Tropical Forests: Increased Carbon Storage in a High CO2 World?

By February 18, 2009 March 1st, 2013 Forest Mortality

A study in Nature last month confirmed ongoing increased CO2 storage in tropical forests – on the gigaton scale – that’s a lot. So the CO2 fertilization effect is confirmed?  Yes of course, we new this all along, CO2 is a fertilizer. But in reality the benefits of the CO2 effect have already been surpassed in northern forests.  Another study in the Journal Science last month found that tree death from climate change had doubled in the western North America in the last decade from 1.5% to 3% per year. This might sound minimal, but because trees are relatively long lived, the researchers found that this increase in mortality would, within several decades, decrease the average tree size substantially, maybe by half or more. Tree size is what natural forest sequestration is all about. Bigger trees mean more storage – Thus the CO2 fertilization effect makes larger trees and sequesters more carbon.  But if warmer temperatures are increasing tree mortality enough to decrease tree size, things start to offset. 

The tree mortality study was done in northern forests; the CO2 fertilization effect in tropical forests.  As important to the environmentalist agenda as tropical forests are, the northern forests store 22 percent of the total carbon on the earth’s land surface, and almost twice as much carbon per unit area as tropical forests. Northern forests cover nearly 17% of the Earth’s surface whereas today, tropical forests only cover about 5% of the Earth’s surface. This makes the carbon content of the northern forests about 7 times greater than the carbon content of tropical forests. The bottom line is that the northern forests are the driver in world forest carbon storage and their capacity to continue to store carbon could be easily and quickly reduced by 75%, conservatively.

More importantly though is the math concerning a halving of tree diameter.  A two foot diameter tree has four times the amount of wood in it than does a one foot diameter tree. When a tree’s diameter is doubled, its volume increases four times, it doesn’t double, so when a trees diameter is halved, it only has 25% of the volume. Before long at all, diminishing CO2 storage in northern forests will outpace increased storage in tropical forests, unless fires increase in tropical forests, which they are projected to do, and unless tree death in northern forests increases further because of insect infestations which were not taken into account in the northern tree death study.  Both of these scenarios are likely and both decrease the efficiency of the worlds forest storage of carbon.

Forest browning, in the words of NASA Earth Observatory scientists, "faster than any scientists thought possible", is occurring today as a sign that the CO2 fertilization effect has been surpassed in our forests of the north.

 

Lewis, et. al., et al Increasing carbon storage in intact African tropical forests Nature February 2009. 

Luyssaert et al Old-growth forests as global carbon sinks Nature July 2008.

http://earthobservatory.nasa.gov/Features/BorealThreshold/boreal_threshold.php

Metzl, et.al., Decadal Increase of Oceanic Carbon Dioxide in the Southern Indian Ocean Surface waters (1991-2007), Deep-Sea Research II, special issue, published online 8 February 2009.

Takahashi, et. al., Climatological Mean and Decadal Change in Surface Ocean pCO2, and Net Sea-air CO2 Flux over the Global Oceans, Deep-Sea Research II, special issue, published online 8 February 2009.

Watson, et. al., Trends in North Atlantic sea surface pCO2 from 1990 to 2006, Deep-Sea Research II, special issue, published online 8 February 2009.

Mantgem, et. al. Widespread Increase of Tree Mortality Rates in the Western United States Science Jan 2009.