Suburban Development Transect: Biodiverse Desert to Trash-filled Parking Lot

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A transect walking a few miles from the indisturbed desert through new housing developments into the city looks like time played in fast-motion.  The ecologist's glasses allow us to see the moving picture of life rather than limiting our vision to the usual single frame.  By substituting space-for-time, we can put on time-travel goggles.  What do we see?

Normally we visit a site for a day, maybe once a year for intensive longitudinal studies, maybe never again.  With this transect we could see the changes in species composition from unique, biodiverse desert with its gnarled shrubs to fresh asphalt streets, planted landscaping and lawns, and a monoculture of weeds in the bladed 'empty patches' between houses and in right-of-ways.  The stream channels were all filled in and replaced with impoundments or concrete-lined ditches.

Eventually we ended up in the back lot behind a storage unit complex.  The slight depression there caught water and supported some of the tallest native flowering trees we'd seen.  The lot was also used, apparently, as a dumping ground and was filled with all kinds of trash.

Later that night, back in my home neighborhood, I saw dual-images of what the land looked like before and after development.  I saw the rocky ground thick with idiosyncratic cacti and weird four O'clock flowers.  And I saw wide asphalt streets, joggers, tall pine trees, oleander, and grass lawns.  It is so difficult to see the past, I felt that my dual-vision was a kind of X-ray superpower, a new found ability to see through reality to what might have been.  Reality has a way of erasing the possibility that things could have turned out differently.

A nice walk in the wilderness can sometimes substitute space for space, so that you can see your neighborhood space as the absence of native wildlife instead of the presence of cars, roads, and lawns.  I suppose some people see nature as empty, and even I see it this way sometimes too: some areas are devoid of active communal life.  For example, on this particular transect we saw no rabbits, no ground squirrels, and no other mammals in the wild.  I don't think we saw a lizard until we got to the rock walls of suburbia.  But I was amazed at the botanical emptiness of our developed landscapes: out of the more than 70 native species of wildflowers and Chihuahan desert shrubs, I saw less than 5 after we crossed the first freshly paved asphalt road.

Of course, there are a diverse mix of landscaping plants, many of them native somewhere, if not in the Chihuahan desert.  Interestingly, the mexican palo verde seems to have escaped cultivation and is now growing up into the wild watercourses that snake off the mountains.  Few other weeds seem able to invade intact ecosystems, although Russian thistle is omnipresent wherever the ground has been cleared.

I think, though, that if the transect had continued further into the past/future, through abandoned neighborhoods or restored areas, the native wildflowers and shrubs would reappear.  Especially with the rains this monsoon, they seem quite happy where they are, and old pipelines have a nice covering of desert marigold, creosote, and javalena bush.  I don't really feel that the desert is destroyed by development...maybe in the long-term view it just goes away for awhile, or changes shape for a spell.  Until the wave of bulldozers breaks and subsides, the desert remains as potential...

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.

Forest Fires versus Forest Restoration

At ESA this year, one discussion turned to the elephant in the room of forest restoration thinning projects: the certainty that thinning will have some deleterious effects versus the probability that a thinned area would actually encounter a forest fire during the approximately 25-year window of restoration effectiveness.  Forest restoration typically involves thinning forests to lower stand densities to discourage the spread of stand-replacement crown fires in historically low-intensity fire regimes.  Yet,  absent the return to presettlement semi-annual burn patterns (i.e. an end to the total suppression paradigm), tree seedlings will recruit and form high density stands within a couple decades.  

What are the odds that a given patch of forest will encounter a mega-fire over a 25-year time span?  According to some participants in the discussion, the odds are quite low.  But that may depend on the forest.  Take, for example, the fire-prone semi-arid mountains of the southwest:

Representative map of burned areas in SE Arizona and SW New Mexico over the last decade.  The Wallow Fire (538,000 acres in the White Mtns) and the Horseshoe Two (223,000 acres in the Chiricahua Mtns) fire, are visible as large yellow areas.

Shaded areas indicate official MTBS 2000-2009 burned area polygons.  Yellow areas are provisional "fire detection" areas from MODIS.  This map was generated using the U.S. Forest Change Assessment Viewer (FCAV), which can map a large number of forest disturbance types, utilizing historical to contemporary GIS layers and current satellite imagery.

Perhaps the ESA discussion participants should read:


Rhodes, J.J. and Baker, W.L., 2008. Fire probability, fuel treatment effectiveness and ecological
tradeoffs in western U.S. public forests. Open Forest Science Journal, 1: 1-7.

Disturbance is good for productivity

Making a "moder" pine forest a little more like a "mull" prarie....