Heterotheca subaxillaris

Heterotheca subaxillaris
Photo by Allen Boatman, Atlas of Florida Vascular Plants
Scientific classification
Kingdom:	Plantae
Division:	Magnoliophyta - Flowering plants
Class:	Magnoliopsida - Dicotyledons
Order:	Asterales
Family:	Asteraceae ⁄ Compositae
Genus:	Heterotheca
Species:	H. subaxillaris
Binomial name
Heterotheca subaxillaris (Lam.) Britton & Rusby
Natural range of Heterotheca subaxillaris from USDA NRCS Plants Database.

Common name: Camphorweed

Taxonomic notes

Synonyms: none.

Varieties: none.

Description

A description of Heterotheca subaxillaris is provided in The Flora of North America.

Heterotheca subaxillaris is an annual in temperate climates and a short lived perennial in subtropical climates. Leaves are simple and are alternately arranged, with one leaf per node. The leaves produce a camphor-like aroma which defers herbivores. The yellow composite flowers are arranged in diffuse, paniculate corymbs. It is a C3 plant in carbon fixation.

Distribution

The range of this plant occurs as north as New Jersey, south to Florida, and west to Texas and Mexico.

Ecology

Habitat

In the Coastal Plain in Florida and Georgia, H. subaxillaris occurs in open sand with sea oats, pine-scrub oak sand ridges, longleaf pine-wiregrass savannas, and upper beaches. It thrives in disturbed areas and has been observed to be a ruderal species that quickly colonizes xeric habitats. It is able to compete well against other species in disturbed habitats due to the overwintering rosettes shading competing seeds. It has been found in disturbed areas such as railroad bridges, powerline corridors, vacant lots, along highways, wet roadside ditches, near phosphate ponds, fire lines, disturbed coastal dunes, and cleared sand pine-evergreen oak scrub. Areas in which it inhabits typically have low levels of soil nutrients and high sand temperature when nitrogen levels are low, it produces an increased amount of allelochemicals and allocates more carbon to root growth. Soil types include loamy sand and sand. Associated species include Andropogon, Baccharis, Setaria, Cenchrus, Distichlis, Paspalum urvillei, P. notatum, Eragrostis oxylepis, Eleusine indica, Digitaria sanguinalis, Cyperus surinamensis, Ambrosia artemisiifolia, Strophostyles helvola, Solanum americanum, S. sisymbrifolium, Daubentonia drummondii, and Sesbania exaltata.

Phenology

H. subaxillaris has been observed to flower March through January and fruit in September. It is self-incompatible and depends on insects for pollination. H. subaxillaris has been observed flowering at different times in different areas, probably an adaptation to a shorter growing season.

Seed dispersal

Seeds are dispersed by wind.

Seed bank and germination

H. subaxillaris is a heterocarpic species, the achenes produced by the ray flowers lack a pappus and require a long period of dormancy. The achenes produced are dimorphic and lack albumin.

Germination occurs in the spring or fall; with seeds that germinate in the fall overwintering in a rosette and forming a taproot. In coastal dunes, there is an absence of a persistent seed bank for H. subaxillaris; however, it has been observed to germinate from a highly disturbed seed bank. Seedlings form dense mats below the dead parent plants in late winter and early spring.

Pollination and use by animals

Heterotheca subaxillaris has been observed at the Archbold Biological Station to be pollinated by ground-nesting bees from the Andrenidae family such as Andrena fulvipennis, sweat wasps from the Halictidae family such as Agapostemon splendens, Augochloropsis metallica, A. sumptuosa, Halictus poeyi, Lasioglossum nymphalis and L. tamiamensis, leafcutting bees from the Megachilidae family such as Anthidium maculifrons, Coelioxys mexicana, Megachile albitarsis, M. brevis pseudobrevis, M. inimica and M. xylocopoides, thread-waisted wasps from the Sphecidae family such as Bicyrtes capnoptera, Microbembex monodonta and Tachytes validus, and wasps from the Vespidae family such as Pachodynerus erynnis, Stenodynerus fundatiformis and Zethus spinipes. Additionally, this species has been observed with ground-nesting bees from the Andrenidae family such as Calliopsis sp., Macrotera sp., Perdita sp., Protandrena sp., and Pseudopanurgus sp., long-tongued bees from the Apidae family such as Anthophora sp., Bombus sp., Ceratina sp., Exomalopsis sp., Holcopasites sp., Nomada sp., Svastra sp. and Tetraloniella sp., plasterer bees from the Colletidae family such as Colletes sp., sweat bees from the Halictidae family such as Agapostemon sp., Dieunomia sp., Halictus sp. and Lasioglossum sp,, leafcutting bees from the Megachille family such as Ashmeadiella sp., Coelioxys sp., Dianthidium sp., Heriades sp., Megachile sp. and Paranthidium sp., thread-waisted wasps such as Sphex ichneumoneus (family Sphecidae), planthoppers such as Pissonotus delicatus (family Delphacidae), plant bugs such as Macrotylus amoenus (family Miridae), stink bugs such as Euschistus servus (family Pentatomidae), and assassin bugs such as Apiomerus crassipes and Apiomerus flaviventris.

In defoliated plants, there is an increased biomass allocation to shoot growth, maintaining a balance between carbon and nutrient income similar to non defoliated plants. Defoliated plants have a slow growth rate, suggesting that even though proportionally greater allocation to the shoots occurs after defoliation, both carbon income and nutrient uptake are reduced.

References and notes

Cheplick, G.P., 2005. Patterns in the distribution of American beachgrass (Ammophila breviligulata) and the density and reproduction of annual plants on a coastal beach. Plant Ecology, 180, 57-67

Olsen, Kenneth M.. “Pollination Effectiveness and Pollinator Importance in a Population of Heterotheca Subaxillaris (asteraceae)”. Oecologia 109.1 (1997): 114–121