Learn · Sources
Sources
Every number on the plant cards, the tolerance lens and the notebook checks comes from one of these.
Evidence grades
Agreement with a source is evidence about the claim, not about your plant.
- A
- Measured on that species in a peer-reviewed study.
- B
- An agency or extension rating.
- C
- Inferred from related species, a multi-species study or general physiology, or a teaching cut-point around a cited anchor.
Data: how common each plant is, and the weather
Three different counts of Philadelphia plants, read on 9 October 2026, plus the air-quality records and the AQI conversion.
Philadelphia Parks & Recreation. Philadelphia Tree Inventory, 2025 snapshot (layer ppr_tree_inventory_2025, 151,726 records). OpenDataPhilly. Accessed 2026-10-09.
Used for: Street-tree counts and shares behind “In Philadelphia” classes.
Nowak DJ, et al. (2016) The urban forest of Philadelphia. Resource Bulletin NRS-106. USDA Forest Service, Northern Research Station. Field data from 2012.
Used for: Share of all woody stems and leaf area by species, citywide.
iNaturalist. Research-grade plant observations, Philadelphia County, PA (place 2983; 1,634 species). Public API, accessed 2026-10-09.
Used for: Record counts and ranks for shrubs and herbs that no census covers.
Philadelphia City Planning Commission (2022) Recommended Planting List.
Used for: All eight field-observation plants are on the city’s planting list.
US EPA AirNow and reporting agencies. Daily data files, 2025, Philadelphia County monitors (preliminary, not fully validated).
Used for: Daily AQI and its defining pollutant for the ozone lens.
US Code of Federal Regulations, 40 CFR Part 58, Appendix G: Uniform Air Quality Index (AQI) and Daily Reporting, Table 2, as amended 6 March 2024.
Used for: Converting AQI back to 8-hour ozone in ppb.
Tolerance evidence: heat, frost, water and ozone
Behind every band in the tolerance lens. Where no study measured the species itself, the band is graded C.
Niinemets Ü, Valladares F (2006) Ecological Monographs 76:521–547. Appendix A: shade, drought and waterlogging tolerance scores (0–5).
Used for: Drought and waterlogging tolerance scores for the woody plants.
USDA NRCS PLANTS Database, plant characteristics (drought, anaerobic and salinity tolerance; minimum temperature). Accessed 2026-10-09.
Used for: Agency ratings; fields that conflict with other authorities are flagged in the report.
Walters RS, Yawney HW (1990) Red maple. In: Burns RM, Honkala BH (eds) Silvics of North America, Vol. 2. Agriculture Handbook 654. USDA Forest Service.
Used for: Red maple cold range limit, drought response and 60-day flood tolerance.
Lahr, Dunn & Frank (2018) PLoS ONE 13:e0197866.
Used for: Red maple photosynthesis declines as leaf temperature rises from 26 °C.
Xie Y, et al. (2015) PNAS 112:13585–13590.
Used for: Moderate heat and drought delayed autumn dormancy; Tmax ≥ 35 °C days advanced it.
Xie Y, Wang X, Wilson AM, Silander JA Jr (2018) Predicting autumn phenology: how deciduous tree species respond to weather stressors. Agricultural and Forest Meteorology 250–251:127–137.
Used for: Twelve NE US species: warmth and drought delay autumn; heat stress and heavy rain advance it.
Augspurger CK (2013) Ecology 94:41–50.
Used for: Frost of −1.7 °C after budburst; damage years with minima of −6.1 °C.
Augspurger CK (2009) Functional Ecology 23:1031–1039.
Used for: Refoliation after the 2007 freeze delayed full leaf area by 16–34 days.
Vitasse Y, Lenz A, Körner C (2014) Frontiers in Plant Science 5:541.
Used for: Emerging leaves of temperate trees: LT50 between −2 and −8 °C.
Granier A, et al. (2007) Agricultural and Forest Meteorology 143:123–145.
Used for: Stomatal closure below 0.4 relative extractable water; respiration falls below 0.2.
Lee EH, Andersen CP, Beedlow PA, et al. (2022) Ozone exposure-response relationships parametrized for sixteen tree species with varying sensitivity in the United States. Atmospheric Environment 284:119191.
Used for: Red maple seedlings: 5% biomass loss at a seasonal W126 of 20.8 ppm-h.
Schaub M, et al. (2003) Environmental Pollution 124:307–320.
Used for: Maple rarely showed visible ozone injury.
Kline LJ, Davis DD, Skelly JM, et al. (2008) Northeastern Naturalist 15:57–66.
Used for: Ozone sensitivity groups; all species showed some symptoms at 90 ppb.
US National Park Service. Ozone-sensitive plant species and bioindicators.
Used for: Milkweeds and buttonbush listed as ozone-sensitive.
Ranney TG, Peet MM (1994) Journal of the American Society for Horticultural Science 119:243–248.
Used for: River birch kept the highest photosynthesis in heat; injury near 49 °C leaf temperature.
Ranney TG (1992) HortScience 27:627d.
Used for: Birch carbon exchange depressed at 40 °C leaf temperature.
Griffin, Ranney & Pharr (2004) Journal of the American Society for Horticultural Science 129:497.
Used for: Redbud photosynthetic optimum of 37 °C.
Abrams MD (1986) Canadian Journal of Forest Research 16:1170.
Used for: Redbud kept conductance at low soil water potential.
Allen RG, Pereira LS, Raes D, Smith M (1998) Crop evapotranspiration: guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56.
Used for: Depletion fraction p used to set water-stress onset for wetland and meadow plants.
The Morton Arboretum. Tree and plant profiles.
Used for: Extension ratings for drought sensitivity and summer leaf drop.
NC State Extension Gardener Plant Toolbox.
Used for: Species facts, reference photos and extension ratings.
Lady Bird Johnson Wildflower Center, University of Texas at Austin. Native plant database.
Used for: Moisture and heat ratings for the herbs.
University of Minnesota Extension. Powdery mildew on flowers.
Used for: Warm dry days after humid nights favour mildew on bee balm.
Methods: how science checks a plant and a model
The field protocols, photo greenness, model checks and scoring used in reports A and B.
Denny EG, Gerst KL, Miller-Rushing AJ, et al. (2014) Standardized phenology monitoring methods to track plant and animal activity for science and resource management applications. International Journal of Biometeorology 58(4):591–601.
Used for: Status and intensity monitoring; onset bracketed by the last “no” and first “yes”.
USA National Phenology Network (2021) Plant and Animal Phenophase Definitions, Version 2.0. USA-NPN technical document.
Used for: The six canopy intensity classes for Leaves and Colored leaves.
Liu G, Chuine I, Denéchère R, et al. (2021) Higher sample sizes and observer inter-calibration are needed for reliable scoring of leaf phenology in trees. Journal of Ecology 109(6):2461–2474.
Used for: Observers differ by about 8 days at budburst; 20–30 trees are needed.
Richardson AD, Hufkens K, Milliman T, et al. (2018) Tracking vegetation phenology across diverse North American biomes using PhenoCam imagery. Scientific Data 5:180028.
Used for: Green chromatic coordinate (GCC) from region-of-interest mean colour.
Sonnentag O, Hufkens K, Teshera-Sterne C, et al. (2012) Digital repeat photography for phenological research in forest ecosystems. Agricultural and Forest Meteorology 152:159–177.
Used for: Why GCC damps illumination changes; timing comparable across cameras.
Hufkens K, Basler D, Milliman T, Melaas EK, Richardson AD (2018) An integrated phenology modelling framework in R. Methods in Ecology and Evolution 9:1276–1285.
Used for: Model error in days against an average-date null model.
Basler D (2016) Evaluating phenological models for the prediction of leaf-out dates in six temperate tree species across central Europe. Agricultural and Forest Meteorology 217:10–21.
Used for: Typical leaf-out model errors of 4–9 days.
Gneiting T, Raftery AE (2007) Strictly proper scoring rules, prediction, and estimation. Journal of the American Statistical Association 102(477):359–378.
Used for: Brier score for checkable predictions.
Dietze MC, Fox A, Beck-Johnson LM, et al. (2018) Iterative near-term ecological forecasting: needs, opportunities, and challenges. PNAS 115(7):1424–1432.
Used for: Predict, observe, score, update as an ecological forecasting cycle.
Schomaker ME, Zarnoch SJ, Bechtold WA, et al. (2007) Crown-condition classification: a guide to data collection and analysis. Gen. Tech. Rep. SRS-102. USDA Forest Service.
Used for: Crown dieback classes, simplified for students.
Smith GC, Coulston JW, O’Connell BM (2008) Ozone bioindicators and forest health. Gen. Tech. Rep. NRS-34. USDA Forest Service.
Used for: Ozone stipple as a visible sign, never a diagnosis on its own.
Meng L, Mao J, Zhou Y, et al. (2020) Urban warming advances spring phenology but reduces the response of phenology to temperature in the conterminous United States. PNAS 117(8):4228–4233.
Used for: City trees leaf out earlier than the airport record suggests.
Zani D, Crowther TW, Mo L, Renner SS, Zohner CM (2020) Increased growing-season productivity drives earlier autumn leaf senescence in temperate trees. Science 370(6520):1066–1071.
Used for: One side of the contested autumn-timing debate.
Lu X, Keenan TF (2022) No evidence for a negative effect of growing season photosynthesis on leaf senescence timing. Global Change Biology 28(9):3083–3093.
Used for: The other side of the autumn-timing debate.
Chuine I, Bonhomme M, Legave J-M, et al. (2016) Can phenological models predict tree phenology accurately in the future? The unrevealed hurdle of endodormancy break. Global Change Biology 22(10):3444–3460.
Used for: One year cannot separate chill-plus-heat from heat-only spring models.