Chapter – Surface elevation is an important factor in achieving mangrove rehabilitation success 3.1 Introduction The number of mangrove rehabilitation programmes implemented world-wide is extensive, the majority of which have been executed by governments and local stakeholders to restore forest cover and habitat functionality of degraded mangrove systems (Katon et al., 2000; Barbier, 2006) This commonly takes the form of planting and replanting (when previous attempts have failed) of mangrove propagules and/or seedlings, with low survival rates (Samson & Rollon, 2008) Yet, rehabilitation programmes can be successful if rehabilitation methods involve matching environmental conditions to the autecology of mangrove species This can be achieved if rehabilitation methods prioritise and optimise structural manipulation to remove barriers (e.g dike walls) and allow for the regeneration of mangroves via natural recruitment and establishment This encompasses hydrological and substrate regrading for re-establishment of appropriate hydrologic regime and elevation ranges (Stanley & Lewis, 2011) since hydrology and surface elevations are major leverage points influencing mangrove establishment, survival and development because of its control on inundation regimes Notably, inundation is one of the primary factors in determining the establishment, survival and growth of mangroves because of its influence on secondary factors such as oxygen availability, salinity and pH (Krauss et al., 2008) Hence, knowledge of appropriate surface elevation and its influence on inundation frequency, period and depth may be one of the more important factors that determine 24 the overall success of mangrove rehabilitation projects (Lewis, 2005; Gilman & Ellison, 2007) Through executing a rehabilitation project, this study investigated the contributions of two aspects of hydrologic restoration to the successful colonisation of mangrove vegetation in rehabilitated sites The study focused on (i) restoring a natural tidal inundation regime via strategic breaching and (ii) regrading selected areas to suitable surface elevations 3.2 Materials and Methods 3.2.1 Study area The Western coastline in South Sulawesi Province, Indonesia, experiences a monsoonal climate The northwest monsoon (December – March) is characterised by high precipitation and strong winds while the southeast monsoon (June – September) brings negligible rainfall (Visser et al., 2004) This coastline experiences a tidal range of 1.6 m with a mean tidal range of 0.95 m, with Mean High Water Spring at 1.33 m Chart Datum This study was conducted in three locations near the fishing villages of Kurri Caddi and Kurri Lompo (5° 01' 57" S, 119° 28' 04" E, Figure 3.1) The main study site consists of 29 disused aquaculture ponds, covering 21.5 Before rehabilitation, the ponds were used for semi-intensive farming of brackish water shrimp and milkfish polyculture These ponds were created in the early 1980s and originally operated by a Korean venture They were then bought and managed by the University of Muhammadiyah, Makassar (UNISMUH) In the past, neighbouring communities had no access or use rights in the ponds However, after an agreement was reached between Mangrove Action Project – Indonesia (MAP-I) (now Blue Forests Indonesia), an NGO, and UNISMUH, MAP-I was allowed to conduct rehabilitation works in the 25 aquaculture ponds The community now holds rights to access non-timber resources in some ponds and collaborate with MAP-I in participatory action research involving mangroves, aquaculture ponds and their own rice-fields Interested members of the community are also involved in a multi-stakeholder working group involving academic, governmental and NGO partners, which serve as to manage and advise mangrove rehabilitation Riverine/lower estuarine mangroves surround these aquaculture ponds Two reference forests, comprised solely of coastal and riverine greenbelts, located at a distance of 2.3 km and 0.05 km from the aquaculture ponds and approximately 200 and 50 m wide respectively, were surveyed (Figure 3.1b) Although only a subset of natural mangrove surface gradients and floral species can be found at the reference forests, these sites nonetheless present the best reference as more than 90% of mangroves in Maros and neighbouring Pangkep District have been converted into aquaculture 26 Figure 3.1: (a) Regional setting of Makassar (black box), South Sulawesi, Indonesia; (b) former aquaculture ponds at Kuri Caddi and reference forests at both Kuri Caddi and Kuri Lompo; and (c) broken lines delineate the disused aquaculture ponds in Kuri Caddi, extracted from Google Earth (dated February 2014) 3.2.2 Field data collection Pre-rehabilitation mapping of abandoned aquaculture ponds – In September 2013, a Trimble Real Time Kinematic GPS (RTK-GPS) was used to establish nine elevation benchmarks throughout the site in the WGS84 coordinate system From these benchmarks, a topographic survey of the former aquaculture ponds was conducted using a Total Station (Topcon GTS-235N; mm relative accuracy), with each 27 sampling point resulted from the mean of three readings taken As the topography of the ponds was not highly complex, surface elevation measurements were sampled along both dike walls and in the ponds at (approximately) every metres Sampling point density was increased when abrupt changes in surface elevation were observed (i.e sudden drop between the dike walls into aquaculture pond) Rehabilitation of aquaculture ponds – In November 2013, rehabilitation works were implemented in selected areas in the ponds through the use of an excavator and community labour with hand tools Dike walls were strategically breached, based on the elevation gradients observed from the previous topographic mapping exercise, in order to restore hydrological flows Some were regraded entirely to produce substrate at an appropriate surface elevation for mangroves, relative to sea level (Figure 3.2) A pile of broken branches was also deployed in one pond, designed to trap floating propagules Across the 29 aquaculture ponds, a total of 16 breaches were made in the dike walls Since seed banks are generally absent in mangroves (Harun-or-Rashid et al., 2009), hand broadcasting of locally-collected propagules was conducted at high tide to overcome propagule availability as a limiting factor Approximately 206250 Aegiceras corniculatum (55 kg; 3750 propagules kg-1), 1500 Avicennia sp (10 kg; 150 propagules kg-1), 2380 Bruguiera gymnorrhiza (70 kg; 34 propagules kg-1) and 8600 Ceriops tagal (47 kg; 183 propagules kg-1) were broadcasted in December 2013 Rhizophora spp., Sonneratia spp and B cylindrica propagules were also broadcasted, but in unknown quantities To facilitate seedling establishment, the ponds were subsequently left untouched 28 Figure 3.2: (a) Dike walls that have undergone strategic breaching, and (b) regrading of selective dike walls to produce substrate at lower surface elevations (foreground) Red arrows point to existing dike walls 3.2.3 Post-rehabilitation vegetation survey in aquaculture ponds and reference mangrove forests A second topographic survey using a Total Station anchored to the existing benchmarks was conducted in June 2014 First, an elevation survey was conducted to measure changes in surface elevation of regraded areas Then, a second survey was conducted to measure the surface elevation at which mangrove vegetation had established inside the newly restored site Vegetation was identified to species level 29 and categorised into three size classes – seedling (< 0.7 m in height), sapling (> 0.7 m in height but < cm Diameter-Breast-Height; DBH) and tree (≥ cm DBH) Separately, this survey was repeated in the reference forests (i.e the two natural mangrove forests near Kuri Caddi and Kuri Lompo) in order to produce the elevation envelope (inter-quartile range) of the natural mangrove elevation range for different species 3.2.4 Genera-specific surface elevation envelopes and prediction maps ArcGIS was used to generate a Digital Elevation Map (DEM) of the ponds using the Linear interpolation algorithm with the chosen grid cell size (resolution) of The statistical computing software R 3.1.2 (R development core team, 2014) was used to define species-specific elevation envelopes This was computed separately for trees and seedlings, and if they were established in reference forests or aquaculture ponds Thereafter, species-specific elevation envelopes for trees in reference forests were checked for normality and equality of variances before conducting a t-test Given that the species-specific elevation envelope between two species (per genus) were statistically similar (p-value < 0.05), they were combined to give a genera-specific elevation envelope for genera Avicennia, Rhizophora and Sonneratia The Raster Calculator in ArcGIS was used to delineate areas in the aquaculture ponds exhibiting the exact elevation range in each genera-specific elevation envelope data This created a prediction map of aquaculture ponds of predicted future mangrove growth for each of the three genera – Avicennia, Rhizophora and Sonneratia 30 3.3 Results 3.3.1 Vegetation established in aquaculture ponds and reference mangrove forests A total of 471 seedlings and saplings and 180 trees were surveyed in aquaculture ponds compared to the 213 seedlings and saplings and 140 trees in reference forests (Table 3.1) In the ponds, Rhizophora was the most abundant seedling/sapling genera (n = 254) with Avicennia as the second most abundant seedling/sapling genera (n = 97) The top three most abundant seedling/sapling species were R mucronata (n = 116), R styolsa (n = 84) and R apiculata (n = 54) For trees in aquaculture ponds, the opposite is observed wherein Avicennia was the most abundant genera (n = 55), followed by Rhizophora (n = 52) The top three most abundant tree species are R mucronata (n = 45), L racemosa (n = 38) and A marina (n = 30) In reference forests, similarly, Rhizophora was the most abundant seedling/sapling genera (n = 138) with Avicennia as the second most abundant seedling/sapling genera (n = 50) The top three most abundant seedling/sapling species were R mucronata (n = 72), R apiculata (n = 58) and A marina (n = 30) For trees in reference forests, similarly, Avicennia was the most abundant genera (n = 59), followed by Rhizophora (n = 40) The top three most abundant trees species were A marina (n = 38), R mucronata (n = 37), and S alba (n = 38) Across both aquaculture ponds and reference forests, Bruguiera spp was observed to be present in low numbers 16 Bruguiera spp seedlings/saplings were surveyed in the aquaculture ponds Bruguiera spp trees were absent in both reference forests surveyed (Table 3.1) 31 Table 3.1: Number of seedlings/saplings and trees surveyed across aquaculture ponds and reference forests Avicennia Species* AC A AA AM Bruguiera AR B BG Rhizophora BS CT EA LR RA RM Sonneratia RS S SA SH Total No of species ponds forests Aquaculture 17 Trees Seedlings/ Reference Seedlings/Saplings 49 41 29 18 20 30 21 12 38 45 23 54 116 14 38 45 10 58 72 37 84 471 13 8 14 180 10 11 213 10 140 Saplings Trees 38 * Abbreviations – AC: Aegiceras corniculatum; A: Avicennia spp.; AA: Avicennia alba; AM: Avicennia marina; AR: A rumphiana; B: Bruguiera; BG: Bruguiera gymnorrhiza; BS: Bruguiera sexangula; CT: Ceriops tagal; EA: Excoecaria agallocha; LR: Lumnitzera racemosa; RA: Rhizophora apiculata; RM: Rhizophora mucronata; S: Sonneratia spp.; SA: Sonneratia alba; SH: Scyphiphora hydrophyllacea 32 Seedlings/saplings surveyed have established at similar surface elevation ranges in both aquaculture ponds (Figure 3.3a; -1.511 m ≤ x ≤ 0.228 m WGS 84) and reference forests (Figure 3.3b; -1.255 m ≤ x ≤ 0.073 m WGS 84) Of all seedling species surveyed in aquaculture ponds, the elevation envelope occupied by A rumphiana was the widest, followed by A marina and B gymnorhiza (Figure 3.3a; -1.085 m ≤ x ≤ 0.218 m WGS 84; -0.994 m ≤ x ≤ -0.269 m WGS 84; -0.861 m ≤ x ≤ -0.219 m WGS 84) In reference forests, the species were R mucronata, R apiculata and Sonneratia alba (Figure 3.3b; -0.850 m ≤ x ≤ -0.503 m WGS 84; -0.934 m ≤ x ≤ -0.614 m WGS 84; -0.930 m ≤ x ≤ -0.737 m WGS 84) 33 Figure 3.3: The interquartile range represents surface elevation envelopes per species of seedling/saplings surveyed in (a) aquaculture ponds and (b) reference forests Whiskers indicate maximum and minimum values and empty circles indicate outliers 3.3.2 Genera-specific surface elevation envelopes and prediction maps of mature mangrove trees in aquaculture ponds The elevation envelopes at which trees occupied in reference forests are summarised in Table 3.2 and Figure 3.4, and were derived from 59 Avicennia trees, 40 Rhizophora trees, 38 Sonneratia trees, and Excoecaria trees Of all the tree species surveyed in reference forests, the surface elevation envelope occupied by Sonneratia spp was the 34 widest at -0.924 m ≤ x ≤ -0.598 m WGS 84, with second being Rhizophora spp of 0.895 m ≤ x ≤ -0.639 m WGS 84 (Table 3.2) Table 3.2: Minimum, interquartile range and maximum surface elevation of established trees surveyed in reference forest sites, based on WGS 84 datum Avicennia Excoecaria Rhizophora Sonneratia 59 40 38 -0.746 -0.677 -1.369 -1.014 1st Qu -0.352 -0.642 -0.895 -0.924 Mean -0.324 -0.608 -0.778 -0.722 3rd Qu -0.283 -0.574 -0.639 -0.598 0.003 -0.539 0.259 0.081 Sample Size Max Envelope Elevation Min Figure 3.4: The interquartile range represents surface elevation envelopes per genus (i.e Avicennia spp., Excoecaria spp., Rhizophora spp and Sonneratia spp., surveyed in reference forest sites Whiskers indicate maximum and minimum values and empty circles indicate outliers 35 Across the 29 aquaculture ponds, surface elevation was within the range of -2.158 m to 2.411 m WGS 84 (Figure 3.5a) Surveyed vegetation in aquaculture ponds have largely established at or below MSL and were more restricted to inter-tidal positions with higher elevations, within the range of -1.511 m ≤ x ≤ 0.228 m WGS 84 Of these 651 individuals, 137 individuals (21%) had established on regarded areas (Figure 3.5) Figure 3.5: Map of aquaculture ponds showing surface elevation changes (i.e grade down, grade up), location of pile of broken branches and established vegetation where each green triangle represents an established individual (surveyed in June 2014) 36 The DEM in Figures 3.6a – 3.6d have been spatially classified into three categories – below zero (-2.152 m ≤ x < m WGS 84), zero and above zero (0 m < x ≤ 2.411 m WGS 84) The green areas serve to represent potential, suitable elevation range wherein Avicennia spp., Rhizophora spp And Sonneratia spp may establish in the future as trees and are reported to be 2.95% for Avicennia spp., 9.80% for Rhizophora spp., and 12.0% for Sonneratia spp of the total area 37 38 Figure 3.6: (a) Each green triangle represents one established vegetation individual (surveyed in June 2014) Predicted elevation ranges where (b) Avicennia spp., (c) Rhizophora spp and (d) Sonneratia spp might establish in the future as trees are represented as green areas 39 3.4 Discussion 3.4.1 Surface elevation affects propagule establishment and seedling development The rehabilitated aquaculture ponds exhibited topographic heterogeneity, with elevations that ranged from -2.158 m ≤ x ≤ 2.411 m WGS 84 Post-rehabilitation surveys highlight that a total of 471 seedlings/saplings and 180 trees, across 13 species, had established in the aquaculture ponds (Table 3.1) The majority of seedlings/saplings surveyed in aquaculture ponds had established at more restricted inter-tidal positions with the range -1.511 m ≤ x ≤ 0.228 m WGS 84 (Figure 3.3a), compared to the full elevation range represented in ponds In general, established vegetation was restricted to the perimeter of aquaculture ponds (Figure 3.5b) where relatively higher surface elevations exist compared to the lower elevations found in the middle of ponds Higher surface elevations around the pond perimeters relates to a lower inundation hydroperiod Inundation hydroperiod encompasses the frequency and duration a location is inundated, and is determined by surface elevation, tidal frequency and amplitude (Crase et al., 2013) Hence, inundation hydroperiod and its inherent link to surface elevation change is a key control on mangrove establishment, forest structure and subsequent long-term stability (Kitaya et al., 2002; Friess et al., 2012) Inundation hydroperiod was first proposed as a key control on mangrove communities more than 80 years ago (Watson, 1928) The distribution of mangrove communities could be delineated based on their tidal regime, surface elevation and inundation frequency This influence of hydroperiod on mangrove distribution has been examined more recently in other field studies Crase et al., (2013) found that S alba trees dominated 40 low elevation areas which were inundated for 68.8% of the year, while C tagal preferred higher elevations experiencing 5.1% annual inundation Similarly, this present study shows that inundation is a key, spatially explicit threshold to the establishment of seedlings in this site The importance of suitable inundation hydroperiod within mangroves has been applied to guide rehabilitation efforts aimed at achieving natural recolonisation of flora and fauna via natural succession processes (Lee et al., 1996; Lewis & Brown, 2014) An example was the successful rehabilitation of mangroves on partially reclaimed land in Singapore (Lee et al., 1996), where hydrological works and surface elevation manipulation maintained tidal connectivity and pre-reclamation inundation frequencies of 40 – 50 times per month Successful regeneration and colonisation of site by Avicennia species and Sonneratia alba was observed only three months after complete rehabilitation On a larger scale, inundation hydroperiod has also been incorporated into the EMR approach that has guided the successful rehabilitation of mangroves spanning sites across the Neotropics and Asia, including the rehabilitation of 400 of community-owned aquaculture ponds on Tanakeke Island, Indonesia (Lewis & Brown, 2014) More relevant to mangroves in early developmental stages, elevation differences have more immediate influences on the germination and growth of mangrove seedlings This is evident in how established mangrove vegetation in aquaculture ponds had colonised similar elevation ranges as surveyed in natural reference forests (Figure 3.3) Also, there existed an “optimal elevation range” as established mangroves were restricted to higher surface elevations around ponds’ perimeter (Figure 3.5) Beyond 41 some optimum thresholds, low elevation and its inherent implications of increased flood frequency, duration and depth, result in reduced seedling physiological efficiency and growth potential (Krauss et al., 2008; Mangora et al., 2014) Mangroves are more sensitive to flooding during earlier life stages and/or when exposed to prolonged flooding periods (Naidoo et al., 1997; Krauss et al., 2006) Flooding appears to stall physiological processes associated with photosynthetic light initiation in seedlings and saplings (Krauss et al., 2006b, 2007), as it reduces oxygen available to roots, depressing the rate of aerobic metabolism and water-use efficiency (McKee, 1996) It also results in the build-up of phytotoxins such as reduced forms of iron, manganese and sulphides that inhibit plant growth (McKee, 1993; Youssef & Saenger, 1998) S caseolaris and S apetala both exhibited inhibition of growth rates, potentially due to depressed photosynthetic capacity (Chen et al., 2013) A germinans seedlings had compromised growth and survival suboptimal intertidal positions that were indicative of either greater or lesser flooding from that of mean water level (Ellison & Farnsworth, 1993) Also, responses are species specific (Kitaya et al., 2002; He et al., 2007) For example, most C tagal seedlings died within a year in lowelevation treatments while S alba seedlings survived (Kitaya et al., 2002) Unlike mature trees, seedlings not form aerial roots and stem lenticels as adaptations to lower soil oxygen levels under prolonged inundation They are limited to employing an anatomical adaptive strategy through increasing root porosity to increase oxygen reserves (Youssef & Saenger, 1996) 42 3.4.2 Low surface elevations hinder propagule establishment: mid-corrective actions are required in approaching successful re-vegetation at rehabilitation site A comparison of these ponds to other rehabilitated sites on South Sulawesi, similar in rehabilitation method, size and time after rehabilitation, highlighted that the ponds had not recruited mangroves in densities equivalent or approaching the documented potential An example would be the village Dande-Dangdere on Tanakeke Island Seven months after initial rehabilitation, the 33 hectares site exhibited an average stem density of 767 (Brown et al., 2014), largely surpassing the stem density of 30 in Kuri Caddi ponds (651 established individuals divided by 21.5 hectares) This could be accounted for by the lack of mangrove establishment in the middle of ponds where large areas of low elevations exist (Figure 3.6) Despite broadcasting large quantities of propagules to overcome propagule availability as a limiting factor, propagule establishment rates were low Only 21% (i.e 137 out of 651 individuals) had established on regraded areas (Figure 3.5b), alluding to a possibility that the remaining 79% were natural recruits that had established before commencement of this rehabilitation project Instantaneous colonisation by Avicennia across tens of hectares and in a few days can occur when conditions are favourable (Proisy et al., 2009), and Avicennia has been shown to be a coloniser of low-elevation, bare tidal mudflats around the world (i.e Clark, 1993; Lee et al., 1996; Osborne & Perjak, 1997) Hence, it is probable that regrading activities were insufficient to raise the internal shrimp pond elevation to a level sufficient for such mangrove colonisation Low surface elevation results in a lack of sufficient inundation-free periods (i.e perpetual flooding) before propagules become unviable (McKee et al., 1995) In this rehabilitation project, locally collected 43 propagules were broadcasted into the ponds instead of the common practice of planting propagules or seedlings With the innate ability to remain buoyant for varying time periods, the most significant threshold that propagules must overcome to transit into the seedling life stage is proposed to be a sufficiently long disturbance-free period that is favourable for propagule anchorage following a dispersal event (Balke et al., 2013) Flume studies and field observations have shown that for seedlings to establish successfully, two critical criteria (among others) must be fulfilled where (1) stranded propagules require a minimum inundation-free period to rapidly develop roots for anchorage, and (2) roots have to be sufficiently long to withstand seedling dislodgement by hydrodynamic forces from waves and currents (Balke et al., 2011) The required inundation-free period is species- and ecosystem-specific Contrasting mangrove ecosystems with river floodplain ecosystems, Rhizophora spp seedlings in mangroves require a disturbance-free period of approximately five consecutive inundation-free days (Rabinowitz, 1978; Balke et al., 2011) whereas Salix spp and Populus spp in floodplains requires a longer duration quantified in years (Bradley & Smith, 1986; Mahoney & Rood, 1998) Hence, mid-course corrections should be implemented to increase surface elevation above mean sea level, thereby promoting the occurrence of sufficiently long inundation-free periods for the establishment of stranded propagules The creation of substrate mounds in the middle of ponds could be practised This is one of the techniques that form the mid-course corrections step in the EMR approach that has been practised and have proved effective in achieving reforestation success (Lewis & Brown, 2014) A lack of “surface roughness” in ponds to aid the retention of propagules may also contribute to the lack of mangrove establishment in ponds Unlike natural mangrove 44 forests, aquaculture ponds and regraded areas exhibit a “smooth” surface due to the absence of vegetation stem, prop roots and pneumatophores, with fewer opportunities to trap and retain propagules The effectiveness of utilising a “rough” matrix to retain propagules has been demonstrated through the trial deployment of a pile of broken branches over an area of 770 m2 and surface elevation range of 0.641 m to 0.748 m WGS 84 Compared to other areas which had been graded up and of similar elevation range (Figure 3.5b), there exist a higher number of established vegetation on the branch structures (a total of 29 individuals – 25 Rhizophora and Avicennia seedlings) Other variables that influence successful colonisation and establishment may be explained by propagule availability and dispersal method Of the six genera of propagules broadcasted into the aquaculture ponds, it was observed that Rhizophora spp represented the highest number of established individuals whereas Sonneratia and Bruguiera represented the lowest (Table 1, n = 254, 17 and 16) Mature Rhizophora propagules are generally available all year round, which contrasts with the seasonal fruiting of Sonneratia spp and low numbers of mature adult Bruguiera trees Also, propagule availability of Rhizophora spp is increased as local communities practice a preferential habit of planting Rhizophora spp in high densities along adjacent river banks and water channels linking aquaculture ponds (pers obs.) 3.4.3 Using surface elevation data to predict future establishment The predicted distribution of mangrove establishment in the aquaculture ponds is low and in discontinuous patches (Figures 3.6b – 3.6d) Only 3.0 % for Avicennia spp., 9.8 % for Rhizophora spp and 12.0 % for Sonneratia spp of the total site were flagged as 45 potential elevation ranges that future mangroves may establish at – the predictions are least favourable for the persistence of Avicennia It is recognised that the predicted distributions may be overly conservative as it was founded on a subset, instead of the full surface elevation gradients, that natural mangrove forests can occupy Specifically, the prediction does not account for the upper elevation range (on the landward side) that mangroves once occupied before conversion into aquaculture ponds, agricultural fields and other urban developments Yet, these low percentages caution that current elevation ranges in ponds might be unfavourable for the long-term persistence of mangroves 3.5 Summary This study has provided evidence for the importance of appropriate surface elevations in influencing successful establishment and colonisation of mangroves, immediately after the broadcasting of propagule The first reason is that surface elevation influences inundation hydroperiod – hence low elevations implies persistent flooding, and for long durations This impedes stranding of buoyant propagules for establishment For seedlings, low surface elevations result in prolonged inundation durations that depresses aerobic metabolism and photosynthetic abilities, thus negatively impacting seedling survival and growth 46 ... species ponds forests Aquaculture 17 Trees Seedlings/ Reference Seedlings/Saplings 49 41 29 18 20 30 21 12 38 45 23 54 11 6 14 38 45 10 58 72 37 84 4 71 13 8 14 18 0 10 11 213 10 14 0 Saplings Trees... aquaculture ponds on Tanakeke Island, Indonesia (Lewis & Brown, 2 0 14 ) More relevant to mangroves in early developmental stages, elevation differences have more immediate influences on the germination and. .. importance of appropriate surface elevations in influencing successful establishment and colonisation of mangroves, immediately after the broadcasting of propagule The first reason is that surface elevation