Recovery of Larrea tridentata following extreme cold across a shrub-grassland ecotone

Background/Question/Methods

The shift from perennial grassland to desert shrubland caused by encroaching woody shrubs is reinforced by positive feedbacks of increased areas of bare soil compared to intact grassland.  At the Sevilleta LTER, Larrea tridentata, a native shrub with a northern range limited by cold temperatures, has encroached on the Chihuahuan Desert grasslands in the last century, altering patterns of soil moisture and winter minimum temperatures. We evaluated the response of L. tridentata across the shrubland-grassland ecotone after widespread foliage dieback caused by minimum temperatures of -30°C in February 2011. We asked: 1) Will the L. tridentata population in shrubland and the mixed-grass ecotone exhibit variation in canopy damage following extreme cold? 2) Will water availability affect the resprouting of L. tridentata impacted by a severe freeze? We measured post-freeze recovery by estimating the percentage of green resprouting in canopies of L. tridentata in untreated shrubland and ecotone and in irrigated and previously droughted plots of an established rainfall manipulation experiment in both vegetation types. 

Results/Conclusions

We found that canopy dieback following extreme cold was greatest within the shrubland, although response to water availability differed across these sites. In the shrubland, the shrubs exposed to water addition exhibited significantly more resprouting than the shrubs in the water removal and control plots (P<.0001). In the mixed-grass ecotone, resprouting of the shrubs in the water removal and addition plots were not significantly different, although both exhibited significantly more resprouting than the shrubs in the control plots (P<.0001). Across all water treatments, shrubs within the mixed-grass ecotone experiment site exhibited greater canopy resprouting than shrubs within the shrubland experiment site (P<.0001).  Although shrubs in the ecotone compete for water with co-occurring grasses, our between site results suggest that shrubs within the mixed-grass ecotone are more resistant to extreme freeze events than shrubs within the shrubland regardless of water availability.  Our within site results suggest that increasing water availability has a greater effect on shrub resprouting within the shrubland than within the ecotone.