Sunday, October 12, 2014

The Long Awaited Summer (Fire) - Will it save our savannas?

1st Summertime prescribed fire conducted in Brunswick Co, NC
Longleaf Pine relict (right), middle sized trees are mostly Pond Pine
no Longleaf Pine regeneration is present
Vegetation response several weeks later (same site as previous)
Longleaf Pine unfazed but significant needle loss on all Pond Pines 
It has been known for decades that regular or frequent fire facilitates the development of open, grassy longleaf pine (Pinus palustris) savannas. Restoring such conditions in existing longleaf pine stands has become virtual dogma across the southeastern United States. However, relatively few sites actually receive the frequency of fire needed to maintain or recreate open savannas, and even fewer receive fires during the growing season when lightning fires would have occurred historically. This has certainly been the case at the Boiling Spring Lakes Plant Conservation Preserve in Brunswick Co., North Carolina where we have been working, with the help of The Nature Conservancy and the NC Forest Service (NCFS), for approximately 10 years to restore fire prone landscapes.  This summer (2014), the NCFS and our small team conducted the first known summer fire in the region (special thanks to Bill Walker, Shane Hardee, Mike Malcolm & Miller Caison for making the burn a reality)

For decades, typical controlled burns across the southeastern NC and the US occurred during the dormant or winter season.  However, it has been recognized that significant parts of the landscape, especially areas dominated by longleaf pine, developed under eons subject to fires started by lightning. Lightning strikes, especially those most likely to start fires, are mostly associated with summer thunderstorms. In North Carolina July is the peak for these storms (1).                                    

Outcalt (2) showed that the largest, tallest trees were preferentially hit by lightning in longleaf stands. These strikes often leave telltale marks (left), sometimes extending to the ground. In certain cases hot strikes ignite the tree itself and the flammable ground cover nearby (right).

Given dry conditions, available fuels, wind to push the fire, and an unbroken landscape, such lightning ignited fires could have extended for miles.  Large, old longleaf pines tend to have concentrated resins that allow them to burn hot, even when drenched with water. These can easily ignite fires even after the passage of rain fall events.

There is some evidence that lightning season fires produce different ecological effects than fires at other seasons; at least 3 syntheses of these differences have been attempted  (3,4,5).  One benefit is the creation of  "seedbeds" for longleaf pines, which drop their seeds in late Fall.  These seeds (which are produced relatively infrequently, and primarily from older, larger trees) need to access mineral soil and are easily "hung up" in understory grass or shrubs. Ironically, typical winter burns occur almost immediately after seeds have dropped thereby destroying many seed crops. Therefore, managing longleaf stands with regular winter fires could eliminate regeneration of the dominant trees. Further, non-lightning season fires (especially if infrequent as they are in most areas) tend to favor development of dense shrub stems which may further hinder longleaf regeneration.

A perfect seed bed for longleaf pine seeds to establish

Serotinous Pond Pine Cone
Immediately after fire
Serotinous Pond Pine Cone
Just opened after fire
In contrast, Pond Pine (Pinus serotina) produces much smaller seeds, produces them at much younger ages, and holds them on the tree for several years until hot fire opens them.

Fires during any season, including winter, could potentially heat the serotinous cones and cause seeds to drop into freshly prepared seed beds.  Pond Pine also has the capability to reprout after even intense fires that could kill similar sized longleaf pines.  Taken together, these factors provide an environment where tree dominance shifts from longleaf pine to pond pine.

Pinus serotina resprouting after summer fire;
in addition to "main" epicormic sprout there are 3 others emerging (front right, back left)
Several previous stems (now blackened and top killed) were present from previous fire
Savanna in Brunswick Co., NC with a rare regenerating Longleaf pine (foreground) amidst taller saplings of Pond Pine
larger trees (rearground) are also Pond Pine

BELOW: Longleaf Savanna replaced by Pond Pine Flatwoods
Lone Pinus palustris persists near middle; dense shrub layer has developed

Longleaf pine relict indicating previous stand composition and open structure
One fire reduced some immediately adjacent & "invading" Pond Pine stems but
high fuel loads remain and threaten remaining Longleaf

BELOW: Longleaf Pine stand decimated by wildfire after heavy midstory development as in previous image;
standing dead are longleaf pines,Pond Pine present were killed and have not reemerged into midstory or overstory
dense tall shrub layer is still present

Although growing season fires are often thought to be important for various ecological reason one of the most important, yet least understood, may be the reversal of longleaf pine savanna replacement from dense, flammable stands of Pond Pine.  

(2) Outcalt, K. 2008. Lightning, fire and longleaf pine: Using natural disturbance to guide management. Forest Ecology and Management 255.
(3) Robbins, L.E. and R.L. Myers. 1992. Seasonal effects of prescribed burning in Florida: a review. Tall Timbers Research Station. Misc. Publ.
(4) Streng, etal. 1993. Evaluating effects of season of burn in longleaf pine forests: a critical literature review and some results from an ongoing long-term study. Proceedings Tall Timbers Fire Ecology Conference 18.
(5) Knapp etal. 2009. Ecological effects of Prescribed Fire Season: A Literature Review and Synthesis for Managers. USDA General Technical Report PSW-GTR-229.

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