Uncharted Territory: NJIT Ecologist Investigates Forest Fate Amid Climate Change, Wildfire

Nearly a decade ago, a bark beetle infestation tore through southeast Wyoming's Snowy Range, transforming lush landscape of Medicine Bow National Forest into a tinderbox of dead lodgepole pine. In Sept. 2020 it ignited — what became known as the Mullen Fire raged beyond the parkland across 176,000 acres over the next month, fueled by the beetle-killed trees and unusually dry conditions.

Xiaonan Tai, assistant professor of biological sciences and director of NJIT’s Ecohydrology Lab, is investigating the fate of the national forest.

Tai has been developing models used to unravel the complex ecological and hydrological processes taking place in North American forests amid historic climate changes.

In the case of Medicine Bow, the Mullen Fire touched off in the midst of a 20-year climate trend in the Rocky Mountains — its high-elevation forests are experiencing reduced snowfall and greater wildfires than at any point in the past 2,000 years.

“The disaster reflects a clear trend in increasing fires across the Western U.S. forests, but the big question now is how these forests recover,” said Tai. “The response of Medicine Bow’s forests could give us a window into how the region’s forests respond to other climate change-related disturbances going forward.”

Tai, who studied the national park’s bark beetle epidemic more than three years ago as part of a CUAHSI PathFinder Fellowship, has returned to collaborate with University of Wyoming researchers through a U.S. Department of Energy grant. The team is conducting a full workup of the land that includes collection of microclimate, vegetation and hydrological measurements over the next three years.

Tai’s ecohydrology modeling is incorporating the field data to paint a picture of the interaction between the Medicine Bow’s groundwater and vegetation health since the fire, answering questions about how the area’s subsurface water shapes forest response and recovery from the disaster.

It’s a tricky equation to calculate.

“The park’s wetlands are where the land’s groundwater tends to accumulate most, so we hypothesize that these are likely refugia sites during fire that are critical to the forest’s response,” said Tai. “A big challenge is that this is an enormous groundwater system where water can transport and affect the fate of trees from 1,000’s of kilometers away. Rainwater doesn’t remain local, but travels based on a mix of regional factors such as topography and the substrate properties.

“Groundwater flow has been missing from ecosystem modeling and future projections, largely because it is difficult to directly observe, and it is computationally challenging to solve… But we have better capability now.”

In the journal Environmental Research Letters, Tai recently established a new state-of-the-art ecohydrological model to quantify the impacts of subsurface groundwater on forest mortality risk for the first time. The NSF-funded study highlighted a key, unseen mechanism affecting how forests respond to drought and elevated CO2 concentrations — the way in which water flows laterally beneath the forest floor.

"Past studies focused on solving water fluxes only in the vertical dimension, ignoring the role of lateral subsurface water exchanges. But with this model, we get more complete descriptions of ecosystem water dynamics than ever before,” said Tai.

Tai says the findings have challenged a previously-held mainstream expectation by climate scientists — that elevated CO2 levels ameliorate drought stress experienced by trees.

“We showed that most water stored doesn’t stay local, but rather, begins to travel laterally to areas that may otherwise be dry. …. It shows we need to reassess future forest predictions by incorporating subsurface flow.”

As Tai’s modeling charts Medicine Bow’s forest-hydrology connections, ecologists could gain a vital roadmap of where and how groundwater influences forest recovery regionally.

“There are similarly impacted forests throughout the Rocky Mountains. It’s our hope that our results will inform effective post-disturbance land management strategies beyond Medicine Bow,” said Tai.