Kevin Robinson, I appreciate your taking the time to thoughtfully respond. It would seem obvious that although some adaptations of some plants work equally well to keep them alive under both fire and grazing regimes, there are clearly all kinds of physical and chemical differences between the two processes. Another reason for managers to be cautious in their selections from the user's tool box.
I also accept that under conditions that have prevailed since the end of the ice age, fire is strongly associated with persistence of many species and ecosystems. But if fire frequencies were low during and before the ice ages, one wonders how now fire-dependent species keep on going. One can speculate that far more infrequent fire is not the same as no fire. Some species, like those in prairies, pine barrens, blue berry barrens, southern conifer forests, and some western conifer forests, etc. may have been able to thrive even if fire frequency was less than what we experience currently. Hard to imagine that grazing was not somewhat involved. A friend has spent time with David Western in Kenya. Western has gone through different opinions on the impacts of elephants on vegetation, but now holds their exclosure is the only way some forests can survive. Africa has a broad suite of grazers and browsers, but still has wildfire. N. Owen-Smith considers lessons from Africa applicable to understanding prehistoric North America, but I am not sure current African conditions are comparable. Nevertheless, good food for thought. Not sure if the good old razor of Occam is as sharp when current conditions are used to make assumptions about to pre-holocene conditions. One can easily imagine that one or more of the many extinct grazers munched on wiregrass. What about one of the extinct musk ox relatives...or ground sloths?. If one species was able to target the tough stuff, others, as you imply, could follow up by continued cropping of tender resprouts. But that said, I think your point still has a lot of validity. On Thu, Oct 11, 2012 at 1:10 PM, Kevin Robertson <[email protected]>wrote: > In response to David Burg’s comments, I think there is evidence that > grazing and fire have different effects, mostly relating to the location of > surviving meristems. This would also depend on what animal is doing the > grazing, but experiments that my graduate student has done in addition to > similar past studies show that clipping woody plants, even very close to > the ground, induces a different resprouting response than fire, including a > greater number of stems, higher genet survival, and greater growth rate of > resprouts, presumably because meristems survive at the base of the stem > whereas fire kills them down to the root crown. **** > > ** ** > > In saying that certain plants have been fire-dependent over the past > several thousand years is certainly not to make claims about what factors > have influenced their survival over the course of evolutionary history, > which could include many proposed processes. However, with regard to > wiregrass and other such species having survived before humans, I feel that > there is abundant evidence that lightning-initiated fires were frequent > enough to maintain fire-dependent ecosystems in the absence of humans. > Such evidence includes dendrochronological data that show very frequent > (mostly 1-3 year interval) fires occurring primarily in the early growing > season which corresponds to lightning rather than fall or winter burning > typical of Native Americans, the presence of many pine-grassland species in > lake cores going back to 40,000 years, and the fact that current large > tracts of native vegetation in the region are lightning ignited at a very > high rate, high enough to maintain frequent fire regimes according to > managers. That is not to say that grazing did not have an important > ecological role, but in the case of wiregrass, this species is famously > unpalatable to ungulates, which has led to hypotheses that it has spread > widely only since cattle grazing, since the cattle do not eat it (unless it > is recently burned). Perhaps most importantly, if you exclude fire from a > pine-grassland, it would require grazers that like to eat wood more than > grass to keep it from changing into a hardwood forest, since resprouting > tree species are pervasive in this community type. Otherwise the southern > cowboys would not have to have burned every year to keep the community > palatable for ungulates. Similarly, work in tallgrass prairie has shown > that bison leave areas pretty soon after burning to find recently burned > prairie, suggesting the overall dependence of the community structure on > fire, even though grazing also has a strong effect. **** > > ** ** > > Invasive species and human disturbance do complicate the picture, but the > pine-grassland communities we are studying have few to no invasive species > and little to no previous soil disturbance, believe or not. That may not > be the case for long, but for the time being we have a good view into how > fire functions in a community of indigenous species. **** > > ** ** > > On a final note, we can use our creativity to think of many ways that > apparently fire-dependent plant species could possibly survive without > fire, but I will stick with Occam’s Razor and the observation that, without > fire, these species disappear. **** > > ** ** > > Kevin Robertson**** > > ** ** > > *From:* David Burg [mailto:[email protected]] > *Sent:* Wednesday, October 10, 2012 6:25 PM > *To:* Kevin Robertson > *Cc:* [email protected] > *Subject:* Re: [ECOLOG-L] FIRE Wildland and Urban Interface Myth or Truth > 1 Fire dependent plants?**** > > ** ** > > Dear fire/urban threaders, > > Since both grazing and fire have lots of variable effects depending on > frequency, intensity, seasonal timing, weather conditions etc, I am > troubled by assertions that fire achieves effects that are not achieved by > grazing. This may indeed be true, but to state that with confidence > wouldn't there have to be a lot more experimental evidence than we > currently have? > > Going back to the Gill paper, Kevin Robertson, one wonders what was wire > grass doing before humans increased fire frequency? In addition to the > variables mentioned above, consider the impacts of multiple species of > grazers, from small rodents to elephants. It would seem that there are > huge numbers of possible combinations of species, and each combo might > produce different impacts on vegetation. Maybe wire grass needed, say, > mammoths and horses? But only at certain seasons? > > As for the map of lightening strikes, Reed Noss, it is very persuasive > regarding current weather patterns. But E.C. Pileou and others show that > climate has fluctuated quite a bit since the glaciers. My concern is that > in defining what is "natural", and therefore a valid target for > preservation and management efforts, we need to think broadly in time and > space and consider a complexity of factors. > > That said, in the local day to day of the present, we have the added > challenge of dealing with novel vegetation systems. In my work to preserve > a small relict coastal oak savanna in the Pelham Bay Park in the Bronx, > (yes, da Bronx) here in NYC we have found that hand-removing a suite of > invasives (porcelain berry, oriental bittersweet, multi-flora rose, Norway > maple and others) as well as certain native species (mostly a polyganum) > has allowed a wide variety of declining plant species (and associated > invertebrates and vertebrates) to flourish. And hundreds of acorns from a > 300+ year old white oak are germinating into seedlings for the first time > in decades. > > All plants were on the site or appeared spontaneously, we have not done > any planting or seeding. But many of the non-native species we removed > are fairly recent additions to the vegetation community of eastern North > America. Difficult to say whether regimes of fire and/or grazing would > have achieved similar results in this new vegetation mix. It would have > been tricky, I suspect, to get the frequency and intensity just right. Ah, > the elusive goldilocks zone. As this discussion thread demonstrates, we > live at a time when we have an over-abundance of opportunities to learn > important details of ecosystem interactions. > > David Burg > WildMetro **** > > On Tue, Oct 9, 2012 at 2:19 AM, Kevin Robertson <[email protected]> > wrote:**** > > Another angle, which might in part have been the intent of the interviewee > in question, is that many plants are dependent on environmental conditions, > including plant community structure, which are dependent on fire, at least > in a natural ecological context. We know that in southeastern U.S. > pine-grasslands that are large percentage of the of several hundred species > of herbs disappear upon excluding fire for several years, some sooner than > later, as well as many species of animals that depend on them. This is > because woody plants that would otherwise be topkilled rapidly grow and > outcompete herbs through root competition and shading, in addition the > removal of fire as a reproductive cue for reproduction and a means to > provide bare mineral soil for seed germination. > > That aside, I am pretty unapologetic about saying that certain plants are > "fire dependent" when talking about this ecosystem. That is not to say > that you could not get the plant to survive and reproduce in a greenhouse > if you knew what specific environment and cues were required, but in an > ecological context it appears to be true that populations of certain > species depend on fire for their survival, at least there is no other > process that we know which would take the place of fire's function in that > population's survival. A well studied case is that of wiregrass (Aristida > stricta), which for a long time was thought (even if illogically) to no > longer sexually reproduce, since no one had ever seen it flower and produce > seed. However, at the time controlled burns were annually applied in the > winter throughout much of the region, preempting lightning initiated fire > later in the growing season. It was discovered later that burning (and > perhaps lightning-initiated or accidental fire) in the growing season, > especially May-June, did cause the grass to produce seed, and this > corresponded to the period when lightning-initiated fires were and still > are most common. Grazing does not seem to have the same effect of fire on > this species with regard to reproduction. Is there any set of > circumstances in which it would flower without fire? Probably. Would that > set of circumstances have occurred historically without human intervention > (it was around before Native Americans)? Probably not, or extremely > rarely. Would wiregrass be one a common grasses throughout the eastern > half of the southeastern U.S. Coastal Plain without fire? Absolutely not. > Thus, for all intents and purposes, in an ecological rather than > theoretical or physiological context, I would say it is a fire-dependent > species. > > Kevin Robertson **** > > ** ** >
