Rocky Volcanic Ash Savanna Kiawe/buffelgrass (Prosopis pallida/Pennisetum ciliare)

Mechanism

State 1 Reference transitions to State 2 Kiawe Invaded in the absence of disturbance such as fire or brush management.

Kiawe seedlings are usually found associated with animal droppings and are intolerant of shade. The continuous Buffelgrass cover typical of State 1 (Reference), Phase 1.1 can limit kiawe establishment. The dominance of perennial short grasses and increasing bare ground typified in State 1, Phase 1.2 and Phase 1.3 can provide kiawe seedlings an advantage to establish. Seedling survival depends primarily on sufficient rainfall during 4 to 6 weeks after germination. As recruitment and dispersal progress, kiawe begins to out compete perennial grasses for water, soil nutrients, and light. Access to water is the greatest limiting factor to plant survival in this site. As a phreatophyte kiawe thrives where it has access to both soil moisture and ground water. It develops an overstory canopy cover that reduces solar radiation penetration to the soil surface. Kiawe is also implicated as a potential cause of changes in local hydrology and mineral cycling due to it’s efficient water harvesting capability and nitrogen fixation (Dudley, 2014).

Constraints to recovery

Conversion from State 2 back to any community in State 1 will require extensive labor, time, and money.

Controlling Kiawe by mechanical means can cause soil disturbance impacts. Kiawe and weedy species from the seed bank may flourish after Kiawe trees are removed. This is exacerbated in areas where the soils are disturbed.

Relevant conservation practices

Mechanism

State 1 Reference transitions and is maintained to State 4 Toxic/Unpalatable Forb primarily by constant heavy browsing and grazing by feral goats and on Maui, deer, such that nearly all perennial grass and forb (forages) are removed.

Large patches of mortality within the grass and forb functional groups are becoming common on the landscape. In areas where ground cover has persisted, the cover and vigor of perennial grass and forbs have become greatly reduced. Litter accumulation is greatly reduced to non-existent, and bare ground patches are large and frequently connected. This results in the creation of large barren areas suitable for toxic forbs to colonize. Access to water is the greatest limiting factor to plant survival in this site. Water infiltration is greatly reduced in areas of bare ground and/or reduced perennial grass cover. Solar radiation penetration to the soil surface increases significantly in areas of bare ground and/or reduced perennial grass cover. These changes create a competitive advantage which favors the establishment and further spread of toxic/unpalatable forbs as well as invasive shrub species.

Constraints to recovery

Many of these forbs behave more like annuals and will die during prolonged dry conditions. As such, these sites are very prone to serious soil erosion by water and wind as indicated by: water flow paths, pedestalled plants with roots exposed, and/or in extreme situations, the mineral horizons become exposed.

Relevant conservation practices

Mechanism

State 3 Native Savanna transitions to State 1 Reference when it is cleared by fire, long-term ungulate disturbance, or mechanical means. While wildfire will clear most native vegetation, prescribed burning is typically not done in Hawaii due to the level of risk to enjoined or embedded relict native ecosystems and development. Desired perennial grass and forbs (typically naturalized forage species) are then re-established.

These disturbances initiate change by affecting the composition, structure, and pattern of vegetation on the landscape (Brown et al 2000 re fire). They affect plants directly, by injury and mortality, and indirectly, by changing resource availability including light, nutrients, pollinators, and mutually beneficial microorganisms such as mycorrhizae, translating into vegetation changes (clearing) at the plant, population, and community level.

Fire Effects: Fires of all intensities, timing, and causes are harmful to most native ecosystems in Hawai`i (Smith and Tunison 1992). Alien species, particularly grasses such as buffelgrass, recover rapidly and fill in the spaces left by native plants, which recover more
slowly, if at all. In Hawaii, fires establish a destructive cycle by increasing fuel loadings of flammable alien grasses, which in turn increase fire frequency and fire size. Subsequent burns further inhibit the reestablishment of native trees species. Some native species easily killed by fire include wiliwili, lama, and akia (Smith and Tunison 1992). Native species tolerant (aerial portions of all native plant species are readily killed by fire, but some resprout or recover from seed) to a very low frequency of fire, recover to some degree after burns, but typically to populations far less than those found in pre-burn conditions because they typically must compete with fire-stimulated alien grasses. Naio (Myoporum sandwicense) resprouts after fire as long as the fire is not too intense. Regrowth is slow and the original canopy cover takes over five years to regenerate. The seeds of `a`ali`i (Dodonaea viscosa) germinate readily after fire, but it is not known if they are fire-stimulated. `A`ali`i may reach higher densities after fire than before. Most native bunchgrasses recover rapidly after burning. Pili grass resprouts readily with a significant increase in cover following low-intensity fire.

Effects of Continued, Long-Term Systematic Herbivory
With continued, long-term systematic herbivory and browsing by goats and, on Maui, deer, nearly all native grass and forbs (forages) and regenerating shrub and tree seedlings are removed allowing buffelgrass to resume dominance.

Constraints to recovery

Prescribed burning is not, on the whole, an effective management tool in Hawai`i, although it may have potential limited management and research value (Smith and Tunison 1992). Moreover, the intensification of fire-prone alien grasses following fire assures the increased incidence of fire, thereby establishing a self-perpetuating alien grass/fire cycle. The effects of fire argue strongly that all agencies responsible for the management of fire in natural areas in Hawai`i should aggressively suppress all fires, whether caused by lava flows, lightning, or humans.

Relevant conservation practices