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Comparison of photosynthesis and water use efficiency between three plant functional types in Hunshandak sandland

NIU Shu-Li, JIANG Gao-Ming~*, GAO Lei-Ming,LI Yong-Geng, JIANG Chuang-Dao, LIU Mei-Zhen, CUI Hong-Xia, DING Li, PENG Yu (Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093,China).  
Stomata controls carbon-water balance in plants by acting as transport for diffusive CO_2 uptake and water vapor loss. Plants with lower photosynthesis and higher transpiration rate may function with less stomatal control and inefficient water usage. To test the hypothesis that tree is improper for the purpose of degraded land restoration in arid or semi-arid environment, we investigated gas exchanges of three plant functional types (PFTs) (3 tree species, 6 shrub species and 25 herb species) in Hunshandak Sandland, China, to compare their carbon assimilation ability and water use efficiency. Our research aimed to look into how those plants with different PFTs design carbon and water cycling in semi-arid areas in order to select the ecologically and economically suitable species for the seek of controlling of sandland degradation. We investigated the typical species of each PFT, i.e. Ulmus pumila (tree), Salix gordejevii (shrub) and Leymus chinensis (herb) to compare their diurnal gas exchange changes. The results showed that trees had significantly lower photosynthetic rates (A) and water use efficiency (WUE), but higher transpiration rate (E) and stomatal conductance (g_s), compared with herbs and shrubs. Tree species decreased their A, E and g_s all along from the maximal values in the morning and could not recover in the afternoon, while shrubs and herbs reopened their g_s after the midday depression. The diurnal changes of g_s indicated the less stomatal flexibility of stomata in trees than herbs and shrubs. Furthermore, all the trees (3 species),shrubs (6 species) and the dominant herbs (25 species) were compared for the stomatal control and water use efficiency among PFTs. Among all the species with different PFTs, there were significantly positive correlations between g_s and A and E. However, tree species displayed lower A but higher E than shrubs and herbs at the same stomatal conductance, indicating that tree species assimilate less CO_2 and transpire more water per unit g_s. Lower photosynthetic capacity and larger transpiration rate induced less water use efficiency in trees than in shrubs and herbs. In addition, g_s in trees were more possible to suffer from the drought as reflected by the sensitivity of g_s to varied leaf water potential. These results might suggest that trees had poor stomatal regulation in controlling carbon-water balance and more sensitive to drought which was common in Hunshandak Sandland. From this study, we recommend that some native herbs, such as Leymus chinensis and Agriophyllum squarrosum and shrubs, like Salix matsudana Koidz, Salix gordejevii L., rather than trees should be selected for the sandland restoration.
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