Overstory dynamics regulate the spatial variability in forest-floor CO2 fluxes across a managed boreal forest landscape

• Published in: Agricultural and forest meteorology 2022-05, Vol.318, p.108916, Article 108916
• Main Authors: Martínez-García, Eduardo, Nilsson, Mats B., Laudon, Hjalmar, Lundmark, Tomas, Fransson, Johan E.S., Wallerman, Jörgen, Peichl, Matthias
• Format: Article
• Language: English
• Subjects: Boreal forest; Carbon dioxide exchange; Forest Science; Forest-floor; Landscape variability; Meteorologi och atmosfärforskning; Meteorology and Atmospheric Sciences; Primary production; Respiration; Skogsvetenskap
• ISSN: 0168-1923; 1873-2240; 1873-2240
• DOI: 10.1016/j.agrformet.2022.108916

•Forest-floor is an annual net CO2 source, which increases with stand age.•Understory production is higher in pine than in spruce stands and declines with age.•Total forest-floor respiration is primarily regulated by its autotrophic components.•Heterotrophic soil respiration remains stable across the managed boreal landscape.•Tree biomass drives the landscape-scale variability of forest-floor CO2 fluxes. The forest-floor represents an important interface for various carbon dioxide (CO2) fluxes, however, our knowledge of their variability and drivers across a managed boreal forest landscape is limited. Here, we used a three-year (2016−2018) data set of biometric- and chamber-based flux measurements to investigate the net forest-floor CO2 exchange (NEff) and its component fluxes across 50 forest stands spanning different soil types, tree species, and age classes within a 68 km2 boreal catchment in Sweden. We found that the forest-floor acted as a net CO2 source with the 10th–90th percentile (used hereafter for describing reported variations) ranging from 149 to 399 g C m−2 yr−1. Among the key landscape attributes, stand age strongly affected most NEff component fluxes, whereas tree species and soil type effects were weak and absent, respectively. Specifically, forest-floor net CO2 emissions increased with stand age due to declining understory gross and net primary production, ranging between 77–275 and 49–163 g C m−2 yr−1, respectively. Furthermore, we observed higher understory production rates in pine than in spruce stands. Across the 50 stands, the total forest-floor respiration ranged between 340 and 549 g C m−2 yr−1 and its spatial variation was primarily regulated by its autotrophic components, i.e., understory and tree root respiration, which displayed divergent increasing and decreasing age-related trends, respectively. Furthermore, heterotrophic soil respiration remained within a relatively narrow range (154–290 g C m−2 yr−1), possibly owing to compensating gradients in forest-floor properties. We further identified tree biomass as the major driver of the landscape-scale variations of CO2 fluxes, likely attributable to modulating effects on forest-floor resource availability and growing conditions. This implies that tree growth responses to forest management and global change will be particularly important for regulating magnitudes and spatial variations of forest-floor CO2 fluxes in boreal forests.