Big Trees, Big Problems?
The New York Times just ran an article about how global warming and increasing droughts could threaten the existence of the Sequoiadendron giganteum (giant sequoia). About 10,000 years ago, most large mammals across North (and probably South) America went extinct: this is termed the megafauna extinction and, from a geologic perspective, it is hard not to think that the arrival of humans near that time was the key difference relative to more than 30 previous deglaciations lacking equivalent extinctions. So humans may have caused extinction of the megafauna. This current threat could be termed the megaflora extinction; sequoia might attract special attention as, to recycle a term, they are charismatic megaflora. They are trees a lot of folks want to save.
It isn’t clear what prompted the NY Times article; the message is nearly identical to that in chapter 53 of volume 2 of the report to Congress of the Sierra Nevada Ecosystem Project in 1996 titled “Ecology and Management of Giant Sequoia Groves” by Nathan Stephenson; this report in turn is based on even earlier work, so worries about the fate of the sequoias have been batted around for 25 years or more. [John Muir embarked on a trip to seek out sequoias because he was worried that the groves near Yosemite that he was familiar with were the dying remnants of a once-widespread species]. Basically the guess is that since sequoia seem to have increased in number and density in the past 4000 years, after the end of a warmer and more droughty time in California history, that a return to higher temperatures and drought would put the trees at risk. As guesses go, this is perfectly plausible.
None of the current worries seem based on any indication of a decline in health of modern giant sequoias (beyond that caused by visitors compacting the ground, increased ozone from the Great Valley, an absence of frequent fire and other very local causes). The logic for sequoias being at risk comes from study of pollen in cores from meadows and lakes. At Log Meadow in Sequoia National Park, Scott Anderson reported that modern groves only emerged about 4000 years ago with few sequoia present before that. Reconstructions of the climate in the Holocene indicate a drier Sierra from 10,000 to 4000 years ago, which is consistent with changes in solar insolation from changes in the earth’s orbit that indicate that Sierran summers in the early Holocene were hotter and drier.
But when you start reading some of the science on giant sequoia, you find a greater air of speculation around them than you’d hope. First off, we don’t really know why they are where they are: giant sequoia grow well in lots of places where you don’t find them naturally; speculation by Muir was that their modern distribution was because of glaciation; later on Axelrod posited the opposite, that it was a product of mid-Holocene drought. We know near the end of glacial time that some were near Pleistocene Lake Russell (where Mono Lake is today) and probably in the lower western foothills of the Sierra, but the travels of this tree remain pretty unclear.
Similarly, it took a long time to realize that the absence of sequoia seedlings within the middle 20th century was because sequoia depend on fire for reproduction (which was dramatically demonstrated with an experimental fire in 1969); furthermore, sequoia are some of the most fireproof of trees. Preventing fires in sequoia groves was actually doing the trees more harm than good. Sequoia seedlings like to grow in openings; they are an invasive species in that respect. In a lot of ways this sounds like a species that might welcome a bit more fire; maybe some drought wouldn’t be so bad?
Here’s the rub: during this whole time that sequoia groves were being established and climates were changing, humans were present and active and interested in modifying the plants they lived with. While the role of humans in modifying the fauna has been hotly debated within the archeological community for quite sometime (Paul Martin’s overkill hypothesis being one extreme), people and plants haven’t quite produced the same level of attention. Fire has been in the human toolkit for probably more than a million years; certainly natives were burning Sierran forests by about 1000 years ago. What if they were burning forests before that? Sequoia like small fires of the kind usually set by Miwok and other Native Californians. Could the emergence of modern sequoia groves be the result of actions of a Miwok Johnny Sequoia-seed? If so, how would that change the story about the creation of sequoia groves relative to climatic factors?
A warning about the possible role of human-ecosystem interaction in the Sierra accompanied that report to Congress by the Sierra Nevada Ecosystems Project:
The scope of this report, as a summary of existing studies of vegetation history, encompasses primarily the vegetation-climate relationship, but for future research it is strongly recommended that prehistoric human modification of the landscape be taken into account. (Wallace Woolfenden, Ch. 5 of SNEP report, 1996)
Does this mean that sequoia aren’t at risk from global warming? No, but what the story shows is that we will remain ignorant of how plants will respond until we understand how much Native activity determined the range of different plants. We have precious little information about what things looked like in the last interglacial 120,000 years ago in these mountains; were there sequoia groves then? Or were they perhaps minor elements of a diverse forest?
Bristlecones might well be better measures of the impact of global warming simply because they live in an environment that seems less likely to have been actively managed by human communities. They are showing serious signs of stress from global climate change (as is mentioned as an aside in the NY Times article); these more ancient of trees might well vanish before giant sequoia do. So is it wise to push the sequoia forward as the poster tree for risks of climate change?