Wednesday, August 4, 2010

PS 53-55: Small-scale differences in duff depth and moisture content: Relevance for re-introduction of fire in longleaf pine forests

Sharon M. Hermann, Auburn University, John S. Kush, Auburn University, Caroline Noble, National Park Service, and Geoff Sorrell, The Nature Conservancy.

Background/Question/Methods: In forests dependent on frequent burns, fire exclusion alters structure and species composition as well as fuel loads.  Duff accumulation makes re-introduction of fire especially challenging when retention of trees is important.  Duff can smolder and kill individuals so a burn objective is to consume duff in ways that minimize tree damage.  In upland forests, small-scale distribution of duff and associated moisture is heterogeneous making no one set of burn parameters ideal.  Better understanding of distribution and condition of duff may facilitate effective re-introduction of fire in many forest ecosystems.  This is true in longleaf pine (Pinus palustris) forests where 40-50% of existing stands suffer from some degree of fire exclusion.  Consequently there is on-going discussion about the utility of altering ignition pattern to decrease duff smoldering.

Objectives are to evaluate patterns in litter/duff biomass and moisture associated with different micro-site conditions and apply the information to burn conditions and ignition patterns.  At Horseshoe Bend National Military Park (Alabama) where fire was excluded for 50+ years, we compared fuel within 0.5m of longleaf to paired points 2-3m away.  Depths of litter and duff were measured and material collected to determine moisture at three depths in the fuel bed

Results/Conclusions:  Duff was deeper near trees (11.4 +/- 3.1cm) compared to away (4.1 +/- 2.8 cm). Duff moisture was predicted to be higher near longleaf compared to away but this was not the case.  Preliminary results revealed little difference in overall moisture content of total litter or duff.  However, except immediately following rain, near longleaf, the top 1cm of duff was significantly drier by ~11% compared to paired sites away from trees.  In addition there were small but consistent differences between duff moisture within 1m down-slope of longleaf trees compared to paired up-slope points. 

Drier near-surface duff is expected to increase likelihood of smoldering near compared to away from longleaf.  We consider how chances of duff ignition near longleaf may be decreased by employing ignitions pattern that minimizes flaming front residence time and so lessen additional drying and ignition potential for the top layer of duff.  Better understanding of micro-site fuel differences may be increasingly important under some climate change scenarios.  Even slightly warmer and/or drier conditions are expected to limit the number of days appropriate for burning in stands with duff accumulation in a wide range of ecosystems, including not only pine but also hardwood forests.