Variation in spring and autumn freezing resistance among and within Spanish wild populations of Castanea sativa

? Genetic variation in freezing resistance was evaluated among and within six populations of Spanish wild chestnut (Castanea sativa Miller). The extent to which frost susceptibility was related to phenology and the relationship between population differentiation and climatic conditions was studied.

? Twigs were collected in March and November from saplings (5-year-old trees) of 41 open-pollinated families from the six populations in a provenance-progeny test, and were subjected to artificial freezing. Damage to each twig was assessed as visible browning of bud and of stem tissues.

? Population differences as regards frost damage traits were highly significant (p < 0.01) in both spring and autumn. Family differences within populations were low, often non-significant, and in all cases smaller than differences among populations. Population means were closely correlated with the parental drought and frost conditions. Populations originating from dry areas or from regions where frost seldom occurs were the least resistant.

? Drought is suggested to be the one of the most important selective agents that shapes population differentiation in Spanish wild chestnut, while frost may be more important in northern Spain. Phenological differences are not always good predictors of the degree of frost damage. Thus, freezing tests should be used to detect frost susceptibility in chestnuts.