From the Field - Crow's Nest Preserve

Amphibian migrations

With it having been so cold lately it might be easy to forget that we are still expecting the spring migration of salamanders and frogs to area wetlands. They usually migrate at night when the ground is wet during or after a rain and it is 40 degrees or more.

Tonight may be the first night this year that fits the bill and area residents are expecting to see some salamanders crossing. To avoid conflict with them on the roads, try to avoid travel at this time, particularly on back roads through wooded areas and near wetlands.

If you'd like more information on local efforts to assist with the road crossings and collect data on the amphibians doing the crossing, please give me a call at 610-286-7955.

Connecting the dots

On May 27, 2005 I wrote about an irruption of a population of fall cankerworms (Alsophila pometaria) that had stripped the woods bare in the spring of 2004 and were doing so again in 2005 (but it never got as bad).

And on December 2, 2005 I wrote an entry called, Night of the moths. I experienced thousands of moths flitting all around me despite the cold weather.

It never occurred to me until yesterday that these two natural events are almost certainly related.

Tim Marasco from the Pennsylvania Department of Conservation and Natural Resources was a speaker at yesterday's conference. He showed a map of forest defoliators that included fall cankerworm; they affected a broad swath of the state following the ridges from northeast to south central PA, sparing the southeast. But there was one non-contiguous area affected that was clearly in and around French Creek State Park. Tim mentioned this anomalous population and that he was in this area when the adults emerged; the moths were all around "just like falling snow."

The light bulb went on.

That's exactly what I saw. The moths I saw in December were likely the adult stage of the larvae I had seen that past spring. Of course there is a multitude of other moth species it could have been, even just in the family that contains the fall and spring cankerworms (Geomedtridae). But it is exciting to have an "aha!" moment even at this late date.

Low-hanging fruit

People often ask if there's something they can do for the environment. Here's an easy one:

Don't move firewood far from its source, for example when traveling to go camping.

Yesterday I attended training by the Pennsylvania Department of Agriculture and Penn State Cooperative Extension. One of the sessions was about forest pests and included a discussion of the emerald ash borer (Agrilus planipennis) a beautiful beetle that threatens ash trees. The beetle (known in shorthand as EAB) has damaged or killed millions of ash trees in affected areas of five states but has not been found in Pennsylvania so far. EAB, a native of Asia, was first discovered in Michigan in 2002.

Counties where it is found have been quarantined but there has been some spread to other states. Firewood is one potential vector for its movement. Ash trees are beautiful and the wood is strong (it's used for baseball bats). Emerald ash borer has its own website.

Student Research: "How to Conserve Native Plants More Effectively"

The Senior Seminar in Ecology last semester at Bryn Mawr College made use of Crow's Nest Preserve for fieldwork and as a case study. The focus of the class was the value of ecosystem services—those functions that support human life and well-being but are not usually commodities bought and sold: clean air and water, erosion control, climate regulation, and pollination.

The work brought up the importance of collecting raw data from the preserve: it would be difficult to know too much about a place!

Daniela Miteva wrote a study entitled How to Conserve Native Plants More Effectively that included an effort to estimate the monetary value of the native plants at the preserve as well as predictions—using flower and bee abundance data—of the impact of habitat disturbance and fragmentation on the native plant communities in Crow's Nest's meadows.

The benefit of the native plant habitat includes intrinsic values of its ability to support wildlife, manifest biodiversity, and contribute to the relative stability of an ecosystem. The report also estimated the value to Natural Lands Trust of the native habitat as a place to host environmental education and research projects, and to promote hiking and walking, bird and wildlife watching and nature photography. The study also estimated the value visitors place on the preserve (how much are they willing to pay to travel to get here) and the increase in market value of homes located near protected open space.

The study also accounts for management costs to maintain natural areas: that minimum intervention to keep a meadow open and free of invasive plants. It doesn't, however, make an attempt to quantify future benefits (with inflation) of open space protected today.

The part of the study that most affects our land management is the flower and bee abundance surveys. One of the conclusions is that the factor that most affected native bee abundance is the presence of forested sites within 2,300 meters; these provide the bees' nesting sites.

Fortunately Crow's Nest's meadows are embedded in a matrix of forested areas; the study also suggests that loss of surrounding continuous forest habitat could increase management costs here long-term.

The study also concluded that bee species diversity is most affected by frequency of mowing. Most of our meadows are mowed once per year to control invasives and keep them open. But sites that had the greatest number of different bee species were sites that had not been mowed for 2 - 3 years, so it may make sense for us to have more "reserves" of less-frequently-mowed areas, something I have begun to experiment with this winter.

Student Research: "An Evaluation of the Ecosystem Services Provided by Beavers"

Beavers have long had a commodity value; this research seeks to identify this and other values associated with them. Rebecca Donatelli conducted her research last semester on the value of beavers for their role in flood reduction, erosion control, water quality, and habitat diversity. We have had a population of beavers on French Creek for a number of years, at the preserve and beyond.

I have long heard that the geomorphology—the very shape of the landscape we live in: rolling hills surrounded by wide, fertile valleys—is a direct result of beavers, including the now-extinct species of giant beaver. The valleys are here as a result of eons of beaver dams trapping sediment followed by a period of abandonment when a forest re-colonizes the site.

Sediment falls out in the relatively still waters behind a dam; since excess sediment is the number one pollutant in Pennsylvania streams this is a major benefit to humans because it offsets flood control and water treatment measures that would otherwise be necessary. The report also suggests that there is more microbial activity decomposing organic matter in beaver flowages. There is a decrease in available nitrates, phosphorus, silica, and aluminum ions and sulfate.

Beavers are called a keystone species because their habitat manipulation influences what other species inhabit a site. The slow-moving open water behind a dam is also warmer and encourages some wildlife: geese, mallard ducks, frogs and toads, and pileated woodpeckers all make use of beaver flowages. Wood ducks are mainly found where there is beaver activity; they make use of cavities in the trees directly over water for nesting.

The report also suggests that beavers need a 150-foot wide corridor around the stream in which to carry out this activity. The photo at top is of recent chewing on a tree along the banks of the creek at Crow's Nest; note that they also chewed through the poison ivy and Virginia creeper vines! The lower photo above is of a diorama of beaver habitat the kids at the daytime WebWalkers program are making this winter.

Student Research: "The Economic Value of Deer Hunting as an Ecosystem Service"

Bryn Mawr student Louisa Smythe wrote her senior project on deer hunting, evaluating the impact of hunter spending, use of deer for food, potential reduction in rates of the human Lyme disease (of which deer are an alternate host) and deer-vehicle collisions, and reduced damage to crops, timber, and residential and nursery plants.

Densities of deer populations increased dramatically in the twentieth century causing a high level of damage to heavily browsed vegetation. The report cites figures from the Pennsylvania Department of Agriculture on the economic cost of deer damage to timber, agriculture, and the nursery industries. High deer densities also can diminish the value of the habitat for other wildlife. And Lyme disease, caused by the spirochete Borrelia burgdorferi and transmitted by infected ticks of the species Ixodes ricinus, has high rates in Pennsylvania and Chester County in particular. (According to the Chester County Health Department there were 660 reported cases in 2005.)

Ms. Smythe surveyed hunters to determine how they spent money on hunting, how they used the deer for food, and gathered data on time spent hunting and success rates. She noted the benefits of outdoor recreation and hunters’ financial contributions to conservation but made no attempt to quantify them.

The report then uses rates of human infection, deer and human population size, and calculations of the cost of treatment at four stages of the disease to estimate the economic savings of lower deer densities for Lyme disease. A similar analysis was also done for deer-vehicle collisions.

Like all of these reports on the values of ecosystem services there are factors that are difficult to quantify and assumptions that limit the applicability of the results. But the studies also highlight how interrelated are the species around us, and the environment in which we live.

Student Research: "Carbon Sink Potential in the Forests of Crow's Nest Preserve"

James Pena, a Haverford College Student enrolled in the Senior Seminar in ecology at Bryn Mawr, wrote a paper last semester that estimates the volume and value of carbon sequestered in the trees growing at Crow's Nest Preserve. A little global background from his paper is necessary here:

The carbon cycle describes the change in carbon from four places: geological (fossil fuels), oceanic, terrestrial (plants & trees), and atmospheric. These are listed in order of decreasing size.

Any place that releases carbon—a net decrease in stock—is a "source" and any place that has a net increase in stock is a "sink."

Trees are sinks because they uptake carbon through photosynthesis from the carbon dioxide in the air. Some of that is returned to the air through cellular respiration but half is converted to sugars, starches, and cellulose and stored in the above- and belowground biomass. Significant carbon storage also occurs in forest soils, particularly in the older growth woods that aren't sequestering relatively as much carbon in rapidly growing young trees.

Estimating the rates of carbon sequestration is much more difficult to do in a diverse, natural forest than it would be in a plantation. In a plantation all the trees are the same species and age and the carbon sequestered is the largely same in each. Research at other forests has used a process called the eddy-covariance technique to measure net ecosystem exchange (NEE) of carbon each hour. Since this is measuring the reduction of carbon dioxide in the air around the trees it is not based on an estimate of the size or growth rate of the trees.

Mr. Pena drew upon these published rates of carbon sequestration in similar forests—particularly the Harvard Forest in Massachusetts—and applied it to the conditions found here at Crow's Nest Preserve. Based on our 158 hectares of forest area (1 hectare = 2.471 acres), and the estimate of +/- two tons of carbon sequestered per hectare per year, our total carbon uptake at Crow's Nest is about 315 tons per year. That's about enough to offset the yearly emissions of 70 average U.S. cars.

To put a monetary value on that uptake, this service can be thought of in terms of carbon credits. Carbon emissions reductions are now traded on a market and have a range of $5 to $12 per ton of carbon dioxide emissions. Based on these figures the value of the forests at Crow's Nest as a carbon sink range from $1,575.62 to $3,781.49 (U.S.) per year. However there are some regulations that would have to change before forest conservation projects qualify for the trading of carbon emission reductions. For more information about these figures go to the Ecosystem Marketplace.

If you have questions about this or any of the research being conducted at Crow's Nest please contact me.

Speaking about Pools

Speaking about vernal pools, Sally Ray, the ecologist from Western Pennsylvania Conservancy who is organizing the seasonal pool registry, will be the speaker at a joint program of the Chester Ridley Crum Watersheds Association and the Habitat Resource Network of Southeastern Pennsylvania.

The talk will be at the Jeffords Mansion in Ridley Creek State Park on Saturday, February 10 at 10:00 am. The program is free and open to the public, but advance registration is requested: 484-678-6200.

Holly greens

Consider the American holly (Ilex opaca). This one species illustrates one aspect of the complexity of conservation. Marlin Corn, Naturalist at the Churchville Nature Center (Bucks County), wrote about American holly—the conundrum of its being both threatened yet occasionally common—in his "Through the Eye of the Dragonfly" column in this month's CNC newsletter, which prompts me to write about the holly here at Crow's Nest.

American holly is both listed as a threatened species in Pennsylvania and yet is common locally. There are two simple reasons—and one complex one—why it is listed as threatened in this state. First, its native habitat is mostly in the Coastal Plain physiographic provence in Pennsylvania, most of which is the heavily urbanized Philadelphia metropolitan area. So habitat loss is one simple reason there's not as much native holly here as there might be.

The second simple reason is that the political boundary of Pennsylvania is arbitrary with respect to plant ranges. There are more American hollies growing in other states, but Pennsylvania has only a little bit of what is thought to be the tree's native range within its boundaries.

And the complex reason—a social factor—is that the greens are used for decorations. You can help protect American holly by asking for holiday decorations that are nursery-grown.

Yet holly is not entirely uncommon in our region. It is widely planted as an ornamental tree in our yards and—since birds eat and pass the red fruit—holly can spread and "escape" in nearby woods (Rhoads & Block, 2000. The Plants of Pennsylvania, University of Pennsylvania Press, p. 151). Holly has spread beyond the Coastal Plain by this partnership of humans planting trees and birds planting the berries.

For example, Crow's Nest is located in the Piedmont physiographic provence—a more upland habitat that usually hosts a somewhat different community of plants than the Coastal Plain. But American holly appears here and there throughout the preserve, on the wooded floodplain of French Creek, and they are probably descendants of hollies planted in neighborhood yards.

But there is no guarantee that our "escaped" hollies are the same genotype (genetic variety) of holly that naturally occurrs in Pennsylvania—in fact it is likely they are not. Many hollies sold for landscaping are grown in nurseries in New Jersey and North Carolina and could represent genetically distinct, disjunct populations.

Ann Rhoads, the chief botanist at Morris Arboretum of the University of Pennsylvania, expressed concern about hollies she saw growing at Crow's Nest some years ago. There is a continuum between the subset of garden plants that escape cultivation and the subset of those that naturalize (establish self-replicating populations in the wild) and then the subset of those that are invasive (grow aggressively and displace other plants to the detriment of these other species). A tree that is threatened in its native habitat can be a pest in another habitat. Wouldn't it be amazing that a single species could be both threatened and invasive in different habitats within the same region or state?

(The tension here is perhaps in the definition of "species," since that term is a container that holds a variety of related individuals and populations, but sometimes so varied as to want to be split into different species.)

Another factor that could (in theory) threaten Pennsylvania's native hollies is that their genetic distinctiveness could be lost by interbreeding with imported ones, creating yet other genotypes. This genetic "swamping" occurs with small remnant populations that become surrounded by a related but distinct population of the same species. I don't know how likely this is to happen with holly, just that it has happened with other species.

One more piece of background is necessary to this story: American holly is diecious: there are male- and female-flowering trees. Only female trees produce the lovely red fruit, but to do this the female's flowers need to be pollinated by an insect that also visited a tree (just one) with male flowers that are open at the same time.

So with the knowledge that I didn't want to spread more holly through the preserve but wanting a holly for screening in the yard, a few years ago I bought a small American holly from a Pennsylvania nursery at a native plant conference. It was listed as a male tree—so no fruit—that would have been grown as a cutting from an open-pollinated native male tree. So this tree wouldn't be setting seed, though of course it could be causing female trees in the same area (within the flight range of a bee, which can be considerable) to be more fruitful.

But look at the photo above. The "male" tree turns out to be bearing fruit! I don't know much about holly, so I don't know whether it was a simple mistake at the nursery or something else. There are plants and animals that change gender based on environmental conditions, a factor that can improve the population's ability to sustain itself. Are these fruit viable? I haven't experimented with them (yet).

I don't plan to remove this holly in the near future, and I won't discourage anyone from growing these beautiful trees, particularly if you can find a local nursery propagating Pennsylvania natives. But it does make you think about how a seemingly simple decision of what to plant could have wide-ranging consequences.

One more plant story

In the story of barberry, wheat, and wheat stem rust (posted below) all three organisms were introduced to this country—none was native. The North American continent was merely the stage upon which this drama unfolded, one of several stages where it also ocurred (or not really, since there may be unknown characters—organisms, conditions, or interactions—that are part of our landscape that may have influenced the outcome of the story.)

Today's story, also from Myers and Bazely, Ecology and Control of Introduced Plants, is about white pines, white pine blister rust (another fungus), and its alternate host, the genus Ribes (currants, gooseberries).

I had known part of the story: that agriculture departments had tried to eradicate currants—native species of shrubs—in an effort to eliminate the blister rust from the more economically valuable white pine. They weren't successful, but . . . I don't know how common currants used to be, but when was the last time you saw one in the wild? There are almost a dozen species of Ribes native to Pennsylvania, but most of them are not common.

And although I know edible native plants are commonly overlooked in our diets, currants are fairly marginal at the supermarket. Perhaps this sentiment is misplaced, but I feel a sense of loss about the currant as a native plant.

Eastern white pines (Pinus strobus) are also native. The white pine blister rust, which is a disease of 5-needle pines, is not native. We are aware of cases where diseases have been introduced on non-native hosts that are resistant to them, with the result that entire species were substantially lost—such as the American chestnut.

But the white pine story has a twist Myers and Bazely describe (and white pines are alive here, though other 5-needled pines in other regions of the U.S. may be imperiled by the rust). Eastern white pines were shipped to Europe as early as the 1700's, where they were planted in entire forests. The rust was later imported to Europe on the Siberian stone pine, with the result that eastern white pine could not be grown there anymore. Nurseries no longer had a market for their seedlings in Europe, so they shipped white pine seedlings to North America. Millions of seedlings were imported between 1890 and 1914. The shipments were inspected for rust, but not properly (pages 152 - 154). And so the eradicating the currants became the means for controlling the rust.

More stories are happening all around us today: sudden oak death, the emerald ash borer, the Asian long-horned beetle, not to mention introductions of habitat-altering invasive plants. There's lots to learn.