Vegetative traits of plants can respond directly to changes in the environment, such as those occurring under climate change. That phenotypic plasticity could be adaptive, maladaptive, or neutral.

We manipulated the timing of spring snowmelt and amount of summer precipitation in factorial combination and examined the responses of photosynthetic rate, stomatal conductance, and intrinsic water-use efficiency (iWUE) in the subalpine herb Ipomopsis aggregata. The experiment was repeated in three years differing in natural timing of snowmelt.

A 50% reduction in summer precipitation reduced stomatal conductance and iWUE. Combining natural and experimental variation, earlier snowmelt reduced soil moisture, photosynthetic rate and stomatal conductance, and increased iWUE.

Earlier snowmelt is a strong signal of climate change and can change expression of leaf morphology and gas exchange traits, just as reduced precipitation can. Stomatal conductance showed adaptive plasticity under some conditions.

We established an experimental manipulation of summer precipitation and snowmelt and then measured floral traits over three years, 2018 - 2020. We used a replicated split‐plot design, with snowmelt manipulated at the plot level and precipitation manipulated at the subplot level. The treatments were applied to the same plots each year. Six 7 m ⨉ 7 m plots were established within a 45 m ⨉ 25 m area of Maxfield Meadow, Gothic, CO, USA and three were randomly assigned early snowmelt treatments; a black 55% woven shade cloth was applied over the entire plot in the spring to accelerate snowmelt. Cloths were set out during spring melt off when snow height reached an average of 100 cm across the study site, monitored, and removed right after bare ground became visible. In 2019, a large avalanche ran through the site and deposited snow and debris, resulting in a later deployment and removal of shade cloth in two plots. The 2019 avalanche added snow that prevented early snowmelt in one plot, so for analysis we recoded it as having normal snowmelt timing.

Within each of the six snowmelt plots, four 2 m ⨉ 2 m subplots arranged in a square were randomly assigned one of four summer precipitation treatments. First, a water addition treatment simulated doubled summer precipitation based on the historical average in July from 1989 - 2006 measured at the EPA CASTNET weather station GTH161, 0.9 km northeast of Maxfield Meadow (www3.epa.gov/castnet/site_pages/GTH161.html). We added 14 L of tap water evenly to each 4 m² subplot every 2 days to supplement daily precipitation by 1.75 mm. Second, a water reduction treatment intercepted approximately 50% of incoming precipitation using a half-covered 2 m × 2 m rainout shelter. The rainout shelters were constructed with a PVC pipe skeleton, with sloping clear corrugated plastic greenhouse roofing slats spaced evenly on top to cover half of the plot's surface area. Intercepted rainwater ran down these slats into an attached gutter, which then fed into a bucket on the ground. The shelter frames were camouflaged with green and brown paint to avoid deterring or attracting pollinators and herbivores. Third, mock rainout treatments controlled for any effects of the physical PVC structures but lacked slats to intercept rain. Fourth, control subplots were unmanipulated and received ambient rainfall. 

We measured leaf-level gas exchange on non-flowering plants on 5 - 8 days each year for a total of 315 measurements of 275 unique plants. Measurements were taken the following number of days after the average unmanipulated snowmelt plot melted: 2018: 47 - 78, 2019: 33 - 60, 2020: 45 - 94. Each day we took measurements from subplots in random order and measured the longest leaf on one haphazardly selected rosette per subplot that had not been previously measured that year and had a leaf longer than 25 mm. Two consecutive measurements were recorded per leaf and averaged. We excluded measurements where the estimated intercellular CO₂ concentrations or photosynthetic rates were negative. Leaf gas exchange was measured using a Li-Cor 6400 XT Portable Photosynthesis system (Licor, Lincoln, Nebraska, USA). All leaf gas exchange measurements taken between 08:00 to 12:00 with saturating light conditions (PAR = 1800 μmol m-² s^-1), a leaf temperature of 27 °C, and a sample CO₂ concentration of 400 ppm, following Wu and Campbell (2007). Gas fluxes were calculated by dividing by the leaf area inside the leaf chamber, measured in ImageJ (National Institute of Health, Bethesda, Maryland, USA). Each value reported is a mean across the plants measured in that subplot and year.

"ReadMe file for Leafgasexchange_by subplot.txt" explains the variables.