Exploring Halophyte Hydrodynamics and The Role of Vegetation Traits on Shaping Salt-Tolerance in Coastal Ecosystems

Abstract

Mangroves are among the most productive ecosystems worldwide against climate change. They act as carbon sinks due their high carbon burial rate, however, their future potential to store carbon is strictly connected to their capability to maintain high productivity in the face of several stress factors, like sea-level rise (SLR), increasing salinization and submersion, changes in climatic drivers and anthropogenic forcing. Mangroves are halophytes, i.e., differently from most terrestrial forests, they can tolerate the presence of salt in the soil water. Nonetheless, their transpiration, productivity, and future response can be strongly limited by salinity. Analogously, their ability to exert a control on climate through the partitioning energy balance at the surface is modulated by salinity, and the strength of this control is proportional to their transpiration rate and productivity. Despite this, Mangrove ecosystems are still unaccounted in Earth System Models (ESMs), i.e., the new generation of climate models incorporating the ‘biosphere-factor' in their predictions. Similarly, ESMs are not considering a multitude of plant-specific traits, like salt and drought-resilience, which play a crucial role in energy partitioning at the vegetation-atmosphere interface. Moreover, carbon dioxide concentration in the ambience is one of the major limiting factors for Mangrove growth. However, atmospheric concentrations of CO2 have been steadily rising. Present projections for CO2 concentrations are to continue to rise to as much as 500–1000 ppm by the year 2100, when it was approximately 315 ppm in 1959. Photosynthetic assimilation of CO2 is vital to the metabolism of plants. Therefore, its retrieving to understand how this will affect the plants and predict their responses to the future continuous CO2 raise. The goal of this thesis is to explore – through a set of Soil-Plant-Atmosphere Continuum (SPAC) models of increasing complexity – the effects of salinity on Mangroves' energy partitioning and productivity. The main rationale behind the work is to produce an ‘easy to parametrize' hydraulic model of Mangroves, incorporating their response to salinity. This model is parsimonious and general enough to allow for the inclusion in ESMs Land models parametrization schemes. The Pre-existing model from Perri et al., (2019) provides the basic machinery to deal with the hydraulics of salt tolerance in Mangroves. This new model was implemented in Python to allow for enhanced accessibility for scientists in the field, and can account for different plant traits, Mangroves species, and regions. The model focuses on the hydraulic functional trait of Mangroves species in response to salinity at the root-soil V interface and simulates carbon, water, and energy fluxes at the terrestrial-aquatic interface. The model is supported with additional database of parametrizations that covers 20 traits for most Mangrove species worldwide which are 60 species over 25 family. This parametrization enhances the understanding of the hydraulic behavior of different species and the responses of different traits to diverse environmental conditions. Further, the available database is incorporated in the model to improve the simulation of Mangrove forest behavior and traits through the representation of stomatal control by water and salinity content in plant tissues. The effects of salinization and atmospheric carbon enrichment on Mangrove traits is analyzed to encapsulate productivity, transpiration, CO2 assimilation and behavior of Mangroves with the different salinity and CO2 concentration. Also, the relation between traits and the different salt tolerant species is evaluated to understand the different behavior expected from each sub-tolerant species. The key pathways by which salinity and atmospheric CO2 concentration impact Mangrove species productivity and behavior was captured for two different species that represent different characteristics in terms of salt tolerance (High Tolerant species is Avicennia Marina and the medium tolerant one Rhizophora Stylosa). Results of study show that the capacity of plants generally to simulate carbon is declining with the increasing salinity. However, the high osmoregulation capacity for Avicennia Marina allow for a non-monotonic transpiration pattern with salinity, with the transpiration rate having its maximum at an intermediate salinity, where the low osmoregulation flux of Rhizophora Stylosa display strictly monotonic decaying transpiration patterns with salinity. On the other hand, carbon enrichment enhances plants productivity through its effects on assimilation and water use efficiency rates, thus, transpiration rates are lowering, and rates of carbon assimilation are increasing. The study concluded that that interplay of salinization and carbon enrichment radically impact the dynamics of Mangrove ecosystems. Salinity affects different species in different ways depending on their salt tolerance and capacity to osmoregulate. CO2 enrichment favors more tolerant species by drastically improving their water use efficiency. However, Salt tolerant species are less productive.

Date of Award	Jul 2021
Original language	American English