Forest impacts of our artificial drought

The green leaves on this dead tree are of living bromeliads it still supports.

We have been impressed by the great tolerance that our forest presented in the face of the severe drought we created. As the moisture stored in the soil was depleted in the dry plot, the trees simply absorbed water from deeper in the soil with their extensive root systems, avoiding most of the visible symptoms of drought stress. By the end of the five-year period of exclusion, many trees in the forest were drawing water from more than 40 feet deep in the soil.

We had anticipated that when the trees could no longer reach water in the soil, they would shed their leaves; while there has been a reduction in leaf area index, leaf litter data show that our trees kept their leaves and instead, rates of growth in trunk diameter decreased. Many smaller trees measuring four to ten inches in diameter simply stopped growing during the end of the dry season of 2000 following the first period of rainfall exclusion, while trees of same species and size continued to grow in the control plot. Trees slow down the amount of water that they lose from their leaves by closing their stomates, thereby reducing photosynthesis; in this case, they adjusted by diverting less sugar and other carbohydrates to wood production. This finding has important implications for climate change, since the amount of carbon removed from the atmosphere during tropical droughts will decrease significantly. There are also important implications for sustainable forest management under drought conditions, as the time between harvests would need to be greater.

More sunlight falls on the panels now that some large canopy trees have died.

Another surprise was that the observed sensitivity of large canopy trees to drought is greater than expected. Once the moisture that is stored in deep soil is depleted, the largest trees - towering 130 to 150 feet above the ground and basking in full sunlight - begin to falter and die. The death of such large trees that may take centuries to reach the top of the forest canopy, increased from about one percent per year to nine percent in the fourth year of the experiment, when soil water was depleted. This sensitivity of large trees to drought means that a decline in rainfall would likely cause a gradual transition from tall, green, lush rainforest towards a shorter, more stunted forest where a great deal more sunlight penetrates to the forest floor.

As the forest becomes shorter and its canopy more open, compromising its remarkable resistance to fire, it is clear that drought in tandem with fire could swiftly push the tall, dense rainforests of the region towards savanna scrub. The amount of carbon that could be released to the atmosphere by this savannization process is significant—equivalent to several years of worldwide carbon emissions —and could accelerate climate change processes already in place.

Canopy density and large tree mortality differed greatly between parcels as early as August, 2003.

In addition to these global effects, drought and fire, a tool of choice among the Amazon’s farmers and ranchers, pose a serious threat to a forest that is home to more plant and animal species, and more indigenous cultures, than any other forest in the world.

With the completion of this first phase of the experiment, attention will turn to another future scenario for the world’s tropical rainforests: namely, how the forest recovers, or not, after prolonged drought. Although it is difficult to imagine a reversal of the current trend in rainforest drying, it is important to understand the forest’s response to this scenario nonetheless. What types of trees will invade the forest that has now been “released” from its imposed drought? Are the trees that survived the drought somehow damaged, and unable to respond to this release? Have the vessels that conduct water from their roots to their leaves become clogged with water vapor bubbles, restricting their growth for years to come? These are just some of the questions that will be explored during the final two years of this experiment.

©Woods Hole Research Center, 2007