Michael G. Barbour

Syntaxonomical approach for classification of the Californian serpentine annual grasslands

A preliminary syntaxonomical approach for the serpentine annual grasslands classification is presented. Our proposals include the phytosociological frame for the syntaxonomy of the annual plant communities growing on ultramafic substrata and distributed throughout the territories of the Californian biogeographical region. All the formal phytosociological high units newly proposed are justified and legitimated on the basis of previous reports and our own data. These are collected in a new phytosociological class: V ulpio microstachyos-Hesperolinetea micranthi that includes, so far, one new order: Eriogono luteoli-Hesperolinetalia micranthi and four new alliances: Hesperevaco sparsiflorae-Hemizonion congestae , Hesperolinion clevelandii , Hesperolino micranthi-Navarretion filicaulis and Streptanthion polygaloidis .

Anomalous diameter growth and population age structure in mature Canary Islands pine stands

Canary Islands pine (Pinus canariensis Chr. Sm. ex DC) is often described as serotinous, even though most serotinous attributes are absent or weakly developed and the trees do not seem to experience a natural fi re regime that would favor and sustain serotiny. We studied the age structure of 22 old-growth stands on the slopes of Mt. Teide on Tenerife, Canary Islands. Statistically robust relationships between trunk diameter at breast height and tree age allowed us to use diameter-age regressions to summarize population age structures and to reconstruct disturbance history. The age structure of only one stand statistically fi t the null hypothesiss expectation of a smoothly declining L-shaped negative exponential, non-disturbance population model. Departures from the model commonly featured high densities of seedlings and saplings in the absence of recent fi res (indicating that regeneration is independent of fi re) and age structures that exhibited one or more peaks of establishment, the average number of years between peaks being 78 yr. Onset of sexual reproduction averaged 46 yr, and the age of mature overstory individuals often exceeded 200 yr. Anomalously, variation in most vegetation attributes, including stem diameter growth, failed to signifi cantly correlate with major abiotic gradients-elevation, temperature, precipitation, exposure to trade wind clouds, slope aspect and steepness, and geological substrate in habitats where P. canariensis dominated.