Crossing the threshold: Invasive grasses inhibit forest restoration on Hawaiian islands

Area of Science:

• Ecology
• Restoration Ecology
• Invasive Species Management

Background:

• Forest removal for grazing can create resilient invasive grass systems, hindering native forest restoration.
• Native woody plants recruit under remnant Metrosideros polymorpha ('ōhi'a) but not in Acacia koa (koa) plantations.
• Understanding factors influencing resilience to invasive grasses is crucial for effective forest restoration.

Purpose of the Study:

• To investigate the influence of grass cover and seed rain thresholds on forest recovery in Hawai'i.
• To determine if native woody understory levels affect recruitment rates under koa and 'ōhi'a trees.
• To identify critical thresholds for invasive grass removal to promote native woody plant establishment.

Main Methods:

• Monitored seed rain and native woody basal area (BA) under 40 koa and 'ōhi'a trees.
• Experimentally manipulated grass removal levels and seed rain density in plots under koa and 'ōhi'a.
• Analyzed survey data of naturally occurring seedlings to assess grass cover thresholds.

Main Results:

• Positive relationship between seed rain and native woody BA; negative relationship between BA and grass cover.
• Seedling emergence was minimal without grass removal, even with high seed densities.
• Native woody recruitment increased 2-3 times when at least 50% of grass cover was removed.

Conclusions:

• Moderate grass removal (~50%) can significantly increase native woody recruitment, suggesting a nonlinear threshold.
• Complete eradication of invasive grasses may not be necessary for successful restoration.
• Identifying critical thresholds is vital for prioritizing resources and improving forest restoration outcomes.