Holocene Vegetation-Fire-Climate Linkages in Northern Yellowstone National Park, USA.

Yellowstone National Park has been an important location for paleoecologic studies that focus on the use of charcoal data to reconstruct past fire activity and on the role of climate variations in shaping past vegetation and fire regimes. One hypothesis, which has been explored in other parts of the western U.S., is the idea that present-day summer-dry and summer-wet precipitation regimes were intensified during the early Holocene as a result of greater-than-present summer insolation and its effect on atmospheric circulation patterns. In Yellowstone, this hypothesis was previously examined at two sites, one in summer-wet and one in summer-dry precipitation regions. The records showed variation in both fire and vegetation history that supported the hypothesis. We present a fire and vegetation history from Blacktail Pond, located in Pseudotsuga parkland in the transitional region. The Blacktail Pond data indicate the following ecological history: prior to 12,000 cal yr BP, the site supported tundra vegetation and fire episodes were infrequent. Between 12,000 and 11,000 cal yr BP, Picea–Pinus parkland was established and fire activity increased; these changes are consistent with increasing temperature, as a result of rising summer insolation. From 11,000 to 7600 cal yr BP, the presence of a closed forest of Pinus and some Picea is attributed to high levels of winter moisture, but high fire activity indicates that summers were drier than at present. After 7600 cal yr BP, the presence of forest and steppe vegetation in combination with high fire activity suggest that middle-Holocene conditions were warm and dry. The decrease in Picea and Betula in the last 4000 cal yr indicates continued drying in the late Holocene, although fire-episode frequency was relatively high until 2000 cal yr BP. The pollen data at Blacktail Pond and other low-elevation sites in the northern Rocky Mountains suggest a widespread vegetation response in summer-wet regions to effectively wetter conditions in the early Holocene and decreased moisture in the middle and late Holocene. In contrast, the more-variable fire history among the three sites implies either that (1) summer moisture stress and fire conditions are related to year-round moisture balance and not well predicted by the hypothesis, (2) the transitional area between summer-wet and summer-dry precipitation regimes experienced complicated shifts in effective moisture through time, and/or (3) fire-episode data have a limited source area that makes it difficult to separate local influences from regional climate changes in understanding long-term variations in fire-episode frequency.

Huerta, M., Whitlock, C. and J. Yale. 2009. Holocene Vegetation-Fire-Climate Linkages in Northern Yellowstone National Park, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 271, 170-181

黄石国家公园一直是古生态学研究的重要区域，相关研究聚焦于利用炭屑数据重建过往火灾活动，以及气候变化在塑造过去植被与火况中的作用。此前美国西部其他地区已对某一假说开展过研究，该假说提出：由于夏季太阳辐射水平高于现今及其对大气环流模式的影响，现今夏季干旱型与夏季湿润型的降水格局在全新世早期得到了强化。在黄石国家公园内，此前已有两处点位对该假说进行了验证，一处位于夏季湿润型降水区，另一处位于夏季干旱型降水区。两处的记录均揭示了火灾与植被历史的变化特征，为该假说提供了支撑依据。本研究报道了位于过渡区域花旗松（Pseudotsuga）疏林中的黑尾塘（Blacktail Pond）的火灾与植被历史记录。黑尾塘的记录揭示了如下生态演化历程：在距今12000校准距今年（calibrated years before present, cal yr BP）之前，该区域以苔原植被为主，火灾事件较为罕见。在距今12000至11000 cal yr BP期间，云杉-松疏林得以形成，火灾活动有所增强；这些变化与夏季太阳辐射升高导致的气温上升趋势一致。在距今11000至7600 cal yr BP期间，区域内形成了以松属与部分云杉属植物为主的郁闭林，这一现象归因于冬季降水充沛；但较高的火灾活动表明当时夏季较现今更为干旱。在距今7600 cal yr BP之后，森林与草原植被共存且火灾活动频繁，表明全新世中期的气候条件温暖干旱。在距今4000 cal yr BP以来的时段内，云杉属与桦木属（Betula）植物的占比持续下降，表明全新世晚期气候持续干旱；尽管直至距今2000 cal yr BP，火灾事件频率仍处于较高水平。黑尾塘与落基山脉北部其他低海拔点位的孢粉数据表明，夏季湿润型区域的植被对全新世早期更为湿润的有效湿度条件，以及全新世中晚期湿度下降均产生了广泛响应。与之形成对比的是，三处点位的火灾历史变化差异更大，这意味着以下三种可能性之一：（1）夏季水分胁迫与火灾状况与全年水分平衡密切相关，而非该假说所能很好预测；（2）夏季湿润型与夏季干旱型降水格局之间的过渡区域，其有效湿度随时间发生了复杂的时空变化；（3）火灾事件数据的源区范围有限，这使得在解读火灾事件频率的长期变化时，难以区分局地影响与区域气候变化。参考文献：Huerta, M.、Whitlock, C. 与 Yale, J.，2009年。《美国黄石国家公园北部全新世植被-火灾-气候关联》。《古地理学、古气候学、古生态学》（Palaeogeography, Palaeoclimatology, Palaeoecology）第271卷，第170-181页。