Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Spring Wildflowers - Some Notes

I.  Species in bloom

1. Spring ephemerals - woodland species
2. Open site species (weeds, prairies)
3. Trees - pollinating
4. Differ from Europe to North America (similar genera of trees/shrubs but different herbs; speculation that the European spring herb flora evolved from summer-dry Mediterranean species)

II.  Features of Spring Habitats

1. Light - lots, no leaves on trees; after canopy closure, little light available
2. Temperature - temperature gradient (cool below soil surface, warmer near forest floor and getting progressively cooler as go into canopy). May explain typical sequence of leafing out:  herbs → shrubs → trees
3. Water - moister, still air near forest floor, greater availability for herbs; trees leaf out later, after adequate water supply insured
4. Forests - some litter, not too wet, not too mineralized

III.  Characteristics of spring woodland plants

1. perennials (some winter annuals like shepherd's purse and some annuals)
2. bulbs, tubers, corms, rhizomes
3. often poisonous - especially underground portions (e.g., Jack-in-the-pulpit - calcium oxalate crystals)
4. usually shorter than summer plants, often rosettes
5. leaves various - usually different than trees/shrubs in habitat, more similar to wetland species
6. dissected leaves - decreases size of boundary layer → increase carbon dioxide uptake (adequate moisture so water loss isn't a major concern) → more photosynthesis
7. higher rate of photosynthesis than summer plants
8. lots of rubisco
9. light saturation point (about 1/2 full sun - spring ephemerals vs. 1/40 full sun for forest species later in summer)
10. carbon dioxide compensation point (0.5 - 2% full sun spring vs.  0.1 - 0.5% forest)
11. lots of nitrogen in leaves
12. require cold to break winter bud dormancy
13. require cold to stimulate flowering
14. require cold to break seed dormancy and stimulate seed germination
15. flowers often white (to be seen more easily against forest floor)
16. flowers often bowl-shaped (unspecialized, available to more types of pollinators which are generally limited in spring)
17. many are self-compatible (to insure seed set)
18. long bloom period (to be available for limited pollinators)
19. less allocation to reproduction (more to vegetative parts)

IV.  Sites to find spring ephemerals

1. extremely shady in summer (shade tolerance important determines species in forest - for example, sugar maple shade tolerant, many young ones in forests, suggests much of St. John's will become sugar maple in future; oaks - not shade tolerant, cannot grow well in own shade, thus need special techniques to propagate (shelterbelts).
2. disturbed by grazing or other
3. soil dry in summer

V.  Data collected

Conclusion:  Tree cover makes a significant difference in amount of light that reaches the forest floor

Conclusion:  The soil warms up more slowly than the air.  Air temperature is often warmest at the surface of the forest floor and gets cooler.  We detected little difference up to two meters.

Last updated:  04/28/2009     � Copyright  by SG Saupe