Limited natural regeneration of unique Scalesia forest following invasive plant removal in Galapagos

Abstract

More than 60% of the flora of the Galapagos Islands is introduced and some of these species have become invasive, severely altering ecosystems. An example of an affected ecosystem is the Scalesia forest, originally dominated by the endemic giant daisy tree Scalesia pedunculata (Asteraceae). The remnant patches of this unique forest are increasingly being invaded by introduced plants, mainly by Rubus niveus (blackberry, Rosaceae). To help large-scale restoration of this ecologically important forest, we seek to better understand the natural regeneration of S. pedunculata after invasive plant control. We monitored naturally recruited S. pedunculata saplings and young trees over five years in an area where invasive plant species are continuously being removed by manual means. We measured survival, height and growth of S. pedunculata saplings and young trees along permanent transects. Percent cover of surrounding plant species and of canopy shade directly above each S. pedunculata individual were determined, as well as distance to the next mature S. pedunculata tree. We identified potential factors influencing initial sapling survival and growth by applying generalized linear models. Results showed a rapid growth of saplings and young trees of up to 0.45 cm per day and a high mortality rate, as is typical for pioneer species like S. pedunculata. Sapling survival, growth and mortality seemed to be influenced by light availability, surrounding vegetation and distance to the next adult S. pedunculata tree. We concluded that natural regeneration of S. pedunculata was high only five months after the last herbicide application but that 95% of these recruits had died over the 5-year period. Further studies are needed to corroborate whether the number of surviving trees is sufficient to replace the aging adult trees and this way maintain remnants of the Scalesia forest. Urgent action is needed to help improve future restoration strategies to prevent further degradation of this rapidly shrinking threatened forest ecosystem.

Invasive blackberry outcompetes the endemic Galapagos tree daisy Scalesiapedunculata

“…To study the effects of such intensive management and thereby the effects of R. niveus presence/absence on survival, mortality, and recruitment of the endemic S. pedunculata , plots were evaluated inside and outside the area subjected to R. niveus removal for the following 7 years. Related studies have evaluated recruitment and young tree survival in the R. niveus removal area but outside of permanent plots (Walentowitz et al, 2021), as well as the effects of removal on plant, invertebrate, and bird communities (Cimadom et al, 2019; Filek et al, 2018; Jäger et al, 2017). However, prior studies have not evaluated S. pedunculata growth or demography.…”

“…However, the experimental layout was dictated by the management authority's decision to concentrate removal efforts on a specific, relatively small area and it was only within this area that plots were permitted. While not ideal, there is no indication of confounding factors such as differences in microclimate or soil‐type (Cimadom et al, 2019; Filek et al, 2018; Jäger et al, 2017; Walentowitz et al, 2021). The groups of plots were ~500 m apart and had comparable slopes, aspects, moisture regimes, tree densities, weather, sun exposure, and were equally far from cleared land.…”

“…The presence of R. niveus may thus have significant effects on S. pedunculata , its main native competitor. Because S. pedunculata is protected, rare (IUCN status “vulnerable”), and now restricted to a fraction of its former habitat on Santa Cruz (Mauchamp & Atkinson, 2010), the authorities of the Galapagos National Park Directorate (GNPD) are executing a removal program for R. niveus (Jäger et al, 2017; Rentería, Gardener, Panetta, & Crawley, 2012; Walentowitz et al, 2021).…”

Long‐term trajectories of non‐native vegetation on islands globally

“…In the Galápagos Islands, large blackberry ( Rubus niveus ) or guava ( Psidium guajava ) populations are thriving where once endemic species (e.g. Scalesia pedunculata on Santa Cruz) were dominant (Urquía et al, 2019; Walentowitz et al, 2021). Our findings open a new perspective on the timing of the development and the extent of novel species assemblages that seem to emerge up to 1000 years earlier than historical records suggest.…”

History of Land Cover Change on Santa Cruz Island, Galapagos

“…Their control methods include manual destruction, the application of pesticides, biological controls, educational campaigns, and alternative forms of farm management [140][141][142]. In addition, several ecological restoration initiatives, such as Galapagos 2050, were launched [143,144]. The changes in the surface area with invasive species is related to the relative success of control strategies and restoration projects and improvements in the technology used to monitor vegetation changes [145,146].…”