The most important biological contributor to soil volume and elevation change in some settings occurs below ground. Mangrove root accumulation has been shown to influence the direction and rate of change in surface elevation in Florida and the Caribbean (Cahoon et al., 2003; McKee et al., 2007; McKee, 2011).
Compared with leaves and wood, roots have a much greater potential to contribute to soil volume and elevation gain, because of their refractory nature as well as the anaerobic soil environment which slows decomposition (McKee & Faulkner, 2000a; Middleton & McKee, 2001; Saintilan et al., 2013). Although root production rates may be lower than litterfall rates in some forests (McKee & Faulkner, 2000a; Cormier, 2003; Castañeda-Moya et al., 2011), the lack of oxygen retards the decomposition of roots (McKee & Faulkner, 2000a; Middleton & McKee, 2001) and increases the relative proportion of root matter accumulating in the soil (Fig. 5). In sediment-deficient locations, root accumulation is the primary organic component contributing to peat formation (McKee & Faulkner, 2000b; Middleton & McKee, 2001). In mangrove forests with minimal terrigenous sediment inputs (e.g. on offshore islands or atolls), vertical land development is often dependent on the accumulation of organic matter (i.e. peat formation; McKee & Faulkner, 2000b; McKee et al., 2007; McKee, 2011). For example, some Caribbean mangroves have built peats to thicknesses of 10 m, allowing these forests to track sea-level rise over the Holocene (McKee et al., 2007). In mangrove forests in Florida and Belize, roots accounted for 1.2–11.8 mm yr−1 of total vertical change in soil elevation (McKee, 2011). Where root production was high, elevation gains were found despite minimal surface accretion of inorganic sediment (McKee et al., 2007). Rates of elevation change in Florida and Belize mangroves were positively correlated with both fine (r = 0.75) and coarse (r = 0.69) root accumulation (McKee, 2011).
Root structural traits
The structural characteristics of mangrove roots may also be important in maintaining soil elevations, especially with respect to resisting compaction. The specific root length (SRL; root length per biomass) is a trait that describes the morphology of root systems. SRL has rarely been assessed in mangroves, but, for a similar growth rate, species with low SRL will contribute a greater volume to soils and thus to elevation gains. In solution culture (i.e. where roots form differently compared with fine sediments; Gill & Tomlinson, 1977), significant differences in SRL were found among mangrove species: 0.55 m g−1 for Rhizophora mangle, 1.05 m g−1 for Avicennia germinans and 1.70 m g−1 for Laguncularia racemosa. Although these measurements were made only on primary roots, which would probably have higher SRL, SRL values from mangroves were low relative to rainforest species which have SRLs that range from 5 to 40 m g−1 (mean of 10 m g−1; Metcalfe et al., 2008). Low values of SRL in mangroves reflect their thick roots compared with rainforest species. In rainforest species, the majority of root diameters are within the 0.2–0.5-mm size classes (Metcalfe et al., 2008). By contrast, < 20% of roots were < 2 mm in diameter in a south Florida mangrove forest (Castañeda-Moya et al., 2011). Thick roots are probably an adaptation to improve oxygen supply to root systems, as SRL was inversely related to the capacity to withstand root zone anoxia among mangrove species (McKee, 1996).
The accumulation of long-lived roots is also a mechanism by which soil volume can be maintained or increased over time. The longevity of mangrove roots in R. mangle-dominated forests (and mixed communities with A. germinans and L. racemosa) in Florida was estimated to be 1.7–4.4 yr for fine roots and up to 25 yr for coarse roots (Castañeda-Moya et al., 2011), whereas longevity in Micronesian mangrove roots (dominated by R. apiculata, S. alba and B. gymnorrhiza) ranged from 5.2 to 25.6 yr (Cormier, 2003). These are extremely long lived in comparison with terrestrial trees, which have much shorter lifetimes (faster turnover rates). For example, in temperate trees, median root lifespan ranged between 95 and 336 d, and, for tropical trees, a mean of 135 d was found (Yavitt et al., 2011; McCormack et al., 2012). Currently, there are no assessments of differences in root lifetimes among mangrove species. However, in terrestrial species, root lifespan increases with root diameter, calcium content, tree wood density and carbon : nitrogen (C : N) ratios of tissues, whereas SRL and plant growth rate are negatively related to root lifespan (McCormack et al., 2012). If mangroves follow similar trends to terrestrial species, we anticipate that species with thick roots and low SRL will have greater root contributions to soil volume than species with thinner roots and higher SRL.
The loss of root volume after the death of roots is also an important factor that will influence soil volume. After death, root structures collapse as a result of loss of cell contents and decomposition, and roots are compressed under the weight of soil and water. Many of these processes (collapse, decomposition, compression) may be influenced by differences in root structure among species. On death, larger roots of R. mangle can form channels that occupy 1–2% of the soil volume, which are often colonized by smaller roots, a response hypothesized to capture nutrients within these more oxygenated sites in the soil (McKee, 2001). Moreover, the collapse of root channels within the top meter of soil can cause subsidence of soil elevation, illustrating the importance of the maintenance of root structure to soil volume. For example, Cahoon et al. (2003) documented peat collapse of up to 11 mm yr−1 following acute mangrove forest mortality in Honduras.
The porosity of roots, which is a measure of the air spaces within the roots, may also be linked to the loss of volume during collapse, decomposition and compression of roots after death. One untested prediction is that low-porosity roots (with a low proportion of air spaces) will maintain soil volume better than high-porosity roots (with a high proportion of air spaces), unless the latter are fortified by secondary thickening (as in the case of major root branches) or possibly metal plaques. Root porosity varies significantly among mangrove species (McKee, 1996; Cheng et al., 2012). For example, root porosity was lowest in R. mangle (c. 9%) and higher in A. germinans (c. 25%) and L. racemosa (c. 20%; McKee, 1996). Cheng et al. (2012) found that porosity in Indo-Pacific mangrove species ranged between 10% and 33%. The lowest root porosities were in R. stylosa (15%) and the genus Heriteria (10%), with higher porosities in the genera Sonneratia, Aegiceras, Kandelia and Bruguiera (c. 30%).
Secondary thickening of smaller diameter (< 1 cm) mangrove roots is limited (Gill & Tomlinson, 1977), but mangrove roots often have a lignified epidermis that can persist and even form channels as described above (McKee, 2001). In addition, in terrestrial soils, chemical stabilization of root C with minerals is important for long-term C storage (Rasse et al., 2005). Although mangroves do not tend to accumulate metals in roots (MacFarlane et al., 2007), there is evidence of metal plaques in mangrove roots (Alongi et al., 2004; Machado et al., 2005; Pi et al., 2011), which may contribute to the stabilization of soil C and the maintenance of soil volume. Larger diameter structural roots can account for up to half of root biomass where they occur (up to 50 kg m−3), although their distribution is patchy (associated with stems; Komiyama et al., 1987). Similar to woody debris from the canopy, these large roots may make an important, but as yet unquantified, contribution to soil volume.