2.2 Background

Understanding of spatial ecology and stand dynamics in mangroves is critical to assessing ecosystem resilience to pressures such as sea level rise, yet has eluded ecologists for the past century. Mangroves were first believed to establish on land that they “built” by trapping sediments in dense aboveground root complexes, effectively prograding the shoreline. Later studies showed that mangroves did not build land per se, but rather came to understand hydrogeomorphological setting as a key control on mangrove establishment. Mangrove colonization as purported by these studies was found to be “opportunistic” in hospitable micro-environments.

Following this recognition, a series of classifications of functionally different hydrogeomorphological settings were established for systems with autochthonous (within the system) versus allochthonous (external to the system) sedimentation patterns. These classifications have served well to frame conceptual understanding of the dominant hydrogeomorphological processes that characterize mangroves to date, with recent studies reshaping them for specific initiatives such as restoration.

Spatial aggregations of mangrove tree species, however, are not fully explained by hydrogeomorphological characteristics and yet are visually apparent at many sites. Seminal work has identified environmental settings that help map forest type to dominant hydrogeomorphological processes; however, significant variation across sites exists. Many sites exhibit zonation in tree species, which is the species-specific assemblage of trees in bands parallel to the coast. Zonation in mangrove species has been among the most well-studied ecological phenomena in mangroves, with a variety of hypotheses proposed to explain it: hydrogeomorphological setting, tidal sorting of propagules, ecophysiology, differential predation, and competition among tree species.

Understanding zonation in mangroves will be critical for managing ecosystem resilience, as shifting hydrogeomorphological processes may have disproportionate effects on particular species. Furthermore, zonation may be important for ecosystem services as variation in aboveground root system morphology has been shown to have differential effects on sedimentation, and different zones of mangrove tree species may have different carbon sequestration potentials.

Although few studies of succession in mangroves exist, mangroves are not thought to conform to classical succession regimes. Rather, cyclical patterns of forest development may occur due to both the dynamism of the intertidal zone as well as frequent and varied disturbance regimes. At present, it is unclear how such systems may respond to shifting baseline environmental parameters, as mangroves may exist at or near environmental thresholds such as time of inundation or salinity levels.

Finally, from a methodologically perspective, while in situ data provides critical understanding of mangrove stand dynamics, it is limited in understanding dynamic processes due to commonly being attained at a single point in time. Remote sensing, on the other hand, conveniently provides an efficient means to monitor the time-dependent nature of environmental processes. Recent studies have noted the value of remote sensing for examining existence of multiple stable states and catastrophic shifts in coastal ecosystems, as well as for mapping of coastal hydrogeomorphology more generally. Nevertheless, remote sensing of mangrove hydrogeomorphology is extremely limited and represents a key knowledge gap within the field.