Aliso: A Journal of Systematic and Evolutionary Botany Volume 6 | Issue 3 Article 3 1967 Hybridization and Variation of Ponderosa and Jeffrey Pines on Mt. San Antonio, California Mark W. Parratt Claremont Graduate School Follow this and additional works at: http://scholarship.claremont.edu/aliso Part of the Botany Commons Recommended Citation Parratt, Mark W. (1967) "Hybridization and Variation of Ponderosa and Jeffrey Pines on Mt. San Antonio, California," Aliso: A Journal of Systematic and Evolutionary Botany: Vol. 6: Iss. 3, Article 3. Available at: http://scholarship.claremont.edu/aliso/vol6/iss3/3 ALISO VoL. 6, No. 3, pp. 79-96 JUNE 30, 1967 HYBRIDIZATION AND VARIATION OF PONDEROSA AND JEFFREY PINES ON MT. SAN ANTONIO, CALIFORNIA 1 MARK w. PARRATT Claremont Graduate School, Claremont, California INTRODUCTION The purpose of this study is to determine whether plants having the character­ istics of Pinus ponderosa or Pinus jeffreyi occur in allopatric or sympatric patterns of ecological distribution. If ecological conditions do permit the two pines to exist in a sympatric situation, occurrences of hybridization would be significant. Several authors have questioned the taxonomic and genetic interrelationships of the ponderosa (western yellow) pine, Pinus ponderosa Dougl. ex P. and C. Lawson, and the Jeffrey pine, Pinus jeffreyi Grev. and Balf. in A. Murr. Although two recognizable extremes do exist, the question of the degree of genetic con­ tinuity between the two pines is unsolved and at present controversial. In 1929, N. T. Mirov established numerous chemical differences between the resins of the ponderosa and Jeffrey pines. The resin of P. jeffreyi contains traces of straight­ chain aldehydes not found in the terpene mixture unique to P. ponderosa. He found that these chemical differences rarely combine in supposed hybrid indi­ viduals. In 1953, D. A. Johansen established morphological criteria for the specific validity of the Jeffrey pine. He summarized some 14 archegonial characteristics of the megagametophyte which differed markedly from those of the ponderosa pine. He also noted several pines in upper San Antonio Canyon ( Mt. San An­ tonio, California) which had a combination of archegonial characteristics dif­ ferent from the ponderosa pine and unlike those of the Jeffrey pine. These trees were previously regarded as variations of P. ponderosa. However, Johansen's analysis of the archegonia showed the individuals to be either hybrids or a taxon constituting an unrecognized species. L. D. Benson ( 1955) and a group of students made a survey of the ponderosa­ Jeffrey pine complex on the southern and eastern slopes of Mt. Pinos (Santa Emigdio Range). At elevations approaching 7,750 ft., the combination of char- 1Based on a thesis presented in partial fulfillment of the requirement for the degree of Master of Arts in the Claremont Graduate School. The work was done at the Rancho Santa Ana Bo­ tanic Garden under the supervision of Professors Lyman Benson, Sherwin Carlquist, and Robert Thorne. [79] 80 ALISO [VoL. 6, No. 3 acteristics followed a relatively pure P. jeffreyi segregation pattern. However, at the 6,300 ft. level, many individuals had characteristics of each species in nearly equal proportions. The data were derived through the quantification of five characteristics using a hybrid-index scale (Anderson, 1949). An important later study by J. R. Haller ( 1962) also used Anderson's hybrid­ index method. Fifteen characteristics were quantified and applied to an analysis of several ponderosa and Jeffery pine populations throughout Californian montane regions. Haller found that the frequency of hybridization was very low in all areas. The marked diversity of characteristics within a given individual was at­ tributed to the wide variability of the taxa. However, Haller's quantification of a given characteristic on only a three point scale (0= ponderosa-like, 1= inter­ mediate, 2= Jeffrey-like) could obscure segregation of characteristics within a putative hybrid zone. A greater, yet reasonable numerical range for a given char­ acteristic enables one to work with degrees of phenotypic differences. This type of analysis is more dimensional and should reveal the nature of hybridization in which multiple-factor segregation is occurring. Various earlier studies and observations have suggested that hybridization be­ tween Jeffrey and ponderosa pine is actively occurring within a certain zone on Mt. San Antonio, California. Since past studies concerning the ponderosa-Jeffrey pine complexes have either been superficial or based upon too few criteria, I undertook an intensive study of the Mt. San Antonio area. Some four years of analysis and field work were undertaken to test the degree and nature of hybridization, to supply comparisons with "pure" populations, and to describe the ecological background of this hybridization. The "pure" or allopatric populations of both species were studied in isolated areas near Mt. San Antonio and elsewhere within the same mountain range (San Gabriel Range). On completing this investigation, I co-operated with a colleague in making a comparative study of the bark chemistry of the ponderosa-Jeffrey pine complex on Mt. San Antonio. Chromatographic analysis of phenols and tannins revealed a distinct pattern difference between the two species.2 Hybrid individuals de­ tected by morphological methods in this paper were verified by plausible inter­ mediate chromatogram patterns (unpublished data). DISTRIBUTION AND ECOLOGY In California, Pinus ponderosa is common in the montane forests from the Ore­ gon border through the Cascades, the inner North Coast Ranges, and the western slopes of the Sierra Nevada. The species occurs sporadically through the South Coast Ranges, becoming more abundant in the higher mountains that comprise the Transverse and Peninsular ranges. The southern limit of the ponderosa pine in California is near Cuyamaca Reservoir in San Diego County. Pinus jeffreyi is restricted to southern Oregon, California, and northern Baja California and its distribution approximates the geographic range of the ponderosa pine. However, the Jeffrey pine is more abundant in the south and occurs in relatively large stands within the Sierra de San Pedro Martir in Baja California. 2The concept of "species" in relation to Pinus will be discussed later. JUNE 30, 1967] PONDEROSA AND JEFFREY PINES 81 Typically, P. ponderosa occupies lower, coastal-facing slopes of the major moun­ tain axes, while P. jeffreyi is characteristic at higher elevations and on dry, desert­ facing slopes. Studies by other authors as well as my observations show that extremes of the ponderosa-Jeffrey pine complex are relatively homogenous. Within the mountain ranges of Southern California, P. ponderosa generally occurs between 5,000 and 7,000 ft., whereas P. jeffreyi characteristically occupies the 7,000-10,000 foot zones. On Mt. San Antonio, the stands between 5,600 and 6,600 ft. appear to be almost uniform P. ponderosa, whereas between 7,000 and 9,500 ft. most individuals are P. jeffreyi. Apparently differential tolerances of the two species to various ecological fac­ tors partially dictate their elevational zonation. On the south-facing slope of Mt. San Antonio, the lower elevationallimit of P. ponderosa is approximately 5,600 ft. Lack of adequate precipitation (under 25-28 inches annually) is often the limit­ ing factor at this lower level. However, weather readings from the Camp Baldy Fire Station ( 1950-1963) indicate precipitation averaging some 31 inches per year at the 4,200 ft. level, a quantity in excess of the minimum water requirement for the ponderosa pine. Apparently other factors combine to limit P. ponderosa at the 5,600 ft. level. Occupation of this area by dense chaparral growth and other conifers may be a limiting factor. The ponderosa pine generally occurs in open montane forests where individual trees are well spaced from other vegeta­ tion. However, in this local situation, large numbers of Pseudotsuga macrocarpa, Calocedrus decurrens, Pinus lambertiana, Quercus chrysolepis, Cercocarpus be­ tuloides, Eriogonum fasciculatum, and other species seem more successful than Pinus ponderosa. High concentrations of soluble soil salts at this elevation may be correlated with the abundant chaparrel vegetation. Using a salinity meter and soil cores, I measured the electrolytic conductivity of the soil. Negligible readings were found between 5,700 and 9,600 ft., while at 5,500 ft. the reading was 2.00 mm/per em. Perhaps P. ponderosa cannot tolerate this saline condition, whereas the chapar­ ral vegetation thrives in such a habitat. The upper limits of P. ponderosa on this mountain (between 6,500 and 6,600 ft.) are probably controlled by low temperatures and lack of adequate precipita­ tion. The cold, dry conditions of Mt. San Antonio's north face continue to some extent on the south-facing slopes. Apparently, environmental conditions enable only a narrow band of ponderosa pine to exist (a vertical 800-900 ft. zone), while the more drought-resistant and temperature-tolerant Jeffrey pine extends through a much wider elevational range (approximately 2,500 ft.). Wide temperature ex­ tremes in the Jeffrey pine belt are evidenced in statistics obtained from the Los Angeles County Flood Control District ( 1964). At 7,800 ft. ( Mt. San Antonio Pass or "the Notch"), the average maximum temperature was 90°F and the average minimum was 4°F over the past six years. Such conditions appear magnified on the north-facing slopes where P. jeffreyi extends down to the 5,000 ft. level. Here the ponderosa pine is virtually absent. The upper limits of P. jeffreyi on the south-facing slope of Mt. San Antonio are near the 9,500 ft. level. Here low temperatures, rapid fluctuations from day to night, and incessant drying winds permit development only of stunted specimens 82 ALISO [VoL. 6, No. 3 of Jeffrey pine (Fig. 1). The climatic conditions here correspond closely with those encountered in a subalpine environment (Horton, 1960). Both species are restricted in their upper distributional limits by low temperature. However, as noted above, P.