Structure and Dynamics of the Root System
51
tribution (Fig. 4.2). In a review of 96 Mediterranean woody species from
Australia, California and Chile the average radius of root horizontal extension was 1.6 ± SD 1.5 m and the maximum extension was 7.0 m (Canadell
and Zedler 1995).
In La Castanya, root grafts were occasionally observed between roots of
the same tree, but never between roots of different trees. On the contrary,
root grafts were somewhat common between holm oak trees in Prades, although no root grafting was observed between different species. Generally,
root grafting is not very common in Mediterranean ecosystems (Keeley
1988); it was observed in only one of nine species excavated in the Chilean
matorral (Hoffmann and Kummerow 1978).
4.4 Lignotubers
One of the most characteristic features of the root system of holm oak is the
development of a large woody swollen structure at the stem base called the
lignotuber (Canadell and Zedler 1995). This structure is genetically determined and appears early during seedling development. There are no studies
on lignotuber development in this species, but it is known that Q. suber, another evergreen western Mediterranean oak, starts developing the lignotuber
structure through an accumulation of bud clusters and starch that occurs
close to the cotyledonary insertion (Molinas and Verdaguer 1993a,b).
The size of lignotubers depends on plant age, environmental and edaphic
conditions, and, most of all, on the disturbance history that individual plants
have experienced (e.g. fire, logging, extreme drought or cold events). In general, disturbances trigger the growth of the meristematic tissue of the lignotuber, which is further enlarged by the fusion of the stem bases of the new
emerging shoots. The presence of large lignotubers was common in Montseny and Prades because both forests were intensively coppiced for charcoal
production until the 1950s. At both sites, individual holm oaks often allocate
as much as half of their total biomass to the lignotuber (Canadell and Roda
1991; Djema 1995; Sabate et al. 1998). In one case at Montseny, the lignotuber
dry weight was 317 kg and accounted for 66% of the total tree biomass.
It is believed that lignotubers have a dual function. The first function,
which is morphologically related, is to store concealed buds that will resprout
after disturbances; the lignotuber generally stores a large number of buds
that enable plants to regrow even in environments subjected to multiple
disturbances. The second function is to store non-structural carbohydrates
and nutrients that will support regrowth after disturbances. Our understanding regarding the latter function has been inferred largely from studies
of plant growth (DeSouza et al. 1986; Castell et al. 1994), anatomical structure
(Montenegro et al. 1983), and tissue analysis of lignotuber nutrient and carbohydrate contents (Mullete and Bamber 1978; Dell et al. 1985). To date, very
51
tribution (Fig. 4.2). In a review of 96 Mediterranean woody species from
Australia, California and Chile the average radius of root horizontal extension was 1.6 ± SD 1.5 m and the maximum extension was 7.0 m (Canadell
and Zedler 1995).
In La Castanya, root grafts were occasionally observed between roots of
the same tree, but never between roots of different trees. On the contrary,
root grafts were somewhat common between holm oak trees in Prades, although no root grafting was observed between different species. Generally,
root grafting is not very common in Mediterranean ecosystems (Keeley
1988); it was observed in only one of nine species excavated in the Chilean
matorral (Hoffmann and Kummerow 1978).
4.4 Lignotubers
One of the most characteristic features of the root system of holm oak is the
development of a large woody swollen structure at the stem base called the
lignotuber (Canadell and Zedler 1995). This structure is genetically determined and appears early during seedling development. There are no studies
on lignotuber development in this species, but it is known that Q. suber, another evergreen western Mediterranean oak, starts developing the lignotuber
structure through an accumulation of bud clusters and starch that occurs
close to the cotyledonary insertion (Molinas and Verdaguer 1993a,b).
The size of lignotubers depends on plant age, environmental and edaphic
conditions, and, most of all, on the disturbance history that individual plants
have experienced (e.g. fire, logging, extreme drought or cold events). In general, disturbances trigger the growth of the meristematic tissue of the lignotuber, which is further enlarged by the fusion of the stem bases of the new
emerging shoots. The presence of large lignotubers was common in Montseny and Prades because both forests were intensively coppiced for charcoal
production until the 1950s. At both sites, individual holm oaks often allocate
as much as half of their total biomass to the lignotuber (Canadell and Roda
1991; Djema 1995; Sabate et al. 1998). In one case at Montseny, the lignotuber
dry weight was 317 kg and accounted for 66% of the total tree biomass.
It is believed that lignotubers have a dual function. The first function,
which is morphologically related, is to store concealed buds that will resprout
after disturbances; the lignotuber generally stores a large number of buds
that enable plants to regrow even in environments subjected to multiple
disturbances. The second function is to store non-structural carbohydrates
and nutrients that will support regrowth after disturbances. Our understanding regarding the latter function has been inferred largely from studies
of plant growth (DeSouza et al. 1986; Castell et al. 1994), anatomical structure
(Montenegro et al. 1983), and tissue analysis of lignotuber nutrient and carbohydrate contents (Mullete and Bamber 1978; Dell et al. 1985). To date, very