Adult mortality of chaparral shrubs following severe drought.

摘要

While drought is an annual event in chaparral systems, adult mortality of the woody shrubs is extremely rare. We examined a chaparral shrub community that was at an ecotone with a desert system and that had experienced widespread dieback following a severe 5-year drought. The initial aim of the study was to quantify patterns of dieback, detect whole plant mortality, and make comparisons of dieback among seven co-dominant evergreen sclerophyllous species. Since mortality did occur, and varied dramatically among species, we attempted to relate the mortality to post fire regeneration strategy and to various physiological parameters such as xylem resistance to cavitation, specific leaf area, predawn and midday xylem pressure potential and leaf stomatal conductance, and transpiration rate. As expected, no general relationship between water stress and mortality was observed. Contrary to predictions, mortality was significantly higher in the two nonsprouting species, which had the most cavitation-resistant xylem and lowest specific leaf area. An explanation for the surprising relationship between mortality and water stress resistance is offered, based on rooting depth. Deep rooted species appear to avoid water stress, possibly by accessing deep water resources available even during the prolonged drought. Shallow rooted species (with roots 1 m deep) are denied this resource so they must tolerate water stress during annual summer droughts. They were apparently unable to do so during the severe 4-year drought from 1998 to 2002.