Climate and Woodland Expansion in the Western Great Plains, USA

西部大平原和落基山脉之间的边界是北美一个突出的生物地理特征。在过去的一个世纪里，在大平原中西部的许多地区，有记载的森林扩张到原来的草原。目前的生物地理学和生态学知识表明，林地向草原的扩张可能受到气候、人类土地利用和/或物种迁移的驱动，这些因素中的任何一个都不可能完全独立于其他因素。这项研究将解开与这种扩张有关的因素，并确定气候的作用。研究人员将调查扩张的时间，以及在过去三到六个世纪里，从内布拉斯加州西部延伸到新墨西哥州东北部的空间梯度上，扩张是如何与气候变化相吻合的。为此，将选择在整个空间梯度上具有统一土地利用历史的林地扩张区域进行研究。树木生态学技术将用于重建黄松（Pinus ponderosa）林地扩张时间。然后，使用现代和重建的古气候数据进行气候分析，将检查每年到百年时间尺度上的气候变化和趋势。最后，将对研究区域进行木冢（Neotoma）冢的调查，以研究该地区可能的全新世早期扩张。通过将林地扩张的时间和地点与当地气候变化和广泛的人类土地利用的开始进行比较，研究气候在19世纪和20世纪林地扩张中的作用。古生态和古气候记录将用于确定过去是否发生过其他扩张，或者19世纪50年代以后的植被运动是否具有前所未有的性质。在过去的一个世纪里，人类对景观的影响可以掩盖植被和气候之间的长期关系。这可能导致一种假设，即最近植被动态的任何变化都是人类活动的结果。在广泛的人类活动开始之前的长期数据使我们能够确定气候和林地扩张之间的长期关系，并确定这些关系在过去一个世纪中是否发生了变化。该项目的结果将揭示气候在其他林地扩张中可能发挥的作用，并阐明未来气候变化对许多地区林地扩张的潜在影响。一个跨学科小组将与美国农业部和短草草原长期生态研究站密切合作，确定林地的长期管理目标和研究议程。

The boundary between the western Great Plains and the Rocky Mountains is a prominent biogeographical feature of North America. Over the past century, woodland expansion into former grasslands has been documented in many areas of the west-central Great Plains. Current biogeographical and ecological knowledge indicate that the expansion of woodlands into grasslands can be driven by climate, human land use, and/or species migration, and none of these factors may be entirely independent of the others. This study will disentangle the factors related to this expansion and identify the role of climate. The timing of expansion will be investigated and how the expansion coincides with climate variability over the past three to six centuries across a spatial gradient that extends from western Nebraska to northeastern New Mexico. To do this, areas of woodland expansion with uniform land-use histories across the spatial gradient will be selected for study. Dendroecological techniques will be used to reconstruct the timing of ponderosa pine (Pinus ponderosa) woodland expansion. Then, climate analyses using modern and reconstructed paleoclimatic data will examine variations and trends in climate over annual to centennial time scales. Finally, the study area will be surveyed for woodrat (Neotoma) middens to investigate possible earlier Holocene expansions in the area. The role of climate in 19th and 20th century woodland expansion will be investigated by comparing the timing and location of the expansion to local climate variations and the onset of widespread human land use. The paleoecological and paleoclimatological records will be used to determine whether other expansions have occurred in the past or whether post-1850s vegetation movements are unprecedented in character.Human impacts on the landscape over the past century can obscure long-term relationships between vegetation and climate. This can lead to the assumption that any recent changes in vegetation dynamics are the result of human activity. Long-term data predating the onset of wide-spread human activity allow us to identify the long-term relationships between climate and woodland expansion and to determine if those relationships have changed over the past century. The results of this project will shed light on the possible role of climate in other cases of woodland expansion, as well as elucidating the potential affects of future climate change on woodland expansion in many regions. An interdisciplinary team will work closely with the U.S. Department of Agriculture and the Shortgrass Steppe Long-Term Ecological Research site to identify long-term management goals and research agendas for woodland areas.