144
Robert Save, Carles Castell and Jaume Terradas
10.10 Biogeographical Implications
The ability of holm oak to maintain relatively low but sustained rates of foliar water loss is an important trait to understand its success in a range of
Mediterranean environments. The water requirements and the degree of
drought tolerance of holm oak preclude, however, the existence of dense
holm oak forests in areas having either a mean annual rainfall lower than
400-450 mm or extended periods with a predawn xylem potential below
-3.0 MPa. This helps us to understand the above-mentioned limitation of
holm oak forests to humid and sub-humid Mediterranean environments.
Most of the extant holm oak forests in NE Spain are found in areas characterized by humid mesoMediterranean conditions with cold winters, especially near the upper limit of their altitudinal range. A very cold winter can
be expected every 10-12 years in northern or mountain sites within the
Mediterranean region (Larcher 1981). This is relevant because the lowest xylem water potentials in montane holm oak forests are often found in winter
(Sala et al. 1988, 1990). Winter transpiration rates and xylem water potential
decline along altitudinal gradients, as we observed at Montseny. We can expect winter water deficits to develop if there is a reduced water uptake when
the evaporative demand is high, due to strong, dry winds, or high radiation.
Our results show a very low xylem potential (-3.0 MPa) in winter at the highest altitude along the gradient (1130 m a.s.l.). Below values of -2.2 MPa, water
loss is exclusively cuticular. Because cuticle thickness increases as the growing season becomes longer and warmer, we might expect cuticular transpiration to increase in colder environments (Tranquillini 1976; Save et al. 1988)
and so trees growing at high altitudes would have a less efficient control of
water losses. Water stress is thus involved in limiting holm oak distribution
not only towards dry climates but also towards cold climates.
References
Acherar M, Rambal S (1992) Comparative water relations of four Mediterranean oak species.
Vegetatio 991100:177-184
Aussenac G, Vallette JC (1982) Comportement hydrique estival de Cedrus arlantica Manettl,
Quercus ilex L. et Quercus pubescens Will. et de divers pins dans Ie Mont Ventoux. Ann Sci
~or 39:41-62
Bazzaz FA, Chiariello NR, Coley PO, Pitelka LF (1987) Allocating resour,ces to reproduction and
defense. BioScience 37:58-67
Berger A, Eckardt FE, Methy M, Heim G, Sauvezon R (1977) Interception de l'energie rayonnante, echange de CO2, regime hydrique et production chez differents types de vegetation
sous climat mediterraneen. In: Moyse A (ed) Les processus de la production vegetale primaire. Gauthier Villars, Paris, pp 1-15
Burriel JA, Calvet S, Sala A, Gracia CA (1993) Angulo foliar en Quercus ilex: modulaci6n por el
ambiente, y contribuci6n a la economia hidrica de la planta. In: Silva FJ. Vega G (eds) Congr
Forestal Espanol, Ponencias y Comunicaciones, vol 1. Xunta de Galicia, Lourizan, Pontevedra, pp 225-232
Robert Save, Carles Castell and Jaume Terradas
10.10 Biogeographical Implications
The ability of holm oak to maintain relatively low but sustained rates of foliar water loss is an important trait to understand its success in a range of
Mediterranean environments. The water requirements and the degree of
drought tolerance of holm oak preclude, however, the existence of dense
holm oak forests in areas having either a mean annual rainfall lower than
400-450 mm or extended periods with a predawn xylem potential below
-3.0 MPa. This helps us to understand the above-mentioned limitation of
holm oak forests to humid and sub-humid Mediterranean environments.
Most of the extant holm oak forests in NE Spain are found in areas characterized by humid mesoMediterranean conditions with cold winters, especially near the upper limit of their altitudinal range. A very cold winter can
be expected every 10-12 years in northern or mountain sites within the
Mediterranean region (Larcher 1981). This is relevant because the lowest xylem water potentials in montane holm oak forests are often found in winter
(Sala et al. 1988, 1990). Winter transpiration rates and xylem water potential
decline along altitudinal gradients, as we observed at Montseny. We can expect winter water deficits to develop if there is a reduced water uptake when
the evaporative demand is high, due to strong, dry winds, or high radiation.
Our results show a very low xylem potential (-3.0 MPa) in winter at the highest altitude along the gradient (1130 m a.s.l.). Below values of -2.2 MPa, water
loss is exclusively cuticular. Because cuticle thickness increases as the growing season becomes longer and warmer, we might expect cuticular transpiration to increase in colder environments (Tranquillini 1976; Save et al. 1988)
and so trees growing at high altitudes would have a less efficient control of
water losses. Water stress is thus involved in limiting holm oak distribution
not only towards dry climates but also towards cold climates.
References
Acherar M, Rambal S (1992) Comparative water relations of four Mediterranean oak species.
Vegetatio 991100:177-184
Aussenac G, Vallette JC (1982) Comportement hydrique estival de Cedrus arlantica Manettl,
Quercus ilex L. et Quercus pubescens Will. et de divers pins dans Ie Mont Ventoux. Ann Sci
~or 39:41-62
Bazzaz FA, Chiariello NR, Coley PO, Pitelka LF (1987) Allocating resour,ces to reproduction and
defense. BioScience 37:58-67
Berger A, Eckardt FE, Methy M, Heim G, Sauvezon R (1977) Interception de l'energie rayonnante, echange de CO2, regime hydrique et production chez differents types de vegetation
sous climat mediterraneen. In: Moyse A (ed) Les processus de la production vegetale primaire. Gauthier Villars, Paris, pp 1-15
Burriel JA, Calvet S, Sala A, Gracia CA (1993) Angulo foliar en Quercus ilex: modulaci6n por el
ambiente, y contribuci6n a la economia hidrica de la planta. In: Silva FJ. Vega G (eds) Congr
Forestal Espanol, Ponencias y Comunicaciones, vol 1. Xunta de Galicia, Lourizan, Pontevedra, pp 225-232