Abstract:
Evaluation of meta-sedimentary bedrock as a source of stored water available
to plants in a mediterranean climate is presented based on: root length distribution
(RLD), root morphology, rock water holding capacity and seasonal pattern of water
depletion. Studies were performed in southwestern Oregon under young stands of
whiteleaf manzanita, ponderosa pine, Douglas-fir and Pacific madrone. RLD analysis
shows that more then one quarter of the total root length of ericaceous plants is located
in the rock layer. Stepwise procedures identified the best fitting multiple-regression
model for relating RLD to depth, bulk density of rock and the space penetrable by roots
(SPR). Linear expressions of SPR and depth were found to be significant explanatory
variables.
Roots of manzanita and madrone occupy rock fissures as small as 100 urn. The
root stele remains cylindrical in shape without visible mechanical stress, the cortex
became flat, creating "wing-like" structures on each side of the stele. Fine particles that fill the space between root cortex and rock matrices create a continuous conduit for
water flow.
Pore volume distribution of rock was measured in four functional ranges of pore
size: gravitational water, available water content (AWC), slowly available water (SAW)
and unavailable water (UW). An appreciable amount of pore volume is present within
the AWC and SAW, ranging downward from 20% of total rock volume in rock at a
density of 2.1 to negligible at density 2.67 grams per cubic centimeters. End-of-summer
water content measured in-site in the field and bio-assay results shows that
plants extract this water from the rock matrix during the typical water stress of a
mediterranean type summer.
Water depletion from soil/rock profile follows distinct patterns according to
vegetation type, based on neutron probe measurements from aluminum access tubes 3.2
m deep. Measurements were taken during two growing seasons (1993 and 1994) under
pure cover of the above species. Both conifers show very limited ability to utilize
water from bedrock. Ericaceous plants were very efficient in depletion of water from
the rock layers, using all available water measured by the access tubes. Water
resources used by both types of plants overlaps considerably and competition will be
very strong in species mixtures. Ability to utilize bedrock water by ericaceous plants
appears to offer a major competitive advantage.
