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In 2006 we established a global change experiment in the Front Range of the Rocky Mountains to investigate how manipulations of warmer summer temperature, N deposition, and increased snowpack would affect the growth of alpine plants. The experiment was implemented on Niwot Ridge, where shrub cover has expanded by over 400% since 1946 (Formica et al. 2014). We established experimental plots north of the Niwot Ridge saddle, in an area of moist meadow tundra where willow shrub (Salix sp.) patches are present. Within experimental plots, Salix glauca seedlings were transplanted in 2006 and 2007 to test whether changing environmental conditions facilitated shrub survival and growth. In 2007 and 2008, phenological observations were recorded for all (2007) or abundant (2008) species in experimental plots. Measurements of plant species composition and aboveground net primary productivity (ANPP) are also made annually or biennially (ANPP, 2017-onward). In 2021, canopy height and NDVI began being measured annually. In July of 2016, a community transplant experiment was implemented to test whether changing environmental conditions support changes in alpine tundra plant communities. Two species characteristic of (1) dry meadow tundra (Tetraneuris acaulis, Erigeron pinnatisectus), (2) snowbed tundra (Ranunculus adoneus, Saxifraga rhomboidea) and (3) subalpine meadow (Trollius albiflorus, Polemonium pulcherrimum) were transplanted into experimental plots. Survival and growth of transplants was documented annually through 2021. 

Abstract Rapid climate warming is altering Arctic and alpine tundra ecosystem structure and function, including shifts in plant phenology. While the advancement of green up and flowering are well-documented, it remains unclear whether all phenophases, particularly those later in the season, will shift in unison or respond divergently to warming. Here, we present the largest synthesis to our knowledge of experimental warming effects on tundra plant phenology from the International Tundra Experiment. We examine the effect of warming on a suite of season-wide plant phenophases. Results challenge the expectation that all phenophases will advance in unison to warming. Instead, we find that experimental warming caused: (1) larger phenological shifts in reproductive versus vegetative phenophases and (2) advanced reproductive phenophases and green up but delayed leaf senescence which translated to a lengthening of the growing season by approximately 3%. Patterns were consistent across sites, plant species and over time. The advancement of reproductive seasons and lengthening of growing seasons may have significant consequences for trophic interactions and ecosystem function across the tundra. 

Observations of changes in phenology have provided some of the strongest signals of the effects of climate change on terrestrial ecosystems. The International Tundra Experiment (ITEX), initiated in the early 1990s, established a common protocol to measure plant phenology in tundra study areas across the globe. Today, this valuable collection of phenology measurements depicts the responses of plants at the colder extremes of our planet to experimental and ambient changes in temperature over the past decades. The database contains 150,434 phenology observations of 278 plant species taken at 28 study areas for periods of 1 to 26 years. Here we describe the full dataset to increase the visibility and use of these data in global analyses, and to invite phenology data contributions from underrepresented tundra locations. Portions of this tundra phenology database have been used in three recent syntheses, some datasets are expanded, others are from entirely new study areas, and the entirety of these data are now available at the Polar Data Catalogue (https://doi.org/10.21963/13215).