Nassella (Stipa) pulchra vs cernua
Can Nassella (Stipa) pulchra and cernua be distinguished macroscopically? - a working post that I'm updating as I learn more and as folks comment on the post. Thanks @aparrot1 , @hkibak, @yerbasanta, and @keirmorse for comments / contributions that have improved the post substantially.
(Note: @keirmorse provided a new key in a comment on this post (see below) on 11/15/23. I haven't had a chance yet to update the body of the post with information from the new key.)
The context here is centered on Fort Ord where pulchra and cernua range widely in a largely overlapping way (Styer 2019), and where one might want to learn more about the how the two species differ in range and habitat without having to pull out the macro lens for every single plant. To do this, one must first know how to identify them (e.g. with a macro lens, hand lens, or dissecting scope) and then extrapolate from that to a recognition of naked-eye characters that might correlate.
There is also some important nomenclatural context. The taxa have changed names several times. The story is quite complicated (Barkworth 1990, Columbus & Smith 2010), but the main thing to know is that Nassella pulchra and Stipa pulchra are synonymous, and Nasella cernua and Stipa cernua are synonymous. The synonymy arises from the history where Nassella was originally a subgenus of Stipa in 1830, then it's own genus in 1853, then a subgenus again in 1901, then a genus again in 1942 and 1947, then deleted (?) in the peer-reviewed thread in 2010. And regardless of the level of Nasella, species pulchra, while having been first described as Stipa pulchra in 1915 was not placed under any version of Nasella until Barkworth put it there in 1990. Species cernua was was first described as Stipa cernua in 1941 (Stebbins & Love), and then also moved by Barkworth in 1990 to become Nassella cernua. The genus Stipa was for a time abandoned in 1993 by Buckworth, all of it's contents having been transferred to other genera in Tribe Stipeae, and then resurrected in 2010 by Columbus & Smith based on genetic work e.g. by Jacobs et al. (2007). Genus Stipa stands today (including pulchra and cernua) in the Jepson Manual (2nd ed., 2012). iNaturalist goes with Nassella pulchra & cernua, not Stipa, being based on Plants of the World Online, which doesn't yet (2022) correspond to Jepson 2nd ed in several areas. If you think I've gotten any of this wrong, let me know.
It's all about the lemmas. The characteristic that pulled all the species into Nassella is the way the lemma wraps completely around, overlapping itself and completely concealing the palea etc. And, it is a lemma characteristic - hairiness - that also separates pulchra and cernua in the latest Jepson Manual treatment.
The story of how cernua differs from pulchra appears to begin in 1941 with a paper by Stebbins & Love. They note:
"Several people, including the present writers, have noticed that this species as recognized in the current manuals actually consists of two distinct types. One, with deep green foliage, relatively broad leaves, stiffer panicle branches, large glumes, thick, fusiform lemmas, and stout, stiff awns, is predominant in the outer Coast Ranges and the wooded parts of the Sierra Nevada foothills. This is typical S . pulchra, of which the type came from Healdsburg, Sonoma County. The other form, with somewhat glaucous foliage, narrower leaves, flexuous, often nodding panicle branches, smaller, narrower glumes, slender lemmas, and slender, often flexuous awns, occurs chiefly in the treeless parts of the inner Coast Ranges, the San Joaquin Valley (in scattered areas undisturbed by cultivation), the valleys of southern California, and the edges of the deserts. The two types have been given different common names, typical S . pulchra being known as purple needle grass, and the slender, interior type as nodding needle grass."
This is wonderful to come across, because it distinguishes the two taxa entirely on naked-eye characters. Obviously, we need the hand-lens characters to be sure, but we can get a pretty good clue from the above before deciding whether to pull out the hand lens.
Stebbins & Love also provide a map of the relative abundance of the two taxa at about 60 locations throughout California. The Monterey area has both, and it's clear from the map that there's potential that within the Monterey area, one would find pulchra in more coastal, mesic, and higher-elevation situations and cernua in more inland, xeric, and lower elevation situations. Interestingly it's hard to discern this from Styer's (2019) detailed maps of the distributions within Fort Ord, perhaps because Fort Ord does not exhibit much variation in the above characters.
The new taxon Stipa cernua started showing up in floras and other broader treatments in a paper on Stipa by Dedecca (1954), then Munz & Keck's state-wide flora (1959), followed in county-wide works, for example, by Thomas (1961) in Santa Cruz county and Hoover (1970) in San Luis Obispo county. It shows up as Nassella cernua in Barkworth's treatment in the Jepson Manual (1993) and in Matthews (1997) for Monterey County. But then it flips back to Stipa cernua in Columbus et al.'s treatment in the 2nd edition of the Jepson Manual (2012, print version, and online version as at May 2022) following a paper by Columbus & Smith (2010) based on genetic work by Jacobs et al. (2007). Note that species cernua didn't appear in the 1951 state-wide Jepson publication, despite this being after Stebbins & Love (1941), presumably because the 1951 Jepson Manual was actually a posthumous re-publication of Jepson's 1923 and 1925 works.
Regardless of the name changes at the genus level, pulchra and cernua have stood since 1941 as two distinct species. But, the characters used to discriminate them in keys have varied widely. For example:
- Stebbins & Love (1941) uses leaf width, lemma shape, caryopsis length, awn shape, awn:lemma ratio.
- Munz & Keck (1959) uses lemma shape, leaf width, leaf color, awn shape, awn:lemma ratio
- Thomson (1961) uses awn:lemma ratio, leaf color
- Hoover (1970) uses awn length
- Barkworth (1993) in Jepson Manual uses awn shape
- Matthews (1997) uses awn length, culm length, glume length, lemma length
- Columbus et al. (2012) in Jepson 2nd ed. uses lemma hairiness
- Matthews & Mitchell (2015) uses lemma hairiness, awn length, culm length, glume relative lengths
One might seek out a definitive differentiation in the current Jepson Manual treatment by Columbus et al. (2012). Here the key (for genus Stipa) separates pulchra and cernua solely on lemma hairs - pulchra being hairier.
Couplet 8:
- 8. Lemma body in age glabrous in distal 3/4 except on veins ..... S. cernua
- 8' Lemma body in age hairy throughout ..... S. pulchra
https://ucjeps.berkeley.edu/eflora/eflora_keys.php?key=11182
The species descriptions by Columbus et al. reveal more distinctions. About 20 characters are listed in the descriptions for both taxa. Some are useful to the question of separating pulchra and cernua. Pulchra has wider flatter leaf blades, and pulchra has hairier leaves and lemmas. Other characters are a bit of a wash, such as stem height, blade length, and inflorescence length.
Notably, several distinguishing characters used in previous floras are absent from Columbus et al.'s treatment. And yet, I suspect that several of them remain helpful.
So, in the quest for clues as to how one might instantly distinguish the two taxa in the field, perhaps it is helpful to step back and look in turn at all the characters that have ever been used to differentiate them. I'll attempt this here, starting with "macroscopic" characters that you might recognize without even breaking step on a hike, and then working down through "mesoscopic" characters that you might recognize with a floret in hand, ultimately to "microscopic" characters for which a at least a hand lens might be needed.
Macroscopic characters
Leaf color
Pulchra foliage usually deep green. Cernua usually glaucous. Stebbins & Love (1941), Munz & Keck (1959), Thomson (1961).
Leaf width
Unrolled pulchra leaves are wider, with little to no overlap with cernua. Pulchra culm leaves 2.4-6 mm. Cernua 1.2-2.4 mm. Stebbins & Love (1941), Munz & Keck (1959), Thomson (1961).
Culm length
Culm length overlaps substantially between the two species. Pulchra 6-10 dm. Cernua 6-9 mm. Munz & Keck (1959). Differing among authors e.g. Matthews (1997), Matthews & Mitchell (2015).
Panicle shape and flexion
Pulchra "spreading, or slightly cernuous". Cernua "slender, flexuous or cernuous". Stebbins & Love (1941).
Glume color (purple or not)
Purpleness of glumes does not seem to be clearly an indicative character. The word "purple" appears in the common name of pulchra, and in casual reports mentioned by Stebbins & Love. But I haven't yet found it mentioned as a clearly differentiating character in any keys or floras (yet), nor have I yet observed it as such in the field. Stebbins & Love's description of cernua refers to the glumes as "pale reddish or purple". Matthews (1st and 2nd ed.) list purple color as a character for pulchra but not an absent character for cernua. Columbus et al. (2012) online key does not mention color in relation to these two taxa. The species descriptions also do not mention purple color, other than in the common name for pulchra. One can find purple glumes in the non-"purple" taxon. For example, here's a Fort Ord cernua with purple glumes: https://www.inaturalist.org/observations/117652776
When it has been mentioned, purpleness only seems to have ever been mentioned in relation to glumes (see references above), which are what you see when you look at an inflorescence as a whole. In contrast, color differences have never been mentioned to my knowledge in relation to lemmas, for example; lemmas being what you see when you pull a floret out of a spikelet, leaving the glumes still attached to the rest of the plant.
Also, presumably, any colors of glumes, lemmas, etc. vary with season - generally becoming more pale with age.
Inflorescence nodding
"Cernua" means "nodding", hence the common name for cernua "Nodding needlegrass". But interestingly, Munz & Keck attribute the "nodding" character to pulchra and not cernua, and Stebbins & Love (1941) attribute some degree of cernuosity to both species.
Location, geomorphic setting, soil type, and associated plant species
In some parts of the state, location should give a strong clue as to which Nassella/Stipa species you're seeing. But in the Monterey area, location within this area seems to be a weak indicator, as evidenced by the somewhat overlapping ranges mapped by Styer (2019), whose 70 mapping units within the former Fort Ord average 400 acres in size. In areas smaller than this where both species occur in a broadly overlapping way, one ought to be able to learn the ways in which geomorphic setting and soil type act to determine which of the Nassella/Stipa species would flourish. And perhaps a similarly determined correlation with other plant species might be learned as a clue to which Nasella/Stipa species to expect. In the Monterey Area, this might align generally with the differentiation between Northern Coastal Grassland and Valley Grassland (sensu Holland & Keil 1995), for example. While broad gradients obviously extend across the Monterey area, much of the pertinent differentiation could be described as a "complex" or "mosaic" rather than a broad contiguous dichotomous boundary.
Mesoscopic characters
Lemma width or shape
When you pluck and examine a Nasella floret and ignore the awn, the lemma is most of what you are seeing. In this genus, the lemma is rolled around on itself completely with opposite margins overlapping. The lemma is like a tube with a bulge. At it's proximal end, there's a "callus", and at its distal end, there's a "crown", followed by the awn. Inside, the palea is concealed, along with the ovary or caryopsis, depending on maturity.
Variation in lemma thickness is one of the first things I noticed when I looked at my first 20 or so macro images of Nassella florets (https://www.inaturalist.org/observations?place_id=any&taxon_id=52806&user_id=fredwatson&verifiable=any) and this variation is also one of the obvious differences evident in this wonderful photo by Keir Morse: https://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0520+0704.
Pulchra lemmas are "thick" and "fusiform". Cernua's are "slender" and "cylindrical". Stebbins & Love (1941), Munz & Keck (1959)
Interestingly, lemma width or shape is not mentioned in the latest Jepson treatment (Columbus et al. 2012).
Awn shape and flexion
Pulchra awn "stout, stiff". Cernua "slender, flexuous beyond the second bend". Stebbins & Love (1941), Munz & Keck (1959), Barkworth (1993).
Awn:lemma ratio
Cernua has longer awns relative to the lemma. Slightly different ratios are reported by different authors:
- Pulchra awn or 5.9-9.8 x lemma. Cernua 8.7-14.5 x. Stebbins & Love (1941).
- Pulchra awn or 7-9 x lemma. Cernua 9-12 x. Munz & Keck (1959), Thomson (1961).
Awn length
Cernua has longer awns in absolute measurement, although there is substantial overlap between the two species. Pulchra 60-90 mm. Cernua 60-100 mm (Stebbins & Love, 1941). See also: Hoover (1970), Matthews (1997), Matthews & Mitchell (2015).
Glume length
Cernua has longer glumes, although there is substantial overlap between the two species. Pulchra lower glume 15-26 mm. Cernua 12-19 mm (Stebbins & Love, 1941). See also: Munz & Keck (1959), Matthews (1997)
Glume relative lengths
Cernua glumes differ more between lower and upper glumes (first and second glumes). I think Stebbins & Love (1941) might have missed this character, but it was picked up by later authors. Pulchra second glume "slightly shorter" than first. Cernua "shorter". Munz & Keck (1959), Matthews & Mitchell (2015).
Microscopic characters
Upper glume venation
Pulchra 3-5. Cernua 3. Stebbins & Love (1941).
Lemma venation
Pulchra 5-9. Cernua 5-7. Stebbins & Love (1941).
Lemma length
Pulchra lemmas are generally longer, but there's substantial overlap between the species. Munz & Keck (1959) state pulchra lemma length 7.5-13 mm, and cernua 5-10 mm. Other authors use slightly different values with the same general theme (Matthews 1997, Columbus et al. 2012).
Columbus et al. (2012) also measure the callus and crown lengths in fractions of a millimeter - with pulchra having a generally longer callus (proximal end of the floret), and longer (but less distinct) crown (distal end of the floret, before the awns).
Lemma hairiness
Pulchra lemma more hairy, described in various ways by different authors. Munz & Keck (1959), Columbus et al. (2012), Matthews & Mitchell (2015).
Lemma papillosity
Cernua lemma papillose. Pulchra not indicated, other than by implication through hairiness. Hairiness and papillosity are somewhat contrasting characters, as far as I can discern from plants in general. Munz & Keck (1959)
Caryopsis thickness at maturity
Pulchra 1-1.4 mm. Cernua 0.6-1 mm. Stebbins & Love (1941).
Epilogue
One thing's for sure, both taxa are deserving of the epithet "pulchra". They are both pulchritudinous - i.e. "beautiful" - especially in May.
It would be great to continue hearing folks' thoughts on all this. My intent would be to try to incorporate information from comments into the main text of the post.
