The Fall Rainbow: Green Islands

The process of deconstructing a leaf to recoup its embodied energy and nutrients generally proceeds as normal from year to year. But occasionally something disrupts the signals (that is, the plant growth hormones), and the leaf doesn’t quite get it right. In these cases you’ll find a swath of green on an otherwise yellow or orange leaf, as in the alternate-leaved dogwood leaves below. There are a couple possible reasons for these “green islands.”

A common defensive response plants utilize to combat disease is to isolate the pathogen and quarantine those plant tissues from the rest of the plant to prevent the disease from spreading (this is one advantage of shedding leaves each fall as an infected leaf can just be whisked away by the wind). This strategy becomes visually obvious on the trunks of beeches infected with Beech Bark Disease. After the beech scale (a tiny insect that feeds on the sap of beech trees much like a mosquito feeds on our blood) inoculates the trunk with fungal spores from the destructive Nectria fungus, the trunk attempts to seal off these diseased sites by killing off circular patches of tissue in the bark. But because the insects colonize large sections of the tree, trunks can be riddled with a field of cracked and fissured plates, as below.

Similar to the beech, the witch-hazel in the leaf below has done essentially the same thing in response to a fungal infection (likely leaf spot). Because the process of breaking down chlorophyll is an active process that happens in response to the presence of certain plant growth hormones (like auxin and ethylene), when the plant walls off part of its tissues from the rest of the leaf, the patch also becomes isolated from chemical signals. While the rest of the leaf is aware that it’s fall, the quarantined zone remains blissfully oblivious to the passing of the seasons, green until the bitter end.

The more devious among the parasites can actually mimic plant growth hormones to confuse the plant for its own selfish purposes. This is true in the parasitic moth, Ectoedemia argyropeza, whose larvae parasitize the leaves of big-toothed and quaking aspen (and much less frequently eastern cottonwood). The larvae produce a chemical that mimics cytokinin, a plant growth hormone that stimulates growth, effectively telling the localized part of the leaf where the caterpillar is residing that it’s not fall and the leaf does not need to do anything to prepare for winter. While the rest of the leaf senesces, the area at the base of the leaf hosting the caterpillar has a wedge-shaped green island.

Virtually all adult moths are female, and the eggs, which are primarily produced parthenogenetically, are laid on the outside of the leaf. After hatching in the late summer/early fall, the larvae tunnel into the leaf through the petiole. Here they nestle in and begin to feed on the plant tissues. The caterpillars over winter as pupae (the cocoon stage) and emerge as adults the following spring. The challenge is that the eggs hatch late in the growing season just as aspen leaves are about to senesce. If they didn’t disrupt leaf senescence they wouldn’t have enough time to eat the green tissues to acquire enough energy to fuel transition into their pupal stage. If instead, they just cut off the entire leaf before senescence begins, while the leaf would have plenty of stored energy to support the caterpillar’s development, the leaf would also remain attached to the branch through winter and the cocoon would spend the entire winter exposed to the elements. Disrupting senescence in part of the leaf provides an optimal balance, and the moth’s success is visible this time of year as the forest floor becomes carpeted with these green and yellow leaves.