At My Signal, Unleash Hell: How Invasive Annual Grasses Use Pestilence Against Their Native Perennial Rivals

Taeniatherum caput-medusae (Medusahead Grass) Inflorescence

In previous posts, I touched upon some of the ways invasive grasses manage to overwhelm native populations. 

Usually, this involves having some direct competitive advantage over their rivals. This may include pathogen release (they leave behind all the pathogens that afflict them back in their own native land), some intrinsic competitive abilities, or through some life cycle advantages (for example, as in the diagram below, the ability of Taeniatherum caput-medusae or Medusahead Grass to create flammable litter that prevents competitor germination and clears the rest via frequent wildfires).

How T. caput-medusae (Medusahead Grass) dominates its competitors

Amazingly, however, a new way that invasive annual grasses manage to dominate their perennial native rivals has been discovered. In California, more than 9 million ha of land have been invaded by European winter annual grasses like Avena fatua (Wild Oats), which have usurped the native perennial bunchgrasses. Vast fields of these invasives have paradoxically enough, created the so-called "Golden Hills of California", which have become iconic to that region.

Golden Hills of California

The ability of these invaders to dominate the landscape was thought to be through direct competition with native perennial bunchgrasses, such as Nassella (Stipa) pulchra and Elymus glaucus. But some studies showed that the exotics were in fact poor direct competitors, and other factors were needed to explain their dominance. 

Avena fatua, an exotic and invasive annual grass in California

One of these factors turned out to be a virus!

The barley and cereal yellow dwarf viruses (B/CYDVs) are a family of viruses that are generalized pathogens of grasses, and can cause stunting and slow growth. They travel from host to host via intermediate vectors, which are various species of aphids. The aphids take in the virus from an infected plant when they suck on the phloem, and then transfer it to another plant when they move to new pastures.

Aphids on spikelets of T. caput-medusae (Medusahead Grass)

The invasive grasses compete against their perennial grass rivals by amplifying the population of the aphids, and this in turn significantly increases the pool of infectious viruses. Researchers found that aphids not only were much more attracted to the exotic annual grasses, but that their fecundity increased significantly in the presence of these species. Aphid densities were up to 800 times greater in areas with dense stands of the invasive annual A. fatua, and the infection rate of native grasses with B/CYDV more than doubled! The negative effect of this pathogen on perennial bunchgrasses is more severe than on the short-lived annuals, and this difference is enough to tip the scales of the competition.

By unleashing the viral pathogens against their more vulnerable perennial rivals, A. fatua and other exotic annual grasses have managed to dominate the California landscape. This unusual type of competition (called "apparent competition"), seems to also be available to other exotic annual grasses such as T. caput-medusae, which typically dominate their own areas through more direct methods.

Golden Medusahead Grass carpeting the ground in Oregon

References:

Borer ET, Adams VT, Engler GA, Adams AL, Schumann CB, Seabloom EW. Aphid fecundity and grassland invasion: invader life history is the key. Ecol Appl. 2009 Jul;19(5):1187-96. doi: 10.1890/08-1205.1. PMID: 19688926.

Malmstrom, C.M., McCullough, A.J., Johnson, H.A. et al. Invasive annual grasses indirectly increase virus incidence in California native perennial bunchgrasses. Oecologia 145, 153–164 (2005). https://doi.org/10.1007/s00442-005-0099-z

The Tree and the Reed: How Wind Affects Grasses

Video above shows Phalaris arundinacea (Reed Canary Grass) being buffeted by wind.

There are many versions of Aesop Fable's The Tree and The Reed, a tale which first appeared in Ancient Greece, and whose moral about pride and humility still resonates to this day. One of the versions goes thus:

A Giant Oak stood near a brook in which grew some slender Reeds. When the wind blew, the great Oak stood proudly upright with its hundred arms uplifted to the sky. But the Reeds bowed low in the wind and sang a sad and mournful song.

“You have reason to complain,” said the Oak. “The slightest breeze that ruffles the surface of the water makes you bow your heads, while I, the mighty Oak, stand upright and firm before the howling tempest.”

“Do not worry about us,” replied the Reeds. “The winds do not harm us. We bow before them and so we do not break. You, in all your pride and strength, have so far resisted their blows. But the end is coming.”

As the Reeds spoke a great hurricane rushed out of the north. The Oak stood proudly and fought against the storm, while the yielding Reeds bowed low. The wind redoubled in fury, and all at once the great tree fell, torn up by the roots, and lay among the pitying Reeds.

The reeds mentioned in these tales were probably a hodge-podge of riverside-living graminoids. The original and most likely species that answers to this name is Phragmites australis (Common Reed), but the term may also refer to other grasses, such as Phalaris arundinacea (Reed Canary Grass), Arundo donax, Neyraudia reynaudiana (Burma Reed), various species of Calamagrostis, as well as a few other assorted members of the Poales. All these plants are herbaceous, with thin leaves that seem to support the central idea of the fable about wind resistance, but the reality is actually more complex. 

It is true that grasses are able to withstand gusts that would topple large trees, but they too can be negatively impacted by continuous strong winds, either due to shaking (seismorphogenic) or rubbing (thigmorphogenic) processes.

Some of the effects of wind include:

• Increased leaf transpiration, which means grasses lose moisture faster through their stomata. 
• Decreased leaf extension, which means the grasses produce less leaves.
• Slower growth rate.
• Damage to leaf surfaces, including displacement and smoothing of the epicuticular waxes, damage to the cuticle, collapse of epidermal cells and fracture of trichomes.

These effects increase with increasing wind speed, and older leaves are disproportionately affected.

In addition, studies of wind effects on wheat showed that there was significant reductions in grain yield when the grasses were exposed to strong continuous wind. There were less heads per sqm, less kernels per head, and each kernel weighed less than normal. All these negative effects show that Aesop's Fables was not exactly right when it marveled at the invincibility of reeds compared to the more arrogant trees during a storm. 

As an aside, I love seeing grasses swaying and dancing in the wind, especially when they occur in large masses. This is why I decorate my home with lots of ornamental grasses; their movement gives a very attractive active component to the home compared to static bushes and trees. But now that I know the effect of wind on these plants, I'm more careful about wishing for the stronger and damaging winds.

References

Pitcairn, C.E.R., C.E. Jeffree, and J. Grace. 1986. Influence of polishing and abrasion on diffusive conductance of leaf surface of Festuca arundinaceae Schreb. Plant, Cell and Environment 9:191-196.

Russell, G., and J. Grace. 1978. The effect of wind on grasses. V. Leaf extension, diffusive conductance, and photosynthesis in the wind tunnel. Journal of Experimental Botany 29:1249-1258.

Smika, D.E., and R.W. Shawcroft. 1980. Preliminary study using a wind tunnel to determine the effect of hot wind on a wheat crop. Field Crops Research 3:129-135.

Goodbye to an Old Forest Friend

I have mentioned how we like hiking, and more times than not we tend to hike in heavily shaded areas like closed canopy forests. This is mostly because my wife is not usually a fan of walking under the full sun. 

In such places, grasses are not the major component of the understory. They tend to occur in small patches or even in single tufts in the dimly lit ground. Where they tend to occur in masses, the species are usually shade-tolerant aggressive invaders like Microstegium vimineum (Japanese stiltgrass) or Oplismenus undulatifolius (Wavyleaf basketgrass).

But one of the more numerous of the grasses that thrive in the shade is one that I personally find attractive. Dichanthelium clandestinum and its ilk have broad dark green leaves and a habit that you can't help but love.

Beautiful broad and dark green leaves

I encounter it relatively frequently here in the Northeast, and I have stumbled upon it once or twice in Florida.

I also see specimens of this grass close to my home in New Jersey whenever I go for my afternoon walk. They sit alongside the paved path around a nearby lake, and I make it a point to always stop and marvel at the beautiful forms that grace the lakeside and the nearby wooded area.

These are such distinctive critters, and interestingly enough, I see them not only in deep shade, but in brightly-lit areas as well, a testament to the adaptability of this grass.

Spikelets and florets in June

This species also has a somewhat unusual flowering scheme, as I mentioned awhile back in another post. During early summer it produces "normal" flowers which are pollinated when open ("chasmogamous"). Then later in the season, the same individual has closed flowers hidden in the sheaths that will be self-pollinated ("cleistogamous").

Spikelets and florets (macro view) in June

I think this will be my last season in this home, so it saddens me that this will also be the last time I see the D. clandestinum that make their living around the lake. They have been my yearly companions in my jaunts under the shade, and I wish them well. Perhaps someday when I come to visit, I will again walk around the lake path, and say hello to these old friends.