Factors maintaining plant diversity in degraded areas of northern Kuwait

Journal of Arid Environments (2003) 54: 183–194 doi:10.1006/jare.2001.0880

Factors maintaining plant diversity in degraded areas of northern Kuwait

G. Brown* Universita( t Rostock, Institut fu( r BiodiversitaK tsforschung, Wismarsche Str. 8, D-18051 Rostock, Germany

Arid and semi-arid regions are jeopardized by land degradation with serious consequences for the natural vegetation, plant biodiversity and sustainable use of the natural environment. This paper describes the major causes of land degradation in northern Kuwait and outlines factors that serve to maintain plant biodiversity in those affected areas that would normally be dominated by the perennial dwarf shrub Haloxylon salicornicum. A conceptual model is presented describing the four major stages of degradation in this community. It is emphasized that stringent conservation measures are needed to protect the landscape from further deterioration and to promote re-establishment of the natural vegetation.  2003 Elsevier Science Ltd. Keywords: biodiversity; desert ecology; desertification; Kuwait; microhabitats; micro-nebkas; vegetation degradation

Introduction Land degradation and desertification are menaces in many parts of the world with serious implications for sustainable use of the natural environment. According to Kassas (1995) and Agnew & Warren (1996), it is particularly the semiarid regions of the world that are most susceptible. At the same time, these regions, both rich and poor, are experiencing some of the highest population growth rates worldwide (Warren et al., 1996). The term ‘desertification’ has been the subject of much controversy in the literature (for a review see Verstraete, 1986), but is used here in accordance with the concept adopted by Dregne (1986). This concept includes the following key features: (a) reduction in vegetation productivity (particularly due to the loss of perennial shrub cover), (b) decrease in species diversity and (c) increase in aeolian processes such as the erosion, transportation and deposition of sand. According to Le HoueH rou (1996), the direct causes of land degradation in arid areas stem from a drastic reduction of the perennial plant cover and simplification of the vegetation structure with a number of serious consequences for productivity, soil structure, water relations and microclimate. The same author is of the opinion that the indirect causes of land degradation are the same throughout the world, namely, ever-increasing pressure on the land from an expanding human and livestock population. Particularly in poorer regions, the collecting of wood for fuel and construction purposes can have a significant impact on the vegetation (Warren et al., 1996). *E-mail: [email protected] 0140}1963/03/010183#12 $30.00/0

 2003 Elsevier Science Ltd.

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Kuwait, a small (ca. 17,600 km2), oil-rich country located in the north-eastern part of the Arabian Peninsula, has suffered severe land degradation in recent decades, as noted by several authors (e.g. Khalaf, 1989; Omar, 1991; Zaman, 1997; Brown & Porembski, 1997, 1998). The main causes of this problem can be attributed principally to overgrazing, but also to recreational activities such as off-road driving and camping, as well as industrial practices, particularly quarrying (Khalaf, 1989). In addition, serious damage was inflicted on the natural environment during the recent Iraqi occupation and the hostilities associated with the liberation of the country during the Gulf War. Perennial plants can often act as effective obstacles to wind-blown sand, and as the sand particles are checked, a mound of sand becomes established around the base of these plants. These mounds of sand are referred to by a variety of names in the literature, for example, ‘micro-nebkas’ (Bendali et al., 1990; Brown & Porembski, 1997; 1998), ‘nebkas’ (Danin, 1996), ‘phytogenic hillocks’ (e.g. Batanouny & Batanouny, 1968, 1969) and ‘rehboub’ (Le HoueH rou, 1986, 1987). Batanouny & Batanouny (1968) provide a list of plant species that form micro-nebkas in Arabia. In contrast to the larger ('1m high), more stable dunes formed around some perennial plants (in Kuwait mainly around Nitraria retusa), and which are usually referred to as ‘nebkas’, micronebkas are not only smaller, but also lack a stratified internal structure of organic and inorganic material (Le HoueH rou, 1986). The main aim of this study is to outline vegetation degradation in a dwarf shrub community dominated by Haloxylon salicornicum, and in which micro nebkas are prominent and ecologically important feature. Study site and methods Study area The landscape of Kuwait is flat to gently undulating, and rises almost imperceptibly from the coast to a maximum altitude of nearly 280 m above sea level in the south-west. The climate is characterized by hot, dry summers and relatively mild winters. Rainfall occurs almost exclusively during the winter and spring months, mainly between November and April, and has a seasonal average of about 115 mm, with extremes of 28 and 260 mm (Halwagy et al., 1982). According to Halwagy & Halwagy (1974) and Halwagy et al. (1982), three distinct vegetation communities are typical of the desert plain which occupies the greater part of Kuwait (Fig. 1). Broadly speaking, most of the southern half of the country would naturally be covered by a dwarf shrub community dominated by the composite Rhanterium epapposum (local Arabic name: ‘arfaj’). Locally, a community occurs in which Cyperus conglomeratus (local Arabic name: ‘thunda’) replaces Rhanterium as the key perennial species, probably as a result of overgrazing. The Haloxylon salicornicum community potentially occurs in many areas of northern Kuwait and is dominated by the chenopod Haloxylon salicornicum (local Arabic name: ‘rimth’), a dwarf shrub that can attain up to about 25% cover in favorable locations. In contrast to Rhanterium, Haloxylon is a C4-plant which experiences its main period of vegetative growth during late spring/early summer. Flowering takes place between September and October, and by January, abundant fruits have been produced and shed (Brown & Al-Mazrooei, 2001). Methods On numerous occasions during a three-year period from September 1995 until December 1998, the author traveled extensively throughout northern Kuwait, and made detailed observations as to the state of the vegetation.

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Figure 1. Vegetation map of Kuwait. (Halwagy & Halwagy, 1974, slightly modified).

Vegetation sampling was carried out as described by Brown & Porembski (1997; 1998) and Brown & Schoknecht (2001), and the results presented are partly based on these studies. Furthermore, 25 randomly placed 1-m2 sampling plots were used to compare species diversity in different community types in a season of abnormally heavy rainfall (ca. 240 mm) in March 1998. Quadrats were placed in the open ground between the dominant perennials, so that only the accompanying annual flora was sampled. Total percentage cover of the vegetation was recorded, as were cover values for each species present. The vegetation data were analysed with multivariate procedures contained in the Cornell Ecology Package (Mohler, 1987). The data matrix was read into the program COMPOSE, and species occurring in fewer than three quadrats were excluded. Detrended correspondence analysis (DCA) was used for ordination of the data, and this was performed with the program DECORANA (Hill, 1979). Further statistical analysis was performed with SPSS for Windows, and details are given below. Procedures for determination of the germinable seed bank were outlined in Brown & Porembski (2000) and Brown & Schoknecht (2001). The nomenclature for plant species follows Boulos and Al-Dosari (1994).

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Results and discussion General features of the Haloxylon salicornicum community Haloxylon salicornicum still forms extensive stands in a number of areas in northern Kuwait, particularly in the northeast of the country. However, many areas are severely degraded, and shrubs are usually absent. Some of the more intact Haloxylon stands were encountered on the mainland west of Bubiyan Island. In northwestern Kuwait, welldeveloped stands are generally much smaller and more isolated, being confined to depressions in which sand accumulates. The situation improves in the demilitarized zone in the border area between Kuwait and Iraq, where grazing and recreational pressures are considerably less intense. Patches of R. epapposum are often interspersed on very sandy soils, especially in depressions which are found predominantly in northeastern and some western areas. Haloxylon salicornicum is accompanied by a large number of winter ephemerals, particularly in years of plentiful rainfall (Halwagy et al., 1982). Such therophytes account for the greatest proportion of species (up to 90%) with respect to the life-form classification system of Raunkiaer (1934). Some of the more frequent associates include Plantago boissieri, Rostraria pumila, Schismus barbatus, Malcolmia grandiflora, Picris babylonica, Loeflingia hispanica, Filago pyramidata, Gypsophila capillaris, Launaea mucronata, Launaea capitata, and Reichardia tingitana (Brown & Porembski, 1998). Micro-nebkas in H. salicornicum stands affected by sand deflation Cover of Haloxylon stands is usually between 2 and 10%. In these open vegetation structures, aeolian processes such as the transportation and deposition of sand become more pronounced (Le HoueH rou, 1987; Nickling and Wolfe, 1994). H. salicornicum grows to a height of about 0)8 m, rendering the plant an effective ‘sand-trap’. The height of these characteristic asymmetric micro-nebkas with their extended lee ranges from a few centimeters to 1 m. Their basal area varies considerably, but is on average about 1 m2. A photograph of large micro-nebkas formed by H. salicornicum and Zygophyllum qatarense in a coastal area of Kuwait is found in Halwagy & Halwagy (1977). In many degraded Haloxylon stands, the accompanying annual vegetation is often concentrated around the base of micro-nebkas, particularly on their leeward side (i.e., the micro-nebka-effect, see Brown & Porembski, 1997, 1998). These studies showed that not only was the percentage cover higher towards the leeward margins of the micro-nebkas, but also that species diversity was generally enhanced. A number of annuals, including Rostraria pumila, Launaea mucronata, Malcolmia grandiflora, Plantago boissieri, Gypsophila capillaris, Reichardia tingitana, Stipa capensis, Launaea capitata, Arnebia decumbens and Anthemis deserti were conspicuously more frequent (i.e. more often found in randomly placed plots) on the micro-nebkas than in the interdune space (i.e. the open area between the micro-nebkas). Conversely, only two species, Gymnarrhena micrantha and Asphodelus viscidulus, displayed a marked preference for the interdune space (Brown & Porembski, 1997). It should be stressed that the role of micro-nebkas as favorable microsites for annual vegetation appears to be restricted to areas where sand mobility is not excessive, and there is a net tendency for sand to be removed rather than deposited. In areas of mild sand deflation, micro-nebkas around Haloxylon salicornicum tend to be rather small and quite stable, and are clearly delimited from the harder, more consolidated ground of the interdune space. In areas where sand is generally deposited (e.g. in many areas of north-western Kuwait), the mounds tend to be much larger, composed of rather coarse sand particles and more mobile (i.e. their shape changes quite suddenly when there is a change in wind direction). In this case, the statement of Bagnold (1941) applies,

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namely, that mobile sand is an extremely poor growth medium for plants because it contains only very small concentrations of finer-grained particles, organic matter and nutrients. Consequently, the micro-nebka effect was not observed there (Brown, unpublished results). Brown & Porembski (1998) attributed higher densities and diversity of annuals on micro-nebkas primarily to their role as moisture reservoirs, particularly as a number of authors stress the importance of water availability for species diversity in desert ecosystems (Danin, 1978; Olsvig-Whittaker et al., 1983; Kutiel & Danin, 1987). However, an enhanced nutrient status of micro-nebka soils may also be a contributing factor in explaining the more favorable conditions for annual plant growth (Charley & West, 1975). So-called ‘fertile islands’ have been described in other semiarid and arid regions of the world. For instance, Mott & McComb (1974) found that relatively denser populations of taller annuals occur on slight mounds under shrubs in Australia, and attributed this to the higher nitrogen content and better water storage capacity of the deeper soil. Schlesinger et al. (1990) and Stock et al. (1999) have suggested that increased spatial heterogeneity of soil resources may be a useful indicator of desertification in semiarid shrublands. Observations show that in a manner similar to that by which mound-forming plants immobilize wind-borne sand, micro-nebkas also represent effective seed traps, and most species found on them possess highly mobile diaspores. This is particularly noticeable in the autumn months, when large amounts of Haloxylon diaspores accumulate around the bases of micro-nebkas. Results from seed-bank studies carried out under controlled laboratory conditions support this, with about six to eight times more seedlings emerging from micro-nebka soils than from interdune soil samples (Fig. 2). Single passage tire tracks as favorable microsites in Stipa-capensis-dominated vegetation In severely degraded areas of northern Kuwait or in areas where hardpan naturally occurs near the soil surface, H. salicornicum is largely missing or confined to small depressions in which sand and dust accumulate. On the shallow, often very firm, concrete-like soils, rather monotonous stands dominated by the annual grass Stipa

12

Emerged seedlings

10 8 6 4 2 0 Wind

Lee

IDS

Figure 2. Mean number of emerged seedlings from micro-nebka (lee and wind side) and interdune space (IDS) soil samples (n"15, respectively). Error bars give standard errors.

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Table 1. Mean species number per plot, percentage cover (with standard deviation in brackets) and total species number of the annual flora in the different stands recognized in Fig. 3

Species number per plot Pecentage cover Total species number

Rhanterium epapposum community (intact)

Cyperus Haloxylon conglomeratus community-I community (no or incipient signs of degradation)

Haloxylon Haloxylon community-III community-IV (advanced (severe stage of stage of degradation) degradation)

16)2 (1)5) a

12)6 (0)55) a

23)8 (1)3) b

14)2 (2)8) a

9)8 (2)5) c

32 (21)3) a

8)8 (1)1) b

73)0 (5)7) c

8)4 (2)2) b, d

2)5 (3)1) b, e

32

19

35

27

20

ANOVA was first performed to confirm statistically significant variation between means (species number: F4,20"38)61, p(0)0005; percentage cover: F4,20"41)99, p(0)0005). Different letters indicate significantly (p(0)05) different means (Bonferroni test).

capensis can cover quite extensive areas (Halwagy et al., 1982; Brown & Schoknecht, 2001). As indicated by Halwagy et al. (1982), and shown in Table 1, Haloxyloncommunity-IV, the number of associated species tends to be rather limited. Furthermore, careful observations in such highly degraded areas have shown that much of the vegetation is found in very small depressions in which wind-blown sand has accumulated. Such depressions can be natural, but very often have been created by vehicles. As a consequence, the vegetation shows a distinct linear patterning and is thus characterized by a high degree of spatial patchiness. However, such tire tracks appear to be favorable microsites for vegetation growth only if they have been caused by the single passage of a vehicle (i.e. single-passage tracks), not on more frequently used tracks (Brown & Schoknecht, 2001). In a study carried out by these authors near the Sabriya oil field in northern Kuwait, a total of 28 species were recorded in the 200 randomly placed vegetation quadrats (100 quadrats in tire tracks and 100 quadrats away from the tracks (intertrack)), all of which are annuals in Kuwait. Twenty-six species were found in the tracks, and only 15 in the intertrack quadrats, i.e. on the characteristic hardcrusted surface. Whereas 13 species occurred solely in the tracks, only two (Gypsophila capillaris and Schimpera arabica) were restricted to the intertrack quadrats, and these each had a single occurrence. Most intertrack quadrats contained either no species (45%) or merely a single species (25%), with between two and four species occurring in the remaining 30% of quadrats, whereas between four and six species were found in the majority of track quadrats. The maximum number of species registered in the track quadrats was 11, but a maximum of only four were recorded in the intertrack ones. Furthermore, seeds that would normally be blown across the desert surface collect in the tire tracks, a fact that was borne out by studies of the seed bank (Brown & Schoknecht, 2001). On average, 28)0 (between eight and 42) seedlings emerged from soil samples taken from the tracks, but only 1)1 (ranging from zero to five) from intertrack samples. This positive track effect may come as a considerable surprise, as it is in marked contrast with results from the Mojave Desert in the U.S.A., which in respect of climate and physiognomy of the vegetation, is broadly similar to the situation in Kuwait. A number of studies undertaken there emphasize the highly detrimental effects of

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off-road driving on the desert vegetation (see Rundel & Gibson, 1996). This undoubtedly applies to much of Kuwait, but in severely degraded areas where the surface sand layer is completely absent, revealing the underlying gypsic horizon, indentations caused by vehicle tracks allow sand, plant debris and water to accumulate with positive effects for the vegetation. Although annuals tended to dominate in the tire tracks of the study area, observations showed that numerous young Haloxylon plants had become established in some older single-passage tracks in adjacent areas where disturbance was low. Model for vegetation degradation in northern Kuwait in areas with net sand deflation Large tracts of the desert environment in Kuwait have been degraded to a large extent with considerable implications for the natural vegetation. Figure 3 summarizes the changes in vegetation type and structure that probably apply to many areas of northern Kuwait which are prone to sand deflation. The information presented here is based on several studies, including Halwagy & Halwagy (1974), Khalaf (1989) and Brown & Porembski (1997). The underlying assumption of the model for vegetation degradation shown in Fig. 3 is that dwarf shrub/graminoid communities represent the potential natural vegetation. Northern Kuwait is now largely dominated by H. salicornicum, but by comparing a vegetation map compiled by Dickson (1955) with their own, Halwagy & Halwagy (1974) concluded that R. epapposum, a species that is favored by sheep, goats and camels (Thalen, 1979; Batanouny, 1990) and that once occurred extensively over much of northern Kuwait, has gradually been replaced by H. salicornicum as a result of overgrazing and ensuing soil erosion. In certain cases, this gradual transition could also take place via the C. conglomeratus community, as Cyperus is much more tolerant to grazing than Rhanterium, and also occurs on sandy soils (Halwagy et al., 1982). In contrast to Rhanterium, Haloxylon is largely neglected by sheep, but is a valuable grazing plant for camels (Thalen, 1979; Batanouny, 1990; personal observations). Within the Haloxylon community, Brown & Porembski (1997) broadly recognized four distinct stages of desertification, and these are depicted in Fig. 3. Stages 1 to 3 involved a gradual decrease of Haloxylon cover, loss of sand and the tendency for annual plants to become concentrated around the base of micro-nebkas. This becomes particularly conspicuous in stage 3 (advanced stage of degradation). The fourth stage, severe degradation, was characterized by the near total loss of Haloxylon cover, severe reduction in vegetation productivity and the local dominance of S. capensis, at least in years of good rainfall. The vegetation in this case is largely restricted to the tire tracks described above, or to other small depressions. Figure 4 shows the results of DCA ordination of the annual vegetation in randomly placed 1-m quadrats in different vegetation units. Despite the rather limited number of plots used, the accompanying annual flora was quite distinct for each vegetation unit, as underlined by the aggregation of samples into distinct clusters on DCA-axes 1 and 2. Mean species number and percentage cover in these units are given in Table 1. Intact Haloxylon quadrats contained by far the largest number of species and had the highest percentage cover of all units. With increasing degradation in this community, species number and percentage cover declined dramatically, with Haloxylon-IV plots (dominated by Stipa capensis) possessing the lowest values. Interestingly, despite relatively high cover values, mean species number in quadrats of intact Rhanterium stands was significantly lower than in those of Haloxylon-I stands, and total species number was also highest in Haloxylon-I stands. These results support the findings of a detailed study by Halwagy et al. (1982), that the Rhanterium community shows a narrower spectrum of species than that of Haloxylon.

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G. BROWN Rhanterium epapposum community Rhanterium = dominant perennial shrub; species-rich ephemeral vegetation; sandy substrate

Cyperus conglomeratus community Perennial shrubs largely absent; Cyperus dominant perennial; species-rich ephemeral vegetation; sandy substrate

Haloxylon salicornicum community - I (no or incipient signs of degradation)

Increasing degradation

Haloxylon = dominant perennial shrub with high cover (up to 25%); large micro-nebkas; species-rich ephemeral vegetation; sandy, gravelly substrate

Haloxylon salicornicum community - II (moderate stage of degradation) Haloxylon = dominant perennial shrub, although with reduction of potential cover by up to 50%; large micro-nebkas; species-rich ephemeral vegetation; gradual loss of substrate, increase in gravel content

Haloxylon salicornicum community - III (advanced stage of degradation) Haloxylon = dominant perennial shrub, although with marked reduction of potential cover from 50-90%; micro-nebkas small; decrease in diversity/productivity; ephemerals concentrated around base of micro-nebkas; severe sand deflation in interdune space

Haloxylon salicornicum community - IV (severe stage of degradation) Haloxylon and other perennials largely absent; Stipa capensis dominant ephemeral in wet winters; marked decrease in diversity/productivity; ephemerals concentrated in small depressions; severe sand deflation

Figure 3. Model of vegetation degradation in northern Kuwait in areas with net sand deflation. Bold arrows: probable sequence of degradation; dashed arrows: possible sequence of degradation.

Conclusions and recommendations It should be emphasized that the main causes of land degradation in Kuwait are anthropogenic, not climatic. This implies that measures can be invoked

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200 180 160

Axis 2

140 120 100 80 60 40 20 0

0

100

200

300

400

500

Axis 1 Figure 4. Plot scores of annual vegetation on the first two axes of DECORANA ordination. Refer to Figure 3 for explanation of the vegetation units. Haloxylon-I ( ); Haloxylon-III ( ); HaloxylonIV ( ); Rhanterium ( ); Cyperus ( ).

to successfully combat desertification in the short to medium term, as discussed below. Much of the desert landscape is used for livestock production (mainly sheep and camels). If a sustainable usage based on livestock production is to be developed, it is vital that the number of livestock be properly balanced with the available forage resources. Based on detailed data collected by Thalen (1979) in southern Iraq, 1 km2 of desert can be expected to be able to support between 12 and 16 sheep throughout a year of normal rainfall (ca. 100–120 mm). In years of drought though, livestock numbers would have to be reduced, if efforts to combat vegetation degradation are to be successful. In order to maintain optimum soil conditions and allow maximum sustainable grazing capacity in Kuwait, it is vital to conserve and, where necessary, re-establish the natural dwarf shrub vegetation, as this is best adapted to the specific local environmental conditions. In accordance with the definitions of Le HoueH rou (1995), there are two ways of achieving re-establishment of the natural vegetation, regeneration and restoration. The first process entails removing the factors that cause deterioration, such as overgrazing and off-road driving. Restoration involves the artificial establishment of the same type of vegetation that existed before destruction. In order to speed up regeneration of the natural vegetation, it may prove feasible to establish enclosures in selected areas for a certain period of time. Le HoueH rou (1996) states that vegetation in desert regions usually responds favorably to permanent enclosure. Omar et al. (1990) and Zaman (1997) provided examples of the desired response of the vegetation to permanent enclosure in the degraded Kuwaiti desert, although there were exceptions where no significant improvement was found. According to Le HoueH rou (1996), desertification is irreversible (i.e. the vegetation will not recover to its pristine condition even after 25 years of total protection) where the environment is drier and where the soil is more shallow. In addition, recovery of the vegetation is presumably dependent on the extent of degradation when remediating measures are invoked, as well as climatic factors. In severely degraded areas, it is possible that the exclusion of grazing and other detrimental practices may not yield beneficial results in the medium term. As noted above, a number of authors, including Vetaas (1992) and Brown & Porembski (1997, 1998), have indicated the importance of microsites created by trees and bushes for plant establishment in natural semiarid ecosystems. Tongway & Ludwig (1996) have recently

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described a successful procedure for restoring productive soil patches in semiarid landscapes in Australia which involved laying piles of branches of native woody plants (i.e. by artificially creating microsites) in open patches and allowing soil, litter and water to accumulate there. The same technique also proved feasible in restoring vegetation patches in the same area (Ludwig & Tongway, 1996). If adequate protection from grazing is afforded, re-planting of seriously degraded parts of Kuwait with Haloxylon shrubs could also prove to be a viable option for accelerating vegetation recovery. Haloxylon seeds germinate readily under laboratory conditions and the plants are easily grown (Thalen, 1979; Brown & Al-Mazrooei, 2001). Plants could therefore be grown in nurseries and later transferred to the field. The shelter afforded by adult plants of these woody species is also beneficial to a number of other species, protecting them from high levels of irradiance. Also, as noted above, litter produced by such shrubs enriches the soil with nutrients and improves the soil-water status (Charley & West, 1975; Shachak et al., 1998). Regeneration of the natural vegetation of Kuwait and the introduction of a sustainable grazing regime require a careful management strategy. It should be stressed that it may not be possible to re-establish the potential natural vegetation entirely, especially in the short term. However, a balance must be sought between the interests of landscape conservation and sustainable livestock economy. According to Krueger (1990), it is possible to develop grazing programs that enhance the productivity of both the overall landscape and the livestock. I would like to thank Kuwait University for financially supporting the fieldwork (SO 068 and SO 073), and Dr Peter Cowan (Kuwait) for assistance and helpful comments on the manuscript.

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