Mangroves, woody plants inhabiting the interface between land and sea in tropical and subtropical regions (Chapman 1976Chapman V.J. 1976. Mangrove Vegetation. J. Cramer, Vaduz, Germany.) in more than 100 countries, cover more than 81 million ha worldwide (Hamilton and Casey 2016Hamilton S. E., Casey D. 2016. Creation of a high spatio-temporal resolution global database of continuous mangrove forest cover for the 21st century (CGMFC-21). Glob. Ecol. Biogeogr. 25: 729-738. https://doi.org/10.1111/geb.12449 ). They provide many ecological services, acting as nursery grounds, a thermal refuge from ocean warming and a foraging habitat for biota (Mumby et al. 2004Mumby P.J., Edwards A.J., Arias-González J.E., et al. 2004. Mangrove enhance the biomass of coral reef fish communities in the Caribbean. Nature 427: 533-536. https://doi.org/10.1038/nature02286 , Giomi et al. 2019Giomi F., Barausse A., Duarte C. M., et al. 2019. Oxygen supersaturation protects coastal marine fauna from ocean warming. Sci. Adv. 5: eaax1814. https://doi.org/10.1126/sciadv.aax1814 , Macreadie et al. 2019Macreadie P. I., Anton A., Raven J. A., et al. 2019. The future of Blue Carbon science. Nat. Commun. 10: 3998. https://doi.org/10.1038/s41467-019-11693-w ). In the last two decades, there has been a drastic decrease in mangrove cover of up to 20% globally and 50% in Southeast Asia, mainly because of massive urban development and increased land usage for aquaculture purposes (Thomas et al. 2017Thomas N., Lucas R., Bunting P., et al. 2017. Distribution and drivers of global mangrove forest change, 1996-2010. PLoS ONE 12: e0179302. https://doi.org/10.1371/journal.pone.0179302 ). One of the few exceptions to this declining trend is the Red Sea coast of Saudi Arabia, wherein mangrove cover has been increasing, mainly thanks to a series of replantation programmes (Almahasheer et al. 2016aAlmahasheer H., Aljowair A., Duarte C.M., Irigoien X. 2016a. Decadal stability of Red Sea mangroves. Estuar. Coast. Shelf. Sci. 169: 164-172. https://doi.org/10.1016/j.ecss.2015.11.027 ). The diversity of mangroves in the Red Sea is limited to two species, Avicennia marina and Rhizophora mucronata, the former of which shows the widest distribution (Price et al. 2007Price A.R.G., Medley P.A.H., McDowall R.J., et al. 2007. Aspects of mangal ecology along the Red Sea coast of Saudi Arabia. J. Nat. Hist. 21: 449-464. https://doi.org/10.1080/00222938700771121 ). This low diversity of mangrove species in the region is likely driven by the extreme environmental conditions, including high temperature, high salinity and ultra-oligotrophy of the ambient seawater and the sediments (Garcias-Bonet et al. 2019Garcias-Bonet N., Delgado-Huertas A., Carillo-de-Albornoz P., et al. 2019. Carbon and Nitrogen Concentrations, Stocks, and Isotopic Compositions in Red Sea Seagrass and Mangrove Sediments. Front. Mar. Sci. 6: 267. https://doi.org/10.3389/fmars.2019.00267 , Saderne et al. 2019Saderne V., Baldry K., Anton A., et al. 2019. Characterization of the CO2 System in a Coral Reef, a Seagrass Meadow, and a Mangrove Forest in the Central Red Sea. J. Geophys. Res. Oceans 124: 7513- 7528. https://doi.org/10.1029/2019JC015266 , Anton et al. 2020Anton A., Almahasheer H., Delgado A., et al. 2020. Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation. Front. Mar. Sci. 7: 597. https://doi.org/10.3389/fmars.2020.00597 ), all of which can limit mangrove survival, growth and height (Bernstein and Hayward 1958Bernstein L., Hayward H.E. 1958. Physiology of salt tolerance. Annu. Rev. Plant Physiol. 9: 25-46. https://doi.org/10.1146/annurev.pp.09.060158.000325 , Clarke and Hannon 1970Clarke L., Hannon N. 1970. The Mangrove Swamp and Salt Marsh Communities of the Sydney District: III. Plant Growth in Relation to Salinity and Waterlogging. J. Ecol. 58: 351-369. https://doi.org/10.2307/2258276 , Burchett et al. 1984Burchett M.D., Field C.D., Pulkownik A. 1984. Salinity, growth and root respiration in the grey mangrove, Avicennia marina. Physiol. Plant. 60: 113-118. https://doi.org/10.1111/j.1399-3054.1984.tb04549.x ).
Mangrove seedlings are influenced by the available nutrient supply and oxygenation at the root level (Ball 1988aBall M.C. 1988a. Ecophysiology of mangroves. Trees 2: 129-142. https://doi.org/10.1007/BF00196018 , Krauss et al. 2006Krauss K.W., Lovelock C.E., McKee K.L., et al. 2006. Environmental drivers in mangrove establishment and early development: A review. Aquat. Bot. 89: 105-127. https://doi.org/10.1016/j.aquabot.2007.12.014 ). Concomitantly, mechanical factors such as burrows by crabs, mainly of the orders Sesarmidae and Oocypodidae (Lee 1999Lee S.Y. 1999. Tropical mangrove ecology: Physical and biotic factors influencing ecosystem structure and function. Austral Ecol. 24: 355-366. https://doi.org/10.1046/j.1442-9993.1999.00984.x , Kristensen 2008Kristensen E. 2008. Mangrove crabs as ecosystem engineers; with emphasis on sediment processes. J. Sea Res. 59: 30-43. https://doi.org/10.1016/j.seares.2007.05.004 ), have been shown to promote seedling growth by increasing soil aeration and nutrient-holding capacity (Ridd 1996Ridd P.V. 1996. Flow through animal burrows in mangrove creeks. Estuar. Coast. Shelf Sci. 43: 617-625. https://doi.org/10.1006/ecss.1996.0091 , Lee 1998Lee S.Y. 1998. Ecological role of grapsid crabs in mangrove ecosystems: a review. Mar. Freshw. Res. 49: 335-343. https://doi.org/10.1071/MF97179 , Gribsholt et al. 2003Gribsholt B.G., Kostka J.E., Kristensen E. 2003. Impact of fiddler crabs and plant roots on sediment biogeochemistry in a Georgia saltmarsh. Mar. Ecol. Prog. 259: 237-251. https://doi.org/10.3354/meps259237 ). Tidal inundation, which promotes root ventilation and nutrient availability (Ball 1988aBall M.C. 1988a. Ecophysiology of mangroves. Trees 2: 129-142. https://doi.org/10.1007/BF00196018 , Krauss et al. 2006Krauss K.W., Lovelock C.E., McKee K.L., et al. 2006. Environmental drivers in mangrove establishment and early development: A review. Aquat. Bot. 89: 105-127. https://doi.org/10.1016/j.aquabot.2007.12.014 ), is also a critical factor for the growth and survival of mangrove seedlings (Smith 1987Smith T.J. 1987. Effects of light and intertidal position on seedling survival and growth in tropical tidal forests. J. Exp. Mar. Biol. Ecol. 110: 133-146. https://doi.org/10.1016/0022-0981(87)90024-4 ). In fact, the structuring of species zonation with diverse mangrove communities is attributed to this mechanical process (Krauss et al. 2006Krauss K.W., Lovelock C.E., McKee K.L., et al. 2006. Environmental drivers in mangrove establishment and early development: A review. Aquat. Bot. 89: 105-127. https://doi.org/10.1016/j.aquabot.2007.12.014 ). However, despite its significance, studies on the influence of tidal inundation in the early development of mangroves are few, both in situ in the field and under laboratory conditions (Krauss et al. 2006Krauss K.W., Lovelock C.E., McKee K.L., et al. 2006. Environmental drivers in mangrove establishment and early development: A review. Aquat. Bot. 89: 105-127. https://doi.org/10.1016/j.aquabot.2007.12.014 ), and no studies have been performed to date in the Red Sea. Thus, the influence of tidal flooding on seedling growth in this region merits further investigation.
The effect of sandstorms on the growth and survival of mangrove seedlings has as yet received very little attention (Tamaei 2005Tamaei S. 2005. Study of gray mangrove (Avicennia marina) afforestation for greening of desert coasts: Gray mangrove afforestation on banks of artificial channel across a sabkha and the established biotic community. Jpn. J. Ecol. 55: 1-9.), despite the negative consequences that have been shown for mangrove seedling survival, growth and tree species richness as a result of varying exposure to burial and soil accretion (Ellison 1999Ellison J.C. 1999. Impacts of sediment burial on mangroves. Mar. Pollut. Bull. 37: 420-426. https://doi.org/10.1016/S0025-326X(98)00122-2 , Thampanya et al. 2002Thampanya U., Vermaat J.E., Terrados J. 2002. The effect of increasing sediment accretion on the seedlings of three common Thai mangrove species. Aquat. Bot. 74: 315-325. https://doi.org/10.1016/S0304-3770(02)00146-8 ). In the Arabian Peninsula, sandstorms occur naturally and frequently, and one study indicates negative effects (e.g. mortality) of a sandstorm on the grey mangrove A. marina (Tamaei 2005Tamaei S. 2005. Study of gray mangrove (Avicennia marina) afforestation for greening of desert coasts: Gray mangrove afforestation on banks of artificial channel across a sabkha and the established biotic community. Jpn. J. Ecol. 55: 1-9.). In contrast, some mangrove species are able to develop morpho-anatomical adaptations that enhance life recovery after burial exposure (Okello et al. 2020Okello J.A., Kairo J.G., Dahdouh-Guebas F., et al. 2020. Mangrove trees survive partial sediment burial by developing new roots and adapting their roots, branch and stem anatomy. Trees 34: 37-49. https://doi.org/10.1007/s00468-019-01895-6 ). In addition, a recent study (Cusack et al. 2020Cusack M., Arrieta J.M., Duarte C.M. 2020. Source Apportionment and Elemental Composition of Atmospheric Total Suspended Particulates (TSP) Over the Red Sea Coast of Saudi Arabia. Earth Syst. Environ. 4: 777-788. https://doi.org/10.1007/s41748-020-00189-z ) identified that dust particles in the Red Sea were enriched in iron and phosphorus, which may provide a vital source for oligotrophic coastal mangrove stands in the Red Sea (Anton et al. 2020Anton A., Almahasheer H., Delgado A., et al. 2020. Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation. Front. Mar. Sci. 7: 597. https://doi.org/10.3389/fmars.2020.00597 ).
To the best of our knowledge, the only study that has been conducted so far on mangrove seedling growth on the Red Sea coasts is a laboratory study that found that the growth of mangroves in the central Red Sea is likely driven by iron limitation (Almahasheer et al. 2016bAlmahasheer H., Duarte C.M., Irigoien X. 2016b. Nutrient limitation in central Red Sea mangroves. Front. Mar. Sci. 3: 271. https://doi.org/10.3389/fmars.2016.00271 ). In addition, Anton et al. (2020)Anton A., Almahasheer H., Delgado A., et al. 2020. Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation. Front. Mar. Sci. 7: 597. https://doi.org/10.3389/fmars.2020.00597 indicated that the short height of adult A. marina trees in the central Red Sea is likely driven by nitrogen limitation in the area. The scant information on natural mangrove habitats in the Red Sea, especially at early stages, led us to carry out this study, in which we estimated in situ growth rate of A. marina seedlings in a relatively pristine environment in the region of Baish, Jizan, in the southern Saudi Arabian Red Sea, in relation to tidal flooding intensity and sandstorms. Specifically, this study aimed to (1) estimate and compare the seedling growth rate at different tidal exposures and in different sampling periods, and (2) investigate the effects of burial (e.g. during a sandstorm) on the early development of mangrove trees.
MATERIALS AND METHODS⌅
The mangrove stands in the northern region of the semi-enclosed bay at Baish in the southern Red Sea (Saudi Arabia, coordinates 17.34904°N 42.32134°E; Fig. 1) have been relatively unaffected by human activities. The environment in this area is hot and arid, and dead and desiccated mangroves are commonly found adjacent to the northernmost part of the mangrove forest. This area experiences a natural gradient of tidal inundation, which is generally lower than in other parts of the bay because of its proximity to the northernmost portion (Fig. 1). The northern easterly winds expose the site to wind stress and dust deposition. Rainfall rarely occurs in the area and is usually associated with thunderstorms. Sandstorms are common in the area particularly in summer (Arishi 2021Arishi A.A. 2021. Classification of Sandstorms in Saudi Arabia. Atmos. Clim. Sci. 11: 177-193. https://doi.org/10.4236/acs.2021.111012 ). Sabkhas (or supra-tidal flats) are found north of the site and are composed mainly of sand and mud. Seawater temperature and salinity in the study site during the study period ranged from 35°C to 37°C and 46.9 to 53.01, respectively.
Data from March to September 2019 within the vicinity of the mangrove site suggested that the mean atmospheric temperature was 38.1°C, with values ranging from 31.7°C (March) to 42.5°C (June); mean relative humidity was 88.1%, with values ranging from 85.2% (March) to 97.2% (August); mean precipitation was 29.2 mm, with values ranging from 0 to 360.6 mm (August).
The study site was subdivided into four distinct ecological zones based on qualitative observations of tidal exposure and fiddler crab burrow density: namely, Z1, low tidal exposure; Z2, medium tidal exposure; Z3, high tidal exposure with numerous crab burrows; and Z4, high tidal exposure with a few or no crab burrows (Fig. 1). The zones were qualitatively classified by tidal exposure based on their distance to the nearest tidal inundation and the observed number of fiddler crab burrows. The only vegetation on the site is the grey mangrove A. marina. The qualitative characteristics of the sediment varied across zones from sand (Z1), to sandy-muddy (Z2, Z3 and Z4).
Newly sprouted and naturally grown seedlings of A. marina from these zones, encompassing 30 sampling locations, were randomly chosen and identified with wooden markers (see Fig. 2), and all seedlings were considered for measurements. Seedling height was measured from the soil surface to the tip of the seedling for all samples (n=53) using a plastic ruler to the nearest centimetre. The study was conducted over a period of three months in the summer of 2017, from 28 April to 27 July. Measurements of seedling height and survival were performed on days 0, 14, 30, 38 and 88.
Measurements of air temperature, relative humidity, wind speed and direction, and dust concentration suspended in the air were conducted on a weekly basis at a permanent station close to the mangroves (Fig. 1). Estimates of weather conditions such as dry bulb temperature (T, °C), relative humidity (RH, %), and wind speed (WS, m s-1) and direction were recorded using a Kestrel 4500 Pocket Weather Tracker. The wind direction was identified in degrees (°) measured from the north clockwise. Estimates of dust particle concentration (PM10 and PM2.5) were obtained using a portable dust monitor (PC-3016A) from a fixed station with a dust meter installed near the mangrove forest (17.336617°N, 42.330517°E) at a height of approximately 1.5 m.
An increase in seedling height between sampling periods was used to calculate growth rates. One-way analysis of variance (ANOVA) was used to test the significance of different growth rates between the four zones across measurement periods (days 14, 30, 38, 88), survival rate and the selected environmental variables. The growth rate was calculated by the absolute difference of two height measurements divided by the number of days between two height measurements. Prior to ANOVA, data were examined for normality and homogeneity of variance. Statistical differences between specific zones and sampling periods were assessed using a Tukey post hoc test.
Table 1 shows the increment (cm), duration (d), growth rate (cm d-1) and location of mangrove seedlings. Site Z3 showed the highest growth rate (0.34 cm d-1), followed by Z4 (0.23 cm d-1), Z2 (0.12 cm d-1) and Z1 (0.01 cm d-1) (Fig. 2A). Sampling zones differed significantly in growth rates (Table 2). Values at Z3 were significantly higher than at Z1 and Z2, but not from those at Z4 (Fig. 2A). Also, values at Z4 were significantly higher than those at Z1 and Z2 (Fig. 2A). Seedling growth rates showed a decreasing trend over time and were significantly different between sampling periods (Table 2) (Fig. 2B). The growth rate of day 14 (0.38 cm d-1) was significantly different from that of days 30 (0.17 cm d-1), 38 (0.1 cm d-1) and 88 (0.03 cm d-1) (p<0.05) (Fig. 2B). Furthermore, the survival rate showed a slight reduction from day 14 to day 38 but a significant reduction on day 88 (Fig. 2C, Table 2). The only seedling survivors were from Z3 (Fig. 3C) with a total of 5.6% (n=3) left from the initial samples. The significant decrease in survival was attributed to burial by sandstorm (Fig. 4).
|Shoot height (cm)|
|Zone||Seedling||Latitude||Longitude||28-Apr-17||12-May-17||28-May-17||5-Jun-17||27-Jul-17||Increment (cm)||Total days of alive seedlings (d)||Growth rate (cm d-1)|
|Variables||Sum of squares||df||Mean square||F||Sig.|
|Growth rate by zone||Between groups||0.37205||3||0.124018||14.11||0.0000027|
|Growth rate by period*||Between groups||0.96665||2||0.48332||32.52||0.0000|
|Survival rate (%)||Between groups||19064.1475||3||6354.72||24.2199||0.000022|
|Temperature (°C)||Between groups||17.55||3||5.85||1.801||0.201|
|Relative humidity (%)||Between groups||513.34||3||171.11||1.664||0.227|
|Wind speed (m s-1)||Between groups||2.92||3||0.97||0.917||0.462|
|Wind direction (degrees)||Between groups||6075.50||3||2025.17||0.989||0.431|
|PM2.5 (µg m-3)||Between groups||12013.70||3||4004.57||2.71||0.092|
|PM10 (µg m-3)||Between groups||116843.45||3||38947.82||2.446||0.114|
* Adjusted sum of squares for tests
Air temperature showed an increasing trend from May to July, rising by 2.4°C (Fig. 3A). By contrast, relative humidity showed a decreasing trend from May to July (Fig. 3B). Wind speed seemingly displayed an indistinguishable pattern, although the highest value was recorded in June (Fig. 3C). The wind consistently blows between 200° and 250° from the north (Fig. 3D). No sandstorms generally occurred between April and June, but they showed a marked increase in July (Fig. 3E, F). All the selected environmental variables showed no significant differences between sampling periods (Table 2).
Our study highlights significant aspects of the growth dynamics and fate of mangrove Avicennia marina seedlings, using in situ field observations in a pristine mangrove forest in the southern Red Sea of Saudi Arabia. We showed that increased exposure of seedlings to tidal inundation could increase growth performance, while a cumulative exposure of seedlings to an intense sandstorm can have a detrimental effect on their survival by burial.
The variation of seedling growth rates can be attributed to the differences in exposure to tidal inundation. This variation can be explained by (1) water and nutrient supply, (2) air temperature and (3) salinity. Firstly, tidal inundation increases water supply and enhances soil moisture and water absorption, which the seedlings need for their growth, especially in an arid environment (Bernstein and Hayward 1958Bernstein L., Hayward H.E. 1958. Physiology of salt tolerance. Annu. Rev. Plant Physiol. 9: 25-46. https://doi.org/10.1146/annurev.pp.09.060158.000325 , Ball 1988bBall M.C. 1988b. Salinity tolerance in the mangroves Aegiceras corniculatum and Avicennia marina. I. Water use in relation to growth, carbon partitioning, and salt balance. Aust. J. Plant Physiol. 15: 447-464. https://doi.org/10.1071/PP9880447 ). In addition, the availability of nutrients carried by the tide could be a necessary factor for seedling growth owing to the oligotrophic condition of the Red Sea (Almahasheer et al. 2016bAlmahasheer H., Duarte C.M., Irigoien X. 2016b. Nutrient limitation in central Red Sea mangroves. Front. Mar. Sci. 3: 271. https://doi.org/10.3389/fmars.2016.00271 , Anton et al. 2020Anton A., Almahasheer H., Delgado A., et al. 2020. Stunted Mangrove Trees in the Oligotrophic Central Red Sea Relate to Nitrogen Limitation. Front. Mar. Sci. 7: 597. https://doi.org/10.3389/fmars.2020.00597 ). Secondly, low tidal exposure and increasing air temperature probably act towards seedling dehydration, as likely happened at Z1 (Hastenrath and Lamb 1979Hastenrath S., Lamb P.J. 1979. Climatic Atlas of the Indian Ocean, part 2. The ocean heat budget. University of Wisconsin Press, Madison., Clough 1992Clough B. E. 1992. Primary productivity and the growth of mangrove forests. In: Robertson A.I., Alongi D.M. (eds), Tropical Mangrove Ecosystems, pp. 225-250. American Geophysical Society, Washington DC, USA., Rasul et al. 2015Rasul N.M.A., Stewart I.C.F., Nawab Z.A. 2015.Introduction to the Red Sea: Its Origin, Structure and Environment. In: Rasul N.M.A., Stewart I.C.F. (eds), The Red Sea. Springer Earth Syst. Sci. https://doi.org/10.1007/978-3-662-45201-1 ). For example, the negative growth rate observed in seedlings 16A, 20B and 30 (Table 1), which all belonged to Z1, may be attributed to dehydration resulting in the bending of the overall structure of the plant. Lastly, lower tidal inundation increases the exposure of the seedlings to high evaporation, which may tend to increase soil salinity owing to a reduced water volume and dilution. Higher salinity may force seedlings to undergo physiological stress, which could affect their optimal growth (Clarke and Hannon 1970Clarke L., Hannon N. 1970. The Mangrove Swamp and Salt Marsh Communities of the Sydney District: III. Plant Growth in Relation to Salinity and Waterlogging. J. Ecol. 58: 351-369. https://doi.org/10.2307/2258276 , Burchett et al. 1984Burchett M.D., Field C.D., Pulkownik A. 1984. Salinity, growth and root respiration in the grey mangrove, Avicennia marina. Physiol. Plant. 60: 113-118. https://doi.org/10.1111/j.1399-3054.1984.tb04549.x , Kraus et al. 2006Krauss K.W., Lovelock C.E., McKee K.L., et al. 2006. Environmental drivers in mangrove establishment and early development: A review. Aquat. Bot. 89: 105-127. https://doi.org/10.1016/j.aquabot.2007.12.014 ).
Sandstorms are an environmental stressor for the mangrove seedlings, affecting the survival of the plant through burial (Balke et al. 2011Balke T., Bouma T.J., Horstman E.M., et al. 2011. Windows of opportunity: thresholds to mangrove seedling establishment on tidal flats. Mar. Ecol. Prog. Ser. 440: 1-9. https://doi.org/10.3354/meps09364 ). The present study warrants their inclusion in the list of previously identified natural stressors of mangroves in the Red Sea. Indeed, it has been shown that the deposition of sand results in intense burial and consequent suffocation of mangrove seedlings, probably leading to stunted growth and/or mortality (Ellison 1999Ellison J.C. 1999. Impacts of sediment burial on mangroves. Mar. Pollut. Bull. 37: 420-426. https://doi.org/10.1016/S0025-326X(98)00122-2 ). Accordingly, Tamaei (2005)Tamaei S. 2005. Study of gray mangrove (Avicennia marina) afforestation for greening of desert coasts: Gray mangrove afforestation on banks of artificial channel across a sabkha and the established biotic community. Jpn. J. Ecol. 55: 1-9. enumerated sandstorms and sand accumulation as one of the leading causes of mangrove A. marina seedling mortality. We observed a decreasing seedling growth rate that aligned with increasing sandstorms during the course of our investigation. Since seedlings exhibit a premature root system that does not provide a strong anchorage to the ground, they are likely vulnerable to physiological stress caused by the combination of sandstorms and strong winds (Balke et al. 2011Balke T., Bouma T.J., Horstman E.M., et al. 2011. Windows of opportunity: thresholds to mangrove seedling establishment on tidal flats. Mar. Ecol. Prog. Ser. 440: 1-9. https://doi.org/10.3354/meps09364 ).
The higher seedling growth rate between sites Z3 and Z4 can be explained by the observed presence of burrows. Burrowing crabs may increase seedling growth by enhancing aeration of the soil (Lee 1998Lee S.Y. 1998. Ecological role of grapsid crabs in mangrove ecosystems: a review. Mar. Freshw. Res. 49: 335-343. https://doi.org/10.1071/MF97179 , Smith et al. 1991Smith T.J., Boto K.G., Frusher S.D., Giddins R.L. 1991. Keystone species and mangrove forest dynamics: the influence of burrowing by crabs on soil nutrient status and forest productivity. Estuar. Coast. Shelf Sci. 33: 419-432. https://doi.org/10.1016/0272-7714(91)90081-L ). Burrowing also creates networks for water flow below the surface, allowing diffusion of nutrients and oxygen across a swamp-bed interface (Ridd 1996Ridd P.V. 1996. Flow through animal burrows in mangrove creeks. Estuar. Coast. Shelf Sci. 43: 617-625. https://doi.org/10.1006/ecss.1996.0091 ). Smith et al. (2009)Smith N.F., Wilcox C., Lessman J.M. 2009. Fiddler crab burrowing affects growth and production of the white mangrove (Laguncularia racemosa) in a restored Florida coastal marsh. Mar. Biol. 156: 2255-2266. https://doi.org/10.1007/s00227-009-1253-7 found that the burrowing of fiddler crabs enhances mangrove height by 27%. In our study, we found no significant differences that were due to the presence/absence of borrows, perhaps because the number of burrows at Z3 (high tidal exposure with numerous crab burrows) did not reach the minimum threshold for the burrows to make a difference in seedling growth. Unfortunately, our empirical approach was qualitative, as we did not quantify the number of burrows at Z3 and Z4, and further investigation is warranted.
The survival of Z3 seedlings could be attributed to the fact that existing mangrove trees act as a buffer against sediment resuspension and deposition caused by strong winds. The wind direction was generally 200° to 250° from the north, and all three zones except Z3 were exposed. Also, the exposure of Z3 seedlings to tidal inundation could also help reduce the impact of sediment burial by washing off the settling sediments into the seawater, especially during flood tide.
After a brief field exploration of seedlings in situ in the arid environment in the southern Red Sea, we observed that there was a correlation between growth of the seedlings and their location and growth period (age). The differences in seedling growth according to location were likely related to exposure to tidal inundation (i.e. higher growth with higher exposure to tidal inundation). On the other hand, the differences in seedling growth according to age, which showed a decreasing trend over the course of the study, could be related to their prolonged exposure to high air temperature, humidity and salinity. Interestingly, the severe decrease in survival of the seedlings in this area coincided with exposure to strong winds, sandstorms and dust deposition. These climatic conditions together resulted in a massive burial of the seedlings, which can be considered an important natural stressor in this area.
Therefore, while tidal exposure is important for the management of A. marina seedlings in arid regions such as that in the study area, the consideration of the above factors is crucial, especially for devising strategies for effective restoration and conservation of mangroves.
DECLARATION OF INTEREST⌅
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.