Non-Fibre Values: Trapping nutrients in an ephemeral root-web
By Frederick W. Schueler

It's a truism that a forest is more than trees, but once we've grown trees on a site, we tend to take the resulting “forest” for granted, without thinking about what more there might be. Scrounging across the Internet recently, I found a paper from the Harvard Forest that addresses how much more there may be, and how long it may take for a second-growth forest to develop an herbaceous ground cover of ferns and spring wildflowers (Jesse Bellemare, Glenn Motzkin and David R. Foster. 2002, Legacies of the agricultural past in the forested present: an assessment of historical land-use effects on rich mesic forests). Once we question whether young woods have their full complement of herbs, it's easy to imagine that the trees are growing a lot faster than herbs are recolonizing the blanket of needles under plantation pines or the scrappy grey leaf-mould under a young stand of ash. In many young forests that are far from seed sources, undergrowth plants seem to occupy patches rather than microhabitats, as if single ancestors have simply spread rather than competing for space with a full community of other species. The Harvard Forest study has actually measured the incompleteness of recolonization in forests more than a century old.

The authors studied forest vegetation in the towns of Conway and Shelburne in western Massachusetts. These towns are now 70% forested, with stands dominated by sugar maple (Acer saccharum, 61% of basal area), though they were only 20% forested in 1830. The forests were classified as either “primary forest” (forested in 1830), “nineteenth century secondary'” (open land in 1830, but forested in 1942), or “twentieth century secondary” (open land or early successional vegetation in 1942). These different classes of forest had recognizably different patterns of vegetation, independent of the richness of their soil or other environmental factors. Herbs with no particular adaptations for dispersal and those with seeds dispersed by ants are less frequent in secondary forests. Environmental differences between primary and secondary forests appear to be less important than age in influencing species' distribution.

When the forests of southern Ontario were cleared for agriculture, forest herbs were often eradicated by direct disturbance and burning, exposure and desiccation, competition with sod-forming grasses, and intense grazing and trampling. They will persist vegetatively through ordinary forest disturbances such as fire, blow-down, and logging, but must colonize second-growth woods by dispersal from existing populations. Often the first colonists are spore-borne ferns and clubmosses, or lady-slipper orchids (Cypripedium) with their spore-sized seeds. These are followed by species carried by the wind or vertebrates, and only slowly by those without adaptations for long-distance dispersal.

Young woods can have a rich herb cover if bedrock outcrops and steep slopes have provided refuges for the herbs during decades of deforestation. In the sugar maple-paper birch woods where the Bruce Trail doglegs out to the shore of Georgian Bay along Little Cove Road, the forest floor is as green as a lawn. Trout lily (Erythronium americanum) covers the floor with a pelt of small leaves and nodding blooms of back-swept petals. There are low plants of yellow violet (Viola pubescens) with small leaves and constellations of small yellow blooms, spring beauty claytonia with coy red-striped pink flowers among patches of small red-stemmed leaves, Hepatica tucked into the shelter of tree roots, the flowers open while the wooly leaves are still furled, and blue cohosh's (Caulophyllum thalictroides) strange blackish patches of purple-black leaves above the trout lilies. Wild leek (Allium tricoccum) forms tall patches of broad green leaves that will grow, yellow, and wither before the flower-stems emerge.

Like all woods on the outer Bruce Peninsula, these are less than a century old, and the largest maples outline what must once have been unprofitable pasture, with limestone bedrock never more than a few centimetres from the surface. Mossy boulders are concentrated at the edges of what was the field, and nearby the bedrock opens into fissures 20-30 cm wide. Old sprawling leggy apple trees are surrounded by whip-thin maples, and a single common juniper bush is a survivor of the pasture (Little Cove Rd./Bruce Trail, Bruce County, 17 May 1997).

The Harvard Forest study found evidence that forest herbs had persisted in local refugia such as bedrock outcrops, rocky slopes and fencerows within the agricultural landscape. In “nineteenth century forest,” plots within 50 m of bedrock outcrops often had herb diversity comparable to that of primary forest, while plots farther from bedrock were significantly less diverse and often lacked species with limited ability to disperse.

As anyone who has tucked a few trout lilies or trillium into a hedge knows, these ephemeral spring herbs often thrive in a wide variety of shaded habitats. And they are not just breathtakingly beautiful and (in the case of wild leek and ostrich fern), wonderfully delicious. Through the early spring their roots retain nutrients that might otherwise be lost to groundwater or runoff before the roots of the trees are actively drawing water from the soil. If we're to build up the nutrient status of our woods in the face of acid rain, a green floor may be as important as a green canopy. If we want the flora of young forests to mature within a century, we'll need to recognize and protect refugia where forest floor herbs persist, and perhaps transplant slowly dispersing species into the regrown woods.

About the author – Fred can be reached at the Bishops Mills Natural History Centre, RR#2 Bishops Mills, Ontario, K0G 1T0 or by e-mail <bckcdb@istar.ca>. Visit the Web site <http://www.pinicola.ca>.

This article appeared in the Spring/Summer 2004 (Volume 35) edition of the S&W Report the newsletter of the Ontario Woodlot Association.

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