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Researchers studying land cover and land use through time know they have a gold mine in a Landsat archive that spans almost 50 years. But it’s not the only rich data source for time series studies.

Color photo of Douglas fir tree in central Idaho
This Douglas fir tree in central Idaho is more than 700 years old. Dr. Chris Crawford of the USGS Earth Resources Observation and Science (EROS) Center used time series data from tree rings to learn about historical climate and precipitation patterns before turning his full-time attention to Landsat research.

Research Physical Scientist Dr. Christopher “Chris” Crawford at the Earth Resources Observation and Science (EROS) Center says he’s also tapped into important lessons about climate and the environmental impacts of large-scale drought and rain events through the historical lens of tree rings.

Those rings—some narrow, some wide, some lighter and others darker—go back much farther than 50 years, too. In many cases, we’re talking hundreds if not a thousand years of important climate data captured in the trunks of trees.

“Trees really provide a temporal record, a chronology of environmental change over time,” Crawford said. “What I think is interesting about this type of work is that it really does sort of bridge very well with the land change science that we do in remote sensing.”

Tree Ring Science Spans a Century-Plus

The discipline of tree ring science, called dendrochronology, has been around for a hundred-years-plus, Crawford said. In 1990, the National Oceanic and Atmospheric Administration (NOAA) Paleoclimatology Program and World Data Center began maintaining the International Tree-Ring Data Bank, which was started 16 years earlier and is the world’s largest public archive of tree ring data.

Those data are acquired by boring to the center of trees and pulling out core specimens, Crawford said. During periods when it is warm or wet, fat rings form in the trunks. During cold or dry conditions, the rings are narrower.

Individual tree rings offer more knowledge than just that. In about 2012-2013, studies started coming out on sub-annual variability in the rings, or the influences of early spring and later summer precipitation and temperature on the trees, Crawford said.

Light coloring in part of a ring gives insight into what is called early wood, when trees are soaking up winter and early spring precipitation and experiencing their greatest rate of growth. Darker coloring in that same ring represents late summer precipitation and slower growth that occurs in anticipation of the dormant season to come.

Rings Offer Impacts of Climate

Through cross-dating, cores taken from multiple trees at one site can tell scientists what the impacts of climate—through precipitation and temperature—were in any given year just by examining the rings.

“By using the practice of cross-dating or pattern-matching of the rings through time, you get an exactly dated year to that ring,” Crawford said. “Then you begin to hone in on what actually happened in, say, 1952, or what happened in 1685.”

With a continental set of exactly dated tree ring records that go back hundreds of years, sometimes a thousand years plus, he said researchers begin to see large spatial patterns in drought and wet periods.

A new North American Seasonal Drought Atlas (NASDA) discussed in a recent Journal of Climate paper provides the ability for scientists to access time series data for anywhere in North America and look at the differences between cool season (November to April) and warm season precipitation (May to July) going back hundreds of years, Crawford said.

NASDA is based on a lot of work done in the late 1990s and early 2000s for a prior atlas called the North American Drought Atlas (NADA)—work Crawford contributed to when he was taking core samples from trees in Idaho and Montana back in 2009 and 2010.

Color illustration of tree ring data
A photomicrograph illustrates tree rings for years 1504-1520 on a Douglas fir core specimen. The schematic illustrates which components would be measured as earlywood (white) and latewood (black). Note that latewood contains independent variability, particularly evident in years when it is disproportionally wide (e.g., 1506, 1509, 1517) or narrow (e.g., 1508).

Studying Tree Rings in Southern Appalachia

His interest in dendrochronology began when, as a graduate student in geography at Virginia Tech, he helped take core samples out of trees in Southern Appalachia to study the migration of forests into open, higher elevation areas. Later, as he started pursuing his doctoral degree at the University of Minnesota, his interests took him to the northern Rocky Mountains.

“I was interested in developing really long tree ring records to support the ability to tease out temperature and precipitation in the northern part of the western United States,” Crawford explained. “So, in 2009 and 2010, I went out and collected 13 or 14 different long-term datasets that spanned a thousand years-plus.”

He ended up writing several papers about those datasets, and saw his work folded into the new NASDA drought atlas. But in time, Crawford ultimately moved on from his dendrochronology interests to begin working full-time with Landsat.

Now the Landsat Project Scientist and agency co-chair of the Landsat Science Team for the USGS, Crawford believes there are lessons in tree ring science that can be applied to the time series remote-sensing work being done with Landsat.

“In many ways, the methods of analysis in tree ring science could be systematically applied in time series analysis for Landsat,” Crawford said. “In fact, the Continuous Change Detection algorithm that receives a lot of attention is quite like time series spectral methods that have been used for a really long time in climate and tree ring sciences. So, the methodology is not new.”

A Window into Historical Patterns

Color chart of time series climate data from tree rings
Tree ring-reconstructed warm season precipitation for the closet North American Seasonal Drought Atlas (NASDA) grid point to USGS Earth Resources Observation and Science (EROS) Center in South Dakota (red line). The time series spans AD 1000-2016 with a 10-year smoothing function (black line) applied to emphasize decadal variability.

What tree ring science offers is a window into the historical patterns of precipitation and temperature, Crawford said. At a half-degree resolution, NASDA provides a baseline to enable use with Earth system models to simulate future North American climate.

Tree rings—like annual agricultural census information, historical databases on reservoirs and military bases, maps of cities and towns as they have expanded and contracted, and climate records—can provide the kind of consistent data that enable researchers to quantify historical land change within an integrated analysis environment.

Ultimately, satellite sensors, supercomputers, and models will help pave the path forward on land surface change and future projections, Crawford said. But tree rings can help to benchmark the historical range of climate variability and inform scientists on what they should be looking for and looking at.

“There’s a real fundamental scientific basis for our understanding of past and present environmental change, and tree rings are an important part of that,” he said. “What I’m saying is, ‘This whole idea that we need to be able to forecast or simulate conditions for planning and for improved scientific understanding is where we’re at today. Tree rings and the data produced from the NASDA project are essential for that type of activity.’”

It’s the kind of environmental data scientists need to consider as they start looking outside the box beyond just satellite data at the different types of Earth proxy records that can assist in what they’re trying to accomplish, Crawford said.

“It’s a bit more of an integrated view of how we actually move forward,” he said. “This tree ring NASDA dataset has turned growth rings into actual estimates of past and modern era climate. As such, I think it becomes much more valuable and easier to integrate into multi-disciplinary studies.”

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