The Interaction of Pinon Pine and Gambel Oak in Plant Succession near Dolores, Colorado

Abstract

-Pinus edulis (piiion pine) and Quercus gambelii (Gambel oak) near Dolores, Colorado, coexist in successional sequence leading to woodland regeneration. The germination of seeds and the establishment of seedlings of Pinus edulis are enhanced under an oak canopy. In a mature pifion woodland, oak exists as scattered understory, and pifion individuals of less than 10-cm diameters constitute 45% of the population. In contrast, 75% of the pifiones have diameters less than 10 cm in a stand which is invading an oak-dominated habitat. Near Dolores, Colorado, the oak stand is viewed as successional, leading to a pinion woodland. The vegetation of the Dolores River Valley, northwest of Dolores, Montezuma Co., Colorado, consists of Pinus ponderosa and Pseudotsuga menziesii at higher elevations of 2,548 m, and a lower pifion-juniper (Pinus edulis, Juniperus osteosperma, J. scopulorum) zone which extends to the river bottom at 2,184 m. Shriubland communities, often on east-facing slopes, consist of Amelanchier utahensis, Quercus gambelii, and Cercocarpus montanus. Considerable overlap exists in the oak and pifion-juniper vegetation types. The present-day vegetation in and around the Dolores River Valley was mapped during the summer of 1979 in an effort to determine the successional seres leading to potential climax vegetation. Comparison of presentday vegetation with 1973 aerial photographic coverage made it possible to detect the interrelationships between Gambel oak and pifion-juniper in this successional sequence. Four studies were designed to elucidate the role of oak in germination, establishment, and early growth of pinion. Stands of oak in the Dolores River valley are dense when young, but the individual trees become relatively spindly at approximately 75 to 100 years of age, permitting increased light penetration into the stand. Litter accumulation alters the seed bed, increasing its moisture retention capacity, and decomposition of oak leaves allows the release of volatile secondary plant compounds. These habitat changes could provide a suitable environment for piiion germination and establishment. It has been reported (Meagher, 1943) that shading of pinion seeds promotes early germination, and that pinion saplings become shade intolerant beyond the seedling stage (Tueller and Clark, 1975). The hypothesis that oaks facilitate germination of seeds and establishment and early growth of pinion individuals was tested in the Dolores River Valley. MATERIALS AND METHODs.-Germination of Pinon Seeds.-Pinus edulis seeds were allowed to imbibe in distilled water for 24 h. Fifty seeds were placed in germination cages under each of the following conditions: (1) under a mature pifion; (2) under a stand of mature oaks; (3) under sagebrush (Artemisia tridentata); (4) in the open, no plant cover; (5) in the open, no cover, and no germination cage. Germination cages were 1 by 1 m, 0.5 m tall, made of 4 wire mesh, and were designed to exclude large seed-predators. Seeds were planted in early June 1979 in approximately THE SOUTHWESTERN NATURALIST 27(2): 143-147 MAY 21, 1982 This content downloaded from 207.46.13.193 on Thu, 08 Sep 2016 04:30:03 UTC All use subject to http://about.jstor.org/terms 144 The Southwestern Naturalist vol. 27, no. 2 2 cm of litter and soil, in rows of 12 and 13 seeds. All plots were within 100 m of each other to control soil and climatic differences as much as possible. Soils consisted of gladel stony fine sandy loam. The latter condition served as the control group. The number of seeds germinated were recorded at 5 weeks and continued daily until snow cover prevented access to the germination cages at the end of October. After 60 days, four ungerminated seeds per treatment were removed and viability was tested. Viability was determined by a positive pink reaction in the megagametophytic tissue and embryo when soaked in 1% tetrazolium chloride, indicating the presence of deh-