Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
4
5
2007
10.5194/bg-4-791-2007
http://www.biogeosciences.net/4/791/2007/
http://www.biogeosciences.net/4/791/2007/bg-4-791-2007.html
http://www.biogeosciences.net/4/791/2007/bg-4-791-2007.pdf
791
802
2007-09-28
Net ecosystem carbon exchange in three contrasting Mediterranean ecosystems – the effect of drought
J. S. Pereira
jspereira@isa.utl.pt
J. A. Mateus
L. M. Aires
G. Pita
C. Pio
J. S. David
V. Andrade
J. Banza
T. S. David
T. A. Paço
A. Rodrigues
Instituto Superior de Agronomia, Universidade Técnica de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal
Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
CESAM & Department of Environment, University of Aveiro, 3810-193 Aveiro, Portugal
Estação Florestal Nacional, Quinta do Marquês, 2780-159 Oeiras, Portugal
Droughts reduce gross primary production (GPP) and ecosystem respiration
(Reco), contributing to most of the inter-annual variability in terrestrial
carbon sequestration. In seasonally dry climates (Mediterranean), droughts
result from reductions in annual rainfall and changes in rain seasonality.
We compared carbon fluxes measured by the eddy covariance technique in three
contrasting ecosystems in southern Portugal: an evergreen oak woodland
(savannah-like) with ca.~21% tree crown cover, a grassland dominated by
herbaceous annuals and a coppiced short-rotation eucalyptus plantation.
During the experimental period (2003–2006) the eucalyptus plantation was
always the strongest sink for carbon: net ecosystem exchange rate (NEE)
between −861 and −399 g C m<sup>−2</sup> year<sup>−1</sup>. The oak woodland and the
grassland were much weaker sinks for carbon: NEE varied in the oak woodland
between −140 and −28 g C m<sup>−2</sup> year<sup>−1</sup> and in the grassland between
−190 and +49 g C m<sup>−2</sup> year<sup>−1</sup>. The eucalyptus stand had higher
GPP and a lower proportion of GPP spent in respiration than the other
systems. The higher GPP resulted from high leaf area duration (LAD), as a
surrogate for the photosynthetic photon flux density absorbed by the canopy.
The eucalyptus had also higher rain use efficiency (GPP per unit of rain
volume) and light use efficiency (the daily GPP per unit incident
photosynthetic photon flux density) than the other two ecosystems.
The effects of a severe drought could
be evaluated during the hydrological-year (i.e., from October to September)
of 2004–2005. Between October 2004 and June 2005 the precipitation was only
40% of the long-term average. In 2004–2005 all ecosystems had GPP lower
than in wetter years and carbon sequestration was strongly restricted (less
negative NEE). The grassland was a net source of carbon dioxide (+49 g C m<sup>−2</sup> year<sup>−1</sup>).
In the oak woodland a large proportion of GPP resulted
from carbon assimilated by its annual vegetation component, which was
strongly affected by the shortage of rain in winter. Overall, severe drought
affected more GPP than Reco leading to the deterioration of NEE. Although
the rain-use efficiency of the eucalyptus plantation increased in the dry
year, this was not the case of evergreen oak woodland, which rain-use
efficiency was not influenced by drought. Recovery after drought
alleviation, i.e., beginning with heavy rain in October 2005, was fully
accomplished in 2006 in the oak woodland and grassland, but slow in the eucalyptus plantation.
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