Phenological patterns of Quercus ilex, Phillyrea latifolia, and Arbutus unedo growing under a field experimental drought

A holm oak forest located in the Prades Mountains (northeast of Spain) was subjected to an experimental drought to determine its phenological responses. Soil water availability was reduced by 15% by plastic strips and funnels that partially excluded rain throughfall and by ditch exclusion of water runoff. We monitored eight phenological events: leaf shedding, leaf flushing, flower bud formation, flowering, flower senescence, fruit growth, fruit maturation, and fruit abscission in the three dominant species, Quercus ilex, Phillyrea latifolia, and Arbutus unedo once a week from winter 1999 to winter 2001. We also collected litterfall in circular baskets randomly distributed on the ground every 15 d from winter 1999 to winter 2001 and every 2 months from winter 2001 to winter 2003. Arbutus unedo showed a higher proportion of individuals flowering and fruiting than Q. ilex and P. latifolia. Arbutus unedo was also the most sensitive species to water availability since drought treatment delayed its phenophases, whereas this treatment did not significantly affect the timing of the other two species' phenophases. The flower and fruit production was also greater in A. unedo than in the two other species, but inter-annual variability was high, and no significant drought effect was found in any of the three species. Stem litterfall was greater in drought plots than in control plots during the overall studied period. In a drier environment, as predicted for Mediterranean areas in the near future by global circulation models, drought-resistant species such as P. latifolia could present greater ability to produce reproductive structures than less resistant species such as Q. ilex or A. unedo. This different response among species could produce changes in seedling recruitment and resprouting ability and, in the longer term, in species distribution.