Building Resilience to Invasive Species in Ohia Forests
ʻŌhiʻa (Metrosideros polymorpha) is the dominant tree in Hawaiian forests, but Rapid ʻŌhiʻa Death (ROD), a newly emerging disease that stands to cause high mortality of this ecologically important tree, may lead to invasions by exotic plants as gaps open in the forest. To better understand if planting of ʻōhiʻa seedlings can be used as a restoration tool, we investigated the survival of ʻōhiʻa seedlings in areas with ROD-affected trees. We also asked how ROD-induced mortality of seedlings may compare to other stressors such as exotic plant competition and damage from feral ungulates.
Background: ʻŌhiʻa (Metrosideros polymorpha) is the dominant tree in Hawaiian forests, the first to colonize new lava flows, and often forms the bulk of biomass. It is the basic building block of habitat for threatened and endangered plants and animals. In addition, it is vital to ecosystem services such as water capture and soil conservation. Rapid ʻŌhiʻa Death (ROD) is a newly emerging disease caused by the fungal pathogens Ceratocystis lukuohia and C. huliohia that has killed at least one million trees and spread to other islands in the state. Many habitats being affected by ROD have invasive plant species with high growth rates and high seed dispersal capabilities in the understory. As slow-growing ʻōhiʻa trees die, it is highly likely that invasive species such as strawberry guava (Psidium cattleyanum) or faya tree (Morella faya) will take their place in the canopy. While planting new ʻōhiʻa could help to stall complete state-shift of native forests to highly invaded ecosystems, there are no data on whether seedling ʻōhiʻa succumb to ROD infected soils.
Overview: The study was designed to test whether actively planting ʻōhiʻa seedlings is a viable restoration strategy in areas with high ROD mortality. ʻŌhiʻa seedlings were planted directly underneath adult ʻōhiʻa that had tested positive for Ceratocystis huliohia or lukuohia, the fungal pathogens that cause ROD. Seedlings were monitored for survival in plots that were fenced to keep feral ungulates (e.g., goats and pigs) out and weeded to keep invasive plants from growing; other seedlings were planted in areas with no fencing or weeding. Seedlings that died were brought back to the lab for testing within 1 month of dying. In the lab all the dead seedlings and soil from around the ROD-affected trees were tested for the DNA of Ceratocystis and tested for evidence of viable fungal spores.
Study Area: The preliminary study was conducted in the Keaukaha Military Reservation (KMR) in Hilo, Hawai‘i, an area with lowland forests with a mix of native and exotic species. The canopy is dominated by native ʻōhiʻa trees but the sub-canopy is dominated by exotic species. The site has high rates of ROD infection and mortality (Fortini et al. 2019). We are establishing similar experiments in different locations to incorporate variation in climate and understory invasion, including Hawaiʻi Volcanoes National Park, Waiakea Forest Reserve, and Kalopa State Park.
Highlights and Key Findings:
- ʻŌhiʻa planted into sites with active ROD infections can survive for at least 1 year; this indicates that seedlings can survive the disease, even when it is found in the local area, and in the soil.
- Although some seedlings planted into sites with ROD died, none tested positive for Ceratocystis, the fungal pathogen that causes ROD.
- ʻŌhiʻa in plots with weeds were 6 times as likely to die as ʻōhiʻa in weeded plots, and ʻōhiʻa in unfenced plots were 3 times as likely to die as ʻōhiʻa in fenced plots.
- ʻŌhiʻa can survive in ROD-affected forests if protected from wild pigs and goats and invasive weeds.
- Controlling invasive plants and animals were more important for survival in the tree’s first year than ROD.
- Keeping forests free of exotic plants and animals, actions already well understood by land managers, remains a good way to promote ʻōhiʻa survival.
- These early results for survival after one year should give managers and landowners hope that active planting can successfully help maintain native ʻōhiʻa forests.
- Results also indicate that naturally occuring ʻōhiʻa seedlings may survive ROD, even in forests with active infection.
Results from the from the first year have been published in the journal Restoration Ecology.
ʻŌhiʻa (Metrosideros polymorpha) is the dominant tree in Hawaiian forests, but Rapid ʻŌhiʻa Death (ROD), a newly emerging disease that stands to cause high mortality of this ecologically important tree, may lead to invasions by exotic plants as gaps open in the forest. To better understand if planting of ʻōhiʻa seedlings can be used as a restoration tool, we investigated the survival of ʻōhiʻa seedlings in areas with ROD-affected trees. We also asked how ROD-induced mortality of seedlings may compare to other stressors such as exotic plant competition and damage from feral ungulates.
Background: ʻŌhiʻa (Metrosideros polymorpha) is the dominant tree in Hawaiian forests, the first to colonize new lava flows, and often forms the bulk of biomass. It is the basic building block of habitat for threatened and endangered plants and animals. In addition, it is vital to ecosystem services such as water capture and soil conservation. Rapid ʻŌhiʻa Death (ROD) is a newly emerging disease caused by the fungal pathogens Ceratocystis lukuohia and C. huliohia that has killed at least one million trees and spread to other islands in the state. Many habitats being affected by ROD have invasive plant species with high growth rates and high seed dispersal capabilities in the understory. As slow-growing ʻōhiʻa trees die, it is highly likely that invasive species such as strawberry guava (Psidium cattleyanum) or faya tree (Morella faya) will take their place in the canopy. While planting new ʻōhiʻa could help to stall complete state-shift of native forests to highly invaded ecosystems, there are no data on whether seedling ʻōhiʻa succumb to ROD infected soils.
Overview: The study was designed to test whether actively planting ʻōhiʻa seedlings is a viable restoration strategy in areas with high ROD mortality. ʻŌhiʻa seedlings were planted directly underneath adult ʻōhiʻa that had tested positive for Ceratocystis huliohia or lukuohia, the fungal pathogens that cause ROD. Seedlings were monitored for survival in plots that were fenced to keep feral ungulates (e.g., goats and pigs) out and weeded to keep invasive plants from growing; other seedlings were planted in areas with no fencing or weeding. Seedlings that died were brought back to the lab for testing within 1 month of dying. In the lab all the dead seedlings and soil from around the ROD-affected trees were tested for the DNA of Ceratocystis and tested for evidence of viable fungal spores.
Study Area: The preliminary study was conducted in the Keaukaha Military Reservation (KMR) in Hilo, Hawai‘i, an area with lowland forests with a mix of native and exotic species. The canopy is dominated by native ʻōhiʻa trees but the sub-canopy is dominated by exotic species. The site has high rates of ROD infection and mortality (Fortini et al. 2019). We are establishing similar experiments in different locations to incorporate variation in climate and understory invasion, including Hawaiʻi Volcanoes National Park, Waiakea Forest Reserve, and Kalopa State Park.
Highlights and Key Findings:
- ʻŌhiʻa planted into sites with active ROD infections can survive for at least 1 year; this indicates that seedlings can survive the disease, even when it is found in the local area, and in the soil.
- Although some seedlings planted into sites with ROD died, none tested positive for Ceratocystis, the fungal pathogen that causes ROD.
- ʻŌhiʻa in plots with weeds were 6 times as likely to die as ʻōhiʻa in weeded plots, and ʻōhiʻa in unfenced plots were 3 times as likely to die as ʻōhiʻa in fenced plots.
- ʻŌhiʻa can survive in ROD-affected forests if protected from wild pigs and goats and invasive weeds.
- Controlling invasive plants and animals were more important for survival in the tree’s first year than ROD.
- Keeping forests free of exotic plants and animals, actions already well understood by land managers, remains a good way to promote ʻōhiʻa survival.
- These early results for survival after one year should give managers and landowners hope that active planting can successfully help maintain native ʻōhiʻa forests.
- Results also indicate that naturally occuring ʻōhiʻa seedlings may survive ROD, even in forests with active infection.
Results from the from the first year have been published in the journal Restoration Ecology.