Paramount to any forester or timberland owner is the question of how fast forests grow. In the United States, an acre of forest land grows approximately .59 cords of wood per year. However, there is great regional difference in these numbers, and much depends on the acreage itself. Let’s examine the factors that can affect forest growth rates.
Forest Growth Rates by State
Looking at a map of forest growth rates by state taken from FIA data, it is easy to see a clear regional pattern: the highest growth rates can be found in the southeast and Pacific northwest. Much of the regional differences come from the climate and length of growing seasons. In Maine, for example, the growing season lasts about 135 days. In Georgia, the growing season is 270 days–twice as long! Clearly, the extended growing season is going to have a profound effect on how fast forests grow.
Growth Rates Depend on Species
The growth rate of a forest can depend heavily on the species that grow there. In both the south and pacific northwest, forests are dominated by softwood. In general, softwoods like spruce and pine grow substantially faster than hardwoods like oak and maple, so in aggregate, these softwood-dominated forests are going to be more productive than forests in states dominated by hardwoods. However, these differences can be seen between acreages within a single state. It is certainly possible for northern states like Minnesota and Maine to grow as fast as 1 cord/acre/year in a nice stand of white pine, for example. On the other hand, the most productive sites in Mississippi could be producing far beyond that, and there are certainly poor quality swamps in Louisiana growing less than .25 cords/acre/year. Like any normal distribution, there will always be a great degree of variation within a group.
Frequent Harvests Increase How Fast Forests Grow
Growth rates are not entirely organic. Forestry practices themselves can have an incredible impact on how fast a forest grows. The growth rate of a tree is not linear. Instead, it follows a sigmoid function. Younger trees grow slowly, but gradually pick up pace until they reach middle age. As the tree grows older, growth tapers off. Thus, if a forest is older, it follows that growth will be slower. If a forest is younger, it will be faster. States and regions with more forest harvest activities will tend to have a balanced age structure, leading to higher growth rates.
This is particularly true where planting is a common practice. Trees are planted specifically to grow fast, often with selective breeding programs to aid the rapidity, so planting can really supercharge growth. Uncoincidentally, planting is most common in the southeast and Pacific northwest. Even without planting, however, forestry activities can have a dramatic impact on how fast a forest grows. Regenerative cuts such as shelterwood cutting can select for species and individuals with higher growth rates, increasing forest growth in aggregate. Pre-commercial thinning can also dramatically increase growth rates.
How Fast a Forest Grows is Highly Variable
While it is useful to look at regional data to devise a quick rule of thumb, the variation in growth rates between different stands of wood is large and can only be determined through careful measurement. In general though, a forest in the United States will grow between .5 and 1 cord of wood per acre per year.