Separating Drought Effects from Roof Artifacts on Ecosystem Processes in a Grassland Drought Experiment

Abstract

1Given the predictions of increased drought probabilities under various climate change scenarios, there have been numerous experimental field studies simulating drought using transparent roofs in different ecosystems and regions. Such roofs may, however, have unknown side effects, called artifacts, on the measured variables potentially confounding the experimental results. A roofed control allows the quantification of potential artifacts, which is lacking in most experiments.2We conducted a drought experiment in experimental grasslands to study artifacts of transparent roofs and the resulting effects of artifacts on ecosystems relative to drought on three response variables (aboveground biomass, litter decomposition and plant metabolite profiles). We established three drought treatments, using (1) transparent roofs to exclude rainfall, (2) an unroofed control treatment receiving natural rainfall and (3) a roofed control, nested in the drought treatment but with rain water reapplied according to ambient conditions.3Roofs had a slight impact on air (+0.14°C during night) and soil temperatures (−0.45°C on warm days, +0.25°C on cold nights), while photosynthetically active radiation was decreased significantly (−16%). Aboveground plant community biomass was reduced in the drought treatment (−41%), but there was no significant difference between the roofed and unroofed control, i.e., there were no measurable roof artifact effects.4Compared to the unroofed control, litter decomposition was decreased significantly both in the drought treatment (−26%) and in the roofed control treatment (−18%), suggesting artifact effects of the transparent roofs. Moreover, aboveground metabolite profiles in the model plant species Medicago x varia were different from the unroofed control in both the drought and roofed control treatments, and roof artifact effects were of comparable magnitude as drought effects.5Our results stress the need for roofed control treatments when using transparent roofs for studying drought effects, because roofs can cause significant side effects.

Design and Manual to Construct Rainout-Shelters for Climate Change Experiments in Agroecosystems

“…After an initial phase with similar soil water content in each of the three treatments, soil water content was constantly lower under the rainout-shelter as compared to the ambient control and the rainout-shelter control plots. The soil water content in the experimental treatments started to differ after the first heavy rain events supporting results of previous studies (Mikkelsen et al, 2008;Vogel et al, 2013). Although the early summer 2017 was characterized by several short drought-like periods, the developed rainout-shelters still resulted in differences in soil water content, making the design also suitable for regions with drier climatic conditions.…”

“…Rainout-shelters may cause lower air temperature due to the interception of radiation (Yahdjian and Sala, 2002), on the one hand, on the other hand a greenhouse effect, enhanced by reduced air flow under shelters, may cause higher temperature (Svejcar et al, 1999;Vogel et al, 2013). Both artifacts may bias the results of rainout-shelter experiments.…”

“…Major problems of permanent roofs relevant for biota and ecosystem processes include in particular passive warming (Svejcar et al, 1999;Fay et al, 2000;Vogel et al, 2013) and reduced PAR (Svejcar et al, 1999;Vogel et al, 2013). Reduced air circulation under complete exclusion roofs may lower the vapor-pressure deficit (VPD) and thereby reduce evapotranspiration, which in turn lowers the water demand of plants.…”

Drought‐Net rainfall shelters did not cause nondrought effects on photosynthesis for California central coast plants

“…Large interception troughs in New Mexico, USA, affected near‐surface air temperatures by up to 4°, and an air temperature effect was found in relation to distance from the troughs (Pangle et al, ). In some cases, effects of rain‐out shelters on air temperature were smaller than our night‐time increase of ~0.6 K (Kundel et al, ; Vogel et al, ), whereas other studies found a larger effect on temperature (Power et al, ). Overhead rain‐out shelters almost always reduce instantaneous PAR (Power et al, ).…”

Drought reduces floral resources for pollinators

“…Each of the 24 plots contained three subplots (1 × 1.5 m) which were at least 1 m from each other and from the edge of the plot. Each subplot was given one of three treatments: (1) Drought (D), covered with a transparent roof to exclude rain, simulating drought; (2) Control (C), not covered with a roof; and (3) Roofed control (R), covered with a transparent roof with 5 cm holes, allowing rain to pass through, but controlling for possible roof effects such as increased temperature and decreased light intensity (Vogel et al., ). Control and roofed subplots received ambient rainfall during the 6‐week period.…”