Eighty foot tall ironwood (Casuarina equisetifolia) with exposed roots due to shoreline erosion. Note the ‘bones’ of an ironwood in the waves just beyond.

Shoreline erosion has become an increasing problem for Hawaii.  Nearly 25% (17 miles) of Oahu’s beaches were lost during the 20th century.  Even greater losses have been reported on Maui.  While it would be nice to blame something sinister and faceless like climate change, the reality is that the problem has been caused primarily by beach-loving people like many of us.  Homeowners with beachfront property have built as close to the beach as possible.  Once the natural shifting of the shoreline occurred, they constructed seawalls, with or without permits, to protect the stability of their homes.  While seawalls may have protected their homes, the scouring action of the waves working at each end of the wall caused the neighboring properties to be more severely affected by the erosion.  Of course, the neighbors then constructed seawalls to protect their properties and so on.  The next thing you know, the entire beach is a line of walls and the sand is gone.  This is a big problem, particularly for a state whose largest industry is tourism with a primary focus on beach activities.

 

Many of the remaining live roots are growing into more stable soil away from the ocean.

So what does all of this have to do with trees?  Plenty.  It’s not just homes that become destabilized.  Beaches, such as Baldwin Beach Park on Maui, have had to be closed temporarily to protect beachgoers from tumbling trees.  Once the trees are removed, the beaches are reopened, but with less shade and beauty than before.   Somehow tree stumps don’t have quite the same ambience as the waving coconut palms in tourist brochures.

 

Another view of the roots extending towards more stable land. Three Certified Arborists inspected this tree. All agreed that the main reason it was still standing was one large buttress root, growing to the right in this view. The ability of the tree to respond through redirected growth shows that the shoreline erosion occurred over an extended period of time.

While arborists may not be able to prevent shoreline erosion, we can learn from it.  Naturally exposed root systems provide a glimpse into the below-ground structure of trees.  I had the opportunity to work on a property with shoreline erosion problems in early 2006.  One ironwood (Casaurina equisetifolia) in particular caught my eye.  This tree, measuring eighty feet (80’) in height, looked as though it was growing on stilts because so much of its root system was exposed.   I thought I would share the photos with my fellow arborists so we can all learn together.  In case you’re wondering about the fate of this tree, it has been reduced to a 5’ high stump but not poisoned so that it can continue to help stabilize the shoreline for as long as possible.  The owner has been instructed to maintain it as a shrub by shearing it regularly to prevent the growth of large watersprouts.

View of the root system facing towards the ocean (makai). The primary buttress root supporting the tree is growing towards the naupaka (Scaevola taccada) in the lower left of the photo.

 

 

 

 

 

 

 

 

Other root features apparent are numerous sinker roots and the lack of a tap root.

 

 

 

 

 

 

 

Dead roots on the ocean side of the ironwood. Even this salt tolerant species has it limits.

 

 

 

 

 

 

 

 

 

References:

Advertiser Staff, Erosion closes popular Maui beach, The Honolulu Advertiser,July 29, 2006, http://the.honoluluadvertiser.com/article/2006/Jul/29/ln/FP607290351.html .

 Mike Leidemann, Advertiser Staff Writer, Beach erosion ‘widespread’, The Honolulu Advertiser,August 6, 2006, http://www.honoluluadvertiser.com/apps/pbcs.dll/article?AID=/20060806/NEWS01/608060345

 Coastal Erosion and Beach Loss in Hawaii, Facts about beach erosion and the Coastal Lands Program at DLNR,  http://www.soest.hawaii.edu/SEAGRANT/CEaBLiH.html

A version of this article was first published in the Western Arborist around Winter or Spring 2006.