Not the usual suspects: studying demography of parasitic plants

This is a short article I wrote for our Biology Department’s newsletter.

A 2012 Nature article titled “There’s more to life than rats and flies” highlighted the fact that experimental biologists use only a handful of species. These are probably familiar to most of you – the mouse and rat, Drosophila, Caenorhabditis elegans, Arabidopsis… These model species continue to be used due to inertia and because their properties are well-known: They are easy to raise and care for, and convenient to study. Although these models have resulted in huge leaps forward for science, the limited diversity brings a cost. Translating the knowledge gained to other species can often be tricky, which is a particular concern in studies of human disease.

The same kind of taxonomic bias exists in other fields including population biology and evolution. Most long-term population studies on birds seem to be done on passerines (and many are on species that will conveniently nest in man-made boxes), similar studies on mammals tend to be on ungulates. Studies on plants are no exception to this bias with most studies being on herbaceous perennial species. I received a grant, with Sydne Record and Rob Salguero-Gomez, from the British Ecological Society where we take a small step to change this by studying European mistletoe (in Danish, Mistelten – Viscum album) a parasitic plant that lives in tree canopies. Not only is this a rarely-studied taxon, it is also a species with a demographically poorly understood lifestyle. This is surprising given the iconic status of the species, and the fact that it is regarded as a keystone species where it is common. It provides winter food for birds (who spread its seeds) and its fallen leaves provide nutrients to plants. It also caters for insects such as the mistletoe marble moth (Celypha woodiana) [pdf], which relies on the plant as a larval food source.

Mistletoe in Denmark (A); A new recruit to the study (B); A mature mistletoe plant (C).

Mistletoe in Denmark (A); A new recruit to the study (B); A mature mistletoe plant (C).

Mistletoe steals water resources from its host but must also photosynthesise. I am particularly interested in how their “cheating strategy”, which enables them to escape much of the cost of being a plant, influences their demography. For example, being large and high in the tree is beneficial for the mistletoe since it will increase reproductive output, robustness and access to sunlight. However, if it is situated too high in the canopy, or becomes too large, it risks causing cavitation (formation of air bubbles) in the xylem of the host. This could kill the host, or the limb on which the mistletoe stands, and thus the mistletoe itself. Therefore a balance must be struck – both for size and for height in the canopy – and this will have an influence on growth and reproductive output/death rates.

Some mistletoes are easier to reach than others!

Some mistletoes are easier to reach than others!

Studying the biology of species that are typically high in tree canopies is challenging, not least because the best time of year to study them is winter when the leaves have fallen from the host (mistletoe retains its leaves, and reproduces in winter). We do a lot of work from the ground using cameras, binoculars and telescopes, with follow-up work using digital image analysis tools. However, we also climb into the canopies using techniques borrowed from cavers and arborists to get hands-on measurements. For the next field season we are also investigating the use of flying drones, with the help of researchers at the University of Southern Denmark’s Faculty of Engineering. These will allow us to get up close to the mistletoe at the very top of the trees without needing to climb.

Although European mistletoe is widely distributed, including in Denmark, our study site is in the UK. I would love to have a field site closer to the University of Southern Denmark, but the distribution in Denmark is very sparse. If you have spotted mistletoe nearby [on Fyn, Langeland, Tåsinge etc.], please let me know!

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