Limited Evidence For Indicator Species of High Floristic Quality Wetlands In The US Southern Plains

Floristic Quality Assessment requires compiling a full list of vascular plant species for the wetland. Practitioners may lack the time and taxonomic skills for full-community vegetation surveys, especially when wetlands are large and complex. In this paper we broadly ask whether oristic quality indicator species may exist for wetlands, specically evaluating indicator species potential for high oristic quality wetlands in the US southern plains region. Indicators were identied for a broader context (wetlands in Oklahoma prairie ecoregions) and narrower context (depressional wetlands in the northern Central Great Plains ecoregion of Oklahoma) based on indicator value, indicator validity, hydrophytic status, and ecological conservatism. No candidate indicators satised all criteria for high oristic quality. Indicator values improved with increasing spatial-environmental context, but many candidates occurred too frequently in non-high quality sites or too infrequently in high quality sites, relative to predicted rates. The best performing indicator (Eleocharis compressa) lacked validity in the broader context and showed high false-positive rates in the narrower context. Combining E. compressa with select other candidates (Amorpha fruticosa, Juncus torreyi, Leersia oryzoides, Schoenoplectus pungens) may compensate for weaknesses but the combinations may rarely be found across the region. Overall, these results do not support relying on indicator species to rapidly identify or verify high oristic quality wetlands in the US southern plains. We recommend similar studies in other regions and testing other quality levels (low, moderate) before broadly concluding that oristic quality indicator species do not exist for wetlands.


Introduction
Floristic Quality Assessment (FQA) is a widely used tool for conservation planning, ecological health assessments, and restoration monitoring. Floristic quality is commonly indexed as the product of species richness and mean ecological conservatism (Kutcher and Forrester 2018). Conservatism is estimated by expert opinion and ranges from nonnative and weedy native species at the low end to species of minimally human-disturbed areas at the high end (Taft et al. 1997). These subjective scores assigned to individual species can align with empirically derived scores (Bried et al. 2018 The main requirement and challenge of FQA is compiling a complete species list for the wetland. Although mean conservatism may be robust to some level of misidenti cations and failed detections (Spyreas 2016), all extant vascular plants should be detected and accurately identi ed to species level.
The requisite eld experience and taxonomic skills may be lacking (Noss 1996;Drew 2011) and many plant species are unidenti able at certain times of the growing season. Furthermore, compiling a full list takes time, whether setting up plots and transects or meandering throughout the wetland. Wetland researchers have begun to propose shortcuts around full-community FQA, such as ignoring graminoids and non-dominant taxa (Chamberlain and Brooks 2016). Indicator species (Siddig et al. 2016) offer another possibility but have not, to our knowledge, appeared in FQA research or applications. Focusing on a few target species linked to preset levels of oristic quality would ease the skill requirement and improve eld work e ciency; in theory a well-supported indicator species could reduce the eld visit to minutes or even seconds depending on how fast its detected.
High quality or minimally altered wetlands are sought for regulatory protection and used as baselines for assessing ecological disturbance and management actions Herlihy et al. 2019). Tools for rapid identi cation of potential reference wetlands are needed, especially when such conditions are rare over large areas containing abundant wetlands. An approach of indicator species, su ciently calibrated (Bried et al. 2019), is one option worthy of consideration. Such species may be used for example to rapidly screen wetlands for potential regulatory protection and follow-up intensive assessments in the development permitting process (Stapanian et al. 2013).
In this paper we ask if certain species can represent high oristic quality as de ned by relatively conservative, species-rich, native plant assemblages. We were speci cally interested in nding these species for wetlands in the US southern plains, a region stretching from Texas through Nebraska between the Rockies and Xeric Plains to the west and the Temperate Plains and Southern Appalachians to the east (Kentula and Paulsen 2019). We focus on the prairie ecoregions of Oklahoma where establishing reference standards is a priority of the Wetland Program Plan (www.ok.gov/wetlands) and where previous research has suggested oristic quality criteria for potential reference wetlands (Bried et al. 2014; Gallaway et al. 2019). Because Oklahoma's prairie ecoregions and dominant land use extend into neighboring states, developing e cient and reliable assessment tools in Oklahoma is a step towards rapid eld identi cation (or veri cation) of potential reference-quality wetlands in the southern plains. Our broader question here, beyond the study region, is whether indicator species of wetland oristic quality may exist.

Study area
Oklahoma is centrally located in the US southern plains and intersected by a dozen Level III ecoregions (Omernik & Gri th 2014), two-thirds of which are largely non-forested and classi ed as "prairie" (Fig. 1).
Historically the prairie ecoregions were dominated by short-grass, mixed-grass, or tall-grass vegetation communities but now they are dominated by pastures and rangeland (Hoagland 2000;Tyrl et al. 2007).
The distribution of vegetation types is broadly divided among the ecoregions and follows a precipitation gradient from driest (< 50 cm annual rainfall) in the High Plains and Southwestern Tablelands to wettest  Table 1 Two sets of candidate indicators for high oristic quality (≥ 20.0 or ≥ 13.0 index score) wetlands in Oklahoma prairie ecoregions; three additional sets are found in Table S1. Each set comes from a randomly drawn ~ sample (Main Samples #1 and #2 shown in Fig. 1) of 74 sites (37 high quality vs. 37 non-high quality). C value -Oklahoma ecological conservatism value; wetland status -regional obligate wetland (OBL), facultative wetland (FACW), facultative (FAC), facultative upland (FACU) status; LCL, UCL -lower and upper 95% con dence limits from 10,000 bootstrap iterations (lowest and highest bootstrap limits in six combinations of speci city and sensitivity thresholds); IV -Indicator Value (Speci city × Sensitivity).  Table 2 Two sets of candidate indicators for high oristic quality (≥ 20.0 or ≥ 13.0 index score) depressional wetlands in the northern Central Great Plains ecoregion of Oklahoma; three additional sets are found in Table S2. Each set comes from a randomly drawn ~ sample (Main Samples #1 and #2 shown in Fig. 1) of 48 sites (24 high quality vs. 24 non-high quality). Terms and acronyms same as de ned in Table 1. Study sites (108 total) were located using National Wetlands Inventory data, the National Wetland Condition Assessment probability sample (Kentula and Paulsen 2019), and several Oklahoma-based sources (Bried et al. 2014;2016). All sites exhibited seasonal to semi-permanent hydroperiods and dominance of scrub or herbaceous communities (i.e., non-forested wetlands). Most sites were geomorphically classi ed as depressional (84%) and the rest as lacustrine-fringe, riverine, or seep We repeated the analysis for wetlands of prairie ecoregions (i.e. all study sites, "prairie wetlands") and for depressional wetlands of the northern Central Great Plains ecoregion ("CGP depressions") in Oklahoma, expecting stronger indicators for CGP depressions due to increased spatial-environmental context (Bried et al. 2019). For both contexts we used randomly drawn ~ Main Samples (e.g. Figure 1) to nd indicator species, giving us 74 sites in the broader context and 48 sites in the narrower context. We constrained each draw to yield equal numbers of high quality and non-high quality sites, repeating the draws and analysis ve times per context to help assess robustness of the indicators. Results from the rst two samples (those shown in Fig. 1) are presented below and results from the remaining samples in the online Supplement.

Indicator validity
Valid indicator species should occur with relatively high frequency in the target group (high oristic quality wetlands) and low frequency elsewhere. We tested discriminative ability in candidate indicators using 34 prairie wetlands and 22 CGP depressions (the remaining ~ of samples) each split evenly into high and non-high quality as above. Indicator validity was measured by frequency of occurrence in highquality sites (observed true positive rate) and non-high quality sites (observed false positive rate) relative to predicted rates. For predicted rates we focused on speci city and used the LCL (lower con dence limit) for true positives and 1 -UCL (upper con dence limit) for false positives. In valid indicators observed true positives should equal or exceed the predicted rate (observed ≥ Speci city LCL) and observed false positives should be at or below the predicted rate (observed ≤ Speci city 1 -UCL). Clearly false positive rates should be zero or reasonably small (say < 0.2) and true positive rates reasonably large (say ≥ 0.5) in any indicators chosen for practice.

Final indicators
We judged indicators for practical use based on indicator value (Speci city × Sensitivity), indicator validity, and two species characteristics: regional hydrophytic status (FACW or OBL according to USACE 2010) and Oklahoma C value (Ewing & Hoagland 2012; B. Hoagland pers. comm.). Hydrophytic status ensures the indicator is more representative of wetlands than non-wetlands (Tiner 1999), and high to moderate C value (≥ 4) helps prevent weedy species from being selected as indicators of intact wetland conditions.

Results
Across the ve samples we extracted 40 candidate indicators of high oristic quality wetlands of prairie ecoregions and 29 candidate indicators for northern Central Great Plains depressions (Tables 1, 2, S1, S2 and Figs. 2, 3, S1, S2). Depending on the sample, the broader context (prairie ecoregions) revealed 12 to 17 indicators and narrower context (CGP depressions) 8 to 12 indicators. The candidate pool for CGP depressions showed overall greater performance (higher indicator values, higher true positive rates, and lower false positive rates on average) than candidates for prairie wetlands. Most candidates were hydrophytes (79.4% OBL or FACW) and the majority had low to moderate C values (70.6% ≤5). High quality and non-high quality sites had similar conservatism distributions (Fig. 4).
The overall best indicator of high oristic quality was Eleocharis compressa based mainly on indicator value (> 0.3 in prairie ecoregions, > 0.4 in CGP depressions) and ecological conservatism (6) ( Tables 1, 2, S1, S2). This was also a robust indicator being the only candidate selected across all ve samples in both contexts. However, in prairie ecoregions E. compressa failed the validity test in all but one sample (Figs. 2, S1). Performance improved in the CGP depressions where it passed the true positive test (observed ≥ Speci city LCL) in all samples and attained near 75% occupancy in four samples (Figs. 3, S2). However, it failed the false positive test (observed ≤ Speci city 1 -UCL) in three samples (Fig. 3, S2).
Some candidates outperformed E. compressa in certain criteria, context, or samples but were lacking overall or in key respects. For example, all candidates showing higher indicator values (Phyla lanceolata, Salix nigra, Typha angustifolia) had lower C value than E. compressa, whereas all candidates with equal or higher C value (Ammannia coccinea, Amorpha fruticosa, Boehmeria cylindrica, Ludwigia repens, Rhynchospora corniculata) showed weaker indicator values (≤ 0.2) than E. compressa. Many candidates appeared to trade off on performance (indicator value and validity) and species characteristics (hydrophytic status, ecological conservatism). For example, relatively conservative (6) and hydrophytic (FACW) Amorpha fruticosa showed evidence of false positive validity only, whereas Rumex crispus passed both validity tests but is introduced (C = 0) and ambiguously hydrophytic (FAC status). Or similarly, wetland obligates Leersia oryzoides and Schoenoplectus pungens had higher indicator values but lower C value (4) than A. fruticosa. One of the best indicators after E. compressa may be Juncus torreyi (C = 6) with its respectable indicator values (0.299 to 0.337) and evidence of false positive validity (~ 0.1 observed rates in two samples), but this applied only to the narrower spatial-environmental context ( Table 2 and Figs. 3, S2).

Discussion
By short-cutting full community Floristic Quality Assessment, indicator species (Dufrêne and Legendre 1997; Siddig et al. 2016) could drastically accelerate eld identi cation of high oristic quality and help to screen or verify potential reference-quality wetlands, including veri cation of predictions made remotely (e.g., Host et al. 2005). Unfortunately, the present results do not support using indicator species to rapidly discriminate high oristic quality wetlands in the US southern plains. In the same study area Bried et al. (2014) reported plant indicator species of potential reference-quality wetlands de ned by multimetric vegetation criteria, including the oristic quality benchmark (≥ 20.0 FQI) used here. They reported some encouraging indicator values and false positive predictions but lacked a validation analysis (i.e. comparison with observed misclassi cation rates).
Several challenges may limit the performance of oristic quality indicator species. First and foremost, the species are being asked to indicate a level of quality derived from community data. We assumed species combinations (De Cáceres et al. 2012) would mitigate this disproportionality, but few species pairs and only one triplet combination met any speci city and sensitivity thresholds. Secondly, indicator species are always xed to a target, but oristic quality is a "moving target" because theoretically many species compositions can result in the same level of quality. It seems indicator species would need strong associations or positive co-occurrence patterns with a large fraction of the assemblage to adequately represent oristic quality. One candidate (Eleocharis compressa) appeared in every scenario and consistently showed among the highest indicator values. This species also possesses a high C value (6) relative to the low-biased C distribution across the dataset (Fig. 4). In western Oklahoma E. compressa can dominate in small depressions (often interdunal swales) on clay soils (Hoagland 2002), suggesting it may occur commonly enough for practice. Some candidates selectively outperformed E. compressa but most of the nal pool struggled on one or both sides of validity. We used the most conservative false positive prediction (1 -Speci city UCL) when measured against observed rates, in part to counterbalance the more relaxed true positive measure (Speci city LCL). Even if we relaxed the false positive prediction (to 1 -Speci city) many of these indicators still would not pass the test, or they would run an unacceptable 20-30% risk of erroneously indicating a site as high quality. Alternatives to traditional indicator value analysis (Dufrêne and Legendre 1997) might lead to improvements. Stapanian et al. (2013), for example, used classi cation and regression tree models to nd indicator species of wetland vegetation quality in Ohio, USA. Their simplest model containing just two species predicted high-quality wetlands with 13% overall misclassi cation rate.
We tested species combinations (De Cáceres et al. 2012) but few combinations appeared in the candidate pool. Perhaps combining the single-species indicators post hoc would help mutually offset their de ciencies. For example, pairing E. compressa with Juncus torreyi, both relatively conservative (C = 6), may reduce misclassi cation error in the narrower spatial-environmental context (CGP depressions). Likewise joining E. compressa with Amorpha fruticosa, Leersia oryzoides, and/or Schoenoplectus pungens may strengthen performance in the broader context of prairie ecoregion wetlands. These latter species also have the advantage of being de nitively recognizable throughout the growing season, unlike E. compressa which owers early to mid-season and can look similar vegetatively to other Eleocharis species in the region (E. albida, E. montevidensis, E. tenuis). The potential drawback is whether such combinations will su ciently occur in the target area, a problem mitigated ad hoc by preset sensitivity thresholds in the analysis of combinations (De Cáceres et al. 2012). Indeed, E. compressa and J. torreyi co-occurred in only 7.4% of our study sites, consistent with Hoagland (2002) who did not detect J. torreyi where E. compressa was most abundant. Similarly, E. compressa co-occurred with A. fruticosa, L. oryzoides, and S. pungens in 4.6%, 6.5%, and 7.4% of sites, respectively, and there was only one site where all four of these species co-occurred.
The present ndings do not preclude the existence of high oristic quality indicators in other regions, especially where wetlands are well classi ed and training samples are given su cient spatialenvironmental context. Nor do our results negate potential for indicators in other ecosystems where Floristic Quality Assessment is commonly applied, namely forests and prairies. However, if wetlands tend to be less oristically diverse than forests and prairies (on average within climate zones) nding strong indicator species in those systems would seem even less likely for the reasons discussed above. Our study also cannot rule out potential for indicators at other levels of oristic quality. Indicators of low quality could be useful in avoiding a costly permitting process (Stapanian et al. 2013) or futile conservation investment, i.e. a site not worth protecting or restoring (a site "beyond repair"). Indicators of moderate quality could help direct protection and management effort to where there is both need and worth. Before broadly concluding that oristic quality indicator species do not exist for wetlands, we recommend exploring indicator potential in other regions and at other quality levels, and perhaps trying other statistical approaches (e.g. Stapanian et al. 2013).

Declarations
Grants awarded through Oklahoma State University; 2016 by the Oklahoma Conservation Commission and EPA National Wetland Condition Assessment; and 2017-2018 by the Department of Natural Sciences at Northeastern State University.

Compliance statement
To the best of our knowledge this study was conducted in compliance with all relevant ethical and legal standards, and all necessary permissions (to access eld sites, to collect and use data) were obtained.
Funding: Please see the Acknowledgments section Con icts of interest/Competing interests: Not applicable Ethics approval: Not applicable Consent to participate: Not applicable Availability of data and material: Available from the corresponding author upon request Code availability: Available from the corresponding author upon request Figure 1 Samples for extraction and validation of indicator species (those reported in Tables 1, 2

and Figs. 2, 3) in
Oklahoma, USA. Floristic quality scores of at least 20.0 (or ≥13.0 west of Interstate 35) separate 'high quality' from 'non-high quality' sites. 'Other' refers collectively to riverine, seep, and lacustrine-fringe wetlands. Prairie ecoregions are labelled and mapped in different colors; forest ecoregions are symbolized in grey.

Figure 2
Indicator validity corresponding with Table 1. Predicted rates are derived from Speci city values in Table   1 and observed rates from the leftover sites (17 high quality, 17 non-high quality) not used in Table 1. Validation results from three additional samples are found in Fig. S1.