Implications of oak decline for the structure and functioning of Iberian forests

Conferencia plenaria presentada en el 4º Congreso Ibérico de Ecología (Coimbra, Portugal). 16-19 Junio 2015

In the last decades widespread tree dieback has been documented in forests all over the world. In the Iberian Peninsula in particular, evergreen oaks Quercus ilex and Quercus suber are among the tree species most severely impaired by problems of defoliation and mortality. To date, the emphasis has been on analyzing the patterns and causes of this decline. However, much less is known about the consequences of oak mortality for the structure and functioning of declining forests. In this talk I will provide an overview of our current knowledge about the impacts of oak decline on plant and soil communities and the ecosystem processes that they control, using examples from recent work conducted in Q. suber forests of southwestern Spain. The evidence thus far suggests that ongoing alterations in health and cover of canopy oaks do translate into changes in understory plants and microbial communities, and that such changes occur more consistently in terms of abundance and functionality than of taxonomic diversity. For example, the decline of Q. suber has been shown to reduce the abundance of seedlings and saplings of tree species, particularly late-successional oaks, but not of other functional groups such as shrubs or lianas. These differential responses among woody species could affect successional trajectories, potentially leading to vegetation shifts. Belowground, the decline of Q. suber has been linked to lower soil respiration rates but higher heterotrophic respiration and microbial biomass, altered probabilities of establishing mycorrhizal interactions, higher pathogen loads, lower nematode abundance, and an increasing role of bacterialfeeder nematodes in soil food webs. Overall, the process of oak decline seem to translate into functionally altered above- and below-ground biotic communities that could threaten the quantity and quality of the goods and services provided by these valuable forest systems.

Peer reviewed