HP Schmid

Karlsruhe Institute of Technology, Carlsruhe, Baden-Württemberg, Germany

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Publications (71)87.26 Total impact

  • Agricultural and Forest Meteorology. 01/2011;
  • Agricultural and Forest Meteorology. 01/2011;
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    ABSTRACT: Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO(2) exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 degrees N). The sensitivity of NEE to mean annual temperature breaks down at similar to 16 degrees C (a threshold value of mean annual temperature), above which no further increase of CO(2) uptake with temperature was observed and dryness influence overrules temperature influence.
    Environmental Research Letters 08/2010; 5:34007-34007. · 3.58 Impact Factor
  • EGU General Assembly 2010, held 2-7 May, 2010 in Vienna, Austria, p. 12039. 05/2010; 12:12039.
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    ABSTRACT: Vegetation albedo is a critical component of the Earth's climate system, yet efforts to evaluate and improve albedo parameterizations in climate models have lagged relative to other aspects of model development. Here, we calculated growing season albedos for deciduous and evergreen forests, crops, and grasslands based on over 40 site-years of data from the AmeriFlux network and compared them with estimates presently used in the land surface formulations of a variety of climate models. Generally, the albedo estimates used in land surface models agreed well with this data compilation. However, a variety of models using fixed seasonal estimates of albedo overestimated the growing season albedo of northerly evergreen trees. In contrast, climate models that rely on a common two-stream albedo submodel provided accurate predictions of boreal needle-leaf evergreen albedo but overestimated grassland albedos. Inverse analysis showed that parameters of the two-stream model were highly correlated. Consistent with recent observations based on remotely sensed albedo, the AmeriFlux dataset demonstrated a tight linear relationship between canopy albedo and foliage nitrogen concentration (for forest vegetation: albedo=0.01+0.071%N, r2=0.91; forests, grassland, and maize: albedo=0.02+0.067%N, r2=0.80). However, this relationship saturated at the higher nitrogen concentrations displayed by soybean foliage. We developed similar relationships between a foliar parameter used in the two-stream albedo model and foliage nitrogen concentration. These nitrogen-based relationships can serve as the basis for a new approach to land surface albedo modeling that simplifies albedo estimation while providing a link to other important ecosystem processes.
    Global Change Biology 01/2010; 16(2):696 - 710. · 8.22 Impact Factor
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    Remote Sensing of Environment 01/2010; 114� �:576-591. · 5.10 Impact Factor
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    ABSTRACT: Vegetation albedo is a critical component of the Earth's climate system, yet efforts to evaluate and improve albedo parameterizations in climate models have lagged relative to other aspects of model development. Here, we calculated growing season albedos for deciduous and evergreen forests, crops, and grasslands based on over 40 site-years of data from the AmeriFlux network and compared them with estimates presently used in the land surface formulations of a variety of climate models. Generally, the albedo estimates used in land surface models agreed well with this data compilation. However, a variety of models using fixed seasonal estimates of albedo overestimated the growing season albedo of northerly evergreen trees. In contrast, climate models that rely on a common two-stream albedo submodel provided accurate predictions of boreal needle-leaf evergreen albedo but overestimated grassland albedos. Inverse analysis showed that parameters of the two-stream model were highly correlated. Consistent with recent observations based on remotely sensed albedo, the AmeriFlux dataset demonstrated a tight linear relationship between canopy albedo and foliage nitrogen concentration (for forest vegetation: albedo=0.01+0.071%N, r2=0.91; forests, grassland, and maize: albedo=0.02+0.067%N, r2=0.80). However, this relationship saturated at the higher nitrogen concentrations displayed by soybean foliage. We developed similar relationships between a foliar parameter used in the two-stream albedo model and foliage nitrogen concentration. These nitrogen-based relationships can serve as the basis for a new approach to land surface albedo modeling that simplifies albedo estimation while providing a link to other important ecosystem processes.
    Global Change Biology 01/2010; 16(2):696-710. · 8.22 Impact Factor
  • 29th Conference on Agricultural and Forest Meteorology. 01/2010;
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    Global Change Biology 01/2010; 17(2):886-897. · 8.22 Impact Factor
  • B. Deng, H. P. Schmid, D. Dragoni
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    ABSTRACT: The observation of latent heat fluxes is critical for the correct interpretation of the energy partitioning between atmosphere and biosphere. The eddy-covariance technique provides high resolution (hourly or semi-hourly) measurements of energy, water and carbon exchange. However, complete data sets of surface fluxes are required for the annual estimation of energy, water and carbon exchange between atmosphere and biosphere. Due to the underlying assumptions of the technique, periods of system maintenance and sensor limitations under certain conditions such as rain or low turbulence, data gaps are unavoidable in any long term measurements of eddy-covariance fluxes. Most commonly used gap-filling strategies are based on the exploitation of stochastic information contained in the valid data before and after a gap, and are thus biased by the conditions causing the gap. In this research, a mechanistic model based on the re-arranged Penman-Monteith equation is developed to fill the data gaps in latent heat flux, and it is evaluated using the observations from the flux tower in Morgan-Monroe State Forest (MMSF), Indiana, US. The core of the model consists of two components: one is for the aerodynamic conductance, which accounts for the influence of atmospheric stability on evapotranspiration, and the other is for the canopy conductance, which accounts for the biophysical regulations of evapotranspiration. To evaluate the model, artificial data gaps are introduced over the leaf-on periods of 2000-2003. Acceptable results for the vegetative season are obtained in years 2000, 2001 and 2002 with R-square values about 0.8 (0.64 for year 2003).
    04/2009;
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    ABSTRACT: In this study, we compare 10 years (1998-2007) of carbon budget estimates obtained with two independent methods: the micrometeorological approach based on eddy-covariance measurements, and the biometric method based on carbon stock increment measurements in a mixed deciduous forest in the Morgan-Monroe State Forest, Indiana, USA. Even though the cumulative estimates of both methods for the ten-year period are within 1% of each other, differences between biometric estimates of net ecosystem productivity (NEP-BM) and the corresponding eddy covariance based estimates (NEP-EC) were large in some years (up to 100%). We focus our analysis on the inter-annual variability in NEP and on the differences between the two methods and what processes they represent. We interpret our results in terms of ecosystem response to inter-annual and seasonal meteorological variability, extreme weather events (like late-spring or early-fall frost and intense and prolonged droughts), and consequences for allocation to different carbon pools. This work confirms the importance of long-term experiments in which results from both approaches are used to reduce overall uncertainty of NEP estimates, increase the understanding of carbon partitioning among different compartments of the forest ecosystem, and helps to explain observed inter-annual variability.
    03/2009; 11.
  • D. Dragoni, K.K. Caylor, H.P. Schmid
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    ABSTRACT: In this work we present experimental evidence in support of a new approach for investigating the dependence of sap velocity on atmospheric water demand and soil moisture supply. In this method, sap velocity is defined as the product of two components: the first describes the ‘shape’ of the radial profile of sap velocity, which is consistent through time and is likely linked to the species-specific anatomical and structural properties of the conducting xylem; the second, which we define as stem conductance, captures the time-dependent component of sap velocity that is mostly governed by shifts in atmospheric water demand and individual tree water supply. The heat pulse technique was used to estimate radial profiles of sap velocity and transpiration from a sample of 16 mature sugar maples (Acer saccharum) located along a topographic transect in a mixed deciduous forest. Our results demonstrate that: (1) stem conductance is strongly correlated with bulk air conditions (with confidence intervals for all the sampled trees greater than 99% and average R2 of 0.43, 0.57, 0.54 for vapor pressure deficit (VPD), PPFD and net radiation, respectively) and atmospheric water demand (average R2 equal to 0.73) on an hourly basis and that it is independent of tree size; (2) sensitivity of stem conductance to atmospheric water demand in sugar maples is also correlated to variation in local soil water availability (P-value = 0.014, R2 = 0.43) which arises due to a mild topographic gradient (i.e. 20 m of relative relief along 140 m) and relatively shallow soil. Although the sampled trees were subjected to a wide range of atmospheric water demands and soil moistures, the response to changes in environmental conditions is entirely explainable by dynamics of stem conductance rather than the relative fraction of sap flow along the radial profile, as some of the previous studies reported. Overall, our results confirm our theoretical approach and the possibility of partitioning sap velocity variability between either xylem properties or changes in environmental conditions.
    Agricultural and Forest Meteorology 03/2009; 149(s 3–4):570–581. · 3.89 Impact Factor
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    Danilo Dragoni, Kelly K Caylor, Hans P Schmid
    Agricultural and Forest Meteorology 01/2009; 149(3-4):570-581. · 3.89 Impact Factor
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    ABSTRACT: The availability of nitrogen represents a key constraint on carbon cycling in terrestrial ecosystems, and it is largely in this capacity that the role of N in the Earth's climate system has been considered. Despite this, few studies have included continuous variation in plant N status as a driver of broad-scale carbon cycle analyses. This is partly because of uncertainties in how leaf-level physiological relationships scale to whole ecosystems and because methods for regional to continental detection of plant N concentrations have yet to be developed. Here, we show that ecosystem CO(2) uptake capacity in temperate and boreal forests scales directly with whole-canopy N concentrations, mirroring a leaf-level trend that has been observed for woody plants worldwide. We further show that both CO(2) uptake capacity and canopy N concentration are strongly and positively correlated with shortwave surface albedo. These results suggest that N plays an additional, and overlooked, role in the climate system via its influence on vegetation reflectivity and shortwave surface energy exchange. We also demonstrate that much of the spatial variation in canopy N can be detected by using broad-band satellite sensors, offering a means through which these findings can be applied toward improved application of coupled carbon cycle-climate models.
    Proceedings of the National Academy of Sciences 01/2009; 105(49):19336-41. · 9.81 Impact Factor
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    ABSTRACT: The influences of thermal stability and seasonal changes in canopy morphology on mean flow and turbulence statistics in a mixed hardwood forest are presented from a year long field experiment at the University of Michigan Biological Station AmeriFlux site. A secondary wind speed maximum at z/h = 0.07 (z is height above ground and h is mean canopy height) below the level of peak vegetation area density (VAD) in the understory (young white pines) is observed more frequently and is more pronounced in fully leafed (closed) canopy than defoliated (open) canopy, and in stable than near-neutral and unstable conditions. A secondary wind speed maximum at z/h = 0.58 is observed only in the closed canopy below the level of peak VAD in the upper canopy (crowns of mature aspen trees), which occurs less frequently and is less pronounced than that at z/h = 0.07. Horizontal mean winds in the forest are observed to flow to the left (counter-clockwise) of that at the canopy top. The degrees of turning of the mean winds increase with increasing depth into the forest except a reversal (clockwise) near the forest floor in the closed canopy. The degrees of turning are greater in the closed canopy than the open canopy but smaller in near-neutral than unstable and stable conditions. The attenuations of Reynolds stress, correlation coefficient and velocity variances with increasing depth into the forest are more rapid in the closed canopy and in stable conditions. But the relative turbulence intensities are greater in the closed canopy than in the open canopy and decrease with increasing stability. In near-neutral stability, the zero-plane displacement height (d) for the closed canopy decreases with increasing wind speed (∼0.81h on average), while d for the open canopy does not show a clear dependence on wind speed (∼0.65h on average). The bulk drag coefficient (CDh) measured at the canopy top is much greater over the closed canopy than the open canopy, contrary to earlier observations over a deciduous forest. But CDh*=CDh/VAI (VAI is vegetation area index) is about the same over the closed and open canopies (∼0.03 in near-neutral stability). The drag coefficient (Cd) for the parameterization of drag force in mean momentum budget equations in closure models increases with decreasing wind speed and varies with height. The drag coefficient (CdLES) for the parameterization of drag force in prognostic momentum equations in large-eddy simulations of airflow in plant canopies is smaller than Cd, and the ratio CdLES/Cd is greater in the open canopy than closed canopy and in stable than near-neutral and unstable conditions due to smaller relative turbulence intensities. All drag coefficients decrease and the displacement height increases with increasing stability, which indicates that these estimated aerodynamic parameters are not entirely the properties of vegetation elements, but are influenced by vertical turbulent mixing of momentum. Both eddy-diffusivity and mixing-lengths for momentum transfer decrease with increasing stability. An evidence of non-local transport is shown by peak values in estimated eddy-diffusivity and mixing-lengths below the crowns of mature aspen trees in the closed canopy. Otherwise, the eddy-diffusivity decreases with increasing depth into the forest, while the mixing-lengths above the level of the peaks are greater in the open canopy and the opposite is true below the level of the peaks.
    Agricultural and Forest Meteorology 06/2008; 148(s 6–7):862–882. · 3.89 Impact Factor
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    agricultural and forest meteorology. 02/2008; 148(2):186-205.
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    ABSTRACT: Ecosystem respiration (ER) was measured with the eddy covariance technique in 14 forest ecosystems in the Upper Great Lakes Region during the growing seasons of 2002 and 2003. The response of ER to soil temperature and moisture was analyzed using empirical models. On average, ER was higher in the intermediate and young than in the mature stands, and higher in hardwood than in conifer stands. The seasonal mean temperature-normalized respiration rate (R10) ranged from 1 to 3 μmol CO2 m−2 s−1 and seasonal mean activation energy (Ea) from 40 to 110 kJ mol−1. The variation in the residuals of temperature response function of ER was best explained by soil moisture content. ER showed higher temperature sensitivity (as indicated by lower Ea) in the young than in the mature stands of coniferous forests, but not in the hardwood forests. The inclusion of soil moisture as an explicit driver of R10 explained an additional 8% (range 0–21%) of variability in ER. Significant moisture sensitivity of ER was detected in only 5 out of 20 site-years and it was associated with bimodal soil moisture distribution. Moisture sensitivity could partially be predicted from statistical moments kurtosis and interquartile range. The data implied greater moisture sensitivity with increasing stand age, possibly due to faster depletion of soil water supplies from a greater evaporative surface in the older stands. Additional limiting factors to ER were implicated.
    Agricultural and Forest Meteorology. 01/2008;
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    ABSTRACT: We measured annual C storage, or net ecosystem production (NEP), from 1999 to 2003 in an aspen-dominated, mixed-deciduous forest in Michigan, USA. Measurements of the annual production of above- and below-ground live and dead mass, foliar herbivory, and soil respiration were used to develop biometric estimates of NEP (NEPB). Eddy-covariance measurements made above the canopy were used to construct meteorological estimates of NEP (NEPM). Over the 5-year study period, C stored annually in live mass (above- and below-ground wood) averaged 2.13 Mg C ha−1 year−1, while annual detritus production (fine roots, leaf litter, woody debris) averaged 4.35 Mg C ha−1 year−1. Fine root inputs were the largest component (41%) of annual net primary production.The forest was a consistent C sink, with annual NEP ranging from 0.80 to 1.98 Mg C ha−1 year−1, comparable in magnitude to other eastern North American forests. When annual NEPB and NEPM were compared in a given year, they differed from each other by 13–148%. However, when compared over 5 years, these independent estimates of C storage converged to within 1% of each other. Differences between same-year biometric and meteorological NEP estimates were explained in part by a lag between late-season net canopy photosynthesis and C allocated to tree growth the following spring. Weekly assessments of bole radial growth in 2001 and 2002 indicated that woody mass growth began prior to positive net canopy photosynthesis in the spring and that >25% of annual photosynthetic C assimilation occurred after growth had stopped in the autumn. Thus, while the temporal separation between photosynthesis and growth reduced agreement between annual NEPB and NEPM estimates, these metrics converged over several years. These results suggest that the allocation of recent photosynthate to storage carbohydrates, rather than to immediate growth, may obscure the relationship between shorter term, annual biometric and meteorological C storage estimates. Moreover, the 5-year convergence of biometric and meteorological NEP estimates serves as an important cross-validation, demonstrating that both approaches can yield accurate forest C storage assessments.
    Agricultural and Forest Meteorology. 01/2008;
  • K. Novick, G. Katul, H. P. Schmid, P. Stoy, C. Wayson
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    ABSTRACT: Quantifying carbon and water fluxes from terrestrial ecosystems remains critical for carbon accounting and mitigation strategies. To date, most regional estimates of carbon balance use a top-down scaling approach that has limited temporal and spatial resolution and can not consider biotic fluxes in isolation. Forward scaling approaches to estimate coarse-scale carbon and water fluxes can retain high-resolution process-level information of source/sink dynamics by combining ecosystem carbon cycling and evapo-transpiration models with meteorological re-analysis and satellite-derived land cover data. However, a key challenge with this approach is the incorporation of non-specific biomass data from satellites into models that typically rely on species- or site- specific parameters. Here, we use in-situ eddy-covariance observations of carbon and water fluxes from nearly 40 long-term Ameriflux sites in Eastern North America to explore inter-specific functional convergences between key physiological parameters and readily obtainable climatic, edaphic and land cover data. Specifically, records from roughly half of the Ameriflux sites are used to train models relating: 1) phenological dynamics to temperature data and climate statistics, 2) evaporation to precipitation data and physical soil properties, 3) canopy transpiration rates to canopy architecture, and 4) ecosystem assimilation and respiration parameters to climatic and edaphic conditions. With the remaining Ameriflux site data, we test the models and assess their utility in an effort to scale up carbon exchange dynamics for a large region of North America between 28° and 50° N and 90}° W to the East Coast.
    AGU Fall Meeting Abstracts. 12/2006;
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    ABSTRACT: Eddy covariance fluxes of CO2 from a twin-engine Beechcraft Duchess were compared to the Ameriflux tower- based platform over a northern hardwood, deciduous forest between June 21 and July 24, 2006. Vertical wind was measured using a combination of a hemispherical pressure sensor probe and GPS/INS system on the aircraft and a sonic anemometer on the tower. The platforms employed NDIR analyzers to measure CO2 concentrations. Six flight legs, 5km in length, were averaged at heights of 150, 210, and 275m and compared with 30min averaged CO2 and sensible heat fluxes at the top of the 46m flux tower. Aircraft-based flux measurements as a function of altitude allow for assessment of flux divergence. In this paper, we compare the aircraft and tower data sets, and assess the differences in averaged fluxes, based in part on the footprint of the two platforms and flux divergence.
    AGU Fall Meeting Abstracts. 12/2006;

Publication Stats

2k Citations
87.26 Total Impact Points

Institutions

  • 2010–2011
    • Karlsruhe Institute of Technology
      • • Institute of Meteorology and Climate Research
      • • Department of Atmospheric Environmental Research
      Carlsruhe, Baden-Württemberg, Germany
  • 2009
    • Indiana University East
      Indiana, United States
    • University of New Hampshire
      • Complex Systems Research Center
      Durham, New Hampshire, United States
    • Klinikum Garmisch-Partenkirchen
      Markt Garmisch-Partenkirchen, Bavaria, Germany
  • 2002–2009
    • Indiana University Bloomington
      • Department of Geography
      Bloomington, Indiana, United States
  • 2008
    • East Carolina University
      • Department of Geography
      North Carolina, United States
  • 2003
    • The Ohio State University
      • Department of Evolution, Ecology, and Organismal Biology
      Columbus, OH, United States