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

每年，随着气温上升和火季延长，火灾频率和燃烧面积增加，野火后再生的幼年针叶林覆盖了美国西部更多的地区。树苗在火灾后的头几年里生长，幼树在最初的几十年里达到生长的顶峰。树木的快速生长在一定程度上是由氮(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期间通过燃烧流失到大气中。在高度严重的火灾后，这种失去的生态系统N是如何补充的尚不清楚，特别是在能够将大气N转化为生物有效N的固氮植物(如豆类)很少见的地方。从历史上看，西部针叶林在再次燃烧之前很早就从火灾中恢复过来。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。在1988年黄石公园火灾后长期研究的基础上，本研究旨在确定和量化火灾后松树幼树氮素回收的来源。以确定生态系统N如何对更频繁的火灾作出反应。研究人员将邀请新闻专业的学生向广大受众传达他们的发现，并开发新的方法来向学生传授森林生态系统中的氮。生物固氮(BNF)是维持林分生长所需新氮的最有可能来源，研究人员假设，新的N输入反映了多个不明显的N源的组合活性。利用野外观测和美国怀俄明州大黄石地区多年的氮肥试验，本研究将解决三个问题：(1)火后40年的加重小尾松林中N输入的来源和速率是什么，以及N的来源和速率是如何随林分结构和火后时间的变化而变化的？(2)随着火灾后时间的延长，N的有效性如何调节幼林(40年生)的N固定和N限制？(3)短时间(30年)林分更替火灾对N固定、N有效性和N储量有何影响？这项研究将测试总固氮率将随着小松生物量和氮库的增加而增加的预期，以及随着树木需求随着时间和空间的增加，生物固氮的来源将从共生的、自由生活的固氮者转变为隐蔽的、自由生活的固氮者。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的加速而预测西部森林未来的能力。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，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.