Archive for strawberry guava

Hawaii native forests vs the Strawberry Guava

Posted in environment with tags on July 5, 2010 by marksun

The strawberry guava Psidium cattleianum  has long been considered an invasive specie in Hawaii, and over the years has successfully established itself in many areas.  It thrives in low to mid elevations below <1500 meter, and  receiving >1000mm of rain per year.  This habitat makes the SG primarily a plant found in windward exposures in and around the fringes of the main trade-wind rainfall cores wherein  the bulk of rainfall occurs in  the Hawaiian islands.   Originally imported from Brazil 150 years ago, the strawberry guava in many areas is agressive and invasive, with the characteristic of  displacing Hawaiian species and other desireable vegetation, contributing to fruit fly infestation from fallen fruit.  The SG has been found to have a possibly drying effect on the micro-climate and hydrology of its habitat by virtue of the efficiency of it’s vascular system to evaporate water from it’s root system through it’s leaves in a process called evapotranspiration.  Recently an insect has been discovered that is believed to target the Strawberry guava exclusively and seems to be on the verge of release into Hawaii stands of strawberry guava in an attempt by the State of Hawaii to introduce a predator  specific to the strawberry guava and thereby aid in its control and eventual eradication.  The State of Hawaii is now preparing a campaign to introduce this insect T.Ovatus.   Preliminary to going forth with this plan, a draft  environmental impact statement (EIS)has been prepared and released for comment.

Maps (Fig 1A) in the EIS show estimates of the potential range of SG based on rainfall and elevation.  The maps make the case that 495000 acres or 38%  of forests in Hawaii State have potential for dense infestation  with SG and that another 680000 acres or 52% of the native forest is threatened by partial invasion of SG.   It is important to note that  the maps reflect the potential extent of infestation and not the actual extent.   It is also important to note that this zone is not exclusively native forest.  Some aerial photographs in the EIS  show many acres of  infestation extent in Wao Kele O Puna on the volcanic rift zone in the Puna district.

Here is the website for HEAR and the draft EIS for introduction of a bio-control for the strawberry guava

The draft EIS is comprehensive and much care and input went into it to represent the many facits of the problem, including cultural impacts,  presented by the presence of the strawberry guava.  The main impact of the insect will be to reduce the vigor of the trees and in fruit production.  The reduced vigor of the plant is expected to slow its spread and prevent new infestations from forming dense thicket growth found in Hawaii.  The growth pattern found in Hawaii is reported to not be found in its native Brazil due to the presence of an insect T. Ovatus,  that reduces the growth and reproduction of SG.

The EIS makes the case that forest managers in Hawaii consider the strawberry guava to be the greatest threat to native forest species within it’s habitat zone because of its ability to completely take over areas and exclude growth of other species where conditions are optimal.  There seems to be a concensus that chemical and mechanical controls are too expensive to halt the continued spread of SG.  The introduction of T. Ovatus is recommended.  Introduction is predicted to reduce the competitive vigor of SG  to the point where native species will at least be able to coexist with it and possibly out-compete it.  There is some concensus that at present, no other specie, indigenous or otherwise can now outcompete SG.

Giambellucca  find that the strawberry guava in effect uses more water in  evapotrasnpiration than does native ohia forest.  The authors cite the smaller trunk diameter of the SG and higher xylem content of SG stands as phisiological reasons for its apparent efficiency at moving water from the ground into the atmosphere through evapotranspiration.   Their measurements indicate SG transpires 27-50% more water than does the ohia.  The EIS concludes from this that SG has an overall negative hydrological impact.

The EIS makes a number of additional cases for the introduction of  T.Ovatus

  1. will not kill or unduly mutilate SG, but suppress it’s biological aggression to levels found in its native Brazil.
  2. galls affect only the leaves, not the wood or fruit
  3. galls reduce fruit production but not the fruit itself
  4. does not attack any known plant species in Hawaii
  5. will be effective over a period of years in slowing the spread of SG
  6. reduces control costs especially employing chemical and mechanical means

Weakness of the EIS

Maps show potential and not the actual extent of full canopy infestation.  There is no estimate of the percent of the potential zone actually infested. No doubt that there are large tracts of forest in this state, but where statistics are cited, it is important not to mislead, or the report loses credibitliy.  For example, on Oahu at Aiea Loop Trail, I personally know of  two trailside areas on the west edge of the Loop running perhaps 100 meters through 100% SG stands which are full infestations of SG.    However, these areas are surrounded by stands of mixed residual native vegetation and other non-native vegetation, forested tree plantings of non-native species (Eucalyptus, paper bark, norfolk island pines for example), and completely invaded areas of non SG vegetation, primarily grasses in  many acres of clear cut forest to make way for power-lines.  These areas  far exceed the area of SG infestation.   Given the degree of human disturbance of Aiea ridge, these areas dominated by SG have always seemed to me of limited, though not insignificant, importance ecologically for Aiea Loop trail.

However there are numerous quotes by land managers  which point to a significant control problem, with one example of how SG hinders the establishment of native Koa for timber development.

A weakness of the EIS is that its purpose is to actively advocate for the introduction of T.Ovatus as a control and thereby carries a taint of not being entirely objective.  The study neglects to point out that the introduction of the SG is merely a side effect of the arrival in the Hawaiian Islands of the most invasive specie of all.  I suppose this is to be expected from an EIS.


The main message of the EIS is that the strawberry guava is gradually spreading and growing out of control in ecologically significant areas of the Hawaiian Islands, already arguably unwilling and unhappy in its role as a  natural laboratory for the study of invasive specie impact on isolated island ecosystems.

The EIS study of T. Ovatus seems to have been professionally carried out as represented by the EIS.  I have not read the study of T. Ovatus, but professional reputations are staked on it’s accuracy, and I have little reason to doubt its scholarship.  As for the hydrological arguments, I am familiar with  excellent scientific work by Tom Giabellucca, which adds credibility to the EIS.  I personally am not convinced that the relatively high evapotranspiration (ET) rates by the SG is significant from a water supply perspective as there are many many factors in the delivery of water in Hawaii.  For example, is ET by SG more important than diversion of streams and other  surface and groundwater resources in the equation of the total commonwealth?   That is a very complex issue, resolved as it were by historical and political forces.  While interesting, I personally do not believe it to be a sufficient reason of itself to control the SG in and of itself.  More compelling, as represented in the EIS,  is the argument that the introduction of T.Ovatus has few relatively few downsides.  It will not mutilate the SG plants or introduce a devastating pestilential disease into what is now part of the Hawaiian ecosystem.  The insect co-evolved with the SG and does not have a biological or evolutionary mandate to kill its host.  The insect creates a structure called a gall on the leaves of the guava plant.  Even these can evidently be  mitigated relatively easily by (vegetable?) oils by those  who wish to continue to cultivate SB for fruit or as an ornamental plant or even a crop for fiber and wood production.  The most convincing argument for introduction of T. Ovatus is that it is a co-evolutionary species with SG and that it has had the ecological effect of mitigating the aggressive biological potential of SG in its native habitat.  Thus one may argue that introducing T.Ovatus  balances the original introduction of SG in a manner that is ecologically and socially responsible.  The phenomenon of aggressive invasive species is becoming well studied and the EIS contributes  to this knowledge base.