Biodiversity decreases in old growth forests for a variety of reasons, but first what exactly is an “old growth forest”?
An aging forest is arguably composed of trees that are over 100 to 120 years old. However, “The Index of Old Growth” describes several roadblocks to finding the elusive universal definition,
“The attempt to define old growth is a mire of different viewpoints, perspectives, motivations, terminology. A simple all-encompassing definition doesn’t exist as the characteristics of an old growth forest varies from locality to locality and from forest type to forest type.”
Some forests are dominated by variegated trees that have different lifespans. This creates uncertainties as to the age of the entire tract, as one tree species or another fights to complete the life cycle.
However “age” is defined, the reasons for decreasing biodiversity in old growth forest include fire, illegal timber operations, large tracts of closely aged trees that naturally die off at the same time, and disease. Also, the introduction of alien plant, insect, reptile, bird and animal species will decrease biodiversity if there are no natural predators to keep the newcomers in check. In the end, as the trees die, the habitats that support all other life will be affected.
The quickest way for biodiversity to drop to zero is through improper timber harvesting, natural disasters like the Mount St Helen eruption, and fires that cannot be reached for containment and control.
Forests that have been allowed to build up excessive biomass from fallen trees or pine needles, for example, are susceptible to catastrophic fires that denude and sterilize the land. Pounding rain deconstructs and erodes the fertile soil. Illegal timber operations are also quick to remove the canopy that protects the delicate soil from pounding rain, encouraging soil erosion and desertification. In these cases, it can take a long time for the conditions to be right, if they are ever right again, for a biodiverse community to re-form.
Some natural die-offs are good for biodiversity. Younger trees are able to get the light that the older trees were blocking. This creates opportunities for lower lying plants to get light and for a new canopy to form.
The denser and older forest canopies can curtail biodiversity by cutting off light. Young trees, grasses, ferns, shrubs and hosts of other ground plants struggle to survive. As the lower level plants struggle to survive, so do the habitats that allow microbes, birds, insects and animals to thrive.
Riparian forest zones have the biodiversity that a healthy creek, stream, pond, bog, marsh or other waterway will provide. If the aging forest dies out, burns, or is harvested, then the health of the waterway will be affected by soil runoff or other factors. This disturbs the natural balance of oxygen that aquatic plants and animals need. Pollution and dirt reduces the light levels that aquatic plants need. Tree falls and solid obstructions destroy existing habitats for all kinds of life forms. The heat from catastrophic fires can sterilize the water of all life.
The Council of Europe provides a comprehensive discussion of many aspects of global forest management. The council expressed a specific concern about managed forests,
“A large proportion of the forests planted in Europe are monocultures with little biodiversity. The Assembly regrets that a substantial proportion of these plantations comprise alien species.”
Thus, even carefully managed forests can lead to decreased, increased and changing biodiversity.
In summary, the death of trees in a forest takes away the habitats of far more life forms than we are aware of. Whether “biodiversity” refers to life forms that are invisible to the naked eye, or biodiversity refers to large and impressive mammals, there is more to a forest than the trees.
