Woodland and forest ecosystems across western North America have experienced increased density and expansion since the early 1900s, including in the widely distributed piñon-juniper vegetation type of the western United States. Fire suppression and grazing are often cited as the main drivers of these historic changes and have led to extensive tree-reduction treatments across the region. However, much of the scientific literature on piñon-juniper expansion dates back only to the early 1900s, which is generally half a century after Euro-American settlement. Yet US General Land Office (GLO) surveys provide valuable insight into the historical extent and density of woodland and forest ecosystems as surveyors would note where on the landscape they entered and exited woodlands or forests and provided qualitative estimates of relative tree density. This study uses these GLO surveys to establish piñon-juniper woodland extent in the late 19th century at the incipient stages of Euro-American settlement in southeastern Colorado and compares these data with 2017 aerial imagery of woodland cover. We found substantial amounts of woodland contraction, as well as expansion: ≈61% of historically dense woodland is now savanna or open (treeless), whereas ≈57% of historically open areas are now savannas or woodlands. The highest rates of expansion occurred on shallow, rocky soil types with low soil available water capacity, which support little herbaceous vegetation and were consequently less likely to be affected by fire suppression or grazing. Meanwhile, the significant contractions in woodland extent occurred on deeper, upland soils with higher soil available water capacity, which were likely where early settlement and tree cutting was most prevalent. Our results provide mixed support for the widespread assumption of woodland expansion since Euro-American settlement in southeast Colorado and suggest that the expansion that has occurred in our study area is unlikely a result of past grazing or fire suppression.

自1900年代初以来，整个北美西部的林地和森林生态系统经历了密度增加和扩张，其中包括美国西部分布广泛的松柏-杜松植物类型。灭火和放牧经常被认为是这些历史性变化的主要驱动力，并导致该地区广泛的减树措施。然而，许多有关杜松扩展的科学文献都可以追溯到1900年代初，这通常是欧美定居后半个世纪。然而，美国土地总署（GLO）的调查提供了有关林地和森林生态系统的历史范围和密度的宝贵见解，因为测量员会注意到他们进入和离开林地或森林的风景，并提供了相对树木密度的定性估计。这项研究使用这些GLO调查来建立19世纪末期在科罗拉多州东南部的欧洲裔美国人定居初期的松柏林地范围，并将这些数据与2017年林地覆盖的航空影像进行了比较。我们发现大量的林地收缩和扩张：大约61％的历史密集林地现在是稀树草原或开阔地（无树），而大约57％的历史开放区域现在是稀树草原或林地。最高的扩张速度发生在土壤可用水量低的浅层岩石土壤上，这些土壤几乎不支持草本植物，因此受灭火或放牧的影响较小。同时，林地范围内的明显收缩发生在土壤深层，土壤具有较高的可用水量的情况下，早期定居和砍伐树木最普遍的地方。我们的结果为自科罗拉多州东南部的欧洲裔美国人定居以来对林地扩张的广泛假设提供了混合的支持，并表明在我们的研究区域内发生的扩张不可能是过去放牧或灭火的结果。