Tree shift rolls downhill

[Originally posted on www.foreststeward.com on Jan 28, 2011]

Article Reviewed: Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species’ Optimum Elevations

By S.W. Crimmins, S.Z. Dobrowski, J.A. Greenberg, J.T. Abatzoglou, and A.R. Mynsberge, published in the journal, Science, Vol. 331, pp. 324-327

Plot line: The authors gathered data collected from the early 1930’s from surveys that measured the elevation where species occurred throughout the forested regions of California (north of about Tehachapi). They also gathered weather data from the same period to see what type of climate the species were occurring under. They then did the same thing (gathered plant and weather data) from the early 2000’s to see if there was a difference in where plants were growing and what climate conditions they preferred. They found that most species (72%) shifted downhill by on average 289 feet in elevation. Although average temperature has increased since the 1920’s (and especially since about 1950), there has also been an increase in precipitation which has actually resulted in a net reduction in drought stress at a given elevation. They conclude that species have shifted downhill in order to limit the amount of drought stress and to expect similar shifts to occur if the pattern of higher temperatures simultaneous with higher precipitation continues.

Relevant Quote: “Plant species in our study area appear to be tracking their climatic niche by shifting their altitudinal distributions downhill in response to decreased climatic water deficit.”

Relevance to landowners/stakeholders

Plants are finicky creatures. They have a certain set of climatic conditions under which they can thrive versus merely persist. This is kind of similar to how we set our thermostats at the temperature where we are most comfortable. Sure there is a wide range of temperatures in which we could survive, but we work best at a very precise range (my optimum is about 68F, but I could probably get by between 55 and 80). But how dry it is and how thirsty I am influences this range greatly. I’ll get heat stroke if I try to work when it is 85 degrees and dry outside, but if I am well hydrated or if it is raining, then I can get by and maybe even be more comfortable at a higher temperature. This study found that plants (many of which were trees) in mainly forested areas of California did what the authors call “niche tracking” as a response to climate change. Even though temperatures have increased recently (which may cause one to predict that species would retreat uphill to stay cool), there was also a corresponding decrease in “water deficit.” Water deficit accounts for changes in both water stress that comes from hot and dry weather as well as from alleviations of that stress that come from increases in precipitation. It appears that the trees (especially those at higher elevations to begin with) shifted downhill where it was hotter but also wetter. This makes sense from a physiology perspective since photosynthetic activity can increase with temperature (up to a point) as long as there are associated increases in water and nutrients. It also makes sense in the Mediterranean climate of the Sierra Nevadas, where annual droughts limit plant growth.

Relevance to managers

I think the primary relevance here is that there remains tremendous uncertainty in how forests will respond to climate change. This study suggests that forests have already responded and will continue to respond, but the details of the response are difficult to predict. As was discussed in a previous post, there is a lot of uncertainty in how precipitation in particular will change. And as this study confirms, the direction of change will have a very important influence in how forests respond.

This study demonstrates that it should not be assumed that species will generally shift uphill and to the north, as many have predicted. At the same time, however, I don’t think that this uphill/north scenario should be ruled out because of this study. Rather, managers need to hedge their bets against change in general, whether they are changes in species locations up, down, east, or west. In the end, it isn’t the fact that these plants have moved down a couple of hundred feet that is worrying. Of more concern is if some species may have not been able to adjust at all. Of even more concern is how disturbances such as fire or how exotic pests and pathogens will interact with these shifts. Uncertainty is the rule. Active Adaptive Management is perhaps the most reasonable solution.

Critique and/or limitations (there’s always something, no matter how good the article is) for the pedants:

From the graphs, it looks like there was a pretty severe drought in the 20’s and 30’s that puts a lot of leverage on the overall increase in precipitation and the decrease in water deficit. Just an observation.

I wasn’t convinced that the sampling bias was completely accounted for. The map of plots from the 30’s show the plots mostly in the central or southern portions of the study area, while the plots from the 2000’s are on average farther to the north. They corrected for differences in elevation and temperature between sample periods, but what about latitude? Species from farther north in California would be expected to occur at lower elevations, which would contribute to the temporal difference found. Perhaps I am missing how difference in latitude is accounted for (maybe elevation and temperature corrections inherently account for it).

I think their suggestion that there is a general assumption among scientists that temperature changes will be the primary factor that drives biotic changes is a slight overstatement. There are plenty of forecasters who have considered changes in precipitation as also being important (see this post, for example). Ecologists in dry western forests are especially aware of the importance of water deficit in driving change.

Lastly, they say that there was a widespread downward shift across the elevation gradient, but from the graph (figure 4), it looks like species that occurred below 750 m (2460’) did not shift down on average.