Learning from Fire

After the fires at Somme Prairie Grove, we study their messages.

Every year, fire wipes the slate clean in some part of the preserve (about half every year). The major changes (recovery of ecological health, or the losses) are evident in the area of the burn. If it was an effective burn, fire-adapted species come gloriously into their own. Malignant growths are reduced, or gone.

Check out the photo below: the superficial lesson was obvious, but the management prescription was not.

A patch of pale, unburned leaves - surrounded by the life-giving black.

There's a simple reason why this year's fire left the area in the center of this photo unburned. All around, there was enough fuel (tall grass and oak leaves) to carry the fire. But the unburned patch had been heavily shaded by basswoods. Crisp, tannin-protected oak leaves make great fuel; fires burn readily. Basswood leaves sog down and rot quickly; fires that reach them tend to go out. And basswood makes such deep shade that no thatch grass fuel developed.

We had watched this failing-to-thrive area for years. Finally, last fall, we cut those invading basswoods (note stumps) and planted seed where the shade had repressed vegetation that would be natural for a savanna. Perhaps we'll have good quality recovering here in a decade or so. But the plan may reverse again. The white oaks were planted about eighty years ago on what was originally prairie. Prairie vegetation survives around the edges. Should the oaks be cut too? Managers and planners are weighing alternatives. More to come on that.

Other photos tell different parts of the story. The burn photo below is from this March. The other is from July, in the same area, after a burn years ago.

In the distance some tall cottonwoods stand in a pond, with much older and shorter bur oaks behind them. But the challenge is in the foreground. Zoom in on the burn photo, and you can see dense shrub stems, mostly buckthorn. During fires in cooler and damper weather, this area doesn't burn well, if at all, so the shrubs have been increasing. If we hadn't successfully burned here this spring and tried to take the July photo this year, we'd see few flowers. Instead, buckthorn would rule - and in time kill the grasses and wildflowers and diverse associated animals.

Why is buckthorn so aggressive here? Perhaps we made a poor decision. Decades ago, this foreground area was dense brush, with few or no herbs underneath. But nearby were many quality species including the site's only population of New Jersey tea, along with violet wood sorrel, meadow parsnip (Thaspium), sanicle, and others. When we cut the brush here, we were hesitant to seed too much; in the absence of oaks, the somewhat aggressive tall prairie grasses might outcompete the special savanna flora. 

Our approach successfully promoted the spread of New Jersey tea and other quality plants. But it failed at restoring a "naturally sustainable" community - that is, one that improves or at least thrives over time with little care beyond burning. Thus, back to the drawing boards again. (Bit by bit, we solve these challenges, as would a doctor with a patient, trying various therapies.) 

Similarly, below, in the wide open, we'd been trying to restore mostly prairie vegetation for four decades. The prairie-like community looks great and burns readily. 

Still, check out the burn photo of this same area, below. You can zoom in and count hundreds of shrub stems. That's pathologically many in a grassland (which a savanna is one type of). In this case, the shrubs are mostly gray dogwood, in part perhaps because we've done a lot of buckthorn control here. But, unlike the previous area, we've done a lot of seeding and burning. But the shrubs in some parts are becoming so dense that there's little diversity left.  

This area was planted originally in little patches, perhaps twenty feet across. Perhaps the patchiness we see today reflects those patches. Perhaps the shrubbiest areas didn't get much seed? As I remember, years ago the patch of dense dogwood, below, was a "problem area" of dewberry. The sprawling berry briars choked out most quality vegetation for years. Then the dogwood took over. We planted acorns and hazelnuts in the middle; perhaps in burned grasslands, oaks survive long enough to be fire resistant in shrub patches that resist burning?

On the other hand, step back a little and take another photo, and patches of rich grassland are developing. Those dark clumps are soon to sprout prairie dropseed and little bluestem grass - and the richness of prairie clovers, leadplants, blazing stars, shooting stars, and all the other stars. So why don't we just cut and herbicide all the dogwood?

Part of the answer is suggested in the photo below. That spreading little bur oak, and scores like it nearby, are part of this system too. This was mesic bur oak savanna on rich soil, a community so rare and battered that it had been given up as lost to the planet. Now we and others are trying to revive it.


Originally, we thought we were doing "prairie restoration" here. That would be possible. But this area has remnants of the savanna biota surviving. We know that from studies of the invertebrate animals, and fungi, and remnant plants. What could be restored and conserved here is of much greater rarity and significance than even the prairie. In mid-summer, parts of this area (as in all that colorful diversity four photos above) appear to be out-and-out prairie. But in a few weeks we'll be struck yet again by four features that indicate otherwise. 1) There will be an unbroken sod of Penn sedge - characteristic of savannas. 2) Mixed with the prairie species will be many others that are more typical of savanna (many sedges, wild hyacinth, sanicle, carrion-flower, and others). 3) Lots of little oaks are trying to become established. And 4) all that gray dogwood just won't stop.

So, for now, in this area, we continue to seek the right stewardship to allow delicate balances to work themselves out in such a way that biodiversity continues to return, as in many areas it has been.

And we're struck by the contrast with the superficially similar area shown in the two photos below: from a different part of Somme Prairie Grove. Take a look:

Here shrubs are minimal, and the "prairie" (if that's what it is) has a higher proportion of high-quality plant species. In this part of the preserve, we're on the edge of the original prairie. Maybe that's why our old prairie planting is doing better here. Or maybe it seems better because this soil was once plowed for crops, and the savanna remnants are gone. Or our plantings were more successful. Or  some other reason.

But see that grassy expanse in the far distance? That's Somme Prairie Nature Preserve. It's more visible in the more recent burn photo, because there's been a major project over the last two winters to cut most of the brush there. Someday, the prairie in the foreground may become functionally part of Somme Prairie.

Remember the photo that began this post? Those white oaks planted on prairie soil are the trees in the distance in these photos that separate this patch from the main prairie. Perhaps should be cut for restoration someday, to make a contiguous prairie. That size difference may determine the sustainability of some prairie biota. (See Somme Prairie: questions and answers - maps and science.)

So, now we begin the excitement of another growing season. What will we learn this year? We appreciate your comments and will try to keep you posted.

Acknowledgements

Thanks to Christos Economou, Cathy Garness, and Eriko Kojima for helpful proofing and edits.