MINNESOTA FOREST TRENDS: Table 5 - 8

Table 5 shows trends in average stand age for each forest type across the 1977, 1990, and 2005 inventories.   Stand age is defined as the average total age (to the nearest year) of the trees in the predominant stand-size class.  For most forest types, stand age remains relatively constant across the inventory periods.  A few forest types peak in 1990 and then decline slightly.  Forest types such as aspen and oak-hickory, hold a near constant average age in 1990 and 2005 inventories.  Beyond Table 5, it is useful to look at the distribution of area across age classes to recognize how forest type area, volume, and habitat will play out in the coming decades.  These distributions are presented in Tables 9 and described further below.

Table 6 describes the area of each forest type from 1977 - 2005.  Several trends are evident including an increase in the area of hardwood forest types including oak-hickory, elm-ash-red maple, and maple-beech-birch.  Recall these forest types represent FIA recognized species conglomerates and might not include individual species in the type designation (e.g., beech is a scarce species in Minnesota).

Another trend to recognize in Table 6 is a dramatic decline in the balsam fir forest type area between 1990 and 2005.  Four factors might help explain this change.  These include the spruce budworm, preferential harvesting, aging, and forest type classification methods.  To consider these, first recall that FIA uses an algorithm to assign field plot forest type codes.  The algorithm is hierarchical divisive, starting by comparing the live tree stocking of conifer and hardwood, then continuing to refine the forest type designation into smaller subgroups.  In this framework, the forest type designation of the subplot will shift if balsam fir drops below a certain stocking level.  This does not mean that all balsam fir stems are absent on the subplot; rather, it means that the balsam fir no longer comprises the dominant stocking relative to other species on the subplot.

The spruce budworm and preferential selection in timber harvesting are probably the two main factors in reducing the stocking of balsam fir in the past decade.  The 1990s saw an increase in selection harvesting and particularly clearcutting with residuals.   Such practices preferentially removed merchantable size balsam fir stems and left less economically viable hardwood (primarily maple and birch) stems standing.  The loss of balsam fir to the spruce budworm and harvesting practices on the affected acres also caused a shift in the species composition in these stands to the maple and birch categories.  A third factor in losses of balsam fir acreage, notably in older forest stand conditions, is the short lifespan of this species.   The fourth factor that could contribute to the decrease in estimated balsam fir forest type acreage is the change in FIA sample plot design and forest typing algorithm between the 1990 and 2005 inventories.

Table 7 presents the estimated number of live stems statewide for selected forest tree species.  Between 1990 and 2005 nearly all species realized an increased number of stems.  Notably, there was a large increase in the number of aspen stems statewide, much of which comes from the less than 5-inch diameter class (from dense young stands of suckers that typically follow harvesting the parent stands).

Other trends apparent in Table 7 include an increase in the number of balsam fir stems.  Initially, this result appears to contradict the loss of the balsam fir forest type area; however, division into diameter classes shows the balsam fir stem recruitment occurred only in the smallest diameter class.   Corresponding to our explanation for loss of balsam fir forest type area, there is a substantial decrease in the balsam fir stems in the merchantable diameter classes.  Further, in spite of increases in oak-hickory, elm-ash-red maple, and maple-beech-birch forest type area, there is little if any increase in the numbers of stems of species that comprise these types.

Table 8 contrasts conifers and hardwood forest type area and number of stems between the 1990 and 2005 inventories.  This table is important from a wildlife habitat perspective because many species use conifers stands as thermal cover.  Although there has been a decrease in total conifer forest type area in the past decade, there is a slight increase in the number of conifer stems. Much of this increase occurred in the smallest diameter class.