This is my first post written from Abilene, TX, in my humble abode just about a block from Hardin-Simmons University (HSU). I am officially a new Rolling Plains resident. I’ve just finished my third week as a new professor of biology at HSU. It’s been hectic but enjoyable, and along with my wife having a scheduled major surgery last week (my family still lives 5 hours from Abilene), I’ve been been quite busy to say the least. My wife is doing great and along with school duties starting to approach a manageable routine I am hoping to devote more time to the Rolling Plains flora and this blog.
Now on to a Rolling Plains resident of the botanical sort. During my daily walk into the HSU campus I cross a street that borders the north side of campus. The north side of the street is bordered with a line of trees in deciduous, leaf-off mode at present. I’m pretty sure these are pecan-Carya illinoisensis-trees. But what really drew my attention to them was the numerous and noticeable “shrubs” that were suspended among the leafless branches of the pecan trees.
Most folks probably know that these aerial shrubs are none other than mistletoe or Phoradendron sp. (this is probably Phoradendron tomentosum), which is a hemiparasite. Hemiparasites like mistletoe live on their host for physical support but also insert what is essentially a root or haustorium into the stem tissue of their host. Being a hemiparasite, as opposed to a strict parasite, means that mistletoes do make at least some of their own food via photosynthesis and extract water and mineral nutrients from their host. Most photosynthetic plants are green because they contain the green photosynthetic pigment called chlorophyll. The leaves of mistletoe are definitely green in color. Here’s a closeup shot of one of the mistletoes:
In the above image note the green leaves and white fruit. At this time of winter mistletoe can be seen all over town here in Abilene. From an ecological sense, hemiparasites like mistletoe do take a toll on their host since they are reducing the host’s water and nutrient availability. If the mistletoe infestation is heavy enough the result could certainly be fatal for the host species. And aside from ecological and biological implications for the host many residents proabably consider them to be an eyesore that detracts from the “natural” habitat. That being said, I was surprised (after doing a little research) that mistletoe is generally considered to be of positive ecological importance.
Recent research has shown mistletoe to be a keystone species. A keystone species is a species that has a disproportionate effect on its environment relative to its abundance (see “Keystone Species” in Wikipedia; or some readable, technical references upon request). Many animals depend on mistletoe for food, eating the leaves and young shoots and both help transfer pollen between plants and disperse mistletoe’s sticky seeds. Maybe even more relevant for us here in the Rolling Plains is mistletoe’s interactions with juniper shrubs and trees that are prevalent just to our south.
Quoting from the Wikipedia entry on mistletoe:
A study of mistletoe in junipers concluded that more juniper berries sprout in stands where mistletoe is present, as the mistletoe attracts berry-eating birds which also eat juniper berries; juniper berries that have passed through a bird’s digestive tract are less likely to remain imprisoned within a hardened outer capsule.[see reference below] Such interactions lead to dramatic influences on diversity, as areas with greater mistletoe densities support higher diversities of animals. Thus, rather than being a pest, mistletoe can have a positive effect on biodiversity, providing high quality food and habitat for a broad range of animals in forests and woodlands worldwide.
(source: http://en.wikipedia.org/wiki/Mistletoe); also: Susan Milius, “Mistletoe, of All Things, Helps Juniper Trees” Science News 161.1 (January 2002:6).
One final reference for the mistletoe as a keystone species idea: David M. Watson, “Mistletoe-A Keystone Resource in Forests and Woodlands Worldwide” Annual Review of Ecology and Systematics 32 (2001:219-249). I can provide access to this for those who would like to dig deeper.
I just may be able to start thinking about mistletoe in a new way. We’ll see.
Stay tuned for further botanical natural history adventures from the Texas Rolling Plains….