AimGlobally, forests at the alpine-treeline ecotone (ATE) are considered sensitive to warming temperatures; however, responses to recent climate change show high variability and many underlying processes remain unclear. This study aims to provide further insight into possible ATE forest responses to climate change by examining spatiotemporal patterns in recent tree regeneration and growth responses to climate across treeline forms.LocationThis study is situated at the ATE in the Rocky Mountain and Columbia Mountain ranges in central British Columbia, Canada.TaxonGymnosperms - subalpine fir (Abies lasiocarpa Hooker (Nutall)).MethodsWe collected tree and stand data from 48 plots across five study sites. Plots were distributed across three treeline stand types: (i) islands; (ii) abrupt; and (iii) fringes of regeneration adjacent to tree islands. We used a dendrochronological approach to analyse the ages of recently established trees in fringe stand types, detect long-term trends in annual tree growth and quantify climate-growth relationships.ResultsSeedling recruitment adjacent to tree islands occurred over a period of approximately 40 years (1960-2000), with two regeneration pulses in the late 1970s and 1980s. Abrupt and fringe trees showed a similar age structure and annual radial growth has increased in most trees over the past 30 years. Across the study region and stand types, summer temperature has the strongest influence on radial growth. Over the past 70 years, growth in tree islands has become increasingly correlated with growing season temperature variables.Main ConclusionsForest growth and structure have changed in coherent spatial and temporal patterns over recent decades at the ATE in central BC. Projections for sustained warming in this region will likely result in increased tree growth and potential continued expansion of forests into untreed areas below the treeline. These changes will have implications for hydrological regimes, wildlife habitat and carbon sequestration.

This study analyzes mid-21st century projections of daily surface air minimum (T-min) and maximum (T-max) temperatures, by season and elevation, over the southern range of the Colorado Rocky Mountains. The projections are from four regional climate models (RCMs) that are part of the North American Regional Climate Change Assessment Program (NARCCAP). All four RCMs project 2A degrees C or higher increases in T-min and T-max for all seasons. However, there are much greater (> 3A degrees C) increases in T-max during summer at higher elevations and in T-min during winter at lower elevations. T-max increases during summer are associated with drying conditions. The models simulate large reductions in latent heat fluxes and increases in sensible heat fluxes that are, in part, caused by decreases in precipitation and soil moisture. T-min increases during winter are found to be associated with decreases in surface snow cover, and increases in soil moisture and atmospheric water vapor. The increased moistening of the soil and atmosphere facilitates a greater diurnal retention of the daytime solar energy in the land surface and amplifies the longwave heating of the land surface at night. We hypothesize that the presence of significant surface moisture fluxes can modify the effects of snow-albedo feedback and results in greater wintertime warming at night than during the day.

We integrated experimental and natural gradient field methods to investigate effects of climate change and variability on flowering phenology of 11 subalpine meadow shrub, forb, and graminoid species in Gunnison County, Colorado (USA). At a subalpine meadow site, overhead electric radiant heaters advanced snowmelt date by 16 d and warmed and dried soil during the growing season. At three additional sites, a snow removal manipulation advanced snowmelt date by 7 d without altering growing season soil microclimate. We compared phenological responses to experimental climate change with responses to natural microclimate variability across spatial gradients at small and landscape scales, as well as across a temporal gradient from a separate study. Both manipulations significantly advanced timing of flowering for the group of species and for most species individually, closely paralleling responses of timing to natural spatial and temporal variability in snowmelt date. Snowmelt date singularly explained observed shifts in timing only in the earliest flowering species, Claytonia lanceolata. Among all other species except Artemisia tridentata var. vaseyana, the latest flowering species, a consistent combination of temperature-related microclimate factors (earlier snowmelt date, warmer soil temperatures, and decreased soil degree-days) substantially explained earlier timing. Both manipulations also extended flowering duration for the group of species, similar to species' responses to natural snowmelt variability at small spatial scales. However, only early flowering species displayed consistent, significant changes in duration, with extended duration related to earlier snowmelt or warmer spring soil temperatures. Soil moisture was generally not a significant explanatory factor for either timing or duration of flowering. Best-fit microclimate models explained an average of 82% of variation in timing but only 38% of variation in duration across species. Our research demonstrates the value of comparing and synthesizing results of multiple field methods within a single study. This integrated approach makes it easier to identify robust community-wide trends, as well as species-specific responses of phenology to climate change. The predicted short-term flowering phenology responses to temperature-related aspects of climate change may lead to longer term asynchronies in interspecific interactions, potentially altering population and evolutionary dynamics, community structure, and ecosystem functioning.