Salinity and species richness determine mangrove canopy height

Mangrove ecosystems play an important role in carbon sequestration of coastal wetlands through litterfall and soil carbon accumulation. Maximum canopy height (H_max) is a key variable in assessing above-ground carbon stocks and productivity, and it is relatively easy obtained from forest inventories or satellite remote sensing products. However, the drivers that regulate canopy height in these ecosystems are still poorly understood. It is a common assumption that climatic drivers, such as precipitation and air temperature, account for most of the mangrove height variability. Nevertheless, local productivity and carbon allocation are known to be largely controlled by salinity, which represents one of the dominant sources of abiotic stress in tidal environments. Yet, the control of salinity on canopy height has received scarce attention in the literature.

In this study, we present a global analysis of H_max as a function of seawater salinity, species richness, and air temperature. Our results identify both salinity and air temperature as major abiotic co-factors in control species richness and H_max. For example, the largest number of species is observed in Southeast Asia (> 25), where high temperature co-occurs with low salinity. In contrast, low temperatures in subtropical zones and high salinity in arid regions strongly limit diversity. Such low diversity is generally associated with mangrove ecosystems with short canopy. The multivariate analysis of both global and regional patterns reveals that salinity is the main limiting factor for H_max, while the air temperature is mostly unrelated to H_max. The effects of salt-stress are particularly evident in ecosystems with low species richness (number of species ≤10), while it does not have a detectable effect on H_max in species‐rich communities (number of species >10).

We hypothesize that high stress induced by salinity reduces the niche breadth and decreases competition for above-ground resources, limiting diversity and H_max. On the other hand, more tolerable salinity conditions might promote species coexistence, competition for light, therefore increasing canopy height and mangrove productivity through complementary resource utilizations.