Department of Biology, University of Utah, Salt Lake City, Utah 84112 USA.
Am J Bot. 2000 Sep;87(9):1287-99.
We studied 15 riparian and upland Sonoran desert species to evaluate how the limitation of xylem pressure (Ψ(x)) by cavitation corresponded with plant distribution along a moisture gradient. Riparian species were obligate riparian trees (Fraxinus velutina, Populus fremontii, and Salix gooddingii), native shrubs (Baccharis spp.), and an exotic shrub (Tamarix ramosissima). Upland species were evergreen (Juniperus monosperma, Larrea tridentata), drought-deciduous (Ambrosia dumosa, Encelia farinosa, Fouquieria splendens, Cercidium microphyllum), and winter-deciduous (Acacia spp., Prosopis velutina) trees and shrubs. For each species, we measured the "vulnerability curve" of stem xylem, which shows the decrease in hydraulic conductance from cavitation as a function of Ψ(x) and the Ψ(crit) representing the pressure at complete loss of transport. We also measured minimum in situ Ψ(x)(Ψ(xmin)) during the summer drought. Species in desert upland sites were uniformly less vulnerable to cavitation and exhibited lower Ψ(xmin) than riparian species. Values of Ψ(crit) were correlated with minimum Ψ(x). Safety margins (Ψ(xmin)-Ψ(crit)) tended to increase with decreasing Ψ(xmin) and were small enough that the relatively vulnerable riparian species could not have conducted water at the Ψ(x) experienced in upland habitats (-4 to -10 MPa). Maintenance of positive safety margins in riparian and upland habitats was associated with minimal to no increase in stem cavitation during the summer drought. The absence of less vulnerable species from the riparian zone may have resulted in part from a weak but significant trade-off between decreasing vulnerability to cavitation and conducting efficiency. These data suggest that cavitation vulnerability limits plant distribution by defining maximum drought tolerance across habitats and influencing competitive ability of drought tolerant species in mesic habitats.
我们研究了 15 种河岸和旱地索诺兰沙漠物种，以评估木质部压力（Ψ（x））限制的空化如何与植物沿湿度梯度的分布相对应。河岸物种是专性河岸树种（Fraxinus velutina、Populus fremontii 和 Salix gooddingii）、本地灌木（Baccharis spp.）和外来灌木（Tamarix ramosissima）。旱地物种为常绿（Juniperus monosperma、Larrea tridentata）、旱季落叶（Ambrosia dumosa、Encelia farinosa、Fouquieria splendens、Cercidium microphyllum）和冬季落叶（Acacia spp.、Prosopis velutina）乔木和灌木。对于每种物种，我们都测量了茎木质部的“脆弱性曲线”，该曲线显示了随着 Ψ（x）的减小，水力传导率从空化中下降的情况，以及代表完全丧失运输能力的 Ψ（crit）压力。我们还测量了夏季干旱期间最小的原位 Ψ（x）（Ψ（xmin））。沙漠旱地物种的空化脆弱性均匀降低，并且 Ψ（xmin）值低于河岸物种。Ψ（crit）值与最小 Ψ（x）相关。安全裕度（Ψ（xmin）-Ψ（crit））往往随着 Ψ（xmin）的减小而增加，并且安全裕度足够小，以至于相对脆弱的河岸物种在旱地栖息地中经历的 Ψ（x）（-4 至-10 MPa）下无法进行水分运输。在河岸和旱地栖息地中保持正安全裕度与夏季干旱期间茎空化的最小增加相关。在河岸带缺乏较不脆弱的物种可能部分是由于空化脆弱性降低和导水效率之间存在微弱但显著的权衡关系。这些数据表明，空化脆弱性通过在不同栖息地定义最大耐旱性来限制植物分布，并影响耐旱物种在湿润栖息地中的竞争能力。
