Honey mesquite canopy responses to single winter fires: Relation to herbaceous fuel, weather and fire temperature

Honey mesquite (Prosopis glandulosa Torr.) canopy responses to fire were me asured following 20 single winter fires conducted in north Texas. Weather c onditions during the fires, understory herbaceous fine fuel (fine fuel) amo unt and moisture content, fire temperature at 0 cm, 10-30 cm and 1-3 m abov e ground, and canopy responses were compared. Ten fires occurred on a site where fine fuel was a mixture of cool and warm season grasses (mixed site). The other 10 fires occurred on a site dominated by warm season grasses (wa rm site). When both sites were included in regressions, peak fire temperatu re at all heights was positively related to fine fuel amount. Fine fuel amo unt, fine fuel moisture content, air temperature (AT) and relative humidity (RH) affected fire temperature duration in seconds over 100 degrees C (FTD 100) at 1-3 m height, but not at ground level. Mesquite percent above-groun d mortality (topkill) increased with increasing fine fuel amount, decreasin g fuel moisture content, increasing AT, and decreasing RH. Percent foliage remaining on non-topkilled (NTK) trees was inversely related to fine fuel a mount and AT, and positively related to fine fuel moisture content. Effect of fire on mesquite topkill and foliage remaining of NTK trees was strongly affected by RH at the warm site (r(2) = 0.92 and 0.82, respectively), but not at the mixed site. This difference was due to RH affecting fuel moistur e content (and subsequently fire behavior) to a greater degree at the warm than at the mixed site, because of the lower green tissue content in warm s ite grasses at the time of burning. Under adequate fine fuel amounts to car ry a fire, mesquite canopy responses to fire (i.e., topkill vs, partial can opy defoliation) were largely determined by AT and RH conditions during the fire. This has implications if the management goal is to preserve the mesq uite overstory for a savanna result instead of topkilling all trees. Two substudies were conducted during 3 of the fires. Substudy 1 determined mesquite response to fire in 2 plots with different understory herbaceous f uel loads (5,759 vs. 2,547 kg/ha) that were burned under under similar weat her conditions. Mesquite topkill was 81% and 11% in the high and low fuel f ires, respectively. Under similar weather conditions, fine fuel was an impo rtant factor in affecting mesquite responses to fire. However, as demonstra ted in the main study, under a variety of weather conditions, AT and RH inf luenced mesquite response to fire as much or more than did fine fuel. Subst udy 2 compared response of mesquite plants with abundant and dry subcanopy fine fuel (3252 kg/ha; fuel moisture 10.4%), or sparse and green subcanopy fuel (1155 kg/ha; fuel moisture 25.9%) to a high intensity fire. All trees were topkilled, including those with low subcanopy fuel, probably from conv ection heat generated from herbaceous fuel in interspaces between trees. In support of this conclusion, thermocouple data from all 20 fires indicated that canopy responses were more related to fire temperature at 1-3 m than a t lower heights. This suggests that the topkill mechanism was due to convec tive heat within the canopy rather than a girdling effect of fire at stem b ases.