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Tilting at Windmills

Gravel mulching, tree canopy loss and the heat island effect

At the Top of Lists We'd Rather Not Be On

Once again, the City of Albuquerque finds itself at the top of some lists that we’d rather not be on.  One of these lists is the list of large cities with the greatest rate of tree canopy loss.  We apparently rank second, with hurricane-ravaged New Orleans taking the lead.  While we haven’t had any hurricanes, we did have that one big wind storm last year, and we have had a few years of ongoing drought.  By the way, let’s not fool ourselves – in a good year, we don’t get enough natural precipitation to support good tree growth.  Irrigation is one of the main keys to tree growth and survival in our fair city, even in relatively wet years.  During drought times, our trees need that irrigation water even more.

The second list we find ourselves sitting in second place on is the list of cities’ heat island effect.  For those unfamiliar with that term, it refers to what happens when all of a city’s hardscape (concrete, asphalt, buildings, etc.) absorb the energy of sunlight and convert that into heat energy.  The temperature in urban cores is often a few, to several, degrees hotter than surrounding suburban and semi-rural areas.  Las Vegas, Nevada takes first on this list.

The Culprit

So, what do those two lists have in common, and what can we hopefully learn as a take-away lesson from this listing?  The reasons for canopy loss and urban heat island effect are complex and intertwined, and there is no one single cause, but I do have a favorite culprit to point to.  This culprit probably accounts for a fair portion of both issues, and yet…it’s something we do time and again, on purpose and by design.  I speak, of course, about gravel and rock mulches.

Trees are Good, but Trees Need Water

Albuquerque has the task of trying to balance some seemingly irreconcilable issues.  We do live in a desert, water is scarce, and so it is incumbent on each and every one of us to reduce water use.  Lush landscapes do take water, so the trend to xeriscaping has been strong and based on sound policy needs.  At the same time, volumes of research and anecdotal evidence point to the urban forest (the trees growing in all parts of the city) as one key piece of any “quality of life” measure:

  • Trees increase property values while lowering stress levels
  • Trees cool landscapes while encouraging pedestrian traffic
  • Trees can help mitigate storm water events
  • Trees reduce infrastructure stress while pleasing the eye and softening the heart.

Okay, so trees are good.  But – and here’s the rub – trees need water.  They are big plants, with lots of leaves, and they need lots of water.  To my mind, there is no other way to run that biological equation.  So, how do we keep our urban forest healthy (and, dare I dream, growing and thriving) while being careful with our water use?

The Importance of Organic Mulch

Back to gravel and rock mulches for trees – let’s dissect that a bit.  If you’ve ever walked through a natural tree stand and looked closely at the ground, I’ll bet you didn’t see a bunch of gravel and rock on the ground.  Even in stony soils, the surface of the ground under a forest canopy is covered with decaying organic matter: tree leaves, small branches, entire large logs.  This is really important, folks!  Research has shown tree growth is strongest when the soil surface is covered with organic mulches, like chipped-up trees.  As this stuff slowly decomposes, it creates soil conditions that really support tree root growth and function, and roots are fundamental to whole-tree success.  So, point one: organic mulches decompose, and as a consequence better rooting conditions develop in the underlying soils.  Gravel and rock do not do this, which is one reason designers and other like them – they are considered “low maintenance”.   The truth is that these gravel mulched areas still need maintenance – weed seeds germinate and grow in the gravel, especially when there is an underlying fabric between the soil and the gravel.  Fallen leaves are blown out of these areas with gas-powered blowers, in a perverse situation in which greenhouse-gas generating mechanisms are used to remove the material which could actually benefit the trees, if it was just left in place.

Heat Stress

Second issue: heat.  Mineral materials like gravel convert the energy of sunlight into heat, and that heat stresses trees.  To the degree trees can deal with this added heat stress, they do so by letting more water evaporate through the leaf surfaces.  However, in most cases, we are barely irrigating enough to keep them alive, not enough to overcome the heat stress as well.  So, the trees suffer and begin to decline.

Soil Compaction

Third issue: weight.  Minerals are heavy, the wheelbarrows used to distribute the gravel are heavy, the boots on the feet of the workers dumping the gravel are heavy, and the big trucks that deliver the gravel are heavy.  All of that weight on the soil surface leads to degrees of soil compaction under the gravel, and soil compaction is really hard on tree roots.  A compacted soil is hard to penetrate, doesn’t hold much water, doesn’t hold much oxygen (which roots need…), and generally leads to very poor rooting.

One, two, three: no soil generation, heat generation, soil compaction.  Trees suffer.  It’s that simple.

So what about those two lists?

Well, it seems just possible to me that the trend towards extensive gravel/rock mulching all over this city has contributed greatly to tree canopy loss, and measurably to the heat island effect.  The loss of tree canopy adds to the heat island effect.  The heat island effect adds to the loss of tree canopy.  And at the bottom of this vicious cycle, we find that blasted gravel mulch.

So that is my current windmill to tilt against – the excessive use of rock mulches in landscape areas where plant material not evolved to grow under hot rock surfaces is planted.  That would be one of the first things I would change if I were king for a day.

Joran Viers
City Forester
Parks and Recreation Department
City of Albuquerque


“Trees are poems that the earth writes upon the sky.”
― Khalil Gibran, Sand and Foam

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