What we
are up
against
Three threats, deeply entangled. Invasive grass fuels fire; fire clears ground for more invasive grass; a hotter, drier climate accelerates both and leaves fragmented remnants with nowhere to retreat. They are set out separately here, but they act as one.
Fountain grass invades open ground between native trees.
The grass cures dry and carries fire fast and hot through forest that never evolved with it.
Native trees are killed; the grass survives and resprouts.
More open ground, more grass, and the next fire comes sooner.
The greatest invasive plant threat to dry forests in West Hawaiʻi is fountain grass (Pennisetum setaceum), introduced at a North Kona ranch as an ornamental close to a hundred years ago. It now covers over two hundred thousand acres across the Kona and Kohala region of Hawaiʻi Island and continues to spread at an alarming rate.
It is both highly fire-prone and fire-adapted — it survives wildfire and spreads further because of it. So it is not only a major contributor to the spread of fires through native forest, it also out-competes natives and prevents their regrowth afterwards. This grass-fire cycle is what has produced the vast fountain grass savannas of West Hawaiʻi.
In 2014 a second major threat to all Hawaiian forests was identified: a fungus, Ceratocystis, causing Rapid ʻŌhiʻa Death. ʻŌhiʻa is the foundational and most abundant tree of Hawaiian native forests — roughly 80 percent — so the loss of this one species is of the utmost concern statewide. The fungus is a wound pathogen: it enters through wounds in the tree, colonises the sapwood, and effectively robs the tree of the water and nutrients it needs to survive.
The main tools protecting dry forest from fire are fuel breaks, cleared of fire-prone grasses and shrubs and then kept clear. Some areas use managed grazing by cattle or goats to hold them. Both tactics are in use at Puʻu Waʻawaʻa Forest Reserve.
Shorter-term climate variation matters too. The El Niño-Southern Oscillation can raise wildland fire risk across the islands, and because the phase and strength of an event can usually be identified several months ahead, managers can adjust restoration schedules and fire precautions in advance.
Greenstrips — bands of fire-resistant vegetation, already used on the mainland — are being investigated for Hawaiʻi as an additional line of defence.
That finding comes from Fortini et al. 2013, a landscape-based assessment of climate change vulnerability for all native Hawaiian plants, where vulnerability is defined as the inability of a species to exhibit the responses necessary for persistence under climate change.
The factors driving that assessment are the same ones that produced the present state of the dry forest: extremely low native cover after the loss of over 90 percent of the forest; high fragmentation of what remains, leaving no pathways for species to migrate; the invasibility of dry forest by highly detrimental non-native species; and a lack of ungulate exclusion and conservation areas.
Depending on location, Hawaiʻi faces increasing winds, much less rain and increasing drought on the leeward sides, or more rain and flooding to windward. Temperatures rise everywhere, including the ocean. Sea level rise, coastal flooding, more frequent and intense hurricanes, extreme heat and rainfall, more wildfire, erosion, desertification and disease all follow.
There is a counterweight, and it is the forest itself. Forests absorb carbon dioxide and store carbon above and below ground. Tropical dry forest sequestration is far from trivial: above-ground biomass is about a third of that in tropical wet and moist forests, but below-ground biomass is comparable across all three — and that is with roughly 10 percent of the dry forest ecosystem left on the planet. The potential in restoration is enormous.
We can rebuild
these forests
Forests capture and direct rainfall to replenish aquifers, mitigate erosion by reducing runoff and wind, and provide shade and cooling against rising temperatures. They protect native species by being the habitat those species need. Restoration is slow and labour-intensive, and it works.