Open-Source Fire Science

Blog Post After the Drought

How Standing Dead Trees Are Shaping California's Forests and Fire Risk

By Pyregence Team

In the wake of California’s devastating 2012–2016 drought, over 100 million trees died across the Sierra Nevada. While much attention has focused on the immediate impacts of this mass tree mortality, a recent study dives deeper into what happens after the trees die – and the surprising ways they continue to shape wildfire risk and forest ecology for decades.

From Snags to Surface Fuels: A Delayed Threat

The study completed by the Pyregence Team looked at how drought-killed trees – called snags – fall over time and how they contribute to surface fuel loads. These fallen trunks and branches accumulate on the forest floor, increasing the risk of severe wildfire. The researchers used long-term forest monitoring data from Yosemite, Sequoia, and Kings Canyon National Parks to answer key questions:
  • How quickly do dead trees fall?
  • How much fuel accumulates from fallen snags?
  • Can current models predict these dynamics accurately?

Their results have major implications for forest managers facing the increasing threat of “mass fires” driven by climate change, dead biomass, and delayed fire behavior changes.

Key Findings

Fuel Loads Are Already Climbing

Between 2017 and 2021:

  • Fine woody debris increased by over 145% across all sites.
  • Litter and duff loads more than doubled at some locations.
  • Total surface fuel loads rose by up to 69%, especially in pine-dominated areas.

Snags Stand Longer Than Expected

Not all trees fall quickly. In fact:

  • Some snags are projected to stand for decades.
  • On average, snag fall rates were 20-40% slower than commonly used models predict.
  • Species matters: Incense-cedar stood the longest, while yellow pines (like ponderosa and Jeffrey) fell the fastest.

Big Pulse of Debris Still to Come

Projections showed that:

  • Sites with high yellow pine mortality (e.g. YOPI) could accumulate up to 136 Mg/ha of coarse woody debris by 2040.
  • Even the slowest site (Sequoia Kings Canyon Parks) will see fuel inputs double over the coming decades.

Modeling Matters: Updating the Tools

The study also found that the widely used Forest Vegetation Simulator – Fire and Fuels Extension (FVS-FFE):

  • Overestimates short-term snag falls.
  • Underestimates long-term fall rates and lingering surface fuels.
  • Uses outdated fall rates based more on expert opinion than empirical data.

Northrop et al. (2024) offer improved, species-specific equations based on decades of annual field observations, providing better tools to forecast fire behavior in snag-heavy forests.

What Can Managers Do?

The coming wave of coarse woody debris presents a window of opportunity:
  • Prescribed burning soon after tree death can reduce fine fuels before snags fall.
  • Salvage logging may be viable where access allows - but it’s time-sensitive and limited on public lands.
  • Monitoring should prioritize snag-rich areas with yellow pine dominance, where fall rates and biomass inputs are fastest.
  • All management strategies must balance fire risk reduction with the ecological benefits snags provide - like wildlife habitat and nutrient cycling.

Bottom Line

Climate-driven tree die-offs aren’t just one-time events – they set in motion a cascade of changes that can alter wildfire behavior for decades. With droughts intensifying and tree mortality rising, forests in the Sierra Nevada are entering a new phase defined by delayed but potent risks.

Forest managers, fire modelers, and policymakers need up-to-date, science-based tools to navigate these changes. This study offers exactly that – a data-rich foundation for the next generation of fire-smart forest management.

Citation:

Northrop, H., Axelson, J. N., Das, A. J., Stephenson, N. L., Vilanova, E., Stephens, S. L., & Battles, J. J. (2024). Snag dynamics and surface fuel loads in the Sierra Nevada: Predicting the impact of the 2012–2016 drought. Forest Ecology and Management, 551, 121521. https://doi.org/10.1016/j.foreco.2023.121521