Kiawe (Prosopis pallida): Hawaii’s Tropical Mesquite

1 Jul

A couple of years ago I was photographing some volunteers at a Beach Cleanup Day at Kanaha Beach park near Kahului on Maui’s windward side. Part of the cleanup involved piling up some large tree branches that had been cut the day before by a Maui County tree-trimming crew. As I and the volunteers began stacking the cut limbs by the side of the access road for the County people to dispose of, a pickup truck full of locals hurriedly pulled up behind us. The driver asked if he and his boys could have all of the cuttings. The crew leader agreed as long as they took the entire pile with them. “No problem, bruddah! Dat’s Kiawe and it’s great for makin’ barbecue fire!” In an instant, the slash pile was thrown into the bed of the truck by its enthusiastic passengers. The truck then sped off down the dirt road, its occupants all waving happily as if it was double-pay day. In this way I was introduced to the Kiawe, Maui’s tropical mesquite tree.

Kiawe tree, Kealia Beach, Maui

A healthy mature Kiawe tree helping to stabilize the berm on Kealia Beach, Ma’alaea Bay, Maui.
(To view a larger version, click on image.)

Prosopis pallida is a perennial that belongs to the Mimosoideae subfamily (mesquite & mimosa) of the Leguminosae/Fabaceae family. It is commonly referred to by its Hawaiian name “kiawe” (pronounced “kee-AH-vay”). As with all other members of this family, it produces its seeds in pods referred to as “legumes”, hence the common name for the family.

Bark & foliage of a Kiawe tree

The outer bark and foliage of the Kiawe tree.
(To view a larger version, click on the image.)

Physical Characteristics — Under ideal conditions, Kiawe trees can grow to be more than 30 m tall and form dense, continuous forest canopies (one of the few species of dry coastal trees to do so). However, they may also develop into gnarled, stunted bush-like trees in dry, wind-swept areas with poor soil such as those found in stabilized sand dunes and beach berms.

Leaves emerge from a common node on the twig, which changes direction often as it grows, giving it a corkscrew or zig-zag appearance when mature. The small, delicate-looking leaves (usually less than 2 cm long by a half a centimeter wide) are doubly compound with 3 to 4 pairs of stemlets branching from the main leaf stem. The twigs and branches include a fair number of very formidable thorns (see image below) notorious for drawing blood from careless beach-goers who seek out the Kiawe’s ample shade.

In the spring, small yellowish-green flowers are borne on long (8 to 15 cm) cylindrical spikes that give way to dense clusters of long (10 to 20 cm) yellowish-brown seed pods. The 10 to 20 seeds per pod (see image below) are encased in a sticky, sugary pulp.  The heartwood is dense-grained and hard, making it a favorite fuel for cooking fires on Maui and throughout the world. Generally, the root system is shallow, spreading out laterally; but in arid soils it easily develops a long taproot.

A typical Kiawe twig

A typical Kiawe twig displaying its characteristically delicate foliage and very nasty thorns.

Kiawe seed pods

Kiawe seed pods fall from a single tree’s branches by the thousands, ensuring that many seeds survive & germinate.
(To view a larger version, click on the image.)

Introduction to Hawaii — Prosopis pallida is native to the arid coast of northwestern South America. In 1828 it was introduced to the Hawaiian Islands by Father Alexis Bachelot, the head of the first catholic mission to Hawaii. He planted a tree on the grounds of the Catholic Mission on Fort Street in Honolulu that he had raised from the seed of a Peruvian tree growing in the royal gardens of Paris. By 1840, the progeny of that single tree became the principal shade trees of Honolulu and were already spreading to the dry leeward plains on all of the neighbor islands, including Maui.

Growth Habits — A Kiawe seedling’s survival depends on receiving sufficient rainfall and sunlight (the seedlings do not tolerate constant shade) during the first few weeks after germination. The seedling can grow more than one meter in its first year under the right conditions. It will produce a strong and rapidly growing taproot system that can penetrate deep into even the hardest soils. Many old trees have been saved as garden and park trees during land development and have grown to large sizes with constant irrigation. Trees that grow on coastal plains where groundwater is shallow and abundant grow to be quite large, but they develop shallow root systems and windstorms can topple them easily. Once mature, growth is very slow compared to that of other trees under similar conditions. On windy or dry sites, Kiawe grows as a shrub or a small twisted tree only a few meters tall. Where it grows in strong trade winds, it is sculpted and shaped by the prevailing winds and lies along the slopes as a rounded bush. Although it is a coastal species, Kiawe is easily defoliated by the windblown salt spray of winter storms. Kiawe trees grows in areas where fire hazard is often extreme. Trees rarely survive slow-burning fires; they are usually killed outright by fire.

Invasive Habits — Once Kiawe was introduced to Hawaii it quickly became a pest species, invading, out-competing, and overwhelming native grass species and woody plants. It is a successful invasive species due to its ability to reproduce in two ways: production of large numbers of easily-dispersed seeds, and vegetative growth (by suckering) to create thick monotypic stands that shade out all other nearby plant species. It requires less than four inches of annual rainfall to establish itself and survive. It survives well in dry environments due to its extremely long taproot. It is so efficient at withdrawing moisture from soil that it can kill nearby plants by depriving them of water. It is often found growing in areas where other plants do not grow, such as sandy, dry, degraded slopes; salty soils; disturbed areas; and rocky cliffs.

Continuous canopy of kiawe trees

A dense, continuous canopy of Kiawe trees growing along a beach front in the Kawalilipoa neighborhood of Kihei, south Maui.
(To view a larger version, click on image.)

Ironically, it is these very traits that have made Prosopis species so valuable in efforts to control soil erosion due to desertification in Africa and South Asia. Kiawe serves a similar purpose on islands such as Maui where beach erosion is a serious problem.

Kiawe @ Kealia Beach

These well-established Kiawe trees rooted in the unstable sands of Kealia Beach protect a delicate intertidal marine habitat from the ravages of wind-blown sand and storm-driven waves.
(To view a larger version, click on the image.)

7 Responses to “Kiawe (Prosopis pallida): Hawaii’s Tropical Mesquite”

  1. mfmsparrow at 6:50 PM #

    Reblogged this on You Don't Say.

  2. GARIE CHAVEZ at 6:30 AM #

    Thank you for posting information on Kiawe tropical mesquite. I am local artist living in Arizona but plan to move Kauai soon and my art consist of wine bottle candle holder and use mesquite as the base. Now, I can say that desert mesquite has bruddah called tropical mesquite and continue to market them as the same.

  3. I don’t think the Halekulani’s tree is the largest on Maui. I have seen some immense kiawe trees in the large stand that fronts on Kealia Pond Beach (near the Kihei power station), and near Kenaha Pond on the windward side.

    As far as other species of trees and/or sizeable schrubs mixed in with kiawe trees, that’s unlikely (but not impossible). Well-established stands of Kiawes manage to keep out natural invasions of other species due to the toxic secretion from their branches that selectively poisons the soil directly under the trees’ canopy. However, where storms or human clear-cutting have created large enough openings in the forest floor, Ironwood trees often move in quickly, sprout fast, and when mature they manage to hold their own against the Kiawe’s poisonous secretions. Thanks for the comment/questions, Jon!

  4. Jon at 10:31 AM #

    Nice site! Last night, I was at the Halekulani Hotel in Waikiki and marveling at the size of the Kiawe tree there next to the beach. That brought up the question, is that the largest Kiawe tree in Hawaii?

    Another question, are there other trees mixed in with the Kiawe in the right foreground of the photo in this webpage captioned “A dense, continuous canopy of Kiawe trees growing along a beach front in the Kawalilipoa neighborhood of Kihei, south Maui.
    (To view a larger version, click on image.)” ? Some look like Opiuma but it is hard to tell from just this photo. Jon

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  6. Jennie at 6:29 PM #

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  1. Another day in paradise… | You Don't Say -

    […] off as soon as we put our towel down on the sand.  Quickly I realized that we were sitting near  Kiawe trees. They leave hidden thorns.  Stepping on one is the second worst thing to ruin a […]

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