Physiological mechanisms of sustained growth despite crown scorch in a young longleaf pine plantation

Background/Question/Methods

Repeated fire is used to perpetuate southern pine ecosystems and protect them from wildfire.  While the benefits of prescribed fire are established, this tool may decrease forest production if whole-crown carbon fixation is reduced.  Conversely, it is hypothesized that physiological processes associated with forest production are unaffected or enhanced by fire if natural mechanisms to restore scorched foliage remain intact.  Using longleaf pine saplings as a model of southern pine forests managed with fire, our objectives were to identify physiological variables that sustain carbon fixation and growth after fire and hypothesize why their effectiveness is seasonal.    

The study is on the Kisatchie National Forest in central Louisiana, USA.  Using a randomized complete block design with five blocks, three treatments were established: Control (C)-- no post-planting management, Burn (B)-- prescribed burning in spring 1998, 2000, 2003, and 2005, and Herbicide (H)—post-planting herbicide application to control vegetation.  Longleaf pine seedlings were planted in 1997.  Predawn needle water potential and root starch concentration, as well as fascicle net photosynthesis (A_sat), stomatal conductance (g_c), and transpiration (E) were measured eight times between May 2003 and October 2005.  Sapling leaf area and stem biomass were quantified by destructive harvests in fall 2003, 2004, and 2005.

 Results/Conclusions

Fires in May were followed by higher rates of A_sat, g_c, and E on the B plots than the C and H plots.  This continued through late summer.  In 2003, post-fire A_sat and g_c averaged 16 and 27% higher on the B plots than the C and H plots; whereas those in 2005, averaged 47 and 90% higher on the B plots than the C and H plots, respectively.  Greater gas exchange in response to fire is attributed to less leaf area and a subsequent increase in water use efficiency.  The magnitude of this response was larger in 2005 than in 2003 because scorch was greater in 2005 than 2003, and saplings experienced greater summer water deficit in 2005 than 2003.  Regardless of scorch in May, by fall sapling leaf area and production were similar on the B and C plots.  Cohort leaf areas in fall and root starch dynamics suggest that energy stored in roots was mobilized to reestablish leaf area.  Sustained gas exchange despite seasonal water deficits and rapid replacement of foliage appear to contribute to the production of southern pine forests managed with fire.  The dependence of these mechanisms on season of fire will be discussed.