Research projects (collaborative research by USDA Forest Service, California State University at Fullerton, and State University of New York at Plattsburg)
The impact of invasive exotic grasses on water-use patterns in the native dry-forests of Hawai’i, as quantified by use of natural-abundance stable isotopes
Using stable isotopes, researchers are tracing rainfall events to determine the potential impact that fountain grass has on native tree and shrub water use. It is believed that fountain grass, with its extensive shallow fibrous root system, out competes native plants for smaller, but more frequent, convective light rainfall events. Native plants with shallow root systems may be competitively excluded by fountain grass, thereby altering the future species composition of native dry forests. In fact, many once common species, such as Kauila (Colubrina oppositifolia) and ‘Aiea (Nothocestrum breviflorum) are presently federally endangered species with less than 300 wild individuals of each left in the state. With new NSF funding (beginning Jan. 15th, 2002), and USDA-NRICGP support (beginning Jan. 2003) researchers will have the resources to analyze field samples for isotopic composition. In January 2002, scientists surveyed leaf carbon and nitrogen isotopes and stem water isotope composition of all of the major native understory and canopy tree species. Twice a year (since summer 2000) leaf, twig, soil, and grass are sampled for oxygen isotope signatures in grass-removed and grass-intact plots. Concurrently, researchers measure water potential and gas exchange in four marked trees per plot. Additionally, all trees in each plot have been measured for diameter at breast height (DBH) with increment measurements each year.
Competition for limiting resources between fountain grass and native Hawaiian dry forest shrubs
The major focus of this project is to elucidate competition between native shrubs and alien grass for limiting resources, and determine how this competition might alter dry forest productivity and other ecosystem processes. To address this objective, in May 2001, a 2-by-2 factorial experiment was established to test the effects of light and water competition on native plant seedling establishment. Sixteen 5m x 5m plots were divided into four treatments:
- grasses 50% clipped, to allow light penetration but maintain competition for water (+ light, – water);
- grasses removed, to release competition for water, and 80% shade added using shade cloth erected over plot (– light, + water);
- grasses removed, no shade added (+ light, + water); and
- grasses left intact (– light, – water).
Dry forest shrub seedlings were raised in a greenhouse in potting soil/cinder mix for six months, then transplanted to the plots in Oct. 2001 (30 per plot) and watered for 30 days to prevent transplant shock. Periodically, two seedlings per plot per species have been sacrificed to determine what water sources are being exploited (as per isotope methods above). In January 2002, survival, stem height, root length, and above and belowground biomass were measured for all remaining plants.
Resource partitioning among native Hawaiian dry forest trees
Researchers are currently studying the mechanisms related to the partitioning of limited water sources of seven co-existing dominant trees from the Ka‘upulehu dry forest project on the island of Hawai‘i. The species are Lama (Diospyros sandwichensis), Kauila (Colubrina oppositifolia), Iliahi (Santalum paniculatum), ‘Ala‘a (Pouteria sandwicensis), Alahe‘e (Psydrax oderata), Halapepe (Pleomele hawaiiensis), and ‘Ohi‘a (Metrosideros polymorpha). Little to nothing is known about growth, phenology, reproduction and function of these tree species, and information gained by this study will provide possible clues to the current and future species composition of Hawaiian dry forests. Preliminary results point to very unique and species-specific resource use strategies. In addition, results from this study will be useful for seed collection, rodent control and species-specific restoration plans.
Fountain grass control in Hawai‘i
The highly invasive, fire promoting species fountain grass (Pennisetum setaceum) is decidedly the most serious threat to native dry forest communities on the island of Hawai‘i. Containment of this species is critical for breaking the regional grass-fire cycle and preserving and potentially restoring dry forest communities. Recent studies have shown that relatively simple and inexpensive techniques may be employed to both control fountain grass and facilitate the establishment of key native dry forest species. Using weedwhackers and multiple follow-up applications of a grass specific herbicide (Fusilade®), researchers managed to reduce fountain grass cover by 90%. Although initial costs of these efforts were relatively high (>$5,000/acre), subsequent methodological refinements (for example, the herbicide glyphosate has been more cost effective than Fusilade) and on-site storage and ownership of equipment have reduced these costs to less than $800/acre. Researchers also found that these efforts are not only facilitating the natural establishment of formerly suppressed native understory species, but once controlled, fountain grass does not appear able to successfully re-invade the same areas it once dominated, at least not in the short term (c. 5 years). This result is surprising and at present researchers do not understand the causal mechanism behind it. It has been suggested (Daehler, unpublished data) and demonstrated (Sanford and Cordell, unpublished data) that fountain grass spread is potentially seed limited since few seeds germinated in seedbed studies using soil taken from areas where fountain grass is abundant. In conclusion, given the highly fragmented condition of native dry forests in Hawai‘i, even small–scale successful restoration projects will create urgently needed habitat for rare species introduction.
Dormancy, germination, and storage of Hawaiian dry forest seeds
The objective of this project is to gain understanding on aspects of Hawaiian dry forest seed ecology and seed storage. Little is known about the dormancy and germination requirements of most Hawaiian species. Quantitative and speculative data indicates that the seeds of most native Hawaiian species form transient or psuedo-persistent seed banks while alien species tend to be more persistent suggesting that aliens are favored to increase in abundance if the forest understory is disturbed. However, such information is non-existent in Hawaiian dry forests where it may be assumed that the various factors that control germination and dormancy such as timing to rainfall events may favor more long term and persistent seed banks. Of equal concern is the need to understand the factors involved in seed storage for many of these Hawaiian dry forest species. Seed storage is a critical component of native forest restoration, and can ensure the survival and perpetuation of all dry forest species. Unfortunately, fire-promoting perennial alien grasses have proven to be particularly devastating to native species in Hawaii and elsewhere. In general, fires were fairly rare and localized in Hawaiian ecosystems prior to settlement by humans roughly 1500 years ago. Consequently, few native Hawaiian plant species today appear able to survive fires or reestablish in recently burned areas. Fires have proven especially destructive to Hawaii’s lowland dry forests, which have been invaded and dominated by fire-promoting alien grasses throughout the archipelago. Development of a dry forest seed storage facility and understanding the relationship between fire and native and alien seeds are the first steps toward a successful restoration program if a devastating fire were to occur. Additionally, seed storage can provide an active “bank” for future large-scale restoration of degraded landscapes.