Measuring soil frost depth in forest ecosystems with ground penetrating radar
Soil frost depth in forest ecosystems can be variable and depends largely on early winter air temperatures and the amount and timing of snowfall. A thorough evaluation of ecological responses to seasonally frozen ground is hampered by our inability to adequately characterize the frequency, depth, duration and intensity of soil frost events. We evaluated the use of ground penetrating radar to nondestructively delineate soil frost under field conditions in three forest ecosystems. Soil frost depth was monitored periodically using a 900 MHz antenna in South Burlington, Vermont (SB), Sleepers River Watershed,North Danville, Vermont (SR) and Hubbard Brook Experimental Forest, New Hampshire (HBEF) during winter 2011–2012 on plots with snow and cleared of snow. GPR-based estimates were compared to data from thermistors and frost tubes, which estimate soil frost depth with a color indicating solution. In the absence of snow, frost was initially detected at a depth of 8–10 cm. Dry snow up to 35 cm deep, enhanced near-surface frost detection, raising the minimum frost detection depth to 4–5 cm. The most favorable surface conditions for GPR detection were bare soil or shallow dry snow where frost had penetrated to the minimum detectable depth. Unfavorable conditions included: standing water on frozen soil, wet snow,thawed surface soils and deep snow pack. Both SB and SR were suitable for frost detection most of the winter, while HBEF was not. Tree roots were detected as point reflections and were readily discriminated from continuous frost reflections. The bias of GPR frost depth measurements relative to thermistors was site dependent averaging 0.1 cm at SB and 1.1 cm at SR, and was not significantly different than zero. When separated by snow manipulation treatment at SR, overestimation of soil frost depth (5.5 cm) occurred on plots cleared of snow and underestimation (1.5 cm) occurred on plots with snow. Despite some limitations posed by site and surface suitability, GPR could be useful for adding a spatial component to pre-installed soil frost monitoring networks.