Ecophysiology of butternut (Juglans cinerea L.) and naturally-occurring hybrids (J. xbixbyi) with japanese walnut (J. ailantifolia Carr.)
Butternut (Juglans cinerea L.) is a relatively uncommon, high-value hardwood that ranges throughout much of eastern North America. Populations have drastically declined since the first report of butternut canker disease caused by Ophiognomonia clavigignenti-juglandacearum (Nair, Kostichka, and Kuntz) Broders and Boland. Forest Inventory Analysis (FIA) data from 1966 to 1986 documented a 77% reduction in butternut throughout the United States and recent surveys found ≥95% infection in northern New England and New York. Overall genetic diversity has not yet been affected by drastic population decline and putatively resistant individuals have been found, indicating that species restoration is possible. Two major impediments to this effort are lack of information regarding site preference for butternut and whether naturally occurring hybrids (J. ×bixbyi Rehd.) with Japanese walnut (J. ailantifolia Carr.) could serve as functional substitutes for butternut or pose a threat to the continued maintenance of the butternut gene pool. ^ To address these issues, open-pollinated black walnut (J. nigra L.), butternut, hybrids, and Japanese walnut seedlings were exposed to control, drought, and flood conditions following transplant in a controlled environment (Chapter 2). There was a strong taxa × treatment interaction. Butternut had a negative response to flood expressed by reduced leaf area (LA), photosynthetic assimilation (A), and chlorophyll fluorescence (Fv/Fm). Japanese walnut exhibited the opposite reaction, with A and LA reduced under drought. Hybrid A and LA were reduced in both treatments. Black walnut was the only taxon which did not develop hypertrophied lenticels following flooding. ^ Similar discrepancies between butternut and naturally occurring hybrids were obtained from cold-hardiness assessment (Chapter 3). Three traits: fall leaf abscission, electrolyte leakage during dormancy, and spring leaf flush, were used to evaluate half-sib families from throughout butternut's range established in a common garden in 2002. More butternuts fully abscised leaves by first frost (28 October 2011), whereas hybrids, overall, did not. In addition, butternut had reduced damage in response to freezing temperatures compared to hybrids in November 2011 (-33°C) and January 2012 (-40°C). Few differences were detected among half-sib families. Results may have been constrained by presence of butternut canker disease in the common garden and limited effect size for electrolyte leakage and spring leaf flush. ^ Due to the differences between butternut and hybrids for two abiotic stresses (moisture and cold) that largely define plant distributions, the overall variability and similarity to butternut in the hybrid gene pool was assessed using discriminant analysis (Chapter 4). Because it does not operate on trait averages, discriminant analysis negates variability among hybrids associated with altered proportions of progenitor genome between and within hybrid classes or even full-sib families. Black walnut, butternut, heartnut, and hybrids were grown from seed and evaluated at a suite of vegetative characters and functional adaptive traits associated with seedling survival. When projected onto the parameter space defined by the discriminant function pulling apart the three pure species, the hybrid response was dependent on assessment suite. In vegetative assessment, most (50.6%) hybrids effectively looked like heartnut with 44.5% falling within JC parameter space (21%) or intermediate to the two (23.5%). In functional trait assessment the majority of hybrids were projected onto black walnut parameter space (36.6%) and remaining hybrids almost equally projected onto butternut, heartnut, or parameter space not defined by any pure species. ^ Results indicate that hybrids, overall, are not completely similar to butternut in terms of moisture or cold stress and are currently undergoing a hybrid swarm effect characterized by high phenotypic variability among progeny when grown under mesic conditions. This suggests the overall hybrid pool is not a suitable ecological substitute for butternut and that efforts should be made to protect pure butternut populations from genetic homogenization and loss of locally adapted genotypes. It does not, however, discount all hybrid individuals from serving as substitutes or in a resistance breeding program. Rather habitat differentiation at two abiotic stresses that largely define plant distributions could be used to efficiently screen hybrids for butternut character in conjunction with disease resistance through nursery prescriptions or by outplanting on sites with low Japanese walnut fitness relative to butternut.^
Douglass F. Jacobs, Purdue University.
Biology, Ecology|Agriculture, Forestry and Wildlife