Photosynthetic responses of different tree species mixtures

Biodiversity regulates several aspects of ecosystem functioning and the delivery of ecosystem services. Many forest ecosystem services, such as timber production and carbon sequestration, are directly related to tree growth and photosynthesis rates.

During the growth of a forest, trees establish relationships  with their neighbours, both at root and at canopy level, depending on different growth rates, space occupation,  strategies and sun/shade tolerance. As far as canopy processes  are concerned, different height and architecture of tree species result in the formation of microenvironments with a variety of light conditions, thus allowing the appearance of shade-tolerant an light demanding species. A mixed forest creates varying illumination conditions which induce different photosynthetic responses in plants at both stand level and within the crown of individual trees.

Plant responses to light can be efficiently measured using chlorophyll fluorescence techniques. Chlorophyll fluorescence allows to quantify the production of chemical energy available for photosynthetic reactions. It provides information on the physiological status of a plant, its ability to utilize sunlight and environmental resources, stress condition and acclimation strategies to environment. In the FunDivEUROPE project the main hypothesis to be tested were:

- There is, at the canopy level, a competition among crowns of different tree species for spaces and light;

- The interactions are species-specific. The kind of the species mixture and the neighboring tree species produces, in mixed forests, more relevant effects than the species richness.

The first results were:

- At Satakunta the maximum quantum yield decreased with species richness in birch, but increased in spruce, because of the two species occupied different positions in the canopy structure;

- At Kaltenborn the maximum quantum yield decreased with species richness in Fagus sylvatica, probably because young and small plants suffer the competition more than vigorous conifers. However the total performance index, upscaled at plot level, increased with species diversity.

Martina Pollastrini - Quantification of the chemical energy available for photosynthetic responses

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