Intra-annual radial growth of beech, Douglas fir and spruce: Differences between pure and mixed stands reveal complementarity between species

Understanding the processes underlying reduced competition in mixed forest stands is essential for assessing forest productivity and stability, yet so far, these processes have rarely been quantified. Here we analyzed intra-annual patterns of radial growth for Fagus sylvatica (beech), Pseudotsuga menziesii (Douglas fir) and Pices abies (spruce) in pure stands and two-species mixtures (beech–spruce, beech–Douglas fir) at a sandy and a loamy site in north-western Germany, recorded with dendrometers from 2019 to 2022. We considered species, site, year, and stand type, and determined the influence of climatic condition. Low temperatures reduced early season growth for beech and Douglas fir. High vapour pressure deficit (VPD) constrained isohydric conifers in summer more strongly than anisohydric beech. Mixing altered growth patterns: spruce concentrated growth earlier in the growing season when mixed with beech; Douglas fir showed a similar shift, but only for the first part of the growing season on the loamy site; and beech concentrated growth later in growing season when mixed with Douglas fir on the sandy site. The mixture effects on intra-annual growth are explained by different leaf phenologies and water use strategies of the species. Linking intra-annual growth with species neighborhoods revealed a complementary use of resources in time between the species.