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
