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Strategies Utilized by Pinus attenuata and Pinus coulteri at Low Elevations in the San Bernardino Mountains
Plants use an isohydric or an anisohydric strategy to cope with declining soil moisture availability and increasing evaporative demand. The goal of this study was to determine the physiological and structural strategies utilized by Pinus attenuata (knobcone pine) and Pinus coulteri (Coulter pine) at low elevations in the San Bernardino Mountains of California, which experienced above-average precipitation after a severe seven-year drought. Based on a previous study, it was hypothesized that P. attenuata should be isohydric and P. coulteri should be anisohydric because P. attenuata has a slower transpirational loss of water than P. coulteri. Physiological, morphological, and anatomical measurements were used to compare the responses of the two species to seasonal environmental changes and induced drought. In the field, P. coulteri had greater water potentials and maximum quantum yields of photosystem II than did P. attenuata; however, gas exchange did not differ with species. In a controlled drought study, young P. coulteri had higher water potentials and gas exchange than young P. attenuata, with P. coulteri continuing to take up CO2 two weeks after P. attenuata had ceased uptake; however, P. attenuata took up CO2 at lower water potentials than P. coulteri. Tracheid diameter was larger for P. coulteri than P. attenuata for bands of late wood only and tracheid cell wall thickness did not differ with species. The species did not differ in plant height, but P. attenuata at low elevations had the smallest diameter at breast height, indicating P. coulteri could transport and store more water where it occurred. Finally, neither species experienced extensive branch or canopy dieback due to the previous drought. In conclusion, based on their physiological responses, both species appear to be isohydric, but also display some traits of anisohydric species.