Leffler A Joshua, Evans Ann S
Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA e-mail:
Oecologia. 1999 May;119(3):311-319. doi: 10.1007/s004420050791.
Water availability is an important factor limiting the productivity of desert plants but little is known about the impact of water-limiting conditions on the physiology of plants in mesic environments. Riparian ecosystems of the western US receive significantly more water than the surrounding desert environments but experience dramatic interannual fluctuations in water availability because both stream flow and precipitation are highly variable over time. This variability results in different growing conditions each year which may influence the physiology of riparian species such as Fremont cottonwood (Populus fremontii), the dominant, native canopy tree species in lowland southwestern US river systems. We wished to determine if the physiology of this species varies among years, what climatic parameters are related to any observed physiological variation and if individuals within a P. fremontii population differ in their physiological response to variation through time. We collected tree ring cores from a central New Mexico cottonwood population and analyzed carbon isotope composition (δC) in each year from 1981 to 1995. We used δC analysis in this study because it allowed us to obtain multi-year estimates of physiological activity. During these years, mean stream flow at our study site ranged over two orders of magnitude from 0.82 to 80.94 m s, precipitation ranged fourfold from 49 to 215 ccmm and mean temperature ranged from 20.5 to 22.6°C during the growing season. δC varied from a low of -26.7‰ in 1984 to a high of -24.7‰ in 1981. Low δC values were associated with years in which stream flow and/or precipitation were high and temperature was low. The opposite was true of years with high δC values. We observed a strong linear relationship between δC and stream flow during years when stream flow was <25 m s but no significant relationship between these variables when stream flow was >25 m s. Additionally, there was a linear relationship between δC and precipitation during years when stream flow was <25 m s but not in years when stream flow was >25 m s. These data suggest that above a threshold of total stream flow, increased flow does not influence physiology. Below this threshold, precipitation can be an important water source. The ten individuals within our study population varied significantly in mean δC values but responded to interannual variation in a similar manner (i.e., all individuals had low δC values when water was abundant). These results suggest that precipitation as well as stream flow are important factors influencing the physiology of this riparian tree.
水分可利用性是限制沙漠植物生产力的一个重要因素，但对于水分限制条件对中生环境中植物生理的影响却知之甚少。美国西部的河岸生态系统比周围的沙漠环境获得的水分要多得多，但由于河流流量和降水量随时间变化很大，其水分可利用性会经历显著的年际波动。这种变异性导致每年的生长条件不同，这可能会影响河岸物种的生理，比如弗里蒙特三角叶杨（Populus fremontii），它是美国西南部低地河流系统中占主导地位的本土树冠树种。我们希望确定该物种的生理是否随年份变化，哪些气候参数与任何观察到的生理变化相关，以及弗里蒙特三角叶杨种群中的个体在对随时间变化的生理反应上是否存在差异。我们从新墨西哥州中部的三角叶杨种群中采集了树芯样本，并分析了1981年至1995年每年的碳同位素组成（δC）。我们在这项研究中使用δC分析，因为它使我们能够获得多年的生理活动估计值。在这些年份里，我们研究地点的平均河流流量范围跨越两个数量级，从0.82到80.94米/秒，降水量变化四倍，从49到215立方厘米/米，生长季节的平均温度范围从20.5到22.6°C。δC值从1984年的低至 -26.7‰变化到1981年的高至 -24.7‰。低δC值与河流流量和/或降水量高且温度低的年份相关。δC值高的年份情况则相反。当河流流量<25米/秒时，我们观察到δC与河流流量之间存在很强的线性关系，但当河流流量>25米/秒时，这些变量之间没有显著关系。此外，当河流流量<25米/秒时，δC与降水量之间存在线性关系，但当河流流量>25米/秒时则不存在。这些数据表明，在总河流流量的一个阈值以上，流量增加不会影响生理。低于这个阈值，降水可能是一个重要的水源。我们研究种群中的十个个体的平均δC值有显著差异，但对年际变化的反应方式相似（即当水分充足时，所有个体的δC值都较低）。这些结果表明，降水以及河流流量是影响这种河岸树木生理的重要因素。
