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Coastal exposures of the Santa Monica Mountains rarely experience freezing temperatures

(0 °C) because of the ameliorating effects of the Pacific Ocean and seawater’s specific heat capacity. In contrast, inland sites of the Santa Monica Mountains frequently experience winter temperatures below -10 °C. This temperature gradient, from coast to inland, may be a major determinate of species distribution patterns. To investigate possible mechanisms by which freezing impacts chaparral distribution patterns, we examined xylem vessel diameter and vessel length of three chaparral species growing at inland freezing sites versus coastal nonfreezing sites (Malosma laurina, Umbellularia californica, and Ceanothus megacarpus). It has been established that vessel size influences freezing-induced embolism and the blockage of xylem water transport from soil to leaves. However, it is not known if this “size effect” is primarily due to vessel diameter, vessel length, or both. We initially hypothesized that matched species-pairs at non-freezing sites would have both longer and wider vessels than at freezing sites. We determined maximum vessel length by injecting air into stems at decreasing segment lengths and mean vessel diameters by using an ocular micrometer in conjunction with a light microscope. Sample sizes were six for each species pairs. For all three species, mean vessel diameters were narrower at freezing than non-freezing sites (P < 0.05) ranging between 13 µm mean differences for C. megacarpus to 20.4 µm mean differences for U. californica. In contrast, we found no significant difference in vessel lengths for any of the species-pairs

(P > 0.05). We conclude that reduction in vessel diameter is more significant than reduction in vessel length for protection from freezing-induced embolism of stem xylem. Furthermore, limits in the genetic plasticity of some species to reduce vessel diameter may preclude their survival at freezing sites.

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