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The invasion of a high elevation burn (3500 m) by Engelmann spruce (Picea engelmannii Parry) was investigated on the Santa Fe National Forest in north-central New Mexico. Population analyses and a field germination test of spruce seeds under various treatments were performed. The resistance of soil moisture blocks and soil temperatures were measured.

The fire which occurred in 1886 eliminated a climax spruce-fir forest and resulted in a subalpine grassland at elevations above the zone where aspen (Populus tremuloides Michx.) has invaded. The estimated crown coverage of trees in the grassland is less than 5%. Engelmann spruce comprises 92.5% of the trees in sampling plots, while limber pine (Pinus flexilis James) and corkbark fir (Abies lasiocarpa var. arizonica (Merriam) Lemm.) comprise 4.1% and 3.4%, respectively. The density of trees in all size classes is 7.3 trees/100 m2. The size class distribution of spruce shows a marked J-shape which is assumed to be an indication of an increasing rate of establishment. More rapid invasion seems to occur on north aspects. Establishment was found to be a non-random process, following a clumped distribution; however, no site factor was found to be correlated with tree density. The diameter growth rate of Engelmann spruce was determined to be 0.77 cm/year. The growth rate for spruce greater than 10 cm in diameter was found to be neither increasing or decreasing with age. The deviations in an age distribution of spruce were positively correlated with June plus July precipitation during this years of establishment.

The treatments had significant effects on the germination and seedling survival of Engelmann spruce. Black plastic mulch increased germination; however, it also increased seedling mortality. Pine bark mulch treatment and greenhouse (polyethylene enclosure) treatment decreased germination, while the disturbed treatment did not appear to affect germination or mortality. The amount and distribution of precipitation, and the surface soil temperatures control the timing, the amount, and the rate of germination and seedling mortality. The lack of germination of seeds in a field plot on a north aspect during the summer of 1973 was probably related to low soil temperatures.

Soil moisture at depths greater than 7.5 cm during the summer of 1972 did not appear to fluctuate greatly from field capacity. The pattern of seasonal soil block resistance readings was significantly correlated with the mean maximum daily surface soil temperature and the cumulative departure of precipitation from the mean seasonal precipitation. Extremes of surface soil temperatures can result in mortality from frost damage, heat girdling, and surface soil drought. The establishment of spruce in the subalpine grassland depends on sufficient surface soil moisture and moderate surface soil temperatures.



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UNM Biology Department

First Committee Member (Chair)

James Roman Gosz

Second Committee Member

Loren David Potter

Third Committee Member

Earl F. Aldon

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