The Landscape of the Piedmont
Arabia Mountain, DeKalb County, Georgia
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Arabia Mountain, part of Davidson-Arabia Mountain Nature Preserve, a DeKalb County park, is the cornerstone of Arabia National Heritage Area*. I've got a blog entry for Arabia Mountain with lots of details from March 14, 2012.
* National Heritage Area is a site designated by the United States Congress intended to encourage historic preservation of the area and an appreciation of the history and heritage of the site. The 49 current areas are not federally owned and Arabia is a combination of state, county and private land coordinated by the Arabia Mountain Heritage Area Allliance.
* National Heritage Area is a site designated by the United States Congress intended to encourage historic preservation of the area and an appreciation of the history and heritage of the site. The 49 current areas are not federally owned and Arabia is a combination of state, county and private land coordinated by the Arabia Mountain Heritage Area Allliance.
The Atlanta area is a very old landscape that does its best to hide the story of its creation. It is largely covered with a young forest of loblolly pine [Pinus taeda Linnæus 1753] growing out of a 30 to 50 foot thick layer of rotten rock called saprolite. Both serve to so thoroughly cover the underlying rock that in most places it takes human disturbance to see what lies below. In a few places, the rock is exposed such as Arabia Mountain in southeast DeKalb County. This bare round mountain rises to 954 feet above sea level, slightly higher than the surrounding round forested mounds. Only nearby Stone Mountain (1,686 feet) and rise as naked rock above the flatness of the Piedmont peneplain here averaging about 900 feet.
Arabia Mountain represents a good starting point for my ramblings on the Landscape of the Southeastern United States. My curiosity of things always makes me ask, "why does it look this way?" The answer for Arabia Mountain serves my purpose well as the answer is at once simple and extremely complicated.
"Arabia" strikes me as a strange name for Georgia and I find no references to its origin and neither Krakow or the Geographic Names Information Server include it. I'll have to take the obvious metaphor literally and look at the mountain as being a desert like Arabia.
The simple answer for this round mountain is that it is made out of a very resistant yet very eroded rock. The complicated answer comes from its geologic history. The highly metamorphosed gneiss creates a gently sloping broad mound with some steep sides. Soil is rare on this rock, yet it is almost entirely covered with life. One of the most famous flowers of late summer is the Confederate daisy, here growing in a sloping solution pit on a late summer dawn. Life on this mountain has to adapt and overcome a myriad of environmental risks, and it has, elegantly.
The scorching heat of summer (I've measured the temperature of the rock surface at 140°F) fries most things, and evaporates what little water can remain on the solid rock after a rain. Plants deal with this with an amazing assortment of strategies.
"Arabia" strikes me as a strange name for Georgia and I find no references to its origin and neither Krakow or the Geographic Names Information Server include it. I'll have to take the obvious metaphor literally and look at the mountain as being a desert like Arabia.
The simple answer for this round mountain is that it is made out of a very resistant yet very eroded rock. The complicated answer comes from its geologic history. The highly metamorphosed gneiss creates a gently sloping broad mound with some steep sides. Soil is rare on this rock, yet it is almost entirely covered with life. One of the most famous flowers of late summer is the Confederate daisy, here growing in a sloping solution pit on a late summer dawn. Life on this mountain has to adapt and overcome a myriad of environmental risks, and it has, elegantly.
The scorching heat of summer (I've measured the temperature of the rock surface at 140°F) fries most things, and evaporates what little water can remain on the solid rock after a rain. Plants deal with this with an amazing assortment of strategies.
Storing It
Make your own canteen!
Make your own canteen!
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Many are unaware that cactus are native to Georgia and the southeast, but we have three species in our flora. The most widespread is Eastern prickly-pear, Opuntia humifusa (Rafinesque) Rafinesque 1830 with five named varieties. In the Piedmont and Blue Ridge, the typical form is our only cactus, but it is not common.
In the Piedmont of the Atlanta area I find it on all of our hard, crystalline rock outcrops such as this patch on Arabia Mountain. Most know, almost instinctively, that cactus have the ability to store water as they thrive in dry climates. What most people don't know as the water is stored in very specialized stem tissue that also carries out photosynthesis. The leaves are reduced to spines. Even more amazing, transpiration only occurs at night to reduce water loss. Cactus use crassulacean acid metabolism and store CO2 as malic acid at night to be used later in the sunlight. |
Dessicating
If there's no water, get used to it!
If there's no water, get used to it!
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Another way to deal with the heat and extreme dryness is to let all the water in the plant to simply evaporate such as with Grimmia laevigata (Bridel) Bridel 1826. Curiously, this moss really has no common name and yet is widespread in North America, South America, Eurasia, Africa, Indian Ocean Islands, and Australia (FNA).
This moss is an early succession pioneer plant on the crystalline flatrock outcrops of the southeast. To do this, it has the ability to almost completely desiccate for long periods, effectively shutting down most physiological functions. The moss becomes hard and brittle and will make a distinct crunching sound when walked on. It has the stunning ability to turn on these functions within seconds to minutes when water is present. I prove this regularly by pouring the contents of my water bottle on a desiccated patch of grimmia and delight at the oohs and ahhs of folks as they watch it turn green before their eyes. |
Dying
"If you can't stand the heat, stay out of the kitchen!" This Trumanism can be extended to plants here.
One of my favorite plants that can't stand the heat is elf-orpine [Diamorpha smallii Briton ex Small] and uses what most would consider the extreme strategy of dying, but it works! It is a winter annual, a plant that completes its entire life cycle without having to deal with the heat of summer. The seeds germinate in late summer or early fall when the great heat of summer has passed and water is plentiful. It grows first as small globes of reddish green in the sandy strata of the solution pits. Come later winter, these globes begin to elongate and grow into a stem that might reach the mighty height of 3 or 4 inches above the hot rock. Come income tax day (for those not familiar with that day in the United States, it's April 15), the plant reaches its zenith with a crown of gorgeous white flowers. The plant can grow in such profusion that the entire sandy soil of solution pits and ponds will be completely covered with them in a sea of white with a red undertone of the barely visible succulent leaves.
When the fruit is completely developed, the plant dies, having lived only about half a year. What was once succulent plant full of water is now a thin, wiry, multi-headed stalk. At the end of each are four sharply pointed capsules that contain the seeds, now held just high enough above the scorching surface to protect them. Careless walkers paths can be seen in spring as perfect footprints without plants where the seeds were baked by the summer sun. Since the heat of summer is coming, the capsules remain tightly sealed, preventing the seeds from dispersing. It is only with the cooler days of late summer that a single opening in the capsule lets the seeds out to now spread on sandy soil that isn't hot enough to bake them.
"If you can't stand the heat, stay out of the kitchen!" This Trumanism can be extended to plants here.
One of my favorite plants that can't stand the heat is elf-orpine [Diamorpha smallii Briton ex Small] and uses what most would consider the extreme strategy of dying, but it works! It is a winter annual, a plant that completes its entire life cycle without having to deal with the heat of summer. The seeds germinate in late summer or early fall when the great heat of summer has passed and water is plentiful. It grows first as small globes of reddish green in the sandy strata of the solution pits. Come later winter, these globes begin to elongate and grow into a stem that might reach the mighty height of 3 or 4 inches above the hot rock. Come income tax day (for those not familiar with that day in the United States, it's April 15), the plant reaches its zenith with a crown of gorgeous white flowers. The plant can grow in such profusion that the entire sandy soil of solution pits and ponds will be completely covered with them in a sea of white with a red undertone of the barely visible succulent leaves.
When the fruit is completely developed, the plant dies, having lived only about half a year. What was once succulent plant full of water is now a thin, wiry, multi-headed stalk. At the end of each are four sharply pointed capsules that contain the seeds, now held just high enough above the scorching surface to protect them. Careless walkers paths can be seen in spring as perfect footprints without plants where the seeds were baked by the summer sun. Since the heat of summer is coming, the capsules remain tightly sealed, preventing the seeds from dispersing. It is only with the cooler days of late summer that a single opening in the capsule lets the seeds out to now spread on sandy soil that isn't hot enough to bake them.