Using Fiber-Optic Distributed Temperature Sensing to Measure Ground Surface Temperature in Thinned and Unthinned Forests

Studies of forest meteorology are often conducted at the stand level, but few studies examine temperature heterogeneity within stands. Differences in canopy structure, whether caused by species composition or disturbances, introduce variation in the amount of light reaching the forest floor, which in turn introduces variation in forest floor temperatures. Furthermore, in temperate latitudes, canopy openings cast light on the forest floor in complex patterns depending on the path of the sun throughout one day and throughout the season. We installed two temperature measurement devices in control, gap, and thinning treatments to capture both the time structure and spatial variability of forest floor temperature. We compared air temperatures measured by meteorological stations to spatially continuous ground surface temperatures measured along 760 m of fiber-optic cable. Using the principle of Raman spectra distributed temperature sensing, we inferred temperature at 1 m intervals along the fiber-optic cable every 30 minutes for 42 days in May - June 2010. In regenerating secondary forests with generally intact canopies, temperatures were spatially correlated throughout the day and night. In thinned forests or in gaps, ground surface temperatures were spatially correlated at night, but spatially heterogeneous during the day, suggesting that meter-scale measurements may be required to adequately characterize these environments. Understory plant species richness was 50% lower where higher temperatures were measured. We also modeled light transmission through the overstory with tRAYci and found that understory plant species richness was highest at 10% of above-canopy light and lower at both lower and higher light levels.