Disturbance distance: quantifying forests' vulnerability to disturbance under current and future conditions
Disturbances, both natural and anthropogenic, are critical determinants of forest structure, function, and distribution. The vulnerability of forests to potential changes in disturbance rates remains largely unknown. Here, we developed a framework for quantifying and mapping the vulnerability of forests to changes in disturbance rates. By comparing recent estimates of observed forest disturbance rates over a sample of contiguous US forests to modeled rates of disturbance resulting in forest loss, a novel index of vulnerability, Disturbance Distance, was produced. Sample results indicate that 20% of current US forestland could be lost if disturbance rates were to double, with southwestern forests showing highest vulnerability. Under a future climate scenario, the majority of US forests showed capabilities of withstanding higher rates of disturbance then under the current climate scenario, which may buffer some impacts of intensified forest disturbance. While climate attributes such as temperature and precipitation are principal determinants of the distribution of the world’s ecosystems (i.e. tundra vs. forestland), natural disturbances suchasfire,wind, andother events can also influence the distribution and properties of ecological systems [1]. Within forested ecosystems, disturbance influences forest structure, function and composition, and thus the ecosystem services they provide [2–5]. Recent studies have highlighted changes in natural disturbance regimes compared to historic norms and the potential for further alterations in disturbance regimes from future climate change on a scale unprecedented in historic records [6–13]. These studies lead to a key question: What levels of disturbance can forests tolerate before they face critical alterations in structure and function and how might their sensitivity to disturbance change under future climate? While field studies that characterize and/or simulate the impact of disturbance continue to be vital to our understanding of changing disturbance impacts to forested ecosystems, it is difficult and often impractical to extend these studies to continental and centennial scales [14, 15]. Thus, process-based prognostic models that can simulate events over larger areas and temporal scales have been used to advance our understanding of regional to global ecosystem dynamics. Previous studies have explored a range of topics such as the modification of global vegetation in a world without fire, to the potential impacts of large-scale deforestation in Tropical and Boreal regions, to the dependence of future climate mitigation stategies on the future rate of natural disturbance rates [1, 16–18]. Given the critical roles disturbance plays in shaping forest structure, function, and dynamics, we propose a framework to assess ecosystem vulnerability to disturbances. Specifically, we sought to address the following questions: (1)What is themaximumrateof disturbance for which current forests can be maintained across the US?; (2) How close are current forests to a fundamental shift in ecosystem structure?; and, (3) How may forest © 2017 The Author(s). Published by IOP Publishing Ltd Environ. Res. Lett. 12 (2017) 114015 Figure 1. Conceptual diagram of Disturbance Distance (D). Given unique environmental growing conditions, forests tolerance to disturbance (λ∗ x-axis) varies from low in areas with poor growing conditions (site-1 and 2) to high in areas with favorable growing conditions (site-3 and 4). At the same time forests sites vary in the actual rates of disturbance experienced (λ) from relatively lower rates of disturbance (sites 1 and 4) to higher rates (sites 2 and 3). Subtracting actual rates of disturbance from an ecosystems threshold rate gives an indication of the additional amount of disturbance a forest can tolerate (D) before a transition to non-forest occurs. ecosystem sensitivity to disturbance change under a potential future climate change scenario? Forest vulnerability to disturbance was determined by developing a simple and flexible framework. First, ecosystemresponses todisturbance are evaluatedunder representative climatic and environmental conditions to determine threshold rates of disturbance (λ∗), the rates that lead to fundamental alterations of vegetation structure (i.e. transition from forest to non-forest based on criteria of plant structure, composition and biomass). While forests with favorable growing conditions recover faster and can thus tolerate higher disturbance, the same level of disturbance on a site with poor growing conditions can be enough to tip the land into a different ecosystem type [19]. Next, estimates of actual forest disturbance rates (λ) are acquired over forested regions. Comparing these observed rates of disturbance to the estimated threshold rates provide estimates of how much additional disturbance an ecosystem may tolerate before a transition threshold is reached, herein termed Disturbance Distance (equation (1)) D = λ∗ − λ. (1) A region’s Disturbance Distance, D, gives insight into its vulnerability to potential increases in disturbance intensity (figure 1). In this report, threshold disturbance rates (λ∗) for which forest conditions could be maintained across the contiguous US were estimated by simulating potential vegetation growth and dynamics under varying disturbance rate scenarios in an advanced mechanistic and prognostic ecosystem model [20] (see methods supplement available at stacks.iop.org/ERL/ 12/114015/mmedia). Following previous studies [3, 21] the forest threshold definition used here, required the maintenance of forest plant functional types and an above ground standing stock of natural cover equivalent to 2 kgC m−2 or greater. While the individual-based mechanistic model was chosen in part due to its capabilities to incorporate sub-models of disturbance that may allow future studies of disturbance interactions and feedbacks [3, 22], to isolate the average disturbance rate leading to non-forest conditions, all sub-models were turned off and annual disturbance rates were held constant in time and space within each model run. The modeled-based results of this simplified disturbance case study indicate that forests in southeastern US can maintain the highest rates of disturbance before non-forest conditions are reached, while southwestern forests were estimated to have the lowest disturbance rate thresholds (figure 2). To estimate how far current forests may be from a transition to non-forest, threshold rates of disturbance (λ∗) were compared to remotely sensed derived estimates of disturbance over 50 US forested Landsat scenes representative of major forest types [23]. The observed average annual disturbance rates (λ), measured as the percent of live forest cover loss persisting 2 or more years between 1986–2010, ranged from 0.4%−3.8% yr−1 with a national average of 1.4% yr−1 (figure 2, figure S1). Over these same forested