Above- and below-ground phenology of four tree species in mixed forests and monospecific stands

Abstract

The phenology of tree root growth is poorly understood especially for mixed forests. To attempt to fill this knowledge gap, we selected four co-occurring broadleaf trees species of the European forests (Quercus robur, Carpinus betulus, Tilia cordata and Acer platanoides). For these species growing in monocultures and mixtures, we monitored root growth, crown development and soil temperature over 2 years. Our investigation revealed three key findings: (1) Root phenology showed species-specific patterns in monocultures and mixtures, with mixed stands showing patterns which reflected the dominate species. Acer and Carpinus showed earlier growth peak than species Quercus and Tilia. Carpinus and Quercus maintained winter root growth at temperatures as low as 4.2oC, whereas Acer initiated growth in spring at higher temperatures (8.7-8.9oC). (2) Above-ground and below-ground phenological events showed clear asynchrony which was significantly amplified in mixed stands compared to monocultures, with the interval between full leaf expansion and peak root production being longer in mixtures (approximately 5 weeks) than in monocultures (2-3 weeks). Root growth initiation consistently preceded leaf emergence in both monocultures and mixtures, but peak production occurred after full crown development. (3) Species mixing seasonally modified the temperature-growth relationship, with positive correlations observed in winter and negative correlations in summer. Our findings demonstrate that the patterns of root growth modified in mixtures which cannot be predicted from the monocultures. These results highlight the importance of considering below-ground dynamics and species interactions when predicting forest ecosystem responses to climate change.