Cyclic patch dynamics in a Macaronesian island forest

R. B. Elias, and E. Dias

We assess if the structural and floristic diversity of Juniperus-Ilex forests from Pico Island (Azores) corresponds to a mosaic of different phases of a forest cycle, by identifying the phases of that cycle and the dynamic relations between them. Eight 100 m2 plots were placed randomly in areas with structural and floristic differences but having in common the presence of live and/or dead individuals of the dominant tree species (Juniperus brevifolia). In each plot the number of seedlings, saplings, and adults as well as canopy height and width and maximum height of live Juniperus brevifolia and Ilex azorica adults were recorded. The floristic composition was determined in a 25 m2 plot, placed inside each 100 m2 plot. Juniperus-Ilex forests show a cyclic patch dynamic triggered by the senescence and death of same cohort individuals of J. brevifolia. In this forest cycle, five phases were identified, such as gap, building, mature, initial degenerative and final degenerative. The first two phases are dominated by J. brevifolia however in the degenerative phases I. azorica is the dominant tree species. The cycle may be divided into an upgrade series (comprising the first two phases) and a downgrade series (between the mature and final degenerative phases). In these forests there was no climax micro-succession detected since changes in the dominant tree species occur in the degenerative phases. This paper brings the first evidence for the existence of a forest cycle in Macaronesian forests.

(in Community Ecology)

Gap dynamics and regeneration strategies in Juniperus-Laurus forests of the Azores Islands

Rui Bento Elias & Eduardo Dias

We asked the following questions regarding gap dynamics and regeneration strategies in Juniperus-Laurus forests: How important are gaps for the maintenance of tree diversity? What are the regeneration strategies of the tree species? Thirty canopy openings were randomly selected in the forest and in each the expanded gap area was delimited. Inside expanded gaps the distinction was made between gap and transition zone. In the 30 expanded gaps a plot, enclosing the gap and transition zone, was placed. In order to evaluate the differences in regeneration and size structure of tree species between forest and expanded gaps, 30 control plots were also delimited in the forest, near each expanded gap. In the 60 plots the number of seedlings, saplings, basal sprouts and adults of tree species were registered. Canopy height and width of adult individuals were also measured. The areas of the 30 gaps and expanded gaps were measured and the gap-maker identified. Juniperus-Laurus forests have a gap dynamic associated with small scale disturbances that cause the death, on average, of two trees, mainly of Juniperus brevifolia. Gap and expanded gap average dimensions are 8 and 25 m2, respectively.

Gaps are of major importance for the maintenance of tree diversity since they are fundamental for the regeneration of all species, with the exception of Ilex azorica. Three types of regeneration behaviour and five regeneration strategies were identified: (1) Juniperus brevifolia and Erica azorica are pioneer species that regenerate in gaps from seedlings recruited after gap formation. However, Juniperus brevifolia is a pioneer persistent species capable of maintaining it self in the forest due to a high longevity and biomass; (2) Laurus azorica and Frangula azorica are primary
species that regenerate in gaps from seedlings or saplings recruited before gap formation but Laurus azorica is able to maintain it self in the forest through asexual regeneration thus being considered a primary persistent species; (3) Ilex azorica is a mature species that regenerates in the forest.

(In Plant Ecology)