Parasitic flowering plants

When we think of parasites, we usually think of tapeworms, ticks, fungi, and assorted micro-organisms. But the world of flowering plants provides some interesting examples of a botanical version of parasitism, in which one plant extracts nutrients from another. We have two quite conspicuous and common kinds of flowering-plant parasites: hemlock dwarf mistletoe (Arceuthobium campylopodium) and northern ground cone (with the resounding name of Boschniakia rossica). Both of these species are entirely parasitic (or almost so). They derive all or almost all of their nutrients from their host plants and are unable to survive without a host.

You may have noticed the ‘witches brooms’ of gnarly, tangled branches that are the result of a mistletoe infection. The mistletoe alters the balance of growth hormones in hemlocks (and sometimes other species), causing the ‘brooms’. Male and female mistletoe plants are separate, with very small, inconspicuous flowers. Pollination is mostly by wind. When the seeds mature, they are discharged explosively; they are capable of travelling ten yards or more. Because they are sticky, they adhere to other branches or trunks and start new infections. This species has been reasonably well studied, because heavy infestations decrease the economic value of the timber.

 

The ground cone pokes up above ground in June. All we see is the cone-like inflorescence of tightly packed, brownish flowers; the rest of the plant is underground. Ground cone is commonly parasitic on the roots of alders, and sometimes other species, including blueberries. Various insects may visit the flowers and accomplish pollination; occasionally we have seen bumblebees probing the flowers. The seeds are minute and very numerous. The biology of this species apparently has not been studied in detail, as I can find no scientific papers on the subject, but the medicinal uses of this plant are reported to be many. We do know, from local observations, that bears frequently dig up and eat the underground base of the flowering stalk.

In addition, all orchids depend on their mycorrhizae (fungal associates) at least at some time in their life history. Some researchers consider the relationship to be mutualistic, with both participants gaining some advantages, and that eventually may be the case when the adult orchid develops green leaves and can provide carbohydrates to the fungus in return for soil nutrients. However, other researchers refer to mycorrhizal associations as a parasitism by the orchids on the fungi. This seems especially applicable in the case of orchids that develop little or no green tissue as they mature, so they can contribute nothing to their fungal associates. We have two species of the genus Corallorhiza (coralroot), one of which has no green tissue at all, while the other has a little and so can make some carbohydrates. They are both parasitic on fungi for all of their lifespans, but the relationships are even more complex: they can indirectly parasitize green plants through their fungi, which attach to the roots of trees and shrubs (which have their own mycorrhizae), and draw nutrients from those hosts.

Less well known are the so-called hemi-parasites, half parasitic and half independent. They have chlorophyll and can synthesize carbohydrates, but also commonly latch on to other plants and capture some nutrients from these hosts. We have a number of hemi-parasites among our local flowers.

The plant called yellow rattle or rattle box (Rhinanthus minor) grows in open areas at low elevations. This species is capable of parasitizing the roots of many other herbaceous species, including grasses. A single Rhinanthus plant can attach itself to the roots of several other plants. Some plants, however, are able to resist invasion by the roots of the parasite. Rhinanthus plants clearly benefit from the parasitic connections, growing and reproducing better if they are connected to a host. Some hosts, such as legumes, are better than others in providing the parasite with nutrients. In addition, the parasite can take up defensive chemicals, such as alkaloids, from the host, and thus reduce the risk of herbivory. The effects on the host plants are negative, because many of their nutrients are drained away by the parasite; parasitized hosts have decreased growth and reproduction; the negative impact is greater in some hosts than in others. There are indirect effects of Rhinanthus parasitism too: parasitized hosts have lower ability to compete with other plants in the community, and lower ability to recover from herbivory, which then affects the relative abundance of various species in the entire community.

Photo by Bob Armstrong

There are three species of Indian paintbrush (Castilleja) in Southeast, but only one has been studied much, and those studies were conducted elsewhere. In general, members of this genus grow better, flower more, and resist herbivory better, when they are connected to a host plant. Legumes seem to be better hosts than other plants, and two or more hosts may be better than one. When a parasitic paintbrush plant dies and decays, decomposition is accelerated, and co-occurring species have increased productivity, which may counter the usual negative effects of the parasite to some degree.

The genus Pedicularis—the louseworts–has about five representatives in our area, which have not been studied directly, as far as I can tell. In general, the negative effects of parasitism by different species of lousewort may vary among host species, and different louseworts have different host preferences. Some louseworts are reported to have mycorrhizal associations too, collecting nutrients via the fungal connection in addition to the usual direct parasitism.

Clearly, the world of the parasitic flowering plants is complicated, and it gets more complex as more studies are conducted. All those pretty (and not so pretty) flowers are intricately involved with many ecological interactions, and what we see above-ground is only a small part of the story.

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Brooms, burls, and pinetree galls

On a recent exploration in the muskegs along the Dan Moller trail, two hikers asked about the dark, knobby growths on the branches of the shore pines. The knobs are called galls, although they are not induced by insects or mites; in this case the causative agent is a fungus known as the western gall rust.

The gall rust fungus spreads by means of airborne spores. When a spore lands on a pine needle, it germinates and grows into the twig, where it taps into the tree’s vascular tissue that conducts nutrients and water throughout the tree. The gall rust is a parasite that lives off the tree but seldom kills the branch. It takes over local control of the tree’s growth hormones and causes extra growth near the site of infection. It is apparently fairly long-lived, eventually maturing and making more spores that fly on the breezes to other pines.

A recent survey in Southeast Alaska found that as many as eighty-six percent of shore pines are infected with this gall rust fungus. Some of the galls are as large as a human cranium, but most are smaller. The majority of galls form on branches, but some afflict the main trunk, where they began when the tree was small and had needles close to the trunk. One third of the pines in the survey had gall rust infections on the trunk.

A quick look around a muskeg shows that there are dead needles ‘outboard’ of the gall, and the end of the branch or the top of the tree has died. Very recent studies have found that it is not the gall itself that usually kills the branch. Instead, it appears that branch death is caused by secondary infections of other organisms. One secondary agent is another fungus (a species of Nectria) that invades the gall and ultimately clogs up the vascular system of the branch, killing it. Other secondary agents are insects (probably beetles or moths whose larvae dig tunnels through the gall and interrupt the flow of nutrients and water to the end of the branch). The survey indicated that about twenty percent of branch deaths could be ascribed to Nectria and about twenty percent to insects (the remainder were unknown). I have to wonder if woodpeckers can find the insect larvae inside these galls and dig them out.

Another common tree parasite in Southeast is hemlock dwarf mistletoe. Dwarf mistletoes have co-evolved with their conifer hosts for millions of years and now about thirty species occur in North America. In our rainforest, the only species is the western hemlock dwarf mistletoe, which infects western hemlocks all over Southeast. However, hemlocks north and west of Cross Sound (up the coast toward Anchorage) are not afflicted, perhaps because some climatic factor limits the distribution of this parasite.

Hemlock dwarf mistletoe commonly causes branch distortions that are called witches’ brooms. The mistletoe has altered the growth-regulating hormones of the tree, producing a dense proliferation of twigs. Sometimes the mistletoe infection starts on the hemlock trunk, where it can eventually create a large swelling called a burl. Mature infections often send up aerial shoots, especially in the upper part of the tree crown. The shoots are typically yellowish-green and leafless, capable of synthesizing little carbohydrate on their own, so the parasite depends on its host plant for food and water.

Hemlock tissues infected by hemlock dwarf mistletoe may be attacked by fungi or insects that can kill the parasite. Surviving mistletoe infections often live for decades and can alter the water needs and shade tolerance of the host tree, as well as reducing growth. Severe infections may kill the tree.

Hemlock dwarf mistletoe is a flowering plant. Male and female flowers are borne on separate plants. The flowers contain nectar and are presumably pollinated by insects. After pollination, seed development takes more than a year. Each seed is enclosed in a fruit that explosively discharges the mature seed. The discharged seed is coated with very sticky material that gums down the seed wherever it lands. If it lands on hemlock bark, on the same or a different tree, it can germinate. Most sticky seeds are flung only for short distances, but some adhere to birds or squirrels and get carried longer distances. The germinating seed sends root-like structure into the hemlock and captures nutrients from the tree’s vascular system. After establishing itself, the new mistletoe may send up its own aerial shoots and eventually flower.

The bark around hemlock dwarf mistletoe infections is sometimes eaten by squirrels. The ‘brooms’ provide nesting platforms for marbled murrelets and other birds, and songbirds often feed on invertebrates that hide in the brooms.

What about the huge burls we sometimes see on sitka spruce trees? Wood-turners love them for the intricate figures exposed when the wood is carved. But what causes those exuberant growths? I was astonished to learn that apparently no one knows!

Many thanks to helpful local plant pathologists, who provided information and photos!