Water balance and carbon sink strength of an European savannah-type woodland during the drought year 2012
2 Agroecosystem Research Department, University of Bayreuth
3 Field Experiments & Instrumentation, Max Planck Institute for Biogeochemistry Jena
4 Instituto Superior de Agronomia, Technical University of Lisbon
O 3.4 in Research Yields: Ideas Pursued to the End
10.10.2013, 16:15-16:30, H6, GEO
Savannah-type ecosystems account for around 30% of global gross primary production (GPP) and are the second most important biome in terms of global GPP after the tropical forests. A typical savannah-type woodland in Europe (montado) consists of a sparse tree layer and a herbaceous understorey layer, covering an area of 2-2.5 million ha on the Iberian Peninsula. These ecosystems are adapted to regular summer droughts depending on life form (Chamaephyte or Therophyte) and access to different water reservoirs. In the recent past, annual precipitation exhibits a significant decrease in winter and spring as well as of total annual rainfall on the Iberian Peninsula.
The objective of this study is to investigate the differences in ecosystem fluxes of a typical European savannah-type cork oak (Quercus suber L.) woodland between the very wet year 2011 and the extreme drought year 2012.
We measured CO2 and H2O fluxes of the trees and the understory with two eddy covariance towers above and below the tree canopy, respectively. We also measured auxiliary ecosystem variables such as precipitation, soil temperature and moisture θ and leaf area index LAI.
Precipitation P was 34% higher in 2011 and 39% lower in 2012 than the long-term average. The intra-annual pattern of P shows a severe drought anomaly in winter to early spring 2012 accounting for up to 68% of the total annual P reduction. However, evapotranspiration (ET) only diminished by 15% due to a decrease in canopy conductance gc while vapour pressure deficit stayed almost constant. Thus, annual ET/P fraction increased from 34% to 80%. This left only a very small amount of water for soil moisture replenishing, resulting in only half the soil moisture in spring 2012 compared to the spring 2011.
While gc was reduced by 15% in 2012 compared to 2011, annual GPP of the trees was reduced by 28%. We could attribute this by fitting the Farquhar-model of photosynthesis to a further reduction of the maximum carboxylation rate Vc,max. The bulk reduction takes place in summer whereas only little reduction occurs during the spring drought anomaly. Understorey GPP was reduced by 87% and was affected rather instantaneously already in winter. Overall is the ecosystem a carbon sink in both years but with a reduced sink strength by 34% in the drought year 2012.
In summary, the savannah-type ecosystem was strongly affected by the drought in 2012. Water availability was reduced due to a strong decrease in precipitation with little effect on evapotranspiration and canopy conductance. The carbon sink strength was strongly diminished by an early die-back of the understory vegetation and a reduced maximum carboxylation rate of the Q. suber trees.
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