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As part of its long-term climate data core collection, the Niwot Ridge LTER has collected daily air temperature at the Saddle site since 1981. The Saddle station is located at 3525 m.a.s.l. and is an important point location to capture local, ambient meteorological conditions for many biological and environmental datasets collected nearby. The location of the Saddle station has also presented challenges to its operation. Freezing temperatures, snow deposition from strong winds following storms, and exposure to lightning are some elements that have disrupted instrument functionality, affected data quality, and made access for research staff difficult over time, especially in winter months. These interruptions have led to missing or faulty data at times and inconsistent data gap-filling. Additionally, a mixture of mechanical hygrothermograph chart and temperature sensors with electronic data loggers have been used since the inception of the Saddle station to measure and record air temperature. Thus, a close inspection of potential influence from instrument turnover and relevant notes from research staff is required for a quality, daily air temperature time series for Saddle. Here we present a quality-controlled, gap-filled, daily time series of maximum, average, minimum, and diurnal air temperatures that accounts for instrument turnover at the Saddle. Methods follow those used to gap-fill long-term daily air temperature at the Niwot Ridge LTER D1 and C1 stations so there is consistency among core collection daily air temperature datasets. Metadata for this data package centralizes the most complete station history for Saddle air temperature and includes notes to data users on aspects and limitations of the dataset to consider when using these data in scientific analyses. 

As part of its long-term climate data core collection, the Niwot Ridge LTER has collected daily precipitation at the Saddle site since 1981. The Saddle station is located at 3525 m.a.s.l. and is an important point location to capture local, ambient meteorological conditions for many biological and environmental datasets collected nearby. The location of the Saddle station has also presented challenges to its operation. Freezing temperatures, snow deposition from strong winds following storms, and exposure to lightning are some elements that have disrupted instrument functionality, affected data quality, and made access for research staff difficult over time, especially in winter months. Here we present a quality-controlled, gap-filled, daily precipitation time series corrected for blowing snow overcatch at the Saddle station. Methods follow those used to gap-fill long-term daily precipitation at the Niwot Ridge LTER D1 and C1 stations so there is consistency among core collection daily precipitation datasets. Metadata for this data package centralizes the most complete station history for Saddle precipitation and includes notes to data users on aspects and limitations of the dataset to consider when using these data in scientific analyses. Because of unresolved data quality concerns with winter precipitation the first several years of the record, gap-filled data for winter months (October–May) 1981-10-01 through 1987-05-30 are removed. 

Precipitation data have been collected at the C1 climate station (3022 m asl) almost continuously from 1952 to the present. C1 is on locally level terrain on the southeastern flank of Niwot Ridge, 9.7 km east of the Continental Divide. Surrounding vegetation is closed-canopy subalpine conifer forest. Through 1964, precipitation was recorded using an unshielded U.S. Weather Bureau standard totalizing gauge, with observations manually recorded on an approximately weekly basis. The gauge was located in an open area with sparse tree cover adjacent to the forest proper. Starting in late 1961, daily precipitation has been recorded using a Belfort Universal weighing-bucket gauge with chart recorder in an 8-m diameter clearing in the forest; the forest provides natural shielding for the gauge. The two records were assessed for an impact of this station change on record homogeneity and merged (see Methods). Missing daily data were infilled and multiday records were parsed to dailies (see Methods). 

Precipitation data have been collected at the D1 climate station (3739 m asl) almost continuously from 1952 to the present. D1 is on a narrow, exposed ridge on the westernmost part of Niwot Ridge, 2.6 km east of and ca. 200 m lower in elevation than the Continental Divide. Surrounding vegetation is low-stature alpine tundra. Through 1969, precipitation was recorded using an unshielded U.S. Weather Bureau standard totalizing gauge, with observations manually recorded on an approximately weekly basis. Starting in 1965, daily precipitation has been recorded using a Belford weighing-bucket gauge with chart recorder and with an Alter-type shield encircled by a Wyoming-snow fence. Overlap in the two records was used to adjust the totalizing gauge record so that it could be merged with the weighing-bucket gauge record (see Methods). Missing daily data were infilled and multiday records were parsed to dailies (see Methods). 

Daily minimum and maximum surface air temperature data have been collected at the C1 climate station (3022 m asl) almost continuously from 1952 to the present. C1 is on locally level terrain on the southeastern flank of Niwot Ridge, 9.7 km east of the Continental Divide. Surrounding vegetation is generally closed-canopy subalpine conifer forest. Temperature minima and maxima were measured with a chart-recording hygrothermograph housed in a Stevenson screen located in an open, sparsely treed area adjacent to the forest proper. Hygrothermograph records were calibrated at the time of chart changes against liquid-in-glass thermometers. Processing included quality checks and infilling of missing daily data (see Methods). 

Daily minimum and maximum surface air temperature data have been collected at the D1 climate station (3739 m asl) almost continuously from 1952 to the present. D1 is on a narrow, exposed ridge on the westernmost part of Niwot Ridge, 2.6 km east of and ca. 200 m lower in elevation than the Continental Divide. Surrounding vegetation is low-stature alpine tundra. Temperature minima and maxima were measured with a chart-recording hygrothermograph housed in a Stevenson screen. Hygrothermograph records were calibrated at the time of chart changes against liquid-in-glass thermometers. Processing included quality checks and infilling of missing daily data (see Methods). 

Abstract A central goal at the interface of ecology and conservation is understanding how the relationship between biodiversity and ecosystem function (B–EF) will shift with changing climate. Despite recent theoretical advances, studies which examine temporal variation in the functional traits and mechanisms (mass ratio effects and niche complementarity effects) that underpin the B–EF relationship are lacking.Here, we use 13 years of data on plant species composition, plant traits, local‐scale abiotic variables, above‐ground net primary productivity (ANPP), and climate from the alpine tundra of Colorado (USA) to investigate temporal dynamics in the B–EF relationship. To assess how changing climatic conditions may alter the B–EF relationship, we built structural equation models (SEMs) for 11 traits across 13 years and evaluated the power of different trait SEMs to predict ANPP, as well as the relative contributions of mass ratio effects (community‐weighted mean trait values; CWM), niche complementarity effects (functional dispersion; FDis) and local abiotic variables. Additionally, we coupled linear mixed effects models with Multimodel inference methods to assess how inclusion of trait–climate interactions might improve our ability to predict ANPP through time.In every year, at least one SEM exhibited good fit, explaining between 19.6% and 57.2% of the variation in ANPP. However, the identity of the trait which best explained ANPP changed depending on winter precipitation, with leaf area, plant height and foliar nitrogen isotope content (δ¹⁵N) SEMs performing best in high, middle and low precipitation years, respectively. Regardless of trait identity, CWMs exerted a stronger influence on ANPP than FDis and total biotic effects were always greater than total abiotic effects. Multimodel inference reinforced the results of SEM analysis, with the inclusion of climate–trait interactions marginally improving our ability to predict ANPP through time.Synthesis. Our results suggest that temporal variation in climatic conditions influences which traits, mechanisms and abiotic variables were most responsible for driving the B–EF relationship. Importantly, our findings suggest that future research should consider temporal variability in the B–EF relationship, particularly how the predictive power of individual functional traits and abiotic variables may fluctuate as conditions shift due to climate change. 

Abstract Fine‐scale microclimate variation due to complex topography can shape both current vegetation distributional patterns and how vegetation responds to changing climate. Topographic heterogeneity in mountains is hypothesized to mediate responses to regional climate change at the scale of metres. For alpine vegetation especially, the interplay between changing temperatures and topographically mediated variation in snow accumulation will determine the overall impact of climate change on vegetation dynamics.We combined 30 years of co‐located measurements of temperature, snow and alpine plant community composition in Colorado, USA, to investigate vegetation community trajectories across a snow depth gradient.Our analysis of long‐term trends in plant community composition revealed notable directional change in the alpine vegetation with warming temperatures. Furthermore, community trajectories are divergent across the snow depth gradient, with exposed parts of the landscape that experience little snow accumulation shifting towards stress‐tolerant, cold‐ and drought‐adapted communities, while snowier areas shifted towards more warm‐adapted communities.Synthesis: Our findings demonstrate that fine‐scale topography can mediate both the magnitude and direction of vegetation responses to climate change. We documented notable shifts in plant community composition over a 30‐year period even though alpine vegetation is known for slow dynamics that often lag behind environmental change. These results suggest that the processes driving alpine plant population and community dynamics at this site are strong and highly heterogeneous across the complex topography that is characteristic of high‐elevation mountain systems.