Old growth forests are natural stands that have developed over a period of time, generally at least 120 years, without experiencing a stand-replacing disturbance. Typical traits of old growth in Wisconsin include:  Advanced stand age  Diversity of tree sizes from saplings to large sawtimber  Presence of very old and large trees  Presence of large, standing dead trees  Increased amounts of dead stumps and coarse woody debris  Pit and mound topography and presence of canopy gaps

Several of these characteristics are measured in the Forest Inventory and Analysis database and can be used to identify forests with old growth characteristics. In this report, we present one methodology for estimating old growth acreage across several inventories.

 Advanced stand age  Diversity of tree sizes from sapling to large sawtimber  Presence of very old and large trees  Presence of large standing dead trees  Increased amounts of dead stumps and coarse woody debris  An estimation of “old growth” acres in Wisconsin  “Old growth” compared to “old stands”

Division of Forestry WI Dept of Natural Resources 8/2014

Advanced Stand Age

Area of timberland by stand age class 1983 1996 2004 2013 5 W isconsin’s forests are aging as almost half of all timberland is over 60 years of age (Figure 1). The 4 amount of very old forest (over 120 years) is far less than 3 what it was in 1983, but has increased 60% in the last 2

acres Million decade. 1 n 1983, over 4% of timberland was at least 120 years of I - age, and 80% of this was in the oak / hickory or maple / 0-20 21-40 41-60 61-80 81-100 101-120 121+ beech / birch forest types (Figure 2). Thirty years later, in Stand age class (yrs) 2013, only 1.2% of timberland is this old and less than Figure 1. Area of timberland by stand age class (Forest Inventory and Analysis). 50% is in either oak / hickory or maple / beech / birch type. Area in timberland over 120 yrs of age 1983 1996 2004 2013 Forest Inventory and Analysis (FIA) approximates stand 350 age by weighting the cored age of several canopy trees of 300 differing sizes by the proportion of that size class in the 250 canopy. This procedure may underestimate the age of 200 very old stands that have trees in many size classes. 150 100 In this analysis, we will see that many stands with old acres Thousand 50 growth characteristics do not have an advanced stand 0 Oak / hickory White / red / Maple / beech / Spruce / fir Elm / ash / age in the FIA inventory and many stands with advanced jack pine birch cottonwood ages do not have typical features of old growth. Figure 2. Acreage of timberland over 120 years old by forest type group (forest Inventory and Analysis).

Diversity of tree sizes from sapling to large sawtimber

Diameter distribution and diversity index: pine forest type As a forest ages, large trees die and fall over. They 200 0-40 yrs (Index: 1.23) open canopy gaps for saplings and poles to fill in and 41-80 yrs (Index: 1.85)

150 the stand becomes a mix of trees of various sizes and 81-120 yrs (Index: 2.05) ages creating a layered canopy. >120 yr (Index: 2.11) 100 In the FIA database, tree ages are not measured but trees peracre trees tree diameters are and in general correlate well with 50 tree age. One measure of the evenness of distribution of tree diameters is the Shannon’s index of diversity*. 0 5-6.9 7-8.9 9-10.9 11-12.9 13-14.9 15-16.9 17-18.9 19-20.9 21.0+ The index increases when the number of trees in each Daimeter class (in) diameter class up to 21+ inches is more evenly Figure 3. The relationship between diameter distribution and diversity index (Forest Inventory and Analysis, 2013). distributed. As stands age, the distribution becomes flatter and the value of the diversity index increases Average diversity of canopy (Figure 3). 1.5 As mentioned, the diversity index increases with age 1.3 (Figure 4). Between 1996 and 2013, however, the 1.0 average value of the index decreased in all age classes. 0.8 1996 2013 The forest types with the highest average diversity 0.5 index are maple/beech/birch, oak/hickory and white 0.3 /red/jack pine. These types have higher numbers of - large diameter trees in old stands. 0-20 yrs 21-40 yrs 41-60 yrs 61-80 yrs 81-100 yrs >100 yrs

Figure 4. Average value of Shannon’s index of diversity by stand age, 1996 and 2013 (Forest Inventory and Analysis). *For an explanation see: http://en.wikipedia.org/wiki/Diversity_index

Presence of very old and large trees

sawtimber

Number of large trees by diameter class 80 Large and very old trees not only provide structural 70 22 60 diversity and a layered canopy but also genetic 20 50 16 21.0+ inches diversity. Trees that have survived challenges from 18 19 40 16 abiotic forces, disease and insects over decades may 14 19.0-20.9 inches 30 12

harbor important genes for survival. trees Million 17.0-18.9 inches 20 32 35 10 23 27 The number of large sawtimber trees has increased 0 steadily since 1996, 50% for trees 17 to 20.9 inches 1996 2004 2009 2013 in diameter and 23% for trees 21 inches and greater Figure 5. Trends in the number of trees greater than 17 inches (Forest Inventory and Analysis). (Figure 5).

The density of trees (trees per acre) over 21 inches Density of trees over 21 inches dbh by stand age in diameter increases significantly with stand age 50 0-80 yrs 81-100 yrs 101-120 yrs >120 yrs (Figure 6). 40

The density of these large trees in stands younger 30 than 80 years is very low. Over 50% of all large 20 trees are in younger stands (<80 yrs) but at a very low density. The density of large trees is 8 times 10 higher in stands over 120 years of age compared to 0 White / red / Oak / Maple / Elm / ash / Aspen / Spruce / fir All types forests younger than 80 years old. jack pine hickory beech / cottonwood birch birch

Figure 6. The density of large trees (over 21 inches dbh) by forest type group (Forest Inventory and Analysis).

Presence of large standing dead trees

Density of standing dead trees over 17 inches 2.50 1.9

2.00 1.7 Large standing dead trees or snags provide 2.0 1.50 vital nesting, foraging and denning sites for 1.0 1996 many species of forest birds and mammals. 1.00 0.7 2013 Trees per acreper Trees 1.0 0.50 The density of large standing dead trees (over 0.8 0.5 17 inches dbh) increases dramatically after a 0.00 0-20 21-40 41-60 61-80 81-100 101-120 >120 stand age of 100 years (Figure 7). Snag Stand age (yrs) density decreased between 1996 and 2013 for Figure 7. The density of large standing dead trees (Forest Inventory and Analysis, 2013). stands less than 120 years of age.

The highest density of large standing dead Density of standing dead trees in stands over 120 yrs old trees occurs on the white / red / jack pine types which also has the highest density of 4.0

large living trees. 0.7 21+ inches 3.0 17-21 inches The major species of large snags in the 0.7 2.0 0.6 database were red oaks and eastern white 2.7 Trees per acreper Trees 1.7 1.6 pine. 1.0 0.4 0.4 0.0 Maple / beech / Oak / hickory White / red / All types birch group group jack pine group

Figure 8. The density of large standing dead trees in stands over 120 yrs old by forest type group (Forest Inventory and Analysis, 2013). .

Increased amounts of dead stumps and coarse woody debris

Carbon density in dead wood and litter

20 Without intervention as stands become older they

accumulate greater amounts of carbon in non-living 15 Litter material such as coarse woody debris (CWD), 10 Standing dead trees

standing dead trees, stumps and litter. These Stumps & CWD components are a major contributor to old growth 5

acreper tonsshort ecosystems not only serving as a reservoir of stored 0 carbon but also supporting a diversity of species. 0 - 20 21 - 41 - 61 - 81 - 101 - 121 - 141 - 161 - 181+ 40 60 80 100 120 140 160 180

Carbon in dead material accumulates steadily with Stand age class (yrs)

age and then levels off after about stand age 140 Figure 9. The density of carbon in coarse woody debris, stumps and litter (Forest Inventory and Analysis, 2013). due to a decrease in litter density in older stands (Figure 9). Carbon, however, continues to Carbon by component in stands over 120 yrs 30 accumulate in stumps, coarse woody debris and

25 standing dead trees past 180 years (sampling error is extremely high as there are only five stands in the 20 database older than 180 years). 15 Litter 10 Standing dead The forest type group which has the greatest

Short tons per acreper tonsShort 5 CWD density of carbon is spruce / fir due to the large 0 amount of litter (Figure 10). The white / red / jack Spruce / fir Maple / White / red / Oak / Elm / ash / All forest beech / jack pine hickory cottonwood types pine type has the highest density of carbon in birch coarse woody debris and stumps. Figure 10. The density of carbon by forest type in stands over 120 years of age (Forest Inventory and Analysis, 2013).

An estimation of “old growth” acres in Wisconsin

In order to define old growth forests, stand age alone is not very helpful since many very old stands have been harvested or disturbed multiple times. For this reason “old growth” status is not solely a function of stand age. A given stand must meet several criteria such as the presence of many large trees (at least 21 inches dbh) plus a few very large trees (at least 30 inches dbh), the presence of large standing dead trees (at least 17 inches dbh), a diversity of canopy tree sizes ranging from sapling to large sawtimber (high diversity index) and a large amount of coarse woody debris.

In order to approximate the acreage of forest having these old growth characteristics, in this analysis we used the average value from the FIA database for all stands over 120 years of age as a minimum criterion for these five features. By summing the acreage of stands which met four of the five Forested acres meeting old growth criteria criteria, we were able to estimate the acreage in old 90 growth as follows: 60,330 acres in 1996, 60,811 in 2004 80 and 77,205 acres in 2013 (Figure 11). Interestingly, 40% 70 of old growth acreage is located in southern Wisconsin. 60 White / red / jack pine Whereas only 2% of all forest in the state is on Native 50 Oak / hickory American land, 15% of old growth acreage is in 40 Menominee County, presumably on reservation land. Maple / beech / birch

Thousand acres Thousand 30 Elm / ash / cottonwood This is only an estimate based on undocumented 20 10 criteria but the estimate for 1996, 60,330 acres, is very - close to the approximation of 57,000 acres by Frelich 1996 2004 2013 and Reich (1996*). Based on this analysis, old growth forests, as opposed to old forests may actually have Figure 11. An estimation of forested acreage meeting old growth criteria. increased since 1996. One of the reasons is the large increase in trees over 17 inches (Figure 5).

* Frelich, Lee and Reich, Peter. 1996. Eastern Old Growth Forests: Prospects for Rediscovery and Recovery. Island Press. Chapter 11 pg 24.

Old growth compared to old stands

The charts on this page (Figure 12) show the difference between old stands (at least 120 years old in the FIA inventory) and “old growth” as estimated in this analysis. The number of large trees is 2 ½ times higher on old growth. The number of very large trees is over five times higher and the number of large snags is almost three times higher on old growth stands compared to stands which are over 120 years old. Many of these old growth stands are not listed as having an advanced stand age in the FIA inventory: almost 50% of “old growth” stands have a designated stand age of less than 100 years.

Number of trees per acre by diameter class Canopy diversity Old Growth Stands >120 yrs Old Growth Stands >120 yrs 50 1.8 1.5 1.5

40 1.2 1.2 30 0.9 20 0.6 Trees per acre per Trees 10 0.3

- 0.0 9-12.9 13-16.9 17-20.9 21-24.9 25-29.9 >30 in Maple / beech / Oak / hickory White / red / jack All types Diameter class (in) birch pine

Dead standing trees over 17 inches Carbon in stumps and coarse woody debris Old Growth Stands >120 yrs Old Growth Stands >120 yrs

8.0 9 6.6 6.0 5.5 6 4.4 4.0 1.9 3 Short tons per acre per tons Short

Snags per acre per Snags 2.0

0.0 0 Maple / beech / birch Oak / hickory White / red / jack All types Maple / beech / birch Oak / hickory White / red / jack All types pine pine

Figure 12. Diameter distribution (top left), canopy diversity (top right), density of standing dead trees (bottom left) and carbon density in CWD (bottom right) on estimated old growth vs. stands which are over 120 years of age in the FIA database.