Pinyon (Pinus spp.) and juniper (Juniperus spp.) woodland encroachment into sagebrush (Artemisia spp.) steppe communities throughout western North America has substantially altered the vegetation structure and hydrologic function of one of the most ecologically important rangeland ecosystems in the world. Various pinyon and juniper tree removal practices are employed to re-establish sagebrush steppe vegetation and an associated resource-conserving ecohydrologic function. The effectiveness of these practices is highly variable owing to the vast domain in which woodland encroachment occurs, climate fluctuations, differences in treatment applications, and myriads of pre-treatment conditions and post-treatment land uses. This study evaluated the long-term (13 years post-treatment) effectiveness of prescribed fire and mechanical tree removal to re-establish sagebrush steppe vegetation and associated spatial patterns in ground surface conditions and soil hydrologic properties of two woodland-encroached sites. Specifically, we assessed the effects of tree removal on: (1) vegetation and ground cover at the hillslope scale (990 m2 plots) and (2) associated spatial patterns in point-scale ground surface conditions and soil hydrologic properties along transects extending from tree bases and into the intercanopy areas between trees. Both sites were in mid to late stages of woodland encroachment with extensive bare conditions (~60–80% bare ground) throughout a degraded intercanopy area (~75% of the domain) surrounding tree islands (~25% of domain, subcanopy areas). All treatments effectively removed mature tree cover and increased hillslope vegetation. Enhanced herbaceous cover (4–15-fold increases) in burned areas reduced bare interspace (bare area between plants) by at least 4-fold and improved intercanopy hydraulic conductivity (> than 2-fold) and overall ecohydrologic function. Mechanical treatments retained or increased sagebrush and generally increased the intercanopy herbaceous vegetation. Intercanopy ground surface conditions and soil hydrologic properties in mechanical treatments were generally similar to those in burned areas but were also statistically similar to the same measures in untreated areas in most cases. This suggests that vegetation and ground surface conditions in mechanical treatments are trending toward a significantly improved hydrologic function over time. Treatments had limited impact on soil hydrologic properties within subcanopy areas; however, burning did reduce the soil water repellency strength and the occurrence of strong soil water repellency underneath trees by three- to four-fold. Overall, the treatments over a 13-year period enhanced the vegetation, ground surface conditions, and soil hydrologic properties that promote infiltration and limit runoff generation for intercanopy areas representing ~75% of the area at the sites. However, ecological tradeoffs in treatment alternatives were evident. The variations in woodland responses across sites, treatments, and measurement scales in this long-term study illustrate the complexity in predicting vegetation and hydrologic responses to tree removal on woodland-encroached sagebrush sites and underpin the need and value of multi-scale long-term studies.

中文翻译：

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树木移除对重新建立山艾树草原植被和地表条件和土壤水文特性的相关空间格局的长期有效性

Pinyon (Pinus spp.) 和杜松 (Juniperus spp.) 林地侵占整个北美西部的山艾树 (Artemisia spp.) 草原群落，极大地改变了世界上生态最重要的牧场生态系统之一的植被结构和水文功能。采用各种松树和杜松树移除做法来重建山艾树草原植被和相关的资源保护生态水文功能。由于林地侵占发生的广阔领域、气候波动、处理应用的差异以及无数的预处理条件和处理后的土地用​​途，这些做法的有效性差异很大。本研究评估了规定的火和机械树木移除的长期（处理后 13 年）有效性，以重建山艾树草原植被和两个林地侵占地点的地表条件和土壤水文特性的相关空间格局。具体而言，我们评估了树木移除对以下方面的影响：（1）山坡尺度（990 平方米地块）的植被和地表覆盖；（2）点尺度地表条件和沿从树木延伸的断面的土壤水文特性的相关空间模式基部并进入树木之间的树冠间区域。两个地点都处于林地侵占的中后期阶段，在整个退化的树冠间区域（约 75% 的域）周围的树岛（约 25% 的域，亚冠层区域）中都有广泛的裸露条件（约 60-80% 的裸地） . 所有处理都有效地去除了成熟的树木覆盖并增加了山坡植被。烧毁区域中草本覆盖的增强（增加 4-15 倍）将裸露的空间（植物之间的裸露区域）减少了至少 4 倍，并改善了冠层间导水率（> 2 倍）和整体生态水文功能。机械处理保留或增加了山艾树，并普遍增加了冠层间草本植被。在大多数情况下，机械处理中的冠层间地面条件和土壤水文性质与燃烧区域的相似，但在统计上也与未处理区域的相同测量值相似。这表明机械处理中的植被和地表条件随着时间的推移趋于显着改善水文功能。处理对亚冠区域内土壤水文特性的影响有限；然而，燃烧确实降低了土壤拒水性强度，并且树下土壤拒水性强的发生率降低了三到四倍。 1998 年，2016 年。 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 总体而言，超过 13 年的处理增强了植被、地表条件和土壤水文特性，促进渗透并限制了占场地面积约 75% 的冠层间区域的产生。然而，治疗替代方案的生态权衡是显而易见的。不同地点、处理方法的林地反应差异