In sagebrush ecosystems, pinyon and juniper tree expansion reduces water available to perennial shrubs and herbs. We measured soil water matric potential and temperatures at 13–30 and 50–65 cm soil depths in untreated and treated plots across a range of environmental conditions. We sought to determine the effects of tree expansion, tree reduction treatments, and expansion phase at time of treatment over 12–13 yr post‐treatment. Because the effects of tree reduction on vegetation can vary with the soil temperature/moisture regime, we also analyzed differences in soil climate variables between the mesic/aridic‐xeric and frigid/xeric regime classifications for our sites. Growing conditions during all seasons except spring were greatly limited by lack of available water, low temperatures, or both. Advanced tree expansion reduced wet days (total hours per 24 hr when hourly average soil water matric potential >−1.5 MPa), especially in early spring. Fire and mechanical tree reduction increased wet days and wet degree days (sum of hourly soil temperatures >0°C when soil is wet per 24 hr) compared with no treatment for most seasons. Burning resulted in higher soil temperatures than untreated or mechanically treated woodlands. Tree reduction at advanced expansion phases increased wet days in spring more than when implemented at earlier phases of expansion. Added wet days from tree reduction were negatively associated with October through June precipitation and vegetation cover, rather than time since treatment, with more wet days added on drier sites and years. The longer period of water availability in spring supports increased growth and cover of not only shrubs and perennial herbs, but also invasive weeds on warmer and drier sites, for many years after tree reduction. We found that sites classified as mesic/aridic‐xeric had warmer soil temperatures all seasons and were drier in spring and winter than sites classified as frigid/xeric. Land managers should consider reducing trees at earlier phases of expansion or consider revegetation when treating at advanced phases on these warmer and drier sites that lack perennial herb potential.

中文翻译：

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半干旱生态系统中树木膨胀和减少对土壤气候的长期影响

在鼠尾草的生态系统中，松树和杜松树的扩张减少了多年生灌木和草药的可用水。我们在各种环境条件下，在未经处理和经处理的样地中，在13–30和50–65 cm土深处测量了土壤水基质势和温度。我们试图确定树木扩展，树木减少处理以及在处理后12-13年内的处理时的扩展阶段的影响。由于树木砍伐对植被的影响会随土壤温度/水分状况的变化而变化，因此我们还分析了我们站点的中，旱，旱旱系土壤气候变量之间的差异。除春季外，所有季节的生长条件都受到可用水的缺乏，低温或两者的共同限制。先进的树木扩张减少了潮湿天数（当每小时平均土壤水基质势大于-1.5 MPa时，每24小时的总小时数），尤其是在早春。与大多数季节未进行处理相比，减少火种和机械砍伐树木增加了潮湿天和潮湿天数（当土壤每24小时潮湿时，每小时土壤温度总和> 0°C）。与未处理或机械处理的林地相比，燃烧导致的土壤温度更高。相比于早期扩张阶段，高级扩张阶段的树木砍伐增加了春季的湿天。树木砍伐所增加的湿日与10月至6月的降水和植被覆盖率呈负相关，而不是自处理以来的时间，在较干燥的地点和年份增加了更多的湿日。春季减水后的许多年里，春季较长的水分供应不仅促进了灌木和多年生草本植物的生长，而且覆盖了温暖和干燥地区的入侵性杂草，从而增加了其生长和覆盖率。我们发现，归类为中/旱旱生区的土壤在整个季节的土壤温度都较高，并且在春季和冬季比归类为冷/旱生旱区的土壤更干燥。在这些缺乏多年生草本植物潜力的较暖和较干燥的地区，土地管理人员应考虑在扩张的早期阶段砍伐树木或在后期阶段进行植被恢复。我们发现，归类为中/旱旱生区的土壤在整个季节的土壤温度都较高，并且在春季和冬季比归类为冷/旱生旱区的土壤更干燥。在这些缺乏多年生草本植物潜力的较暖和较干燥的地区，土地管理人员应在扩张的早期阶段考虑砍伐树木，或在后期阶段进行植被恢复。我们发现，归类为中/旱旱生区的土壤在整个季节的土壤温度都较高，并且在春季和冬季比归类为冷/旱生旱区的土壤更干燥。在这些缺乏多年生草本植物潜力的较暖和较干燥的地区，土地管理人员应在扩张的早期阶段考虑砍伐树木，或在后期阶段进行植被恢复。