Habitat and Regeneration Requirements to Sustain Populations of Schwalbea americana L. Open Access
Coles, Kelly (Spring 2023)
Abstract
Schwalbea americana L. (Schwalbea) is a federally endangered hemiparasitic herb in the family Orobanchaceae. Habitat loss and fire suppression are the leading causes of this pyrophytic species’ decline. This thesis explores propagation, soil ecology, and plant associates of the species and reports on a habitat suitability model. Chapter 1 includes an experiment in which Schwalbea was grown from seed with and without a host and with and without one of two native soil inocula collected from two sites (Parmalee and Rhexia) on Ichauway, a longleaf pine preserve in GA, USA. Seeds were more likely to germinate when inoculated with either of the inocula than when grown in sterile soil. Seedlings grown with soil inoculum from Parmalee lived longer, and lived marginally longer when grown with inoculum from Rhexia, compared to the control. Chapter 2 examines microsite characteristics using soil measurements. It examines fungal associations of Schwalbea roots. No significant differences were observed between soils in which Schwalbea was growing and surrounding soils. pH was marginally higher in soils where Schwalbea grew than those where it did not. Twenty-eight species of fungi found in Schwalbea roots were identified. Thirty-six percent of these have known beneficial effects on plants, 14% have known harmful effects, 36% have neutral-unknown effects, and 14% have mixed effects. One beneficial species identified is Curvularia protuberata, which forms a symbiotic relationship with a virus and the North American grass Dicanthelium lanuginosum (Elliott) Gould, a partnership which helps these species thrive in high temperatures. Chapter 3 reports on indicator species identified at four Schwalbea populations at Ichauway. Eleven species were found to be indicative of Schwalbea habitat at p = 0.01, including four graminoids, three asters, one subshrub, and three non-aster forbs. One more graminoid and non-aster forb were identified as indicators at p = 0.05. Chapter 4 describes the construction and evaluation of a habitat suitability model for Schwalbea. The model performed well with a test AUC of 0.924 and training AUC of 0.988. Nonmetric multidimensional scaling ordination supported future construction of a model utilizing occurrence records from localized regions rather than the southern extent of the range.
Table of Contents
Table of Contents
Introduction. 1
I.1 The Longleaf Landscape. 1
I.2 Schwalbea americana. 2
I.3 Project Overview.. 13
Chapter 1: Schwalbea americana propagation efforts. 14
1.1 Introduction. 14
1.2 Methods. 18
1.3 Results. 21
1.4 Discussion. 26
Chapter 2: Biotic and abiotic soil characteristics in Schwalbea americana populations. 31
2.1 Introduction. 31
2.2 Methods. 34
Abiotic Soil Characteristics. 34
Fungal Relationships. 35
Data analysis. 36
2.3 Results. 36
Abiotic soil characteristics. 36
Fungal relationships. 43
2.4 Discussion. 49
Chapter 3: Indicator species analysis and pH requirements for Schwalbea americana. 54
3.1 Introduction. 54
3.2 Site and Methods. 57
Data Analysis. 60
3.3 Results. 62
Indicator Species Analysis. 62
Functional Groups. 63
pH.. 76
3.4 Discussion. 78
Indicator species. 78
Functional groups. 80
pH.. 83
Chapter 4: Modeling habitat suitability for Schwalbea americana. 84
4.1 Introduction. 84
4.2 Methods. 85
Element Occurrences. 85
Predictor Variables. 86
Maxent Model: Setting Parameters. 87
Model Testing: Site Visits. 88
4.3 Results. 90
4.4 Discussion. 98
Sources. 102
Appendix. 136
Table A1. Site means for soil measurements. 136
Table A2. Site comparison test results for soil measurements. 137
Table A3. Fungal groups identified in Schwalbea americana roots. 137
Table A4. Fungal genera identified in Schwalbea americana roots. 139
Table A5. Species list (Ichauway). 143
Table A6. Fire history of Schwalbea populations at Ichauway. 157
Table A7. Species list (Habitat suitability site visits) 158
List of Tables
Table 1: Number of seeds in experiment.....................................................................................................21
Table 2: Binary Logistic Regression Model Results.................................................................................. 23
Table 3: Soil category means across groups……………………………………....................................... 37
Table 4: Effect on plants of fungal species found in Schwalbea americana roots..................................... 47
Table 5: ISA analysis (Inside-Outside groups)………………………………........................................... 62
Table 6: ISA analysis (Presence-Absence groups)……………………………………............................. 63
Table 7. Variable importance for habitat suitability model 90
Table 8. Species associations with NMS axes for habitat suitability site visits. 96
List of Figures
Figure 1: Schwalbea americana range map ................................................................................................. 4
Figure 2: Schwalbea americana ............................................................ …………………………………...5
Figure 3: Schwalbea propagation experimental design……….. ............................................................... 18
Figure 4: Photographs of Schwalbea seeds................................................................................................. 20
Figure 5: Effect of seed quality on seedling survival over time…………………………………. ........... 24
Figure 6. Effect of Host Plant on Seedling Survival Over Time…………………..……………………...25
Figure 7. Effect of seed source on seedling survival over time. 25
Figure 8. Effect of inoculant type on seedling survival over time. 26
Figure 9. Site mean comparisons for each element measured. 42
Figure 10. DNA Composition of fungal groups found in roots of Schwalbea americana. 43
Figure 11. Fungal DNA richness found in Schwalbea roots. 44
Figure 12. Number of fungal genera in different effect-on-plant categories found in Schwalbea americana roots. 46
Figure 13. Number of root samples with fungal species present 49
Figure 14. Vegetation sampling layout at sites Rhexia 2 & 3. 58
Figure 15. Diagram of plot classifications. 61
Figure 16. Differences in functional group composition between Inside and Outside groups. 65
Figure 17. Mean cover of different growth forms compared across sites. 71
Figure 18. Differences in Wetland Indicator Status composition between groups. 73
Figure 19. Differences in cover of the five Wetland Indicator Statuses across sites. 76
Figure 20. Differences in mean pH between groups. 77
Figure 21. Differences in mean pH across Schwalbea sites at Ichauway. 77
Figure 22. Jackknife of regularized (a) training and (b) test gain for variable importance in modeling habitat for Schwalbea americana. 91
Figure 23. Area under the receiver operating curve (AUC) for Schwalbea americana habitat suitability model 92
Figure 24. Map of habitat suitability for Schwalbea americana. 92
Figure 25. Variable response curves (Maxent-produced). 94
Figure 26. NMS ordination space for habitat suitability. 96
Figure 27. NMS ordination space: State overlay. 97
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