Production of Herbaceous Vegetation in Openings and under Canopies of Western Aspen

Abstract

Extensive forests of quaking aspen (Populus tremuloides Michx.), which occur widely at intermediate elevations in the Intermountain and Rocky Mountain West, constitute an important resource, both esthetically and economically. The aspen type is esteemed among sightseers, picnickers, and campers because of its beauty. Aspen forests protect the soil of an important snow-collecting belt on western mountain watersheds. Although aspen is not one of the important timber species, aspen wood products are in considerable and increasing demand. Finally, the type supports undergrowth capable of furnishing a great amount of cover and forage for wildlife and livestock. On aspen range in good condition one may wade waist deep through a rich mixture of many plants, including species of Heracleum, Mertensia, Delphinium, Osmorphiza, Agastache, Erigeron, Rudbeckia, Senecio, Thalictrum, Agropyron, Bromus, Elymus, and Carex. Most aspen range in the Intermountain region is depleted from prolonged overgrazing, however, and palatable species that were formerly abundant are likely to be scarce or absent. Curiously, much less vegetation is produced in openings, as a rule, than beneath the aspen canopy.3 Usually the vegetation of openings is the sparser and shorter, and it tends to include a smaller proportion of desirable forage species (Houston 1954). It may, indeed, include undesirable species such as the annual Madia glomerata Hook., that are absent under the aspens. We have personally observed such differences in production and species composition on aspen ranges in Utah, Nevada, southern Idaho, western Wyoming, and western Colorado. These observations have included a great variety of sites with respect to soil and exposure, and many variations in character of vegetation. Why should such a difference in ground-cover production exist? Is it a product of those factors. responsible initially for the existence of openings in an aspen forest? Is it caused by differences in microclimate under and away from the aspen canopy? Is it a result of heavier grazing in openings than under the aspen? Or is it the result of interactions between some or all of these factors? Whatever the cause, this widely observed difference seems anomalous, because it would appearthat demands upon the environment by the aspen trees themselves might be expected to infringe on the needs of plants of the shrub and herb layers. Logically, therefore, being free of such competi-tion, the openings should produce more vegetation, not less, than equal areas under aspens. Such a relation has been described by Moinat (1956) in the Quercus gambelii type in southwestern Colorado, a zone somewhat warmer and drier than the aspen-fir zone. Moinat shows that production ofthe field layer in grassy parks is markedly higher than in scrub-oak thickets. Many trenching experiments (reviewed byKorstian and Coile 1938) have demonstrated the adverse effects of root competition by forest overstory on growth of native ground vegetation or artificially planted trees. Shirley (1945), in a study carefully designed to test the relativeeffects of shade and root competition of aspen anct jack pine on tree seedlings and planting stock in Minnesota, demonstrated a root-competitive depression of conifer growth, although not of sur-vival. Shirley's comment is that, In these studies, however intense the root competition of the overwood, its effect on survival was more than offset by the benefits of the shade, provided this did not reduce the light intensity below 20 percent. Weknow of no experiments of this kind involvingwestern aspen except for Pearson's (1914) comparisons of survival of planted Douglas-fir under aspen and in openings. These showed better sur-vival under aspen.