Enjoying and protecting nature are cited as the top two ‘very important' reasons why private forest landowners in the UC ANR Forest Stewardship Workshops own forestland. Many of these landowners express a desire to have their forest return to an ‘old-growth' state. But, what exactly does this mean? UC Berkeley Silviculture Professor Emeritus, John Helms, says this depends on several things.

Some definitions focus on tree or stand age and size, some on whether the area has been previously harvested and some on the structure of the forest in place now. Defining old-growth trees and stands usually requires species-specific descriptions of many characteristics including age, size, crown form, and structure.

Tree age and size: Old growth, or “ancient forest” has been legally defined (CA State Legislature 2002) as a tree having a stem that existed in 1850 (California Statehood) or a forested area with a multistoried canopy, with at least six live trees per acre that existed before 1800 A.D. and are greater than 60 inches in diameter at stump height for Sierra and coast redwoods, and 48 inches in diameter at stump height for all other tree species.

Sometimes age, such as 150 years, has been suggested, although “old” for a species such as lodgepole pine with a lifespan of about 120 years is obviously different from “old” for redwood that may live for thousands of years.

Stand management history: According to the legislature, an ancient forest may be either:

(A) An un-entered forested area covering 40 or more acres with no evidence of commercial timber harvesting and no record of previous harvest activities, or

(B) An entered forested area covering 40 or more acres with previous entry for logging that provides essential habitat elements for old forest-related wildlife species.

Forest structure: Professor Helms also notes that old growth is considered a complex structure because it typically contains diverse age classes, a multi-layered canopy with an understory layer often made up of more shade tolerant trees, snags, and down logs. A forest could be regarded as old growth when it achieves this structure, regardless of age.

The textbook concept of how a forest stand develops is through these stages: stand initiation, stem exclusion, under-story re-initiation and old growth. These four stages are driven by the amount of growing space and resources available to the trees (light, nutrients and water). In stand initiation after a disturbance, there is lots of space and resources available to new seedlings. In stem exclusion, competition for space and resources is high due to the trees maturing, causing some trees to decline in health and die. There is no growing space available for new stems, thus the name ‘stem exclusion'. As trees compete and some die and fall, this opens up space and frees up resources for the remaining trees and for re-initiation of additional tree seedlings in the understory. Old growth is the climax stage with multiple age classes of trees, where theoretically a forest is in equilibrium in the absence of disturbance.

Daniels, Lori & Gray, Robert. (2006). Disturbance regimes in coastal British Columbia. BC Journal of Ecosystems and Management. 7.

However, this text book model may not really be the ideal for all forest types. The forest stand may not actually develop through all of these stages because of repeated disturbances. In fire-adapted forests, low severity fires regularly burn and kill new seedlings in many areas, greatly reducing the extent of forest in the understory re-initiation and old growth phase. Frequent fires favor the more fire and drought tolerant tree species such as pines and oaks over the shade tolerant species that would initiate in the understory such as white fir in the Sierra Nevada or Douglas-fir on the coast. Lack of frequent fires has led to development of more forest biomass than can be sustained over time in the face of increasingly long fire seasons, high temperatures and drought.

Developing old-growth on your property: How a forest develops on your property is a product of the climate, species, fire regime, timber harvest and soil conditions found there. It is also shaped by the management you choose to do. Private forest landowners who want to foster old growth forest conditions on their property will need to identify the current conditions and consider what definition of old growth they are managing for. Professor Helms notes that depending on a landowner's goals for their forestland, “old growth” may not be the stand development stage that will best support their goals.

For example, if after a stand replacing fire or timber harvest, your goal is to grow back trees in a fairly quick timeframe, then you will need to implement practices that protect and promote seedling growth. If you want to reintroduce trees to the land, you will need to establish them early and care for them by limiting shrub encroachment. Starting with seedlings as opposed to seed, reducing competition, and thinning trees to reduce competition is a more efficient way to development of old growth conditions than waiting for it to naturally develop through a stand initiation, stem exclusion and understory re-initiation phase.

Old growth in dry fire-adapted Sierra Nevada forests maybe be more clumpy and have more openings than traditional models suggest.

If your forestland is mostly overcrowded with young trees, you will probably want to thin them out rather than let them compete with each other until some die from competition stress. The classic old growth structure, with multiple layers of trees, could also be considered incongruent with a fire hazard reduction goal because the understory is also a fuel ladder that increases the risk for high severity fire. To reduce fire hazard you should consider removing trees that are ladder fuels and favoring larger fire tolerant trees rather than letting the classic old growth structure develop by allowing shade tolerant trees to grow up and into a multi-layered canopy.

For more information on forest ecology and tree growth and competition, please see Forest Stewardship Series 3: Forest Ecology and Forest Stewardship Series 5: Tree Growth and Competition.