Using tree-ring data and satelite imagery to reconstruct historical dryland cabon storage


satellite image

Some 50% of vegetation consists of carbon, because plants absorb carbon for photosynthesis with sunlight and oxygen to produce sugars. These sugars are used to build plant cell walls, organs, and hormones which are also examples of carbon storage. The rate of carbon (C) storage in vegetation is called net primary productivity (NPP) or the total weight of carbon stored above and below ground in a specific area in a year.

Trees that grow in seasonal environments store this carbon in tree rings which show the annual growth of the tree and in total the tree's age. The ring width varies with the amount of growth in that year where thicker rings indicate higher production and vice versa.

Production in drylands is limited by the amount of water available. Shortages (drought) and abundances of water determine the tree’s ring width. This relationship allows reconstruction of historical rainfall records as far back as 5,000 years. 

Some satellites can measure the amount of reflected red (R) and infrared (IR) sunlight from a plant canopy. IR is highly reflected from leaves relative to R light. Red light is highly absorbed by leaves for photosynthesis and the fraction of light used for photosynthesis is approximately equal to the ratio of IR to R light or IR/R.  This relationship is proportional to the amount of leaves or biomass in the canopy.

If there are many leaves in the canopy then the IR/R ratio increases and if there are less leaves, it decreases. The amount of IR/R summed up over a growing season is approximately equal to NPP. Summed IR/R correlates nicely with tree ring width and variability. This correlation has allowed reconstruction of the satellite sensor's summed IR/R over 200 to 500 years ago.

However, in the past these reconstructions have been done in time and not space. Spatial reconstructions produce time series of maps of NPP from 200 or more years ago, i.e., reconstruction for each year or 200 or more maps of NPP would be produced. We have produced and validated 303 NPP maps of California using a tree ring record from 1700 to 2003. This project is reconstructing the NPP record for Nevada, the US, and North America’s (NA) drylands for the same period using historical archives of tree ring width and satellite data. This will lead to a better spatial and temporal understanding of above- and below-ground carbon dynamics for these different study areas.

For example, we will see in the mid 1700's during particularly wet conditions where Nevada was most productive and during the 1930's, 1950's, 1970's and late 1990s droughts locations where the state was the least productive. Investigators will then relate predicted and observed changes in carbon dynamics to historical and contemporary records of livestock grazing, wildfire, and climate, particularly droughts. This work will establish longer term baselines and hypotheses about what the drivers of these dynamics may be as well as seek solutions from the areas where we see recovery.