Spring Update: NDVI Differences

Last September, I wrote about Finding the Greenest Place in AZ.  This Spring, we have continued to compare and evaluate the different NDVI difference mapping applications and compare them to the actual growth of wildflowers and grasses we see when we go out hiking.  

Methods

I conduct pre-field research to identify predicted greenness/moisture from UA's Droughtview, USGS VegDRI, and NWS Accumulated Precipitation.  I take a screenshot of each product and assign the proposed site a scale from 1 (driest) to 10 (greenest).  We then visit the area and evaluate the plant production, recording example photos of overall landscape greenness, as well as assigning a score.  The data are organized in a OneNote table.  I then compare the numerical scores in an excel table, adding up the differences between each model and the observations.  

Results

So far, the UA model seems to slightly overestimate greenness, the USGS model greatly underestimates greenness, and the NWS precipitation record comes out closest to observation.

For the UA model, I think it might be helpful to have a difference from maximum, instead of the difference from period. The latter overestimates early spring greenness when the denominator NDVI is very small, so any amount of NDVI in the numerator saturates the index.  Using the maximum NDVI for that pixel could help with this phenology issue.  Plus, % of maximum NDVI may be more intuitive than “difference from average”.

The USGS VegDRI index consistently estimates pre-drought to severe drought in areas that have above average precipitation this water year and have an NDVI above average.  This leads me to think that VegDRI 7-Day eVIIRS is either not well calibrated to the desert southwest, or perhaps that it is better used as a predictive index – perhaps these areas are drying out even though they currently appear green?  However, SWCC does not show significant vegetation drying yet in the areas I assessed.  

Examples

Wingfield Mesa:  UA Droughtview shows this area at maximum NDVI (for this time of year)(=10/10), USGS showed it as pre-drought to moderate drought (4/10) , and NWS shows 125-200% of normal year to date precipitation (9/10).  It is quite green, but it is still early in the growing season and the mesquite have not leafed out yet.  We rated it 6 out of 10.  

Dugas Rd:  UA shows above average (8/10), while USGS showed Moderate drought (3.5/10) and NWS showed slightly below average precipitation (75-90%) (3.5/10).  It is quite green right now, but again not quite at maximum greenness production.  We rated it 8 out of 10.   

Land Development Releases Greenhouse Gases

Land use change releases stored carbon and should be counted under Greenhouse Gas (GHG) reporting.  

Example of a wildflower meadow (left) that was bulldozed to create a parking lot (right). This land use change results in direct emissions of stored soil carbon and plant biomass, as well as continuing opportunity costs: the meadow can no longer accumulate sequestered carbon. If this land is owned by the developing company, this would count as Scope 1 Emissions under GHG reporting requirements.

New GHG reporting standards for land use change are due to be finalized in 2024. According to these new standards,

"Companies shall:

-Account for land use change emissions from land carbon stock decreases across all carbon pools (biomass, soil organic carbon and dead organic matter).

-Account for and report direct land use change (dLUC) emissions or statistical land use change (sLUC) emissions in scope 1, scope 2, and scope 3."

This is important because, according to the IPCC AR6 (2023), land use change accounts for approximately 15% of anthropogenic emissions.  Interestingly, the parts of the land and ocean that have not been developed by humans still absorb 30% of our emissions.  As we degrade more and more land and water, the Earth loses this buffering capacity, in addition to the extra emissions created from land use change.

Xeriscaping or Zero-scaping?

 Xeriscaping is a low-water user landscaping practice in the desert Southwest.  Typically gravel or rock is used as the major groundcover, with desert-adapted plants interspersed.  Herbicides are generally used to control unsightly weeds in the gravel areas. 

Zero-scaping is when landowners skip all of the landscaping and just use herbicides to maintain dirt lots.  Unsurprisingly, the result is often phenomenally ugly.  However, this technique is extremely popular.  Why?

Herbicide-maintained zero-scaping.  The property is listed on one of the popular home-rental websites, so it has to look "presentable"!

Property line contrast.  The owner on the right has elected to let their grass grow tall, creating habitat for wildflowers and pollinators.  They usually mow it once or twice a year.

Close-up of herbicide area.  Not what I would call "presentable".

Another property owner trying to make their yard look nice.  The lush growth on the right shows what they are fighting against.

Some zero-scaping is counter-productive.  Here coir logs were used to try to control erosion.  The slope may even have been seeded.  But the over-zealous (or under-caring) landscaping company tasked with controlling weeds on the property has been very thorough in killing everything.  The result is continuing erosion into the waterway.

 

Fence Line Contrasts

Sitting on the fence has become a metaphor for ambivalence, but actual fence lines are some of the clearest lessons in land management.  Fencelines can be the best place to study ecology, because most fences divide two different land management histories.  The easiest places to learn from fence line contrasts is where the land management history is known.  For example, along highway right-of-ways (ROWs), the strip of land between the road and the fence is almost never grazed, whereas the private or public alotment on the other side of the fence has almost certainly been grazed and/or farmed.

However, just because the ROW hasn't been grazed doesn't mean that it has escaped disturbance. While comparing two disturbed areas can yield some insights, the multiple factors at work will make cause and effect deductions extremely difficult. To find a good comparison, look for areas that are relatively far from the road; immediately adjacent to the road is a zone of disturbance, which can include vehicle traffic, trash, mowing, and runoff from the road (i.e. increased moisture).

The best comparison areas occur where the ROW is relatively higher than road (so there is no possibility of runoff and little chance of other human disturbance).  However, areas with cutbanks below them are not good for comparisons, because of excess erosion, different microclimates around bare rock or exposed subsoil, and lowered water table.   A zone of depression in soil moisture can also occur around ditches, trenches, gullies, roadcuts, etc.

The actual fence-line itself may have different species due to fence-line drip of dew and the ability of fences to catch seeds, especially tumbleweeds. (photo).

On the ground immediately beyond the fence there may be an area of extra disturbance due to cattle trails, etc, and any areas near stock tanks or gates are also likely more heavily used (and hence a more extreme contrast).  In cases with less grazing on the private land, such as on steep hill slopes, vegetation and soils may look quite similar across fence-lines.  Of course, there will always be variable disturbance on both sides of the fence, but that is part of the challenge and opportunity of observing fence-line contrasts.

The best comparisons are between areas relatively far from disturbance, close to but not immediately adjacent to the fence-line.  With a good undisturbed ROW as a control, the vegetation on the other side of the fence can be compared to the potential climax community of the site.  

Case Example:

I was recently watching fence-lines along NM highway 285 from Vaughn to Clines Corners, and noticed that typical overgrazed areas are Grama Grass (Bouteloua gracilis) monocultures or low-stature annuals with large amounts of bare dirt.  The ungrazed roadsides still have bare ground, but the vegetation has a starkly different structure and composition:  multiple grass species occur with different growth forms.  But even more noticeable than the grass growth is the shrub encroachment in an area that is pure grassland.  Without fire or grazing, woody growth, especially saltbush (Atriplex canescens) and  Chimisa (Ericameria nauseosa) increases markedly.

While some trees and shrubs (e.g. E. nauseosa) are resistant to grazing when mature, their seedlings are highly palatable.  Cows can completely eliminate woody overstories from riparian areas in a single generation simply be eliminating recruitment (through both grazing and trampling) of Cottonwood and Willow seedlings.  Grazing pressure on seedlings is important, but easily overlooked: as long as there are trees, we describe an ecosystem as a forest. And it may seem strange to say that cows are eating a forest. But without seedling regeneration, no ecosystem is sustainable.

That grazing impacts woody growth as much or more than herbaceous growth is well-known along rivers and wetlands, but I think has been less remarked on in uplands.  From this brief study of fence-line contrasts, it appears that even more of our grasslands would support shrublands were it not for either grazing or fire limiting woody plant establishment.

Range Monitoring on Chihuahan Desert Grasslands

We helped out with a long-term monitoring project at a ranch in Southwestern New Mexico.

Diverse plots contained upwards of 30 perennial grass and forb species, plus half a dozen shrubs.  Degraded areas might contain only three or four annual, weedy species.  We found evidence of ecosystem engineers like termites and banner-tailed kangaroo rats.  Also, the remains of last year's grasshoppers were thick in the better grasslands.  Interestingly, the only mole we found was in the one of the most degraded sites, an area with only sparse annual grasses -- not much roots for a mole to munch!

Reading a vegetation transect line in a thick Giant Sacaton (Sporobolus wrightii) flat.  More photos.  

Cows eat grass...and Rain grows grass (!)

Grasslands can recover from grazing, provided there is sufficient moisture to grow.  Arid environments often lack moisture, so recovery can be extremely slow (Valone et al) or nonexistent.  That is the overarching conclusion of several long-term vegetation studies in Arizona and New Mexico.  Shrub removal can increase grass cover, but at low levels shrubs do not seem to compete with grasses.  At the Santa Rita experimental range, south of Tucson, invasive species pushed aside native grasses, but then all vegetation cover decreased during the following 20-year drought.  

Figure from Mashiri et al.  Basal cover of perennial grasses on the Santa Rita Experimental Range from 1972 through 2006.  SR = Seasonal Rotation grazing; YL= Year Long grazing.  Following the wet 1980's, grass cover increased to the peak in the center of the graph, but has been falling ever since.

It may sound obvious that grazing can decrease grass cover and that it may take several wet years to regain aboveground growth.  Range science has long advocated for differential season of grazing, or intensive grazing, or other management alternatives, but some studies such as Mashiri et al find no long-term differences between management methods.

A recent study from Bestelymeyer et al did find some slight differences between winter- and summer-season grazing, but they were the opposite differences that traditional range science manuals would predict for Black Grama grass! 

Figure from Bestelmeyer et al.  Black Grama grass cover on the Jornada Experimental Range decreased with grazing and increased following grazing.

References:

Bestelmeyer, Brandon T., Duniway Michael C., James D.K., Burkett L.M., and Havstad Kris M. A test of critical thresholds and their indicators in a desertification-prone ecosystem: more resilience than we thought.  Ecology Letters, 01/2013, Volume 16, p.339-345, (2013)

Mashiri, F., M. McClaran, and J.S. Fehmi. 2008. Short- and Long-term Vegetation Change Related to Grazing Systems, Precipitation and Mesquite Cover. Rangeland Ecology and Management 61:368-379.

Valone, T. J., Meyer, M., Brown, J. H. and Chew, R. M. (2002), Timescale of Perennial Grass Recovery in Desertified Arid Grasslands Following Livestock Removal. Conservation Biology, 16: 995–1002.

An Important Point about Grazing-based Land Restoration

"Allan Savory's holistic resource management [was described] as a "promising option," even though there is no science to back up claims about intensive grazing schemes.  The truth is that grasslands are relatively arid environments, and livestock don't make the grass grow: rain does.  And rain doesn't follow the hoof."
--Jeff Burgess, reader response in the November/December 2013 Nature Conservancy magazine

Addendum:  read this comprehensive response to Allan Savory's claims, or this recent direct rebuttal:

The Savory Method can not green deserts or reverse climate change, Briske, David D., Bestelmeyer Brandon T., Brown Joel R.,Fuhlendorf Samuel D., and H. Polley Wayne , Rangelands, Volume 35, Issue 5, p.72-74, (2013)

and this follow-up:

What do Wilderbeast eat in Ngorongoro?

This computer model knows:

Ecohydrology of Wet Semi-Arid Climates versus Dry Semi-Arid Climates

Semi-arid climates are exquisitely balanced on a range of ecosystem properties. Most famously, these savanna ecosystems, which border the truly arid deserts, are susceptible to desertification. Ecologists say that they have a bimodal stability: they can exist in two very different states, either grassland or shrubland, and small changes in land management and climate can 'flip' them from one to the other. In Huxman, T.E. et al. Ecohydrological Implications of Woody Plant Encroachment. Ecology, 308-319 (2005) ecohydrology researchers hypothesize that the response of semi-arid ecosystems differs between wet semi-arid sites and dry semi-arid sites, with subtle, yet important differences between them if they flip from grassland to shrubland.:



"The relative contribution of T to ET will increase for systems dominated by woody plants as compared to those dominated by nonwoody plants in more mesic climate zones, with the opposite effect in semi arid systems. Four zones can be delimitated where changes in the T/E T are likely to occur. In Area 1, there is little change in T/ET because leaf area remains about the same and ET is dominated by E from large expanses of bare ground. In Area 2, E increases substantially in woody plant systems as a result of the loss of herbaceous cover in intercanopy spaces. Area 3 is a transition zone (no further changes in T are being caused b woody plants, and herbaceous vegetation still dominates intercanopy spaces). In Area 4, differences in T/ET are due to increased T by shrubs (shrubs are using ‘‘extra’’ water that, in a grassland system, would become groundwater recharge)."

Grazed Prairie Grassland vs Ungrazed Desert Grassland

...but how do you tell along transitional zones from Desert Grassland to Midwestern Prairie? How many distinct and idiosyncratic ecosystems or ecotones exist?

From Rio Puerco