Unrestricted quality of seeds in European broad-leaved tree species growing at the cold boundary of their distribution.

OBJECTIVE

The low-temperature range limit of tree species may be determined by their ability to produce and disperse viable seeds. Biological processes such as flowering, pollen transfer, pollen tube growth, fertilization, embryogenesis and seed maturation are expected to be affected by cold temperatures. The aim of this study was to assess the quality of seeds of nine broad-leaved tree species close to their elevational limit.

METHODS

We studied nine, mostly widely distributed, European broad-leaved tree species in the genera Acer, Fagus, Fraxinus, Ilex, Laburnum, Quercus, Sorbus and Tilia. For each species, seeds were collected from stands close to optimal growth conditions (low elevation) and from marginal stands (highest elevation), replicated in two regions in the Swiss Alps. Measurements included seed weight, seed size, storage tissue quality, seed viability and germination success.

RESULTS

All species examined produced a lot of viable seeds at their current high-elevation range limit during a summer ranked 'normal' by long-term temperature records. Low- and high-elevation seed sources showed hardly any trait differences. The concentration of non-structural carbohydrates tended to be higher at high elevation. Additionally, in one species, Sorbus aucuparia, all measured traits showed significantly higher seed quality in high-elevation seed sources.

CONCLUSIONS

For the broad-leaved tree taxa studied, the results are not in agreement with the hypothesis of reduced quality of seeds in trees at their high-elevation range limits. Under the current climatic conditions, seed quality does not constitute a serious constraint in the reproduction of these broad-leaved tree species at their high-elevation limit.