With the increasing sunlight of March, a new season is beginning. Before the end of the month, the various plants, animals and fungi that have been dormant for the last three to four months, will be actively engaged in the annual cycle of events that will enable them to eat, survive, and reproduce. It’s truly a time of great transition within the natural communities of our latitudes. During the months of March and April, prior to the foliation of our forests, more energy, or heat, will be absorbed by the soils than at any other time of the year. Within this eight week period, the spring ephemeral wildflowers will emerge, flower, produce fruit and store enough solar energy in it’s roots for next year’s plant. The true spring ephemerals will then wither away; having completed all of its visible life processes, not to be seen again until next spring.
This early spring period is also a wonderful time for visiting vernal pools. These seasonal ponds, which will routinely dry up by late summer, are the breeding grounds for many of our frog and toad friends, or anurans. Vernal pools are chosen over permanent bodies of water due to the lack of predaceous fish, which would prey on the tadpoles. With three months often necessary for the eggs to hatch into tadpoles and then to metamorphose into the terrestrial adult forms, the overwintering adults must reach the pools early in the season to mate and lay eggs. In our region, this will normally occur sometime in March, in concert with a good afternoon and evening rain. However, as with most early spring events, the timing is greatly influenced by the weather. For example, I recall one winter hike with Len Wheat in the southern district of the Park that we took after about five days in the fifties. Even though it was Feb 22, the ridge pond was swarming with hundreds of calling, sex-crazed wood frogs. The same scenario applies to the spotted salamanders. These ‘explosive breeders’ can only be found at the vernal pools for two or three days under ideal breeding conditions. After this quick mating period, they escape back into the woods, and will be very hard to find for the rest of the year.
But the true harbinger of spring goes to the skunk cabbage. In fact, these plants bloom so early in the season, there isn’t enough of the sun’s energy to facilitate the plant’s functions. So, the skunk cabbage produces it’s own energy. It produces sufficient heat to melt through the frozen soil and overlying snow. It does this through metabolism, in which the flower consumes oxygen as a fuel, burning starch stored in the thick rhizome. In experiments, it has been found that skunk cabbage flowers could be 36 to 63 degrees F higher than the ambient air temperature. At air temperatures around 58°F, the air temperatures inside the floral spathe can average some 9°F higher. When the ambient air temperature dropped to about 5°F, the inside air reached temperatures 30° higher or more. Studies conducted by Roger Knutson found that as temperatures dropped from 63 to 45 degrees F, the plants nearly doubled their oxygen consumption.
The eastern skunk cabbage (Symplocarpus foetidus) is a native member of the Arum family (Araceae), which includes calla lilies, green dragon, golden club, water arum and jack-in-the-pulpits. The flowers of the arums are similar in form, consisting of a central roundish flower head, enveloped by a protective spathe. The spongy spathe enhances the heat retention, which is excellent insulation around the heat-producing spadix, and its dark color also absorbs heat energy from the sun. Most arums, like jack-in-the-pulpit, do not produce measurable spikes in temperature, basically because the production is not necessary.
Heat generation by plants is shared by a group of several hundred species in 10 families, including certain philodendron, the Asian water lotus, and the famous Amazon water lily, (Victoria amazonica); the favorite of 19th century aquatic gardens. Self-heating appears to sprout from ancient lineages at the base of the botanical family tree. For example, heat generation has turned up in early magnolias, Dutchman’s pipes, star anises, custard apples, and water lilies. However, thermogenesis has tended to be dropped along the evolutionary trail. Dr. Roger Seymour, zoologist with the University of Adelaide in Australia, suggests that heat-generating flowers “are like nightclubs for beetles”, in that insects would crawl into the spathe, and often be trapped until the next day by erect spines which would eventually wilt, allowing the beetle to exit. Through the evolution of flowers, insects have been able to visit a flower and quickly take a sip of nectar or a snack of pollen before moving off, while still serving the pollinating purposes of the flowering plant. As Dr. Seymour likes to put it, heat rewards died out because “nightclubs were replaced by fast food”.
You might recall just last summer, the media frenzy surrounding the flowering of the US Botanical Garden’s own Titan arum (Amorphophallus titanium), another heat producer. It is on record for having the largest flower of all flowering plants. The five to twelve-foot size of the flower, however, is normally not the first thing visitors notice about the Titan arum. This plant is also known as the corpse flower; due to its fragrance, which is likened to rotting flesh, or warm, mellow road kill. In fact, this noticeable odor is quite common among the heat-generators. The skunky odor of the spadix and rotten meat color of the spathe attract carrion flies and other pollinators to the flower. In the case of the skunk cabbages, this is extremely important in a season of few pollinators. In fact, dominant compounds, called oligosulfides, have been found in both the scent of the flowers and that of a dead seagull. Some of the typical volatile organic compounds released by the decomposing carcass and arum flowers go by graphic names like putrescine and cadavarine.
As you go hiking through some wet lowlands this month, you might recognize the musky odor of the skunk cabbage. Follow the odors lofted through the air by the heat produced within the spadix (principally by sterile male flowers found among the less pungent fertile male and female flowers), and notice the party going on inside the warm spathe. It’s truly a unique and happening place to be this time of the year.