Interactions between mangroves and exotic Spartina in an anthropogenically disturbed estuary in southern China

Abstract

Cordgrass (Spartina alterniflora) was introduced to China in 1979 from the United States for reducing coastal erosion. It grows vigorously in China and has spread over much of the Chinese coast, from Leizhou Peninsula to Liaoning, a range of more than 19 degrees of latitude. On the southern coast of China, S. alterniflora has invaded mangrove-dominated habitats during the last two decades, but little is known about interactions between native mangroves and invasive S. alterniflora. We studied the distribution and competitive interactions between native mangroves and S. alterniflora in the Zhangjiang Estuary at four tidal sites along a salinity gradient: oligohaline upstream, mesohaline, polyhaline, and euhaline downstream. S. alterniflora occurred at all four sites, and several mangrove species occurred at all but the downstream euhaline site. S. alterniflora has invaded the estuary widely and has spread to the lower tidal margins of mangroves. It has not invaded mangrove areas with a closed canopy but has established in the mangrove zone where the canopy was opened by human disturbance. Ramets of S. alterniflora transplanted into the understory of mangrove stands with closed canopies died within 10 weeks, but 37.5% survived and grew well on open mud flats. S. alterniflora had virtually no competitive effect on mangrove seedlings planted at the upstream oligohaline site. However, S. alterniflora competitively reduced biomass of mangrove seedlings to 33% over a period of 14 weeks at the mesohaline and polyhaline sites where human disturbance has opened the mangrove canopy. In contrast, S. alterniflora marginally facilitated growth and survival of experimental seedlings at the downstream euhaline site. In China, mangroves occur along the coastline south of Whenzhou, but they have been severely disturbed and removed widely, mainly by mariculture activities. Natural vegetation patterns and our experimental results suggest that, without intervention, S. alterniflora could gradually replace these mangroves in mid-salinity regions of Chinese estuaries.

Monitoring the Invasion of Spartina alterniflora Using Multi-source High-resolution Imagery in the Zhangjiang Estuary, China

“…This probably explains why numerous clumps appeared on the intertidal mudflat after 2012 in our study. Previous studies have also suggested that once S. alterniflora becomes established in mangrove gaps, this invasive species can suppress mangrove recruitment by competing for nutrients, light, and space [28,48]. Our results also showed that mangrove forests were suppressed by the expansion of S. alterniflora in or around mangrove forests ( Figure 6).…”

“…In addition to land reclamation along the coastal area, some sandbanks were reclaimed for aquaculture ponds or used for mudflat aquaculture under the local land-contract policy [47]. Humans practicing mariculture activities may clear S. alterniflora from most of the broad mudflats in this estuary [48]. Our results are in accordance with these situations and suggest that the losses of S. alterniflora were mainly due to land conversions to aquaculture ponds during the study period; the most evident land reclamation occurred on the big sandbank (Figure 7), and some S. alterniflora stands on the mudflat were removed during 2005-2011.…”

“…Herbicide application and introducing new species are not suitable for this region in view of environmental safety concerns. Cutting and grubbing combined with mangrove replanting may be suitable for excluding S. alterniflora in mangrove gaps; mature and dense S. alterniflora clumps need to be removed first because previous studies have shown that the combination of competition and crab herbivory is sufficient to exclude young ramets or new recruits of S. alterniflora from intact mangrove forests [28,48]. Moreover, it is necessary to reduce human disturbance to the mangrove forests.…”

Responses of endophytic and rhizospheric bacterial communities of salt marsh plant (Spartina alterniflora) to polycyclic aromatic hydrocarbons contamination

“…The displacement of the native plant species has caused a number of ecological impacts on these ecosystems (Wan et al 2009;Zhang et al 2012). In our previous studies, we have only compared the plant uptake of PAHs from sediment and microbial activity in roots of S. alterniflora (Hong et al 2015).…”

Control and consequences of Spartina spp. invasions with focus upon San Francisco Bay

“…S. alterniflora has spread by floating seed within the areas to which it has been introduced: Willapa Bay, WA, San Francisco Bay, CA, New Zealand, and South Africa. Floating seed has played a role in dispersal of S. alterniflora to more than 19 degrees of latitude-from 39°N near North Korea to 20°N near Viet Nam-since its introduction to China in 1979; China is the only place known where Spartina invades and threatens mangroves (Zhang et al 2012). The hybrid swarm of S. foliosa 9 S. alterniflora spread rapidly and widely by seed floating around San Francisco Bay.…”