Fraxinus uhdei
(tropical ash)

Fraxinus uhdei is a fast-growing, medium to large tree (up to 35-40 m tall and 1 m stem diameter) that grows naturally in mixed mountain forests from west-central Mexico to Costa Rica. It has been introduced to Hawaii, Puerto Rico and sub-Himalayan regions of India for watershed protection and timber production. It is also a popular street and shade tree in California, Arizona and Mexico. F. uhdei is shade tolerant and a prolific seed producer, and has become highly invasive in Hawaii where it has spread from cultivation into disturbed forest areas. It has been recorded as naturaized on the islands of Hawaii, Maui, Molokai and Oahu, where it has been included among the alien species most disruptive to native ecosystems. Invasions are more intense on fertile soils and along streams. In some stands of F. uhdei, understorey vegetation is almost completely absent, but stands with sparse canopies can harbour a profuse understorey of native and introduced species. Chemical control with a range of herbicides has proved to be very effective in Hawaii. Monitoring where introduced and replacement with native trees where naturalization is observed would reduce future risks.

The native range of F. uhdei is commonly cited as mainland Central America, from Costa Rica, through Honduras and Guatemala to Mexico, and it is reported in many Mexican states north to Sinaloa, San Luis Potosi and Durango (USDA-ARS, 2016). McClintock (2001) also notes it in the Mexican states of Oaxaca and Chiapas; in Chiapas it has been observed in pine-oak-liquidambar forest, where it generally occurs along drier streams with other deciduous and semideciduous tree species.

USDA-ARS (2016) reports F. uhdei as native to Bolivia, but this is questionable. Records in the Missouri Botanical Garden (2015) confirm its presence in La Paz and Cochabamba; however, all these records are recent (2014-15) with a single other record from 2009. Thus, also noting the disjunct nature of these occurrences from the Central American native range, and that this species is widely planted as a street tree especially at higher altitudes, it is probable that the Bolivian records represent introductions. In her detailed work on the genus Fraxinus, Wallander (2008) also noted the presence of F. uhdei only in Central America (and Hawaii). F. uhdei (called ‘urapan’) is also recorded in Colombia (Bogota, Medellin, Pereira and Manizales) and Ecuador (Quito), where Fraxinus spp. are noted as widely planted in urban and rural settings in the Andes (Filguera et al., 2004). Thus, South American records are taken here as being introduced, and it is possible that it may be more widespread than reported. However, that the species is also possibly native to the Andean region cannot be discounted, and this assumption requires verification.

In the USA, F. uhdei is introduced, naturalized and invasive in Hawaii (PIER, 2016) and, more recently, in California (Hrusa et al., 2002; Calflora, 2016; USDA-NRCS, 2016). It is present but less common as a street tree in Arizona (Bricker and Stutz, 2005). F. uhdei is grown as an ornamental in Taiwan (Shen and Chen, 1993). As well as Hawaii, it has been introduced as a potential forestry species to Puerto Rico (Liogier and Martorell, 1982) and also India, where Shukla and Sangal (1980) reported on trial plantations established at Dehra Dun in the northern state of Uttarakhand (formerly Uttaranchal) in 1960. The India Biodiversity Portal (2016) has a single report (from 2015) of the species in a tropical moist deciduous forest near Dahanu in northwestern Maharashtra.

As an ornamental street tree, it is also possible that it is has been more widely introduced than is reported. 

F. uhdei is reported in a Pacific island risk assessment to be invasive or potentially invasive, with a high risk of invasion score of 11 (PIER, 2016). Acknowledging its use and value as an ornamental and street tree, as well as a forestry species, it is highly likely to be introduced further.

In Hawaii, where F. uhdei is most invasive, it is observed commonly in forests and forest edges, rapidly colonizing disturbed forest areas (Weber, 2003). Invasions are also more intense on fertile soils and along streams (Harrington and Ewell, 1997). In Hawaii, it is an invasive at high altitude zones (PIER, 2016), but in California and elsewhere it also naturalizes at lower elevations.

It thrives in moist soil conditions, is typically confined to water courses and moist areas, but can survive arid conditions. In California where it is used as a street tree it appears to be spreading into canyons below mesas, and is also seen to be invading oak woodland as well as riparian and wetland areas (Council for Watershed Health, 2016).

Reproductive Biology

Section Melioides contains 15 American Fraxinus species, all dioecious trees with unisexual apetalous and wind-pollinated flowers (Wallander, 2001), though flowers of F. uhdei are known to be visited by bees, such as in Colombia (Obregon and Nates-Parra, 2014). F. uhdei regenerates rapidly from seed and trees coppice vigorously when cut and have also been reported to sucker.

Physiology and Phenology

F. uhdei trees are fast growing, reaching 7.5-10.5 m in 10 years. They are fire and shade tolerant; Pimienta-Barrios et al. (2012) report on the ecophysiological responses of F. uhdei to drought and recovery after rainfall. The species has also shown growth and physiological changes along an elevational gradient in Hawaii (Ares and Fownes, 2000).

When Gleason and Ares (2004) examined the physiological and morphological characteristics of Acacia koa (koa) and F. uhdei to understand the possible mechanisms facilitating invasion of koa forests in Hawaii uplands by F. uhdei, they found that F. uhdei was better able to survive defoliation than koa especially under high-light conditions, and seedlings allocated more carbon and nitrogen to storage, favouring their survival in high light under intense herbivory and on sites with seasonal or highly variable N availability. Ares and Fownes (2001) also reported on the competitive interactions between these two species.

F. uhdei is a deep-rooting species; roots in one study were seen to penetrate deeper than those of Quercus crassipes and Q. crassifolia. In fine-textured soil, F. uhdei root and shoot dry weight was significantly greater than in either Quercus species, but this was not the case in coarse-textured soil (Chacalo et al., 2000).

Regarding the chemical composition of plant parts, 18 compounds were isolated from the fruits, leaves and bark of F. uhdei, including secoiridoids, phenylpropanoids, lignans, phenylethanoids and triterpenoids, plus mannitol and β-sitosterol (Pérez-Castorena et al., 1997).

Longevity

Seeds of F. uhdei may remain viable in the soil for at least eight years (Council for Watershed Health, 2016).

Associations

In Mexico, F. uhdei is associated with the following forest plant communities: Quercus forest, pine forest, montane mesophyllic forest and gallery forest (Zepeda Dominguez, 2006). McClintock (2001) noted it in pine (Pinus) - oak (Quercus) - liquidambar (Liquidambar) forest in Chiapas. In the floodplain between Puebla and San Martín Texmelucan in the state of Puebla at an altitude of 2200 m, F. uhdei was found growing along watercourses in association with Alnus glabrata [Alnus acuminata subsp. glabrata], Salix bonplandiana, Buddleja cordata, Schinus molle and Populus species (Rzedowski, 2006).

In western North America F. uhdei is a food plant for larvae of several swallowtail butterflies, including Papilio eurymedon, P. multicaudata and P. rutulus (Calflora, 2016).

Environmental Requirements

F. uhdei is generally a species of subtropical upland areas up to 2750 m above sea level in Mexico, and also occasionally up to higher elevations in the Andes, to 3600 m in Bolivia (Missouri Botanical Garden, 2015), but it is also known to grow, but only rarely, in coastal areas. The species prefers higher rainfall zones, though tolerates dry seasons up to six months. Although it prefers light and fertile soils, according to Calflora (2016) it tolerates fine, medium or coarse soils and very slightly saline conditions. It prefers a soil pH of 5.7 to 7.6 and a temperature range of 4ºC in winter to 32ºC in summer. 

F. uhdei is attacked by two phytoplasmas, Phytoplasma asteris and Phytoplasma fraxini, causing ash yellows. Perilla-Henao et al. (2012) noted P. asteris for the first time on F. uhdei in Colombia, and Filgueira et al. (2004) provided the first confirmed record of ash yellows occurring outside North America, on F. uhdei in Colombia and Ecuador, with significance for Fraxinus species which are widely planted in urban and rural settings throughout the Andean range, as well as natural stands in Mexico. Testing in Arizona, USA, for the presence of ash decline showed symptoms of this phytoplasma disease (progressive tree dieback) on F. velutina but not on F. uhdei (Bricker and Stutz, 2005).

Foliar necrosis, related to the prevalence of Alternaria alternata, was observed in F. uhdei in Azcapotzalco, Mexico, and a potential association between the ash flower eriophyid (Aceria fraxiniflora) and the micromycete Fusarium sporotrichioides was also noted (Reséndiz Martínez et al., 2015).

F. uhdei is also a host to the glassy-winged sharpshooter (Homalodisca vitripennis) which is a vector for Pierce’s disease of grapevines caused by Xylella fastidiosa (Calflora, 2016).

Environmental

• Amenity
• Landscape improvement
• Windbreak

Fuels

• Fuelwood

Materials

• Wood/timber

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical

Ornamental

• garden plant

Boats

Furniture

Pulp

• Short-fibre pulp

Roundwood

• Pit props
• Posts
• Stakes

Sawn or hewn building timbers

• Carpentry/joinery (exterior/interior)
• Flooring
• For light construction
• Wall panelling

Veneers

Wood-based materials

• Fibreboard
• Hardboard
• Medium density fibreboard
• Particleboard
• Plywood

Woodware

• Cutlery
• Industrial and domestic woodware
• Sports equipment
• Tool handles
• Toys
• Turnery
• Wood carvings

F. uhdei was originally described as a variety of F. americana, apparently based on similarities in samara morphology. The abaxial leaf surfaces of F. uhdei, however, lack the papillose cuticular structure characteristic of F. americana and its presumed closest relatives in eastern North America (Nesom, 2010). A channeled leaf axis and a smaller (2.5-4 cm long) samara also distinguish F. uhdei from F. americana.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

The Council for Watershed Health (2016) suggests alternative species to F. uhdei for planting as street trees in areas where it has become invasive in California, including several native species.

Cultural Control and Sanitary Measures

When Friday et al. (2008) assessed whether disturbance from logging facilitated the spread of F. uhdei and other alien species beyond existing plantations on windward Mauna Kea on the island of Hawaii, they found that F. uhdei was able to regenerate following logging more rapidly than native tree species. Although basal area growth of F. uhdei was large enough to offset a decline in native trees and cause an increase in forest productivity, it was recommended that when F. uhdei plantations are harvested, forest managers should plan ways of favouring regeneration of the native Acacia koa, a species more valuable both for timber and for conservation.

Physical/Mechanical Control

Fraxinus trees coppice readily and can propagate vegetatively, so any mechanical control must also remove the roots. Manual or mechanical pulling of seedlings and small saplings is recommended by Tunison (1991).

Chemical Control

Tunison (1991) reports on research in Hawaii which found that undiluted triclopyr amine salt in a continuous ring notch application offered complete control of F. uhdei. Application of triclopyr to cut stumps was more practical for smaller trees. Lower concentrations than that recommended for notching were also effective. Control should begin with outlying plants and the edges of larger stands, progressing inwards toward the centre of the stand.

In other chemical control trials conducted in Hawaii, F. uhdei was controlled completely by cut-surface (continuous ring) applications of 2,4-D, dicamba, glyphosate or picloram. If applied in two notches in the trunk, all these herbicides except picloram also caused severe injury, but trees took 18 months to die. Better control was achieved if herbicide was applied in four drilled holes per tree and continuous ring notching, when using dicamba, glyphosate, metsulfuron or triclopyr. Good control was also reported with triclopyr ester in diesel oil applied to basal bark, and with undiluted triclopyr amine applied to frills, though only saplings were susceptible to triclopyr ester applied to basal bark (Motooka et al., 2002, 2003).

20/01/2016 Invasive Species Compendium sections added by:

Nick Pasiecznik, Consultant, France