DOI: 10.5061/dryad.z34tmpggp
title: Design and performance of an ecosystem-scale forest soil warming experiment with infrared heater arrays

description: How forest ecosystems respond to climate warming will determine forest
 trajectories over the next 100 years. However, the potential effects of
 elevated temperature on forests remain unclear, primarily because of the
 absence of long-term and large-size field warming experiments in forests,
 especially in Asia. Here, we present the design and performance of an
 ecosystem-scale warming experiment using an infrared (IR) heater array in
 a 60-year-old temperate mixed forest at Qingyuan Forest CERN in
 northeastern China. In paired 108 m2 plots (n = 3), the surface soils were
 constantly elevated 2 degrees above control plots with a feedback control
 system over four years (2018-2021). Subsoils down to 60 cm depth were
 warmed 1.2-2 degrees. Soil warming did not affect soil moisture either in
 surface soils or subsoils. Turn-off time due to weather extremes (heavy
 rains, snow) and power outages only accounted for 2.5% of the total
 warming period. In conclusion, we provide a proof-of-principle setup that
 allows long-term analysis of forest response to warming temperatures in
 large-size field plots. Importantly, our warming experiment demonstrated
 the feasibility of IR heater arrays for soil warming in tall-statured
 forest ecosystems.
description: <strong>Study
 site</strong> The research site is located in the Qingyuan Forest CERN (Chinese
 Ecosystem Research Network), Liaoning Province, Northeastern China (41º51′N,
 124º54′E). The site is
 characterized by a continental monsoon climate. Mean annual precipitation
 is 811 mm, with &gt; 80% of the precipitation distributed between May
 and September. Snowfall accounts for less than 6% of the annual
 precipitation. The mean annual temperature is 4.5℃. Daily mean extreme temperatures are
 –37.6℃ to 36.5℃, with an annual frost-free period of
 approximately 130 days. The forest site is located at 620 m above sea
 level with relatively gentle slopes ranging from 10 to
 16º. Soils of this area were classified as Udalfs
 with clay loamy texture (sand: 25.6%, silt: 51.2%, clay:
 23.2%). The site was
 occupied by primary broadleaves and Korean pines
 (<em>Pinus
 koraiensis</em>)
 before the 1930s, followed by decades of unregulated timber logging. In
 the early 1950s, controlled burns were practiced when clearing the
 remaining stands. Thereafter, the site was naturally regenerated to form
 the current closed canopy mixed broadleaved and coniferous forest. The
 dominant tree species, <em>Juglans
 mandshurica</em>,
 <em>Quercus
 mongolica</em>, and
 <em>Larix
 kaempferi</em>
 coexist in the canopy layer. The height of tree stands is 15-20 m with
 crown sizes ranging from 3 to 8 m.
 <strong>Experimental
 design and infrastructure</strong>
 Six rectangular (18 m × 6 m)
 plots in a northwest-facing slope were designated in 2017, with a buffer
 zone of about 5-15 m between adjacent plots to avoid interferences from warmed plots to the control plots (Figure
 2). These plots were
 placed within a pre-selected area of 1 ha of temperate mixed forest, with
 the layout and orientation of each plot based on the maximal number of
 dominant trees and the similarity of understory plants. Plots were
 assigned into three pairs according to their topography, with each pair
 containing a warmed (+2℃) and a control plot. Within each warming plot,
 rod-shaped infrared heaters (8mm diameter ×151 cm long, 2000W, 240 V
 Model, HS-2420 from Kalglo Electronics Co. Inc., USA) with equilateral
 triangle housing were installed as the warming tool mimicking the natural
 climate warming process (Kimball, 2005; Kimball et al., 2011). The
 radiation from the infrared heater initially heats vegetation and soil
 surfaces, then the heat energy transfers deeper into the soil (Liang et
 al., 2017). To ensure uniform warming treatment within each plot, the IR
 heaters were placed in an arrayed manner, thus, achieving the desired
 thermal radiation across the plots (Kimball et al., 2011). Eighteen IR
 heaters were evenly distributed in each warming plot, into 3 × 6 rows and
 suspended 2 m above the ground. To support the IR heater array, 24 stainless
 steel posts were anchored about 50 cm into the ground. Crossbars were then
 attached to the posts at a height of 2.3 m above the slope surface. The
 vertical posts and crossbars formed a durable structure that protects the
 IR heaters from strong winds, rain, and snow, as well as falling branches.
 IR heaters were bolted to the crossbars along the six meters side,
 providing an IR heaters array that stands parallel to the
 slope. The reference plots
 were set up in an identical manner, with eighteen dummy heaters installed with the
 same shape and size as the IR heaters. Such structural control was set up
 to mimic any possible infrastructure effects of the IR
 system. <strong>Monitoring of soil temperature and soil
 moisture</strong> Thermocouples in the feedback control system
 were used to continuously monitor the surface soil temperature at 5 min
 intervals. In each plot, ten soil moisture probes (Computer Network
 Information Center, Chinese Academy of Sciences, Beijing, China) for
 surface soil measurement were evenly installed in 2018 at the 5 cm depth
 of mineral soil. To measure subsoil temperature and moisture, another set
 of probes (Campbell CS655 with data logger CR1000, Campbell Scientific,
 Logan, UT, USA) were installed at depths of 5, 10, 20, 30, and 40 cm at
 the center of each plot and logged at 15 min intervals, with one probe for
 each soil layer of the plot. As a supplement, we deployed another set of
 temperature and moisture probes at 10-60 cm depths (WITU Agricultural
 Technology, Shenyang, China), with a 10 cm interval. Overall, we deployed
 162 sensors at six plots and sensors were periodically recalibrated as
 needed. Subsoil measurements were initiated in 2019. To continuously
 monitor pre- and post- treatment effects of IR heating, the probes were
 running for 12 months a year.