Quantification of Eccentricity in Stems of Two Species of Purshia

Abstract

Plants of Purshia tridentata and Purshia stansburyana, 2 shrub species from the western United States, naturally display eccentric stem growth. The purpose of this study was to quantitatively document stem eccentricity for these 2 species. Stem segments were obtained from 3 different plants of each species at locations in central Utah. Each stem was sawed into 8-mm sections and photographed for analysis. Every stem section was then marked at a 36° interval to produce 10 wedge-shaped sectors. The areas of each sector were determined and analyzed. Stem segment areas were about 40, 60, and 100 mm2 for stem diameters of 6, 8, and 11 mm, respectively, for both species. Consecutive 8-mm segments were compared at 6 and 11 mm for both species. Absolute differences between paired segments ranged between 0.0 and 12.0. On average, 43% of all pairs had absolute differences above 0.40. These results suggest marked differences between consecutive segments. The eccentric growth percentages were also the same for both species. Mean eccentric percentages for 6-mm-diameter segments were 43.4% and 49.6% for the 2 species. Moreover, 11-mm-diameter segments of P. tridentata and P. stansburyana had mean eccentric percentages of 36.4% and 34.0%, respectively. Overall, 81% of all stem segments showed eccentricity. Many long-lived desert shrubs of the western United States exhibit a wide range of wood anomalies. These wood anomalies may be endemic in these species.