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

Abstract

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-yr seed production period prior to germination and a 6-yr 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.

Pleistocene glacial cycles drove lineage diversification and fusion in the Yosemite toad (Anaxyrus canorus)

“…Conversely, low‐elevation lineages that remain in situ may experience little benefit from glacial action, and higher rates of conifer encroachment into meadows as trees recolonize middle elevations (Woolfenden ; Lubetkin et al. ).…”

“…Diversity patterns may also reflect current habitat: although high-elevation lineages must periodically retreat into refugia, their meadows are rejuvenated with new alluvial deposits that may positively influence hydrology for tadpoles (Wood 1975). Conversely, low-elevation lineages that remain in situ may experience little benefit from glacial action, and higher rates of conifer encroachment into meadows as trees recolonize middle elevations (Woolfenden 1996;Lubetkin et al 2017).…”

Spatial patterns of meadow sensitivities to interannual climate variability in the Sierra Nevada

“…An important caveat is that our analysis may have missed some of the most vulnerable meadows, as (>10,000) meadows that did not reach our minimum size threshold and meadows that have already disappeared due to land cover or climatic changes were not included. Also, this analysis was based on climate sensitivity of meadow vegetation occurring within contemporary meadow footprints, so does not explicitly account for meadows that have changed size over the course of the 31‐year study period, a pattern that is well‐documented in this region and is attributable to conifer encroachment (Lubetkin, Westerling, & Kueppers, ). Finally, the climate sensitivity metrics used in this analysis characterize one aspect of climate sensitivity of vegetation relating to late‐season water availability.…”

Insights on the persistence of pines (Pinusspecies) in the Late Cretaceous and their increasing dominance in the Anthropocene

“…In this paper, we use the term “Anthropocene” to refer to the period during which human activity has become the dominant influence on climate and the environment. Many pine species have undergone major range expansions in the Anthropocene by expanding their ranges in the Northern Hemisphere (Améztegui, Brotons, & Coll, ; Jakubos & Romme, ; Lubetkin, Westerling, & Kueppers, ; Prévosto, Hill, & Coquillard, ; Taylor, Maxwell, Pauchard, Nuñez, & Rew, 2016b; Taylor et al., 2016a) and, due to large‐scale plantings and invasions, mainly in the Southern Hemisphere (Essl, Mang, Dullinger, Moser, & Hulme, ; Richardson, Williams, & Hobbs, ; Simberloff et al., ). Pine is the dominant taxon in natural forests over parts of the Northern Hemisphere (Richardson, ), and more than 20% of pine species are invasive in regions outside their native ranges (Nuñez et al., ; Rejmánek & Richardson, ; Richardson & Rejmánek, ; Rundel, Dickie, & Richardson, ; Simberloff et al., ).…”

“…Pines have expanded their ranges in the Northern Hemisphere due to planting by humans (e.g., Brundu & Richardson, ) and by encroaching into previously treeless ecosystems. Examples in North America are where the native species P. contorta has colonized grasslands and shrublands (Figure e; Jakubos & Romme, ; Lubetkin et al., ; Taylor et al., 2016b), in France where P. sylvestris has spread into abandoned lawns and heathlands (Prévosto et al., ), and in Spain where P. uncinata has expanded its range in the Pyrenees (Améztegui et al., ). Pine invasiveness is associated with ruderal strategies, large niche breadth, human use, historical biogeography, and climate (Essl et al., ; Gallien et al., ; Grotkopp et al., ; McGregor, Watt, Hulme, & Duncan, ; Pauchard et al., ; Procheş, Wilson, Richardson, & Rejmánek, ).…”