Non-native and Native Plant Species Distributions and Variability along an Elevation Gradient in

205 Pages·2014·3.07 MB·English

by  Joshua P. Averett

AN ABSTRACT OF THE THESIS OF

Joshua P. Averett for the degree of Master of Science in Rangeland Science presented
on June 30, 2014.

Title: Non-native and Native Plant Species Distributions and Variability along an
Elevation Gradient in the Wallowa Mountains, Oregon.

Abstract approved:
______________________________________________________
Timothy DelCurto, Michael M. Borman

Invasion by non-native plants into natural areas is an important component of
global change that threatens biodiversity and ecosystem structure and function.
Mountains are currently among the least invaded ecosystems, however, these
biodiversity hotspots are increasingly under threat of exotic plant invasion.
Evaluation of plant species distribution patterns in mountain ecosystems can provide
insight into dominant processes of plant invasion and inform management to reduce
the spread of non-native plants in mountains. The objectives of this study were to: 1)
evaluate the main drivers of plant invasions in the Wallowa Mountain Range of
northeastern Oregon, and the extent patterns of native species distributions and their
causes differed from non-native species along an elevation gradient; and 2)
investigate inter- and intra-annual understory vascular plant species variability along
an elevation gradient to inform long-term monitoring of plant invasion dynamics in
the Wallowa Mountains.
We sampled understory vascular plant communities in summer 2012 along
three forest roads in the Wallowa Mountains. Transects (n=20) were evenly stratified
by elevation (60 meters) along each road. Indicator species analysis (ISA) was used
to identify habitat tendencies for non-native species. Non-metric multidimensional
scaling (NMS) related community composition to environmental and species trait
factors. Predictors of non-native and native species abundance and richness were
evaluated using Non-parametric multiplicative regression (NPMR). Canopy
openness and elevation niche widths were modeled for all common species and
evaluated for evidence of high elevation or closed canopy specialization.
Non-native species richness decreased continuously with increasing elevation.
In contrast, native species richness displayed a unimodal distribution with maximum
richness at the mid-elevations. According to NMS and NPMR, elevation, canopy
openness, and disturbance were the strongest correlates of non-native species
abundance. Non-native species were concentrated in low elevation bunchgrass,
roadside, and open forest habitats with high canopy openness, and moderate to high
disturbance intensity. Overlays of species trait factors onto ordinations revealed that
the transition into the subalpine community and over-story canopy closure exceeding
approximately 60 percent were potential barriers to non-native species establishment.
Vegetation sampling was repeated three times (June, July, and August 2013)
during the growing season in summer 2013 for a subset (n=10) of our transects.
Blocked Multi Response Permutation Procedure (MRBP) was used to test the
hypothesis of no change in species composition between years (summer 2012 and
summer 2013) and between each sampling period (June, July, and August) within one
growing season. NMS related community composition at different times to
environmental and trait category factors. Blocked Indicator Species Analysis was
used to identify species with tendencies towards specific sampling periods. Important
predictors of vegetation change were identified and indicator species distributions
were related to NMS axes using NPMR.
Vegetation composition changed both between years and between each
sampling period within one growing season. Species richness increased with
sampling effort, dominant species were consistently found throughout all sampling
periods and rare species showed the highest rate of turnover between sampling
periods. The highest species richness and greatest number of indicator species were
associated with the early summer (June) sampling period. Approximately 25% of all
non-native species showed tendencies towards a specific sampling period where
greater than half favored the June sampling period within lowland bunchgrass
communities. Annual species including the most dominant non-native species within
the lowland and montane zones showed the highest inter- and intra-annual variability
compared to other trait categories. Total species richness was positively related to
turnover and was the most important predictor of relative abundance change between
all sampling periods. Species turnover was highest in species rich montane plots;
however, trait abundance and richness did not statistically differ in montane plots.
These data suggest that elevation, canopy, openness, and disturbance are
important factors structuring non-native plant distributions in the Wallowa
Mountains. Our findings that non-native species tended to be concentrated in low-
elevation, early successional habitats coupled with a lack of high elevation or closed
canopy specialization indicates that non-native plants that occurr at the highest
elevations are likely generalist species that depend on disturbance (primarily open
over-story canopy) for spread into higher elevation sites until they are eventually
filtered out by harsh environmental conditions coincident with the subalpine
transition zone. Collectively these results suggest that interactions between
introduction pathways that favor human assisted dispersal into the low-elevations
where ruderal species are favored and environmental filters (elevation, canopy
openness, and disturbance) are the dominant factors structuring non-native plant
distributions in the Wallowa Mountains.
Our results also indicate that long-term monitoring of vegetation change in the
Wallowa Mountains will be improved through periodic sampling during each
growing season. Sampling once during the spring or early summer and then again
coincident with peak biomass should improve estimates of species distributions and
diversity patterns by capturing both vernal and late season species. Early season
sampling may be particularly important for monitoring non-native plants in the
Wallowa Mountains as most non-natives were concentrated in low elevation semi-
arid bunchgrass communities where spring and early summer specialization is
common.
©Copyright by Joshua P. Averett
June 30, 2014
All Rights Reserved
Non-native and Native Plant Species Distributions and Variability along an Elevation
Gradient in the Wallowa Mountains, Oregon

by
Joshua P. Averett

A THESIS

submitted to

Oregon State University

in partial fulfillment of
the requirements for the
degree of

Master of Science

Presented June 30, 2014
Commencement June 2015
Master of Science thesis of Joshua P. Averett presented on June 30, 2014

APPROVED:

Co-Major Professor, representing Rangeland Sciences

Co-Major Professor, representing Rangeland Sciences

Head of the Department of Animal and Rangeland Sciences

Dean of the Graduate School

I understand that my thesis will become part of the permanent collection of Oregon
State University libraries. My signature below authorizes release of my thesis to any
reader upon request.

Joshua P. Averett, Author
ACKNOWLEDGEMENTS

I am grateful to Tim DelCurto and Michael Borman for taking me on as their
graduate student and for their guidance through my graduate school experience. I
would like to express great thanks to Ricardo Mata-González and Bruce McCune for
their excellent mentorship, technical support, and consistent optimism and
encouragement throughout my research journey. Thank you to Julia Jones for the
great advice and support that helped to steer me back on course whenever I veered.
I also extend great thanks to Catherine Parks for her support with the
conceptual design of this study and assistance with field sampling and Bridgett
Naylor for her support and patience with data extraction and database management.
This research would not have been possible without my outstanding field
crew. A heart-felt thanks goes out to Kent Coe, Rachel Burton, Regina Thompson,
Ryan Limb, and Samantha Pack.
I would like to express gratitude to my family for all of their love and support
through and leading up to this experience. Thanks to my mother Betty, father Jerry,
and brother Jason. Lastly, and most importantly, I would like to thank my wife
Adrienne. Without her love and encouragement to follow my passions, none of this
would have been possible.
CONTRIBUTION OF AUTHORS
Dr. Bruce McCune contributed to the entire thesis through guidance and editing. Dr.
McCune also provided technical support regarding analytical approaches and
implementation of Non-metric multidimensional scaling, Non-parametric
multiplicative regression, Blocked Multi-response Permutation Procedures, and
Blocked Indicator Species Analysis using HyperNiche 2.0 and PC-Ord 6.0. Dr.
Catherine Parks provided guidance regarding research questions, provided edits, and
assisted in the development of the sampling protocol for both chapters. Bridgett
Naylor contributed to chapters 2 and 3 through support of data extraction, data-base
management, and assisted with the development of the sampling protocol. Dr.
Ricardo Mata-González provided guidance and editing for the entire thesis.
TABLE OF CONTENTS
Chapter 1 Introduction .................................................................................................. 1 
Objectives ...................................................................................................................................... 11 
Chapter 2 Non-native and native plant distributions along an elevation gradient in the
Wallowa Mountains, Oregon .............................................................................. 13 
ABSTRACT ................................................................................................................ 14 
INTRODUCTION ........................................................................................................ 15 
METHODS ................................................................................................................. 21 
Study site and site selection ........................................................................................................... 21 
Sampling design ............................................................................................................................ 23 
Statistical analysis ......................................................................................................................... 27 
RESULTS ................................................................................................................... 33 
Dominant non-native species ........................................................................................................ 33 
Invaded habitats ............................................................................................................................. 34 
Community gradients .................................................................................................................... 35 
Species abundance and richness .................................................................................................... 39 
Non-native high elevation and closed canopy specialization ........................................................ 40 
DISCUSSION ............................................................................................................. 41 
Management Implications ............................................................................................................. 47 
ACKNOWLEDGEMENTS ............................................................................................ 52 
FIGURES ................................................................................................................... 53 
TABLES .................................................................................................................... 64 
Chapter 3 Intra- and Inter-annual Vegetation Change along an Elevation Gradient in
the Wallowa Mountains, Oregon ......................................................................... 73 
ABSTRACT ................................................................................................................ 74 
INTRODUCTION ........................................................................................................ 76 
METHODS ................................................................................................................. 81 
Study site and site selection ........................................................................................................... 81 
Sampling design ............................................................................................................................ 83 
Statistical analysis ......................................................................................................................... 86 
RESULTS ................................................................................................................... 93 
Community gradients .................................................................................................................... 94