Tag: Succession

Arrested alder

Very few Sitka alders established during the first two decades after the retreating Muir Glacier exposed my youngest study site (Site 1) at Glacier Bay. Older sites in Muir Inlet had abundant alder plants by the time the surface was 20 years old, and it seems likely that the difference was caused by a shortage of alder seeds arriving at the young site. Today there are a few alders at Site 1, and what they have done (or not done) in the last three decades suggests a more nuanced explanation.

View from Plot 1 at Site 1 in three different years. Two Sitka alder plants visible in 1990 (ellipse) produced seeds which formed an island of alder 80 m across by 2018.

When alder can’t keep up

The young vegetation on the north side of Upper Muir Inlet in Glacier Bay National Park is developing dramatically differently from older vegetation a few km away. Today, five decades after the retreating Muir Glacier exposed my youngest study site (Site 1) to plant invasion, it supports an open shrubland of willows with most of the ground still carpeted with low-growing Dryas drummondii. At the same age (in 1995), the next older site (Site 2) had no Dryas and was a dense thicket of 6 meter tall Sitka alder shrubs. The distinct successional pathways being followed at these sites have critical ecological differences (e.g., alder is an important nitrogen fixer) and suggest that inferring ecological change from a sequence of different aged sites in this part of Glacier Bay does not work.

Can seed rain explain this?

My favorite hypothesis to explain this is that early seed rain of alder differed between the two areas. Ice margins in Glacier Bay have been retreating to the north for almost three centuries, and invading newly exposed terrain requires that plants can migrate as fast as the ice retreats. Alder was apparently doing a good job of chasing the ice along most of Muir Inlet where my reconstructions of invasion histories at four study sites confirm that alder was an important early invader.

Vintage chronosequence

In the late 1980s, I established vegetation study plots at 10 sites along the eastern side of Glacier Bay in southeastern Alaska. These sites were more or less evenly spaced between the retreating Muir Glacier and the terminal moraine which the Glacier Bay ice had built at the end of the Little Ice Age. The youngest site had been exposed by retreating ice in 1968 and the oldest site had been exposed around 1770 soon after the ice melted back from the 1740 moraine. So on average the age difference between “consecutive” sites was about 20 years (202 years ÷ 10).

There are three approaches to learning things from this series of different aged study sites:

  1. You can assume the sites form a chronosequence so that changes inferred from the series of progressively older sites are also changes that have happened at individual sites. According to this assumption my older study sites would have passed successively through stages similar to all of the younger sites.
  2. You can be less trusting of the chronosequence and use evidence at each site to reconstruct its developmental history. This history can be compared to younger sites and to their histories to test the above assumption. Tree rings and paleoecology are potential sources of information about vegetation history.
  3. You can wait until each site attains the age of the next older site at the time of establishment. At that time Site 1 should look just like Site 2 did when you started, and so on. If the chronosequence assumption is valid, each site will have aged predictably and you will have a new, slightly older, and now verified chronosequence. You can have some confidence that inferences about change made from that chronosequence are valid.

Documenting Dryas

I saw Riggs Glacier on my first trip to Glacier Bay in 1984. Of the three tidewater glaciers in Muir Inlet at that time, only McBride Glacier reaches sea level today. In 1990, as one of the final field tasks for my dissertation, I established permanent study plots between Riggs Glacier and Muir Glacier. This summer Galen and I used old sketch maps to find all five of them and recorded GPS coordinates.

Lewis testing his downwind rig at Riggs Glacier. Kodachrome, July 1984

Arrested development

Click photos to enlarge.

Entrance to the Choate mine.

Entrance to the Choate chromite mine at Soldiers Delight in Maryland. Fall 1973.

Entrance to the Choate mine.

Entrance to the Choate mine. Although trash was being removed from the area, fresh appliances had been dumped above the entrance. Fall 1973.

Soldiers Delight would be a lot less interesting to some were it not for its contribution to the economic history of Baltimore County, Maryland. Serpentine outcrops including Soldiers Delight, Bare Hills, and the State Line Barrens in Pennsylvania supplied most of the world’s chromium ore in the mid 19th century. Issac Tyson, and later his sons, owned land and operated mines at these places, shipping all the chromite to Baltimore and monopolizing the industry from the 1820s until after the Civil War. But the long term impact of this activity may have been more ecological than economic.

Stemming the tide

Below is a diagram that was never shown to me in science class. Alfred Wegener proposed his theory of continental drift in 1912, but it was not until 1960 that most scientists began to accept the new paradigm that continents move around. The idea of crust formation at mid ocean ridges came even later in 1966. So when scientists and teachers in the 1950s and 1960s presented a story about the serpentine rock underlying Soldiers Delight, they got it wrong. Serpentinite is formed in the lower oceanic crust, typically at the mid ocean spreading centers. That’s where it picks up its heavy minerals, like chromium, nickel, and magnesium, which are more abundant in the mantle and deep crust. When Africa floated over here 300 million years ago, a little bit of this oceanic rock got pushed along with it and ended up in the Appalachian Mountains, and in Soldiers Delight. Nobody knew that in 1960.

Click images to enlarge

A diagram of magma rising through the mantle and forming new oceanic crust at a mid ocean rise. This is where serpentinite is formed.

A diagram of magma rising through the mantle and forming new oceanic crust at a mid ocean rise. This is where serpentinite is formed.

Copy that

Click photos to enlarge

Nikon D3100 on the 22A copy stand. The lens is a Micro-NIKKOR 55mm 1:3.5 with a 12 mm extension tube.

Nikon D3100 on the 22A copy stand. The lens is a Micro-NIKKOR 55mm 1:3.5 with a 12 mm extension tube.

The film is placed in a negative carrier from my old Omega B-22 enlarger. The light source is a frosted glass lamp shade with a compact fluorescent bulb.

The film is placed in a negative carrier from my old Omega B-22 enlarger. The light source is a frosted glass lamp shade with a compact fluorescent bulb.

I watched eBay auctions for Nikon D3100 cameras for a day, then started watching auctions for D5100 cameras. I assumed that the upgrade from the Nikon D40’s six megapixels had to be at least to 14 megapixels to make a noticeable improvement in copies of old Plus-X film, and I assumed that to really record the film grain, the D5100’s 16 megapixels would be required. But the D5100 was $100 more, and the raw files it makes are 20 MB, compared to 15 MB for the D3100 (and 5 MB for my old D40). The D5100 is also bigger and a bit heavier. After several D3100 auctions completed, I ordered a refurbished one from Adorama for $350, about the price used ones were going for on eBay with a kit 18-55 mm VR lens. This gave me a 90 day warranty and it was easy to order a couple of spare batteries. It arrived in 1.5 days for $6 shipping, way better than most eBay deals.

The succession of precedence

Google has digitized the text of five million books. The old ones are in the public domain and you can read them online at http://books.google.com/. Most of the more recent ones are still protected by copyright law, but that law did not anticipate the ingenuity at Google. The individual words and phrases in all those books are now in a huge database that anyone can search. Maybe you can’t read every book online, but you can learn how book writers have used the language over the past two centuries at http://ngrams.googlelabs.com/. This allows a very new kind of literary research – answering questions without reading.  Although it turns out that some reading is still required.