6 oak tree rings

photo courtesy web.utk.edu

(by Jeremy Caggiano, science & technology committee chair)

According to Forest Inventory and Analysis (FIA) data, it is estimated that over 50 percent of New Jersey’s forests are between 60 and 99 years old. Since establishment, many hardwood forests have received no silvicultural treatment in heavily populated areas of New Jersey’s Bergen County. As foresters know, competition for growing space and limited site resources begins at roughly 60 percent of full stocking in mixed oak forests. Initially, a certain level of tree competition can be positive and encourages rapid growth while suppressing undesirable growing stocks. Under a “no management” scenario, by the time mixed oak forests are 60-99 years of age, they have reached reproductive maturity and have been competing for light, water and nutrients for many years. Therefore not only have the trees slowed in growth but, through light extinction, a percentage of suppressed, intermediate and some co-dominate trees are actively dying. Additionally, stem excluded mixed oak forests, as described above, contain less than the desired quantity of natural regeneration and, on certain sites, an herbaceous layer with poor vigor that doesn’t flower or fruit.

Within Bergen County’s forestland-urban areas, overstocking is often further exasperated by herbivory from white-tailed deer, fire suppression, insects and disease, as well as established and emerging exotic species. When foresters are called upon to mitigate these problematic conditions, one of the first calculations they need to make is the determination of maximum sustained yield (MSY). Simply put MSY is the mathematical calculation of forest in-growth, in terms of volume, over a period of time. MSY is an essential tool, which provides the forester a conceptualization of size and intensity, for which proposed silvicultural treatment(s), aimed at reversing these conditions, can occur in a sustained fashion.

FIA data is an excellent, broad-scaled information resource for many applications, however, when extrapolated it lacks consideration for environmental variables, stressors and the silvicultural appropriateness of certain harvesting plans. The FIA dataset, examined in this case, provides annual growth rates from 2011 and 2012 in Bergen County. Equipped with this information one should reasonably and proportionally be able to calculate MSY for a single property or a combination of properties as illustrated in the sample below:

  • Bergen County 2011 Volume = 44,118,358 cu/ft
  • Bergen County 2012 Volume = 47,118,985 cu/ft
  • 47,118,985 – 44,118,358 = 3,000,627 cu/ft of in-growth in one year
  • 3,000,627 / 44,118,358 = 0.068013 * 100 = 6.8013% in-growth %
  • Total Volume of Samples During Inventory Year = 425,799.2 cu/ft
  • 425,799 * 0.068013 = 28,959 cu/ft
  • 28,959/90 = 322 In-growth Cords
  • Average Sample Size = 150 acres
  • 322/150 = 1 Cords/Acre

It is easy to see how FIA datasets can quickly illustrate a pattern of slow but steady growth for these two forest owners. Unfortunately, many land managers and even third party forest certification entities accept this as a viable means of calculating MSY. More concerning is that applying FIA data to on-the-ground forest management may paint a fundamentally incorrect picture of true forest conditions. These same two properties were inventoried using identical and statistically sound protocols and post processed with US Forest Service’s NED 2 – Forest Ecosystem Decision Support software. The modeling, more comprehensively, took into account the forest’s overstocking and therefore unbalanced size and age class distributions, current lack of regeneration and area occupied by exotic vegetation. The following NED 2 table illustrates the average distribution of basal area, in four basic size classes:

Size class Balanced range Actual % area Difference
Regeneration 5 to 10 0.00 -5.00
Saplings + Poles 35 to 45 0.00 -35.00
Small sawtimber 25 to 35 16.00 -9.00
Large sawtimber 10 to 15 84.00 +69.00

Ironically, NED 2 depicted an average estimated 253 cords of volume loss per year or roughly 1.7 cords per acre. In the case of these two properties, due to stem excluded conditions, overstory trees are self thinning. Under a “no treatment” scenario, as these trees die singly or in small groups, due to their shade intolerant nature, there isn’t adequate regeneration to take their place. Consequently, the stages of succession will be reset and these sites, as well as others like them in the surrounding area, will continue to become predisposed to undesirable or exotic vegetation.

FIA data is an important tool, however if misused by land resource managers or policy makers, it has the potential to be very dangerous. Foresters should be cautious with its applications and realize mathematically extrapolated MSY calculations don’t take into account all the external variables individual forests are incur. Finally, it is important to note that our oak and mixed oak forests have specific degrees of shade tolerance. Only certain silvicultural systems, executed properly, will be rewarded with the natural regeneration required to ensure these forest types and the wildlife that rely on them will be here for generations to come.