NOAA/WDS Paleoclimatology - Yocom fire data from Orizaba 6, eastern Transvolcanic Belt, Puebla, Mexico - IMPD MXOZ6001

1. Surface fire has increasingly been regarded as a critical threat to tropical forests, but much of the research documenting degradation of tropical forests by fire comes from the low-elevation humid tropics. Fire in high-elevation tropical forests has received less research attention, but these forests are of high conservation value because they support unique ecosystems, which are often isolated due to their restriction to widely separated peaks. 2. We investigated the frequency and ecological impact of fire on a high-elevation tropical forest of Pinus hartwegii in Pico de Orizaba National Park in central Mexico. This forest was previously thought to have been degraded by excessive human-caused fires. We assessed human-caused changes to the fire regime as well as the impact of climate on fire occurrence, both previously undocumented in this region. 3. We found no increase in fire frequency or evidence of degradation of the forest. We found that the forest was uneven-aged and contained many large and old trees (maximum age 483 years). In the twentieth century, the forest experienced a frequent surface fire regime, with fires scarring trees in 90 of 100 years. However, most fires were small and asynchronous among sites. Inter-annual climatic variability was not an influential driver of fire, and El Niño Southern Oscillation was not significantly related to the occurrence of widespread fire. 4. Synthesis and applications. Our results show that this high-elevation tropical forest has not been degraded but has existed with frequent fires for at least a century. A trend in the 21st century towards less-frequent fire could be cause for concern, as a decrease in fire frequency could lead to an increase in tree density and a loss of resilience in the face of climate change and other future disturbance. We recommend that managers take into account historical fire regimes in their local areas: frequent surface fires in the case of Pico de Orizaba. It is important to recognize that although fire can be detrimental in many low-elevation tropical forests, it is an integral part of this high-elevation tropical forest ecosystem, and other high-elevation forests may show similar patterns.

1. 地表火（surface fire）日益被视为热带森林面临的重大威胁，但目前多数记录火灾导致热带森林退化的研究均聚焦于低海拔湿润热带地区。高海拔热带森林的火灾研究却相对匮乏，然而这类森林拥有极高的保护价值——它们支撑着独特的生态系统，而这些生态系统往往因局限于彼此分隔的山峰而呈孤立状态。
2. 本研究针对墨西哥中部奥里萨巴峰国家公园（Pico de Orizaba National Park）内的哈特维格松（Pinus hartwegii）高海拔热带森林，调查了火灾发生频率及其生态影响。此前学界认为该森林曾因人为过火过多而发生退化。我们评估了人为活动对火灾格局的改变，以及气候对火灾发生的影响——这两项内容在该区域此前均未被记录过。
3. 研究结果显示，该区域火灾频率并未上升，也无森林退化的证据。我们发现该森林年龄结构不均，且拥有大量高大古树（最大树龄达483年）。在20世纪，该森林曾处于频繁的地表火格局中，100年中有90年的树木都留有火烧疤痕。不过绝大多数火灾规模较小，且不同地点的火灾发生并不同步。年际气候波动并非影响火灾的关键驱动因素，厄尔尼诺南方涛动（El Niño Southern Oscillation）与大规模火灾的发生也无显著关联。
4. 综合与应用。本研究结果表明，这片高海拔热带森林并未发生退化，反而至少在一个世纪以来始终与频繁的地表火共存。21世纪以来火灾频率呈下降趋势，这或需引起警惕：火灾频率降低可能导致树木密度上升，进而削弱森林应对气候变化及未来其他干扰的恢复力。我们建议管理者应将当地的历史火灾格局纳入管理考量：针对奥里萨巴峰区域而言，即需维持频繁的地表火格局。需要明确的是，尽管火灾在多数低海拔热带森林中具有破坏性，但它却是这片高海拔热带森林生态系统不可或缺的组成部分，其他高海拔森林或许也存在类似的格局。