Just as the forest trees are affected by climatic and biological factors and by man's activities, so are the tree seedlings, shrubs, ferns, and herbs-the understory vegetation.  Here too, the changes can be rapid or can occur slowly over a relatively long period of time.  And the changes can be determined only by recording repeated observations. 
    General observations on the Tionesta tract showed that the understory of hemlock and hobblebush declined between 1935 and 1942.  Beginning in 1942 and continuing through 1962, the occurrence of understory vegetation on sample plots was estimated from photographs taken at 5-year intervals.  In 1972, field observations were made of these same plots.  These data were used to determine frequency rates by species.  The frequency rate for any species is the percentage of sample plots in which a species occurs.  Data for selected species-those of high frequency and those that increased or decreased substantially-are reported here. 
    The frequency rates for ferns, beech, wood sorrel, birch, and sugar maple remained relatively stable for the 1942-72 period.  Violets, club-mosses, and hobblebush decreased in frequency during the period, while striped maple, red maple, and hemlock increased.  These frequency trends are shown in figure 11. 
    From these data, a few suppositions can be made about the past and the future forest.  It can be readily seen that ferns and beech have dominated the understory for at least the last 30 years.  The high frequency of beech was brought about partly by deer browsing.  Species more palatable than beech were heavily browsed: beech was left alone.  If this trend continues, the amount of beech will build up in the years ahead. 
    The decline of hobble bush began in 1935, and

Figure 11.- Trends in understory frequency rates by species and year.

this species disappeared from the sample plots by 1952.  Repeated heavy browsing was responsible for most of this decline.  And, as less hobblebush was available, other species were browsed more heavily. 
    Between 1952 and 1972, striped maple, red maple, and hemlock increased in frequency, most of the increases occurring in the last half of the period.  The reasons for these increases are not clear.  More seedlings of these species may have become established because of good seed years and favorable weather conditions.  Also, timber-harvesting around the Tionesta tract produced abundant browse at a time when the deer herd was below the peak levels of the 1930s.  Because of this, many seedlings may have escaped browsing.  If these same conditions persist, red maple and hemlock will become more abundant in the mature forest.  On the other hand, if dense clumps of striped maple develop, these clumps may prevent the regeneration of other tree species. 
    As both understory and overstory vegetation changes occur, habitats change.  This in turn may affect the animal population seeking food and shelter in an altered environment.  But the trend is always toward a balanced community in which plants and animals have adapted themselves to one another as well as to their environment.

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