Implications for mangrove range expansion with changing climate

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This article is a part of the Fall 2019 issue of the Earth Science Matters Newsletter.

The northern range limit of most tropical plants and animals is determined by extreme freeze events. In coastal wetlands along the Gulf of Mexico and Atlantic coasts of the United States, freeze events govern the northern extent of mangrove forests. Many climate model simulations indicate that winter temperatures may warm in the coming decades; if that were to occur, it is likely that tropical, freeze-sensitive mangrove forests would expand northward at the expense of temperate, freeze-tolerant salt marshes. To better anticipate and prepare for potential mangrove range expansion, there is a need to advance understanding of the influence of microclimate.

Macroclimate refers to climatic conditions that occur across very large spatial scales (i.e., typically up to 100 km horizontally and 10 km vertically). In contrast, microclimate refers to climatic conditions that vary across much smaller spatial scales (i.e., typically less than 100 m horizontally and less than 10 m vertically). While macroclimate is governed primarily by continental-scale atmospheric circulation systems, microclimate is also regulated by local factors near the earth’s surface including proximity to vegetation, soil, and water.

freeze damaged mangrove

Freezing temperatures control the northern distribution of mangrove forests in the southeastern United States. This photo shows black mangrove leaf damage following a freeze event near Port Fourchon, Louisiana.

(Credit: Michael J. Osland, USGS. Public domain.)

A recent study by USGS scientists and Florida International University synthesized hypotheses regarding the effects of microclimatic variation on temperature gradients and corresponding mangrove freeze damage. Temperature data from the literature and from temperature loggers placed in the field were used to quantify temperature gradients produced by microclimatic factors. Then, literature-derived mangrove freeze damage data were used to quantify the ecological effects of these temperature gradients. Microclimatic gradients due to local factors (e.g., proximity to water, soil, or vegetation) can determine whether temperatures are below or above a threshold at which mangrove damage and/or mortality will occur. For example, temperatures during a freeze event may be warmer near the ocean, close to the soil surface, and beneath the canopy of larger mangroves; thus, these are areas where mangrove freeze damage and mortality may be reduced by microclimatic conditions.

The paper identifies the following six microclimatic factors, which produce air temperature gradients that influence mangrove responses to winter temperature extremes: (1) distance from the ocean; (2) distance from wind buffers; (3) mangrove canopy cover; (4) height above the soil surface; (5) local slope concavity; and (6) tidal inundation. Variation in these factors produces local temperature differences that range from 2 to 14°C, with associated effects on horizontal and vertical patterns of biological damage from freezing. These results clarify the influence of microclimate on spatial patterns of biological damage and mortality due to winter temperature extremes.

graphs showing the level of mangrove free damage to different climate variables

Hypotheses regarding the effects of microclimate on temperature gradients that control mangrove freeze damage. Microclimatic gradients due to local factors (e.g., proximity to water, soil, or vegetation) can determine whether temperatures are below or above a threshold at which mangrove damage and/or mortality will occur. For more info, see Osland et al. 2019. (from Figure 2 in Osland et al., 2019).

(Credit: Michael J. Osland, USGS. Public domain.)

The largest temperature gradient observed was related to distance from the soil surface. During chilling and freezing events, temperatures were ~9-14°C warmer near the soil surface compared to temperatures at just one meter above the soil surface. These observations indicate that there is a protective buffer zone near the soil surface, in which mangrove propagules, roots, and above-ground material are more protected from freeze effects compared to taller plant sections that are exposed to colder air. As mangroves expand into new areas in response to warming winter temperatures, the protective buffer zone near the soil surface will likely play a critical role to promote ecological resilience. If the newly-arrived individuals can grow and reach the reproductive stage, there is a good chance that their propagules and low-lying plant strata will be thermally protected and able to rapidly regenerate following winter temperature extremes.

As mangrove ranges expand in response to climate change, microclimatic variation is expected to produce both adverse environments where mangrove expansion is prohibited and expansion hot spots where mangroves are protected. Subsequent expansion into newly-available habitat will occur from protection zones, and microclimatic gradients may even produce positive feedback cycles that ultimately accelerate the rate of range expansion in response to warming. Collectively, these findings regarding the role of microclimate can improve predictions of mangrove range expansion in response to changing macroclimate.

The paper “Microclimate influences mangrove freeze damage: implications for range expansion in response to changing macroclimate” was published in Estuaries and Coasts and is available here: https://link.springer.com/article/10.1007%2Fs12237-019-00533-1

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