Croton alabamensis, known as Alabama croton, is a rare species of flowering plant in the spurge family, Euphorbiaceae, that is endemic to Texas and Alabama in the Southeastern United States. It has two varieties whose ranges are separated by more than 1000 km: Croton alabamensis var. alabamensis (Alabama croton) is found in two central Alabama counties, while Croton alabamensis var. texensis (Texabama croton) is found in three counties in Texas.
The Alabama croton is a semi-evergreen monoecious shrub that reaches a height of 5–35 dm. It is the northernmost shrubby species of the genus Croton in North America and the largest species of Euphorbiaceae native to North America. It is grown as an ornamental for its form and foliage and is valued for its drought tolerance, low maintenance, and herbivore resistance. Although normally forming loose clonal colonies, it can be trained as a single-trunked, dense shrub. C. alabamensis and its nominate variety are considered Vulnerable by NatureServe, while C. a. var. texensis is considered Imperiled. Threats include habitat loss and forestry practices.
Description
The Alabama croton is a semi-evergreen monoecious shrub that reaches a height of 5–35 dm[2] The loose, multi-stemmed thickets[3] it forms are colloquially known as "privet brakes".[4]C. alabamensis is the northernmost shrubby species of the genus Croton in North America[5] and the largest species of Euphorbiaceae native to North America.[6]
Leaves
Leaves are clustered at branch tips and lack stipules. The petiole is 0.6–2 cm. Glands are absent at its apex. Leaf blades are elliptic, ovate, or oblong and measure 3–10 × 1.5–5 cm. Blade margins are typically entire, sometimes undulate. The base is rounded to obtuse, while the apex is acute, rounded, or emarginate. Abaxial surfaces are silvery or coppery and densely lepidote, while adaxial surfaces are green and sparsely lepidote.[2] When crushed, leaves emit a fragrance similar to a mixture of banana and apple.[7]
Flowers
The inflorescence is a bisexual or unisexual raceme 2–4.5 cm in length. Racemes have 0–15 staminate flowers and 0–10 pistillate flowers.[2] Flowers are pollinated by bees.[4] Pistillate flowers are formed earlier and are lower in the raceme than staminate flowers. [8] Flowering occurs in the spring, with peak flowering occurring in mid-March.[9]
Staminate (male) flowers
The pedicels of staminate flowers are 2.2–4 mm long. The 5 sepals are 1.1–2.9 mm in length and have a lepidote abaxial surface. The 5 petals are oblong-ovate and 2–3.1 mm in length with an abaxial surface that is glabrous except at margins, where it is stellate-ciliate. Staminate flowers have 10–22 stamens.[2]
Pistillate (female) flowers
Pedicels of pistillate flowers are 2.2–7.5 mm in length, increasing to 7–11 mm when in fruit. The 5 sepals measure 2–4.5 mm in length, have entire margins, an incurved apex, and a lepidote abaxial surface. The 5 petals are pale green, ovate, and 2–3.5 mm in length. The ovary is 3-locular. There are 3 styles 2–5 mm long, usually unbranched, rarely bifid, with 3 terminal segments (sometimes 6).[2]
Fruit and seeds
The fruit is a smooth capsule measuring 1.6–2.5 × 2–3 mm. The columella is 3-angled.[2] Capsules ripen by June, after which seeds are explosively dehisced.[9] Seeds are shiny and measure 6.7–8 × 5.2–6 mm.[2] Seeds require cold stratification in order to germinate.[9]
Varieties
There are two varieties that are separated by more than 1000 km:[5][10]
Texabama croton is restricted to the eastern Edwards Plateau[10] and southern Cross Timbers[14] within Travis, Bell and Coryell counties. Within this limited range, it has a very patchy distribution and is abundant in the few niches where it occurs and is absent elsewhere. C. alabamensis var. texensis inhabits pockets of deep soils within forested, mesic canyons and upland oak mottes[15] at elevations of 200 to 400 m.[16]
Phylogeny and evolutionary history
Croton alabamensis belongs to the monotypic section Alabamenses within the subgenus Quadrilobi.[17] Its closest relatives inhabit primarily mesic regions in the Neotropics, such as members of the former genus Moacroton, which are restricted to serpentine soils in Cuba.[18]Molecular clock analysis suggests it split from its closest relative in the middle Eocene, approximately 41 million years ago. The Alabama and Texas varieties diverged much more recently in the Quaternary, possibly due to allopatric speciation.[5]
Croton alabamensis was discovered in July 1877 by state geologist Eugene Allen Smith on limestone bluffs at Pratt's Ferry near Centreville, Alabama during a geological survey. Specimens of the plant were sent to botanist and pharmacist Charles T. Mohr, who then passed them to botanist Alvan Wentworth Chapman. Chapman formally described C. alabamensis as a new species in the second edition of his Flora of the Southern United States.[9]C. alabamensis var. texensis was discovered in April 1989 at Fort Cavazos in Coryell County by Carol Beardmore and Rex Wahl.[37] Two months later, plants were independently discovered by Chuck Sexton approximately 70 km to the south on the Post Oak Ridge[38] of the future Balcones Canyonlands National Wildlife Refuge in northwestern Travis County.[37][39] Steve Ginzbarg, a botanist at the University of Texas at Austin, formally described it as a distinct variety of C. alabamensis in 1992.[38]
^ abHalward, Tracy; Lowrey, Timothy; Schulz, Keith; Schulz, Keith; Shaw, Robert (1996). "Germination Requirements and Genetic Diversity in Croton alabamensis var. texensis". Southwestern Rare and Endangered Plants Proceedings of the Second Conference September 11–14, 1995, Flagstaff, Arizona. USDA Forest Service.
^Poole, Jackie M.; Carr, William R.; Price, Dana M. (2007). Rare Plants of Texas: A Field Guide. Texas A&M University Press. p. 163. ISBN9781585445578.
^ ab"Croton alabamensis". North Carolina Extension Gardener Plant Toolbox. North Carolina State University. Retrieved 2024-06-01.
^Cullina, William (2020). Native Trees, Shrubs, and Vines: A Guide to Using, Growing, and Propagating North American Woody Plants. Echo Point Books and Media. ISBN9781648370014.