Extended canopy closure under with mild autumns, and its effect on the understorey growing season

In a new paper just out in Agricultural and Forest Meteorology, we explore how extended canopy closure in oak and birch forests at Lammi Biological Station in southern Finland affects the growing season for common understorey forbs.

Undergraduate student Arthur Daviaud sampling the understorey at Lammi Biological Station for the project which lead to this paper. Arthur was supervised by Maxime Durand (University of Helsinki) and Matthew Robson (University of Cumbria) from CanSEE group.

The paper describes an experiment to test the effects of climate warming which may extend the length of time that canopy trees in a forest keep their leaves into the winter. One puzzle associated with climate change is whether plants growing on the forest floor benefit more from the extended warm periods in the spring and autumn, or whether these warm periods mean that canopy trees retain their leaves for longer and thus restrict the light reaching the ground and available there for photosynthesis.

Autumn colouration of iconic understorey forb Hepatica nobilis
Autumn colouration of iconic understorey forb Hepatica nobilis

In an experiment comparing stands of oak, birch and spruce, we tested the potential for photosynthesis, retention of chlorophyll, and colouration of understorey plant species on the forest floor throughout autumn and the start of winter. Our main finding was that the increase in light received and change in its spectral composition following canopy leaf fall was the main trigger of senescence in the understorey. Those understorey species able to keep photosynthesizing effectively into late autumn were benefitting the most from warmer temperature and an extended closed canopy period.

Spruce scene in summer and winter
Seasonal changes in the boreal spruce forest

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