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

随着气温的升高和火灾季节的延长，火灾发生的频率和面积都在增加，每年野火后再生的针叶林覆盖了美国西部更多的地区。树木幼苗在火灾后的最初几年内建立，小树在最初的几十年内达到生长高峰。树木的快速生长部分是由氮(N)推动的，氮是一种关键的植物养分。然而，森林中储存的一些氮在森林火灾中通过燃烧而流失到大气中。在高度严重的火灾之后，这些失去的生态系统氮是如何补充的尚不清楚，特别是在能够将大气氮转化为生物可利用氮的固氮植物（如豆科植物）很少的地方。从历史上看，西部针叶林在再次燃烧之前很久就已经从火灾中恢复过来了。更频繁的高强度火灾是否会进一步减少生态系统氮并影响森林恢复尚不清楚。在1988年黄石大火以来的长期研究的基础上，本研究旨在确定和量化火灾后黑松（Pinus contorta var. latifolia）幼林N恢复的来源，并确定生态系统N如何对更频繁的火灾做出反应。研究人员将让新闻专业的学生向广大受众传播他们的发现，并开发新的方法来教授学生关于森林生态系统中的氮。生物固氮（BNF）是维持林分火灾后发展所需的新氮的最有可能的来源，研究人员假设新氮输入反映了多个不显眼的氮源的联合活性。通过野外观测和美国怀俄明州大黄石地区的多年氮肥试验，本研究将解决三个问题：(1)退化的黑松林（火灾后40年）氮输入的来源和速率是什么，以及火灾后N输入的来源和速率如何随林分结构和时间而变化？(2)随着火灾发生时间的增加，40年黑松幼林的氮素有效性如何调节固氮和限氮？(3)短间隔（30年）林分置换林火如何影响氮素固定、氮素有效性和氮素储量？本研究将验证以下预期：总固氮率将随着黑松生物量和氮储量的增加而增加，生物固氮来源将随着树木需求的增加而从共生向隐生、自由生活的固氮者转变。这些整合良好的研究将直接解决关键的未知因素，这些未知因素限制了随着火灾活动加速而预测西部森林未来的能力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.