Are there many big trees in Oregon?

 

Mike Cloughesy

By Mike Cloughesy 

Because I’m a forester, people often ask me: “Are there very many big trees in Oregon, and are they protected?” That’s a reasonable question, and it reflects genuine public concern about harvesting trees for wood products. And I had a gut feeling about the answer, but frankly, I didn’t know it.

Fortunately, this is just the sort of question the LEMMA team was set up to answer. LEMMA is the Landscape Ecology, Modeling, Mapping and Analysis team, which is a collaborative effort of the Pacific Northwest Research Station of the US Forest Service and the OSU Department of Forest Ecosystems and Society.

The LEMMA team uses data from regional inventory plots, satellite imagery and other GIS layers to create Gradient Nearest Neighbor (GNN) maps. One of the map layers that was created and included in the Oregon Department of Forestry’s Forestry Map Atlas is Oregon Forestland Tree Size Classes.

For more information on this LEMMA project, visit the website.

 

Table 1: Area and Percent of Oregon Forestland by Average Tree Size Class

ClassSize ClassAverage Tree SizeAcreagePercent

1Shrub/seeding<1" dbh964,1963.1%

2Sapling/pole1-10" dbh10,418,67833.6%

3Small tree10-15" dbh9,543,41430.7%

4Medium tree15-20" dbh5,057,48016.3%

5Large tree20-30" dbh3,809,68712.3%

6Giant tree>30" dbh1,244,2954.0%

 TOTAL 31,037,750100.0%

dbh=diameter at breast height    

 

Table 1 and Figure 1 show the area and percent of Oregon forestland by tree size class in 2000. The tree size class acreages indicate the areas with trees whose average diameter is in a certain size class. Thus, Class 5 represents areas where the average dbh (diameter at breast height) of trees is between 20 and 30 inches.

 

Figure 1: Percent of Oregon Forestland by Average Tree Size Class

In Figure 1, the combined Classes 1 and 2, the combined Classes 4, 5 and 6, and Class 3 each represent about one-third of the forestland in Oregon. As trees grow over time, they will move into the next size class. As areas are harvested or burned in wildfires, they start over at Class 1. I believe that having one-third of our forest acres in shrubs, seedlings, saplings and poles (Classes 1 and 2), one-third in small trees (Class 3), and one-third in medium, large and giant trees (Classes 4-6) provides a sustainable cycle so we will have a continual supply of “big trees.”

Large trees

If we consider “big” trees to be those stands with average diameter of 20-inches-plus, the data in Table 1 shows that we have slightly more than 5 million acres of forest with an average diameter that would classify it as big trees, which total about 16 percent of Oregon’s forestland.

There are many “big trees” out there not being counted in this analysis. What we have presented is acres where the average tree size is above a certain minimum, rather than any counts of big trees per se. Areas with a few large trees per acre and hundreds of much smaller trees will not be counted as “big trees” in this analysis.

Map 1 shows areas of trees with average diameter of 20 inches and larger in Oregon. The map shows that big trees are found throughout the forested areas of the state, but are most common and concentrated in the Cascade Mountains and the Coast Range.

Map 1: Large Trees – Average dbh of 20”+

Because of the human eye’s resolution limitations and the scale of Map 1, the red 30m pixels of the “big trees” bleed into each other, creating the illusion of large homogeneous areas of big trees – where in reality it is a patchwork of big trees. Map 2, a zoom of the large trees in the McKenzie River area near Belknap Springs, shows an area of Lane County zoomed in to show that in many areas, the sea of red is really a patchwork of many red areas of big trees, mixed with other size classes.

Map 2: Zoom of Large Trees in the McKenzie River Area (about 160 square miles)

Reserved forest areas

In terms of protection for the big trees, people generally think of trees being protected if they are in some sort of a reserve, such as a National Park, Wilderness Area or Late Successional Reserve, which is an area administratively withdrawn for wildlife habitat purposes.

Map 3 shows the location of Reserved Forest Areas in Oregon. These reserve areas are predominantly found on federal land. Similar to the big trees, they are most common and concentrated in the Cascade Mountains and the Coast Range. Each of these types of Reserved Forest Areas is a bit different. However, they all have in common that little or no timber harvest is allowed within their boundaries. Big trees in Reserved Forest Areas are unlikely to be harvested, but they are not immune to threats such as wildfire, insects and diseases, or wind.

Map 3: Reserved Forest Areas

 
Large trees and reserved forest areas

Map 4 shows the relationship of big trees and Reserved Forest Areas. There is quite a bit of overlap. Big trees growing in Reserved Forest Areas are shown in green. There are also millions of acres of big trees growing on non-Reserved Forest Areas, shown in blue.

Map 4: Large Trees and Reserved Forest Area

Table 2 shows that Reserved Forest Areas total more than 8.3 million acres, or about 27 percent of Oregon forestland. These reserves contain about 2.6 million acres, or about 52 percent, of the large-tree acres.

So the answer to our question, “Are there many big trees in Oregon, and are they protected?” is that there are more than 5 million acres of big trees in Oregon, and about 2.6 million acres of these are growing on permanently protected forestland.

Table 2: Oregon Forested Area - Reserved Forest Areas and Large Trees

Reserve ClassAcres in 
Large TreesPercent of 
Large TreesAcres in 
Medium TreesForested AcresPercent of Forest

Adaptive Management Areas167,3923.3%78,615524,9281.7%

Administratively Withdrawn140,8132.8%96,286525,3441.7%

National Parks, Monuments & Wildlife Refuges56,0191.1%41,527303,3101.0%

Late Successional Reserves1,393,58427.6%549,0963,525,94311.4%

Wilderness Areas519,53710.3%435,5991,998,5406.4%

Key Watersheds outside other Reserves368,9597.3%216,1821,502,2914.8%

Total Reserved Areas2,646,30352.4%1,417,3048,380,35727.0%

      

Total Non-Reserved Areas2,407,67947.6%3,640,17622,657,39373.0%

Total Area5,053,982100.0%5,057,48031,037,750100.0%

 

Figure 2a: Large Trees in Reserved and Non-reserved Forest Areas 

The acreage of large trees in reserved and non-reserved forest areas can be looked at graphically in a couple different ways. Figure 2a shows a tree “cookie” representing all 31 million acres of forestland in Oregon. The blue wedge represents the 8.4 million acres of Reserved Forest Areas. The brown wedge represents the 22.6 million acres of non-reserved forestland. Overlain on the tree cookie is the yellow wedge representing the more than 5 million acres of large trees in Oregon. The green portion of this wedge, where yellow overlays blue, represents the 2.6 million acres of large trees in Reserved Forest Areas. The plain yellow portion of the wedge represents the 2.4 million acres of large trees on non-reserved forestland.

Figure 2b: Large Trees in Reserved and Non-reserved Forest Areas

Figure 2b shows the 31 million acres of forestland in Oregon as arrayed along a ruler. The green area on the left represents the 22.6 million acres of non-reserved forest, while the blue area to the right represents the 8.4 million acres of Reserved Forest Areas. The 5 million acres of large trees is shown as a purple band that crosses the blue-green boundary, with 2.4 million acres of non-reserved large trees shown in green with purple overlay and 2.6 million acres of reserved large trees shown in blue with purple overlay.

Table 2 also shows that there are nearly 1.5 million acres of medium trees in our reserves. These stands of trees with average diameters of 15 to 20 inches are growing and will become big trees in a few decades or less. At a radial growth rate of six rings per inch, which is common in western Oregon, a 10-inch-diameter tree would become a 20-inch-diameter tree in 30 years.

Conclusion

We began this article asking: “Are there many big trees in Oregon, and are they protected?”

After examining LEMMA data and maps on tree size classes and data on Reserved Forest Areas, we can make the following three observations:

• There are lots of big trees greater than 20 inches in diameter in Oregon.
• About half of these big trees are protected in Reserved Forest Areas. 
• Over time there may potentially be many more big trees as medium trees in Reserved Forest Areas become large trees.

Further, fleshing out three observations by adding information suggests that:

• There are more than 5 million acres of large trees more than 20 inches in diameter in Oregon’s forestlands.
• More than one half, about 2.6 million acres, is in Reserved Forest Areas. These trees are unlikely to be harvested. However, a large percentage of this area is disturbed by wildfire annually. Recent studies by LEMMA of change in older forest suggest a slight net loss of older forest across the area of the Northwest Forest Plan, due to wildfire. 
• A bit less than half, about 2.4 million acres, is in non-reserved areas. These trees are subject to harvest, but with the fairly stable harvest levels we have observed in the past 25 years on private land where most of these non-reserved large trees are, harvest of large trees will be offset by natural growth of medium trees into the large-tree class.
• There are about 1.4 million acres of medium-size trees in Reserved Forest Areas, and many of these are destined to grow into large trees in the next few decades.
• Over time, Oregon has the potential to increase from about 5 million acres of large trees to nearly 6.5 million acres of large trees.

Caveats

The LEMMA data that formed the basis of this analysis were very useful. However, like any data set, there are caveats that must be understood:

• The data presented in this paper are based on models and maps that may contain many sources of error. However, they are the best estimates we have.
• A different definition of “big trees” would give us a different answer. For example, if we identified only trees of greater than 30 inches in diameter as “big trees,” we would have 1.2 million acres, or about 4 percent of Oregon’s forest as “big trees.”
• The tree size variable we used in our analysis is based on average tree diameters. There are many “big trees” out there not being counted in this analysis. What we have presented is acres where the average tree size is above a certain minimum, rather than any counts of big trees per se. 
• Finally, “big trees” are not the same as “old growth.” Old growth forests have large trees, but also include snags and down logs, a variety of tree sizes, and patchiness in tree sizes. Some of our areas of large trees would be considered old growth, but not all. The data don’t give us that information. However, identifying areas of big trees is an important step in looking at our forest diversity.

Prepared by: Mike Cloughesy, Director of Forestry, Oregon Forest Resources Institute

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Notes:

Thanks to Andy Herstrom of the Oregon Department of Forestry, Forest Resources Planning shop, for developing Maps 1-4 and for providing other data for this article. 

Thanks to Jordan Benner of OFRI for developing the graphical images that became Figures 2a and 2b.

Thanks to Janet Ohmann, Andy Herstrom, Jordan Benner and Paul Barnum for reviewing drafts and helping improve this article.