What if I told you that the forests you see in our mountains and across our landscape now may be wildly different in the next 100 years? Ok sure, 100 years is a long time, but some of our trees can live longer than 300+ years. So why would forests change in 100 years if the trees can live much, much longer? The answer is the natural ecological processes known as forest disturbance and succession, together making up forest dynamics. While one could talk for days and weeks on this subject, hopefully we can provide you with a decent enough summary so you can observe the forest dynamics next time you go out into the wilderness.
So what is forest disturbance? Forest disturbance can be a number of events that disturb a forest and allow for different plants, animals, and fungi to colonize an area due to the change of environmental factors in the disturbance area. Disturbances can include wildfires, avalanches, landslides, tree falls, construction, and more. These disturbances in the forest change the environmental factors such as sunlight, soil chemistry, humidity, temperature, and more that allow different species to thrive when before they would have struggled. After a disturbance, the affected area then goes into the process of succession over many years and host many different species during this time.
Forest succession is the change in species and environmental factors over time due to the species present at different stages, starting after some form of disturbance. Succession can be “completed” in as short as 100 years, or it can take well over 300 years depending on the location. Also, disturbance can happen during any part of succession, effectively restarting the process. So what does succession look like?
Disturbance: Let’s say there was a dangerous wildfire that burnt a patch of forest to the ground. What happens?
Early Succession: The first round of species, usually referred to as pioneers or colonizers, get established due to the increased light levels and nutrients (or lack thereof) in the soil. You see this happen in gardens a lot, and we refer to colonizers in gardens as weeds. These plants utilize many of the nutrients they find in the air and soil, and generally help add new nutrients and organic matter to the area that other plants can utilize. Once the pioneers are established, then small shrubs are able to get established thanks to the new organic matter and nutrients in the soil provided by the pioneers. These small shrubs can impact how much light is hitting the soil, and the humidity and temperature of the soil, allowing trees and other shrubs to get established and preventing new colonizers from establishing. Eventually, environmental conditions will change and allow more trees to grow, but it will begin with trees that like full sun and grow quickly to try and collect the most light possible.
Late Succession: Once these initial tree species are established, and grown, they cast shade across the forest floor underneath. Since there is less sunlight hitting the forest floor and under-story, the next round of species will be more shade tolerant and can grow faster than the ones that need a lot of light that is now missing. Many of our wet mountainous forests reach a “climax” of shade tolerant species like cedar, hemlock, ferns, moss and more that love dark, wet, mature forests with lots of organic matter. While the climax is considered the end stage for forest succession, eventually there will be some form of disturbance that starts the entire process over and continues the cycle of forest dynamics.
This is important because different stages of successional forests and disturbed areas provide different types of habitats for different plants, animals, and fungi at different times. That means that over time, one area can be host to many of the different organisms of the forest, but not all at the same time. It also means that when a disturbance event happens, it isn’t an entire “loss” of habitat, it is now a different habitat that will foster the growth of different species that live in the area.
So next time you’re out hiking in our public lands, take the time to stop and look at the forest or environment you are walking through and try to figure out where this forest is in its disturbance and succession cycle. The clues are out there! Also, if you made it this far, thank you for reading this long article, and if you’d like to learn more about succession, you can look into this article from Greenbelt Environmental Consulting that talks about Washington’s forest succession and common disturbances.