Abstract
The response of tropical forests to global warming is one of the largest uncertainties in predicting the future carbon balance of Earth. To determine the likely effects of elevated temperatures on tropical forest understory plants and soils, as well as other ecosystems, an infrared (IR) heater system was developed to provide in situ warming for the Tropical Responses to Altered Climate Experiment (TRACE) in the Luquillo Experimental Forest in Puerto Rico. Three replicate heated 4‐m‐diameter plots were warmed to maintain a 4°C increase in understory vegetation compared to three unheated control plots, as sensed by IR thermometers. The equipment was larger than any used previously and was subjected to challenges different from those of many temperate ecosystem warming systems, including frequent power surges and outages, high humidity, heavy rains, hurricanes, saturated clayey soils, and steep slopes. The system was able to maintain the target 4.0°C increase in hourly average vegetation temperatures to within ± 0.1°C. The vegetation was heterogeneous and on a 21° slope, which decreased uniformity of the warming treatment on the plots; yet, the green leaves were fairly uniformly warmed, and there was little difference among 0–10 cm depth soil temperatures at the plot centers, edges, and midway between. Soil temperatures at the 40–50 cm depth increased about 3°C compared to the controls after a month of warming. As expected, the soil in the heated plots dried faster than that of the control plots, but the average soil moisture remained adequate for the plants. The TRACE heating system produced an adequately uniform warming precisely controlled down to at least 50‐cm soil depth, thereby creating a treatment that allows for assessing mechanistic responses of tropical plants and soil to warming, with applicability to other ecosystems. No physical obstacles to scaling the approach to taller vegetation (i.e., trees) and larger plots were observed.
Highlights
Ecosystems worldwide are responding to Earth’s changing climate, with important implications for their structure and function, as well as for determining central feedbacks to future climate (IPCC, 2014)
IR arrays that maintain a constant temperature rise of the heated plots above that of reference plots using proportional integrative derivative (PID) control have been reported for several ecosystems including grass (Nijs et al, 1996; Van Peer, Nijs, Reheul, & De Cauwer, 2004), tundra (Marchand, Mertens, Kockelbergh, Beyens, & Nijs, 2005; Nijs et al, 2000), grazing land (LeCain et al, 2015; Luo et al, 2010), wheat fields (Kimball et al, 2015; Wall, Kimball, White, & Ottman, 2011), paddy rice (Gaihre et al, 2014; Rehmani et al, 2011), soybean (Ruiz-Vera et al, 2013; Siebers et al, 2015), and maize (Ruiz-Vera, Siebers, Drag, Ort, & Bernacchi, 2015), as well as tree saplings (McDaniel et al, 2014; Rich et al, 2015)
The experimental site is located in northeastern Puerto Rico near the USDA Forest Service Sabana Field Research Station in the Luquillo Experimental Forest (LEF; 18°18′N, 65°50′W) (Figures 1 and 2)
Summary
Ecosystems worldwide are responding to Earth’s changing climate, with important implications for their structure and function, as well as for determining central feedbacks to future climate (IPCC, 2014). IR arrays that maintain a constant temperature rise of the heated plots above that of reference plots using proportional integrative derivative (PID) control have been reported for several ecosystems including grass (Nijs et al, 1996; Van Peer, Nijs, Reheul, & De Cauwer, 2004), tundra (Marchand, Mertens, Kockelbergh, Beyens, & Nijs, 2005; Nijs et al, 2000), grazing land (LeCain et al, 2015; Luo et al, 2010), wheat fields (Kimball et al, 2015; Wall, Kimball, White, & Ottman, 2011), paddy rice (Gaihre et al, 2014; Rehmani et al, 2011), soybean (Ruiz-Vera et al, 2013; Siebers et al, 2015), and maize (Ruiz-Vera, Siebers, Drag, Ort, & Bernacchi, 2015), as well as tree saplings (McDaniel et al, 2014; Rich et al, 2015) All of these studies had 3-m-diameter plots or smaller, and most warmed by only 1.5°C during daytime.
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