Summary

Purpose

  • The primary purpose was to test a system for measuring fire behavior developed in grasslands (FM 3 & 1), in a woodland/forest situation (FM 2ish)

  • Secondary purposes included demonstrating fire science and providing data for the Blackfoot TREX.

Methods

Three FeatherFlame thermocouple datalogger systems were deployed ahead of three prescribed fires at the 2025 Blackfoot TREX.

Take-aways

  • Low fuel loads in these forests largely impeded fire spread under prescription conditions. Visual observation indicated that in many cases, fire spread was limited to small patches of needle cast fuels directly ignited by drip torches. In the absence of such fuels, fire rarely spread from ignition lines. Many instances of burning stumps and downed woody debris, and torching trees, were observed.

  • The FeatherFlame sensor is effective in measuring rate of spread in these forest fuels when fire spreads through the array. Substantial variability due to the factors described above means a greater, or targeted, sampling effort is required to adequately sample behavior of freely-moving flame fronts away from their point of ignition.

  • Care must be taken in interpreting FeatherFlame data from burns in which fire spread was poor. Sensors can reflect elevated temperature due to heat in the ambient environment rather than direct flame contact provided by true fire spread.

  • Soil heating in these forest fuelbeds can be significant and organic matter in the duff layer might be more responsible for carrying fire than conventional surface fine fuels.

Raw data visualized

Raw time-temperature curves for three above-surface temperature thermocouples (TC) from each of three datalogger units (F4, F5, F6) deployed in three prescribed fires at the Blackfoot TREX.

Raw time-temperature curves for three above-surface temperature thermocouples (TC) from each of three datalogger units (F4, F5, F6) deployed in three prescribed fires at the Blackfoot TREX.

Raw time-temperature curves for two temperature thermocouples (TC) located at the soil surface from each of three datalogger units (F4, F5, F6) deployed in three prescribed fires at the Blackfoot TREX.

Raw time-temperature curves for two temperature thermocouples (TC) located at the soil surface from each of three datalogger units (F4, F5, F6) deployed in three prescribed fires at the Blackfoot TREX.

Maximum temperatures

Temperature results were challenging to interpret. As seen above, many sensors recorded elevated air temperatures without the distinct peak of flame coming into contact with the thermocouple bead.

Temperatures in the flame zone in arrays through which fire had actually spread only exceeded 100 deg C six times total, and only averaged above 100 for all three sensors at a point twice, both on Baldy C3.

Interestingly, in four cases, including all three loggers at Baldy C3, soil surface temperatures substantially exceeded flame zone temperatures, suggesting these fires spread by smoldering through forest duff and litter?

Mean maximum temperatures reached in the flame zone and soil surface on three prescribed burns at the Blackfoot TREX.

Mean maximum temperatures reached in the flame zone and soil surface on three prescribed burns at the Blackfoot TREX.

For comparison, maximum temperatures recorded in tallgrass prairie fuels at the NerdTREX in northern Missouri were as follows:

Year Level Min Mean Max
2024 Flame 100 352 597
Surface 10 236 492
2025 Flame 422 530 674
Surface 65 388 658

Below are the data from individual sensors summarized above:

Maximum recorded temperatures (Celsius) for individual thermocouples attached to three dataloggers, deployed on three Rx fire events. Note that TC1-3 are located above the surface in the potential flame zone, while TC4&5 are placed on the soil surface.
event logger level TC1 TC2 TC3 TC4 TC5 Mean maximum temp
Baldy C4 F4 flame 58 62 70 63.5
Baldy C4 F4 surface 37 87 62.25
Baldy C4 F5 flame 53 118 66 79.25
Baldy C4 F6 flame 62 62
Baldy C4 F6 surface 258 257.8
Baldy C3 F4 flame 33 32 32 32.42
Baldy C3 F4 surface 51 41 46
Baldy C3 F5 flame 95 80 137 103.9
Baldy C3 F5 surface 374 125 249.5
Baldy C3 F6 flame 184 258 256 233.1
Baldy C3 F6 surface 359 426 392.8
Wedge 2.0 F4 flame 34 36 34 34.33
Wedge 2.0 F4 surface 38 36 36.88
Wedge 2.0 F5 flame 46 134 84 87.67
Wedge 2.0 F5 surface 53 66 59.38
Wedge 2.0 F6 flame 62 44 40 48.67
Wedge 2.0 F6 surface 42 37 39.62

Rate of spread

In addition to temperatures, the FeatherFlame system is designed to calculate rate of spread based on the arrival times of the flame front at each of the three flame-zone sensors.

Fuels at the Blackfoot TREX were very challenging for ROS measurement and interpretation, because the data do not necessarily reflect flame fronts as much as heating from the general fire environment. In fact, in several cases, the area under or even around the sensors remained green after the fires, and thermocouples detected general increases in surrounding air temperature. Data from these arrays were excluded from the ROS presentation.

Rate of spread through 1m triangular arrays as calculated from arrival times of flame fronts from Rx fire operations. Results limited to arrays observed to be surrounded by black after the burn.
date logger event ROS (m/min)
01 May F4 Baldy C4 0.33
01 May F5 Baldy C4 1.3
02 May F4 Baldy C3 0.034
02 May F5 Baldy C3 0.45
02 May F6 Baldy C3 1.7

For comparison, rates of spread through two tallgrass prairie at the 2025 NerdTREX in Northern Missouri, measured with these same instruments, ranged from 5 to 20 m/min, and in three fires in 2024, ranged from 2 to 12 m/min.