Freezing tolerance in high and low latitude populations of Larrea tridentata: Evidence for local adaptation to long-term minimum temperature

Freezing may limit high latitude range expansion of warm desert evergreen shrubs such as L. tridentata, as low temperature events can cause freeze-thaw embolism or damage to the photosynthetic apparatus.  Conversely, high latitude populations may be locally adapted to long-term minimum temperatures (LT_min), having developed more resistant xylem, greater cold acclimation potential or a more deciduous habit.  In summer and winter 2006 I investigated the possibility of local adaptation to freezing in high and low latitude populations in the Chihuahuan Desert using field populations and plants grown in a greenhouse common garden.  Preliminary results indicate significant differences between field populations in gas exchange, dark adapted chlorophyll fluorescence (F_vF_m), water use efficiency, and xylem vessel size distributions.  Lab experiments revealed that high latitude seedlings may be locally adapted to freezing, with no significant reductions observed in hydraulic conductance following freezing events as low as -24 ^oC, which corresponds to LT_min.  In addition, although gas exchange was reduced significantly following freezing events of -10 ^oC, leaves showed no significant reduction in F_vF_m and an increase in photo-protection, indicating that acclimation rather than damage caused photosynthetic reductions.  Furthermore, differences in freezing resistance were observed between leaves and xylem, with complete leaf drop occurring following freezing events of -16 ^oC followed by re-sprouting upon their return to the greenhouse.  Preliminary results point to differences between high and low latitude populations in freezing tolerance, with low latitude seedlings experiencing reductions in gas exchange, hydraulic conductance and F_vF_m following mild freeze events.