Patterns of vegetation change and the recovery potential of degraded areas in a coastal marsh system of the Hudson Bay lowlands

Abstract

Summary 1In recent decades, foraging by increasing numbers of lesser snow geese has led to loss of vegetation and changes in soil conditions in marshes on the Hudson Bay coast. 2 Changes in species composition were recorded in areas unprotected from goose foraging and in exclosures of varying age (5-15 years) erected in intact swards and on bare sediments where foraging had occurred at La Pérouse Bay, Manitoba. 3 In the supratidal marsh, plants failed to establish naturally in either open or exclosed (15 years) plots in bare areas. In moist intertidal soils, vegetative fragments of the asexual species Puccinellia phryganodes readily established and formed a mat in exclosures (5 years). 4 Changes in species assemblages occurred over 11 years in exclosed and adjacent open plots in intertidal and supratidal marshes. Loss of vegetation cover and species richness, particularly dicotyledonous species, and the reversion of later successional plant assemblages to earlier successional assemblages occurred in open plots. In the absence of foraging, late successional graminoids and willow species replaced early successional graminoids. 5 Late successional grasses of the upper intertidal marsh died when transplanted into degraded soils but still survived after one season in control plots, suggesting that an early successional template is needed for establishment. 6 In the absence of goose foraging, natural re-vegetation by clonal propagation can occur only where edaphic conditions are suitable. Within exclosures, vegetation changes resemble those in undamaged areas where goose foraging pressure is still moderate. We propose a state and transition model for vegetation change in the system based on succession patterns, alternative vegetation states and geomorphological events.

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…The intertidal system has been observed to evolve rather rapidly from a low-elevation, bare mudflat state to a high-elevation, vegetated marsh state when a threshold elevation is exceeded; vegetation establishes and the sediment accumulation rate increases in a positive feedback [86]. Disturbance has been shown to induce catastrophic state shifts in salt marsh ecosystems; increased foraging by Lesser snow geese famously caused an irreversible or nearly irreversible shift from the vegetated marsh state to the bare state in some North American salt marshes [182,183].…”

Spatial correlation as leading indicator of catastrophic shifts

“…Some of these shifts are suggested to be critical transitions between alternative states (Scheffer and Carpenter 2003). Such critical transitions have been described, among others, for lakes (Scheffer 1998;Carpenter 2005), for marine and coastal environments (Petraitis and Dudgeon 1999;Daskalov et al 2007), for terrestrial communities (Handa et al 2002;Schmitz et al 2006), and for semi-arid ecosystems (Rietkerk et al 2004;Narisma et al 2007).…”

Spatial patterns of goose grubbing suggest elevated grubbing in dry habitats linked to early snowmelt

“…Furthermore, grubbing exposes the organic layer to wind and flooding, and thereby increases the impact of erosion processes (van der Wal et al 2007). The recovery of grubbed areas is slow, depends on the grubbing intensity and is faster in wet than in dry habitats (Handa et al 2002;Jefferies & Rockwell 2002;Speed et al 2009;Speed, Cooper et al 2010), and can produce persistent stable states characterized by near irreversible changes in soil properties . However, while grubbing in general has negative effects on vegetation cover and productivity, studies have shown that moss removal has positive effects on the soil temperature in the rooting zone of vascular plants and leads to enhanced grass growth (van der Wal 2006;Gornall et al 2009).…”