Holm Oak and Holm Oak Forests: An Introduction
11
makes this effect difficult to test. Sclerophylly and evergreenness are advantageous when nutrients are scarce. Within the holm oak range, on oligotrophic soils, deciduous oaks (Q. humilis, Q. faginea, Q. cerrioides) are sometimes limited to mesic and nutrient-rich sites. Where deciduous trees become
dominant, holm oak refuges are usually on calcareous, non-oligotrophic soils
with relatively low water availability. Water, and water-related competition,
not nutrients, seems to be the usual limiting factor. Experiments aimed at
understanding the relative roles of water, nitrogen and phosphorus availability on holm oak performance are presented in Chapter 13.
Wildfires are an environmental threat to Mediterranean forests. Whereas
Mediterranean pine forests are frequently devastated by fire, holm oak forests score lower in yearly burnt surfaces. For instance, in Catalonia the yearly
burnt area relative to the area occupied by each species is double for Pinus
halepensis than for Quercus ilex + Q. suber. It is well known that chemical
composition and the production of volatile organic substances are related to
flammability. Whereas most other oak species emit isoprene, holm oak, surprisingly, emits monoterpenes, with an emission rate that seems to be apparently higher than any other values reported in the literature for other plant
species (Seufert et al. 1995). This behaviour differs from that of pines, which
accumulate monoterpenes in the leaves, twigs or bark. The relevant aspect
for fire behaviour is that no massive emission will occur from a holm oak
canopy as a result of heating, making it less flammable than pine.
1.6 Management
As noted above, holm oak forests were probably favoured by man in early
historical times. However, they have been heavily exploited for centuries, and
have been progressively restricted to marginal areas, like shallow soils on
steep slopes in mountain areas, due to the spread of agriculture, human settlements and, sometimes, because faster growing pines have been favoured
for timber production.
There are great geographical differences in management. On very oligotrophic soils in central and western Spain and in Portugal, holm oak woodlands are managed as dehesas (montados in Portugal), a multi-secular manmade type of savannah-like ecosystem with large, isolated trees emerging
from a grassland (G6mez Gutierrez 1992; Huntsinger and Bartolome 1992).
About 1.2 million ha is exploited in Spain in this way for grazing and acorn
production, with acorn yields between 400 and 700 kg ha- 1 (Montoya 1993).
Since these acorns are sweet, in the past they were used not only as hog food
but also even for human use, and holm oak in dehesas can be considered as a
semi-domesticated fruit tree (Laguna 1883). Trees also provide shade for
livestock, firewood from pruning and refuge and breeding sites for a large
number of vertebrates, whereas the grassland is used by cows and sheep for
11
makes this effect difficult to test. Sclerophylly and evergreenness are advantageous when nutrients are scarce. Within the holm oak range, on oligotrophic soils, deciduous oaks (Q. humilis, Q. faginea, Q. cerrioides) are sometimes limited to mesic and nutrient-rich sites. Where deciduous trees become
dominant, holm oak refuges are usually on calcareous, non-oligotrophic soils
with relatively low water availability. Water, and water-related competition,
not nutrients, seems to be the usual limiting factor. Experiments aimed at
understanding the relative roles of water, nitrogen and phosphorus availability on holm oak performance are presented in Chapter 13.
Wildfires are an environmental threat to Mediterranean forests. Whereas
Mediterranean pine forests are frequently devastated by fire, holm oak forests score lower in yearly burnt surfaces. For instance, in Catalonia the yearly
burnt area relative to the area occupied by each species is double for Pinus
halepensis than for Quercus ilex + Q. suber. It is well known that chemical
composition and the production of volatile organic substances are related to
flammability. Whereas most other oak species emit isoprene, holm oak, surprisingly, emits monoterpenes, with an emission rate that seems to be apparently higher than any other values reported in the literature for other plant
species (Seufert et al. 1995). This behaviour differs from that of pines, which
accumulate monoterpenes in the leaves, twigs or bark. The relevant aspect
for fire behaviour is that no massive emission will occur from a holm oak
canopy as a result of heating, making it less flammable than pine.
1.6 Management
As noted above, holm oak forests were probably favoured by man in early
historical times. However, they have been heavily exploited for centuries, and
have been progressively restricted to marginal areas, like shallow soils on
steep slopes in mountain areas, due to the spread of agriculture, human settlements and, sometimes, because faster growing pines have been favoured
for timber production.
There are great geographical differences in management. On very oligotrophic soils in central and western Spain and in Portugal, holm oak woodlands are managed as dehesas (montados in Portugal), a multi-secular manmade type of savannah-like ecosystem with large, isolated trees emerging
from a grassland (G6mez Gutierrez 1992; Huntsinger and Bartolome 1992).
About 1.2 million ha is exploited in Spain in this way for grazing and acorn
production, with acorn yields between 400 and 700 kg ha- 1 (Montoya 1993).
Since these acorns are sweet, in the past they were used not only as hog food
but also even for human use, and holm oak in dehesas can be considered as a
semi-domesticated fruit tree (Laguna 1883). Trees also provide shade for
livestock, firewood from pruning and refuge and breeding sites for a large
number of vertebrates, whereas the grassland is used by cows and sheep for