6.5 Mangrove thresholds

6.5.1 Cavanaugh, 2014

“Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events” (Cavanaugh et al. 2014)

Key contribution: This paper provides evidence for a threshold response of mangroves in terms of latitudinal limits, in which frequency of days colder than -4 deg C is key for mangrove presence.

Key notes:

Distribution modeling has informed hypotheses that mangroves and salt marshes exist due to thresholds at northern limits of mangrove range as a result of climatic variables.

The study employs long-term satellite data coupled with historical temperature data spanning the coast of Florida to examine growth in extent of mangrove forest.

Key results:

Mangrove area within the band at the upper range of its limits (29 - 29.75 deg N) doubled over the 28 year period.

Latitude 26.75 deg N existed as a breakpoint, in which areas north of that experience significant expansion and areas south of that did not experience significant expansion in extent.

Expansion of mangroves, however, was not significantly associated with changes in mean annual or mean winter temperatures.

It is unclear to me how exactly they distinguished between mangrove and other vegetation. They simply assume photosynthetic vegetation is mangroves but its possible it may have been another ecosystem type…

6.5.2 Eslami-Andergoli, 2015

“Approaching tipping points: A focussed review of indicators and relevance to managing intertidal ecosystems” (Eslami-Andergoli et al. 2015)

Key contribution: This article reviews alternative stable state theory and early warning signals, with a particular focus on how it relates to intertidal ecosystems. They note the potential relevance of remote sensing as well as paleoecology & soil stratigraphy for understanding long-term shifts in regime over time.

Key notes:

The authors suggest three primary ways in which alternative stable state theory can be examined within intertidal systems:

1. Retrofitting data from long-term studies (i.e., coupling with external data sets to examine perturbations, shifts in state variables, and regime shifts)
2. Using remote sensing to predict regime shifts
3. Use paleoecological data to reconstruct developmental history of intertidal wetlands

They focus largely on gradually changing state variables rather than sharp perturbations, as slow changes are most relevant for management practices.

They note the potential relevance of spatial datasets for understanding tipping points, as they may help identify autocorrelation or state changes. For example, Dale and Dale 2002 use transition matrix to examine shifts in salt marsh plant communities over time, with permanent shifts potentially indicative of a tipping point.

In identifying EWS in intertidal systems, the scale at which one is considering things is important. Role of vegetation versus shifts in hydrology or sedimentation may have different impacts at different scales.

Often may consider shifting expanse of mangroves and loss of sea grass habitat.

Challenges for indicators

The authors identify several key limitations for using indicators to indicate impending transitions:

1. Insufficient data or resolution within the data
2. Inaccurate or imprecise observation
3. Occurrence of large perturbation without any indication of EWS
4. Interactions of multiple tresholds and correlated process errors may obscure EWS
5. Systematic change in external perturbation regime over period leading to a shift
6. Uncertainty involved in identifying and modelling dynamic processes
7. Proposed EWS might not be detectable in some types of transition
8. Underlying stochasticity that is always present in a system, even given shifts in state variables

They make a call for long term monitoring, both from a field perspective as well as leverating remote sensing.

6.5.3 Ellison, 2015

“Vulnerability assessment of mangroves to climate change and sea-level rise impacts” (Ellison 2015)

Key significance: The study reviews vulnerability of mangroves to climate change, and in particular sea level rise, and performs a study in which they seek to assess and measure vulnerabilities across mangrove sites in the Pacific with an end-goal of informing management of these systems.

Key notes:

Vulnerability as defined by Ellison has three components:

1. Exposure to stress
2. Associated sensitivity
3. Related adaptive capacity

For mangroves, exposure to stress is dependent upon different hydrogeomorphological setting, but is perhaps most broadly impacted by sea level rise. Exposure is largely related to shifts in external influences that impinge upon the health of the system.

Sensitivity relates more to the resilience of the system and is a function of characteristics of species or the system as a whole. Vulnerability may be shown by decline in condition, productivity, biodiversity or increase in mortality.

Adaptive capacity may be expressed in mangroves by retreat inland. If sediment accretion rates cannot keep up with sea level rise, then migration is key adaptation response and is dependent upon suitable topography and available areas.

Vulnerability rankings

Studies have tried to rank vulnerabilities of different ecosystems and the factors that drive them, and mangrove and unconsolidated sedimentary shores or coastal wetlands have been ranked as having high vulnerability.

Assessing vulnerability of multiple mangrove sites globally

The study examined mangrove sites in Eastern Africa, Western Africa, and the Pacific Islands to attain measurements of vulnerability and inform management.

The vulnerability assessment methodology seeks to identify aspects that may already be experiencing climate change impacts and may also be more susceptible to future impacts.

The assessments considers range of metrics within the exposure, sensitivity and adaptive capacity categories. In distributing vulnerabilities amongst each of these categories, appropriate management options (a la Johnson, 2017) that are effective may be identified. Sea level rise, for example, may not be able to be controlled whereas barriers to forests migrating inland may be removed via land planning.

The framework is generalized and allows for application at a broad range of sites. It incorporates both quantitative and qualitative metrics of vulnerability for the various sites.

6.5.4 Sasmito, 2016

“Can mangroves keep pace with sea level rise? A global data review” (Sasmito et al. 2016)

Key contribution: This is a well-written study that looks at data of relative sea level rise and accretion rates in mangroves and assesses the abundance of empirical evidence indicating whether or not mangroves may keep pace with sea level rise. The article is summarized in section 3.4.6.

References

Cavanaugh, Kyle C, James R Kellner, Alexander J Forde, Daniel S Gruner, John D Parker, Wilfrid Rodriguez, and Ilka C Feller. 2014. “Poleward Expansion of Mangroves Is a Threshold Response to Decreased Frequency of Extreme Cold Events.” Proceedings of the National Academy of Sciences 111 (2): 723–27. doi:10.1073/pnas.1315800111.

Eslami-Andergoli, L, PER Dale, JM Knight, and Hamish McCallum. 2015. “Approaching Tipping Points: A Focussed Review of Indicators and Relevance to Managing Intertidal Ecosystems.” Wetlands Ecology and Management 23 (5): 791–802. doi:10.1007/s11273-014-9352-8.

Ellison, Joanna C. 2015. “Vulnerability Assessment of Mangroves to Climate Change and Sea-Level Rise Impacts.” Wetlands Ecology and Management 23 (2): 115–37. doi:10.1007/s11273-014-9397-8.

Sasmito, Sigit D, Daniel Murdiyarso, Daniel A Friess, and Sofyan Kurnianto. 2016. “Can Mangroves Keep Pace with Contemporary Sea Level Rise? A Global Data Review.” Wetlands Ecology and Management 24 (2): 263–78. doi:10.1007/s11273-015-9466-7.