Regeneration

Our forests here in the Northeast are very resilient.  When they are disturbed by cutting, fire, storm, infestation or disease, they bounce back rapidly.  Existing seedlings (called advance regeneration) will take advantage of the increased light, water and nutrients.  Others will grow from the damaged trees' stumps and roots.  More seedlings will develop from seeds stored in the leaf duff on the forest floor, and from seed that blows in from nearby trees.  Yet other seeds will be carried into disturbed areas by birds and animals. 

Different tree species have developed different regeneration strategies over millions of years of evolution.  Some trees produce many, small seeds and depend upon the wind for their dispersal.  Other trees produce a few, large seeds and depend upon animals for their dispersal.  Some tree seeds need little light or heat in order to germination; others need much heat and light.  Some sprout from stumps and roots; others do not.

If there are seedlings of desirable species already established when a forest is cut or otherwise disturbed, a new, high quality forest can develop immediately.  Many of our best timber stands developed in this way after heavy cutting in the past.  For whatever reasons, conditions were right for germination and growth of  those seedlings and sprouts before the overstories were harvested. 

Unfortunately however, the opposite is more often the case.  Desirable seedlings were not there before the last harvest; the right kinds of seeds didn't blow in or get carried in, and they didn't sprout from stumps and roots.   The same usually happened when old pastures or fields were abandoned.  Good quality seed sources weren't available, or if they were, the seeds didn't germinate and grow.

When forests are repeatedly high-graded, that is, when the best trees are cut and the rest are left to grow, the results are forests dominated by low-grade species, poor quality trees of grade species, and cull trees of all species.  Such forests would probably have been better off if they had been clearcut because then at least the poor quality trees would have been removed.

In order to control and optimize regeneration, foresters have developed silvicultural systems to regenerate forests of different species mixes and ages.  These systems are designed around the regeneration strategies of the desired species.  They involve manipulation (cutting) of forest stands to influence the seed mix and light/heat conditions on the ground.  Sometimes they also involve manipulation of existing (advance) regeneration and treatment of the seed bed (forest floor).

SPECIES	SEEDS	LIGHT & HEAT	FREQUENCY	DISPERSAL
Oaks	heavy	high	3-5	Squirrels
White Pine	light	moderate	2-4	Wind
Ash	light-medium	high	3-5	Wind
Cherry	heavy	high	1-2	Birds
Sugar Maple	light-medium	low	1-2	Wind
Black Birch	light	moderate	1-2	Wind
Beech	medium	low	2-4	Suckers
Hemlock	light	low	1-2	Wind

There has been much discussion and debate among foresters and forest scientists about the relative merits of even-aged and uneven-aged silvicultural systems.  Even-aged systems try to develop the next forest of seedlings by one or two partial overstory cuts, and then remove the rest of the overstory in a final cut.  Uneven-aged systems periodically remove part of the overstory, but never all of it.

Even-aged systems are recommended for use with shade-intolerant species such as oak, cherry and pine, and are described as shelterwood systems.  The first one or two cuts leave good seed trees and the right amount of light on the ground for their regeneration.  The final result is an even-aged stand of seedlings that grows rapidly into a new stand. 

Uneven-aged systems are recommended for shade-tolerant species such as sugar maple, beech and hemlock, and are described as selection systems.  Ideally, these systems have a broad range of ages and sizes.  In reality, they will usually have several age classes determined by the frequency of cutting in the stands.