In plants, like other organisms, physiological limits define environmental conditions allowing survival and growth. For most species, especially evergreens, the rare extremes of local climate may exceed physiological limits, killing plants or severely limiting their function. The effects of extreme events can persist for decades, affecting the ecological (growth, reproduction, species composition, etc.) and sociological (aesthetic qualities, land value, freshwater, etc.) functions of that ecosystem. This project will test predictions from past measurements of the physiological limits of creosotebush (Larrea tridentata), a widespread evergreen desert shrub dominating millions of acres in the southwest, following an extreme freeze in central and southern New Mexico in February 2011. The project will measure freezing damage and recovery of creosotebush in central and southern NM, test the ability of characteristics identified in recent research to predict the damage caused by the natural freeze and, use satellite images to quantify the broad patterns of plant damage (measured as plant canopy greenness) throughout the season and link them to our present understanding of the physiological limits of creosotebush. We expect known physiological limits will explain immediate post-freeze damage but mortality in plot and satellite measurements will be reduced by regrowth from roots. This project will improve our understanding and ability to predict plant responses to extreme events and the large-scale effects of these responses. We need to understand the response to extreme events because ongoing climate change may alter them and thus their effect on vegetation. This project will train a postdoctoral scientist and two graduate students and results will be disseminated to primary and undergraduate students and the general public.
This project was designed to assess how plants respond to rare severe freezing events, which may have a large effect on natural vegetation even though they occur infrequently. In February 2011, temperatures in New Mexico dropped to -30 degrees Celsius, more than 15 degrees below the normal minimum temperature. This project tested the predictions of previous laboratory studies of the effects of severe freezing on desert shrubs following this natural extreme event. We tagged 1200 plants and followed their response. Consistent with the predictions of laboratory studies of freezing tolerance, we found that nearly all of them lost all of their leaves while a small fraction showed moderate or even no damage. Surprisingly, only 3 of the 1200 plants were killed despite the severitiy of the freeze. The rest resprouted from buds on branches or at the top of the root system and after two years these plants were substantially recovered. We performed physiological measurements on groups of plants that showed different degrees of damage following the natural freeze but these measurements failed to explain why a few plants were not severely effected while most were heavily damaged. Finally, we developed methods for detecting the change in shrub condition following events like the severe freeze of 2011 that use satillite images. The project supported one undergraduate researcher, two graduate students and one postdoctoral fellow. The results are being prepared for publication in the scientific literature.
