Long-term effectiveness of fuel treatments in Whiskeytown, CA

Aims: Fuel reduction treatments are broadly implemented to reduce the risk of extreme wildfire, but research on the long-term effectiveness and impacts of these treatments is lacking. In this study, we examined short- and long-term (2 and 15 year) changes in fuels and understory vegetation after treatment in chaparral and oak-dominated stands. Location: Whiskeytown National Recreation Area, California, USA Methods: Treatments, including hand-thinning, mastication only, burn only, and mastication + burning, were applied randomly to one or two units within each of ten blocks across two vegetation types (chaparral and oak-dominated). Vegetation data was initially 2 years after treatment and remeasured 15 years later with additional sampling of dead surface fuels and shrubs. Fuel and vegetation data were analyzed to examine the effect of treatment, time, and vegetation type. Results: In chaparral stands, hand-thinning, mastication, and mastication + burning reduced shrub height and cover compared to the control. However, only the hand thinning treatment reduced fine woody fuels. Hand-thinning and mastication + burning increased native species richness after two years, but this was also associated with an increase in exotic species richness and cover that persisted after 15 years. In oak-dominated stands, treatments had varied and relatively fewer changes to fuels and vegetation. Shrub height was reduced in both the hand-thinning and burn only treatments, but only mastication reduced shrub cover. Species richness and vegetative cover were largely unaffected by treatment, except for lower native plant cover in mastication and mastication + burning treatments. Conclusion: Treatments varied in their level of effectiveness and most involved trade-offs between their impacts on fuels and vegetation responses that differed by vegetation type. Our findings provide insights for managers interested in balancing these trade-offs when making fuel treatment decisions and emphasize the importance of examining longer-term effectiveness of fuels treatments.

研究目标：
可燃物削减处理（fuel reduction treatments）被广泛应用以降低极端野火风险，但当前针对此类处理的长期效果与影响的研究仍较为匮乏。本研究以查帕拉尔灌丛（chaparral）和以栎树为主的林分（oak-dominated stands）为研究对象，探究处理后短期（2年）与长期（15年）的可燃物与林下植被变化。
研究地点：
美国加利福尼亚州惠斯基特国家休闲区（Whiskeytown National Recreation Area）
研究方法：
本研究在两种植被类型（查帕拉尔灌丛、以栎树为主的林分）的10个区组（blocks）中，随机将人工疏伐（hand-thinning）、仅机械碎化（mastication only）、仅火烧（burn only）、机械碎化+火烧（mastication + burning）四种处理方式应用于每个区组内的1或2个处理单元（units）。植被数据于处理后2年首次采集，并于15年后进行复测，同时额外采集了地表死可燃物与灌木样本。通过对可燃物与植被数据进行分析，以探究处理方式、时间以及植被类型对研究对象的影响。
研究结果：
在查帕拉尔灌丛样地中，相较于对照组，人工疏伐、机械碎化以及机械碎化+火烧处理均降低了灌木高度与盖度。但仅人工疏伐处理可降低细木质燃料（fine woody fuels）储量。处理后2年，人工疏伐与机械碎化+火烧处理提升了本土物种丰富度，但该效果同时伴随外来物种丰富度与盖度的增加，且这一现象在15年后仍持续存在。
在以栎树为主的林分中，各类处理对可燃物与植被的影响相对有限且存在差异。人工疏伐与仅火烧处理均降低了灌木高度，但仅机械碎化处理可降低灌木盖度。除机械碎化与机械碎化+火烧处理下的本土植物盖度有所降低外，物种丰富度与植被盖度基本不受处理方式的影响。
研究结论：
各类可燃物削减处理的有效程度存在差异，且多数处理在可燃物影响与植被响应之间存在权衡取舍，而该权衡取舍因植被类型而异。本研究结果可为致力于在制定可燃物处理决策时平衡此类权衡取舍的管理者提供参考，同时强调了探究可燃物处理长期效果的重要性。