the flux footprint. When information on the spatial distribution of biomass and the flux footprint probability distribution function were included, the CANVEG model produced values of isoprene emission that compared well with micrometeorological measurements. The authors conclude that a mass and energy exchange model, which couples flows of carbon, water, and nutrients, can be a reliable tool for integrating leaf-scale, isoprene emission algorithms to the canopy dimension over dissimilar vegetation types as long as the vegetation is characterized appropriately.
The nose can reveal much qualitative information about the release of organic volatiles by plants, but since it is preferentially sensitive to certain terpenes. . . and rather insensitive to others. . . analyses with a gas chromatograph are needed to obtain a quantitative picture of the volatile organics present in the air at all times. Thus we easily detect the aromaticity of a deciduous forest in autumn, and especially the sweet odor of the leaf litter on the forest floor, and we can tell a coniferous forest at a distance. But we are unprepared for the fact that an oak forest produces virtually as many aromatics as a pine forest, only of a lower odor level.Rasmussen and Went (1965)
Read moreABSTRACT Forests in the south‐eastern United States experienced a prolonged dry spell and above‐normal temperatures during the 1995 growing season. During this episode, nearly continuous, eddy covariance measurements of carbon dioxide and water vapour fluxes were acquired over a temperate, hardwood forest. These data are used to examine how environmental factors and accumulating soil moisture deficits affected the diurnal pattern and magnitude of canopy‐scale carbon dioxide and water vapour fluxes. The field data are also used to test an integrative leaf‐to‐canopy scaling model (CANOAK), which uses micrometeorological and physiological theory, to calculate mass and energy fluxes. When soil moisture was ample in the spring, peak rates of net ecosystem CO 2 exchange ( N F ) occurred around midday and exceeded 20 μmol m −2 s −1 . Rates of N K were near optimal when air temperature ranged between 22 and 25°C. The accumulation of soil moisture deficits and a co‐occurrence of high temperatures caused peak rates of daytime carbon dioxide uptake to occur earlier in the morning. High air temperatures and soil moisture deficits were also correlated with a dramatic reduction in the magnitude of N E . On average, the magnitude of N E decreased from 20 to 7 μmol m −2 s −1 as air temperature increased from 24 to 30°C and the soil dried. The CANAOK model yielded accurate estimates of canopy‐scale carbon dioxide and water vapour fluxes when the forest had an ample supply of soil moisture. During the drought and heat spell, a cumulative drought index was needed to adjust the proportionality constant of the stomatal conductance model to yield accurate estimates of canopy CO 2 exchange. The adoption of the drought index also enabled the CANOAK model to give improved estimates of evaporation until midday. On the other hand, the scheme failed to yield accurate estimates of evaporation during the afternoon.
Read moreForest ecosystems across the globe show an increase in ecosystem carbon uptake efficiency under conditions with high fraction of diffuse radiation. Here, we combine eddy covariance flux measurements at a deciduous temperate forest in central Germany with canopy‐scale modeling using the biophysical multilayer model CANVEG to investigate the impact of diffuse radiation on various canopy gas exchange processes and to elucidate the underlying mechanisms. Increasing diffuse radiation enhances canopy photosynthesis by redistributing the solar radiation load from light saturated sunlit leaves to nonsaturated shade leaves. Interactions with atmospheric vapor pressure deficit and reduced leaf respiration are only of minor importance to canopy photosynthesis. The response strength of carbon uptake to diffuse radiation depends on canopy characteristics such as leaf area index and leaf optical properties. Our model computations shows that both canopy photosynthesis and transpiration increase initially with diffuse fraction, but decrease after an optimum at a diffuse fraction of 0.45 due to reduction in global radiation. The initial increase in canopy photosynthesis exceeds the increase in transpiration, leading to a rise in water‐use‐efficiency. Our model predicts an increase in carbon isotope discrimination with water‐use‐efficiency resulting from differences in the leaf‐to‐air vapor pressure gradient and atmospheric vapor pressure deficit. This finding is in contrast to those predicted with simple big‐leaf models that do not explicitly calculate leaf energy balance. At an annual scale, we estimate a decrease in annual carbon uptake for a potential increase in diffuse fraction, since diffuse fraction was beyond the optimum for 61% of the data.
Read moreWe report on the interannual variability of evapotranspiration ( E ) and energy exchange of an annual grassland in the Mediterranean climate zone of California. They were measured directly with the eddy covariance technique over a 6‐year period that spanned between July 2001 and June 2007 and experienced a large range in precipitation (376 mm to 888 mm). Despite a two‐fold range in precipitation, annual E ranged much less, between 266 mm and 391 mm. We found that pronounced energy‐limited and water‐limited periods occurred within the same year. In the water‐limited period, monthly integrated E scaled negatively with solar radiation and was restrained by precipitation. In the energy‐limited period, on the other hand, the majority of E scaled positively with solar radiation ( R g ) and was confined by potential E ( E p ). E was most sensitive to the availability of soil moisture during the transition to the senescence period rather than onset of the greenness period, causing annual E to be strongly modulated by growing season length. Bulk surface conductance scaled consistently with Priestley‐Taylor α coefficient regardless of interannual and seasonal variability of precipitation, E , and solar radiation.
Read moreA corollary to this law shows how the chemical equilibrium varies with temperature – namely, how, as the temperature increases, more of the one compound is formed at the expense of the other, or vice versa. This corollary can be stated as follows: At low temperature the greater yield is always of that product whose formation is accompanied by evolution of heat.Jacobus H. van’t Hoff, Nobel Prize Lecture, 1901
Read moreForm follows function . . . has been misunderstood. Form and function should be one, joined in a spiritual union.Frank Lloyd Wright, protégé of Louis Henri Sullivan and architect
Read more
