DOI: 10.7280/d12h50
title: Leaf gas exchange in Ipomopsis aggregata under manipulated snowmelt timing and summer precipitation

description: 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.
description: 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 m<sup>2</sup> 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 CO<sub>2</sub> 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-<sup>2 </sup>s<sup>-1</sup>), a leaf temperature of 27 °C, and a sample CO<sub>2</sub> 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.
description: "ReadMe file for Leafgasexchange_by subplot.txt"
 explains the variables.