Abstract:

Acacia mearnsii is one of the most planted Australian Acacia around the world but is known
to be highly invasive and threatens native habitats by competing with indigenous vegetation.
This study was conducted in controlled conditions to determine the impact of the invasion
level severity on soil chemical characteristics, microbial activities and on ECM soil
communities. We hypothesized that shifts in soil microbial functionalities and ECM
community structure triggered by this exotic Acacia species could correlate with a decrease of
the early growth of Quercus suber, a native tree species. Soil samples were collected from 3
different sites along an invasion gradient in the Algerian El Kala Biosphere reserve: site S1:
non-invaded Q. suber forest, site S2: Q. suber / A. mearnsii mixed forest recently invaded by
the Australian Acacia and site S3: A. mearnsii stands introduced for more than 20 years. Plant
growth, ectomycorrhizal community structure associated with Q. suber roots and soil
microbial functionalities were assessed after 6 months of culturing in glasshouse conditions.
The results clearly showed that soil from the A. mearnsii invaded site S3 was far more altered
than soil from the site S2 in its chemical characteristics, microbial functions and
ectomycorrhizal community, as compared to soil from the native Q. ilex stand. These shifts
were correlated to a decrease in the early growth and ectomycorrhization of Q. suber
seedlings. Further studies are needed to identify cultural strategies to improve the
performances of reafforestation programs with native tree species in such degraded areas by
using for instance some indigenous ectotrophic shrub species (i.e. Cistus spp.) known to
promote ectomycorrhizal multiplication and the early growth of native tree species.

Key-words:

exotic plant invasion; Acacia mearnsii; soil microbial community; ectomycorrhiza