plots established as part of a long- term monitoring
network known as RAINFOR, which consists of 136
permanent plots located in old- growth forest across 44
landscape types. These plots have been monitored for 25
years. Data are periodically collected on tree diameter
and wood density. Mathematical models are used to
calculate biomass and rate of change in biomass. For
the 2005 drought, researchers focused on 55 plots; they
measured net biomass change, growth, and mortality,
and compared these data with those of earlier years.
Prior to the 2005 drought, the 55 plots considered
together showed a net increase in aboveground (dry-
weight) biomass, indicative of the forest acting as a
carbon sink.
During the drought year there was no net gain in
biomass, but rather an overall loss. Analysis indicated
that the biomass loss coincided closely with drought
conditions and represented the first time since the plots
were monitored that there was a net biomass decline.
Tree mortality rates were also elevated in forests that
experienced the most severe drought conditions.
The large sample size of the study confirmed that the
biomass loss and mortality increase were widespread,
not confined to just a few areas. Should droughts such
as that of 2005 become more frequent or more severe
(as climate change models predict), we can expect to
see more of these changes in forest composition and
net primary productivity throughout Amazonia and,
presumably, the rest of the tropical world. And the
efficacy of lowland humid forests as carbon sinks
would be very much reduced.Will Too Much Rain Reduce NPP?
Drought is at one end of a spectrum of factors affecting
levels of NPP in tropical forests. At the other end is
precipitation (plate 5- 10). Models of climate change
suggest increases in mean annual temperature (MAT)
and mean annual precipitation (MAP) in the coming
decades. The response of tropical forests to drought
or added rainfall will determine whether these forests
act as carbon sinks or carbon sources. It is clear that
tropical forests are sensitive to drought. How sensitive
are they to increased precipitation?
To answer the question, E. A. Schuur performed a
study using data from a source called the International
Biological Program (IBP) combined with that from a
recent survey of tropical forest NPP. The results showedthat as mean annual temperature increased, so did
net productivity— initially not surprising. But more
important, as mean annual precipitation continued to
increase, productivity peaked and then declined fairly
steeply. The maximum NPP was attained at a MAP level
of 244.5 cm (96.25 in). Above that, NPP declined. In
other words, above a threshold, rain reduced primary
productivity. This result was initially a bit of a surprise.
Why should increased precipitation result in declining
NPP? One reason might be that reduction in total amount
of solar radiation caused by persistent thick clouds
lowers productivity. Another effect of added moisture
might be to decrease the efficacy of nutrient cycling
by leaching nutrients out of the soil and by decreasing
decomposition rates. This could result from rainfall
saturating soil, interfering with the aerobic demands
of root systems and microbial organisms such as fungi
and bacteria, the principal decomposers. There are thus
two possible avenues, reduced sunlight and interrupted
biogeochemical cycling (chapter 6), by which NPP rates
may be driven lower with increased precipitation. Such
an occurrence, as with severe drought, would temper
the function of tropical forests as carbon sinks.
***
It is unclear whether global tropical forests will
ultimately prove to be carbon sinks or carbon sources.
The studies described above show that tropical forests
have the potential to act as carbon sinks but rising global
temperature and more severe and frequent droughts, to
say nothing of increasing anthropogenic impacts, may
collectively outweigh enhanced NPP, making tropical
forests carbon sources, not sinks.Plate 5- 10. In some tropical forests, precipitation may be
sufficiently frequent to limit net primary productivity. Photo
by John Kricher.chapter 5 sun plus rain equals rain forest 79