332
Carlos A. Gracia, Santiago Sabate, Juan Manuel Martinez and Eva Albeza
following thinning and the leaf area index of the forest thus increased, recovering the pre-treatment values in about 6 years. Competition among trees
was reduced since there were more water, nutrients and light available for the
remaining trees. These results agree with those obtained by Abrams and
Mostoller (1995) who found a reduction in the mean leaf lifespan as water or
nutrient availability increased. Brix and Mitchell (1986), Sabate and Gracia
(1994) and Sheriff (1996) found analogous results analyzing the effects of irrigation or fertilization on different tree species.
23.5 Leaf Water Potential
Soil water availability in Prades is closely related to the course of precipitation throughout the year. During the experiment, a period of severe drought
occurred. From September 1993 to September 1994 rainfall was less than
100 mm. Predawn leaf water potential ('Ppd) was measured at three different
levels of the vertical profile: the upper canopy, involving sun-exposed leaves,
lower canopy, involving shade leaves of the canopy, and leaves of the new resprouts. The results obtained for 1 year, from June 1994 to March 1995, are
plotted in Fig. 23.1. In June 1994, after several months of severe drought, 'Ppd
of mature trees in control plots was about -2 MPa, i.e. much lower than in the
thinned plots. In the treatment of maximum thinning the leaf water potential
at that time was about -0.5 MPa, and slightly less than -1 MPa in the medium
thinning treatment. These results show the improved water status of the trees
induced by thinning compared with the trees in control plots. During August
1994, 'Ppd in the control treatment was lower than -4.2 MPa - the maximum
measuring capacity of our pressure gauge - while the trees in the maximum
thinned plots reached values around -3 MPa. In control plots, visual symptoms of foliar damage were evident at this time with the browning of the
leaves of more than 50% of the trees. No brown leaves were observed in the
trees of the thinned plots, which remained green and healthy-looking all
summer. During the drought period, the higher competition for water among
trees and the higher rainfall interception in control plots led to a decrease in
the soil water content compared with the thinning treatments, in agreement
with results of Breda et al. (1995).
Following the severe drought period, more than 450 mm of rain fell in
only 2 days in October 1994. 'Ppd in December 1994 and spring 1995 was
about -0.5 MPa (Fig. 23.1). There were no statistical differences between the
upper and lower canopy, either in the control treatment or the maximum
thinning. However, resprouts in the maximum thinning treatment showed a
slightly better water status (higher 'P pd) than the canopy leaves. Differences
between resprouts were not always significant, but at most times 'Ppd was less
negative in the maximum thinning treatment than in the minimum thinning.
So during March 1995, 'Ppd was higher (P < 0.04) in the maximum thinning,
Carlos A. Gracia, Santiago Sabate, Juan Manuel Martinez and Eva Albeza
following thinning and the leaf area index of the forest thus increased, recovering the pre-treatment values in about 6 years. Competition among trees
was reduced since there were more water, nutrients and light available for the
remaining trees. These results agree with those obtained by Abrams and
Mostoller (1995) who found a reduction in the mean leaf lifespan as water or
nutrient availability increased. Brix and Mitchell (1986), Sabate and Gracia
(1994) and Sheriff (1996) found analogous results analyzing the effects of irrigation or fertilization on different tree species.
23.5 Leaf Water Potential
Soil water availability in Prades is closely related to the course of precipitation throughout the year. During the experiment, a period of severe drought
occurred. From September 1993 to September 1994 rainfall was less than
100 mm. Predawn leaf water potential ('Ppd) was measured at three different
levels of the vertical profile: the upper canopy, involving sun-exposed leaves,
lower canopy, involving shade leaves of the canopy, and leaves of the new resprouts. The results obtained for 1 year, from June 1994 to March 1995, are
plotted in Fig. 23.1. In June 1994, after several months of severe drought, 'Ppd
of mature trees in control plots was about -2 MPa, i.e. much lower than in the
thinned plots. In the treatment of maximum thinning the leaf water potential
at that time was about -0.5 MPa, and slightly less than -1 MPa in the medium
thinning treatment. These results show the improved water status of the trees
induced by thinning compared with the trees in control plots. During August
1994, 'Ppd in the control treatment was lower than -4.2 MPa - the maximum
measuring capacity of our pressure gauge - while the trees in the maximum
thinned plots reached values around -3 MPa. In control plots, visual symptoms of foliar damage were evident at this time with the browning of the
leaves of more than 50% of the trees. No brown leaves were observed in the
trees of the thinned plots, which remained green and healthy-looking all
summer. During the drought period, the higher competition for water among
trees and the higher rainfall interception in control plots led to a decrease in
the soil water content compared with the thinning treatments, in agreement
with results of Breda et al. (1995).
Following the severe drought period, more than 450 mm of rain fell in
only 2 days in October 1994. 'Ppd in December 1994 and spring 1995 was
about -0.5 MPa (Fig. 23.1). There were no statistical differences between the
upper and lower canopy, either in the control treatment or the maximum
thinning. However, resprouts in the maximum thinning treatment showed a
slightly better water status (higher 'P pd) than the canopy leaves. Differences
between resprouts were not always significant, but at most times 'Ppd was less
negative in the maximum thinning treatment than in the minimum thinning.
So during March 1995, 'Ppd was higher (P < 0.04) in the maximum thinning,