Monday, 7 May 2012

Hands In The Forest - Dwarf Mistletoe

Sometimes when walking through a hemlock forest you will see peculiar clusters of  branches. On a dark winter day they conjure up visions of the Mirkwood in Lord of the Rings. These branches are short, thick and bunched close together. Old ones that have lost their needles look like hands projecting from the tree trunk. Usually if one tree has these clusters its neighbors will also have them. They look very different from the normal long tapering branches. What you have seen is abnormal growth caused by a small parasitic plant, hemlock dwarf mistletoe (Arceuthobium tsugense). A hemlock that is not infected does not produce these clumpy branches.

Dwarf mistletoe is a parasitic plant that lives only on coniferous trees. It belongs to the same family as the Christmas-time mistletoe. The coastal form is found almost exclusively on hemlocks. You seldom see the actual plant, just the swollen tree branches. The living plant is only seen on branches that are high up on the tree. After a winter storm, however, look around on the ground for any hand-like clusters which still possess needles. The mistletoe grows on these clusters. It is about 7 cm. tall, of an olive-green color, and has no leaves, just the little olivey shoots. Unlike tree branchlets they are soft, and break off easily.

What is the mistletoe doing? It is growing within the living wood of the tree and taking  food made by the tree for the tree, to build its own tissues. It is a flowering plant but its roots do not grow in soil they grow in wood. The little shoots have some chlorophyll, but most of its nutrition comes from the host. If the shoots get broken off it does not hinder the plant. Its roots just continue to grow within the host. These aerial branches are only needed for reproduction. They produce nondescript little green flowers which mature into sticky berries. These berries, however, are not eaten by birds as most other berries are. They are held tightly inside their skins, and the pressure builds up to such an extent that when mature they are shot into the air with a great deal of force. In fact, they are reported to move at 50 kilometres per hour! If they strike the bark of another hemlock tree they will cement themselves to it,  germinate and send a root into the tree. If they land on the needles the sticky surface can slide down until it contacts the bark. 

Are these peculiar plants just strange oddities, or do they have important ecological and economic effects on the forest? The answer is yes, to the second part of the question. Let’s look at their ecological role first. There are no simple answers to most questions, and if we look at mistletoe in a general way, and ask is it good or bad, we get two conflicting answers. It is a parasite, and is bad for the tree it infects, but it is good for the ecology of a hemlock forest. This parasite steals water and nutrients from the tree and weakens it. It also causes it to grow in a contorted manner, reducing its ability to gather sunlight or transport water and food. The swellings also develop cracks and crevices where disease causing fungi can invade. If the mistletoe becomes established on the trunk of the tree this trunk becomes swollen and weakened, adversely affecting not just individual branches but the whole tree. In a windstorm the weakened trunk is more susceptible to breaking.

Considering the above paragraph, how can mistletoe be healthy for the forest? That revolves around the nature of hemlock trees. Western hemlock has developed ways to maximize its success in competition with other trees and plants. It poisons and shades out its competitors. Noon on a rainy winter day under a hemlock canopy is more like twilight. Densely packed, dark green needles capture most of the light, with very little reaching the forest floor. Only the most shade tolerant plants can survive in such places. Dead hemlock needles are very acidic and low in nutrients. When they fall to the ground they form a duff layer which is also inimical to plant growth. This is where the mistletoe comes in. When trees weakened by mistletoe and fungal infection die, or blow over, gaps are created in the otherwise continuous forest. Sunlight now reaches the forest floor where shrubs such as salmonberry and elderberry can grow, along with a number of wildflowers. These in turn attract various songbirds and insects, thus increasing biodiversity. Mistletoe and hemlock have developed together over thousands of year to such an extent that mistletoe is an integral part of the western hemlock ecosystem. 

Although mistletoe is so important ecologically, economically it creates a considerable problem for British Columbia’s forest industry, weakening and distorting the wood on many hemlock trunks. Hemlock is now an important timber tree, and mistletoe causes more financial losses for hemlock products than any other single cause. A century ago this was not an issue, as hemlock was considered a weed tree. The industry was only interested in Douglas fir at that time. If something removed hemlock forests that was considered a good thing. One of the things that removes hemlocks is fire. Mistletoe infected branches break easily and dry branches on the forest floor increase the probability of fire. Fire acts as a limiting factor for mistletoe because it destroys the host tree. Fire, however, favors Douglas fir which needs sunlight and bare soil to germinate. Ironically fire suppression has probably increased the frequency of mistletoe infection. Western hemlock has gone from weed to forest product. Other forest inhabitants, such as salal and chanterelle mushrooms, have become forest products in recent years. As we do not know what other forest components will become valuable in future years there is an economic incentive for preserving as much biodiversity as possible. 

A small parasitic plant that most hikers never see, which is common in our forests, and controls the structure of the entire forest ecosystem. Is this not another secret of the coastal rainforest? Although you will probably not find the plant itself, you can readily see evidence of mistletoe’s presence. The hemlock forest would not be the same without it.  

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