Strawberry guava — Maui Invasive Species Committee (MISC)

Measuring the impacts of invasive plants in Hawai’i’s watersheds

Posted on June 5, 2020 by MISC

University of Hawai’i professor and researcher Tom Giambelluca is looking to better understand how invasive plants, like strawberry guava on the right, disrupt the water collection function of a native rain forest like that on the left.

In Hawaiʻi, the supply of fresh plentiful water depends on the forest capturing moisture from fog drip and rain, and then move it to streams and aquifers. Scientists and resource managers know that invasive plants disrupt the capacity of a native ecosystem to collect water. In Hawaiʻi, invasive species are the greatest threat to the function of native ecosystems.

But there are many unanswered questions about how and to what degree invasive plants affect the water capture in Hawaiʻi. Water capture, depends on both rainfall and cloud or fog drip, water then moves through the forest and soil to streams and underground reservoirs (aquifers). Some water is lost through evaporation and transpiration – the movement of water from roots through the leaves and into the atmosphere.

Like straws, plants pull water out of the ground and release it into the air. Differences in both the physical structure and growth characteristics contribute to different rates of transpiration between native and invasive species. Tom Giambelluca is looking closely at those differences. Giambelluca is the director of the Water Resources Research Center and a Professor in the Department of Geography and Environment at the University of Hawaiʻi at Mānoa. He has spent much of his career looking at the impacts of invasive species on water resources in Hawaiʻi. His latest research attempts to tease out and quantify how invasive species increase the loss of water back into the atmosphere.

“One of the reasons invasive plants take over might be that they grow faster,” he says. “Faster growing plants tend to use more water” and over the long run, as invasive plants take over the native dominated forest, these differences in transpiration rates could have significant impacts on the cycle of water through the environment and the availability of water for human use.

To quantify these impacts, Giambelluca has relied on an experimental model of setting up monitoring equipment in paired sites where forest dominated by native species is growing immediately adjacent to stands of invasive plants. This site selection eliminates differences that may be caused by soil types and weather patterns in geographically separate locations. Weather can vary dramatically within small distance in Hawaiʻi. He then looks at the differences in transpiration by setting up tall towers filled with monitoring equipment and using measurements of water flow moving up the stems of sample trees.

But suitable study sites are hard to find – and funding additionally limits the number of sites he can potentially set up. With only a limited number of sites, he can’t accurately predict the impacts of invasive species may have on transpiration rates across the landscape.

So, Giambelluca is developing an experimental model wherein he can evaluate sites based on data that can be collected in a single day, therefore enabling his team to gain a better understanding of how individual differences in plant transpiration rates can impact water capture throughout Hawaiʻi. And he’s focusing primarily on leaves.

“Leaves are where the action is,” explains Giambelluca. “It’s where the plants control the uptake of carbon dioxide and release of water vapor.”

He takes a handful of measurements including the leaf area index (the surface area of all leaves over a given area), and leaf gas exchange rates of different plant species. The more leaves in an area, the higher the transpiration rates. Invasive – dominated forest tends to have a higher leaf area index (i.e,. more leaves). Plant species transpire at different rates. After he has enough data to be statistically significant, he can then combine that with distribution maps of plant species across Hawaiʻi, and better assess the impacts invasive species have on the amount of water lost through transpiration.

While still in the proposal stage with results a few years in the future, the potential outcomes from this research could help inform the decisions resource managers and funders face in setting priorities both for which species should be controlled as well as which areas are more sensitive to the impacts of invasion. In the meantime, enjoy a fresh glass of water brought to you by a native forest nearby.

For more information on Giambelluca’s research, check out his lab page at https://sites.google.com/a/hawaii.edu/ecohydrology_lab/

Lissa Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia’i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.

Originally published in the Maui News on November 9th, 2019.

Invasive plants=Less water

Posted on June 27, 2014 by Lissa Strohecker

This rainforest, in Puu Kukui on West Maui, is an example of an open canopy rainforest typical of native rainforest in Hawaii. This forest acts like a ‘sponge,’ absorbing water from rain and gently releasing to recharge streams and aquifers. MISC file photo

When rain falls from the sky by the bucket-load it can be tempting to take water for granted, but the trip from raincloud to tap relies on effective, functioning natural systems. In Hawai‘i, alien plants disrupt the forest’s ability to capture water.

Water follows a cycle through the environment. Water over the ocean evaporates becoming atmospheric moister, as it travels over land it condenses into clouds, falling to Earth as rain or fog drip. Once on the ground, water has three paths: 1) it’s taken up by plants that use it to move nutrients through their cells, then return it to the atmosphere via transpiration; 2) it fills streams, lakes and rivers, eventually returning to the ocean–minus what was used for irrigation or lost through evaporation; or 3) it seeps through the soil into underground pools called aquifers. These aquifers supply most of the water we use to drink, bathe, and grow our food.

Forests have evolved differently to take advantage of prevailing water cycles in their regions. In South America and Africa the greatest diversity of plants is found up in the canopy. In Hawai‘i most plant species are found in the lower third of the forest, closer to the forest floor. Hawaiian understory plants need sunlight filtering through an open canopy. Koa and ‘ōh‘ia, the dominant Hawaiian canopy trees, grow in such a way that light reaches the shrubs and groundcover below.

Koa and ‘ōh‘ia pull water from passing clouds as fog drip; the curved leaves of koa provide surface area to collect moisture from clouds and channel it down leaf tips to the forest floor. Spongy plants on ‘ōh‘ia branches and trunks absorb moisture from passing clouds. Alien species disrupt the system.

Strawberry guava, Psidium cattleanium, has invaded Hawaiian rainforests, growing so dense and so

Strawberry guava invaded forest in Makawao Forest Reserve

The invasive tree strawberry guava chokes out native plants. As it sheds bark, strawberry guava provides no habitat for the plants growing on the trunks of native species. Photo courtesy of Forest and Kim Starr.

fast that it chokes out other plants. A strawberry-guava-invaded forest captures water differently than our native forests. In Hawai‘i Volcanoes National Park researchers found that a forest dominated by strawberry guava delivered less water to the forest floor than an intact native rainforest. Researchers noticed several differences between the two forest types: more rain ran down the stems of strawberry guava than ‘ōh‘ia but less water was pulled from the passing clouds. This could result from structural differences. ‘Ōh‘ia have aerial roots and furrowed bark carpeted with mosses and small plants called epiphytes. The epiphytes and aerial roots soak up water from passing clouds. Strawberry guava bark is smooth and regularly shed; few plants grow on its trunk and branches.

Other invasive species disrupt the water cycle by sheer size—leaf size. Miconia trees with three-foot long leaves sweep into the forest, stealing light from the understory and transforming it into darkened forest, the soil bare but for miconia seedlings. As another researcher on Hawai‘i Island discovered, the huge leaves are more than just light-hogs. They collect water as if they were huge tarps, and, as anyone who has stood near the edge of a tarp in a rainstorm knows, it’s only a matter of time until the water

The forest floor under a miconia invasion is bare, often with exposed roots. Not a good sign for water collection. MISC file photo

dumps down. The drops running off of miconia turn out to be the largest drops ever measured. Larger drops hit the ground harder, and where miconia has smothered the understory, those drops fall on bare soil. Huge drops compact soil particles, preventing water from seeping down to aquifers, instead causing it to run off, carrying topsoil into streams and eventually the ocean where it can smother coral reefs.

These are only some of the ways invasive plants alter the forest’s ability to capture water. Water-hungry plants like Himalayan ginger pave the forest floor in tough roots. Huge trees like eucalyptus require more water to carry nutrients to their crowns than smaller trees. Unfortunately, the list goes on. Healthy forests mean reliable sources of water. Keeping invasive plants out of native forests is one way to help keep the water flowing from raincloud to tap

By Lissa Fox Strohecker. Originally published in the Maui News, March 11th, 2012 as part of the Kia‘i Moku Column from the Maui Invasive Species Committee

Positive spin on invasive species by artists, chefs

Posted on October 7, 2011 by Lissa Strohecker 1 Comment

Crews harvest saplings of invasive eucalyptus for use in a sculpture by artist Patrick Dougherty. The sculpture is at the Hui No'eau Art Center in Makawao. Photo by Rose Gabriel

One day I was pulling a weed and I realized there was a little grain of hostility there toward this invasive plant,” said Washington D.C. artist, Patterson Clark, in an interview with National Public Radio. “And I stopped and thought: I don’t want to be this way in nature. I don’t want to be in an adversarial mode when I’m in nature. How can I change my attitude to make this more of a positive experience? And the word ‘harvest’ came to mind.” Patterson shifted his perspective and began turning weeds into resources — brushes, inks, and even paper.”

It’s happening throughout the country — artists, chefs, hunters, and fisherman are using their talents to harvest invasive plants and animals, transforming them in innovative ways–each example regionally appropriate. In the Southeast, artists weave kudzu into lampshades, baskets, and sculpture. In the Pacific Northwest, printmakers are making paper out of Japanese knotweed. In Maryland, the forest-choking vine, oriental bittersweet, is morphing into lighthearted sculpture. In some places, it’s work you can sink your teeth into. “Invasivores” control pests by dining upon them: Asian carp, Himalayan blackberries, even nutria, the semi-aquatic rodent that invades the wetlands of the South, can be dinner. Fishing derbies have tapped into the competitive spirit, controlling venomous lionfish in Florida and removing voracious roi from Hawaii’s reefs. This month, on the lawn of the Hui No’eau art center, sculptor Patrick Dougherty will be twisting watershed-choking strawberry guava into imaginative shapes.

sculpture on display at the Hui No'eau Visual Arts Center was made out of strawberry guava, eucalyptus, and white ash.

The best an invasive species can look! This sculpture on display at the Hui No'eau Visual Arts Center was made out of strawberry guava, eucalyptus, and white ash. Photo by Hui No'eau.

Many of the plants and animals that overrun our environment have useful attributes: they taste good, have beautiful wood or nice flowers. But the value of the species may come at a high cost– the health of an ecosystem, quiet nights, locally grown produce or livestock.

By harvesting (or overharvesting!) these invaders we can recycle unwanted species while helping the environment. The carbon footprint of using locally available material is less than importing it from the mainland and the process provides a connection with the land while giving back to the place we live.

There are risks. Invasive plants and animals carry a reputation for jumping the fence and escaping. Keep the following points in mind when using invasive materials and you’ll help ensure that your work is making a difference, but not planting the seeds of a new invasion, literally and figuratively.

How does this species spread? Tiny seeds, resilient vegetative roots? If you are going to move an invasive species, be sure to bag the flowers and fruit, and make sure fruit, flowers, and roots are disposed of properly (not Green Waste!).

Toxic invaders: Many species have chemicals harmful to people and animals. Research the species and how it can safely be used. Roi are toxic to eat, every part of oleander is toxic, and the sap of some plants, like the pothos vine, can cause a bad skin rash.
Do you have permission? Check with the landowner before removing any species.
Is it legal? State laws restrict the transport of some plants and animals. Some species are on the noxious weed list or targets for control by local invasive species committees. Check with them for guidance.

Finally, remember to stay slightly detached from the material which, ideally, will become harder and harder to find. Perhaps then it will be time to find a new invasive to work with. It’s unlikely that a hunting tournament, new recipe, or art sculpture will lead to the eradication of an invasive plant or animal, but by harvesting invasive species you’re helping restore balance to the ecosystem.

So by all means, jump in. Lend a hand to control invasive plants and animals. Cook with ‘em, carve, weave, or sculpt them, mash a weed into paper, or even turn your project into a contest! Tell people why and what you are doing. Just be extra careful you’re not accidentally making the problem worse.

For more information on the sculpture at the Hui visit their website and check out the day-by-day building of the sculpture entitled On The Wild Side at their blog.

Dining on invasive species? Check out invasivore.org for recipes.

By Lissa Fox. Originally published in the Maui News, September 11, 2011 as part of the Kia‘i Moku Column.Check out all of the MISC articles in the Kia`i Moku series at: www.hear.org/misc/mauinews/

Biocontrol precision is weapon against invaders

Posted on September 6, 2011 by Lissa Strohecker Leave a Comment

Lloyd Loope

Last month I began to present my point of view as a conservation scientist relevant to current “controversy” over biocontrol of strawberry guava (Psidium cattleianum), Hawaii’s most damaging invasive plant species. I touched upon the generally accepted theory that many highly invasive plant species, held in check by insects and pathogens in their home range, become unbeatable competitors in their introduced range where those insects and pathogens are lacking. Strawberry guava exemplifies the “enemy release” theory; it is attacked by virtually no insects or pathogens wherever it invades on many tropical islands of the world.

Though I’ve always supported the concept of biocontrol as a potential tool for Hawaii’s most widespread and most invasive plants, I recall a personal pessimism in the 1980s that biocontrol success might be unlikely in this case because guava is in the myrtle family (same as Hawaii’s ‘ōhi‘a) and commercially important common guava (Psidium guajava) is such a closely related sister species.

A strawberry guava tree in Curtiba, Brazil is heavily infested with the galls of the scale insect Tectococcus ovatus. This is the typical effect of the insect on the plant. It was a result of a naturally developing infestation of the insect in the plant’s native range. U.S. Forest Service photo

But state and federal natural resource agencies in Hawaii have managed to cobble together a remarkably stellar program to address the rampant invader. In 1991, the National Park Service and the University of Hawaii began to collaborate with the Federal University in Curitiba, Brazil, to identify and evaluate potential biocontrol agents. After 10 years of field observations and other testing working within the native ranges of both strawberry guava and common guava in Brazil researchers found 133 insect species in 12 orders and 80 families that fed on strawberry guava. Thirty-one of those species fed only on plants in the myrtle family, and five species fed only on strawberry guava and not at all on common guava. The collaborative program zeroed in on Tectococcus ovatus, a scale insect that feeds on the new growth, creating galls on the leaves of strawberry guava. This scale insect was the choice for more intensive work based on the relative severity of damage inflicted on P. cattleianum and the ease of handling it.

Tectococcus ovatus was brought to containment facilities in Hawaii and Florida, where strawberry guava is also invasive, for intensive experimental testing to ensure its safety as a biocontrol agent. Many related native and non-native plants were tested as potential host species. Strawberry guava is the only plant in Hawaii that this insect is able to feed on. Dr. Tracy Johnson of the U.S. Forest Service in Hilo has brought the program close to fruition over the past eight years.

The insect reduces the plant’s energy and nutrients available for growth and reproduction. Heavy infestation is expected to accelerate leaf drop and reduce fruit and seed production. Some fruit will be produced and the plants are not expected to die. If the scale insect thrives, it will level the playing field between the invasive strawberry guava and native forest species.

What other species in the ecosystem will be affected by the expected decline in strawberry guava fruit production? Feral pigs and a few species of non-native birds will lose a seasonal carbohydrate diet supplement during the September-December fruiting season. These same species are fully able to thrive in areas where there is no strawberry guava.

Hawaii is confronted by intractable invasive species problems, but by integrating prevention, early detection, rapid response, and biocontrol we can begin to address these problems. Biocontrol is an essential part of invasive plant management strategies in New Zealand, Australia, and the U.S. mainland. Florida has become a leader in biocontrol in the past decade. For example I recently learned that thanks to two insects introduced about 10 years ago as classical biocontrol agents, the notorious Melaleuca, or paper bark tree is no longer invasive in the Florida Everglades! My scientific opinion is that biodiversity conservation in Hawaii will not succeed without the enlightened use of biological control.

Lloyd Loope is a research scientist with the U.S. Geological Survey stationed at the Haleakala Field Station. He holds a doctorate in botany from Duke University and is an active Maui Invasive Species Committee member.

Originally published in the Maui News, October 12, 2008 as part of the Kia‘i Moku Column.
You can see all the articles in the Kia‘i Moku series http://www.hear.org/misc/mauinews/

Strawberry guava sows seeds of infestation

Posted on August 30, 2011 by Lissa Strohecker 1 Comment

The fruit if strawberry guava are eaten by birds that spread the tiny seeds. Photo by Forest & Kim Starr

When I came to Maui 28 years ago to work as a research biologist at Haleakala National Park, one of the first things I learned was that strawberry guava (Psidium cattleianum) was regarded as the most damaging invasive plant in the state. A year later, I was in Kipahulu Valley and experiencing the amazing guava fruiting and dispersal season. The 10-30 foot tall trees left a sea of red fruits that still lurks in my memory. There were dense, extensive guava thickets of tens of acres at the 2,000-3,000 ft elevation level in the valley. One could scarcely walk through; there were literally no other species present. These devastated areas contrasted sharply with the rich biodiversity in nearby areas with intact native understory vegetation.

A dense infestation of strawberry guava, or waiawī , in the Makawao Forest Reserve. Photo by Forest and Kim Starr

At that time, C.H. Diong, a graduate student at UH-Manoa, was putting the final touches on his Ph.D. thesis. Diong showed that feral pigs had been the primary agent spreading strawberry guava from the mouth of the valley upslope for the past 30 years or so. The pigs fed almost exclusively on guava fruits during September-December. The seeds pass through the pigs’ digestive tracts with no loss of viability. Fruiting was occurring only below about 3000 ft elevation but the seeds could germinate and grow much higher, up to over 4000 ft in the valley.

Years later, my colleague Dr. Art Medeiros did his Ph.D. thesis on the biology of weeds in Kipahulu Valley and East Maui. Art estimated that strawberry guava already occupies over one-third, or 8900 acres, of its potential area of dominance on East Maui. Although feral pigs had been eliminated and fenced from Kipahulu Valley since the late1980s, Art found that non-native birds, primarily the red-billed leiothrix, were also spreading strawberry guava seeds. Strawberry guava plants were still not fruiting above 3000 ft, but seeds planted experimentally (or those carried by birds or pigs) germinate at much higher elevations. Strawberry guava now reaches as high as 5300 ft on East Maui in at least one location.

Strawberry guava has infested the outlined area on Mahana Ridge, just to the east of Honokahua Valley in West Maui. Photo by Randy Bartlett.

Newly established guava plants can spread vegetatively by root sprouts. On all the major islands, nearly monotypic guava stands infest thousands of acres of moderately wet (mesic) and very wet forest. Strawberry guava is considered a serious threat to native forests due to its ability to invade relatively undisturbed areas and form thickets with dense mats of feeder roots. Dense invasion allows only about 5% of sunlight into the understory. The guava at first invades under native canopy, then expands and closes the mid-level canopy position, preventing the establishment and regeneration of all other species, including the dominant native forest plants – ohia and koa. Although it continues to expand into relatively pristine native forest areas, strawberry guava is now so widespread in Hawaii that its future impacts are expected to consist mainly of filling in areas creating monotypic stands.

As one of the founding members of the Maui Invasive Species Committee, I never considered taking on strawberry guava as a MISC target given its wide distribution and abundance and MISC’s limited resources. We always hoped for the eventual possibility of identifying an effective biological control.

A generally accepted theory of biological invasions is that certain plant species, held in check by insects and pathogens in their home range, become invasive in their introduced range where those insects and pathogens are lacking. Strawberry guava seems to fit the “enemy release” theory perfectly. In Hawaii, it is currently attacked by no insects or pathogens.

The first biocontrol researcher for this species, Dr. Charles Hodges, traveled to coastal Brazil in 1988 and reported that strawberry guava occurs not in dense stands but as scattered trees and rarely in small clumps. He noted that “few trees were observed that had not been attacked by at least one kind of insect,” and “often four or five types of insect damage could be identified on the same tree.” After 20 years of research to determine the best potential biocontrol agent, the scale insect Tectococcus ovatus has jumped through all the hoops but one – public opinion.

Lloyd Loope is a research scientist with the U.S. Geological Survey stationed at the Haleakala Field Station. He holds a doctorate in botany from Duke University and is an active member of the Maui Invasive Species Committee.

Originally published in the Maui News, September 14, 2008 as part of the Kia‘i Moku Column.
You can find all the articles in the Kia‘i Moku series http://www.hear.org/misc/mauinews/

Measuring the impacts of invasive plants in Hawai’i’s watersheds

Invasive plants=Less water

Positive spin on invasive species by artists, chefs

Biocontrol precision is weapon against invaders

Strawberry guava sows seeds of infestation

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