Vegetation Conditions for the Coterminous U.S.

It is difficult to map vegetation conditions, period. It’s especially difficult to do it for large places. For one, condition implies good or bad-compared to what? Second, where does the data come from? Here I explore the way LANDFIRE defines and maps condition with a metric called “Vegetation Departure”.

Randy Swaty https://www.landfire.gov/vdep.php
11-19-2020

Green is good…

Vegetation Departure for the lower 48
LANDFIRE's Vegetation Departure.  Green areas represent low levels of Vegetation Departure, white areas high levels, and black represents urban and agricultural land use, and water.

Figure 1: LANDFIRE’s Vegetation Departure. Green areas represent low levels of Vegetation Departure, white areas high levels, and black represents urban and agricultural land use, and water.

at least if you are comparing “reference” to current vegetation structure using LANDFIRE’s Vegetation Departure data. What does all of that mean?

In the map above green pixels or areas represent ecosystems that are closer to having reference amounts of the states compared to the white areas. The black areas represent agricultural or urban land uses, and water.


About these so called “Reference” conditions

As noted above reference conditions essentially equal how much of each state an ecosystem had historically, or pre-European colonization. To get reference conditions The Nature Conservancy’s LANDFIRE team held dozens of workshops with hundreds of experts to build ecosystem models in software called SyncroSim. In each model we entered detaisl such as:

We then ran the models to get the expected amounts of each state. We then mapped how much of those states are on the landscape today and did a little math to come up with a value from 0 to 100 for each ecosystem. The figure just below illustrates the basics for 3 ecosystems.

For much more information, and even tutorials for how to explore and change the models visit https://thenatureconservancy.github.io/landfiremodels/index.html, a website built by The Nature Conservancy’s LANDFIRE team.


Examples of how Vegetation Departure plays out
Example of how Vegetation Departure works. Current and reference successional classes for three ecosystems. Departure from reference conditions can be caused by (A) increases in uncharacteristic vegetation, as in Great Basin Salt Desert Scrub, (B) increases in closed canopy successional classes, as in Ozark Oak Woodland, or (C) increases in early successional classes, as in Cascades Western Hemlock Forest. These vegetation changes are the expected outcomes of biological invasion, fire suppression, and logging, respectively.  From Swaty et al., 2011

Figure 2: Example of how Vegetation Departure works. Current and reference successional classes for three ecosystems. Departure from reference conditions can be caused by (A) increases in uncharacteristic vegetation, as in Great Basin Salt Desert Scrub, (B) increases in closed canopy successional classes, as in Ozark Oak Woodland, or (C) increases in early successional classes, as in Cascades Western Hemlock Forest. These vegetation changes are the expected outcomes of biological invasion, fire suppression, and logging, respectively. From Swaty et al., 2011

If green is good, is white bad?

It depends. The white areas in the map above indicate areas where the amounts of states within the ecosystems vary a lot between reference and current. For example, if historically an ecosystem had little of a “tall and dense forest” succession class but has substantial amounts of that today such as in B above, Vegetation Departure would be high.

Good and bad are in the eye of the beholder. If high levels of departure mean there is more of a succession class that the humans in the area like, or value then they might think that high levels of departure are a good thing. Often though that is not the case. In the western United States the white areas are often ones that are likely to have catastrophic wildfires.

Additionally, reference conditions look backwards. They are super valuable for understanding how an ecosystem used to look and work, and can inform managers in a changing climate, but do not incorporate invasive plants and pests, human uses (e.g., logging), climate change or other modern disturbances.

Whether or not a particular is good or bad is ultimately up to local stakeholders to decide. I just hope these and other datasets promote informed discussion focused on issues and collaborative learning about the world around us, and how we interact with it.