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ASSESSMENT OF SALINITY OF GROUNDWATER IN SWAMP TARO (CYRTOSPERMA CHAMISSONIS) "PULAKA" PITS IN TUVALU
Abstract and Figures
Pits on all islands of Tuvalu (except Niulakita) were surveyed between January and April 2006. At the time of study, only three islands (Nukulaelae, Niutao and Funafuti) had pits which showed salinity concentrations thought to be too high for successful swamp taro growth, (≥ 3 000, 4 000 and 5 000 μS cm–1, respectively). In the case of both Nukulaelae and Niutao these high readings were restricted to one pit area. Otherwise, conductivity readings in the remaining pits on these islands were generally low (≤1 000 μS cm–1) and adequate for swamp taro growth. There is anecdotal evidence to suggest that causeway engineering activities in Niutao’s central lagoon may have contributed to higher salinity in localised areas. Alternatively, sampling in Funafuti (Fongafale) showed that all pits were either too saline or very marginal and swamp taro production is unlikely to succeed anywhere on Fongafale islet.
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... The question of whether or not giant swamp taro is salt tolerant is a controversial issue. Some researchers have stated it as a salt tolerant crop (Dunn, 1976;Ivancic, 1992;320 Hather, 2000;Thaman, 2002), whereas others argue that its tolerance is limited (Webb, 2007). As this crop is so important to atoll communities for food security, and for its linkages with local tradition and culture, it is essential that this question gets addressed (Thaman, 2002;Iese, 2005;Manner, 2009). ...
... The highest groundwater salinity level recorded in Tuvalu in 2007 (Webb, 2007) was 4,252 µScm which is about 2-3 ppt (0.2-0.3% salinity) and in 2010 (Rao, 2010) 14,790 µScm which is 5-15 ppt (0.5-1.5%). Therefore, it appears that the cultivars 'Ika raoi' and 'Katutu' are relatively salt tolerant cultivars, as they can tolerate salinity more than the current ground water salinity levels detected in Tuvalu. ...
Low-lying islands in the Pacific, such as atolls, are observing increased groundwater salinity, attributed to the impact of climate change, which is affecting the production of giant swamp taro (Cyrtosperma merkusii). Giant swamp taro has been cultivated successfully in the atolls providing food and nutritional security for atoll communities. In addition, this unique crop has high cultural status. Despite the importance of this crop to the atoll communities and the Pacific in general very little research has been carried out on this species. Little is known about the extent of diversity and any variability in salinity tolerance that might exist. To better understand whether or not salinity tolerance exists within the genepool of swamp taro and the extent to which it might vary, a rapid screening method is needed. This paper reports on the diversity in the Pacific with specific reference to the island of Tuvalu. It also describes an in vitro screening method, which assesses the impact of salinity levels of 0, 0.5, 1.5 and 2% salt on two cultivars from Kiribati, 'Ika raoi' (larger cultivar) and the 'Katutu' (smaller cultivar). Two approaches are discussed, one in which "saltwater" is applied on an increment basis to avoid shock to the plant, and to mimic, as much as possible, inundation; the other incorporates the salt directly into the culture medium. The criteria for monitoring the experiment include biomass measurements and visual toxicity responses. Plants showed high tolerance to salinity in the first four weeks of the experiment but exhibited slight toxicity from the fifth week.
... It is the main source of water supply that supports atoll vegetation. Climate change related sea level rise greatly threatens this ground water lens by means of salt water intrusion and sea water inundation, both of which result in increased groundwater salinity levels (Dunn, 1976; Woodraffe, 1989 Woodraffe, & 2008 White et al, 1999; Webb, 2007; White and Falkland, 2010). This increase in turn, threatens the food security of the atoll islands as crop production which is greatly reduced. ...
... This particular aroid is known for its ability to survive harsh atoll environments. However, it is currently also threatened by the increased salinity levels in the ground water lens (White et al, 1999; Webb, 2007; White and Falkland, 2010). The loss of this aroid would not only mean reduced food security on the atolls, but also a loss of their identity. ...
Giant Swamp Taro (Cyrtosperma merkusii) is a staple food crop in the Pacific, especially in the low lying atoll islands such as Tuvalu and Kiribati. This is owing to its ability to survive under poor soil conditions and harsh environments. However, as a result of the effects of climate change such as sea water inundation and intrusion into the fresh ground water lens, this crop is now under threat. To address this issue an adaption approach was taken whereby, Cyrtosperma merkusii was screened in vivo for salt tolerance. The epistemology followed random selection of two cultivars Ikaraoi and Katutu. These two cultivars were subjected to 0% (0 parts per trillion), 0.5% (5 ppt), 1% (10 ppt), 1.5% (15 ppt) and 2% (20 ppt) of salt in Yates's advance seedling common potting mix. Both cultivars were able to tolerate salinity levels up-to 5ppt which is significantly more than the salt tolerance in glycophytes of 2.83 ppt. This research provides an insight into the variation of salt tolerance that may exist in C.merkusii gene pool, which can be used to adapt to natural disasters and buffer its impacts.
... Reviews of salt tolerance in coconut and citrus (lemon and lime) trees have, respectively, been carried out by Carr (2011) and Storey and Walker (1998). Salt tolerance in giant swamp taro has been studied by Mourits (1996), Rao (2011) and Webb (2007). ...
Societal Impact Statement
With global weather patterns becoming more extreme and unpredictable, sourcing reliable irrigation water is vital for improving food security and conserving drinking water in drought‐prone areas. Emerging desalination technologies, which are still in the development phase, could potentially provide large quantities of slightly saline water for irrigation. However, we must first ensure that any benefits of alleviating drought using this water outweigh the negative impacts of salt stress. We examine the viability of such a system, using Tonga as a case study, with the aim of advocating for future use of emerging desalination technologies for irrigation in low‐ and middle‐income countries.
Summary
Emerging desalination technologies have the potential to be a cheap and energy‐efficient source of irrigation water that could be used to alleviate short‐term droughts in low‐ and middle‐income countries. However, the water produced is unlikely to be completely salt‐free, potentially increasing the risk of salt stress in crops. In this review, we give an overview of the various emerging desalination technologies. We then use Tonga as a case study for assessing the viability of temporarily irrigating crops with moderately saline water (≤100 mM salt) and assess whether the benefits of alleviating drought outweigh the negative impacts of salt stress. We conclude that, in Tonga, important food security crops are likely to have higher survival and growth rates if they are provided with moderately saline water during drought, including taro, pumpkin and yam. Water derived using the new technologies would not be prohibitively expensive to produce. Moreover, it would minimise the need to divert a diminishing supply of water away from drinking to irrigation. The continued improvement of emerging desalination technologies, together with field trials, will help to optimise the use of moderately saline water for irrigation. This is likely to be especially beneficial for achieving and maintaining food security in low‐ and middle‐income countries in increasingly capricious conditions for agriculture.
... For many remote atoll communities, the availability of clean, potable freshwater is already a major challenge for public health, especially as overcrowding on some islets has long since reached critical levels (Spennemann, 2006;White et al., 2008;White et al., 2010). Subsistence cultivation is similarly hampered: groundwater quality in taro pits monitored across the atolls of Tuvalu between January and April 2006 by SOPAC 3 (Webb, 2007) revealed that salt concentrations in some pits on Nukulaelae, Niutao and Funafuti are already too high for successful swamp taro growth, while the interior swamps of Fongafale islet repeatedly experience problems of saline flooding by spring tides (Yamano et al., 2007). ...
Many human populations on remote atoll islands in the Pacific and Indian Ocean rely for their survival on the existence of fragile freshwater lenses, but sea-level rise (SLR) and discrete episodes of marine inundation (e.g. caused by tropical storms or tsunamis) both pose challenges for continuing groundwater viability. Mathematical modelling in this study is used to examine the effects on an atoll freshwater lens (FWL) of 10, 20 and 40 cm projected SLR scenarios. Subsequently, cyclone-driven coastal washover is simulated to examine the immediate FWL responses and later gradual recovery from rainfall recharge. A key feature of model design is a topographic depression containing a low-lying swamp in the atoll island centre (often absent in modelling domains), which accommodates seawater during flooding.
Results indicate that the FWL shrinks in thickness by approximately 50%, develops a brackish centre and changes morphology after 40 cm SLR. Following coastal inundation, steep salinity gradients demonstrate the formation of a strong salt plume at shallow depths, but reveal also the preservation of an undisturbed fresh horizon deeper within the substrate under present conditions and the modest 10 cm SLR scenario. Salt concentrations are maintained in this preserved fresh horizon within thresholds for human use (i.e. < 1.5 g/L) for over a year. This picture contrasts markedly with the 20 and 40 cm SLR scenarios, where already-diminished freshwater lenses then display less resilience and slower recovery from saline damage over similar post-disturbance timeframes. Overall, findings imply that advancing groundwater salinisation may render Pacific atolls uninhabitable long before they are completely submerged or physically eroded by sea-level rise.
... For instance, some seatolerant cultivated species like coconut tree (Cocos nucifera) or pandan (Pandanus) living along the shoreline may be damaged by storms or by shore erosion (Prasad 2013), while inland species like breadfruit (Artocarpus altilis) are going to be affected by the progressive salinization of land and groundwater. Another example is the taro (Colocasia esculenta), a traditional crop across the South Pacific; grown in swamp pits, its cultivation seems to be already affected by sea-level rise in the nearby archipelago of Tuvalu (Webb 2007). Raw resources or processed agricultural products like copra (coconut oil) are also a significant source of income for small insular communities who rely on trade and food importation to subsist. ...
Among the various adverse effects of climate change, sea-level rise is expected to increase the severity and frequency of flooding events impacting the vulnerable, low-lying islands of French Polynesia. It has long been understood that sea-level changes are not spatially uniform, yet this aspect is not taken into account in the decision-making. Notably, no projections of future sea level have been produced specifically for this region so far, partly because the processes driving sea-level changes remain poorly constrained. To approach the issue, we present a detailed reconstruction of sea-level changes for the mid-to-late Holocene, based on the observation of coral proxies. This dataset is then used to calibrate a sea-level model in order to estimate the contribution of glacial isostatic adjustment to regional sea-level changes and to infer past variations in global ice volume. Building upon this baseline and exploiting recent outputs of climate models, we project that in a “worst-case” scenario, sea level would rise 1.05 meters by 2100 in French Polynesia, exceeding the value adopted in the French adaptation strategy by 0.45 meters. We conclude that spatial variability of sea-level rise should be considered in future risk studies for this and other regions.
Atoll nations such as Tuvalu are considered to be amongst those most vulnerable to the effects of climate change. Here we present a national‐scale coastal flood hazard assessment for Tuvalu based on high‐resolution Light Detection and Ranging (LiDAR) topography and bathymetry. We follow a fully probabilistic approach, considering sea level anomalies, tides, and extreme wave conditions from a mixed climate (i.e., from distant extra‐tropical storms and local tropical cyclones). Nearshore processes such as wave setup and runup are also accounted for. Hazard maps were calculated for the present sea level, as well as for sea level rise projections corresponding to different shared socioeconomic pathways (SSP2 4.5 and SSP5 8.5) and time horizons (2060 and 2100). With a mean elevation of 1.55 m above mean sea level (1.37 m above mean high water spring) >25% of land area is inundated once every 5 years and >50% of land area floods once every 100 years nationally. Results indicate that present day 1‐in‐50 years floods (>45% of land area flooded) will occur more than once every 5 years by 2060 (annual exceedance probability >20%), even under the moderate SSP2 4.5 sea level rise projections. Results of this study highlight the pressing need for ambitious and large‐scale adaptation solutions which are commensurate with projected sea level rise and marine hazard impacts. The methodologies presented in this paper can easily be applied to other low‐lying islands in the tropical Pacific, where mixed climates (i.e., regular and TC conditions) and non‐linear nearshore processes dominate extreme water levels and flooding.
Taro (Colocasia esculenta (L.) Schott) is a staple food crop in the Asia-Pacific region in areas where rising sea levels are threatening agricultural production. However, little is known about its response to salinity. In this study, we investigated the effects of salinity on the growth, morphology, physiology, and chemical traits of taro to predict the impacts of rising sea levels on taro production and nutritional value in the Pacific. We grew taro (approximately 4 months old) with a range of NaCl treatments (0–200 mM) for 12 weeks. Full nutrient, micronutrient, and secondary metabolite analyses were conducted, including measures of calcium oxalate (CaOx), an irritant that reduces palatability. Significant reductions in growth and biomass were observed at and above 100 mM NaCl. Concentrations of macro- and micronutrients, including sodium, were higher on a per mass basis in corms of plants experiencing salt stress. Foliar sodium concentrations remained stable, indicating that taro may utilize a salt exclusion mechanism. There was a large amount of individual variation in the concentrations of oxalate and phenolics, but overall, the concentrations were similar in the plants grown with different levels of salt. The total contents of CaOx and phenolics decreased in plants experiencing salt stress. Taro’s ability to survive and produce corms when watered with a 200 mM NaCl solution places it among the salt-tolerant non-halophytes. The nutritional quality of the crop is only marginally affected by salt stress. Taro is, therefore, likely to remain a useful staple in the Pacific region in the future.
Root and tuber crops are important to agriculture and food security of many countries, ranging from the tropics through to temperate countries for potato. Climate change research has mainly considered and evaluated the impact on major food crops grown in continental environments. This chapter considers the optimum growing conditions for root and tuber crops and their tolerances to abiotic and biotic stress. It discusses the potential vulnerability of these crops based on climate change projections. Potato is discussed separately because more research has been conducted on this crop. Conservation agriculture (CA) which comprises minimum soil disturbance, retention of crop residues and crop diversification is promoted as an approach for making farming systems more resilient to climatic changes as well as mitigating climate change through soil carbon sequestration. Breeding crops to resist biotic and abiotic stresses will be an essential tool in managing climate change impacts.
Land degradation in many Pacific Island Countries (PICs) has become an emerging concern in recent years. The causes of land degradation in PICs include: deforestation, inappropriate agricultural practices, overgrazing, mining, population pressure, land tenure issues and changing climate. Deforestation and inappropriate agricultural practices especially on sloping lands often lead to soil erosion, a key process leading to land degradation. On-site effects of accelerated soil erosion include decline in soil physical properties, loss of soil organic carbon and loss of plant nutrients and subsequently low crop productivity. This threatens the environment and food security for the growing population in PICs. The PICs are embarking on sustainable land management practices to address land degradation and safeguard the resource for their future generation. However, limited resources, lack of capacity and awareness on land degradation amongst the population are serious obstacles to implementation of sustainable land use plans and management.
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Historical and projected sea-levels for islands in the tropical Pacific and Indian oceans are a subject of considerable interest and some controversy. The large variability (e.g. El Niño) signals and the shortness of many of the individual tide-gauge records contribute to uncertainty of historical rates of sea-level rise. Here, we determine rates of sea-level rise from tide gauges in the region. We also examine sea-level data from the TOPEX/Poseidon satellite altimeter and from a reconstruction of sea level in order to put the sparse (in space and time) tide-gauge data into context. For 1993 to 2001, all the data show large rates of sea-level rise over the western Pacific and eastern Indian Ocean (approaching 30 mm yr− 1) and sea-level falls in the eastern Pacific and western Indian Ocean (approaching − 10 mm yr− 1). Over the region 40°S to 40°N, 30°E to 120°W, the average rise is about 4 mm yr− 1. For 1950 to 2001, the average sea-level rise (relative to land) from the six longest tide-gauge records is 1.4 mm yr− 1. After correcting for glacial isostatic adjustment and atmospheric pressure effects, this rate is 2.0 mm yr− 1, close to estimates of the global average and regional average rate of rise. The long tide-gauge records in the equatorial Pacific indicate that the variance of monthly averaged sea-level after 1970 is about twice that before 1970. We find no evidence for the fall in sea level at the Maldives as postulated by Mörner et al. (2004). Our best estimate of relative sea-level rise at Funafuti, Tuvalu is 2 ± 1 mm yr− 1 over the period 1950 to 2001. The analysis clearly indicates that sea-level in this region is rising. We expect that the continued and increasing rate of sea-level rise and any resulting increase in the frequency or intensity of extreme sea-level events will cause serious problems for the inhabitants of some of these islands during the 21st century.
Cultures of taro, Colocasia esculenta var antiquorum (L.) Schott, established from shoot tip explants were used to select for salt tolerance. Presently, cultures are being maintained and produce plantlets in 10–70 per cent artificial seawater. The results indicate that in vitro selection techniques for salinity tolerance may prove useful in the development of salt tolerant taro cultivars.
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