Agriculture Reference
In-Depth Information
Figure 1. Total summer water balance assessed in the period from 08
th
to 18
th
August, 2004 in our study
area. Ebs, Es and Ep = total evaporation from bare soil, soil under vegetation and
S. tenacissima
stand
transpiration respectively, calculated from a multi-source evapotraspiration model (Clumped model).
Wbs and Ws = total water gains from water vapour atmospheric adsorption process in bare soil and
under vegetation soil respectively. ΔS = difference of the soil water storage at a depth of 15 cm, Water
? = water not accounted for by the water balance. Adapted from Ramírez et al. (2007a).
3. Soil Availability
We carried out an ecophysiological assessment of three
S. tenacissima
stands
characterized by three gradients, namely a gradient in the cover of
S. tenacissima
, in average
soil depth and altitude (see Table 2). The study was carried out in summer 2004 (season with
high water stress). The stand with lower soil depth and higher variability of this variable
(hereafter stand III, Table 2) showed the greater water stress symptoms. Accordingly, this
stand obtained lower average gas exchange and F
v
/F
m
(maximum photosystem II
photochemical efficiency) at dawn and at midday (0.33±0.03 and 0.18±0.02, respectively)
than stands I and II (Table 2). Stand III was located in the upper part of the micro-basin, a
place characterized by the presence of rock outcrops where the tussocks are into “soil
pockets”, and where the
S. tenacissima
stand cover is spatially clumped because it depends on
the clumped distribution of the soil available (Figure 2). The root system of
S. tenacissima
tussocks into “soil pockets” is limited only to the soil under vegetation (Figure 3). These
tussocks have no connection with the bare soil, and therefore the root water harvest from the
important bare soil water gains detected in summer by Ramírez et al. (2007a) cannot happen.
On the other hand, the stand with higher soil depth (stand I) showed higher gas exchange
values (Table 2). In this micro-basin's sector the
S. tenacissima
cover shows a random
distribution pattern again following the pattern of soil availability (Figure 2). The higher soil
