Allison L. Dunn

Evaluation of remote sensing based terrestrial productivity from MODIS using regional tower eddy flux network observations

The Moderate Resolution Spectroradiometer (MODIS) sensor has provided near real-time estimates of gross primary production (GPP) since March 2000. We compare four years (2000 to 2003) of satellite-based calculations of GPP with tower eddy CO2 flux-based estimates across diverse land cover types and climate regimes. We examine the potential error contributions from meteorology, leaf area index (LAI)/fPAR, and land cover. The error between annual GPP computed from NASAs Data Assimilation Offices (DAO) and tower-based meteorology is 28%, indicating that NASAs DAO global meteorology plays an important role in the accuracy of the GPP algorithm. Approximately 62% of MOD15-based estimates of LAI were within the estimates based on field optical measurements, although remaining values overestimated site values. Land cover presented the fewest errors, with most errors within the forest classes, reducing potential error. Tower-based and MODIS estimates of annual GPP compare favorably for most biomes, although MODIS GPP overestimates tower-based calculations by 20%-30%. Seasonally, summer estimates of MODIS GPP are closest to tower data, and spring estimates are the worst, most likely the result of the relatively rapid onset of leaf-out. The results of this study indicate, however, that the current MODIS GPP algorithm shows reasonable spatial patterns and temporal variability across a diverse range of biomes and climate regimes. So, while continued efforts are needed to isolate particular problems in specific biomes, we are optimistic about the general quality of these data, and continuation of the MOD17 GPP product will likely provide a key component of global terrestrial ecosystem analysis, providing continuous weekly measurements of global vegetation production

Redefinition and global estimation of basal ecosystem respiration rate

Basal ecosystem respiration rate (BR), the ecosystem respiration rate at a given temperature, is a common and important parameter in empirical models for quantifying ecosystem respiration (ER) globally. Numerous studies have indicated that BR varies in space. However, many empirical ER models still use a global constant BR largely due to the lack of a functional description for BR. In this study, we redefined BR to be ecosystem respiration rate at the mean annual temperature. To test the validity of this concept, we conducted a synthesis analysis using 276 site-years of eddy covariance data, from 79 research sites located at latitudes ranging from similar to 3 degrees S to similar to 70 degrees N. Results showed that mean annual ER rate closely matches ER rate at mean annual temperature. Incorporation of site-specific BR into global ER model substantially improved simulated ER compared to an invariant BR at all sites. These results confirm that ER at the mean annual temperature can be considered as BR in empirical models. A strong correlation was found between the mean annual ER and mean annual gross primary production (GPP). Consequently, GPP, which is typically more accurately modeled, can be used to estimate BR. A light use efficiency GPP model (i.e., EC-LUE) was applied to estimate global GPP, BR and ER with input data from MERRA (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate resolution Imaging Spectroradiometer). The global ER was 103 Pg C yr (-1), with the highest respiration rate over tropical forests and the lowest value in dry and high-latitude areas.

Assessing interannual variation in MODIS-based estimates of gross primary production

Global estimates of terrestrial gross primary production (GPP) are now operationally produced from Moderate Resolution Imaging Spectrometer (MODIS) imagery at the 1-km spatial resolution and eight-day temporal resolution. In this study, MODIS GPP products were compared with ground-based GPP estimates over multiple years at three sites-a boreal conifer forest, a temperate deciduous forest, and a desert grassland. The ground-based estimates relied on measurements at eddy covariance flux towers, fine resolution remote sensing, and modeling. The MODIS GPP showed seasonal variation that was generally consistent with the in situ observations. The sign and magnitude of year-to-year variation in the MODIS products agreed with that of the ground observations at two of the three sites. Examination of the inputs to the MODIS GPP algorithm-notably the fraction of photosynthetically active radiation (FPAR) that is absorbed by the canopy), minimum temperature scalar, and vapor pressure deficit scalar-provided explanations for cases of disagreement between the MODIS and ground-based GPP estimates. Continued evaluation of interannual variation in MODIS products and related climate variables will aid in assessing potential biospheric feedbacks to climate change

Changes in net ecosystem productivity of boreal black spruce stands in response to changes in temperature at diurnal and seasonal time scales.

Net ecosystem productivity (NEP) of boreal coniferous forests is believed to rise with climate warming, thereby offsetting some of the rise in atmospheric CO(2) concentration (C(a)) by which warming is caused. However, the response of conifer NEP to warming may vary seasonally, with rises in spring and declines in summer. To gain more insight into this response, we compared changes in CO(2) exchange measured by eddy covariance and simulated by the ecosystem process model ecosys under rising mean annual air temperatures (T(a)) during 2004-2006 at black spruce stands in Saskatchewan, Manitoba and Quebec. Hourly net CO(2) uptake was found to rise with warming at T(a) < 15 degrees C and to decline with warming at T(a) > 20 degrees C. As mean annual T(a) rose from 2004 to 2006, increases in net CO(2) uptake with warming at lower T(a) were greater than declines with warming at higher T(a) so that annual gross primary productivity and hence NEP increased. Increases in net CO(2) uptake measured at lower T(a) were explained in the model by earlier recovery of photosynthetic capacity in spring, and by increases in carboxylation activity, using parameters for the Arrhenius temperature functions of key carboxylation processes derived from independent experiments. Declines in net CO(2) uptake measured at higher T(a) were explained in the model by sharp declines in mid-afternoon canopy stomatal conductance (g(c)) under higher vapor pressure deficits (D). These declines were modeled from a hydraulic constraint to water uptake imposed by low axial conductivity of conifer roots and boles that forced declines in canopy water potential (psi(c)), and hence in g(c) under higher D when equilibrating water uptake with transpiration. In a model sensitivity study, the contrasting responses of net CO(2) uptake to specified rises in T(a) caused annual NEP of black spruce in the model to rise with increases in T(a) of up to 6 degrees C, but to decline with further increases at mid-continental sites with lower precipitation. However, these contrasting responses to warming also indicate that rises in NEP with climate warming would depend on the seasonality (spring versus summer) as well as the magnitude of rises in T(a).

Modeling acclimation of photosynthesis to temperature in evergreen conifer forests.

• In this study, we used a canopy photosynthesis model which describes changes in photosynthetic capacity with slow temperature-dependent acclimations. • A flux-partitioning algorithm was applied to fit the photosynthesis model to net ecosystem exchange data for 12 evergreen coniferous forests from northern temperate and boreal regions. • The model accounted for much of the variation in photosynthetic production, with modeling efficiencies (mean > 67%) similar to those of more complex models. The parameter describing the rate of acclimation was larger at the northern sites, leading to a slower acclimation of photosynthesis to temperature. The response of the rates of photosynthesis to air temperature in spring was delayed up to several days at the coldest sites. Overall photosynthesis acclimation processes were slower at colder, northern locations than at warmer, more southern, and more maritime sites. • Consequently, slow changes in photosynthetic capacity were essential to explaining variations of photosynthesis for colder boreal forests (i.e. where acclimation of photosynthesis to temperature was slower), whereas the importance of these processes was minor in warmer conifer evergreen forests.

Tree Productivity Enhanced with Conversion from Forest to Urban Land Covers.

Urban areas are expanding, changing the structure and productivity of landscapes. While some urban areas have been shown to hold substantial biomass, the productivity of these systems is largely unknown. We assessed how conversion from forest to urban land uses affected both biomass structure and productivity across eastern Massachusetts. We found that urban land uses held less than half the biomass of adjacent forest expanses with a plot level mean biomass density of 33.5 ± 8.0 Mg C ha-1. As the intensity of urban development increased, the canopy cover, stem density, and biomass decreased. Analysis of Quercus rubra tree cores showed that tree-level basal area increment nearly doubled following development, increasing from 17.1 ± 3.0 to 35.8 ± 4.7 cm2 yr-1. Scaling the observed stem densities and growth rates within developed areas suggests an aboveground biomass growth rate of 1.8 ± 0.4 Mg C ha-1 yr-1, a growth rate comparable to nearby, intact forests. The contrasting high growth rates and lower biomass pools within urban areas suggest a highly dynamic ecosystem with rapid turnover. As global urban extent continues to grow, cities consider climate mitigation options, and as the verification of net greenhouse gas emissions emerges as critical for policy, quantifying the role of urban vegetation in regional-to-global carbon budgets will become ever more important.

Landscape heterogeneity, soil climate, and carbon exchange in a boreal black spruce forest

We measured soil climate and the turbulent fluxes of CO2, H2O, heat, and momentum on short towers (2 m) in a 160-yr-old boreal black spruce forest in Manitoba, Canada. Two distinct land cover types were studied: a Sphagnum-dominated wetland, and a feathermoss (Pleurozium and Hylocomium)-dominated upland, both lying within the footprint of a 30-m tower, which has measured whole-forest carbon exchange since 1994. Peak summertime uptake of CO2, was higher in the wetland than for the forest as a whole due to the influence of deciduous shrubs. Soil respiration rates in the wetland were approximately three times larger than in upland soils, and 30% greater than the mean of the whole forest, reflecting decomposition of soil organic matter. Soil respiration rates in the wetland were regulated by soil temperature, which was in turn influenced by water table depth through effects on soil heat capacity and conductivity. Warmer soil temperatures and deeper water tables favored increased heterotrophic respiration. Wetland drainage was limited by frost during the first half of the growing season, leading to high, perched water tables, cool soil temperatures, and much lower respiration rates than observed later in the growing season. Whole-forest evapotranspiration increased as water tables dropped, suggesting that photosynthesis in this forest was rarely subject to water stress. Our data indicate positive feedback between soil temperature, seasonal thawing, heterotrophic respiration, and evapotranspiration. As a result, climate warming could cause covariant changes in soil temperature and water table depths that may stimulate photosynthesis and strongly promote efflux of CO2 from peat soils in boreal wetlands.

Correction to “Seasonal controls on the exchange of carbon and water in an Amazonian rain forest”

Citation: Hutyra, L. R., J. W. Munger, S. R. Saleska, E. Gottlieb, B. C. Daube, A. L. Dunn, D. F. Amaral, P. B. de Camargo, and S. C.Wofsy (2007), Correction to ‘‘Seasonal controls on the exchange of carbon and water in an Amazonian rain forest,’’ J. Geophys. Res.,112, G04099, doi:10.1029/2007JG000573.

Simulating Peatland Methane Dynamics Coupled to a Mechanistic Model of Biogeochemistry, Hydrology, and Energy: Implications to Climate Change

Northern peat lands have a strong potential to modify climate through changes in soil organic carbon (SOC) and methane (CH4). A dynamic interaction among climate, soil physical properties (e.g., temperature and moisture), and biogeochemistry (e.g., quantity and quality of SOC) determines the peat land system. Due to this interaction, CH4 production, oxidation, and transport dynamics changes dramatically under climate change. To appropriately study the future CH4 in a predictive manner, a simulation model must be able to reproduce the inherent dynamic interaction in the peat land system. Here, these complex interactions were simulated simultaneously in a biogeochemical peat land model coupled with mechanistic soil hydrology and thermal dynamics (ED2.0-peat). The model successfully reproduced soil physical profiles and the resultant SOC and CH4 observed in a poor fen of northern Manitoba. With an experimental simulation of 4°C warming, a significant long-term decline in CH4 emission was found, caused by a loss in substrate and prevalence of aerobic conditions. However, there was a transient increase in CH4 emission shortly after warming because of time lag between the temperature dependence of microbial activity (a fast response to climate change) and the loss in peat depth (a slow response). 17 Climate Change and Variability 328