Forests of the future: Climate change impacts and implications for carbon storage in the Pacific Northwest, USA

Highlights

• Longer summer dry periods, less snowpack, increased drought stress for forests.

• More wildfires, less growth and mortality will lead to less carbon storage.

• We need better soil carbon knowledge, monitor ecotones and improve PBM models.

• Case studies illustrate impacts on forests and offer relevant management responses.

Abstract

Rising greenhouse gases are changing the Earth’s climate and adversely affecting ecosystems that currently provide a suite of invaluable benefits, from cleaning water to sequestering carbon. Some of the world’s most productive forests grow in the Pacific Northwest region of North America, but our understanding of climate change effects on these forests and their carbon is still emerging. Here, we synthesize the current state of research (including empirical, paleo, and modeling studies), discuss the implications on forest growth and carbon storage in Pacific Northwest forests, and identify key knowledge gaps and future research opportunities based on a combination of published studies and expert opinion. Two case studies are presented that illustrate the expected effects of climate change on moist and dry forest ecology and carbon storage. In response to these impacts, we highlight a number of appropriate regional forest restoration and management adaptation strategies. Filling in knowledge gaps will improve the accuracy of forest carbon accounting, a crucial part of the strategy to meet climate mitigation targets and prevent the most severe impacts of climate change.

Introduction

Rising greenhouse gases (GHGs) and resulting climate change is already affecting the Earth’s terrestrial and aquatic systems (Steffen et al., 2018). Current mitigation efforts are largely aimed at curbing human caused greenhouse gas emissions; however, to keep global temperatures below 1.5 °C and avoid some of the worst impacts of climate change, these strategies must also prevent additional CO₂ from being released from land and sea ecosystems and soak up as much atmospheric CO₂ as possible (IPCC, 2018, USGCRP, 2018). One such removal technology – natural climate solutions – is found within our ability to conserve and restore forests and improve natural resource management practices, which results in increased carbon storage and/or a reduction in GHG emissions (Fargione et al., 2018, Griscom et al., 2017). A critical piece of natural climate solutions is maximizing carbon storage and sequestration, the ability to remove CO₂ from the atmosphere and store it long-term. Natural climate solutions for forests can include afforestation, reforestation, or protecting forest carbon storage and sequestration potential by minimizing impacts (Law et al., 2018, Moomaw et al., 2020, Zomer et al., 2008).

Globally, forests absorb 15–20% of annual human carbon emissions (Le Quéré et al., 2018), most of which is stored either aboveground in tree biomass or belowground as soil carbon. The Pacific Northwest (PNW) region of North America has some of the greatest natural terrestrial carbon storage potential within North America (Hudiburg et al., 2009, Smithwick et al., 2002). For instance, in the US, live tree aboveground carbon density in PNW moist coastal forests regularly contains more than 80 megagrams (Mg) per hectare (ha) of carbon compared to other regions - forests in Midwest states (40–60 Mg/ha) and forests in Northeast states (60–80 Mg/ha) (Wilson et al., 2013). PNW moist coastal forests are highly productive and have historically stored carbon for long periods of time in part because of their longevity and the high amounts of precipitation they receive, which also manifests in a historically long fire return interval. Further, forests in the West Cascades ecoregion of the PNW are estimated to have more than double the amount of total carbon stored compared to forests in the East Cascades ecoregion (Law et al., 2018). However, there is substantial heterogeneity in PNW forests, and they can differ greatly in response to climate, steep elevation gradients, soils, and complex disturbance and land-use histories (Franklin and Dyrness, 1988). Moreover, there are diverse environments and ecological systems across the PNW that respond differently to a warming climate, changes in disturbance patterns, and natural resource management (Halpern and Spies, 1995).

Broadly, PNW forested systems can be categorized as moist, dry, and subalpine forests. Geographically, moist and dry forest types are generally found on the western and eastern sides of the Cascade Divide, respectively, while subalpine forests are located at high elevations (Fig. 1). Each forest type has a unique set of defining characteristics, disturbance factors, and responses to a changing climate (Fig. 1). However, many climate mitigation targets that specifically rely on forest carbon storage and sequestration do not incorporate these differences. For example, California’s Forest Climate Action Plan (Forest Climate Action Team, 2018), does not account for responses of individual forest types to climate change. Moreover, there are current gaps in our understanding of how climate change will influence forests and how those effects will impact carbon storage and sequestration. These knowledge gaps have the potential to derail the path to current mitigation targets.

In this review, we synthesize how climate change is expected to affect major forest types in the PNW and highlight how carbon storage will likely be impacted. We first summarize the existing literature on the direct and indirect climate change impacts on forests. Second, we identify key climate change impacts on PNW forests and their ability to store carbon. Third, we combine scientific studies and expert opinion to explore some of the critical knowledge gaps of how climate change will impact forests and carbon and offer several actionable strategies in response. These key knowledge gaps form a foundation for future forest carbon research opportunities and, once addressed, will help lead to more accurate projections of climate change impacts on forests and carbon and thus more effective management strategies. To demonstrate this process, we briefly present two case studies that illustrate how climate change is anticipated to affect moist and dry forests and their carbon storage. Finally, we highlight appropriate adaptation strategies that are currently or planned for implementation. This review is intended to be used by regional conservation organizations to better inform their forest restoration and management strategies as well as prepare for an uncertain future.