Salt marshes develop most fully in temperate latitudes, whereas mangrove forests are most luxuriant in the tropics. From a global perspective, the data indicate that mangrove forests attain peak biomass near the equator with a decline towards temperate latitudes. The proportion of biomass vested below-ground differs between mangroves and marshes, reflecting the structural differences between trees and grasses. Salt marshes and mangrove forests are commonly believed to be highly productive ecosystems. Numerous estimates of above-ground, net primary production indicate that, on average, mangroves are more productive than salt marsh grasses. The relationship between these halophytes and edaphic characteristics is fundamental to understanding what limits plant growth and the subsequent transfer of organic matter to detritus food chains. Mangroves and marsh grasses grow over a wide range of temperatures and light conditions. Most species of marsh grasses and mangroves are classified as either facultative or obligate halophytes but vary widely in ability to tolerate salt.