Asteraceae Elevation Ranges And Bloom Times At The Grand Canyon, Arizona
Grand Canyon National Park spans elevations of roughly 1000 to 9000 feet, and hence it is an interesting area in which to study the distribution of species with altitude, and when those species bloom. The bloom times are also interesting in that this area receives almost exactly half its annual rainfall in the winter months (Dec - May) and half in the summer monsoons (June - September).
The following analysis is based on the Grand Canyon National Park Asteraceae Checklist With Additional Information.
The maximum elevation is plotted versus the minimum elevation for a given species in the following plot:
The diagonal lower border of the points represents those species that are known from only a single or a few locations, and hence have a minimum elevation roughly equal to their maximum elevation.
The plot shows three areas of concentration. The cluster at lower left represents species whose minimum elevation is ~1000-~3000 feet, and whose maximum elevation is ~3000-~5000 feet. These consist primarily of plants that can tolerate the hot desert conditions of the Tonto Platform and below, and similar exposed slopes above the Tonto Platform.
The cluster at upper right represents species whose minimum elevation is ~6500-~8000 feet, and whose maximum elevation is ~8000 - ~9000 feet. These consist of the plants adapted to the cold Rims.
The looser cluster at upper left represents species whose minimum elevation is ~1000-~3000 feet, and whose maximum elevation is ~7000-~9000 feet. These are the minority of species that can adapt to many climatic conditions, such as Gutierrezia sarothrae, Artemisia dracunculus, Solidago canadensis var. scabra, Heterotheca villosa var. minor, etc.
The plot clearly shows a distinct segregation by elevation, with very few species having a minimum or maximum elevation of ~5000 - 6500 feet. This is seen very clearly in the following histograms:
Histograms are given separately for the minimum elevation and for the maximum elevation for each species. Both histograms show a strong minimum at elevations of ~5000 - ~6500 feet.
There are at least two possible reasons for that gap. First, it looks like there is not much area within Grand Canyon National Park at elevations of 5000-6500 feet. (I haven't seen a histogram of the area versus elevation, but this seems correct based on perusal of topographic maps.) The large area above the rims is mostly above 6500 feet, and the next largest area is along the Tonto Platform and Esplanade, mostly at 3000-5000 feet. Since the number of species is proportional to a power of the area, the gap is caused at least in part by the lack of area at those elevations.
Second, 5000-6500 feet is a transition area in the Canyon between the hot desert below and the cold forest above. Hence the desert species have their highest elevation at 5000 feet and below, and the forest species have their lowest elevation at 6500 feet and above.
The histogram of the minimum elevation is especially interesting, and strongly supports the last reasoning. That histogram consists primarily of two separate curves, one with a maximum at 1000 feet and declining smoothly to 6000 feet, and another with a maximum at 7000 feet declining smoothly to 9000 feet. This is another way of looking at the first two clusters discussed above in the previous plot that gives additional insight to them.
Consider the first curve, with a maximum at 1000 feet, and consider a plant that can survive the hot exposed conditions on the Tonto Platform at 4000 feet. Conditions at 1000 feet are not much different from conditions at 4000 feet, and hence it is likely that a plant that can survive at 4000 feet will also be able to grow at 1000 feet. The histogram above shows that most plants at the Grand Canyon in fact do that, and have a range in elevation that spans all those conditions.
Of course, not all plants can do that, since many species are adapted to special niches (soil type, slope exposure, amount of shade, etc.) that may not be found at all elevations. Hence some species have a minimum elevation above 1000 feet.
Similarly, any species that can grow at 9000 feet on the North Rim is likely to be able to grow in similar conditions at 7000 feet.
The histogram of maximum elevation is truncated by the lack of elevations much above 7000 feet on the South Rim and 9000 feet on the North Rim, and hence the upper elevation part of that histogram does not contain much interesting information.
The lower elevation part of that histogram shows that few species have 1000-2500 feet as their uppermost elevation, which supports the conclusions drawn above from other plots. The gradual fall-off of the maximum elevation above 4000 feet probably represents the effect of cold winter temperatures limiting the upper range of species that cannot tolerate freezing temperatures, and the lack of flat slopes required by many species.
The final plot here shows the number of species blooming per month:
Three curves are shown. The uppermost is for all species, the middle one (yellow) is for species with a maximum elevation below 6000 feet, and the lower one is for species with a minimum elevation above 6000 feet.
Considering all elevations, peak bloom occurs from May to September, with August being the absolute peak. But that curve of all elevations hides two distinct trends seen in the separation by elevation.
The lower elevation species show two clear peak bloom periods: April - May and August - October, which are clearly driven by the two rainy periods. Compare that curve to one from the Santa Rosa Plateau in coastal southern California, at ~2000 feet elevation, where all the rain falls in winter, and there is only one peak bloom period, the same April - May.
The upper elevation species show only one peak bloom period of July - August. The single peak bloom period is produced by constraints on each end. The climate is so cold at these elevations that most species cannot produce blooms until June, and the seeds and fruit must have matured by the first frosts in October.