Seasonal variation in the potential herbage production and response to nitrogen by kikuyu grass (<i>Pennisetum clandestinum</i>)

2016

Restoration Ecology

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.

Section: Discussionmentioning

confidence: 99%

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

2016

Restoration Ecology

“…2004, Yelenik 2017). We expected that an available N reduction in turn would reduce kikuyu growth, which has been shown elsewhere to benefit from high soil N (Colman and O'Neill 1978). We chose to use ʻōhiʻa litter because it can be collected on site, has a higher C:N ratio than koa (Appendix : Table S2), can be an important substrate for woody plant germination and seedling growth (Rehm et al.…”

Section: Discussionmentioning

confidence: 96%

“…Our addition of ʻ ōhiʻa litter was based on a similar premise: a relatively high C:N litter of ʻ ōhiʻa compared to koa (i.e., C:N = 53.2 vs. 28.8) would stimulate microbial immobilization of soil N (Johnson and Cheng 2000), which is otherwise highly available under koa (Scowcroft et al 2004, Yelenik 2017. We expected that an available N reduction in turn would reduce kikuyu growth, which has been shown elsewhere to benefit from high soil N (Colman and O'Neill 1978). We chose to use ʻ ōhiʻa litter because it can be collected on site, has a higher C:N ratio than koa (Appendix S1: Table S2), can be an important substrate for woody plant germination and seedling growth (Rehm et al 2020), and does not require bringing in an external source of materials in this remote environment.…”

Section: Addition Of C To Reduce Available Nmentioning

confidence: 99%

Can the impact of canopy trees on soil and understory be altered using litter additions?

Ecological Applications

Rehm

D’Antonio

2021

Trees can have large effects on soil nutrients in ways that alter succession, particularly in the case of nitrogen‐(N)‐fixing trees. In Hawaiʻi, forest restoration relies heavily on use of a native N‐fixing tree, Acacia koa (koa), but this species increases soil‐available N and likely facilitates competitive dominance of exotic pasture grasses. In contrast, Metrosideros polymorpha (‘ōhi‘a), the dominant native tree in Hawaiʻi, is less often planted because it is slow growing; yet it is typically associated with lower soil N and grass biomass, and greater native understory recruitment. We experimentally tested whether it is possible to reverse high soil N under koa by adding ‘ōhi‘a litter, using additions of koa litter or no litter as controls, over 2.5 yr. We then quantified natural litterfall and decomposition rates of ‘ōhi‘a and koa litter to place litter additions in perspective. Finally, we quantified whether litter additions altered grass biomass and if this had effects on native outplants. Adding ‘ōhi‘a litter increased soil carbon, but increased rather than decreased inorganic soil N pools. Contrary to expectations, koa litter decomposed more slowly than ‘ōhi‘a, although it released more N per unit of litter. We saw no reduction in grass biomass due to ‘ōhi‘a litter addition, and no change in native outplanted understory survival or growth. We conclude that the high N soil conditions under koa are difficult to reverse. However, we also found that outplanted native woody species were able to decrease exotic grass biomass over time, regardless of the litter environment, making this a better strategy for lowering exotic species impacts.

“…In addition, we hypothesized that recovery of naturally recruiting or planted native species in a D. viscosa ‐dominated plant community would be greatest in high‐suitability sites. We also hypothesized, however, that invasive C. clandestinus ‐dominated sites would have the greatest recovery of the invasive grass, which can resprout from rhizomes and favors high‐nutrient sites (Colman & O'Neill, 1978), in high‐suitability sites, leading to competitive interactions and low recruitment and lower survivorship and growth of outplanted native species.…”

Section: Introductionmentioning

confidence: 99%

The role of microtopography and resident species in post‐disturbance recovery of arid habitats in Hawaiʻi

Ecological Applications

Rose

Cordell

et al. 2022

Habitat-suitability indices (HSI) have been employed in restoration to identify optimal sites for planting native species. Often, HSI are based on abiotic variables and do not include biotic interactions, even though similar abiotic conditions can favor both native and nonnative species. Biotic interactions such as competition may be especially important in invader-dominated habitats because invasive species often have fast growth rates and can exploit resources quickly. In this study, we test the utility of an HSI of microtopography derived from airborne LiDAR to predict post-disturbance recovery and native planting success in native shrub-dominated and nonnative, invasive grass-dominated dryland habitats in Hawaiʻi. The HSI uses high-resolution digital terrain models to classify sites' microtopography as high, medium, or low suitability, based on wind exposure and topographic position. We used a split-plot before-after-control-impact design to implement a disturbance experiment within native shrub (Dodonaea viscosa) and nonnative, invasive grass (Cenchrus clandestinus)-dominated ecosystems across three microtopography categories. In contrast to previous studies using the same HSI, we found that microtopography was a poor predictor of predisturbance conditions for soil nutrients, organic matter content, or foliar C:N, within both Dodonaea and Cenchrus vegetation types. In invader-dominated Cenchrus plots, microtopography helped predict cover, but not as expected (i.e., highest cover would be in high-suitability plots): D. viscosa had the greatest cover in low-suitability and C. clandestinus had the greatest cover in medium-suitability plots. Similarly, in native-dominated Dodonaea plots, microtopography was a poor predictor of D. viscosa, C. clandestinus, and total plant cover. Although we found some evidence that microtopography helped inform post-disturbance plant recovery of D. viscosa and total plant cover, vegetation type was a more important predictor. Important for considering the success of plantings, percent cover of D. viscosa decreased while percent cover of C. clandestinus increased within both vegetation types 20 months after disturbance. Our results are evidence that HSIs based on topographic features may prove most useful for choosing planting sites in harsh habitats or those