Biotic invasions take place when a non-native species is introduced to an ecosystem through varying means of transportation. Invasive species can be dispersed by many natural and anthropogenic causes such as animals or vehicles; the greatest cause of dispersal is due to human activity whether it be intentional or accidental. Once established in a site, even the smallest satellite population of an invasive species can competitively spread and displace native species while effectively altering important ecological functions and services within an ecosystem (Mack et al.
, 2010). Biotic invasions are frequently followed by an overall decrease in ecological biodiversity and species richness which can be difficult to restore after it is lost. Globally, biotic invasions have become an increasing concern as diversity and evolution plummets, species are driven towards extinction, and habitats are reduced beyond repair (Mack et al., 2010).
Very few ecosystems, if any, remain untouched by biotic invasions and fewer can withstand the competitiveness of increasing populations of invasive species. With humans being the primary cause of non-native species dispersal, it is our responsibility to assist these invaded ecosystems to mitigate against further invasion and decrease of ecological integrity and health.
Eliminating established populations of invaders is often a difficult and unsuccessful task because it can require extensive management and monitoring of a site to ensure that invasive populations are eradicated, otherwise, re-establishment can easily occur from a single individual (Mack et al., 2010). Disturbance events can promote the spread of invasive species by altering spatial and temporal attributes of an ecosystem and creating opportunities for encroachment to take place.
The site I chose to observe is located around Blaine, Oregon near Beaver, Oregon in Tillamook County. The Nestucca River Recreation Area is a scenic site that is adjacent to the Nestucca River that hosts campgrounds and other recreation activities while being managed by the Bureau of Land Management. Roughly 8 to 9 miles up a road off of Nestucca River Road, following Elk Creek, is a plot of forested land that has been invaded by populations of scotch broom (Cytisus scoparius), Himalayan blackberry (Rubus armeniacus), and Herb Robert geranium (Germanium robertianum). Scotch broom is a perennial, bushy shrub that is native to regions of Europe and Africa; populations were exported throughout the world for its use as ornamental vegetation. Himalayan blackberry is an evergreen shrub that can spread aggressively throughout an ecosystem and produces berries that are harvested by animals and humans. Herb Robert geranium is a low-growing, annual invasive species that can rapidly establish on sites and displace native vegetation.
I chose this site because during my time working for the Bureau of Land Management I was sent to the area to remove the identified invasive species; I wanted to revisit the site and see if the encroaching populations had recovered over the years (they had). Upon arriving at the site, it was obvious that the scotch broom populations had efficiently re-established in the open areas of the forested plot where canopy cover was more open and all along the roadside in much denser and mature populations. The blackberry populations were sparser in comparison to the scotch broom; appearing more so along the Elk Creek banks and bordering the road. Geranium populations were observed in dense clusters on the roadside that borders the creek.
To acquire my measurements, I used a pencil and notebook for recording, a diameter tape for determining DBH, and a simple PVC pipe measured out to 1 meter in length for general comparisons and observations. I first began by identifying the invasive species present in the site and then recorded the dominant native species as well. Using the diameter tape, I measured the DBH of a total of 5 Douglas-fir (Pseudotsuga menzieseii) trees, and recorded them. I used the PVC pipe for a general measurement of scotch broom height and vegetative width to compare the stature differences between individual plants. The work was done during November 18th, 2018 and began at 1:30 in the afternoon and took about an hour to acquire all of the desired data and observations.
The overstory of the site is dominated by older populations of Douglas-fir and represent a relatively dense stand and closed canopy. However, overstory vegetation following the banks of Elk Creek are primarily dominated by red alder (Alnus rubra) and big-leaf maple (Acer macrophyllum) in scattered, less dense populations with generally open canopy cover. The understory of the site is scattered with native populations of vine maple (Acer circinatum), but otherwise the stand is not represented by a well-established understory. In areas where the dense canopy cover opens to allow plenty of light, scotch broom populations can be found thriving and dominating the shrub layer with little competition from other species excluding the overstory trees.
Along the Elk Creek and the boarding roadside—covering the banks and any other areas with plenty of light and minimal canopy cover—are dense populations of scotch broom that have completely out competed other plant species and now dominate whatever space they occupy. Also along the banks of Elk Creek are varying populations of dense Himalayan blackberry shrubs that seem to be mature and healthy. Groundcover of the stand has scattered populations of swordfern (Polystichum munitum) and dwarf Oregon grape (Mahonia nervosa) that are more prevalent around the Douglas-fir populations and less so around any of the scotch broom. Herb Robert geranium is also present on the forest floor, but is more established along the creek and roadside; the geranium populations seem relatively new to the area and are beginning to spread along the ground. The most dominate biotic invader of the site is scotch broom which as effectively displaced native vegetation and is continuing to spread to other areas. The height of the scotch broom populations varied between 1.5 to 2 meters tall for the more mature individuals with seedlings scattered in different areas. The base of more mature scotch broom individuals had a diameter thickness similar to the size of my wrist and forearm. The most dominant overstory species in the stand is Douglas-fir and the measured DBH of the trees measured ranged between 90-127 centimeters.
The invasive component of this site could be classified as being a dominant component of the stand. Although the invasion of scotch broom has been limited to the open areas that are lacking sufficient canopy cover, should a disturbance visit the Douglas-fir populations and alter the structure and density of the stand, scotch broom could efficiently encroach upon the forested site should it be provided the opportunity and sunlight.
The stand is characterized by a uniform canopy that is dominated by Douglas-fir trees varying in ages from 80 to 130 years old. In the early 1900’s, much of the area was exposed to frequent fires caused by human activity in hopes of supporting homesteading and grazing practices (Barczak, 1998). Seedlings were planted after clearcutting operations took place in the 1960’s and much of the older stands were commercially thinned. The thinning removed the understory trees and vegetation, snags, and valuable downed wood to increase timber regeneration for future logging operations (Barczak, 1998). In the 1990’s, the land use objectives in the area shifted from timber harvesting to restoring late-successional forest areas in response to the application of the Northwest Forest Plan and the protection of spotted owl habitats (Barczak, 1998). Since the desired timber at the time was predominantly Douglas-fir, when land-use shifted, stands were being reforested with Douglas-fir seedlings and caused present day stands to have such uniform canopies and trunk densities with similar tree ages.
Since the stand I chose is located near a road and is adjacent to Elk Creek, it is likely that these invasive species could easily arrive to and spread throughout the site. Himalayan blackberry develop fruit to produce and spread their seeds; many animal and bird species eat blackberries which allows the plant to spread to many different areas whether it be a few feet away or miles. The blackberry populations are also directly next to the creek which can provide transportation for berries that fall off the plant into the water and are carried downstream. Herb Robert geranium is only capable of reproducing and spreading through seeds, but seed capsules can be shot from the plant up to 20 feet when it becomes disturbed. Since the geranium species are located along the road and creek bank, it is likely that disturbance could occur frequently and cause seeds to be released into the creek and carried downstream or picked up by animals and vehicles to be transported elsewhere. Scotch broom populations begin creating seeds around the age of 2 and can potentially produce thousands of pods that could contain an average of 8 seeds per pod (FEIS).
As the seed pods reach maturity they snap apart and can launch seeds an average of 3 feet from the plant; this in addition to the number of seeds produced has most likely allowed the species to quickly establish in the site. Scotch broom could have been brought to the site though multiple means such as animals, recreation, vehicle operation, and creek transportation. Seed banks of scotch broom are also capable of persevering in an ecosystem for many years, even as long as 50 or more dependent upon site conditions; even after scotch broom populations are removed, the thousands of seeds are capable of repopulating an area for years (FEIS).
Furthermore, scotch broom supports a nitrogen-fixing bacterium at its roots so it can persist even in nitrogen deficient sites. Should a disturbance occur in the area that reduces canopy cover or disturbs the soil, scotch broom could spread rapidly throughout the stand and further displace established natives. I predict that scotch broom will continue to spread along the creek and roadside until it has established in all the open areas that can support it; the ecosystem will become so fragile that any disturbance—fire, logging, blowdown—that occurs on the site will create an opening for scotch broom to invade. Scotch broom is a species with high shade intolerance and an increase in canopy cover, or some other means of creating shade, could assist in reducing the overall population.
Future impacts of the observed invasive species create a great difficulty for management because the ecosystem has a high potential for being overrun by non-native species with even the smallest disturbance. Overall biodiversity is likely to decline should invasive populations continue to spread at their current rate and further displace natives from the habitat. Invasive species will have to be reduced if managers wish to retain the area as a diverse habitat for future use, otherwise, continued recreation in the area will only increase the spread of biotic invaders.