Why Mature Forests are Considered Important in Protection Strategies
Old-growth forests, a subset of primary forests (forests of any age with no industrial logging or development), are rare in the US with most old growth concentrated on the Tongass rainforest in Alaska and other areas scattered in pockets in the lower 48 states. Logging of old-growth forests began with European colonization centuries ago and expanded westward following railroads and development. Logging on federal lands ramped up during and after WWII as single tree log trucks took out most of the largest trees. Thus, most (~95%) of old-growth forests in the lower 48 states were replaced by agriculture, tree plantations, and developments lacking in biodiversity and ecosystem services.
Remaining old-growth forests are primarily on federal lands, mostly managed by the US Forest Service (92%) with additional smaller but important areas managed by the Bureau of Land Management (BLM, 9%) and National Park Service (3%) (some mapping errors exist in the datasets). There are also old-growth forests on state and private lands but less than on federal lands.
In some regions, forests are maturing from prior logging (e.g., Appalachia, northeast, Great Lakes) and are within years-decades of having many old growth features. These forests are considered mature and if allowed to continue to grow – by protecting them from logging – old-growth characteristics will fully emerge. The following are important characteristics of mature forests where old-growth characteristics are emerging:
Most dominant/co-dominant trees reach the culmination of mean annual increment (CMAI or Age of Biological Maturity) at 60-80 years depending on site conditions – foresters use CMAI to know when to log a forest for profit. CMAI means that the annual growth rate of trees slows down as trees age – therefore, they are mature.
Structure– large trees, vertical/horizontal layering, snags/logs, canopy branching begin to show up in mature forests at about 80 years or so across most forest types
Processes – gap phase dynamics, predator-prey food webs, pollination, nutrient cycling are at an advanced stage at this age
Functions - climate buffering, water regulation, ecosystem services are all the more advanced as forests mature in 80 years or so
Setting the protection (from logging) threshold >80 years is reasonable for a national standard for all federal mature forests– the Northwest Forest Plan in the Pacific Northwest is an example of mature forest defined at 80 years when the emergence of structure, processes, functions are approaching old growth characteristics.
Age is but one of the diagnostic features for determining forest maturation and not all studies or field methods use age of the dominant/co-dominant trees. For our study, we used three indicators of structural development leading to mature/old growth: (1) tree height; (2) canopy cover; and (3) above ground living biomass. Forests were considered advanced structurally – i.e., mature – when their combined scores for these 3 indicators were highest relative to their surroundings using 30-m pixel mapping. Because we did not distinguish mature from old growth, those structural classes can be considered combined in the mature forest metric.