American beech in winter. Photo by Sixflashphoto, Wikimedia Commons
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(Please note: the Campus Nature Trail at Penn State New Kensington is no longer accessible for hiking. The notes and observations about the trail that are included in this essay were made in the early 2000’s.)
Walking in the woods in the winter is good time to assess the number of beech trees and saplings present because of their tendency to retain their senesced leaves through the winter. This phenomenon called marcesence (Finley, 2012).
Here is a short excerpt from my 2010 “Winter Walk on the Nature Trail:”
“The American beech trees, in their usual fashion, have hung onto to last year’s leaves. Their “autumn” will not end until their new buds begin to open in the late spring. The dry, brown leaves rattle in the breeze making one of the few sounds we hear along the trail.”
Leaf loss, as I have written before, is a purely “economic” decision for a tree. Leaves are the tree’s organs for photosynthesis and energy acquisition, but leaves also lose incredible quantities of water via transpiration. With the approaching winter the leaves for all deciduous trees are shed primarily to help the trees to withstand the very dry conditions of winter. This seasonal leaf loss also recognizes that the freezing of leaf cells’ cytoplasm and interstitial fluids would cause such widespread damage to their cellular structures that they would be incapable of any future photosynthesis.
When the deciduous trees get ready to shed their leaves, they undergo several well defined stages of change. First, in response to the length of the dark period of the day reaching a critical length, they begin to generate large numbers of cells right at the junction of the leaf’s stem and its branch. These cells greatly increase in number but not, at first, in their individual sizes. This layer of cells (the “abscission layer”) slowly starts to interfere with the flow of sugars out of the leaf and the flow of nutrients into the leaf.
The lack of nutrients causes the leaf to stop synthesizing new chlorophyll molecules. Chlorophylls are, of course, the functional pigments of photosynthesis and also the pigments that give plants their characteristic green colors. Initial cessation of chlorophyll production makes the leaves appear a bit paler and less intensely green than they were during the height of summer. These trapped chlorophylls in some leaves may also trigger the synthesis of anthocyanin pigments (red or purple pigments).
Autumnal leaves of the American beech. Photo by N. Toneli, Flickr
As the chlorophyll levels continue to decline other pigments (the “accessory” pigments of photosynthesis: the carotenoids and xanthophylls) that had been present in the leaves all summer long are revealed. These pigments, then, “turn” the leaves orange (from the carotenoids) or yellow (from the xanthophylls) before they finally fall from the tree. The golden-bronze leaves of the autumn American beech, then, is a mix of all three of these types of leaf pigments.
In most tree species, the cells in the abscission layer secrete enzymes that break the attachment of the leaf to its stem. A sudden frost or freeze may reduce the production or action of these abscission enzymes preventing the release of the leaves from their tree. There are, though, some species of trees in which these abscission enzymes are not immediately active at all. These include many types of oak, witch hazel, hornbeam and hophornbeam and American beech. These trees, then, especially their smaller/younger individuals, keep their leaves throughout the winter (Finley, 2012).
What is the benefit of marcescence? The attached, rattling leaves may discourage deer browsing (Svendsen, 2001). It may also give the leaves a period of photodegradation that makes their eventual decomposition more efficient. It may also keep the leaves from being decomposed during the winter (the non-growth period for the tree) and thus make the leaf nutrients more directly available to the growing tree in the spring and summer (Angst et al., 2017).
Hiking along the Rock Furnace Trail in Kiski Township in 2007, I made the following observation:
Beech drops. Photo by Blue Ridge Kitties. Flickr
“We continued down to the Roaring Run stream. All along the path the bright, red berry clusters of trillium stood out sharply against the brown leaf litter and the dark green fronds of the ferns. In the spring, this section of the trail is a riot of white trillium flowers. The evidence of the pollinating success of the plants was now all around us. The trillium berries are commonly gathered, dispersed, and buried by ants as a cold season food source. Lost and forgotten berries germinate in the spring and advance the spread of these magnificent wildflowers. Less obvious than the trillium berries were the brown, foot high stems of a plant called “beech drops” (Epifagus virginanis). Beech drops look at first like the dry, dead stalks of some woody herb. On closer examination, though, the soft flexibility of the stalks and the tiny, delicate brown flowers indicate that these plants are, indeed, alive. The total lack of green color in the plant parts, though, identifies beech drops as a non-photosynthesizing, parasitic plant that is similar in ecology to the plants “Indian pipe” and “squawroot” (both of which are found on our campus nature trail). The stems and flowers of beech drop connect to an extensive, underground root system that is intimately intertwined with the roots of the surrounding beech trees. The plants draw nutrients out of the tree roots and live in a parasitic symbiosis with them. Recent studies involving Indian pipes, though, have shown that this type of symbiosis may be more complex than was formerly thought. There is the suggestion that some trees in association with Indian pipes actually grow better and obtain more nutrients from their supportive soil volume. The symbiotic connections of the tree and plant roots actually increase the absorptive surface area of the root system to the great benefit of both the tree and the plant. Possibly beech drops also assists the root system functioning of the beech tree in a similar manner.”
Beech drops, though, are classified as a root parasite of American beech trees not a mutualistic symbiont, although it is universally stated that the attachment of the beech drops does not adversely affect the health of the beech trees. Further, the presence of beech drops is a forest is considered to be a sign of overall forest health and stability (Tsai and Manos, 2010).
Beech bark disease, Photo by E. Sagor, Flickr
A very significant disease of American beech trees is Beech Bark Disease (BBD). BBD is caused by the interaction of the alien exotic insect the beech scale (Cryptococcus fagisuga) (which was introduced into North America from Europe in 1890) and the fungus Necatria coccinea (also an introduced, exotic species) or the native fungus Necatria galligena Houston and O’Brien, 1983). This disease kills 50 to 85% of infected trees within ten years of their initial infection. BBD has spread from its initial introduction site in Nova Scotia across southeastern Canada, New England, New York, Pennsylvania, West Virginia, west to Michigan and Wisconsin and south down the spine of the Appalachian Mountains (Forest Invasives Canada, 2015). Ongoing research is examining enzyme characteristics and bark phenolic concentrations to try to understand the mechanism of the observed resistance of some beech trees to BBD (Houston and Houston, 1994)(Ostrofsky et al., 1984).
So, I am back in front of the large American beech along the Ravine Trail. A hundred years ago or so the parental tree of this beech flowered in early May shortly after its leaves began to emerge. The parental tree, like all beeches, had both male and female flowers. The female flowers on a particular tree mature first to reduce the chances of self-pollination. The pollen from the male flowers is then dispersed on the wind. Beech flowers are often under the threat of destruction by a late spring frost which, with some frequency, derails the production of beech nuts (Tubbs and Houston, 1990). But, since we ended up with a tree, at least some of the flowers survived and were fertilized this particular year.
After fertilization, the beechnut slowly developed and matured through the rest of the growing season. It ripened between September and November and fell from the parental tree when the encasing burr opened shortly after the first hard frost (Tubbs and Houston, 1990). The nuts, as we mentioned earlier, are eaten by many species of birds and mammals. These animals, though, can also, especially in years when there are very abundant beechnuts, be actively involved in spreading the nuts. Blue jays and gray squirrels are especially important dispersers of beech nuts. Most of the nuts, though, that fell from the parental tree simply landed on the soil and leaf litter under its branches and, if not eaten, waited out the winter for possible spring germination.
Photo by D. Ramsey, Wikimedia Commons
Germination of the moisture loving American beech, ironically, occurs most efficiently on micro-sites that are themselves not excessively wet. These potential germination sites could be either on exposed mineral soil or on piles of leaf litter. They could also be under fern or raspberry cover. The emerging seedling survives best under some degree of shading. Small, protected, partially shaded openings are especially favorable for beech seedling development, although, even under very dense, continuous stands of trees, large numbers of very slowly growing beech seedlings can be found (Logan, 1973). The seedlings are growing so slowly, in fact, that after 6 years the seedling may only be one foot tall and after 25 years may only reach heights of seven feet (Tubbs and Houston, 1990).
Our tree, then, grew very slowly under shade suppression for many years, possibly for several decades. Some fortuitous, for the beech, event in the overstory opened the canopy enough to allow entrance of sufficient sunlight to fuel the beech’s growth. One hundred years later, after avoiding serious damage from the many species of decay fungi which plague the species and the many types of sucking and defoliating insects that can stress or even kill the tree, the tree before us stands large and sound.
References (for “American Beech, Parts 1 and 2”):
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Angst, Šárka; Cajthaml, Tomáš; Angst, Gerrit; Šimáčková, Hana; Brus, Jiří; Frouz, Jan. “Retention of dead standing plant biomass (marcescence) increases subsequent litter decomposition in the soil organic layer”. Plant and Soil. 418 (1–2) (2017): 571–579
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