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assessed light rainfall value (1.59 mm day
-1
). Pugnaire et al. (1996) studied the water
response of
S. tenacissima
after watering in summer season and still the simulated rainfall
assessed was 31 mm day
-1
, a figure much higher than our assessed summer rainfall figure.
The architectural structure of
S. tenacissima
is an interesting factor to conjure up at this
point of our reasoning, trying to explain the rapid response of
S. tenacissima
to a very small
amount of rainfall. The plant is characterized by a thick tussock caused by a vertical leaf
green arrangement (Valladares and Pugnaire 1999) and accumulation of litter from dead
leaves (Domingo et al. 1996). This architectural configuration facilitates high interception
rates mainly under light rainfall events, as shown by Domingo et al. (1998). Shortly, they
demonstrated, using a comparative approach, that
S. tenacissima
has lower canopy water
drainage (when the storage water is low) than
Retama sphaerocarpa
and
Anthyllis cytisoides
(other prominent species in semiarid areas in Spain). They also highlighted that the spatial
structure of
S. tenacissima
promotes low stemflow and throughfall. Due to its high sensitivity
to atmospheric conditions, namely a high evaporation demand such as that characteristic of
summer in our study area, the intercepted water is quickly evaporated: the daily average total
potential evaporative energy in August 2004 in our study area was 5.18 mm day
-1
(Ramírez
2006). These results would help explain the rapid response of
S. tenacissima
to very low soil
water inputs via summer light rainfall events. Taking into consideration that the tussock
structure of
S. tenacissima
promotes great interception of light rainfalls, a root water harvest
from bare soil close to the tussock can be expected.
One of the mechanisms developed by
S. tenacissima
that help avoid high water stress in
the summer season is widespread stomatal closure (Pugnaire and Haase 1996; Balaguer et al.
2002). Indeed, the obtained figures for stomatal conductance during this water-stressed
season were very low (between 0 and 0.08 mmol H
2
O m
-2
s
-1
). However, Ramírez et al.
(2007a) found that our assessed
S. tenacissima
grassland showed atypical high stomatal
conductance values (between 21.8 and 43.1 mmol H
2
O m
-2
s
-1
) in the 2004 summer.
Therefore the question arises as to why the water status of this
S. tenacissima
stand does not
correspond with other stands in the summer season.
2. Stand Evapotranspiration and WVA
Ramírez et al. (2007b) estimated stand evapotranspiration using two methods (scaling
procedures and a multi-source evaporation model) in the spring and summer seasons in our
study area. In the latter season, the evapotranspiration value assessed during a 10-day period
was very similar to the water gains from the WVA process (Figure 1, Ramírez et al. 2007a).
This result suggests the importance of the WVA process in our study area as a resource for
the evaporative demands from evapotranspiration during the season with the highest water
stress. Ramírez et al. (2007a) highlighted the importance of the WVA of the bare soil above
soil under vegetation in the water demands from the
S. tenacissima
stand evapotranspiration,
this arguments is supported due to the fact that, like to other semiarid Mediterranean
environments (Kosmas 1998, 2001; Verhoef et al. 2006), the WVA values in bare soil were
higher than in soil under vegetation.
