Strict forest protection: A meaningful contribution to Climate-Smart Forestry? An evaluation of temporal trends in the carbon balance of unmanaged forests in Germany

Abstract

The role of unmanaged forests is discussed controversially in the context of climate change. One of the key questions is, whether they can contribute to the mitigation of climate change as a carbon sink and storage. However, carbon dynamics of forests after the cessation of management are not well understood. We analyzed a set of 64 forest sites, covering wide gradients of time since abandonment (0–68 years) and stand age (65–261 years) in even-aged, unmanaged beech forests. Five sites that were unmanaged for >100 years complemented the main dataset. We compiled site-specific carbon balances, distinguishing six carbon-compartments: Carbon in aboveground living and dead biomass, carbon in belowground living and dead biomass, and carbon in the organic layer and the mineral topsoil (0–30 cm). We found positive effects of increasing TSA on the carbon stock in living biomass and aboveground dead biomass for up to 50 years after management ceased. The average increase of the total carbon stock over 50 years of TSA was ≈ 80 Mg C ha–1. The effect of stand age on aboveground living biomass showed a convex relation. Aboveground dead biomass increased logistically with TSA, while belowground dead biomass decreased. On average, the five sites unmanaged for >100 years held lower total carbon stocks compared to the observed biomass peak around 50 years of TSA. However, they contained considerably higher amounts of deadwood. Carbon in the mineral soil did neither change with TSA nor with stand age and was driven by pH. Carbon stocks in newly unmanaged forests increased almost linearly for approximately 50 years after cessation of management. Subsequently, a stabilization or medium-term decrease in carbon stock was observed, likely due to the initiating transition from even-aged to multi-aged structures. We conclude that, besides their value for biodiversity and ecosystem functions, the potential of naturally developing forests as a medium-term carbon sink and long-term stable carbon storage should be considered as a valuable contribution to Climate-Smart Forestry.

Forest carbon stock development following extreme drought-induced dieback of coniferous stands in Central Europe: a CBM-CFS3 model application

“…All these objectives are well substantiated in the context of Czech forestry, where soil remains notably disturbed [ 29 , 30 ] and the average amount of deadwood remains low [ 31 ]. Hence, increasing the carbon pool of dead organic matter and indirectly soil represents—at least for medium term [ 32 ]—additional mitigation potential, besides enhancing biodiversity, for forests that have been intensively managed earlier.…”

Natural forests for a safe climate: Enhancing ecosystem integrity, biodiversity and adaptive capacity for climate mitigation

“…The studied set-aside forests appeared to be still below the ecosystem's carbon carrying capacity: the carbon stock change rates were not lower for plots with high initial carbon stocks (no clear trend in Figure 3), indicating that the carbon accumulation did not yet slow down and the carbon stock continued to increase in forests that already built up a large carbon stock over time. Studies in German set-aside forests indeed indicated that the in situ carbon sequestration-and hence additionality of setting aside forests-remains high during at least the first 50 years after set aside, before they approach their ecological carrying capacity and related dynamic steady-state level Nagel et al, 2023).…”

“…In regions with a long history of intensive forest management and no remaining primary forests (e.g., north-western or central Europe), the aboveground carbon stock in forests is expected to be distinctively lower than the natural ecosystem carbon carrying capacity Nagel et al, 2023). Forests in these regions thus represent a high potential for carbon storage.…”

“…In the absence of primary forest reference sites, however, we have no clear indication of the forests' ecosystem carbon carrying capacity. Forests set aside from active management may gradually provide an estimate of the carbon sink potential of forests at specific site conditions (Hoover et al, 2012;Nagel et al, 2023).…”

Dynamics of bio-based carbon dioxide removal in Germany

“…Permanent set-aside and the associated abandonment of management by harvesting beeches from age 100 until the end of their life initially has a positive effect on the annual CO2 removal potential of about 14-16 t CO2 ha -1 a -1 assuming today's set-aside beech stands of the age classes 120, 140 and 160 years in the public forest if no calamities occur (Bolte et al 2022; Figure 3h). However, after a certain time since abandonment, the culmination is reached as the additional growth potential is exhausted, the removal potential decreases, and beginning decay can also reduce the carbon stocks accumulated until that time (according to Nagel et al 2023, this occurs around 50 years after abandonment). In the case of temporary set-aside over a period of 25 years, for example, the further development will depend on whether and in what way management is resumed.…”