Salvaging and replanting 300 mangrove trees and saplings in the arid Arabian Gulf

Keywords: Avicennia marina, mangrove relocation, root damage, tidal inundation, tree salvaging.

Marine and coastal habitats and their associated species are under increasing pressure from anthropogenic developments worldwide, including in the Arabian Gulf (Sheppard et al. 2010; Erftemeijer and Shuail 2012; Milani 2018). The ongoing and cumulative effects of coastal construction, extractive resource exploitation and urban and industrial expansion have resulted in the significant loss of coastal habitats, particularly mangroves (Halpern et al. 2008; Polidoro et al. 2010). In response, the past two decades have seen increasing efforts to prevent such impacts through improved selection of construction sites to minimise habitat losses (Gilman 2002), the conservation of critical areas, environmental impact assessments (Naser 2015) and the restoration of damaged areas (Lewis and Streever 2000; Bayraktarov et al. 2016). Salvaging critical and sensitive marine habitats by physical relocation is often seen as a last resort when destruction cannot be avoided (Arlidge et al. 2018). Published cases have highlighted the ability to translocate coastal habitat successfully, but these are few and often small scale (but see Gayle et al. 2005; Rodgers et al. 2017; Park et al. 2018). Reviews of plant translocations suggest that these are relatively high risk, high cost and challenging but, if successful, can reduce the time required for plants to mature and produce viable seeds as a source of second-generation recruitment (Silcock et al. 2019).

Although there are many reports on transplanting nursery-reared mangrove seedlings, there are only three previously published case studies on the relocation of mangrove trees. Pulver (1976; also reviewed in Snedaker and Biber 1996) reported on the successful translocation of 120 mangrove trees (0.5–1.5 m high, comprising three species) in Florida (USA), describing transplanting techniques, the importance of root ball preservation and the beneficial effects of pruning. Pulver (1976) also describes two earlier attempts to excavate (with a crane and backhoe) six Rhizophora mangle trees in Dade County, Florida (five of which were still surviving after 2 years) and the transplanting of ‘some’ Avicennia germinans trees near Marco Island, with 33% survival. Saenger (1996) documented the transplanting of mangrove propagules, seedlings and small shrubs and trees to compensate for the loss of mangroves due to construction works associated with the expansion of Brisbane Airport in subtropical Queensland, Australia. Survival after outplanting was inversely proportional to plant size, with low survival of larger plants >50 cm tall. Abbot and Marohasy (2014) reported on the excavation of ten 7-year old Avicennia marina trees (up to 1.8 m high) in Queensland, Australia, and their subsequent successful cultivation in containers with an automated irrigation system for experimental research. Eight of the ten translocated trees survived under these conditions for over 2 years.

Although these case studies of mangrove trees included valuable information for designing the present study, they were all conducted under ideal humid tropical conditions, so that their findings were not necessarily applicable to salvaging and replanting mangroves in the environmentally extreme Arabian Gulf. Erftemeijer et al. (2020) reported on the long-term restoration of mangroves on Mubarraz Island in the Arabian Gulf, where performance and survival proved to be limited by exceptionally harsh conditions, particularly high salinity, wide temperature variation, extremely low rainfall and nutrient-poor calcareous soils (Al-Habshi et al. 2007). The mangrove restoration program involved planting of ~500 000 A. marina seedlings over 30 years, enabling documentation of methods to improve success, first with wild-collected propagules and then with nursery-reared seedlings, thus providing background on appropriate habitat conditions (site preparation, soil amendments, duration of tidal inundation) for the present study on tree relocation. Seminal work by Lewis (2005) also emphasised the fundamental importance of ensuring appropriate tidal flooding depth above mean sea level (MSL) and duration (inundated <30% of the time) if success were to be achieved at any mangrove restoration site. Recent research demonstrated a beneficial effect of freshwater treatment (spraying of leaves) on the growth of A. marina trees at arid sites, further contributing to treatments with potential for enhancing the survival of relocated mangrove trees in extremely harsh, arid environments (Steppe et al. 2018; Fuenzalida et al. 2019).

Construction works along a causeway at Mubarraz Island, an offshore site in the United Arab Emirates off Abu Dhabi, necessitated the relocation of >300 mangrove trees and saplings (A. marina) that had been planted 5–9 years ago as nursery-reared seedlings (Erftemeijer et al. 2020). The relocation was not a legal requirement, but an example of corporate citizenship on the part of the oil company. Despite their age, these trees had only achieved heights of up to 1.5 m, an indication of the very slow growth rates in such an extreme climate. Here we describe the results of this large-scale mangrove relocation project (undertaken in September 2019), which provided an opportunity to examine the effects of some environmental and biological variables on the success of the relocation.

The objectives of this study were to: (1) assess the feasibility of large-scale mangrove relocation and evaluate the benefits of salvaging larger trees compared with the planting of new seedlings; and (2) determine the effects of environmental conditions at the recipient site on the survival and performance of the salvaged trees and saplings. The hypotheses were that: (1) the survival of relocated mature mangrove trees would be similar to that of transplanted seedlings, but greater benefits would be conferred owing to their larger size; (2) the survival, growth (as an increase in height) and leaf health would be less for relocated trees than control mangroves at the same site; and (3) the survival, growth and leaf health of relocated trees would increase with watering (fresh water addition), decrease with initial tree size and decrease with longer tidal inundation (greater bed depth).

Mubarraz Island is located in the Arabian Gulf, some 100 km north-west of Abu Dhabi, United Arab Emirates. The island is surrounded by a large sand shoal (up to 5 m deep) with extensive seagrass meadows and fringing coral reefs. The Mubarraz shoal features several oil fields (mostly situated in the northern parts of the shoal) that are under concession by the Japanese oil company Abu Dhabi Oil Co. (Japan) Ltd (ADOC). Typically for the arid Arabian Gulf region, environmental conditions are extreme, with water temperatures varying from 17 to 36°C and salinity ranging from 40 to 48. Water levels are governed by a diurnal tide, with a maximum tidal amplitude of 2.3 m. During 1983–85, a channel was dredged across the shoal to facilitate navigational access. The dredged material was used for the construction of a 17-km causeway that protects several oil pipelines and power cables and serves as a road connection between the oil production areas and the main island. Mass planting of ~500 000 nursery-reared mangrove A. marina seedlings along the causeway and island over the past 30 years resulted in the successful establishment of mangrove vegetation along 6.7 km of shoreline, covering an area of 16.5 ha (Erftemeijer et al. 2020).

Approximately 300 m of existing mangrove vegetation (planted by ADOC several years prior) had to be removed at two sites along the causeway shoreline to make way for the construction of culvert structures. A new site was created elsewhere along the causeway to translocate the excavated mangrove trees (Fig. 1). The new site consisted of a tidal channel ~200 m long and 6 m wide, excavated parallel to the causeway, as in previous mangrove planting programs at Mubarraz (see Erftemeijer et al. 2020).

Fig. 1.  Mangrove relocation at Mubarraz Island, Abu Dhabi (United Arab Emirates), showing (a) the shovel method used for excavating saplings, (b) the backhoe used for the excavation of trees, (c) root balls wrapped in burlap, (d) the front loader used for the transportation of excavated trees, (e) the newly created tidal channel before planting, and (f) the replanted mangrove trees and saplings in the tidal channel.

All mangrove saplings and trees to be removed were counted and their height and number of pneumatophores measured before relocation. Three randomly selected small trees were manually excavated to investigate the depth penetration and horizontal spread of their root systems and to establish a relationship between tree size and pneumatophore abundance.

At the relocation site, bed levels were adjusted (during low tide) with the use of a CAT 966G front loader with a bucket width of 3 m to create the appropriate tidal inundation characteristics conducive to mangrove growth (van Loon et al. 2016). Large limestone rocks were removed from the channel bed before replanting. To allow testing of the relative effect of tidal inundation on the survival and growth of relocated mangroves, two sections were established within the channel that differed in bed depth (shallow, +50 cm above MSL; deep, +30 cm above MSL), corresponding to mean tidal inundations of ~3 and 6 h day^–1 respectively. These levels approximated the lower and upper ends of the locally established, site-specific inundation tolerance limits of the mangroves at Mubarraz, based on a detailed hydrological study (DAMCO Consulting 2020; Erftemeijer et al. 2021).

Two excavation methods were selected for the relocation works (Fig. 1a, b), based on previous literature, an initial investigation of tree sizes to be moved and logistical and environmental constraints, including an assessment of available equipment and staff at Mubarraz, namely manual and mechanical excavation.

Manual excavation, with shovels, was used for the excavation at low tide (Fig. 1a) of smaller saplings up to a height of ~60 cm that had not yet developed extensive cable root systems with pneumatophores. Because of the hardness of the soil (consisting of a mixture of rocks, compacted coral rubble and coarse sand), a pickaxe sometimes had to be used to help loosen the soil around the trees. Given the harsh climatic conditions at the time of the relocation works (with daily air temperatures typically over 40°C), a maximum of 40 saplings could be removed and replanted with this method per day, using a team of 4 people.

Mechanical excavation using an excavator at low tide (backhoe; bucket width 75 cm; Fig. 1b) worked particularly well for the larger trees (from >60 cm up to 2.1 m). The excavator was used to first cut away a square section of root ball, measuring ~70 cm in width. The large excavator was carefully guided by staff standing nearby, who sometimes helped in the excavation with shovels. Care was taken to minimise root damage as much as possible, and secateurs were used to prune exposed and damaged roots where feasible. The root balls of excavated trees, which included the roots and surrounding soil, were wrapped in hessian burlap (Fig. 1c) or plastic. The diameter of the root balls ranged from 25 to 65 cm (mean ± s.e., 38 ± 9 cm; n = 36) depending on the size of the tree.

Special care was taken to try to minimise damage to the root systems of the mangroves during excavation, but this was nearly impossible to avoid, especially for larger trees. The soil in the channels often consisted of predominantly coarse friable material, causing some of the root balls to ‘collapse’ upon excavation, making it difficult to keep the soil and roots together in an intact root ball during excavation.

Excavated mangroves trees were wrapped in shade cloth to minimise water loss from excessive evapotranspiration, transported by front loader (Fig. 1d) to the new site and then replanted at the new site (at low tide) within 2 h of excavation. Prior to replanting, individual holes were dug by shovel to a depth similar to the size of the root ball and ~200–300 g of peat moss was added to each planting hole. The mangrove trees and saplings were replanted at the same depth as in their original position while making sure that their pneumatophores (if any) were not buried. All plantings were given 6–8 L of fresh water (approximately one bucket per tree) immediately after replanting.

To compensate for the loss of root mass, larger-sized trees (>70 cm) were moderately pruned upon transplanting in an effort to avoid excessive water loss through evapotranspiration but taking care not to affect plant height. This reduced the leaf canopy by ~20–30%, following recommendations by Pulver (1976).

All mangroves were subsequently watered daily (during low tide) with fresh water for the next 7 months. Mangroves in separate experimental plots were given two types of freshwater treatment, either ~2 or ~6 L of fresh water per tree. This differential treatment allowed for testing of the effect of fresh water addition on the survival and growth of the relocated mangroves. Although these volumes (2 and 6 L) were arbitrary (there are no guidelines for the watering of mangroves), they are in the same order of magnitude (m^–2) as water volumes commonly applied in the irrigation of agricultural crops and trees (see the Australian irrigation calculator, www.agric.wa.gov.au). The freshwater treatment was applied by spraying the plots with a hose from a water tanker equipped with a metered pump, allowing for the application of approximate measures of watering volumes. Freshwater treatments were continued daily for 7 months after the relocation and then discontinued due to pump breakdown. All plots in the channel (and controls) were also inundated naturally by seawater for several hours during high tides every day.

Relocated mangroves were transplanted in the newly excavated tidal channel according to a randomised block design, with eight plots representing two treatment levels for tidal inundation (bed depth high or low) and two treatment levels for watering regime (daily freshwater treatment 6 or 2 L). This resulted in four treatment combinations (High 6 L, High 2 L, Low 6 L and Low 2 L), each of which was replicated twice within the channel. Excavated trees and saplings of varying sizes were transplanted randomly at a spacing of 1 m across all plots, with a total of 40 mangroves (10 rows of 4) in each plot (except Plots 7 and 8, which only had 32 and 28 seedlings respectively due to insufficient mangroves available for transplanting after relocation). Two control plots along the causeway each consisted of 40 mangroves that were not excavated but had been planted during the same period as those selected for relocation (i.e. 5–9 years earlier). In these control plots, peat moss had also been added with the initial plantings. Controls were monitored for the full duration of the experiment. All mangroves in the experimental and control plots were monitored for survival, plant height and leaf health (% green) at t = 0 (shortly before the relocation) and then at 1 week (i.e. immediately upon completion of the relocation) and 3, 7 and 12.5 months after relocation. Plant height was assessed to the nearest millimetre using a rule measuring from the sediment surface to the top of the tallest branch. The increment in mean height (averaged per plot) between consecutive monitoring events was calculated and taken as proxy for growth. Leaf colour (expressed as a percentage of all leaves on a tree or sapling that were green as opposed to yellow, brown or black) was assessed by visual estimation and used as a metric for leaf health and senescence, in accordance with Benner et al. (1990) and Duke et al. (2005). Trees and saplings that had no leaves (or on which all leaves were black and withered) were presumed to be dead, which served as a metric to determine survival. At the start of the monitoring, we tested for observer bias by repeating monitoring assessments of the same plots by different observers and found a maximum variability of 5% in leaf health estimates (and <2% in plant height measurements) between observers, with no significant difference in the overall assessment results per plot between observers.

Statistical analyses were completed using PRIMER software and the PERMANOVA+ add on to PRIMER (ver. 7, see https://www.primer-e.com/, accessed March 2021; Anderson et al. 2008). A univariate permutational multivariate analysis of variance (PERMANOVA) was completed to test for significant (P < 0.05) differences within plant height, plant survival and leaf health (percentage green) between each treatment by using a three-factor design: (1) Treatment: fixed, five levels (High 6 L, High 2 L, Low 6 L, Low 2 L and Control); (2) Time: fixed, four levels (1 week and 3, 7 and 12.5 months); and (3) Plot: nested within treatment, random, 10 levels (Control Plots 1 and 2 and Experimental Plots 1–8). Additional univariate PERMANOVAs were completed to assess for the effect of plant height, watering regime and tidal inundation on leaf health and plant survival. Plant height was tested using a four-factor design: (1) Treatment: fixed, five levels (High 6 L, High 2 L, Low 6 L, Low 2 L and Control); (2) Plant height: fixed, four levels (0–30, 31–60, 61–90 and >91 cm); (3) Time: fixed, four levels (1 week and 3, 7 and 12.5 months); and (4) Plot: nested within treatment, random, 10 levels (Control Plots 1 and 2 and Experimental Plots 1–8). Bed depth and watering regime were grouped within Treatment in plant height analysis for within and between Plot comparisons.

The effect of watering regime and tidal inundation was tested using four-factor design: (1) Bed depth: fixed, two levels (high and low); (2) Watering regime: fixed, two levels (6 and 2 L); (3) Time: fixed, four levels (1 week and 3, 7 and 12.5 months); and (4) Plot: nested within watering regime and bed depth, random, eight levels (Experimental Plots 1–8). Analyses to determine the effects of tidal inundation (bed depth) and watering regime were treated as individual factors to determine whether each treatment independently affected leaf health or plant survival. All designs including the nested variable plot were used to account for the repeated measurements of the same plants throughout the experiment. PERMANOVA comparisons with 9999 permutations were completed on all plant variables (Anderson et al. 2008) using a Euclidean distance dissimilarity matrix. Pairwise post hoc tests were used to explore significant (P < 0.05) interactions or differences obtained from the PERMANOVA. When there were insufficient permutations (<100) to conduct a rigorous statistical test, Monte Carlo bootstrapping was used to obtain a suitable P-value (PMC; Anderson et al. 2008). We chose to use PERMANOVAs because they are less susceptible to deviations from the assumptions that underlie parametric approaches in repeated-measures analyses of variance (ANOVAs). Our data had considerable variation between and within treatments due to mortality from transplanting that violated sphericity assumptions of a repeated-measures ANOVA. However, PERMANOVAs are largely unaffected by the correlation structure that is associated with sphericity (Anderson and Walsh 2013). The strength of the correlation between the number of pneumatophores per tree and their maximum distance from the stem was explored against mangrove height using regression analyses in RStudio (ver. 4.0.0, see https://rstudio.com/, accessed March 2021). All figures were produced using RStudio. Full statistical results are presented in the Supplementary material (Tables S1–S9).

In all, 300 mangroves were selected for relocation from the two construction sites (Fig. 1f), consisting of small trees and saplings, with a mean (±s.e.) height of 54.3 ± 15.0 cm (maximum 210 cm). Only 17 trees were taller than 1 m, and most (n = 204) were smaller than 55 cm (Fig. 2). The small size of these mangroves, which had been planted during multiple planting campaigns over the past 9 years, is typical of both natural and planted mangrove stands in the arid Arabian Gulf region (Al-Khayat and Balakrishnan 2014; Erftemeijer et al. 2020). Trees and saplings for relocation were similar in size to those in the two control plots, with comparable size frequency distribution at the start of the relocation program (Fig. 2). Mean tree height differed between the two control plots (69 v. 39 cm), but was within the range of mean tree heights (42–76 cm) recorded in the treatment plots immediately after transplanting (t = 0). Mean pneumatophore abundance ranged from 0.1 to 15.6 per tree at the control plots and from 0.2 to 6.6 per tree (after transplanting) at the relocation plots.

Fig. 2.  Size frequency distribution of the mangrove trees to be relocated before the start of relocation.

The root systems of the trees and saplings rarely extended beyond ~1 m from the stem and did not generally penetrate much deeper than 30 cm. There was a strong exponential relationship between lateral extension (R² = 0.697) and number of pneumatophores (R² = 0.724) with the size of the tree (Fig. 3). Mangroves smaller than 60 cm generally had few or no pneumatophores, but the number of pneumatophores increased markedly with tree size for mangroves taller than 60 cm. The maximum distance of pneumatophores from the stem was generally not more than tree height in the trees selected for relocation. Pneumatophore height ranged from a mean (±s.e.) of 12.5 ± 7.4 cm for smaller trees (n = 14) to 23.8 ± 9.8 cm (n = 17; overall mean 18.9 ± 10.4 cm).

Fig. 3.  Relationship between mangrove size and (a) the number of pneumatophores per tree and (b) their maximum distance (cm) from the stem.

The survival of mangrove trees and saplings was significantly less in experimental than control plots (P < 0.05), indicating relocation mortality (Table S1). The survival of mangroves showed a relatively consistent response to relocation across all plots, with an initial mean loss of 24% following excavation and transplantation (Week 1) and a further loss of 43% during the first 3 months after transplantation (Fig. 4). Mean survival stabilised at ~33% at 3 months after transplantation, and remained stable over the subsequent months, with 31% of the trees surviving after 12.5 months. After 7 months, only low levels of ‘new’ mortality were recorded in the treatment plots over time until the end of the experiment, similar to the levels of mortality observed in the control plots (Table S2). Survival of mangroves in shallow tidal plots (41%) was significantly (P < 0.05) greater than in deep plots (19%) (Tables S3, S4). There was no significant effect of initial plant height or watering regime on the survival of the relocated trees.

Fig. 4.  Mean (±s.e.) survival of the relocated mangroves for the different treatments compared with the controls over the 12.5-month monitoring period. High and Low treatments (bed depth) correspond to shorter (~3 h) or longer (~6 h) tidal inundation; mangroves were also watered daily with 2 or 6 L of fresh water.

A significant interaction between treatment and time was found in the leaf health of mangroves (Table S5). Mangroves in all experimental plots (except the High 6-L treatment) showed a significant decrease in the percentage of green leaves (P < 0.05) compared with the control plots immediately after transplanting, indicating stress associated with relocation (Fig. 5a; Table S6). The percentage of green leaves (indicative of the health of the surviving trees and saplings) increased during the last 5 months in all plots (Fig. 5a; Table S6). Initial plant height did not have a significant effect on the leaf health of the relocated trees (Table S5). However, there was significant variation in the leaf health of mangroves between and within different size classes and treatments (Tables S7, S8). Most notably, mangrove leaf health was significantly lower in Low 6-L treatment plots for every size class (0–30, 31–60, 61–90 and ≥91 cm), and in the High 2-L and High 6-L treatment plots for trees >91 cm compared with control plots (Tables S8, S9). There was a significant (P < 0.05) effect of duration of tidal inundation on leaf health, with the leaf health of mangroves in shallow tidal plots (34%), being significantly greater than that of mangroves in deep plots (12%; Tables S3, S4).

Fig. 5.  (a) Leaf health and (b) tree height of the relocated mangroves in the different treatment groups compared with the controls over the 12.5-month monitoring period. Data are the mean ± s.e.

At the start of the monitoring period immediately after transplanting (and in some cases pruning), mean (±s.e.) tree height in the experimental plots was comparable to that of the control plots (55.8 ± 0.7 v. 54.5 ± 2.7 cm respectively). Trees in the control plots showed a mean height increase of ~20 cm year^–1. Growth in the control plots showed marked seasonality (Fig. 6), with substantial growth during summer months but none during the cold winter months. Trees in the treatment plots did not show any significant mean height increase throughout the experiment. The height increase observed in trees in the treatment plots was variable, whereas the mean height of trees in five of the treatment plots decreased by up to 19 cm, possibly as a result of the death of some of the larger trees in these plots (Fig. 5b).

Fig. 6.  Mean (±s.e.) seasonality of growth of mangroves in control plots.

The overall height distribution of relocated mangroves (regardless of treatment) was similar to that of trees in the control plots during the first 7 months (Fig. 7). At the end of the 12.5-month monitoring period, the mean height of mangroves in the control plots had increased substantially due to summer growth, whereas there was no increase in the height of mangroves in the treatment plots (Fig. 7). Variations in the initial plant height had no significant effect on the growth of relocated trees.

Fig. 7.  Violin plots showing the height distribution of mangroves in (a) control plots and (b) experimental plots (all treatments taken together) over the 12.5-month monitoring period (including before and after relocation).

The abundance of pneumatophores in Control Plot 1 increased from a mean (±s.e.) of 15.6 ± 3.7 per tree at the start of the monitoring (t = 0) to 30.2 ± 7.1 per tree after 12.5 months. In all other plots, pneumatophore numbers were low (generally less than six per tree, and many with none, depending on size) and remained low until the end of the 12.5 months with no apparent effect of any of the treatments. No substantial flowering or fruiting was observed in any of the plots over the entire monitoring period, with only a few trees bearing fruits (up to a maximum of 3 of 40 trees (8%) in Control Plot 1 at the onset of monitoring).

This study investigated the feasibility of excavating and relocating mature mangrove trees and saplings during a salvage operation at an offshore island site in the Arabian Gulf. By comparing the results of the present study with those of a previously published evaluation of three decades of mangrove planting efforts using seedlings at the same site (Erftemeijer et al. 2020), we were able to consider the relative success, costs and benefits of salvaging large trees compared with the planting of new seedlings. Our results showed that when relocating mature mangrove trees, minimisation of root damage during excavation and ensuring appropriate tidal inundation at the recipient site were the most important factors determining survival, whereas the effects of initial tree size and freshwater treatment were not significant.

The outcome of this study demonstrated that large-scale relocation of mangrove trees is practically feasible, and that salvaging and replanting larger trees can be advantageous over the planting of new seedlings. Although the overall costs of the tree relocation were not dissimilar to those of the conventional planting of nursery-reared seedlings, the replanting of salvaged trees conferred greater benefits in terms of enhanced survival and faster provision of ecosystem services owing to their larger size.

The overall survival of relocated A. marina mangroves (31% after 1 year) was higher than the long-term survival (26%) of nursery-reared mangrove seedlings in earlier plantings at this same location (Erftemeijer et al. 2020) but relatively low compared with survival rates (<50–90%) reported from other geographic areas (Pulver 1976; Saenger 1996; Abbot and Marohasy 2014). The low survival observed in the present study is attributed to the environmentally extreme conditions in the Arabian Gulf region, further exacerbated by the hot summer conditions at the time of relocation (September 2019), with daily air temperatures up to ~48°C, which is likely to have further increased the water stress experienced by the trees and saplings during relocation.

The overall success of any mangrove relocation will be a function of multiple factors, related to both the method of excavation and replanting, as well as site conditions at the new transplanting site and other environmental and climatological constraints. Our results suggest that the following factors are particularly critical to ensure success: minimising root damage upon excavation, avoiding the extreme temperatures of summer (particularly in extreme climates such as the Gulf region) and ensuring optimal site suitability at the destination site (tidal hydrology, soil conditions).

Tidal hydrology, in particular the mean duration of inundation during high tide, is known to be an important variable determining site suitability for mangroves (Choy and Booth 1994; van Loon et al. 2016; Alsumaiti and Shahid 2019). Although the response of A. marina and other mangrove species to waterlogging is generally well understood (Kozlowski 1997; van Loon et al. 2007, 2016), their specific tolerance thresholds to the duration of tidal inundation may reflect site-specific factors (Friess et al. 2012). The effect of tidal inundation on the health and survival of the transplants in the present study is comparable to observations by Gorman and Turra (2016), who found differences in survival for mangrove seedlings established at sheltered v. exposed locations that reflected differences in tidal height and sediment grain size. Soil conditions are also known to be important, with particle size distribution (in particular the proportion of the coarse sand fraction), nutrient availability, the occurrence of an anaerobic layer in the profile, and surface layer salinity influencing both the establishment and growth of mangrove plants (Duarte et al. 1998; Bhat and Suleiman 2004).

Because field measurements of root biomass in mangroves are both labour intensive and difficult, there is considerable uncertainty regarding estimates of below-ground biomass in mangroves (Njana et al. 2015; Adame et al. 2017). Published data further suggest that there is substantial variation in root biomass in relation to environmental conditions (Adame et al. 2017). It is therefore difficult to give a reliable estimate of the extent of root loss as a result of the excavation of the mangrove trees and saplings for the present relocation. However, a considerable proportion (~34%) of the total below-ground biomass in A. marina is stored in the root crown (Njana et al. 2015). Given that the root crown of the excavated trees was mostly intact and large quantities of cable roots with pneumatophores were excavated successfully in most cases, the damage and loss of root mass in our study is estimated to have been <60% for most trees (depending on their size), and much less for saplings.

Our results did not reveal a significant effect of initial height of the transplanted mangroves on their survival. However, Saenger (1996) reported the survival rates of relocated mangroves in Queensland to be inversely proportional to tree size, finding that for plants >50 cm in height, survival rates fell below 50% within 1 month. This is believed to be primarily related to the fact that it takes longer for larger transplanted trees to re-establish a root : shoot ratio comparable to non-transplanted trees (Watson 2005). Rapid regeneration of the root system is essential for successful re-establishment of any transplanted trees (Watson and Himelick 1982). Transplanting stress is a temporary condition of distress resulting from root injuries and depletion due to impaired function (a reduction in the acquisition and assimilation of water and essential minerals and expenditure of stored carbohydrates to regenerate new roots). Similarly, ‘transplant shock’ is largely due to stresses resulting from the removal of a substantial portion of the transplanted tree root systems, which creates a root : shoot imbalance (Watson 2005). Moderate canopy pruning upon transplanting may alleviate this imbalance and the associated transplant stress, even in mangrove trees (Pulver 1976). Regardless of pruning, in most trees it takes ~1 year for the root system to regenerate to an extent where the shoot : root ratios and pretransplant growth rates are restored (Watson 2005).

Freshwater treatment in the present study did not have a significant effect on the health and survival of relocated mangroves. In fact, some of the data even seem to suggest that a high dosage of (daily) fresh water may have had an adverse effect on plant survival and health or growth, although this difference was not significant due to high variability in the data. Our findings contrast with physiological studies of A. marina by Steppe et al. (2018) and Fuenzalida et al. (2019), who found that leaf wetting events allowing direct uptake of fresh water by the canopy during episodic rainfall can be important for A. marina, especially in arid areas where rehydration from the soil is limited. The growth of A. marina seedlings is known to be adversely affected by high salinity, showing higher growth at reduced salinities, with optimum growth and biomass observed around a salinity of ~15 (Patel et al. 2010; Nguyen et al. 2015; Santini et al. 2015). The mean height increase of trees (~20 cm year^–1) measured in control plots in the present study are well within the range of growth rates reported for A. marina from other locations within the arid Arabian Gulf region (Bhat et al. 2004; Loughland et al. 2020). The slow growth (or even reduction in height) recorded in the experimental plots in the present study is attributed to the stress of excavation and transplantation, and the priority for the trees to first restore their severed below-ground root biomass following the relocation (sensu Watson and Himelick 1982).

Although the results of this study and some previous literature may suggest that relocation of mangrove trees and shrubs is technically feasible with reasonable survival rates, it is always preferable to avoid relocation in the first place. In this context, a common principle that is widely applied in environmental considerations of development projects is ‘mitigation hierarchy’ (avoid-minimise-remediate-offset) to guide development activities towards limiting negative effects on biodiversity (Arlidge et al. 2018). It would be wrong to think that mangrove trees can be easily moved and to use that as an excuse to approve developments that would result in widespread losses of mangroves rather than explore non-destructive alternatives. Salvaging should only be considered as a last resort, such as in situations where damaging effects on mangroves or other valuable habitats cannot be avoided, or the loss of threatened species is imminent (Silcock et al. 2019).

This also poses the question whether it would make more sense to plant new mangroves from nursery-raised seedlings (e.g. Loughland et al. 2020) rather than relocate the older trees and saplings. Does size matter? In laboratory studies in Queensland, under ideal tropical conditions mangrove propagules planted in laboratory pots outgrew well-developed saplings that had been translocated from the field into similar laboratory pots within 2 years (N. Duke, pers. comm.). However, in arid regions (including the Arabian Gulf), where soils are nutrient poor and mangroves are typically dwarfed and very slow growing (Naidoo 2009), it may take well over 10 years for the seedlings to reach heights comparable to natural mangrove stands (AboEl-Nil 2001; Ochieng and Erftemeijer 2002; Bhat et al. 2004; Almahasheer et al. 2016, Erftemeijer et al. 2018, 2020).

Some advantages of larger trees and saplings over seedlings may include that they are less likely to be uprooted and washed away, they offer a promise of greater and faster shoreline protection and they are more likely to rapidly serve as a source of propagules for natural recruitment and further replanting. All these should be given due consideration when evaluating the cost-effectiveness of relocation (older trees and saplings) v. new plantations (nursery-reared seedlings). Apart from some expert technical guidance (10 days) and the involvement of a backhoe excavator (5 days), the costs of relocating trees in this project were not really different from the costs of planting new seedlings because the same channel preparation, man-power and time investment were involved. It should be noted that access to heavy equipment is rarely a constraint within the context of larger construction projects, and the costs for relocation may constitute only a fraction of the overall project construction costs.

In conclusion, this study of relocating 300 mangroves in extreme environmental conditions highlighted two factors that were paramount to success: minimisation of root damage during excavation and ensuring site suitability at the replanting site, particularly the appropriate tidal hydrological conditions. Our findings further demonstrate that the salvaging of adult mangrove trees is worthwhile, especially because relocated larger trees may provide greater and faster ecosystem services, adding another viable option to the usual approaches of planting propagules and nursery-reared seedlings (Vanderklift et al. 2020) that are currently used across the globe to achieve marine coastal restoration (www.decadeonrestoration.org).

The authors declare that they have no conflicts of interest.

This study was funded by the Abu Dhabi Oil Co. (Japan) Ltd.