Quantifying the impact of management on the three-dimensional structure of boreal forests

Forest management practices modify the composition and three-dimensional structure of boreal forests. Such changes affect light regimes, microclimate and habitat availability in forests, thus contributing to the regulation of ecosystem functioning and biodiversity. Compared to traditional even-aged forestry, prevalent in Fenno-scandia since the mid-20th century, uneven-aged forestry has emerged as an alternative approach that may lead to more diverse and resilient forests, as well as a more sustainable provision of ecosystem services. However, the biophysical impacts of uneven-aged strategies have not yet been robustly quantified. Here, we present a detailed assessment of the structural characteristics of even-aged and uneven-aged boreal forest stands in southern Finland, aiming to clarify how these different management approaches regulate three-dimensional forest structure. Using terrestrial laser scanning, we derived and compared several structural traits closely linked to forest functioning. We found that, although different management types result in similar patterns in total plant material, they differ significantly on how plant material is allocated within vertical strata. Moreover, we demonstrated that commonly used structural diversity metrics cannot not capture the morphological intricacies arising from the three-dimensional dispersion of plant material inside the forest stand. Our results highlight the dynamic nature of even-aged forests, where the structural characteristics change with the rotation, whereas uneven-aged forest structures are more stable over time. Precise quantification of forest structure will help to understand which attributes impact the environmental functions and conditions that modify ecosystem services, and how these are altered with forest management.