A safe breeding ground: genetically improved maritime pine for growth and stem form has more efficient but not more vulnerable xylem.

Through repeated cycles of selection and recombination, tree breeding programs deliver genetically improved varieties for a range of target characteristics such as biomass production, stem form, resistance to biotic stresses, wood properties, etc. However, in the context of increased drought and heat waves, it is not yet known whether growth performance will impede drought resistance. To address this question, we compared the hydraulic properties, such as hydraulic efficiency and hydraulic safety, in four varieties over successive varieties of genetically improved maritime pines (i.e., Pinus pinaster Aït.) for growth and stem form. We measured 22 functional traits related to hydraulic efficiency, hydraulic safety, xylem anatomy and wood density. We found that improved varieties presented higher hydraulic conductivity with larger tracheid lumen size and tracheid lumen fraction, and smaller wall thickness reinforcement and tracheid density, but not at the cost of reduced embolism resistance. The reported absence of trade-off between hydraulic conductivity and embolism resistance is a strong asset to improve biomass productivity, through increased hydraulic efficiency, without impacting drought resistance, and should enable new maritime pine varieties to cope with a drier climate. Our study is one of the first to reveal the hydraulic mechanisms over successive varieties of genetic improvement for tree growth. It provides guidelines for sustainable forest management through breeding for other forest tree species.

Field evidence of colonisation by Holm Oak, at the northern margin of its distribution range, during the Anthropocene period.

A major unknown in the context of current climate change is the extent to which populations of slowly migrating species, such as trees, will track shifting climates. Niche modelling generally predicts substantial northward shifts of suitable habitats. There is therefore an urgent need for field-based forest observations to corroborate these extensive model simulations. We used forest inventory data providing presence/absence information from just over a century (1880-2010) for a Mediterranean species (Quercus ilex) in forests located at the northern edge of its distribution. The main goals of the study were (i) to investigate whether this species has actually spread into new areas during the Anthropocene period and (ii) to provide a direct estimation of tree migration rate. We show that Q. ilex has colonised substantial new areas over the last century. However, the maximum rate of colonisation by this species (22 to 57 m/year) was much slower than predicted by the models and necessary to follow changes in habitat suitability since 1880. Our results suggest that the rates of tree dispersion and establishment may also be too low to track shifts in bioclimatic envelopes in the future. The inclusion of contemporary, rather than historical, migration rates into models should improve our understanding of the response of species to climate change.