Cold and heat tolerances of Juglans cinerea, Juglans ailantifolia, and their hybrids

<p>Hybridization could incorporate traits to native species for surviving detrimental global change. However, to serve as an effective substitute for restoration, hybrids must survive and reproduce in the same distribution as the target species. An ecophysiological evaluation of hybrids versus progenitor species could contribute to predicting potential hybrid distribution and utilization areas. <em>Juglans cinerea </em>L. is an endangered, exceptionally cold hardy North American tree species that can hybridize with the non-native <em>Juglans ailantifolia </em>Carr. Preliminary evidence indicates their hybrids could hold resistance to <em>Ophiognomonia clavigignenti-juglandacearum</em>, the fungal disease threatening <em>J. cinerea.</em> Consequently, hybrids are being evaluated as a possible conservation tool. This study aims to compare relative cold and heat tolerances within <em>J. cinerea</em> provenances (USDA plant hardiness zones 4, colder, to 7, warmer) and between <em>J. cinerea</em>, <em>J. ailantifolia</em>, and their hybrids (F1 hybrids - <em>J. cinerea </em>× <em>J. ailantifolia</em>, and backcross hybrids - F1 × <em>J. cinerea</em>). For the cold test, current-year twigs were subjected to five freeze treatments (-38 to 5 °C). Resulting damage was estimated using electrolyte leakage. In the heat test, leaflets were subjected to six hot water bath treatments (30 to 54 °C) and damage was estimated as change in chlorophyll fluorescence. Cold tolerance differed more than heat tolerance among <em>J. cinerea</em> provenances and across species. Within <em>J. cinerea, </em>trees from colder areas exhibited less cold damage than those from warmer areas. Differences in heat damage between hardiness zones occurred, but did not follow a clear trend. Consequently, at the intraspecific level cold and heat tolerance were not correlated. However, at the interspecific level cold and heat tolerance were negatively correlated. <em>J. cinerea</em> exhibited greatest cold tolerance, <em>J. ailantifolia</em> exhibited greatest heat tolerance, and hybrids were intermediate. Thus, the utility of hybrids for restoration could be limited at the ecophysiological extremes of species’ distributions.</p>