- Soil moisture depletion trends under five plant species growing
on the clear-cuts of the Marys Peak Watershed near Corvallis,
Oregon, were followed during the summers of 1963 and 1964. The
species were Holcus lanatus, Lotus crassifolius var. subglaber,
Gaultheria shallon, Berberis nervosa, and Acer circinatum; and
were dominant plants of several stages in a successional sere
occurring on the clear-cuts.
Sampling of the moisture trends was limited to two clear-cut
areas in order to reduce the variability due to location. On one
area, Gaultheria, Berberis, and Lotus were growing in individual,
pure stands. On the other, Acer was growing in closely grouped
clumps and Holcus occupied the areas in between,
The soil moisture was measured by an electrical resistance
method. Plaster-of-paris blocks were installed at 6-, 12-, and 24-inch depths at nine locations in each species. The measurements
were taken two or three times a week with an ohmmeter
and expressed as an average in terms of atmospheres of tension
for each day and depth.
Supporting information on the precipitation, root distribution,
and soils was also obtained. A root count for roots less than two
millimeters in diameter was made from the face of a trench dug
in each species. Soil descriptions made of the trench profiles, a
particle size analysis, and 15 atmospheres determinations indicated
that the soils of the two clear-cuts were similar.
Each species had characteristic moisture depletion trends
during the two years. Trends for Gaultheria indicated slow rates
of moisture loss at all three depths and very little influence of
precipitation. Gaultheria, an evergreen shrub, has thick, leathery
leaves (characteristics which are generally associated with few
stomata) and has a long period during which new stems emerge.
The roots were concentrated near the surface just under the one
and half to two inches of litter, and a few were growing inside
large, dead roots of Douglas-fir.
Depletion trends associated with Berberis were similar to
Gaultheria except moisture losses at the 6-inch depth were more
rapid. Berberis plants are also evergreen shrubs and have thick,
leathery leaves but grow during a short period in the spring. Most of the roots developed in the top few inches of soil. Litter accumulation
was slightly less (one to one and a half inches) and was not
as uniformly distributed.
Under Lotus, moisture trends at 6 and 12 inches fluctuated
considerably. Depletion rates were rapid at all three depths.
Lotus, a herbaceous species, grew quickly and flowered in the
spring, then died back in mid-summer, evidently allowing increased
infiltration of the rainfall. The rhizomes penetrated
throughout the profile. Litter accumulation was about an inch,
and the soil surface was somewhat rocky.
Holcus trends at 6 and 12 inches showed an early and rapid
moisture loss but a slightly delayed and slower loss at 24 inches.
Holcus, a perennial bunch grass, also grew rapidly in the spring
and then died back about mid-summer. Regrowth occurred following
a substantial rainfall. The erect culms and dense mat of
grass leaves (about two inches) surrounding the base of each plant
created high air temperatures which may have caused high evapo-transpiration
rates. Root concentration was greatest near the
surface and decreased sharply with depth.
Moisture losses under Acer were rapid at all three depths and
very consistent without any fluctuations. Acer clumps grew during
the spring and maintained their leaves throughout the summer.
Such comparatively large plants (six to seven feet high) evidently had a high transpirational stress, and along with the one to two
inches of litter, intercepted most of the precipitation. Root concentration
decreased gradually with depth.
This study provides a partial explanation for the replacement
of Holcus by Lotus. It appears that Lotus is able to invade Holcus
by producing rhizomes which grow underneath the dense root system
of Holcus and utilize the moisture there. Lotus then increases
in dominance by sending up shoots from the rhizomes.
Possible explanations for the replacement of other species
were not evident; however, there were some interesting correlations
of the results with the sequence. With each advancing stage
the depletion trends became more consistent. And, except for
Lotus which had the smallest number of roots, the root count decreased
with each advancing stage. The decrease was a reflection
of an increase in the relative size of the roots.
Results of this study are applicable to forest regeneration
problems. In terms of influence upon soil moisture Acer stands
would be very competitive with tree seedlings. Holcus and Lotus
stands would also be competitive, Lotus perhaps more so at the
deeper levels. Gaultheria and Berberís stands, on the other hand,
would not be nearly as detrimental to tree seedling establishment.