A study of Fraxinus americana L. and Fraxinus pennsylvanica Marsh. in the Ottawa district


Dugal, Albert William.




Fraxinus americana L. and F. pennsylvanica Marsh. were studied to determine whether these species are distinct in the Ottawa district. Five populations (two red ash, two white ash and one mixed population) were selected for study. Each tree was mapped and tagged, and from 1966-I969, fruiting and flowering behaviour was studied. In 1966, using twigs collected from the study populations plus an additional group of trees on King Mountain, measurements were made of growth of new twigs, and length of the first, second, third, fourth and fifth leaves. The ontogenetically second leaf (usually the largest in both species) was selected for intensive study of the following characteristics: length, width, distance from apex to point of greatest width, and petiolule length of apical pinna; the same measurements for one of each pair of lateral pinnae; length of petiole; and length of rachil sections. Morphological types of the base and apex of pinnae of the second leaf were determined. Leaf and twig morphology was investigated in an attempt to discover new distinctive characters for white and red ash. The possibility of a distinction in growth rates was investigated using leaves and twigs of several seasons. Teratologies were noted and recorded for the two species. A hybrid index, using twelve characteristics, was produced for the red and white ash. Finally, to attempt an answer to the question of potential species compatability, an experimental cross between a red ash female and a white ash male was successfully made in 1969, and the resultant seeds were planted.

In flowering and seed production the red ash proved to be superior to the white ash. Its blooming period was generally earlier than the white ash, although each year the flowering periods of some members of both species were found to overlap. Temperature appeared to be the prime factor which influenced flowering; white ash requiring higher mean day temperatures to commence flowering than the red ash. Low temperatures retarded flowering and vegetative growth, while frosts often damaged male inflorescences (female inflorescences being more frost resistant) and developing leaves and twigs. Fruit set for both species was highly variable, and high rates of seed failure were not uncommon.

Measurements of vegetative parts, with the exception of petiolule length, showed that these characters strongly overlapped in the two species. The red and white ash also displayed a similar range of apical and basal pinna shapes. However, there are at least nine reasonably distinctive characters which can distinguish between the red and white ash, so that the problem of using a key based on flowers and fruit (which are often lacking in ash specimens may be successfully bypassed.

Eleven apparently natural teratologies were found in the two species; of these the most abundant teratology was "irregular leaflet apices".

Although most of the trees qualified as distinct species, the hybrid index suggested that about 5% of the entire study group might qualify as introgressed hybrids. The possibility of hybridity was proven with the successful interspecific cross made in 1969, and the subsequent 80% seed germination.

The red and white ash are not particularly allopatric, although in this study the majority of these trees were found to be more or less, ecologically distinct. The red ash (growing on xeric, mesic and wet sites), however, exhibited a wider range of ecological amplitude than the white ash (which grew on mesic and xeric sites).

Taxonomically, the red ash and white ash, as they occur in the Ottawa area, would appear to function as distinct species, although they are not reproductively incompatible, so that wherever the two species are found in close proximity, hybridization of the two species becomes a possibility. The presence of atypical ash trees must therefore be expected, wherever the two species are found in the same stand.



Ash (Plants)




Carleton University

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