Abstract
Abstract. Forest ecosystems play an important role in the global climate system and are thus intensively discussed in the context of climate change mitigation. Over the past decades temperate forests were a carbon (C) sink to the atmosphere. However, it remains unclear to which degree this C uptake is driven by a recovery from past land use and natural disturbances or ongoing climate change, inducing high uncertainty regarding the future temperate forest C sink. Here our objectives were (i) to investigate legacies within the natural disturbance regime by empirically analyzing two disturbance episodes affecting the same landscape 90 years apart, and (ii) to unravel the effects of past land use and natural disturbances as well as the future climate on 21st century forest C uptake by means of simulation modeling. We collected historical data from archives to reconstruct the vegetation and disturbance history of a forest landscape in the Austrian Alps from 1905 to 2013. The effects of legacies and climate were disentangled by individually controlling for past land use, natural disturbances, and future scenarios of climate change in a factorial simulation study. We found only moderate spatial overlap between two episodes of wind and bark beetle disturbance affecting the landscape in the early 20th and 21st century, respectively. Our simulations revealed a high uncertainty about the relationship between the two disturbance episodes, whereas past land use clearly increased the impact of the second disturbance episode on the landscape. The future forest C sink was strongly driven by the cessation of historic land use, while climate change reduced forest C uptake. Compared to land-use change the two past episodes of natural disturbance had only marginal effects on the future carbon cycle. We conclude that neglecting legacies can substantially bias assessments of future forest dynamics.
Highlights
Carbon dioxide (CO2) is responsible for 76 % of the global greenhouse gas emissions and is the single most important driver of anthropogenic climate change (IPCC, 2014)
To address our second objective, i.e., the evaluation of the impact of past land use and natural disturbance as well as the future climate on the 21st century carbon sink strength, we extended our factorial simulation design to account for the second disturbance episode and different future climate scenarios
We evaluated the ability of individual-based forest landscape and disturbance model (iLand) to reproduce past natural disturbance and land use as well as the resultant forest vegetation dynamics on the landscape by comparing simulations of the baseline scenario with independent empirical data for different time periods; the simulated amount of timber extracted was compared to historical records for three time periods signifying major technical system changes during the 20th century (Fig. 2)
Summary
Carbon dioxide (CO2) is responsible for 76 % of the global greenhouse gas emissions and is the single most important driver of anthropogenic climate change (IPCC, 2014). During the period 1990–2007, established and regrowing forests were estimated to have taken up 60 % of the cumulative fossil carbon emissions (Pan et al, 2011). This carbon (C) sink strength of forests has further increased in recent years (Keenan et al, 2016), resulting from multiple drivers. Possible factors contributing to an increasing sink strength of the biosphere are CO2 (Drake et al, 2011) and nitrogen (Perring et al, 2008) fertilization in combination with extended vegetation periods resulting from climate warming (Keenan et al, 2014). The accelerated carbon uptake of forests might be a transient recovery effect of past carbon losses from land use and natural disturbances (Erb, 2004; Loudermilk et al, 2013)
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