Forest canopy height: why do we care?

Wow!  This post was selected as PLoS ONE’s Blog Post of the Month (July 2010) – thanks!  Check out the runners-up and previous winners here.

If you’ve been on the internet at all in the past week, you’ve probably seen these lovely images from NASA, visualizing the height of tree canopies around the world.  They’ve been on science sites along with art ones.  In a sense, that alone is useful: using beautiful visuals to make people think about the world on a larger scale.  But where did these data come from, and what do they really mean?

Why is it important?

The two main sources of anthropogenic influence of atmospheric carbon are (1) the burning of fossil fuels, releasing carbon dioxide into the air and (2) deforestation, removing trees which store large amounts of carbon.  20-50% of carbon in the atmosphere is currently not accounted for in climate models – a huge amount.  Knowing where exactly this carbon is coming from is important for both conservation and making socioeconomic decisions regarding energy use.

Some scientists hypothesize that these unaccounted-for changes in carbon flux are due to poor knowledge of forest stands – both where deforestation has occurred, and where forests are recovering after previous deforestation.  By knowing the size of forest stands, as larger trees can store more carbon, and keeping track of changes in the sizes of these stands, the hope is to have more accurate models for carbon storage and atmospheric carbon.

In the face of climate change, these kind of data are also useful, if updated over time, in seeing how rising temperatures and increased carbon dioxide affect tree growth (and thus carbon sequestration).  Since plants get their carbon from the air, it seems natural that increased carbon dioxide would increase tree growth.  Thus canopy height maps can help us to test this hypothesis.

From where did these data come?

Most of the data comes from work by Michael Lefsky et al. in a 2002 paper pubslihed in Global Ecology and Biogeography and a 2005 paper published in Geophysical Research Letters.  In the 2002 paper, Lefsky and his team measured canopy height using LIDAR (Light Detection and Ranging technology), which essentially sends down a laser beam from which distance can be measured based on return time.  LIDAR uses a shorter wavelength than typical radar, making it more sensitive to smaller objects such as particles in the air.  Thus the name, Light Detection and Ranging.  It also has a narrower beam than radar and thus is more specific in its measurements.  In the paper, Lefsky and co. took LIDAR measurements on temperate forest stands and compared them with field measurements of forest height, finding that they matched up very closely.  They also were able to do field measurements on the complexity of forest stands, meaning how much undergrowth lives beneath the canopy, and were able to create an accurate equation for predicting biomass of a forest stand based on LIDAR alone.

The 2005 study was another sort of proof-of-purpose paper, once again showing that LIDAR can predict above-ground biomass.  The authors did not have complexity variables this time, but were still very accurate in their predictions.

For the beautiful figures of canopy height released by NASA, Lefsky combined his own data with that from NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer), which is housed on a satellite and images the earth’s surface every 1-2 days.  Combining the data on terrain from MODIS with his own canopy height data, Lefsky crafted these images over several years.

What does it mean?

Previously, large-scale vegetative modeling was only able to be measured in 1 km swaths of land, done by MODIS alone.  Due to deforestation, there is a great deal of variation in canopy height on a smaller than 1 km scale – and finally we have the tools to create maps of forest canopy and thus help us better track carbon storage at these sites.  The map isn’t perfect – it is a model after all – but it is far more accurate than anything we’ve seen yet.  Expect interpretation from Lefsky and others in the near future.

But this isn’t the end.

As I mentioned earlier, traditional thinking assumes that forest growth will assist in carbon storage through increased growth due to higher carbon dioxide in the atmosphere.  But a recent paper (July 21, 2010) in PLoS ONE by Lucas Silva (“silva” means forest in Latin, lollers), Madhur Anand, and Mark Leithead suggests otherwise.

A major tradeoff for growth in trees (as I’ve discussed elsewhere) is that the opening of leaf stomata (pores) to absorb carbon dioxide also causes water loss through evaporation from these same stomata.  The authors of the PLoS paper looked at tree growth through tree rings and compared it with isotope analysis to measure water loss.  If the trees they studied had increased growth due to increased carbon dioxide, they would also expect more water conservation, as more carbon dioxide would be able to enter the leaves without as much water loss.

Studying 4 species of tree at 4 forest types in Canada, they found a 53% increase in water use efficiency over the last century.  (They looked at both young and old trees to account for varying growth rates and energy use.)  This seems like good news – the trees are absorbing carbon dioxide more readily.  However, they also saw a decline in growth overall.  This suggests that other stresses, such as water, nutrients, and temperature, are limiting their growth despite the ease of access of carbon dioxide.

The next step: can we learn about tree growth from Lefsky’s maps?  Are they accurate enough?  It would be great to measure biogeochemical measures, such as water use efficiency, and compare this to large-scale forest size data.  A girl can dream…

Okay, class: what have we learned?

Lasers are cool!  The LIDAR technology, originally created for studying atmospheric chemistry, reapplied to study canopy heights has allowed us to visualize our forests in a new way.  (And make some beautiful pictures.)  There was a lot of work put into it – and to accurately measure how our forests are changing, increasing work will have to be done to keep the maps updated to create an index of canopy height on our planet.

However, we’ve also learned that we cannot necessarily rely on traditional hypotheses in times of climate change.  While trees have the capacity to remove carbon from the atmosphere and store it, other factors can confound these effects, as we read in the PLoS ONE paper.  While more work certainly needs to be done on this front (using large-scale climate measures for growth instead of dendrochronology, for example), their results are certainly sobering.

So, as usual, we need to do more work!  We need to learn more!  We have to challenge our hypotheses, and challenge new results that support or disprove them.  It’s always easier when you have a mystery to solve: where is all that carbon anyway?

Cohen, W., Harmon, M., Wallin, D., & Fiorella, M. (1996). Two Decades of Carbon Flux from Forests of the Pacific Northwest BioScience, 46 (11) DOI: 10.2307/1312969

Lefsky, M., Cohen, W., Harding, D., Parker, G., Acker, S., & Gower, S. (2002). Lidar remote sensing of above-ground biomass in three biomes Global Ecology and Biogeography, 11 (5), 393-399 DOI: 10.1046/j.1466-822x.2002.00303.x

Lefsky, M., Harding, D., Keller, M., Cohen, W., Carabajal, C., Del Bom Espirito-Santo, F., Hunter, M., & de Oliveira, R. (2005). Estimates of forest canopy height and aboveground biomass using ICESat Geophysical Research Letters, 32 (22) DOI: 10.1029/2005GL023971

Running, S. (1999). A Global Terrestrial Monitoring Network Integrating Tower Fluxes, Flask Sampling, Ecosystem Modeling and EOS Satellite Data Remote Sensing of Environment, 70 (1), 108-127 DOI: 10.1016/S0034-4257(99)00061-9

Silva, L., Anand, M., & Leithead, M. (2010). Recent Widespread Tree Growth Decline Despite Increasing Atmospheric CO2 PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011543