Plants of Southern California: Analysis Pages: Matchweed (Gutierrezia)
Note added 26 September 2015:
This page was done by a naive beginning botanist (myself 13 years ago!), in which I interpreted the key in the Jepson Manual quite literally to try to determine the Gutierrezia plants in two nearby places in southern California. While literally the results indicated that, according to the key, there were indeed two separate species here, I now am pretty sure that the species are the same in the two places, G. sarothrae. For more information, see Gutierrezia californica and G. sarothrae.
I've left this page otherwise intact as it was on 27 August 2002 so others can see what happens when one takes a key a bit too literally, instead of stepping back and realizing the overall similarity of the plants in the two places.
This page investigates some distinguishing properties of Gutierrezia californica and Gutierrezia sarothrae observed at two locations in the southern Santa Ana Mountains.
The Jepson Manual (JM) key primarily uses the clustering property of the flowering heads to discriminate these two species:
2. Heads generally solitary .......................Gutierrezia californica
2'. Heads generally in clusters of 2-5 .........Gutierrezia sarothrae
However, in my experience at the Santa Rosa Plateau Ecological Reserve (SRPER) and Santa Margarita Ecological Reserve (SMER), the clustering property wasn't so easy to distinguish in practice, since casual observations showed that about half the branchlets had solitary heads and about half had multiple heads. Hence the following investigation was done to further my understanding of what this key element means, as well as to verify the identity of the species observed by me at the two Reserves.
An important caveat to keep in mind for this analysis is that these two species are said to intergrade in the central and southern South Coast (SCo) area defined in the JM. Although these two reserves are both in the Peninsular Ranges (PR), they are fairly close to the Temecula/Murrieta boundary of the SCo area and it is possible that some hybridization occurs there as well.
On 31 July and 3 August 2002, I analyzed plants at SRPER on Waterline Road between the Visitor Center and the Granite Loop Trail crossing. I histogrammed the number of heads per branchlet using multiple stems per bush for four separate plants spaced the length of this section of road. I also measured samples of heads taken from several of these bushes.
On 3 August 2002, I similarly analyzed plants at SMER on Red Mountain Truck Trail at two separate locations, at miles 0.84 and 1.03 from the South Field Station.
Some results were kept separate for each specimen. Analysis showed that the plants at each location were consistent with each other (i.e., within the location, not between the locations), and hence I have combined the results for each location separately.
This analysis showed that the plants at SMER were indeed Gutierrezia californica, and the plants I observed at SRPER were Gutierrezia sarothrae. Hence in the following, the plants will be identified as these species.
Number Of Heads Per Branchlet
The number of heads per branchlet is shown in the following plot:
The number has been normalized to be the percentage of branchlets with a given number of heads. I measured 148 branchlets from four different plants at the SRPER and 92 branchlets from ~ten plants at SMER.
The separation between the species is clear, and indeed is consistent with the JM key. Gutierrezia californica indeed had heads generally solitary, with 64% of the branchlets having only a single head. Gutierrezia sarothrae indeed had heads generally in clusters of 2-5, with 59% of the branchlets having 2-5 heads.
Note that the JM key is ambiguous. If one does the statistics by the number of heads, the key would not correctly identify Gutierrezia californica. Thus although 64% of the branchlets for that species have a single head, those branchlets contain only 41% of all the heads. Hence one must interpret the key as "heads per branchlet" and not as "normalized by the number of heads". (To make this clear, suppose a plant has half its branchlets with a single head and half with two heads. Then 50% of the branchlets have a single head, but two-thirds of all the heads are found in clusters of two.)
This plot makes it clear why a casual observation can not easily tell the difference between the species: one must observe at least ~40 branchlets before one can statistically separate 60% from 40% at 95% confidence (the precise number of branchlets required depends on the unknown-to-me variance in the number of heads per branchlet from stem to stem and plant to plant).
These limited data suggest that a much easier observation in practice is simply to look for a significant number of branchlets with 4 and 5 heads. The JM description says that Gutierrezia californica has 1-3 heads per branchlet and Gutierrezia sarothrae has 1-5. My data show that 15% of the branchlets for Gutierrezia sarothrae have 4-5 heads, compared to only 2% of the Gutierrezia californica. Of course, to elevate this suggestion into a key element would require checking many more plants in many locations to see if it applies in general.
Although it does not enter the JM key, there is a much more dramatic difference in the peduncle lengths for the two species. Two-thirds of the peduncles have zero length for Gutierrezia sarothrae, whereas two-thirds of the peduncles are equal to or longer than 1.5 mm for Gutierrezia californica.
The following plot bins the data using bins of 0.5 mm, with the points plotted at the center of each bin. The bin from 0 to 0.5 mm contains points with peduncles of 0 mm but not points with peduncles exactly equal to 0.5 mm, etc.
Again the plot is normalized to a percentage. I measured 19 peduncles at the SRPER and 40 peduncles at SMER.
Clearly the easiest test to separate the species at these two places is simply to check for a high percentage of sessile heads.
These results are largely consistent with the information in the JM, which reports that peduncles are generally larger than 1.5 mm for Gutierrezia californica, and less than 1.5 mm or sometimes sessile for Gutierrezia sarothrae. However, note that 32% of the peduncles in my data for Gutierrezia sarothrae were in fact larger than 1.5 mm.
I measured a few other possibly distinguishing parameters, with the results in the table below:
Parameter Gutierrezia californica Gutierrezia sarothrae # of ray flowers 6-8 5-7 # of disk flowers 3-6 5-7 # series for phyllaries 3 2-3 involucre length (mm) 3.5-4.0 3.0-4.0 involuce width (mm) 1.3-2.0 1.2-2.0
Only the # series for phyllaries gives a clear distinction between the two species; the observed values are exactly as reported in the JM.
The SRPER plants have non-clustered, linear, almost thread-like leaves which look exactly like the ones from SMER. This is inconsistent with the JM description of lance-linear if single, thread-like if clustered for Gutierrezia sarothrae. Thus this is either due to hybridization, or the JM description needs to be enlarged to include linear leaves for Gutierrezia sarothrae.
The SMER plants have phyllaries with obscurely thickened herbaceous tips. This feature is not mentioned by Munz for Gutierrezia californica. Thus this is again either due to hybridization, or, more likely, is simply a characteristic not reported in the Munz description.
The species determined here that is found at SMER, Gutierrezia californica, is the sole Gutierrezia species on Michael Simpson's Vascular Plants of the Santa Margarita Ecological Reserve, and thus these results are consistent with previous work.
However, the species determined here that is found at SRPER, Gutierrezia sarothrae, is not listed in A Flora of the Santa Rosa Plateau by Lathrop and Thorne (1985; Southern California Botanists Special Publication No. 1), which lists only Gutierrezia californica. Thus this species is at minimum a new addition to the SRPER plant list. Further field work is needed to see if any Gutierrezia californica can presently be found at the SRPER. Perhaps my samples this year are just one end of the range of possibly-hybrid plants at the SRPER, and Lathrop and Thorne happened to sample the other extreme of these possibly-hybrid plants.
It should be kept in mind that:
- all the plants observed here are ones that somehow managed to bloom despite the severe drought this year. Thus my observations are of drought-stressed plants which are among the most drought-tolerant individuals in the population. Parameter values may be significantly different for the general population in a plentiful-water year. In addition, if Gutierrezia sarothrae is more drought-resistant than Gutierrezia californica, it is possible that only that species would be found blooming at the SRPER this year.
- all the plants I analyzed may actually be hybrids between the two species, and thus parameter values for the pure species may differ from the results above.
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Copyright © 2002 by Tom Chester
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Last update: 27 August 2002