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Comparison of mangrove and neighbouring cleared habitat



5.6.3 Comparison of mangrove and neighbouring cleared habitat

Spatial variability in biomass collected from Sites 1, 2 and 3, in February 2007, is evident in Figure 5.4, which displays the contribution of fine and structural roots to sediments of both the mangrove and neighbouring cleared habitats at these locations. All sites show less biomass in the cleared area, when compared with the adjacent mangrove habitat, with Site 2 and Site 3 showing significant differences in a T-test (p < 0.05 and < 0.01 respectively). The only significant

difference in total below-ground biomass found in an ANOVA post-hoc Tukey test, however, was between the cleared zones of Site 1 and Site 3 (p < 0.01).

Apparent decomposition can be loosely interpreted from biomass differences between vegetated and cleared habitat. Results suggest the greatest decomposition occurred at Site 3 where approximately 50% less biomass was evident in cores in the cleared site compared to the neighbouring mangrove habitat. Conversely, apparent loss between mangrove and cleared habitat at Site 1 was < 10% (Figure 5.4).

Figure 5.4 Below-ground biomass (+SE) of mangrove habitat (M) and neighbouring intertidal flats cleared of above-ground vegetation (Cl). Cores collected in February 2007 (n=6).

5.6.4 Morphometrics of dead (cleared) below-ground biomass

An investigative sampling regime was undertaken in 2005 to identify spatial variability of below-ground biomass across one site that had been clear-felled.

The location had been cleared in two stages, 3 months apart. Results could therefore be interpreted as decomposition 2 months and 5 months after tree mortality. Below-ground biomass differences were significant (p < 0.01) between locations. An average of 4.7 kg m-2 (+ 0.4) was found across cores nominated ‘2 months post clearance’, compared to 2.5 (+ 0.24) kg m-2 for the ‘5 months after post-clearance’ location (Figure 5.6a). This observation was coupled

The proportion of root biomass with depth (Figure 5.6c) shows some variability, although there is no consistent decline in biomass with depth. The greater mass of root material occurred within the top 5 cm of the core, and also at 15-20 cm below the surface. It was found that fine root material contributed < 50 % of the total biomass (Figure 5.6d). The highest total organic content (TOC) from Loss on Ignition was found in sediments at a depth of 15-20 cm (13% + 4), whereas TOC measured in the overlying sediments ranged from ~ 6 to 8% (Figure 5.6e).

5.6.5 Litterbag results

Linear regression identified patchy results across the 20 month sampling period and no significant trend of litter weight loss associated with decomposition time could be identified (Figure 5.5). An apparent linear decrease in biomass for the first 3 months (up to 13%) is thereafter skewed by variable results for the remaining 17 months.

Figure 5.5 Relationship between percentage of dry weight (+ SE) of mangrove roots lost and time in months after burial in litterbags 10 cm below the surface (n = 4).

R2 = 0.09

Figure 5.6 (a) Total below-ground biomass (dry weight kg m-2) from cores collected to a depth of 20 cm in an area cleared of mangroves 2 months previous, and 5 months previous to collection; (b) mean grain size (microns) of sediments collected in biomass cores; (c) total biomass (dry weight) found in 5 cm vertical sections of 5 month clearance cores (n = 5); (d) % fine roots of biomass with depth; - 5 month post-clearance (n = 5) (e) % total organic content of sediments with depth for 5 month post-post-clearance cores (n = 5).


b) c)

d) e) f)

Figure 5.7 Images: (a) Mangrove stand behind a pile of debris from mangrove clearing activity (April 2006); (b) Typical plant structure of 10-20 year old plants at Site 3 – mean heights approx. 0.7 m; (c) Typical plant structure of older plants closer to landward margins; (d) cleared habitat at Site 1, approximately 18 months after clearance; (e) cleared debris piles, 2005; (f) anoxic black muds and root material of cleared


The mangrove populations in Waikaraka Estuary consist of densely populated

‘low’ trees of 1 to 1.25 m, and ‘stunted’ plants of < 1 m, following physiognomic descriptions from mangroves elsewhere in New Zealand (Kuchler, 1972;

Woodroffe, 1985).

Below-ground biomass of mangrove populations in Waikaraka Estuary ranged from approximately 20 t ha-1 to 40 t ha-1 (2 – 4 kg m-2). This is far from the higher end values of >200 t ha-1 reported for tropical primary mangrove forests (e.g. Komiyama et al., 1987). Avicennia populations near Sydney with tree heights of 6–7 m were estimated to accommodate 147 and 160 t ha-1 of below-ground biomass including pneumatophores (Briggs, 1977), similar to 109 – 126 t ha-1 reported by Mackey (1993) for an Australian site at a lower latitude. A further study of sub-tropical Avicennia reported below-ground biomass estimates of 30 to 80 t ha-1 (Saintilan, 1997a). At first glance this latter finding appears similar to this study, however direct comparison is complicated by the different approach used to determine below-ground biomass, whereby Saintilan (1997a) separated live and dead root material to produce an estimate of living biomass, while elsewhere (and in this study) all root material was included in the measurements (Briggs, 1977; Komiyama et al., 1987; Mackey, 1993).

The low values of below-ground biomass reported in this study are not surprising, considering the lower growth form of the populations, the higher latitude which would influence photosynthetic productivity (Beard, 2006), as well as the young age of most of the trees (Cintron and Novelli, 1984; Mackey, 1993; Komiyama et al., 1987). Allometric studies of mangrove biomass allude to a relationship between above-ground and below-ground biomass, although the reported ratio’s of such have been found to vary because of differences in conditions such as salinity (Saintilan, 1997a; Saintilan, 1997b; Sherman et al., 2003), nutrient supply (Saintilan 1997b) and tree age (Komiyama et al., 1987; Mackey, 1993; Tamooh et al., 2008). With this in mind, it seems reasonable to expect that short trees with narrow canopy diameters would produce relatively lower biomass, and indeed a

strong correlation between canopy diameter and biomass for low trees was identified by Woodroffe (1985).

5.7.1 Decomposition of below-ground biomass

Three methods were used in this study to investigate the decomposition of mangrove below-ground biomass. Interestingly, results from each of the three methods provided different estimates.

Results from the litterbag study suggest an initial loss in biomass of 14% within 3 months, however this apparent trend in the data is subsequently blurred by variable and inconsistent values of biomass loss across the remaining 17 months of the study. Similarly, Van der Valk and Attiwill (1984) recorded intial weight loss of fine roots over the first 40 days after which no more weight loss was detectable, a trend which was also observed by Albright (1976). A similar process was reported by Woodroffe (1985) when measuring decay of mangrove leaves which were found to lose half their weight rapidly and then degrade at a much slower rate.

Van der Valk and Attiwill (1984) suggests there is some error inherent in litterbag studies, however the 15% loss in fine roots over a 270 day period reported therein contrasts greatly to the apparent 14% loss in 90 days recorded in Waikaraka Estuary. Fine roots appear to decay more quickly than main roots (Van der Valk and Attiwill, 1984), and the exclusion of the main structural roots from the litterbags in this study may partially explain the initial high rate of decay.

The second approach used in this study was to collect sediment cores within two adjoining cleared zones. One zone was cleared 2 months prior to core sampling, while the neighbouring seaward zone was cleared 5 months prior to sampling. As such, results provide an indication of biomass loss over a 3 month period. The results suggest that 52% of below-ground biomass was lost over that 3 month period, which appears to be unusually high when compared to other studies (Albright, 1976; Van der Valk and Attiwill, 1984). This could be explained by the fact that the cores were collected on the same day, in two different plots, rather

removed. Therefore, these core results provide a comparative result only and age differences in the mangroves that were cleared could be partially responsible for this result.

The third field method identified comparative differences of below-ground biomass between core samples collected in existing mangrove habitat, and those collected within adjoining cleared zones. The results provide a snapshot of spatial trends in both mangrove below-ground biomass, and biomass degradation. The percentage difference (total dry weight) between cleared and vegetated habitat ranged from 8% at Site 1 (18 months post-clearance) to 54 % at Site 3 (11 months post-clearance). In comparison, Albright (1976) found that after 7 years, a patch of dead mangroves had lost 69% of its roots and 55% of its pneumatophores, extrapolated out to a degradation rate of 12% per year. Middleton and McKee (2001) reported mangrove tissue degradation rates of 0.098 % loss day-1, which is roughly in the middle of Albright’s estimate and the apparent degradation of

>54% per year reported in this study (Site 3). The mangrove stands in Waikaraka Estuary experience semi-diurnal tidal inundation, and it is possible that daily inundation provides sufficient surface flushing of decomposed organic matter to promote faster decomposition of the remaining material at the water/sediment interface (Albright, 1976). It is also possible that the cleared areas had less biomass to start with, when compared to the adjacent remaining mangroves, as a consequence of their location seaward of the nominated ‘mangrove habitat’ which would therefore deem them relatively younger.

Within areas cleared of above-ground vegetation in Waikaraka Estuary, surface elevation fell at an average rate of 14 mm yr -1 (Stokes et al., 2009 – Chapter 4).

Substrate collapse has been observed after mass mangrove mortality from hurricane activity, however at a slightly lower rate of 11 mm yr-1 (Cahoon et al., 2003). Cahoon et al. (2003) suggest the topographical change was driven by subsidence of the mangrove peat, whereas in Waikaraka some sediment erosion is occurring, as evidenced by an increase in grain size over time. Elsewhere, deficits in surface accretion have been linked to shifts in groundwater during drought conditions (Rogers et al., 2005). It is likely that some root compaction is also

occurring in Waikaraka, however the separation of erosional and subsidence processes were not attempted in this study.


This study has investigated the below-ground biomass of a developing, temperate mangrove system. Low mean plant height and small canopy diameter is reflected in low values of below-ground biomass. This can be explained partly by the growth-limiting climate, while site differences of biomass can be attributed to stand age. Spatial variability of apparent decomposition rates was evident from results of core analyses. Decomposition rate estimations in this study were mostly higher than has been reported elsewhere, and this may be attributed to a combination of daily tidal inundation, and low initial biomass. However, it could be expected that it will be a number of years before all below-ground biomass of felled mangrove habitat will degrade in Waikaraka Estuary, and this will influence rates of fine sediment release, the contribution of dissolved organic carbon, and the resultant intertidal topography.