Angelo, C.L. and Daehler, C.C. (2012). Upward expansion of fire-adapted grasses along a warming tropical elevation gradient. Ecography, 36, 551-559. http://dx.doi.org/10.1111/j.1600-0587.2012.07754.x

Summary

Wildfires in Hawaiʻi have increased in size and frequency over the last 100 years fueled largely by invasive grasses and shrubs, which cover 25% of Hawaiʻi’s land area. These fires have led to the expansion of flammable exotic grasslands, often at the loss of native forests, as many native Hawaiian trees are not fire-adapted. Most recent fires in Hawaiʻi have occurred at lower elevations, yet warming due to climate change could shift fire risk up in elevation where the majority of intact native ecosystems remain. Therefore, it is critical to understand how a warming climate affects the distribution of flammable invasive grasses and how this may impact wildfire severity and risk. The authors used vegetation surveys to compare native and non-native grass distributions along an elevational gradient in Hawaiʻi Volcanoes National Park between 1966-1967 and 2008 to determine whether C3 and C4 grass distributions shifted upward in response to climate and fire.

Results

They found that over this 40-year period, average temperatures at Park Headquarters increased by 0.7°C (~1.3°F), with no significant change in annual rainfall. Fires that occurred along the transect were larger during the later period (1969-2000) compared to the earlier period (1924-1966). Also, the C3 to C4 transition point (where % cover is equal) was at ~3940 ft. in 1966-1967 but moved up to ~4843 ft. in elevation in 2008 (a shift upslope of ~903 ft.). Most grass species had higher elevation maxima in 2008. However, a few species declined in elevation over the same time, potentially due to a release from grazing, biotic responses, or wildfire. Finally, significantly more fire-adapted grass species moved up in elevation, and by a greater amount (~1490 ft.) compared to the non-fire-adapted species (~896 ft.). The two fire-adapted grasses with the largest increases in elevation were Melinis minutiflora, molasses grass (introduced in 1913), and Melinis repens, Natal red top (introduced in 1895) with upslope shifts of 3773 ft. and 3921 ft., respectively.

Management Implications

• Grasses with the C4 photosynthetic pathway are better adapted to warmer environments compared to C3 plants, which dominate under cooler conditions. As such, their distributional patterns are significantly correlated with temperature, and warming may result in increased abundance, density, or cover of C4 grasses.
• Consider strengthening biosecurity protocols to prevent additional non-native, fire-adapted C4 plants from establishing as they may further increase risk of wildfire.
• Consider increased monitoring efforts for higher-elevation areas to detect invasive grasses that may be moving upslope with warming.
• Consider increasing fire pre-suppression and planning efforts for higher elevation areas that may see an increase in wildfire size and frequency due to warming and fire-adapted grasses moving upslope.

Take Home Points

• 100% of the fire-adapted grasses in the study shifted up in elevation at Hawaiʻi Volcanoes National Park over the last 40 years. These shifts are consistent with warming conditions (0.7°C, ~1.3°F increase in the summer, and 0.7-0.9°C (1.3-1.6°F) increase in the winter between survey periods).
• The largest increases in maximum elevation change over 40 years were non-native and fire- adapted C4 grasses including 2 species with the largest increases, Melinis minutiflora, molasses grass, and Melinis repens, Natal red top, which moved 3773 ft. and 3921 ft. up in elevation respectively.
• Increased fire frequency is linked to upward range expansion of fire-adapted grasses (i.e., grasses that promote and/or tolerate fire).