Guest post by Larry Fields
The last Ice Age razed all of the coniferous forests in Finland. After the ice sheet retreated, trees from elsewhere–like the Scots Pine–gradually colonized the vacant niches. On a smaller scale, the same thing happened in many high mountains of the Earth’s temperate regions, including the Sierra Nevada Range of California. We can learn a thing or two about climate history from Alpine dendrology.
Round Top Lake, at 9340 feet elevation in the Northern Sierra near Carson Pass, is my favorite place for informal climate history research. Whitebark Pine trees grow in tight clumps around the North half of the lake.
Other high altitude conifers–like Lodgepole Pine, Mountain Hemlock, and Red Fir–also grow in the Carson Pass area. But Whitebark Pines can grow at slightly high elevations than these other trees.
At Round Top Lake, the Whitebark Pines in any given group are nearly identical genetically, since they reproduce asexually. New tree trunks grow outward from an existing root system. This is called suckering. The seeds that do sprout can’t endure the harsh Winters at that altitude. Walking 100 yards downhill from the lake on the main trail, one can see Whitebark Pines that have grown in a more normal way.
Naturalist Jeffrey P. Schaffer mentioned Round Top Lake in the 1989 edition of his book, The Tahoe Sierra: A Natural History Guide to the 106 Hikes in the Northern Sierra. Here’s a link to a review of a more recent book by Schaffer.
Question: After the last Ice Age, how did the Whitebark Pines reach the lake in the first place?
My answer: At some time after the last Ice Age, the Northern Hemisphere was warmer than it is these days. The Whitebark Pines sprouted from seeds at that time, with a little avian assistance.
The Clark nutcracker helps to spread seeds from the conifers in the Carson Pass area. The bird caches seeds that it gathers in anticipation of the Winter food shortage, but often stores more than it needs. Some of the forgotten seeds sprout a fair distance–and even uphill–from the parent trees.
Several years ago, I was surprised to see a knee-high Whitebark Pine seedling outside the half-circle of clone clusters hugging Round Top Lake. However it did not survive.
If the Northern Sierra climate heats up in a big way, I’d expect individual seed-sprouted Whitebark Pines at Round Top Lake to eventually supplant the clumps of small trees. Over the last 1000 or more years, the clones have been gradually accumulating random mutations, which should put them at a slight competitive disadvantage with any future surviving seed-sprouted progeny. When I see a lot of isolated Whitebark Pine seedlings that grow to 6 feet in height at Round Top Lake, then I’ll believe that the Northern Sierra climate is the warmest that it’s been since before the last Ice Age.
Jennifer Marohasy kindly allowed me to guest-post a guest article about this preliminary investigation on her blog. That report is similar to this one, but with less detail. On 29 August 2009, co-investigator James Mayeau and I visited Round Top Lake for more detailed study. Here’s a link to James’ account of our exploits on that day, in a second guest-posting at Jennifer’s blog.
This blog post includes a link to more of James’ photos from Round Top Lake.
Although the overwhelming majority of Whitebark Pines at Round Top Lake were clones growing in clumps, we did manage to find a few medium-sized individual trees near the sunnier West end. I’m guessing that their growth rates were pretty slow, given the marginal nature of the habitat. They probably predate the latest global warming that started in the late 1970s. If any of the West-end trees date from the 1930s, that would be additional evidence that the last round of’ global warming in the 80s and 90s was not a big deal, even by 20th Century standards.
A good follow-up project would be to count the growth rings in these few West-end trees, in order to determine their ages. Alas, my academic background is in analytical chemistry; I’m not qualified to drill for core samples in living trees.
Studies that emphasize tree ring analysis tell parts of several different stories that Nature has woven together into the fabric of climate history. These stories are about precipitation (in the case of Bristlecone Pines in the semi-arid Basin and Range Region of the Western US), insolation, wind, and yes temperature. Modern dendroclimatology often requires sophisticated techniques to tease out the temperature parts of tree-ring stories. One lesson from the recent Keith Briffa controversy: In modern-dendroclimatology-based climate history studies, the methodological uncertainties stemming from sampling error, inter alia, are huge.
On the other hand, Seat-of-the-Pants Dendroclimatology (SPDC) emphasizes straightforward early 20th Century technology: shoeleather, cameras, maps, field guides, and optional hand-operated tree-boring devices. The key insight of SPDC: For a given topography, temperature is the single unambiguous control variable that governs tree reproduction mode just below Alpine timberline. When clumps of clone trees dominate the landscape there, one can be certain that the climate was appreciably warmer at some time in the past.
Side note. At timberline in the Northern Sierra, it is difficult to find Whitebark Pines in the form of trees; instead they typically grow as Alpine shrubs–known as krummholz. This is the subject of ongoing climate-related investigation by other researchers.
Summary. In the Northern Sierra, Seat-of-the-Pants Dendroclimatology can give a more accurate indication of pivotal warming events in Earth’s geologically recent climate history than can modern dendroclimatology. In terms of the debate surrounding the ‘unprecedented’ global warming of the late 20th Century, Round Top Lake is ‘the elephant in the room’.
Cluster of Whitebark Pines at Round Top L. Photo by James Mayeau.