Effects of forest management on stand structural variability and microclimate in boreal forests

In boreal forests, uneven-aged management has recently become an attractive alternative to even-aged rotation forestry. These different management types, based on selection felling and clear-cutting, respectively, can result in substantial differences in the structural characteristics of  forest stands. Therefore, management modifies net surface radiation and heat fluxes and further regulates microclimatic variability important for forest organisms and ecosystem processes. Yet, the magnitude and structural drivers of microclimate variability in managed boreal forests have remained poorly understood. Here, we studied the stand structure and microclimate of 20 study plots including even-aged and uneven-aged forest stands in the Vesijako Research Forest in southern Finland. We used terrestrial laser scanning (TLS) to quantify the structural characteristics of the sites and measured soil and air temperature with 80 microclimate loggers in 2021–2022. The TLS data showed that the total amount of plant material did not differ between the management types. However, there were significant structural differences in vertical layering and horizontal heterogeneity of vegetation. Our preliminary results show significant differences in microclimate temperatures depending on stand age category. These differences were clearest at the hottest and coldest times of the year. Air and soil temperature variability in uneven-aged stands resembled at stand level the variability that is encountered in even-aged management only across larger areas including young, immature and mature stands. Uneven-aged stands may therefore support more diverse habitats than even-aged forests. We show that the total amount of plant material was a stronger structural modifier of air temperatures than the vertical arrangement of vegetation. We expect our results to clarify how forest management contributes to shaping microclimates experienced by organisms, which has potential consequences on biodiversity and ecosystem resiliency.