Oaks…. the foundation of successful fleets for centuries; those of Norse viking expeditions, the English expansion, until they ran out and turned to India for teak but did leave Robin Hood’s Major Oak in Sherwood Forest. There is, of course, the iconic USS Constitution, the Old Ironsides of American defense. Oaks are broadly categorized in to Red or White and co-occur in tree communities across North America. Choosing the right oak for ships was vital. How to tell the difference? Red oak leaves have pointed lobes terminating in bristles (sharp – “Red” for danger) and their acorns have beret caps that only cover a quarter of the acorn. White oak leaves have rounded lobes.
Their common ancestor in North America arose in Canada some 45 million years ago, evolving into 52 species here which now account for more biomass than any other woody plant genus on this continent. Long-lived, oaks as old as 800 to 1000 years have been reported and may be 50 years old before they produce acorns. Tannic acid levels in oaks are high and oak bark has been valued for tanning leather since Roman times. Members of First Nations have used powdered inner bark from oaks, combined with that of other trees, for a heart medicine as well as mild astringent tannic acid solutions applied topically for skin abrasions and taken internally for diarrhea and hemorrhoids.
Back to ships; a dense leak-proof cladding is needed and it is the white oaks that reign. Tannic acid provides durability but red oaks have porous wood and are relegated to railroad ties, fence posts, indoor flooring , furniture and the like. White oaks have tannic acid too but the interior wood is very dense and the pores blocked – the water-conducting tubes of the xylem tissue can be plugged with membranous balloons (tyloses) which makes these oaks more resistant to rot and fungal infections such as oak wilt and importantly for mariners, making the wood water-impermeable. Old Ironsides had Live oak ribs, because their branches are curved, combined with an exterior cladding of classic white oak – both species generate tyloses in their xylem for dense lumber.
Successful navigation certainly helps to feed thousands through trade and distribution but oaks also are significant direct food sources. First Nations members extracted tannins from both red and white acorns by repeated boiling before grinding them into a meal rich in protein and fat for bread. At least1000 species of insects forage on oaks. The most notable and most destructive pest of northeastern oaks is the gypsy moth (Lymantria disbar). Imported in the late nineteenth century by a silkworm entrepreneur, naturally, there were escapees. Peak hatching is in spring, followed by what can be massive defoliation but during moth irruptions cuckoos, white-breasted nuthatches, house wrens, red-eyed vireos, warblers, orioles and others invade for a caterpillar feast. Fifty species of leaf miners also feed on oak leaves are among other many insect feeders such as leaf rollers and hairstreak butterfly caterpillars and walking sticks.
Acorns are the dominant oak energy source for vertebrates; deer, rodents, grouse, bobwhites, turkeys, rufous-sided towhees; grackles too. Gray squirrels bury acorns from red oaks but eat white oak acorns promptly – these acorns may be more palatable to foragers but tend to germinate quickly and rot on the ground.
Major food suppliers, flotilla transport
Oaks are standout but they particularly stand out in northern woods because their leaves stay well past classical fall; present but not conducting physiologic business. Northern deciduous trees lose their leaves to conserve water and protect against frost damage. Approaching winter, most deciduous trees produce the growth inhibitor, abscisic acid, which is involved in formation of a cork membrane abscission layer containing waxy suberin which cuts off the leaf at its junction with the twig. The leaf falls off leaving a scar. Oaks do not complete the abscission layer until spring; they exhibit marcescence. Several reasons for this phenomenon have been offered. Dry, tannic acid-ridden leaves may be un-palatable to foraging deer and protect buds and branches and marcescence is, indeed, more often seen in young trees and on lower branches. Decomposition of leaves could be slower than when on the ground and dropping closer to spring would deliver a timely punch of fresh organic mulch just as the tree resumes growth. Possibly, perhaps as well, marcescence is ecologically neutral; a genetic leftover from the time aeons ago when all trees were evergreen. Red and white oaks here may yet evolve to be full partners in the almost audible sigh of Fall leaf-drop