How Tree Stand Phenology Determines Understorey Senescence - a Case Study from Boreal Forests

Leaf fall in the autumn opens the forest canopy, allowing more solar radiation to be transmitted to the forest floor. Those understorey species that remain physiologically active into the autumn may benefit from the sunlight received by extending their growing season, to assimilate additional carbon while conditions remain favourable. We monitored leaf water and pigment content, as well as photosynthetic capacity in understorey species growing in adjacent stands differing in their canopy tree species. Leaf fall, transmitted light, and microclimate were monitored in each stand. We found that overstorey leaf fall started earlier in the birch (Betula pendula, L.), than in the oak (Quercus robur, L.) stand, and light transmission changed accordingly. Concurrently, understorey leaf senescence was generally earlier in the birch than in the oak stand, itself earlier than in the evergreen spruce stand (Picea abies, L. H. Karst.). Neither atmospheric CO2, humidity, nor temperature differed between stands. A change in light quality and/or increase in quantity following leaf fall drove the difference in the timing of senescence in the understorey. Understorey species with later senescence were able to use the increased light more after leaf fall. Together these findings help to provide a mechanistic foundation to predict how ecosystem functioning and ultimately carbon balance will be impacted by phenological shifts in response to global changes.