Linking dominant Hawaiian tree species to understory development in recovering pastures via impacts on soils and litter

Abstract

Large areas of tropical forest have been cleared and planted with exotic grass species for use as cattle pasture. These often remain persistent grasslands after grazer removal, which is problematic for restoring native forest communities. It is often hoped that remnant and/or planted trees can jump-start forest succession; however, there is little mechanistic information on how different canopy species affect community trajectories. To investigate this, I surveyed understory communities, exotic grass biomass, standing litter pools, and soil properties under two dominant canopy trees-Metrosideros polymorpha ('ōhi'a) and Acacia koa (koa)-in recovering Hawaiian forests. I then used structural equation models (SEMs) to elucidate direct and indirect effects of trees on native understory. Native understory communities developed under 'ōhi'a, which had larger standing litter pools, lower soil nitrogen, and lower exotic grass biomass than koa. This pattern was variable, potentially due to historical site differences and/or distance to intact forest. Koa, in contrast, showed little understory development. Instead, data suggest that increased soil nitrogen under koa leads to high grass biomass that stalls native recruitment. SEMs suggested that indirect effects of trees via litter and soils were as or more important than direct effects for determining native cover. It is suggested that diverse plantings which incorporate species that have high carbon to nitrogen ratios may help ameliorate the negative indirect effects of koa on natural understory regeneration.

Exotic weeds and fluctuating microclimate can constrain native plant regeneration in urban forest restoration

“…, Overdyck and Clarkson , George and Bazzaz , McAlpine et al. , Yelenik ). Many exotic weeds are nutrient‐demanding and will compete aggressively for belowground resources in forests (Gilliam , Peltzer et al.…”

“…Many exotic weeds are nutrient‐demanding and will compete aggressively for belowground resources in forests (Gilliam , Peltzer et al. , Yelenik ). Some also have specific symbiotic mycorrhizae that can cause advantage in uptake capability (Cornelissen et al.…”

“…, Overdyck and Clarkson , McAlpine et al. , Yelenik ), while basal area (Burns and Dawson , Hirata et al. ) and humidity fluctuation (McCune and Antos , Callaway et al.…”

“…We predicted that canopy openness would have direct effects on native tree regeneration (Gerhardt 1996, Olesen and Madsen 2008, Bertacchi et al 2016, soil temperature fluctuation (Chen et al 1993), humidity fluctuation (McCune and Antos 1982), and herbaceous weed cover (McAlpine et al 2015). Our model hypothesized that soil temperature fluctuation (Dodd et al 2011) and herbaceous weed cover would directly constrain native tree regeneration (Davis et al 2005, Kuijper et al 2010, Dodd et al 2011, Overdyck and Clarkson 2012, McAlpine et al 2015, Yelenik 2016, while basal area (Burns andDawson 2005, Hirata et al 2009) and humidity fluctuation (McCune and Antos 1982, Callaway et al 2002, Winkler et al 2005, Zotz and Bader 2009 would directly constrain epiphyte colonization.…”

Rapid colonization of a Hawaiian restoration forest by a diverse avian community

“…Nectivores and fruigivores had a slower response to the reforestation efforts than more insectivorous birds. The koa reforestation areas are largely dominated by an understory of exotic grass and are in many areas lacking understory shrubs that provide nectar and fruit (Yelenik ). Nectivorous ′I′iwi and ′Apapane were detected in young restoration forest, but this may have been visits to relic flowering ′ohi′a spread across the landscape, and not directly in response to the new koa trees.…”