Effects of a prescribed burn on water use and photosynthetic capacity of Pitch pines (Pinus rigida L.) in the New Jersey Pine Barrens

Background/Question/Methods: Wildfires are an important ecological component of the New Jersey Pine Barrens with its sandy, excessively well-drained soils.  However, increasing encroachment by human development has made it necessary to manage these systems using early-spring prescribed burns.  Spring prescribed burns, while occurring during the natural wildfire window, tend to have lower fire intensity, and a longer duration of heat flux, given the ignition patterns of these fires.  This contrasts with wildfires that occur later in the season, have a greater tendency to burn the crowns of mature trees and create significant negative effects on the soil including loss of nutrients and microbial communities.  Little is known about the effects of spring prescribed burns on the physiology of the overstory trees found there.  Therefore, we tested the hypothesis that water use and photosynthetic capacity increase after a prescribed fire both in the short-term and throughout the growing season.    We monitored sap flow rates prior to a controlled burn and throughout the growing season via heat dissipation sensors in a ‘burn’ site and an unburned, ‘control’ site.  We also made photosynthetic measurements using a LiCor 6400 before the burn, one month after the burn and during late summer at both sites.

Results/Conclusions: Water use was similar in both sites before the burn, but about 27% lower in burned trees immediately following the fire.  This was likely due to heat scorch damage to the lower needles, and possibly root damage.  However, after about a month, water use by burned trees was about 10 to 20% higher than the control site, and these differences lasted into the summer.  Trees in the burned site likely experienced decreased competition from surrounding shrubs following the fire, which resulted in their increased uptake of water.   Maximum photosynthetic assimilation at saturating CO₂ and carboxylation efficiency also increased significantly between pre- and post-fire measurements in the burned trees.  This suggests increased photosynthetic capacity after the fire, possibly due to nitrogen mineralization by the fire.  These results suggest that pitch pines receiving early-season prescribed burns will likely experience more favorable growing conditions through higher water availability and presumably higher N availability.  Both of these are important given the sandy soils of the Pine Barrens.  In conclusion, these data are important in elucidating the responses of trees to prescribed fires in the Northeast, as well as modeling carbon and hydrologic dynamics in ecosystems that routinely experience prescribed fire as a management strategy.