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

本论文项目是关于森林火灾后环境变化对树木再生的影响。自20世纪80年代初以来，世界许多地区森林生态系统中野火活动的急剧增加与变暖趋势有关。问题是，火灾后的恢复将是同一类型的森林还是不同的，可能是非森林植被类型。对过去大约25年里在科罗拉多前山脉燃烧过的较低林木线森林的初步观察得出这样的假设：与19世纪末至20世纪中期发生的火灾事件相比，在较温暖的气候下，火灾后树木的再生较少。尽管大量的回顾性研究记录了19世纪末至20世纪中期火灾后树木再生的总体成功，但这些研究对树木再生的实际时间和丰度以及与年尺度气候变化的潜在关系并不明确。很少有研究考察气候变化如何改变森林对未来与气候有关的干扰（如火灾）的恢复能力，而本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，落基山低树线森林对野火的恢复能力如何降低。研究目标是：1)通过检查自20世纪80年代中期以来烧毁的一系列低树线站点上针叶树幼树的密度，量化火灾后针叶树的建立和生存；2)检查针叶树幼树密度的空间变异性与站点因素有关，如火灾严重程度（由树木死亡率百分比表示）、与草本和木本物种的竞争、到种子源的距离以及地形变量（包括海拔和坡向）；3)分析火灾后针叶树更新与年气候变率的关系，确定气候条件是否限制或有利于针叶树的建立和存活；4)通过增加气温，研究温度升高的直接影响和相对湿度和土壤湿度变化的二次影响，实验操纵小气候，确定对针叶树幼苗存活和生长的影响。这项研究将有助于了解为应对持续变暖对落基山脉中部森林的预期影响而进行的植被适应和可能的减缓活动。尽管这些森林在历史上对野火表现出很高的恢复能力，但火灾严重程度的变化可能限制了针叶树在火灾后重建的能力。如果能更好地了解导致火灾后森林恢复格局时空变化的因素，土地管理者的决策可能会得到改善。这项研究将通过让本科生参与野外和实验室工作，以及在K-12、大学和社区层面进行广泛的教育推广，来促进科学教育。本科生参与研究和其他教育推广工作的目标是纳入代表性不足的群体。研究结果将通过同行评议出版物、会议报告、在线数据共享和包括博物馆展览和录像纪录片在内的公众教育工作，传播给其他研究人员、土地管理者和更广泛的公众。作为博士论文研究进步奖，该项目将为有能力的学生开展独立研究事业提供宝贵的支持。

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.