Are changes in solar radiation and spectral composition reflected in leaf optical properties of forest understorey species?

Sunflecks and shaded areas on the forest floor in one of
our field sites at Lammi Biological Station

In the understorey, shade-loving plants live their whole life on the forest floor. Processes like leaf reflectance, transmittance and absorbance, determine the composition of radiation that reaches the understorey. We assist to the formation on the forest floor of micro-sites with different light conditions: shade and sunflecks, a brief increase of solar irradiance that occurs in the understorey when sunlight is able to directly reach the forest floor. As a result, understorey species are exposed to a very dynamic light environment during the day, meaning that they must develop mechanisms to quickly exploit the fluctuating PAR (Photosynthetic Active Radiation) in conditions where light is limiting to growth. The plants’ capacity to assimilate carbon under different species canopy cover determines the community structure and consequently alters nutrient cycling and the forest ecosystem  functioning. As my PhD focuses on the impact of spectral composition on ecosystem processes, we thought it would be interesting to understand how this function varies as a consequence of different lights conditions to which the understorey plants themselves are exposed every day. 

The aim of this study is to monitor the response of understorey plants to the transit of a sunfleck across the predominately-shaded forest environment and to assess how the season and structure of forest canopies, affecting the quantity and quality of irradiance, moderate the photosynthetic response of understorey plants to sunflecks. I focused on four early-spring ephemeral species (Anemone nemorosa L., Oxalis acetosella L., Fragaria vesca L., Hepatica nobilis Schreb.). Furthermore, I characterised the duration and intensity of several sunflecks in forest stands with different canopy species in Finland and Spain during spring and summer 2016 and 2017. Changes in leaf properties passing from shade through a sunfleck have been monitored through time-series of chlorophyll fluorescence (measured with a Walz mini-PAM) and leaf temperature (measured with an infrared thermometer).

My PhD supervisor Matt Robson with me
observing sunflecks in our field site

Preliminary results show a very fast response of the photosynthetic apparatus to sunfleck passage, on a timescale of seconds. The photosynthetic yield changed quickly when the leaves were exposed to different light conditions and recovered quickly from its sunfleck-depression on return to shade. Furthermore, in some of the studied species we observed an increase of the photosynthetic yield just before the leaves reached the edge of the sunfleck. This suggests that the plant is primed for the arrival of the sunfleck, probably by a spectral cue. The temperature of the leaves also changed during the transit of the sunfleck, just before the arrival of the sunfleck the temperature decreased, indicating that the plant can perceived the light changes as a cue to open the stomata and increase transpiration.

A coloured-film filter over beech seedling
during a sunfleck at our filed site in Spain

In order to detect the spectral cues that might be responsible of the priming of the plants we repeated the experiment in summer 2017, but now filters have been used to manipulate the solar spectrum. This meant that we could experimentally test the plant response to different light qualities in an attempt to identify the spectral cue that enables the plant to detect the sunfleck just before its arrival. This is only a small part of my PhD but it can have very important impact on the analysis and modelling of carbon cycle in the forest ecosystem. I am looking forward to go on with my project on this interesting topic and I wish to thank Lammi Biological Station for the opportunity to perform my research and the help that I received. 

Marta Pieriste of the University of Helsinki is a 2016 grant recipient