The effects fire history and position along a hydrologic gradient on the yearly growth of Pinus palustris and Pinus serotina in a pine savannah ecosystem

Background/Question/Methods

Prescribed fires are important for maintaining pine savannah ecosystems.  However, relatively little research has investigated the impact of fire history on growth of trees.  The net effect of fires on trees is determined by the balance between increased growth due to removal of competition and decreased growth due to stress.   The impact of fire on growth is likely to vary by tree species and by environmental conditions.  To test the importance of these factors, we cored 159 Pinus palustris and 79 Pinus serotina from multiple sites across Fort Bragg, NC, a location with data on prescribed burn history at each site dating back to 1991.  At each site, individuals were sampled along a hydrologic gradient from xeric uplands to streamside wetlands.  We compared annual basal growth with fire history, species, and position along a hydrologic gradient.  We predicted that burned trees would show reduced growth and productivity during burn years with trees closer to the wetlands experiencing less detrimental effects.  We also expected greater negative effects for P. serotina than P. palustris during burn years.  We built a linear mixed effect model to calculate the contributions of our three test factors on both basal area growth and ring width growth.

Results/Conclusions

Our model revealed that fire history was not a significant contributor to yearly growth among our test sample.  There was also no significant difference in growth rate by species or hydrologic position.  Temperature, precipitation, and previous growth were the only signigicant contributors to yearly growth in our model.  We suspect that a combination of minor stress from the low intensity prescribed burns, which typically do not cause needle fall in mature trees, and minor benefits from reduced water and nutrient competition post-fire combine to cloud any fire signal in the tree cores.  We also suspect that both P. palustris and P. serotina have similar growth rates in the pine savannah and that each is able to draw water from the water table, negating the effects of hydrologic position.  This may also indicate that both species share similar fire hardness or that hydrologic position in the pine-wiregrass ecosystem does not affect burn intensity.