Differences In Drought Adaptation Between Subspecies Of Sagebrush - Artemisia Tridentata

INTRODUCTION

Sagebrush (Artemisia tridentata) is emblematic of the arid Great Basin region of the United States. The shrub dominates over 150 000 [km.sup.2] of the landscape (McArthur and Ott 1996). Within its wide geographic range, sagebrush also has a broad ecological distribution from dry valley floors, where annual precipitation can be less than 160 mm per year, to mesic mountain tops with double or triple the valley precipitation (West 1983). Along this gradient are found three major subspecies (Cronquist 1994), ssp. wyomingensis at the low and dry end of the spectrum, ssp. vaseyana at the high and wet end, and ssp. tridentata intermediate [ILLUSTRATION FOR FIGURE 1 OMITTED]. Presumably, the morphological differences defining these taxa are associated with adaptive physiological differences. This paper examines physiological differences with respect to drought adaptation in A. tridentata subspecies.

Morphologically, the subspecies differ in stature, inflorescence structure, and ploidy levels. Subspecies wyomingensis is short ([less than] 0.5 m) with floral stalks that arise throughout the crown, and is consistently tetraploid (McArthur et al. 1981). Subspecies tridentata is tall (generally [greater than] 1.5 m in height) with floral stalks resembling those of ssp. wyomingensis. Plants of ssp. tridentata can be either diploid or tetraploid with no known morphological or ecological features distinguishing plants with different ploidy levels (McArthur et al. 1981). Subspecies vaseyana is intermediate in height, usually diploid, and is distinguished from the other subspecies by reproductive shoots that arise from the upper crown and extend above the vegetative structures, giving the plants a flat-topped appearance (McArthur et al. 1981). The differences in morphology between the subspecies are maintained in common garden, suggesting that they have a genetic basis (McArthur and Welch 1982, Barker and McKell 1986, Booth et al. 1990).

Few studies have evaluated physiological differences between the subspecies. Since the differences in habitat include major differences in soil water availability (Sturges 1979, Barker and McKell 1986) one would predict differences in drought adaptation between subspecies, with ssp. wyomingensis being the most drought-adapted and ssp. vaseyana being the least [ILLUSTRATION FOR FIGURE 1 OMITTED]. Comparative water relations of ssp. wyomingensis and tridentata growing contiguously in nature suggested little difference in functional rooting depth (Shumar and Anderson 1986). A common garden study found that ssp. tridentata had the highest rates of photosynthesis and highest [[Delta].sup.13]C, followed by ssp. wyomingensis, and that vaseyana had the lowest average values (Frank et al. 1984). Since [Delta].sup.13]C values can be used as a proxy for water use efficiency, this suggests that the two low-elevation subspecies (ssp. wyomingensis and tridentata) were the most water use efficient, consistent with them occupying areas that are drier than ssp. vaseyana. However, patterns in [[Delta].sup.13]C differed between the 2 yr studied, and physiological distinctions between the subspecies remain ambiguous.

The present study evaluated the differences in drought experience and drought tolerance between the subspecies. Drought experience was quantified in terms of seasonal patterns of xylem pressure and transpiration rates in natural populations. Drought tolerance is a result of many traits, of which we focused on one, vulnerability to xylem cavitation. Xylem cavitation is the rupture of the water column held under negative pressure in the xylem. We emphasized cavitation because of its explicit link to the plant's potential for gas exchange; plants cannot conduct water if their xylem conduits are cavitated. As a secondary drought tolerance parameter, we quantified the turgor loss point. The turgor loss point is the water potential at which protoplasmic pressure reaches atmospheric. Its physiological significance is more ambiguous than cavitation resistance, with potential consequences including the inhibition of cell growth, disruption of tissue structure, and induction of stomatal closure (Kramer 1988, Schulze et al. 1988).

Our hypothesis was that the subspecies' vulnerability to cavitation and turgor loss points would differ according to the degree of drought they experienced, and that this would parallel the qualitative ranking shown in Fig. 1. Vulnerability to cavitation was measured in both natural populations and a common garden population to evaluate genetic vs. environmental influence on the trait. In a theoretical discussion of the results, a model of the soil-plant continuum (Sperry et al. 1998; see Appendix) was used to determine the extent that differences in water use between subspecies could be linked to different hydraulic limitations imposed by cavitation.

METHODS

Study sites

Natural populations of A. tridentata were studied at three sites within 16 km of each other. These sites were in the vicinity of the East Tintic Mountains of central Utah. The ssp. wyomingensis site was at 1620 m (112 [degrees] 00 [minutes] N, 36 [degrees] 57 [minutes] 30 [seconds] W), the ssp. tridentata site was at 1780 m (112 [degrees] 10 [minutes] N, 39 [degrees] 50 [minutes] W), and the ssp. vaseyana site was at 2040 m (112 [degrees] 05 [minutes] N, 39 [degrees] 57 [minutes] 30 [seconds] W).