Rock Creek Park's richest forest grows where the rock allows it

The richest forest in Rock Creek Park covers just 90.9 acres, and it grows where it does because of the rock underneath. The park's ecologists call it the Basic Mesic Hardwood Forest. It holds a greater diversity of native plants than any other upland community in the park — some ferns and wildflowers grow nowhere else there — and almost all of that comes down to one element: calcium.

Why here, and only here

Much of the park sits on acidic rock. In a few places the bedrock is mafic — gabbro and tonalite full of dark minerals such as biotite, hornblende and pyroxene — and as it weathers it releases calcium and magnesium into the soil. Groundwater carrying dissolved minerals does the same. The result is soil that is less acidic and more fertile than the park around it.

Position matters as much as chemistry. The forest grows in deep, shaded ravines and on low, concave slopes just above the stream floodplains, where soil collects instead of washing away. Those soils hold moisture but drain well — wet enough for ferns, never waterlogged like the park's seepage swamps. Most of these soils lie west of a geologic fault; small patches turn up elsewhere where enriched groundwater reaches the surface. An exceptionally large tree can be the clue.

"Basic" here is an ecologist's word, not a chemist's. The soils are seldom basic in the chemical sense, above pH 7; they are simply less acidic, which is what most plants want. Gardeners do the same thing on purpose when they lime a bed to between 6.2 and 6.8.

How to recognise it

It is easy to confuse with a Successional Tuliptree Forest, which can look just as lush. The difference is the canopy: after a clearcut, tuliptrees come back alone, and the beech and oaks are still catching up underneath.

The maple mystery

Sugar maple grows here, and nobody is sure it belongs. The clues run backwards. Several old, protected ravines over basic rock — ideal sugar maple habitat — have no mature sugar maples at all. The oldest, most abundant ones grow mostly on acidic soil, not far from Klingle Mansion, the home of Joshua Peirce, a nurseryman who raised trees for street planting in the first half of the 1800s. Out in the best habitat, there are only a few young ones.

The likeliest story is that someone planted the first grove over a century ago and its seed has been spreading outward ever since, reaching the prime ravines only recently. A DNA comparison with the nearest native populations could test it — though even a match would not say whether birds, Native Americans or the Peirce family first brought the seed.

What threatens it

The same fertile soil suits invaders: Norway maple, English ivy, Japanese stiltgrass, Chinese wisteria and more. Deer browse wildflowers, oak seedlings and shrubs — everything except pungent spicebush and pawpaw. The park has managed deer since 2012, treats gypsy moth with bacterial and viral insecticides and parasitic wasps, works to reduce stormwater runoff, and watches for emerald ash borer. The best places to see the forest are Cross Trail 2 near Riley Springs Bridge and the ravines along the Melvin Hazen Trail.

Sources

Written from the National Park Service's field guide to Rock Creek Park's natural communities, a work of the United States government in the public domain, with map data from NatureServe and the National Park Service National Capital Region.