Will El Niño save struggling trees?

Ask the Arborist


Dave Mortimer

Dave Mortimer

Hopefully, in the middle of next March we will be sitting by our fireplaces and wondering when the incessant rain is going to end, drifting into fond memories of the hot days of summer 2015 and wondering how it all came to this. Meanwhile, we will see on TV how the state is loading all the California tree species two by two onto the “tree ark” as the water inches up to our chins.

At this point measurable rain and lots of it is just a dream—a longing for the once-beautiful green hills in the Conejo Valley that are now a brown stubble of days gone by. If you’ve ever wondered if we live in a desert, just look around at the barren hills that surround us.

I am asked quite a bit about El Niño and the effect the reservoir-filling rain will have on trees that are in decline. Will they bounce back or won’t they? The answer is both yes and no, depending upon the tree.

Trees and other plants seem to like rainwater better than tap water. The several hours of hard rain we got July 18 and then again the second week in September helped out a lot of plants.

It seems there must be something added to or removed from the tap water that diminishes its beneficial effects on plants.

Water officials, however, say the chemicals that are added to the drinking water to ensure its purity quickly dissipate and that nothing is being done to the water that makes it less beneficial to plants, turf or trees.

It may be possible the combination of fewer “unnatural” chemicals in rainwater, as well as natural nutrients available in rainwater makes it a little more effective on plant growth. And when it rains for several days, your trees, grass and shrubs may be getting more inches of rain in those few hours than you give them in a month or more with your sprinkler system..

Many trees are struggling because of the drought. The most obvious are the indigenous, naturally germinated oaks all over the valley. I’ve observed hundreds of Coast Live Oaks that once had thick crowns of lush green leaves reduced to thin canopies you can see all the way through to the sky.

Many of these trees, which rely on natural aquifers for their water, have died due to the depletion of the underground water. If we get a lot of rain, aquifers will be replenished, helping some struggling native trees to recover. But for others, it will be too little too late.

Trees often struggle due to a decline in the density of the roots. Fewer roots bringing in less water and nutrients means reduced upper-crown growth and overall health of the tree. As the tops and branches die back, there are fewer leaves to produce food to feed the rest of the tree, including the roots. It’s a downward spiral.

On another note, if we do get a lot of rain and the associated strong winds that seem to accompany heavy rain, we are going to see trees falling over. As the ground gets saturated, the tree roots lose their natural friction with the soil and a tree is more readily blown over.

In past storms the most common trees that failed were pines and eucalyptus, especially those on hillsides. That potential for failure is even greater for trees that are too thick and create wind sails and by trees that are very tall. Consider this when you look around your yard in preparing for El Niño.

We can only imagine what 30-plus inches of rain could do. It would initially be great for all living things that have suffered from the water shortfall. Whether or not we will soon be wishing for the warm days of summer, my attitude is: Bring it on. We need the rain.

David D. Mortimer is a certified arborist and risk assessor with 40 years of experience in the tree care industry. Email him at dmortimer@roadrunner.com.


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