Variability in needle morphology and water status of Pinus cembroides across an elevational gradient in the Davis Mountains of west Texas, USA1

Journal of the Torrey Botanical Society, Apr-Jun 2007 by Poulos, H M, Berlyn, G P

POULOS, H. M. AND G. P. BERLYN (Yale School of Forestry and Environmental Studies, Yale University, Greeley Memorial Laboratory, 370 Prospect St., New Haven, CT 06511, USA). Variability in needle morphology and water status of Pinus cembroides across an elevational gradient in the Davis Mountains of west Texas, USA. J. Torrey Bot. Soc. 134: 281-288. 2007.-The pinyon pines (Pinaceae, Pinus subsections Cembroides and Rzedowskianae) are a widely distributed group of site generalist species that dominate many of the middle to upper elevation semi-arid regions of North America. We investigated the physiological and morphological response of Pinus cembroides var. bicolor Little across an elevational gradient in the Davis Mountains of west Texas to test the hypothesis that variability in needle morphology, relative water content, and transpiration allow this species to exist across a range of elevations and local site conditions. Results from our study showed significant increases in P. cembroides needle length, mass, and area and plant water status (relative water content and transpiration) with elevation. Our findings suggested that this species is able to adapt to changes in local environment over short distances, which is an important factor responsible for the wide distribution of pinyon pines in North America.

Key words: elevation gradient, gradient analysis, needle morphology, Pinus cembroides, pinyon pine, transpiration, pinyon-juniper woodlands, water relations.

Pinyon pines (Pinaceae, Pinus subsections Cembroides and Rzedowskianae) are some of the most widely distributed trees in the semi-arid regions of the northern hemisphere (Fig. 1). They dominate over 325,000 square kilometers in North America. Their latitudinal range spans from southern Idaho in the United States (42°N) to southern Puebla, Mexico (18°N), and their longitudinal distribution covers a similarly large portion of North America from Puebla (97°W) to California (120°W) (Fig. 1) (Aldon and Springfield 1973, Barger and FfoUiot 1972, Little 1999). The biogeographic range of the pinyon pines is large compared to many other pine species in southwestern North America (i.e., Pinus strobiformis Englem., P. arizonica Englem., P. flexilis James, P. leiophylla Schltdl. & Cham., P. engelmannii Carrière, P. ayacahuite Ehrenb. ex Schltdl.), which are generally more restricted in their geographic and elevational distributions (Hanawalt and Whittaker 1976, Neiring and Lowe 1984, Bailey and Hawskworth 1987, Allen et al. 1991, Poulos et al. in press).

Water availability is the most limiting factor to tree growth in the Southwest (Meko et al. 1995, Hidalgo et al. 2001, Adams and KoIb 2005), and the drought tolerance of pinyon pines in this region is considered the primary factor regulating their distributions across climatic and soil moisture gradients (Barton 1993, Barton and Teeri 1993, Lajtha and Getz 1993, Linton et al. 1998). Moisture and temperature can vary more than twofold over elevational gradients inhabited by Southwestern pinyon pines (West 1988), with a generally increasing trend in moisture and decreasing trend in temperature with elevation (Barry 1992). These changes in local environmental conditions with elevation are presumed to increase water availability to plants, allowing for increased carbons sequestration and growth (Barnes 1986, Padien and Lajtha 1992, Lajtha and Getz 1993).

The wide distribution of pinyon pines across the elevational gradients of southwestern North America has been quantified in a range of studies (St. Andre et al. 1965, Whittaker and Niering 1965, 1968, 1975, Niering and Lowe 1985, Barton 1993, Martens et al. 2001, Poulos et al. in press). However, virtually no information exists that identifies the anatomical, morphological, and physiological variability in pinyon pines that underscores their wide distribution across North America (but see Jaindl et al. 1995).

This study examined the ecophysiological mechanisms responsible for the extensive range of Mexican pinyon pine (Pinus cembroides var. bicolor Little) by measuring a suite of morphological and physiological traits on adult trees that spanned the elevational gradient of The Nature Conservancy Preserve of the Davis Mountains in west Texas (DMTNC). To our knowledge, no previous studies have been conducted on the physiological ecology of this species, though it is the most common and economically important pinyon pine in Mexico (Suzán-Aspiri et al. 2002). We hypothesized that Mexican pinyon pine would respond to changes in environmental conditions along the elevational gradient through variation in needle morphology, plant water status, and transpiration.

Materials and Methods. SITE DESCRIPTION. The Nature Conservancy Preserve of the Davis Mountains is located in Jeff Davis County in the Trans-Pecos region of western Texas (Fig. 1). The Davis Mountains (30° 50' N; 103° 50' W) comprise the largest mountain range in Texas, spanning from 1525 to 2555 m in elevation. The forests of DMTNC are dominated by pinyon-juniper-oak and mixed-conifer woodlands. Chihuahuan desert grasslands bound the site at lower elevations, and relict montane conifer forests form the upper elevational boundary (Hinckley 1944). The Davis Mountains are 35-39 million years old, and originated in the same Eocene to Oligocène orogeny that formed most of the Front Range of the Rocky Mountains (Turner 1977). Geologic substrates in DMTNC are derived from the erosional remnant of the once widespread Davis Mountains volcanic field, and consequently, the underlying rocks are predominantly extrusives, consisting of lavas and pyroclastics (Savage and Morin 2002). Soils are generally shallow to moderately deep, and are volcanic in origin.