title: Warming and provenance limit tree recruitment across and beyond the elevation range of subalpine forest.
abstract: Climate niche models project that subalpine forest ranges will extend upslope with climate warming. These projections assume that the climate suitable for adult trees will be adequate for forest regeneration, ignoring climate requirements for seedling recruitment, a potential demographic bottleneck. Moreover, local genetic adaptation is expected to facilitate range expansion, with tree populations at the upper forest edge providing the seed best adapted to the alpine. Here, we test these expectations using a novel combination of common gardens, seeded with two widely distributed subalpine conifers, and climate manipulations replicated at three elevations. Infrared heaters raised temperatures in heated plots, but raised temperatures more in the forest than at or above treeline because strong winds at high elevation reduced heating efficiency. Watering increased season-average soil moisture similarly across sites. Contrary to expectations, warming reduced Engelmann spruce recruitment at and above treeline, as well as in the forest. Warming reduced limber pine first-year recruitment in the forest, but had no net effect on fourth-year recruitment at any site. Watering during the snow-free season alleviated some negative effects of warming, indicating that warming exacerbated water limitations. Contrary to expectations of local adaptation, low-elevation seeds of both species initially recruited more strongly than high-elevation seeds across the elevation gradient, although the low-provenance advantage diminished by the fourth year for Engelmann spruce, likely due to small sample sizes. High- and low-elevation provenances responded similarly to warming across sites for Engelmann spruce, but differently for limber pine. In the context of increasing tree mortality, lower recruitment at all elevations with warming, combined with lower quality, high-provenance seed being most available for colonizing the alpine, portends range contraction for Engelmann spruce. The lower sensitivity of limber pine to warming indicates a potential for this species to become more important in subalpine forest communities in the coming centuries.
authors.affiliation: Sierra Nevada Research Institute, University of California, Merced, 5200 North Lake Road, Merced, CA, 95343, USA.|Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, 74-316C, Berkeley, CA, 94720, USA.
authors.forename: Lara M
authors.initials: LM
authors.lastname: Kueppers
authors.affiliation: Energy and Resources Group, University of California, Berkeley, 310 Barrows Hall, Berkeley, CA, 94720, USA.
authors.forename: Erin
authors.initials: E
authors.lastname: Conlisk
authors.affiliation: Sierra Nevada Research Institute, University of California, Merced, 5200 North Lake Road, Merced, CA, 95343, USA.|Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, 74-316C, Berkeley, CA, 94720, USA.
authors.forename: Cristina
authors.initials: C
authors.lastname: Castanha
authors.affiliation: Sierra Nevada Research Institute, University of California, Merced, 5200 North Lake Road, Merced, CA, 95343, USA.|Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, 74-316C, Berkeley, CA, 94720, USA.
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Moyes
authors.affiliation: U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, Boise, ID, 83706, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Germino
authors.affiliation: Department of Environmental Science, Policy and Management, University of California, Berkeley, 130 Mulford Hall, Berkeley, CA, 94720, USA.
authors.forename: Perry
authors.initials: P
authors.lastname: de Valpine
authors.affiliation: Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, 74-316C, Berkeley, CA, 94720, USA.|Energy and Resources Group, University of California, Berkeley, 310 Barrows Hall, Berkeley, CA, 94720, USA.
authors.forename: Margaret S
authors.initials: MS
authors.lastname: Torn
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, CO, 80309, USA.
authors.forename: Jeffry B
authors.initials: JB
authors.lastname: Mitton
mesh_terms: D002980:Climate; D065928:Forests; D028222:Picea; D028223:Pinus; D014197:Trees
keywords: 
  Picea engelmannii
  ; 
  Pinus flexilis
  ; Engelmann spruce; alpine treeline; climate change experiment; limber pine; seedling demography; species range shift