Count me in with the 27% of Americans who prefer fall over the other three seasons. There’s too much going on for me to ignore: the cooler days, Halloween, Thanksgiving, football, pumpkin everything, and the lovable contrarians who decry the ubiquity of the festive orange gourds in our lattes, pies, and on our porches. Perhaps the most uniquely autumnal occurrence, however, is the changing of leaves, or “leaf senescence.” Spring has its blooming and budding, summer its oppressive heat, winter its dormancy, but only in fall does Georgia get to experience the natural paintings where our trees are canvases and an interplay of natural pigments within their leaves dictate the palettes.

Why and how do leaves change?

As photosynthetic beings, trees rely on light from the sun to create their own energy. When the seasons change and the days get shorter, deciduous trees respond by halting production of green chlorophyll, which covers up natural orange/yellow carotenoid and xanthophyll pigments present in the spring and summer. In some trees, anthocyanins are produced as chlorophyll is breaking down (and its nutrients to be reabsorbed). Anthocyanins are produced to cover and protect the leaf, which adds a red or purplish pigmentation. As temperatures fall off, production of anthocyanins is halted and eventually only the brown pigment from tannins is left. Below is a picture of a chestnut oak leaf that shows chlorophyll hanging on only along leaf’s venation, with other pigments following suit from the outside of the leaf margin in:

Everything in a species’ survival is a trade-off. Deciduous trees such as oaks, hickories, maples, beeches, and poplar have a strategy that requires substantial energy and resources to produce hundreds of thousands of broad “solar panels.” These solar panels work six to seven months out of the year to provide the tree with a net gain in energy. The trade off is that the trees enter and remain in a dormant state for several months while the days are short and cold. To contrast, pines also produce these solar panels, but their leaves take the form of tiny needles. Their canopies aren’t always as large as a broadleaf deciduous tree’s, but the needles have the ability to photosynthesize year-round. This is especially helpful in a mixed pine/hardwood forest in the winter when a normally crowded canopy is open with more direct access to sunlight.

On the whole, we’re a few weeks past peak color in metro Atlanta (I noticed it around 11/2 or 11/3), but our healthy variety of deciduous trees still sport an array of color. White oaks are clinging to a burnt-orange hue as tannin brown slowly takes over leaf margins. American beeches are peaking out in all their bright-yellow glory. Landscaped and naturally-occurring red maples are in various stages of leaf senescence, with some half-green, some bright red/orange, and some mostly bare. Tulip poplar- which is notorious for losing some of its leaves in the late summer/early fall is mostly done. So where could I – an autumnophile to the end – go to see the best remaining foliage this late in the season? In honor of it being GIS Day, I decided to find out using ArcMap.

First, I’d need to change my latitude. Early autumn foliage predictors such as the Old Farmers Almanac and The Weather Channel subdivided the country into tiers largely based on latitude. They’re not exact, but most predictors had Georgia divided by a northern region ending just south of Atlanta, an interior portion covering half the Piedmont and most of south Georgia, and a third far southern region that I presume is affected by moisture from the Gulf and the Atlantic. I’d need to go south.

To maximize the the amount of broadleaf deciduous trees I could see in one place, I brought in a landcover raster layer produced by the U.S. Geological Survey called the Gap Analysis Project (GAP). This data provides a detailed look at landcover, ranging from sandy beaches to bay/gum swamps to high-density development in Atlanta, Savannah, Macon, Augusta, and Columbus:

As a visual guide, I decided to turn off symbology for any habitat type that wasn’t a.) in the Piedmont (I had a UGA/Kentucky game to drive back and watch, after all) and b.) wasn’t predominantly deciduous. With that, I overlaid a polygon layer showing conservation lands, which tend to have public access:

There was a spot in the southwest end of the Piedmont that looked like it was a perfect intersection of publicly-accessible land and hardwood habitat:

This area turned out to be the Pine Mountain Ridge of Harris/Meriwether Counties. I’ve written before on my sister blog about my love of this area and how ecologically unique it is. Its southerly latitude and relatively sharp elevation relief create a scenario in which coastal plain, Piedmont, and Appalachian plants co-exist in the same area. Here is a map I made conveying the area’s physical geography:

In looking for an access point in Google Earth, I saw one spot in particular that piqued my interest. At this spot, the ridge ran pretty much west to east and as such had a northern and southern slope. Highway 190 runs atop the ridge and serves as the divider between slopes. The background aerial image in Google Earth was taken in early spring. I noticed the trees on the north slope were already budding but those on the southern slope were mostly bare. At first I thought the green on the north might be pines, but sliding the historical images back to a February date revealed barren grey/brown only.

When I got to the site, the foliage was mostly yellowish-brown, chartreuse, and barren grey- a noble color palette in its own right, but hardly emblematic of autumn in its full glory. There weren’t many pines in sight, and the irony of the mountain’s name was not lost on me.

I scaled down the north slope first a few hundred feet. What I found was a healthy hardwood forest with a canopy dominated by chestnut oak. Northern red oak, red maple, and sourwood composed most of the understory trees.

The further down I scaled, the more red maples I ran across, and thus, the more color I found:

The other side of the mountain revealed the importance that aspect (the direction a slope faces) has on species composition. In general, north-facing slopes are not as exposed to direct sunlight as south-facing slopes. Thus, north-facing slopes tend to be cooler and more mesic (moist) and south-facing slopes tend to be warmer and more xeric (dry). This quickly became apparent as I made my way down the trail onto the south side of the ridge. Chestnut oak still dominated, but the canopy was significantly more open.

The open canopy created a scenario in which a wider variety of understory trees and herbs were present, such as lowbush blueberry, broomsedge, dogfennel, sweetgum. The dominant understory species seemed to be red maple, farkleberry, and what I later confirmed to be the rare Georgia oak. Georgia oak is most prominent on granite outcrops in DeKalb County- notably Stone and Arabia Mountains. However, according to the International Union for the Conservation of Nature, there is also an occurrence on Pine Mountain. The bristle tips of the leaves gave it away.

Georgia oak and farkleberry are also known to grow well in xeric conditions, which fits the description of the mountain’s south slope.

The canopy opened up further as I walked, then near a bend in the trail it quickly closed up with great shade- this was the location of the greatest color I saw on my hike, largely due to the prevalence of understory maples:

Why is this? Turns out the slope faces toward the west instead of the south. A different microclimate is created as it only receives direct sunlight for part of the day and is not as dry and warm as its adjacent slope. Here is a map I put together of the slope aspect where I was on Pine Mountain:

Here is where the trail curved and I reached the colorful and more mesic western-aspect slope:

When I hiked further and reached the next curve along a south-facing slope (east of the red circle above), I observed the trail’s canopy open up again with many trees at the end of the 2017 leaf senescence.

I went to Pine Mountain with the help of GIS mapping with the hopes of seeing some great remaining hold-out 2017 autumnal color, but I ended up stumbling into a great field ecology lesson instead.

I ended up finding a better color scheme than yellowish-brown/chartreuse/grey at another spot along the top of the ridge, but I was too exhausted by then to figure out why it may differ from my original hiking location… and, more importantly, I had to get back to watch the Bulldogs beat the Wildcats in resounding 42-13 fashion.