Different stomatal behaviors among species under drought conditions may affect species-specific seasonal variations in photosynthesis and water use efficiency (WUE), and could enable species to follow differing growth strategies. Here we monitored leaf gas exchange, intra-annual radial growth, leaf stoichiometry, and microclimate of three conifers with contrasting stomatal behaviors (isohydric Picea wilsonii and Pinus tabuliformis vs. anisohydric Juniperus przewalskii) over three growing seasons, and collected tree-ring records of the monitored species, on the cold and arid northeastern Tibet Plateau. We used these data to assess the species-specific seasonal variations in net photosynthetic rate (A_n), transpiration efficiency (TE) and intrinsic water use efficiency (WUEi), and their impacts on growth.

Our observations show that the A_n of all three conifers was greatest in autumn. This peak was closely related to increasing stomatal conductance following alleviation of drought stress, rather than to variations in leaf nitrogen concentrations. WUEi of the three conifers did not exhibit a regular seasonal pattern, but TE was significantly lower in summer than in spring and autumn, due to the regulation by vapor pressure deficit (VPD). Increasing summer VPD reduced TE mainly by increasing transpiration rate for J. przewalskii and by decreasing A_n for the isohydric conifers.

The increases of A_n from summer to autumn were much greater for the two isohydric Pinaceae plants than for J. przewalskii. This was consistent with the growth-climate relationships, in which the autumn drought in the previous year limited radial growth of the isohydric conifers more strongly than in the anisohydric conifer. Therefore, we suggest that the growth of isohydric species may profit more from the photosynthates accumulated during the previous post-growing season in the cold and arid Tibetan Plateau.

干旱条件下物种之间不同的气孔行为可能会影响光合作用和水分利用效率 ( WUE ) 的物种特定季节性变化，并使物种能够遵循不同的生长策略。在这里，我们监测了三种具有对比气孔行为的针叶树（等水云杉和油松与等水杜松）在三个生长季节的叶气体交换、年内径向生长、叶子化学计量和小气候，并收集了监测的物种，在寒冷干旱的青藏高原东北部。我们使用这些数据来评估净光合速率 ( A _n)、蒸腾效率 ( TE ) 和内在水分利用效率 ( WUEi ) 及其对生长的影响。

我们的观察表明，所有三种针叶树的A _n在秋季最大。这个峰值与干旱胁迫缓解后气孔导度的增加密切相关，而不是与叶片氮浓度的变化有关。WUEi三个针叶树的没有表现出一个定期的季节性图案，但TE在夏天显著低于在春季和秋季，由于蒸气压赤字（调节VPD）。增加夏季VPD主要通过增加普氏野猪的蒸腾速率和降低等水针叶树的A _n来降低TE。

两种等水松科植物从夏季到秋季的A _n增加量远大于普氏野猪。这与生长气候关系一致，其中前一年的秋季干旱限制了等水针叶树的径向生长，而不是等水针叶树。因此，我们认为在寒冷干旱的青藏高原上一个生长后期积累的光合物质可能对等水物种的生长有更多的好处。