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.

Study site

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 > 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 (Pinus koraiensis) 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, Juglans mandshurica, Quercus mongolica, and Larix kaempferi coexist in the canopy layer. The height of tree stands is 15-20 m with crown sizes ranging from 3 to 8 m.

Experimental design and infrastructure

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.

Monitoring of soil temperature and soil moisture

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.