Monday, February 23

Parasitic Plants, Part One: Little Foot and the Bearberry

What if you could function without providing for yourself? What if, instead of going to work and toiling at your job to make ends meet, you simply took your necessary resources from someone else and were able to live a fulfilling and successful life, reproducing and passing on your genes to another generation? This life strategy, when adopted by Homo sapiens, is at least frowned upon and at most can result in incarceration. But there are entire families of plants that make their living doing little or no work on their own. We affectionately call them parasites, and some of them are quite lovely. ­­­

According to my dog-eared 1971 copy of Dictionary of Biology, parasitism is “a form of symbiosis in which two organisms live in close association with each other, the one, a parasite, depending upon the other, the host, for some essential food factor.” Though quaintly simple and written with too many commas, this conveys a basic concept – one individual is taking resources from another. And on the sun-dappled slopes of Sobrante Ridge, our parasitism parody is playing out. Two native western plants are struggling for survival. One of them is struggling decidedly less than the other. 

Entering the Manzanita Loop Trail

The pallid manzanita (Arctostaphylos pallida) is certainly the star of Sobrante Ridge. Clinging to life on the rocky slopes, this species is making its last stand in a few scattered locations around the East Bay. Originally described by Canadian-American botanist Alice Eastwood, pallid manzanita was probably never very common. It has exacting habitat requirements – siliceous, nutrient-poor soils above 200 meters, and (unlike its chaparral brethren) within the reach of coastal fog. It is currently known from only two major stands in Alameda and Contra Costa Counties; consequently it is federally listed as “threatened” and California state-listed as “endangered.” 

Arctostaphylos pallida in bloom, dangling urn-shaped flowers that will mature into "bear-berries."

The main threat to its existence is decades of fire suppression – without fire, this particular manzanita species has a hard time reproducing. Unlike its burl-forming relatives, A. pallida cannot propagate itself through sprouting. This leaves only the traditional method of seeding for regeneration, and pallid manzanita likes it hot. Fire-scorched hot, in fact – the seeds require fire scarification to germinate, and prefer to sprout in fire-sterilized soils, free from competing plants, allelopathic leaf litter from their parents, and potential fungal infections. It is estimated that without a regular fire regime to control undergrowth and promote seedlings, A. pallida will be outcompeted by native understory plants after approximately fifty years – and the population at Sobrante Ridge has gone over 100 years without a fire. Yet to stroll the shady paths of the Manzanita Loop Trail, you would think this plant is doing well – over 2,000 individuals were once counted here.

Pedicularis densiflora, trailside

Accompanying the maroon trunks on this breezy February afternoon are an abundance of crimson flower spikes, originating from a basal rosette of fern-like leaves. Indian warrior (Pedicularis densiflora) is an early bloomer, and is particularly prolific this year at Sobrante Ridge. But digging in to this plant would literally expose something odd – roots that are connected to those of the nearby manzanitas. Barring excavation of endangered plant habitat (never a great idea), we can glean these facts simply by knowing the flower’s family – it is an Orobanchaceae, and comes from a long line of thieves. 

The “Orobanchs” are legendary for their parasitic lifestyles – of the more than 2,000 species in the family, only 18 entirely provide for themselves. The rest rely on other plants for either some or all of their water and nutritional needs, growing a special type of root called a haustoria to tap into their host plant’s vascular system. And our little-footed friend P. densiflora is no expection. But why, if the plant relies on another for nutrition, the green leaves? 

P. densiflora, growing under A. pallida
Plants in the genus Pedicularis are hemiparasites, meaning they aren’t complete freeloaders. They possess chlorophyll and are capable of photosynthesis, and will grow on their own if they can’t find a nearby host (known as facultative parasitism). But for the plants at Sobrante Ridge, a host isn’t far away – they find supplemental nutrients from the ancient pallid manzanitas under which they grow. Pushing their haustoria into the manzanita roots, they form a xylem bridge – a direct connection to the host’s vascular system. Then, boosted by the additional sugars produced by the host’s canopy, P. densiflora can burst forth with massive spring blooms unequaled by their gaunt, non-parasitic neighbors.   

Pedicularis densiflora flower spike

If there is an upshot to this ghastly tale of carbohydrate thievery, it is that the Indian warriors aren’t killing the pallid manzanita. That strategy, known as parasitoidy, would not benefit future generations of Pedicularis, for they would have to spread and find new hosts on which to grow. That’s a lot of work for a perennial plant that could instead come back in the same place, with the same host, year after year. But for the tired old manzanitas, Pedicularis' parasitism is one more tax to pay on their ever-increasing cost of living.

If you make it to Sobrante Ridge in the next week or so, and admire those abundant stands of crimson flowers, know that something else is paying the price for that splendor. 

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