Collaborative research: Nitrogen recovery in postfire lodgepole pine forests: cryptic sources, uncertain futures

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Young conifer forests regenerating after wildfires are blanketing more of the western US each year as fire frequency and area burned increase with rising temperatures and longer fire seasons. Tree seedlings establish during the first few years after fire, and young trees reach their peak growth within the first few decades. Rapid tree growth is partly fueled by nitrogen (N), a critical plant nutrient. However, some of the N stored in forests is lost to the atmosphere via combustion during forest fires. How this lost ecosystem N is replenished after high-severity fires is not known, especially where nitrogen-fixing plants (such as legumes) that can convert atmospheric N to biologically available N, are rare. Historically, western conifer forests have recovered from fire long before they burn again. Whether more frequent high-severity fires will further reduce ecosystem N and affect forest recovery is not known. Building on long-term research since the 1988 Yellowstone Fires, this research aims to identify and quantify the sources of N recovery in young postfire lodgepole pine (Pinus contorta var. latifolia) forests and to determine how ecosystem N responds to more frequent fire. The researchers will engage journalism students to communicate their findings to broad audiences and develop new approaches for teaching students about N in forest ecosystems. Biological N fixation (BNF) is the likeliest source of the new N needed to sustain postfire stand development, and the researchers hypothesize that the new N inputs reflect the combined activity of multiple inconspicuous N sources. Using field observations and a multi-year N-fertilization experiment in Greater Yellowstone (Wyoming, USA), this study will address three questions: (1) What are the sources and rates of N inputs in aggrading lodgepole pine forests (< 40 yrs postfire), and how do sources and rates vary with stand structure and time since fire? (2) How does N availability regulate N fixation and N limitation in aggrading young (< 40 yr) lodgepole pine forests with increasing time since fire? (3) How do short-interval (< 30 yr) stand-replacing fires affect N fixation, N availability, and N stocks? This research will test expectations that total N fixation rates will increase with lodgepole pine biomass and N stocks, and that sources of biological N fixation will shift from symbiotic to cryptic, free-living N fixers as tree demand increases over time and across space. These well-integrated studies will directly address critical unknowns that constrain the ability to anticipate the future of western forests as fire activity accelerates. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.