Forest composition and density shaped long-term fire regimes and catchment-lake interactions in the temperate-mixed mountain forests of Central Europe

Abstract

The projected rise in fire activity due to climate change challenges forest conservation efforts worldwide. Current non-intervention approaches, which rely on natural processes for ecosystem conservation, often overlook palaeoecological data depicting long-term, local interactions between fire regime components, forest structure and composition, soil, aquatic environment and anthropogenic disturbance. To explore whether palaeoecological information supports non-intervention strategies in the Picea abies – Fagus sylvatica – Abies alba closed temperate mountain forest under future fire risks, we conducted a multi-proxy, retrospective analysis (charcoal morphology, pollen, plant macro-remains, sedimentology, palaeoclimate models) on a Holocene lacustrine archive located in the Bavarian Forest National Park, Germany. Results show that climate shifts directly induced vegetation reorganizations and fire regime changes around 11.1, 9.3 and 7–6 ka BP, and gradually allowed the expansion of closed mixed forests dominated by the late successional Picea abies – Fagus sylvatica - Abies alba under a remarkably stable, low-fire environment. We emphasize the possible role of shade-related traits of these species as key bottom-up drivers of biomass burning over the past 6000 years likely due to their effects on microclimate, offsetting the species-specific fire traits. Although these forests tolerate low-severity surface fire, more subtle impacts were identified at the level of soil cover and lacustrine environment. First, our data demonstrate that lake browning processes developed naturally since the Early Holocene with the progression of chemical weathering and podzol formation, but an ecological threshold due to browning intensification was crossed ca. 6000 years ago, under maximum forest density and in the absence of fire disturbance. Second, fire likely reduced lake water browning and soil podzolisation processes by mineral erosion and the decrease in litter and soil organic matter due to direct burning or indirect aeration. Soil and aquatic ecosystem components appeared highly interconnected and responded even to low disturbance severities. Consequently, these components may react more sensitively to the predicted increase in fire frequency and severity, with yet unknown ripple effects on the forest ecosystem resilience. Given the critical role of the Picea abies - Fagus sylvatica - Abies alba closed-canopy forests in shaping the microclimate and catchment-lake interactions, our study suggests that non-intervention strategies are suitable for preserving ecosystem stability in these types of forests. By promoting natural recovery of shade-tolerant tree species and minimizing direct anthropogenic impacts, this approach would protect the soil, regulate erosion and delivery of soil organic substances, and potentially mitigate the effects of climate change on fire hazard and on individual forest tree species.