The Last Green Thing in the Forest

 

November is bleak in the Twin Cities. Over the four weeks that usually separate the fall of the last leaf from that of the first snow, the day shortens by more than an hour. For many of the humans that live here, that hour is the one whose loss we most acutely feel, of the nearly seven we lose in total between the summer and winter solstices. Other months have their compensations. October’s changing leaves make the world a kaleidoscope in slow motion. December’s early-season snowfalls remake the world afresh. November offers no such diversions:  only bare trees, bare ground, and the relentless, clock-like loss of two minutes of sunlight every day.

OK, maybe there’s one diversion. If you spend any time in the ample forests in and around the Cities, you might have noticed it. A break in the pattern. Here and there, against the gray expanse of leafless trees – a spot of green, vibrant as mid-summer. If you’re the curious type, you might even have approached it for a closer look. If so, what you found was almost certainly a shrub or small tree with a distinctive pattern of banded pores in its bark, and short oval leaves with gently curving veins. The green of those leaves is even more striking up close. The contrast between it and the barren crowns of its neighbors can leave you with the impression that the plant is lost; that it has unwittingly wandered somewhere it doesn’t belong.

As it turns out, there is some truth to that impression. The plant is Common Buckthorn, Rhamnus cathartica in scientific terms, and it’s not from around here. Its home range is vast, covering most of western Europe, edging into North Africa to the south, and central Asia to the east. It did not get lost here on its own, however. Early American colonists imported it, most likely as a hedge. Benjamin Franklin, himself, grew the plant, noting in his 1751 guide to medicinal plants of the colonies that it “grows well from seed I had from Europe”. It does make an excellent hedge, quickly forming impenetrable thickets that require very little care from the horticulturalist. 

At some point between Franklin’s time and ours, buckthorn’s independence became a problem. Franklin noted in his guidebook that it grew “no where wild in our Provinces”. That is no longer the case. In the United States, buckthorn now grows wild from coast to coast. The center of density is a broad band surrounding the Great Lakes that includes Minnesota. Around the Twin Cities, once you have noticed it, you will see it everywhere. It especially thrives along the edges of forests, often forming hedge-like thickets, just a few paces deep, that can surround the forest perimeter. In wooded patches small enough to be ‘all edge’, or those in which high tree mortality has thinned the canopy, buckthorn is visible this time of year as a continuous layer of green spread across the whole understory.

So the obvious question:  Why? Why is buckthorn green when nearly all other plants have shut down for the winter?

The answer seems to lie in its origins. Buckthorn is not the only invasive plant in North America that is green when native plants are dormant. In a study published in the journal Nature in 2017, researchers at Munich University took a close look at this phenomenon. They compared nearly 400 species of trees and shrubs originating from all over the globe:  eastern and western North America, east and west Asia, South America, and Europe. Over a period spanning 2014 and 2015, they carefully noted what ecologists call the phenology of each species; in other words, the timing of different life history events, like leaf emergence in the spring and color change/leaf drop in the fall.  They used the dates of these events to calculate the length of the growing season for each species in their study. Because the plants all grew in the same location – the Munich Botanical Garden – any differences they observed in phenology could be attributed to genetics rather than environment. They found that, on average, species from eastern North America had a genetically programmed growing season that was 18 days shorter than European species and 24 days shorter than east Asian species. 

Three extra weeks of photosynthesis can make a big difference to a forest-dwelling shrub. This is especially true during periods when the crowns of the overstory trees are empty, like early spring or late autumn, giving the shrub access to sunlight at full intensity. In the late 80’s, researchers in Wisconsin found that 3 to 4 extra weeks of photosynthesis under an open canopy could provide nearly 40% of the annual carbon (i.e., food) budget for buckthorn, and nearly 50% for invasive honeysuckle (Harrington et al., 1989). That boost could give the invaders a big edge over the native species they compete with.

This leads to another obvious question:  why are native North American species genetically programmed to leave this advantage on the table? If there is a predictable resource available in those 3 to 4 extra weeks, shouldn’t natural selection have led native species to capitalize on it?

The answer to this question likely involves two factors:  1) the cost of taking that advantage, and 2) climate change. First, the cost. When the temperature eventually does drop below freezing for the first time in winter, the still-green invasive plants lose their leaves to frost damage, along with all the hard-to-acquire nutrients they contain. The controlled dormancy of native species allows them to scavenge those nutrients for reuse in the spring. Incidentally, this is why their leaves change color in the fall. By contrast, the invaders’ leaves simply die ‘on the vine’. If they can’t capture enough energy before they die to make up for the nutrient loss their death incurs, this strategy will not pay off. Importantly, this payoff needs to be fairly consistent to be favored.

This brings us to the second factor, climate change. The authors of the Munich study examined the role that climate differences might play in creating the regional differences they observed in plant phenology. They used global climate data collected from 1901 to 2013 that were publicly available through the WorldClim project (https://www.worldclim.org/). In their analysis, differences between North America and both Europe and East Asia in simple variables like average temperature were not large enough to explain differences in phenology between the regions. What was more different was how much the average temperature changed from year to year. In North America, temperature fluctuations between years were much larger than in the other two regions, particularly in the spring. In other words, in any given year, a plant living in North America would be less able to predict the best time to sprout leaves in the spring, or shut them down in the fall, without danger of losing them to frost than a plant living in Europe or East Asia. Under such unpredictable circumstances, the safest option is to overcorrect:  on average, shutting down ‘too early’ in the fall (i.e., a fair bit before the first frost), and starting up too late, compared to the optimal timing for that year.

The climate difference that underwrites that strategy appears to be changing. In parallel with accelerating climate warming over the past few decades, the seasonal ‘danger zones’ that occur in spring and fall seem to be getting less dangerous. In response, North American plants are leafing out earlier and retaining leaves later in the year than in previous decades (e.g., Bertin 2008). Around Ordway, a few of the still-green plants that catch my eye in November turn out to be Black Cherry or Ash, both native species.

For the time being, however, most of them are still buckthorn, their conspicuous color betraying the phenological edge they brought with them from the Old Country. It will be interesting to watch what happens. For me, and other ecologically-curious Minnesotans, it might even help brighten up those bleak Novembers to come.

Further Reading

Bertin, R. I. (2008). Plant phenology and distribution in relation to recent climate change. The Journal of the Torrey Botanical Society, 135(1), 126-146.

Harrington, R. A., Brown, B. J., & Reich, P. B. (1989). Ecophysiology of exotic and native shrubs in southern Wisconsin: I. Relationship of leaf characteristics, resource availability, and phenology to seasonal patterns of carbon gain. Oecologia, 80, 356-367.

Kurylo, J. S., Knight, K. S., Stewart, J. R., & Endress, A. G. (2007). Rhamnus cathartica: Native and naturalized distribution and habitat preferences1. The Journal of the Torrey Botanical Society, 134(3), 420-430.

Short, T., Bartram, J., Paulli, S., Ray, J., Franklin, B., Hall, D., & Muhlenberg, H. H. (1751). Medicina Britannica. Philadelphia re-printed and annotated by B. Franklin, and D. Hall, at the post-office, in Market-Street.

Zohner, C. M., & Renner, S. S. (2017). Innately shorter vegetation periods in North American species explain native–non-native phenological asymmetries. Nature Ecology & Evolution, 1(11), 1655-1660.