For a tree renowned for its vibrant, golden autumn leaves, this year was more than a little dull. Our friends at the New York Botanical Garden noticed a trend appearing in ginkgoes across the city: instead of bright yellow leaves covering the trees and then carpeting the sidewalk in color, the ground appeared awash with dark green leaves usually indicative of late summer. The trees seem to have skipped the middle step that so often characterizes autumn, the yellow coloration that we know and love.
What happened? This trend isn’t isolated to New York–people have noticed the same
trend in Chicago, and to a lesser extent here in Virginia. These events most likely reflect the trees being “fooled” by a warmer than usual fall. The final leaf-drop is triggered by cold temperatures. This year, when the weather became cold over the period of a few days rather than over the course of the season, the trees dropped their leaves prematurely. The absence of fall ginkgo color is likely another subtle impact of global warming. Although the absolute temperatures we saw this year are not entirely out of the ordinary, the overall warmer season put the plants out of kilter.
We may see the same in the spring, when an overall warmer winter speeds flower development before a late cold snap. That is exactly what happened in the spring of this year: just as flower buds began to appear, a late freeze killed the young buds. For that reason, the Cherry Blossom festival in Washington, D. C. took place amidst a subdued spring tree canopy. In 2016, a similar late cold spell happened, leading to fewer ginkgo seeds as the frost killed the trees’ reproductive parts right at the time of “flowering.”
The long term consequences of such unusual fluctuations are unknown. They may be nothing –ginkgo has survived in its current form for around 60 million years or so. On the other hand, however, the climatic fluctuations of the last 10 million years have stressed ginkgo and even led to a long-term decline before human hands intervened. Only time will tell.
Increases in stress due to changing climates does, however, impact ecosystems around the world. As we discussed in our blog about the wooly bear caterpillars, repeated cycles of freezing and thawing can disrupt the caterpillars’ hibernation patterns and result in greater mortality. For the ginkgo trees, leaf senescence–the process of leaves losing their chlorophyll and changing from green to yellow–is an important part of preparation for winter. As the leaves change color, nutrients are “recovered” and brought back into the tree before the leaf drop. This nutrient storage probably did not happen so effectively this year, and if repeated frequently, over the long haul, might lead to a weaker tree.
Much of this is speculation. All we have now is the sight of sidewalks covered in green leaves and bare ginkgo branches outlined in the air. But it does showcase the pervasive impact of climate change, as the daily processes we see around us are impacted in strange new ways. For now, we can hope to enjoy whatever fall leaves remain before winter sets in for good.