Guest Post by Willis Eschenbach
Anthony recently pointed out a new paper called “Dramatic Response of Montane Plants to Climate Change in the Southwest” by Brusca et al., available here. In that paper, the authors have done an interesting repeat of an earlier study.
In the Brusca2013 study the authors report having done a new plant count following the uphill route along the same road used in a study done by Whittaker and Neiring in 1963 (W&N63). The road runs from Tucson to the top of Mt. Lemmon. They claim that their results show that warmer, drier weather is forcing Arizona plants uphill.
Could this be happening? Sure, it’s possible. Have the authors of Brusca2013 shown it to be a) actually happening and b) the result of climate change? Good questions. Here’s their summary of the results:
ORIGINAL CAPTION, BRUSCA2013 STUDY: Figure 1. Summary of elevation range of the 27 most common upland montane plants along the Catalina Highway. White bars are 1963 elevational range data from Whittaker and Niering (1964), the two terminal (stippled) 1000-ft bands denoting Whittaker’s upper- and lowermost 1000-ft vegetation bands. Black bars represent 2011 elevation data from this study. To be as conservative as possible, a change in a species elevation limit (high or low) was noted only if that species was found outside (above or below) the upper- or lowermost 1000-ft band. Thus, if anything, we underestimate the elevational change in the species since 1963 (see Materials and Methods). Following this protocol, 15 species show an unambiguous increase in lower elevation, four show an increase in upper elevation, and eight show a decrease in upper elevation.
Note that in the original W&N63 study, they only recorded the elevational band (e.g 3,000-4,000 feet) of the lowest and highest specimens. They did not record the actual elevation (e.g 3,232 ft) of the specimens at the extremes. This has created some difficulties in the Brusca2013 authors’ interpretation of their results. For example, look at the grass “Muhlenbergia porteri” (second line from the bottom). It is counted as one of the fifteen species whose lower elevation has been raised. The problem is, the lowest elevation looked at in the new study is 3,500 feet, viz:
Thus, our plant quadrat sampling began at 3500 ft/1067 m and ended at 9111 ft/ 2777 m, excluding desertscrub at the base of the mountains.
Since the new study only began at 3,500 feet, we cannot say that the lower range of this species is increased. They have incorrectly counted it as having an “unambiguous increase in lower elevation”. Does this one error invalidate the study? No. But this kind of error, in favor of their conclusion, does make a person wonder if the authors might have a solution (climate change) and be looking to fit evidence to that solution.
And what makes me wonder even more is that there are three other records with the same problem …
The difficulty is in their analysis of their own data. They want to compare their results to W&N63, which is a good thing, they should do so. But we need to have an apples to apples comparison. To do that, the very first thing you have to do is to convert their data to the “elevational bands” intervals used by W&N63. Yes, you need to throw away information to do it, but if you want to compare the two studies, you have to do it. For an example of what this does, look at Agave schottii, the first succulent in the purple band. It looks like the elevational range has shrunk considerably. But if the proper method were used, counting only by elevational bands as in W&N63, we’d see that there is absolutely no difference between the two records. W&N found it in three bands, and so did the latest study. Now, to their credit, they count it as unchanged. But it is incorrect to present their data for comparison with W&N63 in absolute elevations, rather than elevational bands as in the original study. To compare apples to apples, they have to first convert their data to elevational bands. Their failure to do so has lead them to false conclusions.
With that in mind, look at the bottom row, the grass “Urochloa arizonica”. When we convert their data to the W&N63 elevational bands, we see that there is no evidence of an upward trend at the bottom end, because the new survey only started at 3,500 feet. In terms of elevational bands, all we know from the new data is that that grass is still found between 3,000 and 4,000 feet, just as in the original W&N63 survey. For all we know, the current authors may have found the exact same patch that W&N found in 1963. As a result, we have no new information establishing the bottom end of its range.
The same is true for the woody shrub Mimosa aculeaticarpa, and the grass Aristida ternipes. Once we are comparing apples to apples by converting the data to elevational bands, it’s obvious we know nothing new about the lowest elevations at which they can be found. In all cases, they used to be found between three and four thousand feet, they still are, and we have no new information below 3,500 feet. From that we cannot conclude anything at all about changes in their lower boundaries.
This means that the authors have improperly identified no less than four of the records as showing an increase in lower elevations when the data does not support that claim.
Once that error is corrected, this leaves us with a curious result:
• 11 species have increases in lower elevation. Their range has shrunk from the bottom.
• 8 species have decreases in upper elevation. Their range has shrunk from the top.
• 4 species have increases in upper elevation. Their range has increased at the top.
I would hardly call that a convincing case for much of anything … however, they say it is a consequence of “climate change”, which they are defining as follows
Since the previous survey in 1963, the rainfall has gone up and then back down, not much change there. Arizona is always dry. However, as they point out, the temperature definitely has gone up … in Tucson, down on the valley floor far below the mile-high plants being studied. Measured at an airport which, unlike in 1963, is now half-surrounded by the city. So that might reflect changes up in the mountains … or not, we don’t know.
In addition, with changes in eleven lower and eight upper altitudinal limits in opposite directions, we can’t say the plants are moving up the mountain as they claim. Statistically, those two are no different.
And how is that collection of contradictory results supposed to happen from climate? Are we really to believe that the climate has driven the grass “Muhlenbergia porteri” (second line from the bottom) from 6,000 down to 3,000 feet, and if so, how does that work?
Finally, we have to consider confounding factors, which unfortunately they have ignored. The biggest one of these is the huge interaction between the plants and the animals in any ecosystem. For example, for the first time in decades the bears in Yellowstone Park are feasting on berries as they store fat for the winter. Is the increase in the number of berries a result of climate change modifying the berries’ elevational limits?
In a word … no. Curiously, it’s because of the return of the wolves to Yellowstone Park. The berries have been getting grazed to the bone by the elk for decades, but now that the wolves are keeping the elk in check, the berries are coming back, and the bear are getting their chance.
And although there are no elk in the area of this study, this is Arizona Game Management Unit 33, and the list says “Species within this unit: Javelina, Mule Deer, White-tailed Deer, Cottontail Rabbit, Dove, Tree Squirrel, Quail”. Pigs, rabbits, mule and white-tail deer … how have their numbers changed over time, and what effect has this had on the local plant species? Where I live, the deer exert a huge control over the shape and nature of the biome. And pigs are noted for their effect on the local plant ecology. What’s happening with the pig population?
• The study has significantly over-counted the number of species whose lower limits have increased. The number of species whose lower limits increased drops from their claim of over half (56%) of the species studied, down to 41% of the species.
• Over a quarter of the species studied had upper limits that moved down the mountains, not up as might be predicted.
• The study used temperature data which is from the valley floor, which has a good chance of being contaminated to an unknown degree by urban heating effects.
• The study failed to consider changes in local animal populations as an alternate explanation for at least some of the plant changes.
• Some species showed huge changes in their range, beyond what a few degrees in average temperature might reasonably explain. This indicates that other factors are likely at play.
• There was no discussion of natural fluctuations in the ranges of the plants. Plants are subject to a host of ever-changing forces. Is the current variation in range outside their normal variation, whatever that may be?
Overall, while it is an interesting study and I commend them on repeating the earlier transect, I’d say they have totally failed to demonstrate that the plants are responding in any meaningful or predictable way to the vagaries of the local climate …