Air humidity as an ecological factor for woodland herbs: leaf water status, nutrient uptake, leaf anatomy, and productivity of eight species grown at low or high vpd levels

Abstract

Summary Ecologists have widely neglected the long term impact of vpd (the water vapor saturation deficit of the air) on plant growth and development. This paper tests the hypothesis that the prevailing vpd level during growth has a significant influence on the ecology of temperate woodland herbs, independently of soil moisture status. Eight herbaceous species ( Scrophularia nodosa, Digitalis purpurea, Campanula trachelium, Rumex sanguineus, Geum urbanum, Hieracium sylvaticum, Hordelymus europaeus, Brachypodium sylvaticum ) were grown in climate chambers at four different saturation deficits (vpd = 120, 580, 930 or 1210 Pa) with unlimited water supply in solution cultures in order to experimentally analyse long-term effects of vpd on shoot water relations, plant morphology and plant growth rate. In their natural forest floor environment these species are exposed to vpd levels of 200 to 600 Pa on summer days. After 50 to 90 days of cultivation I compared the four vpd treatments with respect to dry matter production, leaf anatomy, leaf conductance, transpiration rate, bulk leaf water potential (χ l ), leaf water content (θ l ), and shoot nutrient concentrations. Productivity was significantly higher at 120 than at 580, 930 or 1210 Pa in seven of eight species. Plants grown at low vpd had larger leaves and leaf epidermal cells, and a lower stomatal frequency than plants of high-vpd treatments which indicates a stimulation of leaf expansion by high air humidities. Low saturation deficits also affected leaf anatomy (less compact leaf mesophyll with larger fractional air space) but did not alter mesophyll cell dimensions. Leaf water potential and leaf water content were more favorable at 120 Pa than at higher vpd levels in all eight species although a clear dependence of leaf water status (χ l and θ l ) on transpiration rate did not exist. Shoot nutrient concentrations generally were lower at low than high vpd which reflects a nutrient dilution effect caused by an increased carbon assimilation of the low vpd plants. In contrast, no correlation was found between transpiration rate and shoot concentrations of Ca or other mineral nutrients. I conclude that vpd acts as a soil water-independent growth-controlling factor for hygromorphic woodland herbs. Environments with a vpd of 100 to 500 Pa improve leaf water status and stimulate leaf expansion of these species which leads to enhanced growth. Other growth-relevant parameters (leaf conductance, transpiration rate and nutrient uptake), which were also affected by vpd, had only a minor or no influence on productivity.