Graduate Thesis Or Dissertation
 

Some factors controlling the establishment and distribution of Quercus agrifolia Nee and Q. engelmannii Greene in certain southern California oak woodlands

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  • The structure of two oak woodland types in the Santa Rosa region of the Santa Ana Mountains of southern California has been described by Zuill (1967). One type called the grass oak woodland (GOW) is composed mainly of Q. engelmannii on hills and slopes. The other type called the dense oak woodland (DOW) is composed equally of Q. engelmannii and Q. agrifolia with twice the density of the GOW and is located in valleys with granite rock outcrops. An apparent lack of seedlings and saplings was noted in the area. The present study focuses on an explanation for the distribution patterns of the two oak species in the field and the factors which might be most important in influencing seedling establishment. Quercus agrifolia is restricted to cool moist protected locations on the north side and in fractures of rock outcrops because its acorns are sensitive to moisture loss on air drying, require constantly moist conditions for one to five weeks for water uptake to effect germination and have very rapid shoot development after germination. Seedlings may be intolerant of high temperatures in open grass litter, and seedlings and saplings appear less tolerant of fire than Q. engelmannii, which would also tend to restrict Q. agrifolia to protected locations around rock outcrops. The distribution of Q. engelmannii trees with respect to rock outcrops does not differ from a random distribution and the few that are associated with outcrops show a random distribution with respect to position around outcrops. The occurrence of Q. engelmannii in more open exposed habitats may be due to its acorns' tolerance to moisture loss on air drying, their ability to germinate with little or n6 additional water uptake, its self-planting mechanism and delayed shoot development, and the drought deciduous habit of seedlings and their tolerance to fire. In the laboratory the drought capable of killing 50% of the seedlings is about 100 bars soil moisture tension for Q. engelmannii and about 250 bars for Q. agrifolia. The survival of Q. agrifolia under a greater degree of soil drought is mainly attributable to its higher root: shoot ratio under these particular conditions. Seedlings of both species would be expected to survive any drought recorded in the field between 1968 and 1970 at their expected rooting depth. After a very wet rainy season none of the mature trees experienced any moisture stress by the end of the dry season even though the near surface soils were quite dry. After a quite dry year trees in the DOW showed significantly greater moisture stress than those on a hilltop in the GOW. The moisture stress measurements of trees and soil moisture determinations down to 86 cm indicate that all the mature trees are rooted mainly below 86 cm. At least one large tree is apparently rooted down to 6 m. A planting of Q. agrifolia acorns that were buried to simulate squirrel caches survived to establish many more seedlings than surface planted acorns due mainly to protection from predation and drying and to conditions affording good radicle penetration. In an open planting cattle destroyed all the seedlings that were established while most of the seedlings remained in a planting closed to cattle. The complete lack of oak reproduction in open habitats and the restriction of reproduction for both species to rock outcrops since 1910 is attributable to cattle grazing, which has been continuous since about 1910. The last reproduction of Q. engelmannii in the GOW's without outcrops and the peak reproduction of Q. agrifolia in the DOW occurred between 1890 and 1910. This may be explained partly by the fact that for at least 10 years during this time livestock were excluded due to a dry land farming operation in the area. More than half of the living Q. engelmannii were established before any known influence of European men and their livestock (1820) and there has been a general decline in establishment since then. This might be expected since they usually occur in open and exposed habitats which would be most subject to grazing pressure. There was a simultaneous establishment and rapid buildup of the Q. agrifolia population around rock outcrops in the DOW with the entrance of European man and his livestock which may be due to ground squirrels depositing acorns around their burrows which are often associated with rock outcrops. These squirrels are known to come into new areas after disturbance by man. Also about this time filaree, a new preferred food source for squirrels (in addition to acorns), was being established on California range lands which probably resulted in an increase in the squirrel population. An effective squirrel poisoning program was begun in the 1920's and this may also account for the decreased establishment of Q. agrifolia since this time even around rock outcrops.
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