Big herbivores shape plant community response to global warming

ResearchBlogging.orgThe cover article from this week’s PNAS has important implications for how we plan for, and deal with, climate change. Post and Pedersen report that the way an arctic plant community changes in response to warming depends heavily on the presence of large herbivores [$-a], like muskoxen and caribou.

Photo by Giant Ginkgo.

Previously, it was thought that one effect of global climate change would be for woody shrubs and dwarf trees to become more common in arctic and subarctic plant communities. This increase in woody plants could trap more atmospheric carbon and increase the albedo of the land – meaning more heat could be reflected back into space. Both of which effects might help slow a warming global climate.

However, Post and Pedersen show that large herbivores can reduce this shift in community composition. In huge five-year study, they set up experimental plots from which caribou and muskoxen were either excluded by fencing, or not excluded. Within each class of plot, they also placed 1.5-m “open-topped chambers” (OTCs) made of fiberglass – basically, cylinders that help trap solar heat, warming the ground inside. In the fenced plots, the plant communities inside the OTCs shifted toward more woody species; but in the unfenced plots, where large herbivores could reach in and graze inside the warmed cylinders, plant communities didn’t develop greater cover by woody species.

Now, it’s not surprising that large herbivores can have a profound effect on the plant species that grow in their grazing land. Where I come from, in the northeast U.S., large swaths of forest have been dramatically altered [$-a] by a population explosion of white-tailed deer freed from their natural predators. But Post and Pedersen have drawn a connection between this effect and the ways in which natural communities may respond to the most dramatic environmental change in human history. It just goes to show what a massively complex system we humans are tinkering with, and how little we know about what that tinkering may ultimately do.


E. Post, C. Pedersen (2008). Opposing plant community responses to warming with and without herbivores PNAS, 105 (34), 12353-8 DOI: 10.1073/pnas.0802421105

F.L. Russell, D.B. Zippin, N.L. Fowler (2001). Effects of white-tailed deer (Odocoileus virginianus) on plants, plant populations and communities: A review The American Midland Naturalist, 146 (1), 1-26 DOI: 10.1674/0003-0031(2001)146[0001:EOWTDO]2.0.CO;2

Climate change: bad for native plants

[Correction/clarification appended]

This is how I can justify blogging as a scientific activity: once in a while, I find something really useful. Case in point is this post on the ‘blog of Pamela Ronald, the chair of the University of California Davis plant genomics program, which points to a new in the last volume of PLoS ONE that predicts (perhaps not surprisingly) climate change is going to be bad for rare plants in California.

The effect of climate change on plant communities is a major concern for me, because the range of my favorite woody monocot, the Joshua tree, may have to change quite a bit to compensate for a warmer climate. (For reference, see the photo of me setting up a pollination experiment on a Joshua tree in front of the Yucca Valley United Methodist church.) Previous projections have suggested that Joshua trees are going to be in trouble under a warming climate. Back in 2006, Science ran a cover article suggesting that climate change may make wildfires more frequent [$-a]. That’s a very real problem for Joshua tree’s range in the Mojave Desert – my lab has already lost field sites to brush fires in only about half a dozen years of focusing on Joshua trees. Another, more recent study has suggested that climate change is going to make the southwest U.S. even more arid&nbsp[$-a], which is also, obviously, a bad thing for plants (and people) in the region.

Earlier work of this sort usually modeled how climate change might increase or decrease the distribution of individual plant species – big, showy things like Joshua tree, Saguaro cactus, giant Sequoias. Loarie et al. improve over this by projecting changes in whole plant communities across the California floristic province. And they predict that up to 66% of plants endemic to California will lose more than 80% of their ranges. That’s a lot of diversity – more than just my study organism – at stake.

In the original version of this post, I conflated the state of California, which does include a lot of Joshua tree’s range, with the California floristic province, which doesn’t. So Loarie
et al.‘s new paper doesn’t directly impact Joshua trees. But it’s still cool/alarming, and decidedly post-worthy. In making that correction, I’ve also inserted a more recent study of climate change in the U.S. southwest, by Seager et al.

Loarie SR, BE Carter, K Hayhoe, S McMahon, R Moe, CA Knight, and DD Ackerly. 2008. Climate change and the future of California’s endemic flora. PLoS ONE 3:e2502.

Seager R, M Ting, I Held, Y Kushner, J Lu, G Vecchi, H-P Huang, N Harnik, A Leetmaa, N-C Lau, C Li, J Velez, and N Naik. 2007. Model projections of an imminent transition to a more arid climate in southwestern North America. Science 316:1181-4.

Westerling AL, HG Hidalgo, DR Cayan, and TW Swetnam. 2006. Warming and earlier spring increase western U.S. forest wildfire activity. Science 313:940-3.

Life on 2000 Watts

How much energy can each man, woman, and child on Earth use sustainably? According to a consortium of European scientists, it’s 2000 watts. That’s 17,520 kilowatt hours per year per person. Like most nice, tidy numbers, that number is probably more or less fictitious (there’s the question of where the energy comes from, and how you calculate the per-capita consumption, just off the top of my head), but it’s good to have a starting point for thinking about it. And an article in this week’s New Yorker, by Elizabeth Kolbert, does a pretty good job of working through that thought process.

My household electricity usage comes to a little less than 7,200 kwh in the last year – my provider, Avista actually has some great online tools for assessing home energy efficiency, and even allows me to specify that I only buy power from wind and other renewable sources. Unfortunately, my personal energy budget includes more than home light and heating: there’s auto fuel, electricity used at work, and the energy used to produce and transport almost everything I buy, just to name a few. It’s a pity there’s no good way to sum all those up.

91-octane from algae

Over on Sapphire Energy says it will be producing up to 10,000 barrels of carbon-neutral petroleum in the next five years. How? By extracting the oil from algae. That’s biofuel made from pond scum. It sounds promising to me; algae should be able to grow with minimal energy inputs and with much less land use than even switch grass.

Hey! I know that tree!

Tonight’s All Things Considered hits a long overdue topic as part of NPR’s ongoing “Climate Connections” series: the fate of Joshua trees in a warming world. I say overdue, of course, because two chapters of my dissertation will be on the population genetics and phylogeography of Joshua tree, and it’s hard to spend much time with Joshua trees and not wonder about how they’ll hold up under global warming.

I’ve spent two spring flowering seasons in Joshua Tree National Park, where much of the story centers. The Park is right at the southernmost boundary of Joshua tree’s current range, where (all else being equal) you’d expect to see the impact of warming earliest. As the NPR story points out, there does seem to be low recruitment (growth of young trees to replace old ones as they die) in the southern populations. On top of that, drier conditions are contributing to more frequent wildfires across Joshua tree’s range, and sprawl from Las Vegas and Los Angeles is rolling right through Joshua tree woodlands. The Park staff I’ve talked to (including naturalist Joe Zarki, who’s interviewed for the story) are seriously considering that they may have to take drastic measures to prevent Joshua Tree National Park from losing its namesake trees.