Data from: Climate and landscape drive the pace and pattern of conifer encroachment into subalpine meadows

Mountain meadows have high biodiversity and help regulate stream water release following the snowmelt pulse. However, many meadows are experiencing woody plant encroachment, threatening these ecosystem services. While there have been field surveys of individual meadows and remote sensing-based landscape-scale studies of encroachment, what is missing is a broad scale, ground-based study to understand common regional drivers, especially at high elevations, where land management has often played a less direct role. With this study we ask: what are the climate and landscape conditions conducive to woody plant encroachment at the landscape scale, and how has historical climate variation affected tree recruitment in subalpine meadows over time? We measured density of encroaching trees across 340 subalpine meadows in the central Sierra Nevada, California, USA, and used generalized additive models (GAMs) to determine the relationship between landscape-scale patterns of encroachment and meadow environmental properties. We determined ages of trees in 30 survey meadows, used observed climate and GAMs to model the relationship between timing of recruitment and climate since the early 1900s, and extrapolated recruitment patterns into the future using downscaled climate scenarios. Encroachment was high among meadows with lodgepole pine (Pinus contorta Douglas ex Loudon var. murrayana (Balf.) Engelm.) in the immediate vicinity, at lower elevations, with physical conditions favoring strong soil drying, and with maximum temperatures above or below average. Climatic conditions during the year of germination were unimportant, with tree recruitment instead depending on a 3-year seed production period prior to germination and a 6-year seedling establishment period following germination. Recruitment was high when the seed production period had high snowpack, and when the seedling establishment period had warm summer maximum temperatures, high summer precipitation, and high snowpack. Applying our temporal model to downscaled output from four global climate models indicated that the average meadow will shift to forest by the end of the 21st century. Sierra Nevada meadow encroachment by conifers is ubiquitous and associated with climate conditions increasingly favorable for tree recruitment, which will lead to substantial changes in subalpine meadows and the ecosystem services they provide.

山地草甸具有极高的生物多样性，且能在融雪脉冲后调节溪流径流量。然而，当前多数山地草甸正面临木本植物入侵的威胁，损害其提供的各类生态系统服务功能。尽管已有针对单个草甸的野外调查，以及基于遥感的景观尺度木本入侵研究，但目前仍缺乏大尺度地面调查研究，以明晰木本入侵的通用区域驱动因子——尤其是在土地管理干预通常较弱的高海拔区域。本研究旨在解答两大科学问题：其一，景观尺度下利于木本植物入侵的气候与景观条件是什么？其二，历史气候波动如何长期影响亚高山草甸的树木定植过程？我们在美国加利福尼亚州内华达山脉中部区域的340处亚高山草甸中，对入侵树木的密度进行了采样调查，并借助广义加性模型（Generalized Additive Models, GAMs）分析景观尺度木本入侵格局与草甸环境属性间的关联。我们还对30个调查草甸中的树木树龄进行了测定，结合实测气候数据与GAMs模型，构建了自20世纪初以来树木定植时间与气候因子间的关联模型，并通过降尺度气候情景数据将定植格局推演至未来。研究发现，当草甸周边紧邻扭叶松（Pinus contorta Douglas ex Loudon var. murrayana (Balf.) Engelm.）、所处海拔较低、土壤物理条件易发生强烈干旱化，且极端最高温度偏离平均水平时，该草甸的木本入侵程度通常较高。树木定植当年的气候条件对入侵过程影响微弱，树木定植实际取决于定植前3年的种子生产阶段气候，以及定植后6年的幼苗建群阶段气候：当种子生产阶段积雪量充足，且幼苗建群阶段夏季极端最高温偏高、夏季降水量充沛且积雪量充足时，树木定植率较高。将本研究的时间序列模型应用于4个全球气候模型的降尺度输出结果后发现，到21世纪末，典型草甸将完全转变为森林生态系统。内华达山脉的针叶树入侵草甸现象已十分普遍，且与愈发利于树木定植的气候条件密切相关，这将导致亚高山草甸及其提供的生态系统服务功能发生显著改变。