Soil moisture depletion trends under five plant species present on the Douglas-fir clear-cuts of Mary's Peak, Oregon Public Deposited


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  • 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.
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