Marcescence in beech trees

Kaleigh Wood is a senior Environmental Studies in the Rubenstein School at UVM. She enjoys exploring outside, running and cooking.

A couple of weeks ago I was on a stroll through Centennial Woods (CW) for a class assignment. The leaves littered the ground with their colors and gave my steps a pleasant crunchy sound as I made my way deeper into the woods. I looked up to find that most of the trees had shed their leaves. I kept walking and suddenly I was surrounded by this beautiful tree that was still full of its vibrant orange leaves. While I haven't taken dendrology and my tree identification skills are sub-par, I did know this one. The tree that was still holding onto its exciting leaves was an American Beech (Fagus grandifolia). I started to wonder, why do Beech trees hold onto their leaves after all the other deciduous trees let theirs fall?

I went on a run with my trusty running buddy, Lily Morgan, who happens to have a strong passion for trees. I was explaining what I had seen in CW and started to ask her what she knew about the American beech; she started rambling some things in Latin and I quickly stopped her and asked her about the leaves and why they stayed on for so long. She gave me a lesson from Reading the Forested Landscape by Tom Wessels on beech. We found out that beech and oak trees are actually somewhere in between evergreen and deciduous trees, explaining why those tree species can hang on to their leaves throughout  the winter.

We wanted to know more, we found out the genetic reasoning why Beech can hang on to their leaves, but what was the ecological advantage of it? I explored the internet and found an article in Northern Woodlands magazine. The author, Michael Snyder, (who is the Chittenden County Forester in Vermont) explained that some plants will retain dead plant matter, known as marcescence. It turns out there are many theories to the advantages of marcescence. One theory is it’s an adaptive advantage for trees growing in dry and infertile areas (where Beech trees grow!), because if they do not drop their leaves onto the ground they deprive the soil of organic matter which other trees need. Beech and oaks don't need the added nutrients so they  continue to thrive. Keeping leaves on their branches and not on the ground allows them to out-compete other species surrounding them. Additionally oak leaves are high in tannins and reluctant to decompose, further reducing available nutrients to other would-be competitors.

Other advantages of marcescence are, trapping snow which leads to more moisture at the base of the tree, the dead leaves on the limbs protect buds from frost in the winter, and finally the leaves can protect the trees from browsers such as deer and moose. The dead leaves can hide buds so they are less likely to be eaten and are more likely to grow new buds and twigs in the spring. Not only do Beech trees provide CW with great views after most trees have dropped their leaves, but there are also advantages to this interesting adaptation.

More little leaves

What: So in response to Ryan's comment about sugar maples being considerably later than their breathren to leaf out I set out to document some of the later-to-burst buds. A loose pattern I thought I found was that the later species to bud out seem to be the ones that are more shade tolerant. But my sample is too small to make anything other than a very gross generalization (and probably not that correct). I also didn't notice whether or not there's a difference between individuals in the canopy and individuals in the understory. I couldn't find great data on the order of when species leaf out (Project Bud Bursts data set isn't that great) so looks like I'll have to keep investigating.

I know that there is variation within a population due to small-scale climatic features. In the posting on dandelions I described finding dandelions that had gone to seed earliest in south-facing spots adjacent to buildings or rocks. Trees in similar warm areas tend to leaf out earlier. I followed up on the shade-tolerance piece and didn't find a whole lot pertinent to leafing out. It is pretty well established that a woody species' shade tolerance is most relevant in the seedling/sapling stages. The following table is condensed from Humbert et al.'s 2007 paper "A shade tolerance index for common understory species of northeastern North America". It gives ranks each species from 1-9, with 9 being the least tolerant of shade and 1 being the best, an X means that it has a wide range of tolerances. The real table has more trees, lots of herbaceous plants and even some mosses and liverworts.

Latin name	Common name	Shade tolerance index
Abies balsamea	Balsam fir	3
Acer pensylvanicum	Striped maple	3
Acer rubrum	Red maple	5
Acer saccharum	Sugar maple	4
Acer spicatum	Mtn maple	5
Alnus incana ssp. rugosa	Speckled alder	7
Betula alleghaniensis	Yellow birch	5
Betula papyrifera	White birch	7
Comptonia peregrina	Sweet fern	7
Cornus alternifolia	Alt leaf dogwood	4
Cornus canadensis	Bunchberry	X
Corylus cornuta	Witch hazel	4
Fagus grandifolia	American beech	2
Fraxinus nigra	Black ash	5
Larix laricina	Tamarack	9
Ledum groenlandicum	Bog Labrador tea	7
Lonicera canadensis	Fly honeysuckle	X
Myrica gale	Sweetgale	8
Ostrya virginiana	Hophornbeam	X
Picea glauca	White spruce	4
Picea mariana	Black spruce	X
Pinus resinosa	Red pine	8
Pinus strobus	White pine	6
Populus balsamifera	Balsam poplar	9
Populus grandidentata	Big-tooth aspen	9
Populus tremuloides	Quaking aspen	9
Prunus pensylvanica	Pin cherry	9
Prunus serotina	Black cherry	8
Prunus virginiana	Chokecherry	X
Quercus rubra	Red oak	7
Salix discolor	American willow	8
Salix humilis	Prairie willow	9
Sambucus racemosa	Red elderberry	5
Taxus canadensis	Canada yew	2
Thuja occidentalis	White cedar	3
Tsuga canadensis	Hemlock	3
Ulmus americana	Elm	6
Vaccinium angustifolium	Lowbush blueberry	7

The other columns that I would like to see in here would be simple vs compound leaf, ring-porous vs diffuse porous, successionary stage, and dry vs wet habitat preference, freezing killing temperature, herbivore pressure, leaf drop date, flowering date, furthest north latitude, center of evolutionary group's range, among others. So it's a start, but for now mostly just pretty pictures and a cool table.