Morphology, Nurse Plants, and Minimum Apical Temperatures for Young Carnegiea gigantea

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Abstract
The northern limit of C. gigantea (Engelm.) Britton and Rose apparently depends on minimum apical temperatures. Diameters, apical spine coverage, and effects of nurse plants on incoming long-wave [IR] radiation, all of which affect apical temperatures, were therefore determined for stems of C. gigantea up to 4 m tall at four sites along a north-south transect in Arizona [USA]. A simulation model indicated that the increase in diameter accompanying stem growth raised the minimum apical temperature more than 3.degree. C. Thus, plants with the shortest stems would be expected to be the most vulnerable to freezing damage; indeed, freezing damage on stems < 0.5 m tall without nurse plants was fairly common at the colder sites. Nurse plants obstructed a greater portion of the sky for C. gigantea at the colder sites; e.g., the effective environmental temperature for IR radiation at such locations was raised more than 10.degree. C for stems under 1 m tall. It the northern limit of C. gigantea reflects wintertime survival of juveniles, nurse plants could extend the range by offering some protection against freezing.