The boreal forest is an important global carbon sink, but its response to drought remains uncertain. Here, we compiled biometric- and chamber-based flux data from 50 boreal forest stands to assess the impact of the 2018 European summer drought on net ecosystem production (NEP) across a 68 km² managed landscape in northern Sweden. Our results reveal a non-uniform reduction in NEP (on average by 80 ± 16 g C m⁻² yr⁻¹ or 57 ± 13%) across the landscape, which was greatest in young stands of 20–50 years (95 ± 39 g C m⁻² yr⁻¹), but gradually decreased towards older stands (54 ± 57 g C m⁻² yr⁻¹). This pattern was attributed to the higher sensitivity of forest-floor understorey to drought and its decreasing contribution to production relative to trees during stand development. This suggests that an age-dependent shift in understorey–tree composition with increasing stand age drives the drought response of the boreal forest NEP. Thus, our study advocates the need for partitioning ecosystem responses to improve empirical and modelling assessments of carbon cycle–climate feedbacks in boreal forests. It further implies that the forest age structure may strongly determine the carbon sink response to the projected increase in drought events across the managed boreal landscape.

Boreal forests are important global carbon (C) sinks and, therefore, considered as a key element in climate change mitigation policies. However, their actual C sink strength is uncertain and under debate, particularly for the actively managed forests in the boreal regions of Fennoscandia. In this study, we use an extensive set of biometric‐ and chamber‐based C flux data collected in 50 forest stands (ranging from 5 to 211 years) over 3 years (2016–2018) with the aim to explore the variations of the annual net ecosystem production (NEP; i.e., the ecosystem C balance) across a 68 km² managed boreal forest landscape in northern Sweden. Our results demonstrate that net primary production rather than heterotrophic respiration regulated the spatio‐temporal variations of NEP across the heterogeneous mosaic of the managed boreal forest landscape. We further find divergent successional patterns of NEP in our managed forests relative to naturally regenerating boreal forests, including (i) a fast recovery of the C sink function within the first decade after harvest due to the rapid establishment of a productive understory layer and (ii) a sustained C sink in old stands (131–211 years). We estimate that the rotation period for optimum C sequestration extends to 138 years, which over multiple rotations results in a long‐term C sequestration rate of 86.5 t C ha⁻¹ per rotation. Our study highlights the potential of forest management to maximize C sequestration of boreal forest landscapes and associate climate change mitigation effects by developing strategies that optimize tree biomass production rather than heterotrophic soil C emissions.

In semi-arid regions, afforestation with fast-growing species cultured with low irrigation can be an effective approach for environmental protection. An experiment was conducted to evaluate the stem biomass production of Paulownia in a semi-arid climate and clay soils under contrasting low-irrigation and fertilization treatments. The stem biomass at the stand level was estimated by applying allometric equations fitted in sample resprouts and inventory data. The results show that biomass production improved when either irrigation or fertilizer was added, but the combination of a higher dose of irrigation and fertilization did not lead to the highest biomass production; thus water availability was the main factor controlling biomass production. Under the higher dose of irrigation, the absence of a fertilizer effect would be due in part to the fertile soil, which could supply sufficient nutrients for Paulownia growth at the higher level of soil moisture. The stem biomass estimated ranged from 2.14 to 4.50 t×ha⁻¹ (lower irrigation dose without fertilization, and higher irrigation with fertilization). The greater production was similar to other studies in the Mediterranean area receiving more irrigation. Thus, this study permitted us to understand the potential of Paulownia to provide biomass in semi-arid environments with low irrigation due to water use restrictions.