Rooting out ecosystem vulnerability in salt marshes

What is it about?

What you see is not always what you get. Take plants, for example. We know that the foundation of plants exists underground and out of sight: the roots. To help us think about whole-plant health, we can compare growth below- and aboveground, referred to as the root to shoot ratio. Most trees, with their woody trunks, have a root to shoot ratio around 1:6 (one part root to six parts shoot). Grasses, on the other hand, typically have a greater proportion of roots, with ratios of 1:1 to 2:1. As a salt marsh ecologist, I think about root to shoot ratios a lot for two reasons. First, salt marsh grasses have really high ratios of up to 4:1 – meaning when you look out into a salt marsh, there can be four times as much underground root networks than shoots and leaves! Second, these dense root networks are vital to marsh resilience. Tidal salt marshes face extreme pressure from sea-level rise. They are at risk of drowning if not for their dense root networks helping them gain elevation by accumulating organic material and sediment. So, a healthy marsh producing more roots, or belowground biomass, is more resilient. However, until now, we haven’t been able to quantify marsh root growth across the landscape. Using an observational modeling technique we recently developed, we applied satellite, climate, tide, and elevation data to estimate both above- and belowground biomass over the past decade across salt marshes in coastal Georgia. We focused on marshes dominated by the salt marsh cordgrass, or Spartina alterniflora, which dominates the ‘low marsh’ zone, the area that takes the brunt of sea-level rise impacts.

Featured Image

Photo by ag zn on Unsplash

Photo by ag zn on Unsplash

Why is it important?

What we found was not entirely intuitive, and it has major implications for visual assessments of marsh health. Our estimates showed that aboveground biomass increased across the majority of the marsh over the past decade, while simultaneously, belowground biomass broadly decreased. In other words, the marsh may appear to the naked eye to be thriving but is suffering beneath the surface. These losses were strongly associated with increases in flooding pressure, which are expected to continue with sea-level rise, suggesting further vulnerability. This root loss can have drastic effects for the marsh. In locations where we identified substantial decreases in root biomass, we later saw declines in the aboveground biomass. In some situations, aerial imagery confirmed sizable areas of marsh dieback and ecosystem transition to unvegetated mudflats. In total, 27% of the marsh area we examined could be vulnerable to this fate.

Perspectives