Distribution of Spotted Knapweed in Michigan

Abstract

Our study aimed to determine the distribution of spotted knapweed (Centaurea stoebe) throughout Michigan from 1920-2018. Spotted knapweed is an invasive species that was first documented in the United States in 1883 (Jacobs, 2012). Gradually, the populations have varied through the Midwest Herbaria's online collection.

We took every specimen available on the online herbaria in 25 year increments and mapped them by county to determine distribution. The populations of spotted knapweed began sparse and then became increasingly prevalent from 1970-1995 (Figure 3). Then they tapered off from 1995-2018 (Figure 4).

The change in distribution of spotted knapweed could be influenced by management practices or the lack of collecting specimens for herbaria collections.

Introduction

Spotted knapweed (Centaurea stoebe) has been an invasive species known to drive out native vegetation, alter ecosystem balance, and affect the diets of native wildlife (Carson and Landis, 2014). Originally found in Europe spotted knapweed has invaded into the United States as an invasive species (Jacobs, 2012). Throughout history, spotted knapweed was first documented in the United States in British Columbia in 1883 (Jacobs, 2012). Spotted knapweed has a negative effect on native vegetation by creating areas of sparse growth especially in dune and prairie ecosystem types (Spotted knapweed). Spotted knapweed is seed dispersed in the reproductive stage and is considered a perennial plant (Spotted knapweed).

Spotted knapweed is an invasive within the United States and there are various methods used to eradicate this weedy species. Herbicides is a typical method used to reduce populations but is only a short-term fix to the issue (Jacobs, 2012). Picloram is the typical pesticide used to reduce populations although populations near a water source can contaminate the surrounding areas. This restricts were this method can be used (Jacobs, 2012).

Another method discussed is hand pulling this species (MacDonald, et. al, 2014). Hand pulling spotted knapweed is more beneficial in the juvenile stage rather than the adult stage although careful and thorough removal is required as well as a combination of removal methods (MacDonald, et. al, 2014). This method is used on small populations of spotted knapweed and in conjunction with herbicidal treatment of the area (Jacobs, 2012).

A third method used to remove the invasive is prescribed burning (MacDonald, Scull, & Abella, 2007). Prescribed burning is a beneficial method of removing spotted knapweed and promoting native vegetation. The prescribed fires need to be timed correctly in the life cycle and reproduction season for the native vegetation to flourish (MacDonald, Scull, & Abella, 2007).

Lastly, introducing weevils into areas where spotted knapweed is prevalent is an effective method to reduce the spread of this invasive (Carson and Landis, 2014). Carson and Landis studied the effects of releasing populations of weevils in different counties in Michigan to track the distribution of this beetle in correlation to spotted knapweed. Weevils have shown to be a positive method of reducing spotted knapweed populations (Carson and Landis, 2014).

The study objective was to determine overtime does the species appear more widespread or localized regarding herbaria collections in Michigan specifically the Midwest Herbaria online collection. In continuation are the measures to remove the invasive species effective based on the prevalence of spotted knapweed in herbaria collections.I hypothesize that overtime spotted knapweed will become less prevalent and more localized in herbaria collections due to removal efforts of the invasive species.

Methods

Research was conducted through online herbaria collections. The data was found through the Consortium of Midwest Herbaria's online collection. We looked at the specimens throughout the 1900s to present day, to track the distribution and prevalence in Michigan. We recorded every specimen present within both collections and created maps in 25 year increments starting in 1920 as the first specimen was collected in 1921. The upper parameter is inclusive while the lower is not pertaining to the timeframe of distribution. We recorded the county and the year it was found to generate distribution maps of the 116 specimens.

Results

In regards to the Midwest Herbaria's online collection there was a substantial difference in the distribution of spotted knapweed in Michigan from 1921 to present day (Consortium of Midwest Herbaria). The herbaria collection began in 1921 with no specimens before that time. From 1920-1945 the distribution was sparse only found in five counties located in the northern, southeastern, and western parts of the lower peninsula (Figure 1). From 1945-1970 the distribution increased to 13 counties in both the lower and upper peninsula (Figure 2).

The distribution was concentrated in the northwestern part of the lower peninsula. From 1970-1995 the distribution widespread including 44 counties (Figure 3). The distribution was found in large parts of both the upper and lower peninsula. Spotted knapweed was found mainly in central to northern lower peninsula. From 1995-2018 the distribution was sparse found in only 11 counties (Figure 4). The composition was focused mainly on the western coast of the lower peninsula but also found in the upper peninsula. The distribution of spotted knapweed varied throughout the years (Consortium of Midwest Herbaria).

Discussion

The composition of counties where spotted knapweed occurred varied throughout the four time periods studied. The distribution in the early to mid 1900s was sparse and we can infer that at that time, spotted knapweed was beginning to invade into Michigan and specimens were beginning to be collected (Figure 1). As the years progressed it became more prevalent in various counties with the widest distribution during 1970-1995, which we can assume was before eradication methods and management plans were put into place (Figure 3).

In figure 4, the distribution began to decrease after the height of the spread of spotted knapweed. We can hypothesize that the use of prescribed fire, herbicide, insects, and other eradication methods aided in the decreased numbers (Spotted knapweed). Weevils had an impact on the decrease in populations of spotted knapweed, they are used as an aid for controlling the spread of spotted knapweed (Carson and Landis, 2014). Researchers Carson and Landis conducted a research project to release weevils into different areas in Michigan and along the Michigan-Indiana border.

They found healthy populations of weevils from May to July with correlation to the data we researched. From Figure 3 to Figure 4, there was a decrease in spotted knapweed occurrences and Carson and Landis released populations of weevils in south to central Michigan. Spotted knapweed largely decreased in that time which the weevils could influence the decreasing numbers (Carson and Landis, 2014). Eradication methods could be in part to the decreasing numbers of specimens, although there may have not been a need to collect further samples.

Researchers and land managers have been studying and working to manage populations of spotted knapweed. Although various management practices have been put into place the online herbaria collection may not be an accurate way to measure the prevalence of this invasive species in Michigan. Herbaria collections can vary due to the researchers collecting specimens.

Specimens may have been heavily collected from 1970-1995 as spotted knapweed was being introduced into Michigan and then as time progressed they did not have the need to continue collecting specimens (Consortium of Midwest Herbaria). Another hypothesis could be, some digital collections have not been fully updated with every specimen. Though this could be of influence to the decrease in specimens overall the distribution became more localized.

The research aimed to determine if the distribution of spotted knapweed in Michigan was more widespread or localized into a general region of the state. Through the process questions arose. What type of allelopathic affects does spotted knapweed have on different native vegetation? Does it strongly affect certain species more than others? Other studies have been conducted in Michigan to determine different management practices as well as other distributions of spotted knapweed. The research yields to substantially different distributions throughout time, showing widespread as well as a decrease and localization of spotted knapweed.

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