Tuesday, February 20, 2007

Wetland engineers


I first noticed the sprawling beaver den last December. It was constructed in the only part of the park that had any water, the historical deep swamp (elev. 289'), and it was doing a fine job of holding water when the rest of the park was dry. Beavers were active in the swamp all summer and their dens likely serve as a home to the otters who continue to rip apart turtles on the banks of the lake. Since September, water levels have remained high throughout the park; water is standing even in the canebrakes, the areas with the highest elevation (292'). Beaver activity has increased dramatically the past few weeks and the evidence of their work is seen everywhere in the park.

During the autumn months, beavers were selectively gnawing on green ash trees. Every day I would see a stately ash fall and then summarily stripped of its bark. For the past three months, the beavers have moved into the virgin forest, an area that has not seen persistent standing water since before the drainage ditches were installed in the 1930s. Beavers are taking down the forest's utterly enormous willow trees in the course of two or three nights. When the drainage ditches were initially built, forest composition rapidly changed: willows sprang up in an area that was once a pond of sedges and cypress trees. Green ash and maples grew up in an area that was once dominated by pumpkin ash and cypress trees. The existing cypress trees are roughly 80 to 100 years old and they are slowly dying off. Without water in the wet forest, cypresses cannot grow. Without light to the forest floor, cypresses cannot regenerate. A 1995 survey revealed that cypress trees regenerated for the last time 80 years ago, during that initial draining of the historical Grassy Pond. The water levels of the pond fluctuated rapidly and caused a huge population to grow. Since the pond has been completely drained, the forest has been drying out and filling up with maples, hackberries, green ash and willows. I found young (4-5 years old) cypress trees in only one spot: exactly where the beavers had been active 5 years ago. Cypress trees only regenerate where breaks in the forest canopy and dynamic hydrology occur. Beavers were singlehandedly responsible for the canopy breaks that allowed the growth of 22 new cypresses in the wet bottomland forest. They were also responsible for building dens that held water long enough for the water-dependent trees to survive their first 4 growing seasons. 22 cypresses won't regenerate a forest, but it's a start. With the recent beaver activity, the willow and ash-dominated forest is being cleared to make way for new cypress trees and rich understory production.

The large den in the swamp has caused water to pool for many months now. It was explained to me last spring that the water distribution in the park is contingent on beaver populations and their distribution. Unfortunately, beavers aren't only in the swamp and the bottomland forest. They are also building dens in the elaborate ditching system throughout the county, causing persistent flooding of already saturated fields. A state agency is trapping beavers all over the county to help with the drainage of southeast Missouri, but they're not trapping in the park. No farmer wants a standing lake where corn should be growing, especially with corn prices as high as they are.

Beaver breeding is currently underway. A single litter consists of 1 to 8 animals, but most often 3 or 4. Young beavers are able to swim immediately but they often don't come out of the den until they are 1 month old. They live with their parents for 2 years and leave the den voluntarily, establishing new colonies on the fringes of the existing one. The park's beavers are providing such a great service that I hope new colonies stay well within the park's boundaries, where their natural habits are greatly appreciated.

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