90
Javier Retana, Josep Maria Espelta, Marc Gracia and Miquel Riba
tervals (about every 20-25 years) are followed by sprout regeneration of
stumps rather than sexual reproduction (Chap. 5; Ducrey and Boisserie 1992;
Ducrey and Turrell 1992; Retana et al. 1992). Under this management, the
seedling bank plays only a minor role in stand regeneration because seedlings are less competitive than stump sprouts. Nevertheless, as has been suggested for other oak forests (Matsuda et al. 1989; Nowacki et al. 1990), the
importance of seedlings may be viewed in relation to ecological phenomena
such as the persistence of plant populations, genetic variability or potential
forest decline. This chapter focuses on the effects that the heterogeneity of
the physical environment and the main natural and man-made disturbances
of Mediterranean ecosystems have upon seedling recruitment in holm oak
forests.
7.2 Variability of Holm Oak Recruitment According
to Environmental Conditions
The inherent heterogeneity of Mediterranean landscapes (Naveh and Lieberman 1984) is evident across different temporal and spatial scales (Barbero
1988; Romane et al. 1988; Floret et al. 1989). In the Mediterranean region, the
regional climate is an important factor in environmental heterogeneity: there
is a general north-to-south gradient of hydric stress for organisms, which is
associated with a rising gradient of radiation and temperature. There is
therefore a transition from more mesic conditions in the north to more xeric
ones in the south. This determines the distribution of tree species. Recent
studies (Tretiach 1993) suggest that holm oak is, to a certain extent, more
sensitive to drought than expected. Within the Mediterranean region, the
dominance of holm oak decreases as the water deficit increases, but the role
of climatic features is less evident in relation to seedling density: no correlation has been found between water availability (measured as total annual
rainfall) and density of holm oak seedlings, for a wide range of old holm oak
stands distributed throughout Catalonia (NE Spain; data from the Forest Inventory of Catalonia).
A smaller scale of heterogeneity occurs in the species composition associated with gradients of aspect, elevation or soil depth (Aschmann 1984;
Quezel and Barbero 1989; Pigott and Pigott 1993). The distribution of holm
oak seedlings follows topographical patterns more closely than regional gradients. In holm oak stands of the Figar6 area at Montseny (see Chap. 2 for
site description), the evaluation of holm oak regeneration throughout the
elevation gradient (between 300 and 750 m) indicates that the density of
holm oak seedlings (number ha- 1 ) increases with elevation (m) (seedling
density = 39.6 x elevation - 15261.0, r = 0.79, P = 0.001). Aspect also influences holm oak regeneration because of the difficulties that holm oak recruits experience withstanding water stress. The density of holm oak seed-
Javier Retana, Josep Maria Espelta, Marc Gracia and Miquel Riba
tervals (about every 20-25 years) are followed by sprout regeneration of
stumps rather than sexual reproduction (Chap. 5; Ducrey and Boisserie 1992;
Ducrey and Turrell 1992; Retana et al. 1992). Under this management, the
seedling bank plays only a minor role in stand regeneration because seedlings are less competitive than stump sprouts. Nevertheless, as has been suggested for other oak forests (Matsuda et al. 1989; Nowacki et al. 1990), the
importance of seedlings may be viewed in relation to ecological phenomena
such as the persistence of plant populations, genetic variability or potential
forest decline. This chapter focuses on the effects that the heterogeneity of
the physical environment and the main natural and man-made disturbances
of Mediterranean ecosystems have upon seedling recruitment in holm oak
forests.
7.2 Variability of Holm Oak Recruitment According
to Environmental Conditions
The inherent heterogeneity of Mediterranean landscapes (Naveh and Lieberman 1984) is evident across different temporal and spatial scales (Barbero
1988; Romane et al. 1988; Floret et al. 1989). In the Mediterranean region, the
regional climate is an important factor in environmental heterogeneity: there
is a general north-to-south gradient of hydric stress for organisms, which is
associated with a rising gradient of radiation and temperature. There is
therefore a transition from more mesic conditions in the north to more xeric
ones in the south. This determines the distribution of tree species. Recent
studies (Tretiach 1993) suggest that holm oak is, to a certain extent, more
sensitive to drought than expected. Within the Mediterranean region, the
dominance of holm oak decreases as the water deficit increases, but the role
of climatic features is less evident in relation to seedling density: no correlation has been found between water availability (measured as total annual
rainfall) and density of holm oak seedlings, for a wide range of old holm oak
stands distributed throughout Catalonia (NE Spain; data from the Forest Inventory of Catalonia).
A smaller scale of heterogeneity occurs in the species composition associated with gradients of aspect, elevation or soil depth (Aschmann 1984;
Quezel and Barbero 1989; Pigott and Pigott 1993). The distribution of holm
oak seedlings follows topographical patterns more closely than regional gradients. In holm oak stands of the Figar6 area at Montseny (see Chap. 2 for
site description), the evaluation of holm oak regeneration throughout the
elevation gradient (between 300 and 750 m) indicates that the density of
holm oak seedlings (number ha- 1 ) increases with elevation (m) (seedling
density = 39.6 x elevation - 15261.0, r = 0.79, P = 0.001). Aspect also influences holm oak regeneration because of the difficulties that holm oak recruits experience withstanding water stress. The density of holm oak seed-