Elevated atmospheric CO2 and humidity delay leaf fall in Betula pendula, but not in Alnus glutinosa or Populus tremula x tremuloides

Context Anthropogenic activity has increased the level of atmospheric CO2, which is driving an increase of global temperatures and associated changes in precipitation patterns. At Northern latitudes, one of the likely consequences of global warming is increased precipitation and air humidity. Aims In this work, the effects of both elevated atmospheric CO2 and increased air humidity on trees commonly growing in northern European forests were assessed. Methods The work was carried out under field conditions by using Free Air Carbon dioxide Enrichment (FACE) and Free Air Humidity Manipulation (FAHM) systems. Leaf litter fall was measured over 4 years (FACE) or 5 years (FAHM) to determine the effects of FACE and FAHM on leaf phenology. Results Increasing air humidity delayed leaf litter fall in Betula pendula, but not in Populus tremula × tremuloides. Similarly, under elevated atmospheric CO2, leaf litter fall was delayed in B. pendula, but not in Alnus glutinosa. Increased CO2 appeared to interact with periods of low precipitation in summer and high ozone levels during these periods to effect leaf fall. Conclusions This work shows that increased CO2 and humidity delay leaf fall, but this effect is species-specific.