Green Islands

Autumn is here and all across the northern hemisphere deciduous trees are putting on a show unlike anything else in the natural world. The range of colors are spectacular both from afar and up close. If you're like me then every single leaf is worth investigation. The trees are shedding their leaves in preparation for dormancy. The leaves aren't dying outright. Instead, the trees are reabsorbing the chemicals involved in photosynthesis as a way of getting back some of the energy investment that went in to producing them in the first place. 

If you look closely at some leaves, however, you may notice green spots in an otherwise senescent leaf. Why is it that certain parts of these leaves are still photosynthetically active despite the rest of the photosynthetic machinery shutting down around them? The answer to this question is way cooler than I ever expected. 

These "green islands" as they are called are almost always associated with an insect. If you look closely towards the base of these spots you will usually find a tiny leaf mining larvae of a moth busy munching away at the remaining photosynthetic tissue. The most obvious conclusion at this point would be to say that the moth larvae are the cause of the green islands. However, it is not that simple. 

When researchers raised the moth larvae under sterile conditions, they did not produce the green island effect. This proved to be a bit of a conundrum. Why would this happen in the wild but not under sterile conditions in a lab? The answer is bacteria. 

It would appear that the moth larvae have a symbiotic relationship with bacteria living on their bodies. These bacteria interact with the tissues of the leaf and alter the production of cytokinins. In the leaf, cytokinins inhibit leaf senescence. When the plant switches into dormancy mode, cytokinin production is shut down. The bacteria, however, actually ramp up cytokinin production throughout the tissues surrounding the larva. The result of which is a small region or "island" of tissue with prolonged photosynthetic life. 

Because of this, the larvae are able to go on feeding well into the fall when food would otherwise become nonexistent. By harboring these bacteria, the moths are able to get more out of each seasons reproductive efforts instead of simply stopping once fall hits. This is the first ever evidence of insect bacterial endosymbionts have been shown to manipulate plant physiology, though it most certainly will not be the last. 

I would like to thank Charley Eiseman for the use of this photo as well as inspiring this post. Charley is the man behind one of my all time favorite blogs Bug Tracks ( so make sure to visit and like Northern Naturalists.

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The Deciduous Conifer Conundrum

Broad leaf trees get all the glory come fall. Their dazzling colors put on a display for a few weeks every year that is unrivaled. However, it isn't just broad leaf trees that are preparing for winter in this manner. There are some conifers doing the same. The handful that have evolved this deciduous strategy are just as dazzling as their broad leaf neighbors.

The most famous of these are the larches (genus Larix), however, there are others such as baldcypress (genus Taxodium) and the dawn redwoods (genus Metasequoia). So, why have these conifers evolved to be deciduous? There are likely many reasons these genera utilize this strategy but it most likely comes down to cost versus benefit. Needles that last for years are costly to make despite their advantages. They are no guarantee of success either, especially for the larches, which often grow in areas that experience some of the harshest winters on the planet. Heavy snow pack and deep winter chills can take their toll on conifers and many evergreen species show signs of frost damage and broken limbs from snow loads. The habitats in which deciduous conifers are found can be tough places to eek out a living.

By shedding their needles, the larches can get around these issues a bit. They also tend to grow in swampy areas where getting the nutrients needed for survival can be extra difficult. By producing relatively weak needles that are easily replaced from year to year, trees like larches and cypress may get around having to waste resources on more robust needles. Finally, it should be noted that this strategy is by no means less efficient. These genera do quite fine with their deciduous nature. For the most part, these trees are nonetheless successful and can live for centuries. It is mysteries like these that keep the wonderful world of botany interesting.

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