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.