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

随着气温上升和火灾季节延长，火灾频率和烧毁面积增加，野火后再生的针叶林每年覆盖美国西部更多地区。树苗在火灾后的头几年建立，年轻的树木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），一个关键的植物营养。然而，一些储存在森林中的氮在森林火灾期间通过燃烧损失到大气中。这种损失的生态系统氮如何在严重火灾后得到补充尚不清楚，特别是在能够将大气氮转化为生物有效氮的固氮植物（如豆类）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N并影响森林恢复尚不清楚。建立在长期的研究，因为1988年黄石大火，本研究的目的是确定和量化的来源，在年轻的火后lodgepole松（松contorta变种。latifolia）森林，并确定生态系统N如何响应更频繁的火灾。研究人员将让新闻专业的学生向广大受众传达他们的发现，并开发新的方法来教授学生关于森林生态系统中的N。生物固氮（BNF）是维持火后林分发展所需的新N的最可能来源，研究人员假设新的N输入反映了多个不明显的N源的组合活动。本研究利用大黄石（美国怀俄明州）的实地观察和多年氮施肥实验，解决三个问题：（1）退化的黑松林（火灾后40年）中氮输入的来源和速率是什么？以及来源和速率如何随着林分结构和火灾后时间的变化而变化？(2)如何氮的有效性调节氮固定和氮限制在加积年轻（40年）lodgepole松林随着时间的推移，因为火灾？(3)短间隔（30年）的林分替代火灾如何影响氮固定，氮的可用性，和N股票？这项研究将测试的期望，总固氮率将增加与黑松生物量和氮储量，生物固氮的来源将从共生转移到神秘的，自由生活的固氮树的需求随着时间的推移和空间的增加。这些综合研究将直接解决关键的未知因素，这些因素限制了预测西部森林未来的能力，因为火灾活动加速。该奖项反映了NSF的法定使命，并被认为值得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来支持。

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.