A substantial amount of the tropical forests of South America and Africa
is generated through moisture recycling (i.e., forest rainfall
self-reliance). Thus, deforestation that reduces evaporation and dampens
the water cycle can further increase the risk of water-stress-induced
forest loss in downwind areas, particularly during water scarce periods.
However, few studies have investigated dry period forest rainfall
self-reliance over longer records and consistently compared the
rainforest moisture recycling in both continents. Here, we analyze
dry-season anomalies of moisture recycling for mean-years and dry-years,
in the South American (Amazon) and African (Congo) rainforests over the
years 1980-2013. We find that, in the dry seasons, the reliance of
forest rainfall on their own moisture supply (ρfor) increases by 7%
(from a mean annual value of 26% to 28%) in the Amazon and up to 30%
(from 28% to 36%) in the Congo. Dry years further amplify dry season
ρfor in both regions by 4-5%. In both the Amazon and Congo, dry season
amplification of ρfor is strongest in regions with a high mean annual
ρfor. In the Amazon, forest rainfall self-reliance has declined over
time. At the country scale, dry season ρfor can differ drastically from
mean annual ρfor. In for example Bolivia and Gabon, mean annual ρfor is
~30% while dry season ρfor is ~50%.
The dry period amplification of forest rainfall self-reliance further
highlights the role of forests for sustaining their own resilience, and
for maintaining downwind rainfall at both regional and national scales.