Revista Chilena de Historia Natural 78 (4): 711-722, 2005
RESEARCH ARTICLE
Don’t camp beside the river: structure and dynamics of Andean alder
(Alnus acuminata) forests affected by river floods, northwestern Argentina
Alnus acuminata (Betulaceae) is a pioneer tree that
dominates mountain riparian forests throughout the eastern Andes. Early studies associated these forests with particular microclimatic
conditions along river beds, but they were later linked to the periodical disturbance regime of mountain rivers. To understand the
dynamics of these forests, we analysed tree age and spatial arrangement in two plots along Potrero river and another plot along La
Horqueta river, Tucumán Province, north west Argentina. Through intensive tree coring and age reconstruction we identified similar-
aged tree cohorts with maximum age ranges of eight years. Tree mapping and Moran’s I spatial autocorrelation analysis showed that
tree cohorts were spatially clumped, and provided further evidence that coetaneous patches of A. acuminata arise from
massive regeneration following catastrophic flooding events. We tested the association between cohort initiation dates and regional
maximum river discharge records by means of a randomization procedure and found weak evidence that establishment occurs after
devastating floods, which eliminate previous vegetation and create bare sites suitable for A. acuminata
regeneration. Dates of tree floodscars sampled along Potrero river precede by one year the most recent cohort initiation date
and provide further support of this association. Although previous studies found a positive correlation between El Niño events and mean
annual precipitation in northwestern Argentina, we did not detect an association between El Niño and annual maximum stream flow for
rivers in Tucumán Province. Therefore, factors driving devastating floods and A. acuminata stand initiation require further
study.
Key words:
Alnus
acuminata, river floods, riparian forests, forest dynamics, El Niño/ENSO
