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A scientist from Eesti Maaülikool gives a new explanation for enhanced competitiveness of evergreens under global change

Ülo Niinemets from Eesti Maaülikool, the Estonian University of Life Sciences together with Spanish colleagues proposes a new hypothesis explaining the movement of biomes (macroecosystems) under globally changing climate. 

Top journal in ecology Trends in Ecology & Evolution publishes in its March issue an article by Professor Ülo Niinemets from the Estonia University of Life Sciences, Jaume Flexas from the Universitat de les Illes Balears and Josep Peñuelase from Universitat Autònoma de Barcelona, which provides a novel explanation for the expansion of evergreen vegetation in many Earth ecosystems, and adds a new perspective to the research of the biological effects of increasing atmospheric CO2.

«In recent decades evergreen trees and bushes have expanded their habitat at the expense of deciduous forests in several Earth ecosystems», explained Ülo Niinemets. «For example, in Western Europe holly (Ilex aquifolium), a broad-leaved evergreen, has spread northwards. In the Mediterranean countries, evergreen holm oaks (Quercus ilex) and common box (Buxus sempervirens) have been shifting towards higher elevations in the mountains, where they replace beech forests. Similar shifts are occurring in Central and West Europe where several non-native evergreen shrubs have invaded deciduous forest understories».

So far the increased competitiveness of evergreen plants has been explained by global warming and higher temperatures, but recent severe winters have raised the question whether and to what extent the warming climate is really responsible for the increased spread of evergreens.

In their article in Trends in Ecology & Evolution Niinemets, Flexas and Peñuelas state that the well-documented global change — an increase of atmospheric CO2 itself can be responsible for the increase of the competitiveness of evergreen plants.

This can be explained by the fact that the foliage structure of evergreen plants differs from that of the deciduous plants. As the evergreen leaves have to «endure» several growth periods, the cells in their leaves have been densely packed and have thick cell walls. They contain more structural tissue not involved in photosynthesis. Due to their more robust structure, the CO2 diffusion into chloroplasts, where photosynthesis takes place, is limited to a greater degree, and consequently, CO2 limits photosynthesis more than in deciduous plants. Thus, the increase in ambient CO2 concentration enhances the photosynthesis of evergreens more than in deciduous species.

For more information: Ülo Niinemets, Professor at Eesti Maaülikool, phone 5345 7189