Agriculture Reference
In-Depth Information
Thus the coarsed-grained scale can be briefly characterized and we can now focus in the
“within stand” processes, that is, the interactions between water and soil at the individual
plant level.
Some authors have measured (Puigdefábregas and Sánchez 1996) and pointed out
(Maestre and Cortina 2006) the importance of water transported via runoff to the tussock
terrace because this peculiar type of water notably improved morphological (leaf length and
number per stem) and physiological performance (assessed using foliar δ
13
C, δ
15
N, nitrogen
concentration and carbon to nitrogen ratio) in
S. tenacissima
. The lower root fines (Ø < 5mm)
weight of this species found in bare soil (compared to those found under tussock) have been
the main argument to play down the importance of water use via the tussock root system from
bare soil (Puigdefábregas and Sánchez 1996; Puigdefábregas et al. 1999). However, the
occurrence of water inputs from non-rainfall water phenomena and some ecophysiological
traits that we shall show in this communication question the classical paradigm about the use
of soil water in semiarid steppes.
T
HE
P
ROCESS
In order to better address the role of soil water in semiarid steppes we firstly bring about
the physical phenomenon product of the energy exchange between the soil and the
atmosphere known as water vapour adsorption by soil (WVA). This phenomenon is likely to
occur when the humidity of the soil pores is less than that in the adjacent atmosphere (see
Agam and Berliner 2006, for a detailed explanation of this process). Kosmas et al. (1998)
have highlighted the importance of water from WVA in semiarid environments. Indeed, these
authors calculated that the total WVA was +26.25% of the rainfall water during February -
August, 1996 period in Athens-Greece. Moreover, Kosmas et al. (2001) estimated that water
gains from WVA can supply the 55.7 to 70.7% of the soil water evaporation demands. Agam
and Berliner (2004, 2006) proved the paramount relevance of WVA, superior even to that of
dew formation in the soil water gains in the Negev Desert - Israel. The main argument of
these last authors was that the soil surface temperature did not reach the dew point
temperature. Due to the importance of this type of energy exchange between the soil and the
atmosphere, a brief account of the environmental factors that facilitate and limit the
occurrence of WVA in arid and semiarid areas is shown in Table 1. It is noteworthy that some
recent works have presented evidences of the occurrence of WVA in southern and
southeastern areas of the Mediterranean Spain (Verhoef et al. 2006; Ramírez et al. 2007a).
Moreover, Kosmas et al. (1998, 2001) suggested that WVA is an important water resource for
vegetation in semiarid Mediterranean areas mainly during periods of high water stress. Below
we provide with further evidences that support this hypothesis in a
S. tenacissima
steppe in
SE Spain.
E
VIDENCES
The evidences we aim to present in this review were obtained from ongoing studies
carried out in
S. tenacissima
stands belonging to a South-facing micro-basin (19 ha, altitude
