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

Abstract

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

Invasive‐dominated grasslands in Hawaiʻi are resilient to disturbance

“…Previous work in Hawaiian dry forests has shown that sites that are favorable to native species also tend to favor an abundance of invasive grasses (Questad et al, 2012), but due to the lower growth rates of native species, invasive grasses remain persistent or take over sites over time (Yelenik, Rose, Cordell, et al, 2022). Native grass E. atropioides also showed significant declines within disturbed relative to control plots, with some increases in bare ground and B. catharticus, suggesting a slight shift in species composition.…”

“…This variation in colonizer versus competitor syndrome occurred both between species and also within species among different habitats. This points to the importance of resident communities and their effects on species interactions in shaping a plant community's response to disturbance (Hulvey & Aigner, 2014 ; Yelenik, Rose, Cordell, et al., 2022 ). These results are also likely driven by seed availability, or, in the case of C. clandestinus , rhizome availability (HilleRisLambers et al., 2010 ; Hulvey & Aigner, 2014 ).…”

“…As such, C. clandestinus may represent a threat to continued dominance by D. viscosa in the face of future disturbance, even though the outplanting experiment showed that D. viscosa seedlings in their own habitat type. Previous work in Hawaiian dry forests has shown that sites that are favorable to native species also tend to favor an abundance of invasive grasses (Questad et al., 2012 ), but due to the lower growth rates of native species, invasive grasses remain persistent or take over sites over time (Yelenik, Rose, Cordell, et al., 2022 ).…”

“…We implemented a disturbance experiment to assess how plant communities would reassemble (see also Yelenik, Rose, Cordell, et al., 2022 ). We chose the five most dominant species from the random plot survey as the vegetation types, which included four invasive grasses ( C. ciliaris , C. clandestinus , C. setaceus , and M. repens) and one native shrub ( D. viscosa ).…”

Out of the frying pan and into the fire: effects of volcanic heat and other stressors on the conservation of a critically endangered plant in Hawai‘i

“…Finally, the relationships between species and specific biophysical variables can be used to predict sites where planting or translocation efforts will be most successful based on microhabitat (Questad et al 2014). These techniques for threatened and endangered species, however, rarely incorporate biotic interactions, such as competition from invasive species, potentially leading to spurious conclusions about how species shift given climate change (Alexander et al 2015) or altering the utility of habitat suitability models (Yelenik et al 2022).…”