AN ABSTRACT OF THE THESIS OF

Janet L. Ohmann for the degree of Doctor of Philosophy in Forest Science

presented on November 29, 1995. Title: Regional Gradient Analysis and Spatial

Pattern of Woody Plant Communities in Oregon.

Abstract approved:

Thomas A. Spies

Knowledge of regional-scale patterns of plant community structure and

controlling factors is largely qualitative and based on numerous local studies. Data

from a subsample of 10,000 field plots were used to quantify and map

compositional gradients of woody plant communities across Oregon forests.

Canonical correspondence analyses explained 9-14% of the total species variation

at three geographic scales. Climate contributed most to total variance explained

(44-58%), followed by geology (6-16%), disturbance (7-13%), and topography (4-

8%). The dominant compositional gradient was associated with climate, from the

lower elevation, moderate, maritime climate of the coast to the higher elevation,

drier, continental climate of the interior. The second canonical axis followed a

gradient from the warm, dry growing seasons of the interior valleys and eastern

Cascades, to cooler, wetter mountainous areas. Only those parent materials that

present extreme growing conditions, primarily ultramafic soils, were significant at

the regional level. Ecological relationships differed substantially among subregions

within the state. Except for topography, which contributed more to explained

variation at finer geographic scales, ecological differences among subregions

overshadowed effects of geographic extent. Topographic effects were stronger and

beta diversity greater in eastern Oregon, where moisture is more limiting for plant

growth, and weakest near the coast where climate is more favorable. The

secondary importance of topography, disturbance, and substrate can be attributed

to their influence on relative abundances of species within a local area, rather than

on species presence or absence within broader regions. Community structure

varied at a finer spatial scale in eastern than in western Oregon, and species

turnover along gradients was greater for shrubs than trees. Amount of unexplained

variation in the species data was high but not atypical of gradient analyses. Yet

spatial structure in the species data that was uncorrelated with explanatory factors

suggests potential to improve the canonical correspondence analysis models,

particularly in eastern Oregon. Study findings have implications for considering

biological diversity in regional conservation strategies, in planning for global climate

change, and in the design of regional inventory and monitoring programs.

Regional Gradient Analysis and Spatial Patternof Woody Plant Communities in Oregon

byJanet L. Ohmann

A THESISsubmitted to

Oregon State University

in partial fulfillment ofthe requirements for the

degree of

Doctor of Philosophy

Completed November 29, 1995Commencement June 1996

Doctor of Philosophy thesis of Janet L. Ohmann presented on November 29, 1995

APPROVED:

Major Professor, representing Forest Science

Head of Department of Forest Science

Dean of 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.

Janet L. Ohmann, Author

TABLE OF CONTENTS

Page

INTRODUCTION 1

Current Knowledge of Regional Patterns in Forest Communities 4

Research Objectives and Hypotheses 9

METHODS 12

Study Area 12

Physiography, Geology, and Soils 12Climate 20Vegetation 24Disturbance and Landscape Patterns 27Paleoecology 31

Vegetation Data 32

Explanatory Variables 34

Field-Recorded Attributes 34Temperature 36Precipitation 37Solar Radiation 38Geology 38Spatial Position 39

Gradient Analyses 39

CCA with Variance Partitioning 40Stepwise CCA 41Indirect Gradient Analyses 44Mapping Geographic Patterns of Dominant Gradients 45

Community Classification 45

RESULTS 46

Variance Partitioning Analyses 46

Importance of Spatial Position and Environment 46Importance of Local and Regional Explanatory Variables 48

TABLE OF CONTENTS (Continued)

Page

Stepwise CCA 50

Gradient Structure 50Dominant Gradients at the State Level 50Dominant Gradients at Half-State and Subregion Levels 72

Positions of Plant Growth Forms Along CCA Gradients 83

Spatial Patterns in Fit of the CCA Model 87

Comparison of Direct and Indirect Gradient Analyses 95

Comparison of Tree and Shrub Strata 96

Community Patterns 98

Ecotones in Oregon Plant Communities 104

Ecological Differences Between Land Ownerships 108

DISCUSSION 112

Determinants of Vegetation on a Site 112

Spatial Structure in the Species and Environment Data 112Temperature and Moisture 114Topography 117Overstory Canopy Cover 119Disturbance 120Geology 126

Within-Region Variation in Patterns of CommunityStructure and Environment 126

Sampling and Scale Effects 128

Sample Size and Rare Species 129Effects of Geographic Scale 131Resolution of the Species Data 134

Limitations and Sources of Unexplained Variation 135

TABLE OF CONTENTS (Continued)

Page

Summary of Research Hypotheses 137

Objective One 137Objective Two 140

IMPLICATIONS FOR CONSERVATION AND RESOURCE MANAGEMENT . 141

Regional Biodiversity Conservation Strategies 141

Planning for Global Climate Change 144

Monitoring Biodiversity in Forest Ecosystems 145

CONCLUSIONS 147

BIBLIOGRAPHY 150

APPENDICES 161