May 24, 2007

Know Your Biomes VII: Temperate Forest

Walking through a streamside copse of eastern hemlock in the ancient Appalachians is revealing for several reasons. First, the sheer size and age of these virgin stands can be humbling - at 45+ meters high, one tree may have been alive for more than 600 years. Second, a closer look at the forest's composition can tell ecologists two things: By assessing the pollen contained within pond sediment, you learn that these hemlocks started repopulating the eastern US about 12,000 years ago, following in the "footsteps" of the maple genus (Acer spp.) after the retreat of the massive glaciers covering most of the United States. We also learn that eastern hemlocks tend to hug water sources, giving way to deciduous trees as the incline of the valley steepens. Mixed forests like these are principle in most of the Appalachian mountains.

But the Appalachian mixed forest is only one small ecoregion in a much larger biome, the temperate forests. Named for relatively mild temperatures and moderate annual precipitation, they stretch across the globe between 30 and 40 50 degrees latitude, from the Gondwanaland throwback Valdivian forests of Argentina and Chile (they resemble forests in New Zealand and Australia), to the home of the pandas.

Temperate forests vary greatly in the amount of rain they receive, anywhere from 650 mm 3,000 mm. On the high end of the scale are regions like the Pacific northwest, where redwoods and sequoias live in what is sometimes classified as a temperate rainforest due to the high levels of precipitation, mid range for a tropical rainforest. They're seasonal. Deciduous (and one or two conifers like the larch) drop their leaves during the winter to conserve energy.

The soils of temperate forests are typically fertile, but their specific properties depend on the composition of the forest. In deciduous dominated forests (oak-hickory, beech-maple, etc.), nutrients cycle quickly, creating a substrate rich in organics. Soils in coniferous dominated forests are much more acidic and nutrient cycling tends to be more conservative.

Fire is important to nutrient cycling and population regulation. Many conifers have specially adapted thick bark to ward off the effects of fire and the cones of some species, the "fire-climax" pines like the pond pine or the Monterey pine, often depend on the touch of flame to open.

Like the tropical rainforest, temperates are vertically stratified, with organisms living and growing in the canopy, a shorter layer of mature trees below, the shrub layer and, of course, the understory, where nematodes, fungi and bacteria break down the thick mat of leaf litter into organically rich soil. Light is relatively abundant in the forest understory, allowing ferns and herbaceous plants to thrive. Mosses and lichens cover tree trunks and rock in the more moist portions of the forest.

Vertebrate life is equally diverse. In China, the red panda and the giant panda live in the same general area and subsist on the same food - bamboo - without being in direct competition. They fill very specific niches, however, predominantly eating different parts of the plant and browse slightly different regions. White-tailed deer, grouse, bobcats and black bear dominate the Appalachian forest. In eastern Russia the and leopard, both highly endangered, found refuge in Manchuria, which the last ice age left untouched by glaciers.

Humans have affected temperate forests more than any other biome due to the habitability, fertility and resource richness of these areas. Forest covered most of the eastern US and western Europe until civilizations moved in to urbanize.

Next time: Taiga

May 9, 2007

Global warming linked to White Syndrome in corals

Global warming is not only stripping corals of their food source, it is opening the door to rapid, widespread infection.

A group of researchers (led by a Dr. John Bruno) published a paper in PLoS Biology this week looking into a possible correlation between the spread of white syndrome among schleractinian corals and warmer temperatures at the Great Barrier Reef (GBR). The study found a positive correlation between warmer temperatures and outbreak.

More on schleractinian corals and how global warming is affecting them below the fold.

Schleractinian corals - such as the brain coral pictured - are known as "reef-builders"; literally, reefs exist because of this order of animals. Most of the schleractinians found on shallow reefs like the GBR are zooanthellates, meaning their tissues are home to tiny, dinoflagellate algae symbiotes that photosynthesize sugars for the corals (zooanthellae). The pigments of the symbiotes is also responsible for color

Which brings us to an important distinction: White syndrome is not bleaching. Coral bleaching is the result of the expulsion of these zooanthellae from the coral's tissues due to stress. This stress can be caused by any number of factors, but recent evidence ties the proliferation of bleaching to global warming.

Warming ocean temperatures are disrupting seasonal growth. The prolonged warm temps keep the zooanthellae from proliferating and subsequently the corals from producing adequate body mass in the winter, stressing the organism's physiology and leaving them susceptible to infection. The temps also seem to cause rapid growth in the pathogen.

The pathogenic cause(s) of white syndrome are virtually unknown, though Bruno and his colleagues suspect that it may have similarities to the "white diseases" found in corals in the Caribbean, which are caused by a group of pathogens.

The white syndrome was previously thought to be caused by a bacterial infection, but researchers back in March announced that they had found evidence that the corals were somehow committing suicide; their cells were prematurely going into programmed cell death (PCD). At the time, one of the researchers from the ARC Centre of Excellence for Coral Reef Studies had an idea about what was causing this:

"I think the smoking gun is climate change. We have had a series of hot summers recently in which corals in the shallows become bleached and literally 'worn out'. This leads to mechanisms that allow the coral to retract tissues that are no longer functioning due to this stress," says Professor Hoegh-Guldberg. "This is somewhat similar to Eucalyptus trees dropping branches when they run short of water."

This study would seem to support his assertion. Samples taken of infected corals at the GBR have been collected and are currently being studied.