Influence of carbon and nitrogen fluxes on the water and energy budget of the terrestrial biosphere of the Baltic Sea drainage basin (CANIBALT)
Dr. Kirsten Warrach-Sagi, University of Hohenheim, Germany (web site)
Funded by the Federal Ministry for Education and Research (BMBF) within the frame of the German Climate Research Programme (DEKLIM) from April 2001 to March 2004
Summary
The project shall provide a physical-biogeochemical land surface model for regional atmospheric models, that has been validated for a deciduous broadleaf and an evergreen coniferous forest. The aim is further to contribute to the answer of the following questions: (1) Does the terrestrial biosphere regionally serve as a sink or source of atmospheric CO2 within the baltic sea drainage basin? Which are the responsible processes? (2) Of which order of magnitude is the interaction between the carbon and nitrogen cycle at the land surface and the water fluxes? (3) How large are the regional changes of the nitrogen cycle at the land surface that are associated with changes in the components of the water cycle? To which extent does this influence the vegetation?
Introduction
In this century, global anthropogenic climate change will start to affect significantly the development of ecosystems and agriculture in Europe. Since the landscape impact on weather and climate increases with decreasing scale, it is essential to study the combined carbon, nitrogen, water and energy fluxes at the land surface at the regional scale. The following research questions are still of fundamental interest for understanding the processes. Of which order of magnitude is the interaction between the carbon and nitrogen cycle at the land surface and the water fluxes? How large are the regional changes of the nitrogen cycle at the land surface that are associated with changes in the components of the water cycle? To which extent does this influence the vegetation? The Baltic Sea itself is impacted by nitrogen transports from the rivers. Since nitrogen accumulates in the soil and and is partly flushed non-linearly from the land surface into the rivers, mainly during periods of snow melt and strong rainfall events, high frequent measurements in the river system and sophisticated regional models are of need for nitrogen transport budgets (e.g. McHale et al., 2002). The DEKLIM funded project “Interaction of carbon and nitrogen fluxes and the water and energy budget of the terrestrial biosphere” aimed at providing a physical-biogeochemical land surface model for regional atmospheric models. Further the model’s validation and application were subject of the project.
The Model
The soil-plant-land-atmosphere-TOPMODEL model (SPLAT, e.g.Engel et al., 2002) serves as the baseline model for partitioning and allocating GPP (gross primary productivity) to plant tissues in both amongst lowland and upland trees. As the canopy model of SPLAT determines only GPP and foliar respiration, a scheme for carbon allocation and respiration for the non-foliage parts of the vegetation was added (Warrach et al., 2005). Following Scanlon et al. (2001) and Waelbroeck and Louis (1995) a soil nitrogen module was developped and added to SPLAT.
Validation and Application
Validation of such model demands a great deal of data, i.e. many vegetation and soil parameters and long time series of meteorological data. The model requires diurnal cycles of atmospheric variables for multi year periods. Further its validation in forests requires at least time series of tree inventories including alive and dead/died trees. Soil carbon and nitrogen data and atmospheric nitrogen deposition are needed as well as at least daily runoff measurements including nitrogen loads.
In the Baltic Sea drainage high resolution, long-term, quality checked data for validation of such a model are rare. The EUROFLUX station data from 1996-1998 (Falge et al., 2001 a and b) e.g. still include errors such as wrong wind speed and radiation (Hyytiälä) or no measured precipitation data in winter (Flakaliden and Norunda). Further the EUROFLUX data set does not contain plant parameters such as leaf nitrogen at Flakaliden and Norunda. Further the EUROFLUX station data does not contain soil nitrogen and runoff data.
Black Rock Forest
In the project the model SPLAT and its carbon module were validated and applied at the Red Oak dominated Black Rock Forest (BRF) in the northeastern USA. BRF is a secondary growth forest and is a natural preserve since the 1930s, the tree inventory is updated since then every 5 years and annualy since 1990. Extended plant parameter sets are collected since the 1990s and meteorological data is available from nearby stations. Warrach et al. (2005) describe the data, the model and its validation at BRF in detail. A linear relationship between NPP (net primary productivity) and GPP was found for Red Oaks, however the relationship depends on climatic conditions. A major result is the significant (99% level) correlation (R2=0.66) between summer precipitation and the NPP/GPP ratio, a measure of the carbon storage capability of the trees.
Hyytiälä
Within the Baltic Sea drainage basin the model was applied to the pine forest at Hyytiälä (61°51’N, 24°17’E) for 2001 and 2002. Plant parameters if not measured at Hyytiälä were calibrated applying the SCE-UA method (Duan et al., 1992) for 2001 comparing the simulated and measured latent heat flux. The latent heat flux data from 2002 was used for validation (Fig. 1). Tree inventory data from Hyytiälä was available from 1926 onwards but since it only includes living trees and since the forest is also used for timber extraction, the carbon module of SPLAT cannot be validated at Hyytiälä. However, annual growth rates may be calculated and compared well with about 10 m3/ha/year in the 1990s.
Figure 1. Measured (data provided by Timo Vesala) and simulated (SPLAT) latent heat fl ux at Hyytiälä (Finnland)
Ovre Lansjarv
In the project a soil nitrogen module for regional application was added to SPLAT. The module allows for the simulation of rapid nitrogen outflow during strong precipitation events and during snow melt as it is described e.g. by McHale et al. (2002). Since there were no daily or more frequent nitrogen measurements available to the project, a validation is not possible. However, a preliminary qualitative study was carried out for the Ovre Lansjarv catchment in northern Sweden. For this 1340 km2 sub-basin of the Torneälv extended meteorological and hydrological data is therefore available on a 0.25° grid resolution for the 1980s and ‘90s (Bowling et al., 2003). Tree inventory data were purchased, the inventories don’t include dead trees and natural preserves and national parks. Annual atmospheric nitrogen deposition in the Ovre Lansjarv region is 138 mg/m2 (EMEP). The model simulation was started in 1988. Figure 2a shows simulated and observed runoff in 1992 and 1993. Figure 2b shows the simulated runoff and nitrogen outflow. Note the nitrogen outflow can only demonstrate the qualitative simulation since no data for validation was available. However the simulation shows the nitrogen outflow mainly during runoff events due to snow melt and strong precipitation.
 a b
Figure 2. a) Measured and simulated runoff, and b) simulated runoff and nitrogen outfl ow of the Ovre Lansjarv
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Discussion and Outlook
During the project carbon and nitrogen were included into a land surface models for regional model applications in the Baltic Sea drainage basin. The project revealed the importance of complete quality tested data sets with respect to meteorological, hydrological, plant, soil, carbon and nitrogen data. The compilation of such quality tested data sets would be valuable for further studies of this complex issue in BALTEX.
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