Plants of Southern California: Baccharis emoryi / B. pilularis at Upper Newport Bay
Table of Contents
Abstract
I. Introduction
II. Fieldwork
III. Analysis
Some specimens of Baccharis emoryi and B. pilularis are difficult to identify readily in the field. We measured eight specimens at the Upper Bay of Newport Beach, California, on 2 December 2006 in order to see how reliably such specimens could be identified, whether there was any evidence of hybridization, and whether our "at-a-glance" field determinations were accurate. We find that only the pistillate involucre length and pappus length, both measured in full fruit, are reliable in identifications. Most of our field determinations were in fact confirmed.
Baccharis emoryi and B. pilularis are usually easy to distinguish in the field. B. emoryi is a taller plant, with more upright branches (especially noticeable in the inflorescence) and larger leaves. In fruit, female plants of B. emoryi generally produce stunning displays of white from its longer pappus, compared to generally less stunning displays from B. pilularis. B. emoryi is confined to drainages and moist areas, whereas B. pilularis is found in those habitats as well as drier ones.
However, some plants are problematic, since there is considerable overlap in habit, as well as in length of pappus in fruit. For example, there is one B. pilularis specimen growing next to a drainage at the Santa Rosa Plateau that seems a clear B. emoryi from its large leaves and many upright tall branches. Yet it very clearly proved to be B. pilularis from measurements in full fruit in two successive years. (The first author checked its fruit in two successive years since he wanted to double-check the determination from fruit from a single year!)
The Back Bay of Newport Beach is a moist environment that has many specimens of Baccharis emoryi. However, some specimens look a lot like B. pilularis, and others seem hard to determine just by looking at them. Hence on 2 December 2006 we measured eight specimens to see how well they separated.
The bract shape only works well to separate the species if the bracts are clearly obovate (a value of 2 in the above plot), which is in the sense of the Jepson Manual key. However, note that one specimen of B. pilularis had linear bracts, indicating that its branch of the key needs the word gen as well.
All other measured characteristics only weakly separate the species, if at all, as seen in the following plots and in a principal component analysis (PCA). We performed PCA on all measured characteristics and on subsets of the characteristics. All sets showed clean separation between the species, but nearly all the separation comes from just three characteristics, the bract shape and the involucre and pappus lengths:
A plot that includes the ratio of bract to leaf sizes in the PCA looks identical to the above plot. Plots that include all measured quantities show greater numeric separation of the species, but also have greater scatter since the analysis is dominated by the variance in the leaf and bract lengths (see below).
It is unlikely there is any significance to the apparent curvature in the points for each species, which is produced for each species by a single outlying specimen. The first plot above shows that it is likely that the curve would become a filled rectangle if more specimens were sampled.
The following plots have all eight specimens shown; the previous plots excluded specimen 2 MSW due to its lack of a involucre length measurement.
B. emoryi has bigger leaf and bract sizes on average, but there is much overlap between the species:
Note in particular the one B. pilularis with a bract size of 23 x 10 mm (2 MNE in Table 1). In the field, we called this a possible B. emoryi, possibly in part due to its large leaves and bracts.
In fact, the Jepson Manual description gives 8-55 mm for the length of the leaves of B. pilularis. Since all our measured leaf lengths from both species are within that range, clearly leaf length cannot be relied upon to make a species determination in general. However, in small areas like here, where the plants are growing together in similar conditions, separation by leaf size may work fairly well.
The Jepson Manual description gives 35-70 mm for the length of the leaves of B. emoryi; that is clearly too small of a range. The largest leaf we could find on specimen 1 above was 28 mm long; it had many shorter leaves.
Taking the ratio of the bract size to the maximum leaf size appears to separate the species better:
Note that B. pilularis specimen 2 MNE now no longer falls within the B. emoryi range; in fact, it is now the farthest point from the B. emoryi range, the isolated point at the upper right!
However, the majority of the specimens are close enough together that this doesn't make a very useful test for an individual plant.
Finally, some specimens of B. emoryi drop some of the leaves by the time they are in fruit, but many plants had all their leaves still present:
The height of the plant is clearly not useful as a discriminant here; the tallest and shortest plant were both B. emoryi.
While writing this up, we realized that our field determinations depended at least in part on how densely spaced the leaves were, and on the color of the leaf. We didn't think of measuring the leaf spacing, but will try to do that in the future. The color of the leaf was not measured since that is most useful only in comparing samples side-by-side; it is nearly impossible to use in the field by beginners.