HEART ROT

GEORGE H. HEPTING, JAMES W. KIMMEY.

Heart rots, which are caused by fungi that attack the wood of living trees, are to blame for an estimated annual loss of 1.5 billion board feet in our commercial forests. In money, the loss lies somewhere between the approximately 10 million dollar value given the cull as stumpage and the 47 million dollar value given it as logs.

Every timber species in the United States is subject to attack by one or more species of the fungi, but fortunately a large part of the losses can be prevented by proper management.

. In trees that have a clearly defined heartwood oak, ash, and most conifers, for example the heart rots are usually confined to the true heartwood. In many other hardwoods, normal heartwood forms irregularly, and decay of the inner sapwood is also called heart rot. The term "sap rot" is used for the decay of dead or dying sapwood.

When a fungus that is decaying the heartwood of a tree has developed for a number of years, it often produces a spore-bearing structure like a mushroom or a bracket-shaped conk. Each year one such structure can produce millions of tiny spores, which are carried about by air currents. When a spore comes to rest upon exposed wood and conditions are suitable, it germinates and sends fungus filaments into the wood. By means of these threads the fungus spreads through the tree, feeding upon and rotting the heartwood as it goes. Some fungi, which cause some of our common root and butt decays, rarely produce spores, but spread largely by growth through the soil.

Entrance points for rot fungi are usually provided by the exposure of heartwood when the trunk, top, limbs, or roots are wounded by fire, logging, or storms. Butt rot in sprout hardwoods usually is transmitted from the rotting stump to the attached sprout. Some of the most important heartwood destroyers gain entrance through branch stubs or branches killed by natural suppression.

THE HIGH DECAY CULL in many eastern hardwoods reflects mostly fire-scarring, ice damage, and abandonment of defective trees in past logging. Decay cull in most eastern softwoods and in the southern pines now has reached a small percentage because their cutting ages have been reduced. Improved timber management probably will keep the losses from decay at a low figure for those species.

The basic problem of timber management in the West now is to bring hitherto unmanaged forest land into maximum production. The two principal problem types are forest lands that have been cut-over or burned (on them new growth is inadequate) and stagnated virgin stands of overmature old-growth timber. Heart rots are involved in the management of both types. Through good forest practices, heart rots in future timber stands of the West may be kept at a minimum if the factors leading to heart rot are fully understood.

Decay factors affect silvicultural practices throughout the country in seven important ways: In the determination of the cutting age; in the system of harvest cutting; in the choice of trees to be cut in partial-cutting systems; in requiring special salvage cuts in timber burned or otherwise damaged; in managing mistletoe-damaged stands; in requiring the early treatment of hardwood stump sprouts; and in pruning and similar operations. Each is discussed here.

In most of our eastern and southern species, the age at which the trees will be cut (based upon the rate of return from the land) will be lower than the age at which decay ordinarily becomes a critical factor. This is true, for example, of the southern pines, white oak, yellow-poplar, sugar maple, and many other species. But in some species decay definitely limits the desired cutting age. In aspen in the Northeast and the Lake States, stands much older than 50 years are likely to be badly decayed. Decay should limit the cutting age of balsam fir to about 70 years. Most of the oaks will pass 150 years without major decay losses but decay cull usually results in the serious break-up of scarlet oak stands over 80 years old.

In the West, the thrifty, uninjured young forest trees are generally free from heart rots. After the virgin stands have been replaced by second growth, the most profitable cutting age occurs before heart rots become serious.

FREQUENT LIGHT CUTS in the large-crowned hardwoods result in a maximum of logging damage. Clear cutting in strips or blocks or adopting a minimum number of cuts per rotation consistent with good silviculture will cause the least logging injury and the lowest subsequent decay. Logging injuries provide good opportunities for the entrance of heart rot fungi. Careless felling and frequent cutting can cause considerable breaking of the tops and branches of residual trees. Wounds exposing only sapwood in resinous species often become covered with pitch so that fungi are largely excluded. Such wounds in nonresinous species, however, readily permit the establishment of sapwood fungi, and the subsequent checking and sloughing of the decayed sapwood exposes the heartwood beneath to heartwood destroyers.

Selective logging with heavy tractors often causes extensive wounding of residual trees unless special precaution is taken. All forms of damage, including branch and top breakage, felling scars, and butt injury from skidding and yarding, increase as the frequency of cutting in a given stand increases. Heavy partial cuts in old spruce and fir result in wind breakage to the remaining stand, because these old trees are commonly heavily butt-rotted. Under such conditions some form of clear cutting should be considered in place of partial cutting.

Where partial cuts are made, the forester always aims to retain the trees that are increasing the most in volume. He marks for cutting the heavily defective trees, particularly those that are losing more wood from decay than they are adding through growth. Aids are available for estimating internal defect from external signs in some eastern and western species. The timber marker who can estimate the decay situation in a given tree can greatly enhance the net growth in selection systems of silviculture by eliminating defective trees in the earliest cuts.

Heart rots in the overmature stands of the West present a major problem in forest management. Whether such stands are clear-cut or selectively cut, all highly defective trees should be cut whether they are merchantable or not; unless it is necessary to leave them for seed trees to restock the area. In some stands there are so many cull trees that the sound timber available will not pay for their cutting and still leave a profit for the operator. Even if all were felled, considerable damage would be done to young trees and other timber left standing on the area, new young growth would be obstructed, and a serious fire hazard would develop. If they are left standing, they occupy a large percentage of the area that should be taken over by vigorous young trees.

How to dispose of the obviously worthless trees under these circumstances is a challenging problem. This same problem arises in connection with large areas of high-graded timberland in both the East and West. On these areas only the best trees were removed, leaving a considerable stand of near-worthless timber. The systematic elimination of these trees, most of which are badly decayed, is now a prominent phase of the forest land-improvement operations in many sections of the country.