Doctoral Dissertation Research: Spatial and Temporal Variability of Post-Fire Conifer Regeneration in Lower Treeline Forests of the U.S. Rocky Mountains

本论文研究的是森林火灾后环境变异性增加对树木更新的影响。自1980年代初以来，世界许多地区森林生态系统的野火活动急剧增加，这与变暖趋势有关。关于火灾后的恢复是针对相同类型的森林还是针对不同的、可能没有森林覆盖的植被类型，出现了一些问题。对科罗拉多前线山脉较低林线森林在过去大约25年中被烧毁的初步观察得出了这样的假设，即与19世纪末至20世纪中期发生的火灾事件相比，在气候变暖的情况下，火灾后树木再生较少。虽然许多回溯性研究记录了19世纪末至20世纪中期火灾后树木更新的总体成功，但此类研究对树木更新的实际时间和丰度以及与年度尺度气候变化的潜在关系含糊其辞。很少有研究研究气候变化可能会如何改变森林对未来气候相关干扰(如火灾)的适应能力，这项研究解决了一个悬而未决的问题，即在更温暖、更干燥的条件下，较低的树线落基山脉森林对野火的适应能力如何降低。研究目标是：1)通过检查自20世纪80年代中期以来燃烧的一系列较低林线地点的针叶树幼树密度来量化火灾后针叶树的建立和存活；2)检查幼树密度与立地因素的空间变异性，例如火灾严重程度(由树木死亡率表示)、与草本植物和木本植物的竞争、到种源的距离以及包括海拔和坡向在内的地形变量；3)分析火灾后针叶树更新和年度气候变化之间的关系，以确定某些气候条件是限制还是有利于建立和存活，4)通过提高气温和研究气温升高的直接影响以及相对湿度和土壤湿度变化的次要影响，通过实验操纵小气候来确定对针叶树苗存活和生长的影响。这项研究将有助于理解植被适应和可能的缓解活动，以应对持续变暖对落基山脉中部森林的预期影响。尽管这些森林历来表现出对野火的高度恢复能力，但火灾严重程度的变化可能限制了针叶树在此类火灾后重建的能力。通过更好地了解导致火灾后森林恢复模式的空间和时间变化的因素，可以改善土地管理人员的决策。这项研究将通过本科生参与实地和实验室工作以及在K-12、大学和社区层面的广泛教育推广来促进科学教育。本科生参与研究和其他教育外展工作将以纳入代表性不足的群体为目标。研究结果将通过同行评议的出版物、在会议上的陈述、在线数据共享以及包括博物馆展览和视频纪录片在内的公共教育努力，向其他研究人员、土地管理人员和更广泛的公众传播。作为博士论文研究改进奖，该项目将为有能力的学生开始独立的研究生涯提供宝贵的支持。

This dissertation project is about the impacts of increasing environmental variability on tree regeneration following wildfires. Sharp increases in wildfire activity since the early 1980s in forest ecosystems in many parts of the world have been linked to warming trends. Questions have arisen about whether post-fire recovery will be to the same type of forest or to a different, possibly non-forested vegetation type. Preliminary observations of lower treeline forests in the Colorado Front Range that burned during the past approximately 25 years have led to the hypothesis that under a warmer climate there is less post-fire tree regeneration in comparison with fire events that occurred in the late-19th to mid-20th centuries. Although numerous retrospective studies document the general success of tree regeneration following fires in the late-19th to mid-20th centuries, such studies are ambiguous about the actual timing and abundance of tree regeneration and potential relationships to annual-scale climate variability. Few studies have examined how climate change may alter forest resiliency to future climate-related disturbances such as fire, and this research addresses the unanswered question of how lower treeline Rocky Mountain forests may be less resilient to wildfire given warmer, drier conditions. The research objectives are to: 1) quantify post-fire conifer establishment and survival by examining the density of conifer juveniles across a range of lower treeline sites that have burned since the mid-1980s, 2) examine the spatial variability of juvenile conifer densities in relation to site factors such as fire severity (as indicated by percent tree mortality), competition with herbaceous and woody species, distance to seed source, and topographic variables including elevation and slope aspect, 3) analyze relationships between post-fire conifer regeneration and annual climate variability to determine if certain climate conditions limit or favor establishment and survival, and 4) experimentally manipulate microclimate to determine effects on conifer seedling survival and growth by increasing air temperature and examining both the direct effects of warmer temperatures and the secondary effects of changes in relative humidity and soil moisture.This research will contribute to the understanding of vegetation adaptation and possible mitigation activities in response to expected impacts of continued warming on forests in the central Rocky Mountains. Although these forests historically exhibited high resiliency to wildfires, changes in fire severity may be limiting the capacity of conifer trees to reestablish following such fires. Decision-making by land managers may be improved by a better understanding of the factors that drive spatial and temporal variability in patterns of post-fire forest recovery. This research will foster science education through involvement of undergraduates in field and laboratory work as well as extensive educational outreach at the K-12, college, and community level. Undergraduate involvement in research and other educational outreach efforts will target inclusion of underrepresented groups. Findings will be disseminated to other researchers, land managers, and the broader public through peer-reviewed publication, presentation at conferences, online data sharing, and public education efforts including a museum exhibit and video documentary. As a Doctoral Dissertation Research Improvement award, this project will provide invaluable support for a capable student to launch an independent research career.