NOAA/WDS Paleoclimatology - Huffman fire data from Ellison Creek - IMPD USELL001

Ponderosa pine (Pinus ponderosa) forests occur at their warmer, drier environmental limits in the Mogollon Highlands ecoregion (MHE) of the Southwestern United States, and are commonly found in stringers or discrete stands that form ecotones with interior chaparral. These "rear edge" forests are likely to be highly vulnerable to rapid changes in structure and composition with climate warming, drought, and wildfire. There is increasing interest in understanding historical conditions, ecosystem changes, and restoration needs for MHE forests. However, comprehensive reconstruction analysis of fire regimes and stand structure has not been done for these systems, which differ from many montane ponderosa pine forests by having an abundance of understory shrubs. In this study we used demographic data from field plots, fire scar samples, and dendroecology to reconstruct historical fire regimes and landscape structure at ponderosa pine-dominated sites that spanned a range of environmental conditions on the Prescott and Tonto National Forests. We found strong evidence of historical surface fire regimes with mean fire intervals ranging 1.3-15.6 years across the five MHE sites during the period 1700-1879. We found very little evidence of historical high-severity fire at any study site. Historical forest structure was open with tree densities ranging 84.7-136.4 trees ha-1 and stand basal area (BA) ranging 4.5-8.4 m2 ha-1. Historical composition showed codominance of ponderosa pine, Arizona white oak (Quercus arizonica), Emory oak (Q. emoryi), and Gambel oak (Q. gambelii). Thus, oak species and likely other hardwoods were important historical components of these ecosystems. Contemporary forests are greater in stand density and BA by 359-703% and 285–502%, respectively, compared to historical estimates. In addition, we observed contemporary shifts in species composition. Changes related to disruption of historical fire regimes have increased susceptibility of ponderosa pine forests in the MHE to rapid shifts in structure and composition that may come about with climate change and high-intensity wildfire. Meeting fuels reduction and ecological restoration goals will be challenging for land managers due to vigorous regeneration responses of shrubs to tree thinning, prescribed burning, or other management activities. Managers will be required to balance attention to historical reference conditions, conservation of biological diversity, and needs for fuels management.

黄松（Ponderosa pine，学名Pinus ponderosa）林分分布于美国西南部莫戈隆高地生态区（Mogollon Highlands Ecoregion，MHE）的暖干环境极限范围，常以林带或离散林分的形式分布，与内陆灌丛群落形成生态交错带。这类“后缘”林分极易受气候变暖、干旱及野火影响，发生结构与组成的快速改变，因此具有极高的脆弱性。学界对莫戈隆高地生态区林分的历史状态、生态系统变化及修复需求的研究兴趣与日俱增。然而，相较于多数山地黄松林分，该区域林分林下灌木占比极高，目前尚未针对这类系统开展过火制度与林分结构的全面重建分析。本研究依托野外样地种群统计数据、火疤痕样品及树轮生态学（dendroecology）方法，对普雷斯科特国家森林与通托国家森林内不同环境梯度下的黄松主导样地，开展历史火制度与景观结构的重建工作。研究结果显示，1700–1879年间，莫戈隆高地生态区的5个样地均存在显著的地表火制度证据，其平均火间隔期为1.3–15.6年；所有研究样地均未发现历史高烈度火的相关证据。历史林分结构呈开阔状态，林木密度为84.7–136.4株·公顷⁻¹，林分断面积（basal area，BA）为4.5–8.4 m²·公顷⁻¹。历史群落组成以黄松、亚利桑那白栎（Quercus arizonica）、埃默里栎（Q. emoryi）及甘贝尔栎（Q. gambelii）为共优种，由此可见，栎类及其他阔叶树种曾是该生态系统的重要历史组成部分。与历史基准值相比，当前林分的林木密度与断面积分别提升了359%–703%与285%–502%。此外，研究还发现当前群落组成发生了显著转变。历史火制度被打破所引发的一系列变化，加剧了莫戈隆高地生态区黄松林分对气候变化与高强度野火的敏感性，使其更易发生结构与组成的快速转变。由于林下灌木对林木疏伐、计划火烧或其他管理措施会产生较强的更新响应，土地管理者在实现燃料削减与生态修复目标时将面临诸多挑战。管理者需要在历史参照状态保护、生物多样性保护与燃料管理需求之间寻求平衡。