The difference in maintaining a safety margin with regard to hydraulic conductance between pine and oak species influences their distribution in a region. Chir pine (Pinus roxburghii) and banj oak (Quercus leucotrichophora) are the principal species of Central Himalayan forests between 1000 and 2000 m elevations. Nearly 80% of annual precipitation of ~ 1400 mm in the region occurs during monsoon, from mid-June to September, whereafter droughts of varying length and intensity are common. The main objective of the study is to find out the responses of these two evergreen tree species to pre-monsoon (March to mid-June) water stress and topographical heterogeneity that occur in Central Himalaya. We measured soil and tree water potential and osmotic adjustment across five seasons on three slope positions, namely, hill base, mid-slope, and hill top, on north and south slope aspects. Chir pine showed an early response to pre-monsoon drought by restraining daily change in Ψ to 0.89 MPa, while predawn Ψ (Ψ_PD) was still moderate (isohydric tendency). In contrast, the daily reduction in Ψ of banj oak kept on increasing up to 1.49 MPa, despite severely low Ψ_PD (anisohydric tendency). In both tree species, Ψ was invariably lower on south aspect than north aspect and declined from hill base to hill top. Such responses to slope aspect and position, however, were relatively less apparent in chir pine, which tended to maintain a wide safety margin when under stress. As for soil Ψ, banj oak site retained monsoon rainwater more effectively than chir pine.

松树和橡树树种在水力传导方面保持安全裕度的差异会影响它们在一个地区的分布。樱桃松 ( Pinus roxburghii ) 和班吉橡 ( Quercus leucotrichophora) 是海拔 1000 至 2000 米之间喜马拉雅中部森林的主要物种。该地区约 1400 毫米的年降水量的近 80% 发生在 6 月中旬至 9 月的季风期间，此后不同长度和强度的干旱很常见。该研究的主要目的是找出这两种常绿树种对喜马拉雅中部发生的季风前（3 月至 6 月中旬）水分胁迫和地形异质性的响应。我们在北坡和南坡的三个坡度位置，即山底、中坡和山顶，测量了五个季节的土壤和树木水势和渗透调节。樱桃松通过将 Ψ 的每日变化限制在 0.89 MPa 来显示对季风前干旱的早期反应，而黎明前 Ψ (Ψ _PD) 仍然适中 (等水倾向)。相比之下，尽管 Ψ _PD（异水性趋势）非常低，但 banj oak 的 Ψ 每日减少量持续增加至 1.49 MPa。在这两种树种中，Ψ 总是在南面低于北面，并且从山脚到山顶递减。然而，这种对坡向和位置的反应在樱桃松中相对不太明显，在压力下往往会保持较宽的安全裕度。至于土壤 Ψ，banj oak 站点比松树更有效地保留季风雨水。