A relict population of Fagus grandifolia var. mexicana at the Acatlan Volcano, Mexico: structure, litterfall, phenology and dendroecology

Aim Fagus grandifolia var. mexicana (Martinez) Little has an extraordinaril y restricted distribution in the Mexican montane cloud forests. Isolated Fa gus (beech) populations have been recorded in less than 10 small areas (2-4 0 ha) in the eastern Sierra Madre at altitudes from 1400 to 2000 m. The obj ectives were to determine tree and seedling age, forest structure, phenolog y, litterfall patterns and the relationship between mast and climatic varia bles. Location We report on three Fagus stands at the Acatlan Volcano, Veracruz, Mexico. Methods Changes in forest cover were determined using aerial photographs. W ithin each stand, basal area, density and tree species composition were det ermined in a 0.1-ha band transect. Additionally, litterfall production was quantified and phenophases were recorded monthly over a 3-year period, and 60 tree cores were collected to determine age distribution and tree-ring gr owth. Results The forest was atypical in several respects. Fagus was the only dom inant tree species in the crater stand, although in the rim and at the top of the volcano it was codominant with other tree species. Juveniles occurre d only on the rim, but there was a seedling bank in the crater. Although fo rest cover in the area increased between 1968 and 1993, the Fagus stands di d not change in size. Leaf production peaked in March and April, and leaf f ail occurred from October through February. Litterfall production was the h ighest in November. During mast years, flowering started in February and be tween mast events there were no flowers or fruits. Minimum temperatures wer e highly correlated with Fagus litterfall and leaf fall. Seedlings ranged i n age from 2 to 18 years and were 13-60 cm tall. Tree cores ranged from 76 to 120 years, but trees were older than the core samples. Main conclusions Although beech is considered a gap regeneration species th at reaches the canopy after alternating periods of release and suppression, the trees in the crater were released when less than 1.5 m tall and have s uffered few periods of suppression since. The results indicate that the cra ter stand was established after a severe disturbance destroyed the existing forest. We conclude that the relict beech population should be able to mai ntain itself, if not severely disturbed by humans or by climatic changes re lated to global warming.