50
J. Canadell, A. Djema, B. L6pez, F. L., S. Sabate, D. Siseart and C.A. Gracia
I A)
o
1m
o
1m
(8)
Fig. 4.2. Root systems (roots of 0 > 1 em) of single-stemmed holm oaks in A the mesic site and
B the xeric site at La Castanya, Montseny. (Drawings by Meree Cartafia)
growing on a compact matrix of breccias. Interestingly, holm oak trees
growing on the schist soils were more tolerant to drought events. These observations suggest that trees having more roots in deep soil horizons (where
water is stored for a longer time) are better prepared to withstand extreme
drought events. Under these conditions, the interaction between climate and
substrate type plays a critical role in reshaping communities and allowing
deep-rooted individuals to survive.
The lateral root system of holm oak can be quite extensive. Lateral roots
can grow far beyond the canopy projection and were observed at 5.9 m from
the main stem in Prades (Djema 1995); large roots (0 > 1 cm), however, do
not usually extend further than 2.5 m. The smaller diameter roots account
for the largest fraction of root length, which is important in terms of soil occupation and water and nutrient uptake. Most of the horizontal large roots
grow between 20 and 60 cm from the soil surface, and only a few roots are
found close to the surface, where dry-wet cycles are more frequent and
therefore where harsher environmental conditions for root growth are
found. It is worth noting that when trees grow on steep slopes, most of the
lateral root system grows up-slope yielding a highly asymmetrical root dis-