Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur

Outlier Stands of Quaking Aspen in the Davis Mountains of West Texas: Clone or Clones?

Populus tremuloides (quaking aspen) is found from the Pacific to the Atlantic Ocean in the northern United States and Canada, and at higher elevations in the western United States and northern Mexico. While P. tremuloides can reproduce sexually or asexually, it is primarily a clonal species in the intermountain west, reproducing vegetatively via root sprouts, yielding genetically identical stems or ramets. In west Texas, isolated, outlier stands occur in the Guadalupe, Davis, and Chisos Mountains at an elevation of approximately 2300 m. This study utilized seven microsatellites or simple sequence repeats (SSRs) to examine leaf samples from 10 widely separated stems in 10 isolated P. tremuloides individual stands within the Davis Mountains to determine the level of clonal and genetic diversity. We then examined differentiation among stands. Each stem sampled within a stand was genetically identical to all stems examined in that stand or was part of a clone. There were eight genetically identical clones from these ten stands, with three stands being genetically identical or part of the same clone. Many of the genotypes shared several of the same alleles and the remaining alleles were only a few base pairs apart. Some of these alleles have been previously identified in other western North American P. tremuloides stands. Microsatellites identified several triploid patterns consistent with possible aneuploidy, which is concurrent with previous studies