Marcesence - Photo essay

What: One of my students, Kaleigh Wood, posted about marcescence a couple of years ago. Marcescence, which is common in trees in the oak family, fagaceae (e.g. oaks and beeches), is the process in which the leaves die in the fall before the abscission layer completes the process of releasing the leaf from the tree. I think that these trees just have poor timing and cold weather kills the leave. Others suggest that the dead leaves help protect from the cold, water loss due to wind, or deer browse.

The cost of keeping leaves on is that harsh winter winds could rip the leaves off the tree, exposing the branch to insect predators and pathogens. I suppose another overlooked benefit could be that oak leaves are highly acidic and have high concentrations of tannins. There might be incentive not to drop leaves, where the acids and tannins would leach directly into the soil, making growing conditions for the tree more difficult. By retaining the leaves, the acids and tannins may break down before reaching the soil.

 Whatever the reason may be, the leaf petioles on this red oak (growing along the edge of an open field) had been shredded. Some leaves still barely clung to the branches, but mostly all that remained were the frayed fibers of the petioles.

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.