Regrowth of desert shrubs following a natural extreme cold event

Background/Question/Methods

Many of the world’s grasslands include competitive woody species that can encroach on grasslands, altering community dynamics and ecosystem function. Most research examining shrub response to climate change focuses on altered precipitation regimes or increased mean temperature, yet discrete extreme climate events could greatly influence shrub encroachment. Larrea tridentata (creosote bush) is a native shrub in warm deserts of southwestern USA and currently encroaches Chihuahuan Desert grasslands. While L. tridentata commonly experiences sub-freezing temperatures, low temperatures are thought to strongly influence the northern range boundary. To examine how L. tridentata responds to natural extreme low temperature events at a scale unapproachable by experimental manipulation, we measured populations at the Sevilleta National Wildlife Refuge in central New Mexico USA following a 25- to 50-year extreme freeze. On February 3, 2011 T_min reached -31 °C, roughly 20 °C below mean T_min. Following the freeze, plots were established at 6 sites to evaluate shrub response and regrowth following extreme freeze. Initial surveys indicated that freeze-induced canopy death was extensive. Shrubs were re-surveyed in winter and spring 2012 to determine canopy regrowth and reproduction responses during the year following the extreme freeze event. 

Results/Conclusions

During the year following the extreme cold event, canopy regrowth varied greatly across the sites. In general, areas with lower shrub density within the grassland-shrubland ecotone experienced higher regrowth than dense areas within shrublands. Shrubs in high density shrublands, where initial die-back was greatest, experienced the slowest regrowth, with canopy sizes averaging 6 to 13% of pre-freeze canopies. Regrowth at ecotone sites averaged 20-30% of previous canopy sizes. One exception was a mixed shrubland site where regrowth reached 57% of previous sizes. Despite massive canopy die-back, many shrubs were reproductive during the following season, but seed set also varied with shrub density. Within shrublands, ~1% of shrubs produced seeds while within ecotones 40% of shrubs were reproductive. At the ecotone site with substantially greater regrowth, nearly 80% of shrubs produced seed. Although the natural extreme cold event caused extensive canopy dieback, shrub death was extremely low and only 9 of 1133 shrubs showed no regrowth. In general, ecotone shrubs had higher resistance and resilience to the extreme low temperature event. Although low winter temperatures dictate the northern limit of L. tridentata, periodic extreme cold events do not result in extensive mortality that would slow shrub expansion within semiarid grasslands.