TDR-MEASUREMENT FOR THE STUDY OF THE SEASONAL VARIATIONS OF SOIL
MOISTURE ON Quercus ilex DEHESAS
Elena CUBERA1, Gerardo MORENO, Alejandro SOLLA
Departamento de Biología y Producción de los Vegetales. Universidad de Extremadura.
Ingeniería Técnica Forestal. Av. Virgen del Puerto 2, 10600-Plasencia, Spain
1
For correspondence: ecubera@unex.es
Keywords: Soil water content, scattered trees, root distribution, leaf water potential
___________________________________________________________________________
ABSTRACT
Most evergreen oak forests growing in the flat areas of the southwestern Iberian
Peninsula have been gradually transformed into a unique kind of pastoral woodland, the
Spanish dehesas and Portuguese montados, by means of an agroforestry use. Eagleson and
Segarra (1985) have emphasised that, where a marked seasonality in water availability occurs,
a mixed formation of grasses and woody plants is the only stable state of equilibrium. A
positive effect of trees on soil bulk density, water-holding capacity and water content has
been shown by Joffre and Rambal (1988) in dehesas.
We have investigated how holm-oak trees use the soil water in four dehesas of CW
Spain (39º 41’ N, 6º13’ W) by measuring the soil moisture at different distances from the tree
trunk (maximum 30 m), from the soil surface until a maximum depth of 2 m, in intervals of
20 cm. Soil moisture was measured monthly by TDR technique, between May 2002 and
October 2003. Additionally, tree and herbs root systems have been studied by mean of 2 mdepth soil cores (Obrador et al., 2003) and the water stress experienced by trees have been
characterized by mean of leaf water potential (Montero et al., 2004).
We did not found any significant difference in soil moisture with regards to distance to
the tree (Fig 1), which differ from those reported by Joffre and Rambal (1988). S oil water
depletion beyond the tree canopy projection continued even when herbaceous plants dried up
(mid-may) (Fig 1), indicating that trees could use water from open areas, a clear benefits from
tree spacing on soil water consumption. Results have also shown a high dependence of holmoak on deep water reserves throughout late spring and summer, which contributes to avoiding
competition for water between trees and herbaceous vegetation (Fig 2).
In conclusion, we have not found a positive redistribution of soil water near the trees,
however we have found an very extended tree rooting system in dehesas (both in depth and
distance; Fig. 3), which allows them to use efficiently the soil water of a huge volume of soils
and to maintain a near-optimum water status along the summer (Fig 4).
References
Eagleson, P. S. and Segarra, R. I. (1985): Water limited equilibrium of savanna vegetation systems. Water
Resources Research 21:1483-1493.
Joffre, R. and Rambal, S. (1988): Soil water improvement by trees in the rangelands of southern Spain.
Oecology Plantarum 9: 405-422.
Montero, M.J., Obrador, J.J., Cubera, E. and Moreno, G. (2004): Importance of soil management on tree water
status in dehesas of Central Western Spain. Advances in Geoecology , 37 (in press).
Obrador J.J., Bordet, M., García, E. and Moreno, G. (2003): Root distribution in intercropped dehesas of
Central-Western Spain. International Symposium on Sustainability of Dehesas, Montados and other
agrosilvopastoral systems. European Soil Science Society. Cáceres, Spain.
Sala A. (1999): Modelling Canopy Gas Exchange during Summer Drought. In: Rodá F., Retana J., Gracia C.A.,
and Bellot J. (eds). Ecological Studies vol 137. Ecology of Mediterranean Evergreen Oak Forests. Springer
Berlin, pp. 149-159.
Date
20-12-02
30-3-03
8-7-03
16-10-03
24-1-04
3-5-04
26%
2,5 m
20 m
Soil moisture, % weight
23%
5m
30 m
10 m
Figure 1.- Seasonal variation of soil
20%
moisture (average form dat of 1 m
depth) at different distance of holm-
17%
oaks in dehesas.
14%
11%
8%
Soil moisture, % weigth
0
5
10
15
20
25
30
35
0
Figure 2.- Monthly variation in soil
moisture profile from soil surface till 2
between May and June.
50
Depth, cm
m. Most of the water was depleted
100
1-5
7-6
3-7
7-8
150
Beyond tree canopy
200
Mar
Distance, m
0
5
10
15
20
25
Apr
May
Jun
Jul
Aug Sept Oct
0,0
Depth, cm
25
50
75
100
125
150
Herbs
175
Trees
200
Figure 3.- Profiles of Root Lenght Density (RLD, Km
Leaf Water Potential, MPa
0
-1,0
-2,0
-3,0
Predawn
Midday
-4,0
Figure 4.- Monthly variation in leaf water potential of
m ) of herbs and trees in dehesas at different distances
holm-oaks in dehesas. Arrows indicate the thresholds of
of the trees (holm-oak).
significant water deficit (Sala, 1999).
-3
Acknowledgement
This study was supported by E.U. (SAFE project), Spanish goverment (MICASA project, MCyT) and Junta de
Extremadura (CASA project and E. Cubera was awarded a grant).