If freezing limits establishment of warm desert evergreen shrubs at high latitudes shrubland distributions may be altered as a result of rising global temperatures, however, local adaptation to freezing at high latitudes may ameliorate the negative effects of low temperature events. In high latitude deserts competition for water resources and freezing stress may place conflicting demands on xylem and trade-offs could arise between these two important physiological characters. Both hydraulic conductance and xylem vulnerability to freezing embolism are closely linked to xylem vessel size; larger diameter vessels have high hydraulic conductance but are more vulnerable to freezing embolism while smaller diameter vessels have low hydraulic conductance but greater freezing tolerance. Predicted changes in wood structure under freezing stress include decreased investment in more efficient, large vessels and increased redundancy of smaller, safer vessels. In addition, plastic responses to temperature regime, rather than genetic differentiation, may be responsible for differences in vessel size across latitude. To determine whether local genetic adaptation to freezing is present in the warm desert evergreen genus Larrea we investigated xylem vessel diameter and wood structure in greenhouse-grown saplings and field-grown adults from high and low latitude populations of Larrea tridentata, L. divaricata and L. nitida.
Significant differences in mean vessel diameter were evident across latitude in both field and greenhouse populations; low latitude plants had larger diameter vessels than high latitude plants and maintained more of their total hydraulic conductance in larger vessels. Little to no freezing embolism was predicted for greenhouse-grown saplings, and for all three species 35% - 56% of sapling vessels were < 10 mm in diameter. The differences across latitude were smaller when comparing greenhouse-grown vs. field-grown plants, suggesting both genetic and plastic components contribute to differentiation in wood characteristics across populations. Among high latitude field-grown adults we observed two patterns: 1) L. divaricata retained juvenile wood structure including extremely small mean vessel diameter and low vessel density, 2) L. tridentata and L. nitida had higher than expected proportions of large diameter vessels, accompanied by high density of the smallest vessels, a reduction in the number of intermediate sized vessels and ring-porous or semi-ring porous wood structure. At high latitudes seasonality of the climate increases, and maximizing water use during times of high water availability may improve ability to compete for water resources, while investment in a large number of small vessels would provide safety as conditions become less favorable.