Water regime of herbaceous plants on a soil moisture gradient. I. Transpiration rate

摘要

The transpiration rate was analysed for 61 species of herbaceous plants from 16 communities growing on a wide soil moisture gradient. On a soil moisture gradient all array of input data of transpiration rate has average value x=1.538 gH2O gdry mass-1 per h. The typical values of transpiration rate of herbaceous plants on a soil moisture gradient do not exceed 3 gH2O gdry mass-1 per h. The ranges of temperature and air moisture deficit optimal for transpiration are 20-28 degC and 5-24 mb, respectively. The most striking distinctions in transpiration velocity are observed between monocotyledonous and dicotyledonous plant species. The respective average values of this character are 1.358 and 1.759 gH2O gdry mass-1 per h. Plants with denser mesophyll (other things being equal) evaporate slower, than species with more intercellular space in leaves. In all habitats leaves of monocotyledons are denser than those of dicotyledons, that apparently determines distinctions in the evaporation velocity by theseplant groups. Monocotyledonous succulents evaporate water much more intensively than monocotyledonous sclerophytes. However, water deposits in sclerophytes are refreshed twice faster than in succulents. The width of a dispersion zone of species average levels is increased from arid to humid part of the gradient. It is illustrated by comparison of trends of species average values of transpiration rate minimal and maximal for each community. A study of coefficients of variation, a gradual dilating is revealed of a zone where these coefficients exist against the soil moisture gradient. In humid part of the gradient the variation of transpiration rate is on the same level for all plants. As the holard decreases, the range, where species average coefficients of variation are observed, extends. The suppositions are made: (i) in grassy communities of arid ecosystems a rather high resemblance of realization of transpiration process in different plants can be determined by a rigid ecotopic selection and weaklyexpressed competition between plants. An observable diversity of average levels of water evaporation in humid ecotopes is a reflection of econiche differentiation; (ii) the interspecific distinctions on the soil moisture gradient are "structure-specific", that is depending on belonging to taxa of such a high rank as monocotyledons and dicotyledons; (ii) in conditions with limited soil water store plants avoid an interspecific competition for water by not only differentiation of econiches, but also owing to different lability in use of the same soil resource; (iv) a low flexibility of transpiration rate in humid ecotopes is related to a low complexity of organization of water regime of the plants.