Trees in mixed forests can compete or complement each other, depending on conditions

Public University of Navarre press release: Mixed Scots pine and beech forests can grow more as they complement each other in the use of resources, unless rainfall is low

In an article published in Journal of Ecology, researchers from the NUP/UPNA-Public University of Navarre and the CSIC (Spanish National Research Council) have analysed the two species using data from the Navarrese Pyrenees.

Complementarity between conifers (Scots pine) and broad-leafed species (beech) in the use of the available resources, such as water, may increase the growth of mixed forests comprising both species compared with pure forests, those comprising only one species. However, the lack of rainwater would reduce this advantage in those species that, like the Scots pine, do not tolerate shade, because the increased competition for water would not allow them to take advantage of the lesser rivalry for light.

On the other hand, the effect of growing in a mixed forest is less important for high competitive and shade-tolerant species, as in the case of the beech. This is one of the conclusions reached by researchers at the NUP/UPNA and the Instituto Pirenaico de Ecología of the Spanish National Research Council (CSIC) after studying both species in the Navarrese Pyrenees using data going back three decades.

Their work is included in an article published in one of the world’s leading journals on ecology; Journal of Ecology, published by the British Ecological Society.

The authors of the article are Juan A. Blanco-Vaca, Federico J. Castillo-Martínez, Ester González de Andrés, Bosco Imbert-Rodríguez and Yueh-Hsin Lo of the NUP/UPNA’s Department of Environmental Sciences; and Julio Camarero-Martínez and Gabriel Sangüesa-Barreda of the Pyrenean Institute of Ecology based in Zaragoza.

mixed forest

The study focused on Scots pine (left) and European Beech (right). Photos: Wikipedia Commons

“Mixed forests of conifers and broad-leafed species are increasingly being regarded as ecosystems that could provide strategies for adapting forestry management to climate change,” said Ester González de Andrés, lead author of the article.

Yet in these mixed forests, “little is known about the combined effects of competition between individual trees, the increase in the concentration of atmospheric CO2, and climate,” added Juan A. Blanco.

To shed light on this question, NUP/UPNA and CSIC researchers reconstructed chronologies using data obtained in the course of 34 years (from 1980 to 2013) on annual growth and efficiency in water use in two mixed forests of Scots pine and beech in the Pyrenees, located in the Navarrese towns of Aspurz (municipality belonging Navascués Town Council) and Garde (Roncal valley). They also gathered information on Tree to tree competition (measured as the number of neighbouring specimens and the distances from them to each of the trees studied) and the climate variables provided by nearby weather stations.

“Our work shows how the growth of the two species declined when competition between individuals of the same species increased,” explained Ester González de Andrés. “The Scots pine benefitted from the presence of the beech because there was complementarity between them in the forest located in Garde at a higher altitude and with higher rainfall, but not in the drier forest of Aspurz. Yet the beeches did not display any significant response to the presence of the pine trees in the same forest probably because they are naturally highly competitive and can survive in shady environments created by pines as well as by other beech trees”.

The researchers also discovered that the increase in competition between species modified efficiency in water use. “Although the pine trees are more efficient in the use of water than the beeches,” pointed out Juan A. Blanco, “over the last few years the beeches have become more efficient, apparently to adapt better to dry spells. This has been translated into an increase in growth in the forest of Aspurz, more likely to be affected by drought”.

In the view of the authors of the article, these different behaviours have repercussions on the way these mixed forests can withstand drought, which is expected to gradually increase owing to climate change. “This would take place, above all, in the forests located on the southern limit of the distribution of these species, as is the case of the one located in Aspurz. That is why the effects of species complementarity and their relationship with climate need to be taken into consideration to avoid overestimating the degree of atmospheric carbon fixation which can be carried out by mixed forests of conifers and broad-leafed species,” concluded Ester González de Andrés.

Author lay summary:

Mixed forests of deciduous and conifer trees can be more productive than forests dominated by one species, as long as species in the mixture complement each other when using resources such as light, water or nutrients. However, our research has found that such an advantage can be lost if climate is increasingly drier, as a light-demanding species such as Scots pine cannot take advantage of the better growing conditions created in the mixture by European beech. In contrast, for a shade-tolerant species such as beech, the influence of other conifers seems not to be so important due to its high competitive ability. Both Scots pine and beech trees have increased their efficiency to use water in the later years likely due to the higher concentration of CO2 in the atmosphere and increased frequency and intensity of droughts, which allows trees to capture the same carbon but losing less water through the leaves. When growing in mixtures the efficiency to use water is enhanced for pine and lessened for beech as a result of a different effect of one species over the other. However, using less water for growing only resulted in higher growth for beech trees and this effect has been declining throughout the 20th century. From a management point of view, our results have implications for using mixed forests as a strategy to adapt forestry to climate change, as the impact of droughts changes for tree species and site characteristics. Close to the southernmost point where these species are found, Scots pine may be more benefited from growing in mixtures although the impacts of the drying climate seems to have strong impacts on growth, while European beech seems to be more resilient to drought than previously thought, likely making their mixed forests more resilient to climate change than pure forests but also less able to sequester carbon than previously thought.

Read the full paper; Tree-to-tree competition in mixed European beech-Scots pine forests has different impacts on growth and water-use efficiency depending on site conditions

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