As growing seasons in the northwestern USA lengthen, on track with climate predictions, the mixed conifer forests that dominate this region will experience extended seasonal drought conditions. The year of 2015, which had the most extreme drought for the area on record, offered a potential analogue of future conditions. During this period, we measured the daily courses of water potential and gas exchange as well as the hydraulic conductivity and vulnerability to embolism of six dominant native conifer species, Abies grandis, Larix occidentalis, Pinus ponderosa, Pinus monticola, Pseudotsuga menziesii and Thuja occidentalis, to determine their responses to 5 months of record-low precipitation. The deep ash-capped soils of the region allowed gas exchange to continue without significant evidence of water stress for almost 2 months after the last rainfall event. Midday water potentials never fell below -2.2 MPa in the evergreen species and -2.7 MPa in the one deciduous species. Branch xylem was resistant to embolism, with P 50 values ranging from -3.3 to -7.0 MPa. Root xylem, however, was more vulnerable, with P 50 values from -1.3 to -4.6 MPa. With predawn water potentials as low as -1.3 MPa, the two Pinus species likely experienced declines in root hydraulic conductivity. Stomatal conductance of all six species was significantly responsive to vapour pressure only in the dry months (August-October), with no response evident in the wet months (June-July). While there were similarities among species, they exhibited a continuum of isohydry and safety margins. Despite the severity of this drought, all species were able to continue photosynthesis until mid-October, likely due to the mediating effects of the meter-deep, ash-capped silty-loam soils with large water storage capacity. Areas with these soil types, which are characteristic of much of the northwestern USA, could serve as refugia under drier and warmer future conditions.

中文翻译：

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爱达荷州北部的六个同时出现的针叶树种在极端干旱的一年中表现出连续的水力策略。

随着美国西北部生长季节的延长以及气候预测的发展，主导该地区的针叶林混交林将经历长期的季节性干旱条件。2015年是该地区有记录以来最严重的干旱年份，提供了可能与未来状况相类似的年份。在此期间，我们测量了六种主要原生针叶树种Abies grandis，Larix occidentalis，Pinus tankerosa，Pinus monticola，Pseudotsuga menziesii和Thuja occidentalis的水势和气体交换的每日过程以及水力传导率和对栓塞的脆弱性，确定他们对5个月以来的最低降水的反应。在上一次降雨事件发生后的近两个月中，该地区深厚的灰烬土壤使气体交换得以继续进行，而没有明显的水分胁迫迹象。常绿树种的午间水势从未低于-2.2 MPa，一种落叶树种的午间水势从未低于-2.7 MPa。分支木质部具有抗栓塞性，P 50值为-3.3至-7.0 MPa。然而，根木质部更易受伤害，P 50值从-1.3到-4.6 MPa。黎明前的水势低至-1.3 MPa，这两个松属树种的根系水力传导率均可能下降。仅在干燥月份（八月至十月），所有六个物种的气孔电导率才对蒸汽压显着响应，而在潮湿月份（六月至七月）则没有明显响应。虽然物种之间有相似之处，它们表现出连续的等水度和安全系数。尽管干旱严重，所有物种仍能够继续进行光合作用直至10月中旬，这很可能是由于米深，灰分覆盖的粉质壤土具有较大的蓄水能力所引起的。这些土壤类型的地区是美国西北大部分地区的特征，在干燥和较暖的未来条件下可以作为避难所。