Re: Why do plants growing along the side of a road wither and die?

The image I got of your road and the wilted plants is one reminiscent of 
desertification.  I myself am already in the desert, so I apologize if my 
view is a bit skewed.

When a swath of land is cleared, especially in an ecology like yours, water 
is allowed to escape freely.  Normally, plants will grow to different 
heights and use each other to sustain themselves.  The shortest plants stay 
the wettest and don't get much light.  The tallest ones hog the light.  The 
whole package ends up acting as an insulating layer that keeps the surface 
of the soil moist by both blocking out the wind and direct sun, but also 
actively adds moisture when the stomata are open(around here, that's 
usually only nighttime).  When the plants are cleared from an area, an 
opening for the heat and wind to penetrate develops.  The soil of the 
cleared area dries, and because nature abhors a vacuum, the moisture gets 
sucked from the soil nearest the roots of the plants.  They wilt, and 
sometimes die, but a decaying plant provides an insulating layer as well, 
so what would seem a domino effect is arrested after only a few feet or 
less.

Because of this rapid change in ecosystem, the microbiota of the soil also 
changes. Activity in dried soil (vs naturally dry soil) slows to a crawl 
and over time, the dried soil is noticeably less productive, less fertile. 
 Weeds grow because all of the native herbaceous plants either require 
moisture to germinate or soil rich in humus (the dark organic goo) to 
survive beyond a few weeks.  The weed growth tends to mean the end of the 
wilting of the native plants, but only the beginning of a scraggly-looking 
roadside.     
 
Another possibility which is cause for just as much concern is the 
byproducts of the road building, but you'll have to discern that for 
yourself- oil is the most common runoff that has any significant ecological 
impact.  If it rains frequently where this road has been built, you may see 
a relatively thick film floating in any puddles-the scum from oiled 
pavement is anything but subtle, I think you'll have a hard time missing 
it.  I don't know if this can be prevented, but I have seen areas with 
protected waterways running beneath (or sometimes during the rainy season, 
over) a newly built road.  The highway department places hay bales along 
the edge of the construction site, and it is amazingly effective in keeping 
the oils contained.
 
I'm glad someone else has noticed these things, and I sure hope that the 
plants in question aren't in your garden!