June can be defined as the month of sunlight. In fact, from May 17 through July 26, the sun here in Washington, DC is higher in the sky than it is on the equator. What a difference it is in our mid-latitudes versus a place near the equator like Quito, Ecuador. For example, our monthly average high temperatures has a range from 42º F in December to 88º F in July; a difference of 46º, as compared to a monthly average variation of only 3º F in Quito (64º vs. 67º)! This significant difference in temperatures can be related to the varying amounts of available sunlight. Here, our daily sunlight throughout the year varies by 5 hours, 27 minutes (from 9 hours 26 minutes on 12/21 to 14 hours, 53 minutes on 6/21), while in Quito it varies by only 2 minutes and 2 seconds!
Life forms throughout the world must adapt to the amount of sunlight found at their respective latitudes. The longer day length triggers mating behavior in birds and insects, while the shortening days instinctively cause birds to migrate south and chrysanthemums to bloom (as well as goldenrods and asters). Dragonfly nymphs will metamorphose into adults in the early spring while day lengths are increasing at three minutes a day. Once day length increases by less than three minutes, the nymphs won’t change until next spring.
Plants all have their own biological clock that determines when they emerge and flower. A hormone called phytochrome regulates the plant growth and development, including timing of flowering. Many plants are long-day plants, blooming when the days are getting longer. Short day plants bloom as the days are shortening; others are day length neutral. Still others are long-short plants that bloom in summer after the longest days have passed and day length begins to shorten. In the case of some plants, including poinsettias, it’s not the length of day light that triggers the flowering, but rather the length of absence of light, or nighttime.
Certain plants are extremely vulnerable to day lengths. A variety of rice is known to perform best within a ten-minute range of sunlight. An extreme case is a variety of soybean that must grow within a 50-mile radius belt of a given latitude.
In many cases, the daylight hours, more so than climate, dictates the geographic range of plants. This explains why many plants that can survive at freezing northern temperatures cannot succeed in southern climes. It is the lack of short days, necessary to conduct essential hormonal seasonal responses, rather than cold, that prevents their normal growth cycle.
Our deciduous broadleaved forests can be found around the globe in Europe, in central China, and in a few other areas that lie 30 to 60 degrees of latitude from the Equator, but are found most significantly in our eastern United States.
There is a famous eastern Asian-eastern North American connection of plants and animals. Not only are they located within the same latitudes, but due to their similar locations on their respective continents, the climates of the two regions are also similar. Perhaps not surprisingly, many of the same or closely related plant and animal populations occur both in eastern Asia and eastern North America, including genera with species confined exclusively to these two geographically disparate, but ecologically similar habitats. For example, there are only two species of tulip poplars in the world, one here in our region and the other in China. Likewise, the closest relative to the Carolina hemlock is found in eastern Asia.
Nearly 120 genera in 60 families of plants have disjunct populations (geographically isolated) in eastern Asia and temperate North America, relicts of the once widespread flora. Disjunct species are known between these two distant regions among certain ferns, ginseng, mayapple, jack-in-the- pulpit, skunk cabbage, lichens and mosses. Among orchids, more than two-thirds of the orchids of temperate North America have related species in eastern Asia, and eight genera have disjunct species pairs. In contrast, not a single orchid genus grows exclusively in North American and Europe. The genus Panax, that includes ginseng, the well-known medicinal plant, is also found only in eastern US and eastern Asia.
Disjunct species found in these two regions include animals as well. The American alligator and the Chinese alligator are the only two alligator species in the world.
The copperhead has a species in the same genus in southeastern Asia, and our hellbender has an Asian relative.
Looking at fossils collected from western Canada and Alaska confirm a band of continuous deciduous forest ten to twenty million years ago connecting North America to central Asia. Many circumpolar plant species that were once widespread throughout the entire northern hemisphere were wiped out by glaciation 13 million years ago but survived in temperate North America and eastern Asia.
The classic eastern North American - eastern Asian pattern is undoubtedly the best known and most often cited example of the disjunct occurrence of closely related taxa on two continents separated by thousands of miles. It has been said that the similarities of the forests of the southern Appalachians, Japan, and central China, in appearance as well as in ecological associations are in many instances so great that a sense of déjà vu is experienced by botanists by one of the regions visiting the other.
(As some of you are aware, I’ve been writing monthly articles for the PA newsletter since 2001 with only a few exceptions. Recently, the editorial staff has determined that they cannot assure me that there will be room in the PA for my monthly articles and have requested that I submit alternate monthly submissions. That is quite acceptable to me. I say this just to let you know when to expect my articles in the future.)