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Possible role of fauna and humans?

Posted by drpaleo on 05 Mar 2008 at 03:44 GMT

Although the authors do not consider these factors, there are two major changes of biota in central Alaska ca. 14,000 cal BP: 1) humans arrive (earliest dates of 12,300-11,700 14C years bp from Swan Point) and 2) mammoths and horses go extinct, while elk and moose arrive (like humans, from Siberia) (see Dale Guthrie's dates in Nature 2006, v. 441, pp. 207-9). A common pattern observed in several records (Madagascar, new Zealand, Hawaii, possibly SE New York) is that soon after humans arrive, megafauna perish, vegetation they had previously digested accumulates and frequently burns (see papers by David and Lida Burney and Guy Robinson). The major impact upon tundra environments caused by removal of megafauna, perhaps largely due to hunting, has also been suggested by Zimov.

RE: Possible role of fauna and humans?

phiguera replied to drpaleo on 06 Mar 2008 at 01:06 GMT

Dr. Fiedel,

Thank you for your comments and question. You raise an important point about the possible roles of humans and other large mammals as contributors to the changes reported in our study. [In the Nature paper cited in your comments, Guthrie suggests that the development of the birch-dominated shrub tundra, which is a focus of our paper, was partially a consequence of grazer extinctions (which in turn likely had at least a partial human cause).] While the ultimate cause of the vegetation change is important to understand, we focus on the likely causes of increased fire occurrence with the transition from herb- to shrub-dominated tundra. In this context, the cause of the vegetation change, whether climate and/or mammal related, is secondary to the impacts of the vegetation change on landscape flammability. We discuss climate over the period of our records to provide a context for understanding increased fire frequencies, and we suggest that low relative moisture played a role in facilitating frequent fires. We do not suggest that climate was the primary driver of increased fire frequencies with the transition to the shrub tundra: “Given our current understanding of late glaciation and the early Holocene, increased burning in the Shrub Tundra Zone was not a simple function of climate change.” Fire frequencies increased distinctly with the transition to shrub-dominated tundra, and although not highlighted in the paper, fire frequencies then decreased when Populus trees arrived in region ca. 10,000 years ago, creating deciduous woodland vegetation. Both patterns suggest an important role of vegetation in determining fire regimes. I would certainly be interested in other studies documenting a similar pattern of increased fire frequencies coincident with vegetation changes. Can you provide some references for the work you mentioned by Burney and Robinson?

The mammalian paleorecords raise at least two interesting questions from the fire ecology perspective. First, why didn’t the herb-dominated tundra burn? The parsimonious explanation is that fuels were lacking and climate / weather conditions were unfavorable for ignition and fuel drying. Thus, climate and vegetation were both important limiting factors for fire. A lack of fuel (i.e. low productivity), however, contrasts with interpretations of a productive steppe sustaining large populations of megafauna. In a more complicated scenario, climate could have favored fire (although this seems unlikely), but grazers consistently reduced fuel abundance such that fires could rarely spread. Remove grazer, *change* the vegetation type, accumulate fuels, all while climate is gradually warming, and suddenly the system can sustain frequent fires. Although the vegetation context is quite different (i.e. forest vs. tundra, human introduction of grazers) grazing by domesticated animals played an important role in reducing fire occurrence in ponderosa pine ecosystems across the western U.S. in the early 20th century (e.g. Heyerdahl et al. 2001. Ecology 82:660-678).

A second interesting question remaining is if humans played a significant role in modifying ignitions. The timing of human arrival makes this a possibility, but I am doubtful that an increase in human ignitions alone could explain the increased fire frequencies during the shrub tundra period. Even with population densities what they are today across Alaska, most human-caused fires are small (even in the absence of suppression efforts) and most area burned results from lightning ignitions (see DeWilde and Chapin. 2006. Ecosystems 9:1342-1353).

Thanks again for your comments.

Philip Higuera