title: Responses of sap flow, leaf gas exchange and growth of hybrid aspen to elevated atmospheric humidity under field conditions.
abstract: An increase in average air temperature and frequency of rain events is predicted for higher latitudes by the end of the 21st century, accompanied by a probable rise in air humidity. We currently lack knowledge on how forest trees acclimate to rising air humidity in temperate climates. We analysed the leaf gas exchange, sap flow and growth characteristics of hybrid aspen (Populus tremula × P. tremuloides) trees growing at ambient and artificially elevated air humidity in an experimental forest plantation situated in the hemiboreal vegetation zone. Humidification manipulation did not affect the photosynthetic capacity of plants, but did affect stomatal responses: trees growing at elevated air humidity had higher stomatal conductance at saturating photosynthetically active radiation (gs sat) and lower intrinsic water-use efficiency (IWUE). Reduced stomatal limitation of photosynthesis in trees grown at elevated air humidity allowed slightly higher net photosynthesis and relative current-year height increments than in trees at ambient air humidity. Tree responses suggest a mitigating effect of higher air humidity on trees under mild water stress. At the same time, trees at higher air humidity demonstrated a reduced sensitivity of IWUE to factors inducing stomatal closure and a steeper decline in canopy conductance in response to water deficit, implying higher dehydration risk. Despite the mitigating impact of increased air humidity under moderate drought, a future rise in atmospheric humidity at high latitudes may be disadvantageous for trees during weather extremes and represents a potential threat in hemiboreal forest ecosystems.
authors.affiliation: Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia.
authors.forename: Aigar
authors.initials: A
authors.lastname: Niglas
authors.affiliation: Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia.
authors.forename: Priit
authors.initials: P
authors.lastname: Kupper
authors.affiliation: Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Kreutzwaldi 5, 51014 Tartu, Estonia.
authors.forename: Arvo
authors.initials: A
authors.lastname: Tullus
authors.affiliation: Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia arne.sellin@ut.ee.
authors.forename: Arne
authors.initials: A
authors.lastname: Sellin
keywords: Atmospheric humidity; canopy conductance; climate change; net photosynthesis; photosynthetic capacity; relative stomatal limitation; stomatal conductance; water-use efficiency.