Vegetation-environment relationships in a Negev Desert erosion cirque

Journal of Vegetation Science 4: 83-94, 1993 0 IAVS; Opulus Press Uppsala. Printed in Sweden

83

Vegetation-environment relationships in a Negev Desert erosion cirque

Ward, Olsvig-Whittaker, & Lawes,

‘Ramon Science Center, Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer Campus 84993, Israel: Current addresses: 2Department of Zoology, University of British Columbia,

6270 University Boulevard, Vancouver, B.C. V6T 124, Canada: *Fax + I 604 822 2416; E-mail: [email protected]. 3Mitrani Center for Desert Ecology, Institute for Desert Research, Sede Boqer 84993, Israel:

4Department of Zoology & Entomology, University of Natal, P.O. Box 375, Pietermaritzburg 3200, South Africa

Abstract. The relationship of desert vegetation to topographic and edaphic factors in Makhtesh Ramon, an erosional cirque in the Negev Desert of Israel, was analyzed using redundancy analysis ordination. Altitude and geological substrate had the most significant correlations with vegetation patterns. In particular, significant and monotonic increases in both total species richness and the percentage of annual species were found with increasing altitude. A comparison was made of the effectiveness of Whittaker’s diversity sampling plots (Shmida 1984) versus the log-series survey method of McAuliffe (1990). We concluded that at small sample sizes, the McAuliffe plots were more likely to yield interpretable ordination results, even though they included only the perennial flora.

Keywords: Desert vegetation; Makhtesh Ramon; Multivari- ate analysis: Ordination: Sampling method: Species richness: Wadi.

Nomenclature: Feinbrun-Dothan, Danin & Plitman (199 1).

Introduction

Many ecological studies have emphasized the importance of rainfall as a causal factor for determining the structure and composition of desert vegetation (Hillel & Tadmor 1962; Noy-Meir 1973; Shmida 1985). Noy- Meir (1973) terms rainfall ‘the master input’ or ‘driving variable’ in the desert ecosystem, with topography as an important related factor because of its effects on the run- off of water and potential or free water evaporation. The effect of evaporation on plant distribution is evident in the differences in vegetation between north- and south- facing slopes, for example (Noy-Meir 1973; Danin 1983; Evenari, Shanan & Tadmor 1982). However, the role of geological substrate in a desert environment has received scant attention (Lipkin 1971; Danin, Orshan & Zohary 1964, 1975; Danin 1983), even though striking

contrasts in geology are quite evident in many deserts (Goodall 1976; Evenari, Shanan & Tadmor 1982).

For several decades, Israeli scientists have conducted detailed studies on the role of climate and topography in determining patterns of vegetation, particularly in the Negev desert (e.g. Lipkin 1971; Halevy & Orshan 1972; Danin, Orshan & Zohary 1964,1975: Danin 1983; Loria & Noy-Meir 1979; Olsvig-Whittaker, Shachak & Yair 1983). These relationships between vegetation and climatic factors and topography have been fairly well described for the Negev north of Makhtesh Ramon (Danin, Orshan & Zohary 1964, 1975). Since most vegetation of the Negev Desert occurs on limestone, the role of substrate type and its interaction with topography and climate are less well-known. The exploration of these relationships in one part of the Negev Desert is the subject of this study.

In 1990, we conducted a survey of the vegetation of the Makhtesh Ramon, a nature reserve established within an unusual anticlinal erosion cirque - ‘makhtesh’ (He- brew) = ‘mortar’- in the Negev Highlands (30” 35’ N, 34” 45’ E). This cirque contains a wide range of geological substrates and, although the vegetation had been generally described (Danin 1983; Zohary 1962; Lipkin 197 l), the full vascular plant-species composition and its response to the complex topographic and edaphic variability within the area was poorly known. One objective of our research was to select representative sites for long-term monitoring stations; for this purpose, a quantitative analysis of the vegetation was essential.

To determine both vascular plant-species composition and species richness in the Makhtesh flora, we used ‘Whittaker diversity sampling’ (Shmida 1984) as a method for rapid sampling. Our main objective was to relate patterns in vegetation to patterns in geological and topographic environmental parameters. We decided to test the effectiveness of new methods of direct ordination, specifically the redundancy and partial redundancy

84 Ward, D., Olsvig-Whittaker, L. & Lawes, M.

analysis options in the program CANOCO (ter Braak 199 la,b) to determine the relationship between vegetation coenoclines and known environmental factors.

In addition, we compared Whittaker diversity sampling of annual and perennial vegetation with the log- series survey method for perennial vegetation of McAuliffe (1990). The latter technique is a rapid survey technique appropriate for desert vegetation, where plant density is low (<ca. 25 96 total cover; McAuliffe 1990). We wished to determine whether the latter technique, in addition to being rapid to perform and simple enough for most field managers to conduct, provided sufficient information about the plant communities to compare with the results of Whittaker sampling.

(1) the environmental factors most important in determining patterns of vegetation in Makhtesh Ramon; (2) the importance of edaphic vs topographic variables; (3) community parameters (apart from species composition) correlated with environmental gradients, notably species richness, equitability, and plant growth forms; (4) variation in sensitivity to substrate differences of different groups of vascular plant species; (5) the efficacy of the Whittaker and McAuliffe sampling methods for describing gradients in desert vegetation.

Thus, our primary questions deal with:

Study area

The Makhtesh Ramon Nature Reserve (Fig. 1) is an area of about 200 km2 located at the southern boundary of the Negev Highlands, running SW - NE at 30" 00' - 30" 15' N, 34" 40' - 34" 5' E. The erosion cirque is formed by the action of the intermittent river Nahal Ramon eroding through layers of Eocenic limestone and chalk to reveal a variety of substrates otherwise not exposed in this area. These include sandstones, gypsum, limestone, dolomite, various clays and plutonic crystalline rocks (Danin 1983) which form soils with edaphic properties very different from the more prevalent calcareous par- ent materials (Issar 1983; Ben-David & Mazor 1988). In addition to these sedimentary rocks, hills of basalt which are the vestiges of a number of small, extinct volcanoes from the lower Cretaceous are scattered in Makhtesh Ramon, particularly in the west.

The north rim of the cirque consists of a sheer 200-m drop; the cirque floor is tilted downward toward the northeast so that while the rim altitude ranges between 800 and 900 m, the cirque floor ranges from ca. 800 m at the SW edge to ca. 400 m in the NE basin. The southern rim is less steep, with an altitude of about 5 10 m. There is a corresponding very sharp drop in rainfall with the drop in altitude, from 8 5 m d y r on the north rim to 56 m d y r in the central cirque.

Fig. 1. Schematic diagram of the Makhtesh Ramon erosion cirque. Dotted lines = major wadis (sand-alluvium substrate associated with this); stippled areas = sand dunes (upper = Machmal valley; lower = Ardon valley); hatched = limestone- dolomite; black = basalt; cross-hatched = gypsum; hatched + stippled = laccolith; clear = sandstone.

The strong environmental changes in this area are reflected in both vegetation and fauna. Makhtesh Ramon is considered part of a zoogeographic boundary, the Ramon-Zohar line (Shkolnik 1982) separating the Medi- terranean from the desert species. Zohary (1962) considered this area a transition from the Irano-Turanian phytogeographic region (steppe vegetation) in the north to the Saharo-Arabian region (true desert) in the south. We examined the possibility of aphytogeographic boundary at Makhtesh Ramon (Ward & Olsvig-Whittaker in press) and concluded that the latitudinal gradient is disjunct here, with more species and floristic associations being shared with the true desert vegetation to the south than with the steppe vegetation to the north.

Methods

Sampling design

A high percentage of the flora in this area consists of winter-spring ephemerals. Therefore, all sampling took place in April and May in order to record maximum vascular plant-species diversity. It should be noted here that the annual vegetation will vary between years de- pending on rainfall.

We followed the gradsect design of Gillison & Brewer (1985) and Austin & Heyligers (1989) for analysing transects incorporating significant environmental gradients, an effective means of sampling the maximum potential variation in plant-species composition in large areas. Gillison & Brewer (1985) have shown statistically that gradsects capture more information about vegetation attributes than randomly placed transects of similar

- Vegetation-environment relationships in a desert erosion cirque - 85

length. Climatic, lithological and topographic character- istics of the study area are incorporated into a series of gradsects. It was evident from examination of the study area that there was considerable variation in vegetation patterns with the decrease in altitude from west to east in Makhtesh Ramon. Nahal Ramon traverses almost the entire length of the long axis of the cirque, proving a convenient basis for the gradsect of primary interest, namely altitude. Non-wadi samples were selected to represent the geological formations and topography as evenly as possible along the length of the cirque. In the wadis, we usually placed our plots equidistant from one another along any particular stream bed.

Thus, following the gradsect sampling scheme, plots were arranged with altitude as the primary gradient (along the length of the cirque) and replicated along the length of the gradient by placing one plot on either side ofeachofthecentral wadi plots. In addition to theseplots, plots in substratarepresenting other major substrata were sampled at random.

In total 56 Whittaker diversity sampling plots and 75 McAuliffe sampling plots were selected to cover the range of observed vegetation in the erosion cirque. Since most of the vegetation was concentrated in wadis, which were very heterogeneous, we concentrated our sampling on this topographic type. - Note: ‘wadi’ (Ara- bic) = ‘nahal’ (Hebrew) = dry stream bed. ‘Nahal’ is the officially used adjective, but ‘wadi’ is the more common Middle Eastern noun -.

Whittaker diversitlt, sampling

With the ‘Whittaker plots’ (Shmida 1984) we determined the composition and pattern of species accumula- tion in the vegetation. A 20 m x 50 m plot was placed in representative vegetation in each of the study areas along the aridity gradient, with the long axis parallel to the wadi. This plot was divided into 10 1 m x 1 m plots, two 5 m x 2 m plots, one 10m x 10m plot and one 50m x 20 m plot, nested from the center. This sampling method enables us to obtain richness at four levels of area: 1, 10, 100 and 1000 m2, in order to draw a species/ area curve. Hence, this sampling design facilitates direct assessment of the relationships between sampling area and species richness. Our sampling design differs from that described by Shmida (1984) in one small, but significant, factor: five of the 1 m x 1 m samples were placed on one side of the axial tape and the other five on the other. This avoids the possibility of exaggerating the importance of species that may be growing along a single line, as is often the case in wadis.

We determined Simpson’s (1949) dominance index from the Whittaker-plot data, and defined equitability of the distribution of individuals among species as the

inverse of dominance. Abundance values were estimated by weighting the presence of a species in plots of different sizes in the following way: presence in 1-m2 plots = 20: in 10 m2-plots = 10; in the 100-m2 plot = 5; in the 1000-m2 plot = 1. Although these weightings are arbi- trary, they provide a measure of relative abundance of the plants at each plot size. Robustness of the gradients produced by these weightings was checked by re-running the analyses with both log-transformed and equal weightings. Similar results were obtained with these alternative weightings.

McAulifSe’s log-series survey sampling method

This method of quantifying density and cover of perennial vegetation is appropriate for use in arid envi- ronments where perennial ramets are fairly distinct and separate (McAuliffe 1990). Numbers of individual ramets for each species within a plot are assessed. These numbers are used to assign species to logarithmic density classes equivalent to the logarithmic (base 2) values of actual abundances. The species is assigned to a log, canopy class based on the median canopy diameter of the species in the plot. Log, total cover per plot is determined by adding logarithmic density and cover classes. Percent cover is derived by taking the antilog of the difference between log, cover per species and log, total plot area.

Environmental factors

Geological data were obtained from 1 : 20 000 and 1 : 50 000 geological maps drawn up by the Geological Survey of Israel (Zak 1968: Segev 1982). Altitudinal data were obtained from 1 : 25 000 maps.

Environmental variables used in the canonical ordinations were: (1) altitude; (2) geological substrate factors - presence of basalt, sand alluvium, limestone-dolomite, loess-limestone, sand-loess, quartz alluvium. loess alluvium, gypsum, laccolith (submerged volcano eroding through gypsum): (3) eastern vs western sands, which differ in the quantity of loess material; (4) dune sands in Biq’at Machmal and Biq’at Ardon (note: biq’at (Hebrew) = valley); (5) inundation categories (flooded each year, flooded in wet years, not flooded); (6) topography (wadi, slope and plain).

This list, though not intended to be exhaustive is a reasonable sample of environmental variables that may be used by subsequent field workers as a basis for comparison.


Analytical techniques

We used direct and indirect ordination programs contained in the CANOCO multivariate analysis program (ter Braak 1991a,b). To determine the correlation of vegetation and physical environmental factors, we used the canonical form of principal components analysis, redundancy analysis, RDA. Since RDA and canonical correspondence analysis (CCA) gave very similar results in a full ordination with all factors, we used RDA in all later trials in order to get better statistical interpre- tation. - Note: when gradients are short, as in this case (major axes < 2 s.d.), the relationship between vegetation response and environmental variables is likely to be linear. Jongman, ter Braak & van Tongeren (1987) recommend use of RDA for linear reladonships and CCA for uni-modal relationships (major axes <4 s.d. - between the species and the environmental variables). Partial RDA was also used to control for the effects of covariables not of particular interest in certain analyses.

Forward selection of variables (Montgomery & Peck 1982; Escoufier & Roberts 1979) was used to determine the relative importance of environmental variables in the input data, and the variance explained by them. The explained variance is a straight sum of squares of regression in RDA (ter Braak 199 1 a).

Monte Carlo permutation tests were used to determine the statistical validity of the association between vegetation ordination scores and the environmental in- dependent variables. These tests are conducted by random permutation of the sample (= plot) numbers in the environmental variables. The environmental data are randomly linked to the species data. We ran 99 simula- tions in order to achieve a normal distribution with which to compare our data with the random combination thereof, using analysis of variance.

For each random data set, Monte Carlo tests provide one or two test statistics, namely the first eigenvalue and/or the sum of all eigenvalues (= the trace statistic). If the species react to the current environmental variables, then the test statistic calculated from the observed data will be larger than most of the test statistics calculated from the random data. If the observed value is among the 5% highest values, then the species are significantly related to the environmental variables. Tak- ing the trace as the test statistic gives an overall test of the effect of the environmental variables on the species. A potentially more powerful test is obtained by using the first eigenvalue rather than the trace as test statistic. This latter test shows whether the first canonical axis is significant. The second canonical axis may be tested for significance by entering the first ordination axis as a covariable.

Results

Ordination and numerical classification

Forward selection of variables indicated that altitude accounted for more of the variance (9 %) than any other variable, assuming a linear response of the vegetation to the environmental variables. Thereafter, sand alluvium (7 %), limestone-dolomite (5 %) and basalt (5 %) explained most of the observed variance. The discovery that altitude accounts for much of the variance is indeed obvious from superficial observation of the vegetation patterns in Makhtesh Ramon. A gradual transition is apparent from diffuse semi-desert shrub-steppe vegetation (vegetation widely dispersed over the habitat) in the higher southwestern end, to typical desertic ‘contracted’ vegetation (restriction of vegetation to moister areas, resulting in clumping) in the drier northeastern end. The relative unimportance of several variables in this initial analysis (determined by the percentage of variance explained) determined their exclusion from subsequent analyses.

Our first ordination involved RDA of all environmental variables against species and samples (Fig. 2). The sum of all canonical eigenvalues was 0.48 (i.e. 48 % of the total variance explained). We had significant trace and first axis statistics (P<O.Ol). The scatter plot showed good discrimination of two complex orthogonal gradients, one of which primarily represents altitude and the other a combination of edaphic factors. We then ran ordinations for each of the important factors sepa- rately, i.e. using each factor in turn as the single environmental variable. Each of these important factors had significant primary axes ( P < 0.01). The results of these ordinations for species are shown in Tables 1 - 5.

Fig. 2 also indicated to us that the second axis ordinated plots according to a soil texture gradient. Thus, we then ran an ordination using four ordinal categories of a single soil texture variable grading from loess to sand. This showed a good spread of samples and species (sum of all canonical eigenvalues = 0.19) and significant trace and first axis scores ( P < 0.01). With the exception of four plots in the high altitude areas of the western Makhtesh, the samples spread out linearly on a single complex gradient (Axis 2, Fig. 3).

Three topographic factors explained 12% of the variance among sites; ‘plain’ (3 %), ‘slope’ (4 9%) and ‘wadi’ (4 %). Further analysis was not pursued because sampling was concentrated primarily in wadis and information on the other topographic positions was therefore sparse.

The sandstone areas of the western Makhtesh differ from those in the east in that the latter possess no loess. Loess is a wind-borne material and is effectively blown


I

I . I

AXIS 2

I I Altifude 1 x

Sand alluvium x x t

AXIS I

Fig. 2. RDA ordination of the primary and secondary axes relating species to geological features and altitude. Data derived from Whittaker diversity samples.

out of the cirque in the wide, open areas of the eastern sector. We postulated that, because loess is an important soil-forming material (Evenari, Shanan & Tadmor 1982), there should be significant differences in the plant- species composition between these two areas. Using presence of loess as a dummy environmental variable in a redundancy analysis, we found that there was indeed a significant difference in plant-species composition between the eastern and western cirque’s sandy areas (Monte Carlo permutation test, Trace = 0.09, F = 2.58, P=O.Ol; first axis - P = 0.02).

Examples of dominant species that differ between these two substrates are: (i) Western Makhtesh Ramon (loess + sand) -Gymnocarpos decandrum, Zygophyllum dumosum, Reaumuria hirtella; (ii) Eastern Makhtesh Ramon (sand only) - Crotalaria aegyptiaca, Zilla spinosa, Farsetia aegyptiaca. The three latter species are ‘typical’ Saharo-Arabian species while the dominant species of the western Makhtesh are common Irano- Turanian species (Zohary 1962).

Comparison of McAuliffe and Whittaker sampling techniques

We compared Whittaker and McAuliffe techniques using the same environmental variables in both analyses. We found that, of those samples and species that showed strong relationships to particular environmental variables, about half were congruent between the techniques

I

1

X I I X

I x

I

Sand X Altitude

AXIS 2

x x

Sand x x

Altitude x

I < AXIS I x x

i

I I ,

x

x

x

x

x i

I X I

a

Fig. 3. RDA ordination of sample scores from Whittaker survey data relating samples to soil texture and altitude and the interaction effect of these two variables. Primary and secondary axes are shown.

and half were not. A significant positive correlation of shared species scores on the primary ordination axis (Spearmanrankcorrelation=0.67, P < 0.0001)indicates that the species scores from the two techniques are arranged in a similar way by redundancy analysis, although the amount of variance explained was relatively low.

The lack of congruence between these two techniques may be a result of both the unequal sampling of habitat types between the two survey techniques and the absence of annual species in the McAuliffe survey. Annual species account for 43% of the total species richness in Makhtesh Ramon during the late winter and spring. Thus, we continued the comparison of these two techniques using only perennial species in the ordinations and samples where both techniques were used in the same site ( n = 13 sites). The McAuliffe technique produced a significant ordination of the entire set of environmental variables (Trace statistic = 0.11, F = 3.19; P = 0.01; first axis: F = 4.17; P = 0.01). In contrast, the Whittaker ordination was not significant (Trace statistic = 0.32; F = 1.41; P = 0.07; first axis: F = 2.22; P = 0.07). Thus, with the small number of shared samples, only the McAuliffe technique gave us an interpretable ordination, with a significant relationship between environmental variables and vegetation gradients.


Ordination results: species responses in the Whittaker plots

Tables 1-3 present the species responding most strongly to the environmental factors we found most significant in our analysis of the Whittaker plot data. In Table 1, showing the response of species to altitude, of the ten species concentrated at high elevation, six are annuals, whereas only one, Blepharis ciliaris, is found concentrated at the lowest altitudes. This is congruent with the trend of increasing annual species richness with altitude (i.e. rainfall).

The vegetation of Makhtesh Ramon is interesting largely because of its differentiation on strongly con- trasting substrates (Tables 2, 5). Limestone-dolomite species (Table 2 ) are typical rocky-slope species found in the Negev Highlands: species such as Zygophyllum dumosum, Bellevalia desertorum, and Erodium hirtum are perennial species which are abundant where they can exploit the water trapped in crevices in the rocks (Olsvig-Whittaker, Shachak & Yair 1983; Herwitz & Olsvig-Whittaker 1989).

In contrast, the sand-alluvium communities contain many small annuals (e.g. Diplotaxis harra, Euphorbia chamaepeplus, Scabiosa eremophila, Erodium deserti) that are well adapted to quickly utilize the relatively high moisture conditions in surface sand during the winter, and finish their life cycles before the summer drought.

Several of the species preferring basalt (Tables 2 , 5 ) are species that are considered to be ruderals, e.g. Anabasis setifera, Lycium shawi, Ochradenus baccatus, and Asphodelus tenuifolius. Danin (1983), discussing basalt vegetation in the Sinai, noted that the very high silt and clay content of the basaltic soils create very arid conditions, and only support vegetation in the wadis.

Table 1. Species responding strongly to the effect of altitude. Species are divided into two groups based on their extreme scores on the primary axis of a redundancy analysis, using only altitude as an environmental variable. Species with high positive scores show a positive correlation with altitude, and species with low negative scores the converse. Eigenvalue of primary axis = 0.092 ( P = 0.01). Letters following species names denote growth forms: A = annual, C = chamaephyte, G = geophyte, S = tall shrub, T = tree.

High scores Low scores

Astragnlus tribuloides Ixiolirion tataricum Suaeda monoica Artemisin herba-albn Annbnsis setifera Adonis dentatn Gymnnrrhena micrantha Anthemis melampodinn Atractylis phaeolepis Reseda muricatn Scnbiosa eremophila

A G S C C A A A C A A

Blepharis cilinris A Zilla spinosa C Zygophyllum dumosum C Moricnndin nitens C Tnmarix nilotica T Echiochilon fruticosum C

Hnmmndn salicornica C Pnncrntium sickenbergeri G

Fngonia arnbica C

Anvillen garcinii C

This does not appear to be quite the case in Makhtesh Ramon where the basalt hills are relatively well veg- etated, although these hills exist as scree slopes which may provide unstable substrates for plant establishment.

The higher level of loess deposition on sand substrates in the western end of the makhtesh, compared to the eastern sand, is reflected in the vegetation (Table 3). Species such as Onobrychis cristagalli, Trigonella stellata and Gagea reticulata can be found as readily on loess as on sand in the Negev, whereas Artemisia monosperma and Retama raetam are considered typical and diagnostic in vegetation on sand (Danin, Orshan & Zohary 1975).

Table 2. Species responding strongly to the effects of basalt, limestone-dolomite and sand-alluvium. Species listed had high positive scores on the primary axis of a redundancy analysis, using only that variable (e.g. basalt) as an environmental variable. Species with high positive scores show a positive correlation with that variable. Eigenvalue (EV) for primary axis is shown at the base of the columns, with significance value in parentheses. Letters following species names denote growth forms: A = annual, C = chamaephyte, G = geophyte, H = hemicryptophyte, S = tall shrub.

Basalt Limestone-Dolomite Sand-Alluvium

Scorzonern papposa Astrngalus asterins Asphodelus tenuifolius Reseda muricatn Ochradenus bnccntus Scrophulnrin deserti Salvia lanigern Gngea reticulata Lycium shawi

EV = 0.02 (P = 0.01).

Zygophyllum dumosum Atriplex hnlimus Limonium pruinosum Bellevalia desertorurn Lycium shawi Gymnocnrpos decnndru Astragnlus spinosus Ochrndenus baccatus Scorzonera judaicn Erodium hirtum EV = 0.026 (P = 0.01)

C C C G S

f m C C S H H

Diplotaxis hnrrn Euphorbin chamaepeplus Erodium arborescens Renumuria hirtelln Astrngalus nsterias Centnuren negyptincn Erodium deserti Scnbiosn eremophila Vnrthemia iphionoides Herninria hemistemon EV = 0.019 (P = 0.01)

A A H C H C A A C H


Table 3. Species responding strongly to the effect of the dichotomy: eastern and western sand substrates. Species listed had high positive scores on the primary axis of a redundancy analysis, using only the dummy variable east sand-west sand as an environmental variable. Species with high positive scores show apositive correlation with western sands with high loess content. Low negative scores indicate a positive correlation with eastern sands with low loess content. Eigenvalue for primary axis = 0.029 ( P = 0.01). Letters following species names denote growth forms: A = annual, C = chamaephyte, FB = facultative biennial, G = geophyte, H = hemicryptophyte, S = tall shrub.

High Scores (West) Low Scores (East)

Onobnchis cristagnlli Trigonella stellata Gagea reticulata Teucrium polium Plnntago cylindrica Erodium hirtum Annbnsis articulatn Mntthiola arnbicn Diplotaxis hnrrn Gymnocnrpos decandrum

A A G C A G C H A C -

Artemisin monosperma C Retnma raetnm S

Pnracarjum rugulosum FB Helianthemum salicifolium A Bromus fasciculatus A Crotalnria negyptiacn S Scrophulnrin deserti C Calendula arvensis A Trifolium tomentosum A

Astrngalus kahnricus H

Perennial vegetation on different rock types

The diversity of dominant species in the wadis is high (Table 1) and these alternate in which is the most dominant, with no apparent pattern. In western Makhtesh Ramon, A. herba-alba and A. halirnus drop out fairly early asNahal Ramon moves eastward (central Makhtesh Ramon). As one moves further east, the wadi becomes more open with wider terraces where Gyrnnocarpos decandrurn is most abundant. In disturbed areas, Ochradenus baccatus and Lyciurn shawi become more abundant and may even be dominant. In the last 1 km before the wadi exits the cirque, the dominant species is Tarnarix nilotica, with some Acacia raddiana. This is concomitant with the presence of groundwater (Issar 1983; Y. Avni pers. comm.). Tarnarix nilotica is well- known to occur where groundwater is high, and is often found near cisterns built by Nabatean people at the beginning of the Common Era (Danin 1983). - Note: historians, archaeologists and anthropologists use the terms ‘BeforeCommonEra’ (B.C.E.) and ‘CommonEra’ (C.E.), which are the same time periods indicated by the Christian religious terms ‘Before Christ’ (B.C.) and ‘Anno Domini’ (A.D.), to avoid religious terminology -

A Sudanian species, Acacia raddiana, predominates in two small wadis that run parallel to the southeastern wall of the cirque. This appears to be related to exten- sive granite intrusions and high run-off. In Sinai, a desert with large granitic intrusions, A. raddiana is also found associated with this substrate (Danin 1983).

Table 4. Comparison of dominant plant species (from McAuliffe plot % cover data) in the wadis in the eastern and western Makhtesh Ramon. Letters following species names denote growth forms: C = chamaephyte, H = hemicryptophyte, S = tall shrub, T = tree.

West

Retnmn rnetam Atriplex hnlimus Artemisin herba-alba Moricandin nitens Echinops polycerns Piturnnthos tortuosus Achillea fragrantissimn Ochrndenus baccntus Lycium shawi

- S C C C H C C S S

East

Retama raetnm Thymelnen hirsuta Artemisin monosperma Moricandin nitens Crotalaria negyptincn Anabasis articulatn Stipngrostis raddinnn Ochradenus bnccatus Lycium shnwi Acacia raddinnn Tnmnrix niloticn

S S S C S C H S S T T

The most common non-wadi substrate in the cirque is sandstone. The compacted sandstone substrate in the cirque is dominated by Anabasis articulata and Gyrnno- carpos decandrurn. This is the case in both compacted sandstone areas and in consolidated dunes in the eastern Makhtesh. It appears that dominance on sandstone is related more to the compaction of the substrate than to other factors such as sand-grain size, for example. In the west, Arternisia herba-alba and Reaurnuria hirtella are common on this substrate.

The most contracted vegetation (i.e. restricted to moister areas) occurs in the central part of the cirque on

Table 5. Comparison of dominant plant species on five major types of parental material in Makhtesh Ramon, using percentage cover from McAuliffe plots as indicator of dominance. Letters following species names denote growth forms: A = Annual, C = chamaephyte, G = Geophyte, H = hemicryptophyte, S =tall shrub.

Limestone-Dolomite Zygophyllum dumosum Anabasis articulata Renumuria hirtella Gymnocarpos decnndrum Helinnthemum kahiricum

Gypsum Hammnda snlicornica Capparis ovatn Herninria hemistemon Moricandin nitens

Loess Anabnsis syriacn Stiun cnuensis

C C C C C

C S H C

H A

Basalt Anabnsis setifern Ixiolirion tntaricum Pternnthus dichotomus Reboudia pinnntn Scabiosa eremophila Stipa capensis Gymnocnrpos decnndrum Erodium brjonifolium

Sandstone (Non-Wadi) Annbnsis nrticulatn Gymnocnrpos decnndrum Reaumuria hirtella Artemisin herba-alba

C G A A A A C A

C C C C


Table 6. Comparison of dominant plant species in Biqat Machmal and Biqat Ardon sand dunes. Letters following species names denote growth forms: A = annual, C = chamaephyte, G = geophyte, H = hemicryptophyte, S =tall shrub.

Biqat Machmal Biqat Ardon

Pnncratium sickenbergeri G Pancratium sickenbergeri G Cutnndia memphiticn A Calligonum comosum s Echiochilon fruticosum C Blepharis cilnris A Erodium hirtum H Iflogn spicatn A Hippocrepis unisiliquosa A Lotus hnlophilus A Matthioln livida A Stipa pawiflorn H Reboudia pinnata A Arnebin tinctoria A

sandstone plateaux with loess covering and scattered basalt stones. The only vegetation present occurs in narrow washes (average diameter 1-2 m), where Gymno- carpos decandrum and Zygophyllum dumosum are most frequently found.

The two sand dunes differ considerably in species composition, both from other sandstone habitats and from each other. Of the 34 species in both dunes, only 21 are common to both. Moreover, the only dominant species common to both sand dunes is the Madonna lily Pancratium sickenbergeri (Table 6).

Significant differences are also observed between vegetation of the western Makhtesh sands, with high loess content and sands of the eastern Makhtesh, with less loess (Table 3). Species occurring primarily in the west include Onobrychis cristagalli, Trigonella stellata, and Gagea reticulata, all species which can be found on loess or s a d . Species strongly associated with the eastern sands include Artemisia monospennu, Retama raetam and Astragalus kaharicus, all of which are typical of sandy habitats in the Negev (Danin 1983).

Richness and equitability

We recorded 150 vascular plant species in 63 0.1 ha Whittaker diversity plots in Makhtesh Ramon. Of these, 65 species were annuals, 5 1 chamaephytes, 11 geophytes and 9 grasses. Only two phanerophytes (Tamarix nilotica and Acacia raddiana) were recorded.

There was a significant positive relationship between the number of species per 0.1 ha plot and altitude: Log,, Species Richness = 0.001 Altitude +0.986; r2 = 0.15; sy = 0.22; F = 9.20; P = 0.004; n = 56. Much more variance in species richness was explained by altitude when only wadis, the most-sampled habitat type, were included (r2 = 0.46; P < 0.001). There was no evidence of a unimodal relationship between species richness and altitude (i.e. no more variance was explained using a second-order polynomial regression).

Species richness was lowest on the pure loess substrate (2 speciedo. 1 ha), followed by gypsum, sandstone plateaux, compacted sandstone, basalt, limestone-dolomite, sand dunes and then sand alluvium in wadis.

Equitability, lDominance, was positively correlated with altitude: 1Dominance = 0.01 Altitude + 4.17; 9 = 0.07, sy,? = 4.39; P < 0.01; n = 56. No more variance was explained using a second-order polynomial regression.

Annuals

In another study of plant-species diversity over a large area of the Negev (Ward & Olsvig-Whittaker in press), we found that annuals constituted 46% of all species. In the cirque, a similar percentage of annual species (43 %) was found. These figures are somewhat lower than those mentioned by Danin & Orshan (1990). There was a significant positive relationship between the percentage of annual species (arcsin square-root transformed) and altitude: 9% annual species = 0.001 Altitude + 0.179; r2=0.11; s =0.22;F=6.65; P=0.013; n=56. However, considerably more variance was explained by the relationship between the number of annual species per 0.1 ha (Whittaker diversity plot) and altitude: No. of annual species = 0.03 Altitude - 5.76; 9=0.30; sYX=4.85; t=23.69; P 0.001; n=56. No more variance was explained using a second-order polynomial regression for either of these regression relationships.

Y X

Discussion

The over-riding importance of altitude as an environmental factor affecting plant-species associations in Makhtesh Ramon is not surprising, considering its close correlation with rainfall (Danin, Orshan & Zohary 1975; Evenari, Shanan & Tadmor 1982), the factor most lim- iting in deserts (Noy-Meir 1973). Thus, the detection of this factor as the most important in our forward selection of variables, as well as its important effects on the number of annual species per 0.1 ha plot, was not unex- pected. The importance of several geological factors was striking. This is congruent with the data of Danin, Orshan & Zohary (1975) for the northern Negev and Judean deserts, where they were able to differentiate plant communities on the basis of geological substrate.

The fact that geological variables were more important than factors such as topography and inundation was surprising, considering the direct importance of the latter factors in water availability in this desert (Shmida,


Evenari & Noy-Meir 1986; Olsvig-Whittaker, Shachak & Yair 1983). Orshan (1986) also considers the role of the substratum to be important in Middle-Eastern desert plant communities. Our result, and Orshan’s (1986) review, thus contrasts with the result of Noy-Meir, Orshan & Tadmor (1973) from an arid shrubland some 30 km north of our study site, where topographical factors were found to be more important than edaphic ones.

Although geological features were more important, topography and inundation were still significant factors in affecting vegetation patterns. For example, topography was clearly very important on the stony plains, where the only vegetation O C C U ~ ~ in the lower-lying areas where run-off water collects. This is also the case in the gravel desert of Sinai (Danin 1983) and Egypt (Kassas 1952; Kassas & Imam 1959). Similarly, the wadi terraces which are inundated only in years of exceptional rainfall differ considerably in species composition from the wadis themselves. For example, Gymnocarpos decandrum and Anabasis articulata are dominant on the terraces, while these species seldom occur in the wadis themselves.

Austin (1978) and Austin, Cunningham & Fleming (1984) have shown that rock type, rainfall and temperature have a strong influence on the distribution of plant species. Rainfall and temperature negatively covary in the Negev (Danin, Orshan & Zohary 1975; Evenari, Shanan & Tadmor 1982), so rainfall alone need concern us here. At a local scale, aspect and topographic position are known to have a strong influence on vegetation composition (Austin 1978; Austin, Cunningham & Fleming 1984), requiring measurement of crests, exposed slope (north-facing), sheltered (south-facing) and a drainage line. We found that there was no difference between plots on exposed and sheltered slopes (plots were very close together or even non-differentiable on the first and second axes of the redundancy analyses). This is probably due both to the relatively low gradient of north- and south-facing slopes in Makhtesh Ramon and the west-east orientation of the cirque, thereby minimizing differences in exposure between these aspects. However, the situation in the valley and slopes of Nahal Elot, some 5 km north of Makhtesh Ramon (Danin, Orshan & Zohary 1975; Danin 1983), militates against this argument. The valley in Nahal Elot is steep-sided with a strong north-south aspect in parts, yet there is no differentiation between aspects. The over-riding factors there appear to be rock and soil type, which affect water run-off and its availability to the plants.

Spatial and temporal variabilitj

We found thatthere was considerable spatialvariabil- ity in vegetation, even within a single vegetation type.

This may be typical for deserts (e.g. Sonoran Desert, Hastings &Turner 1965; Chihuahuan Desert, Wondzell, Cornelius & Cunningham 1990), largely as aresult of the patchiness of rainfall (Shanan, Evenari & Tadmor 1967; Sharon 1970, 1972). Noy-Meir (1973) recognizes soil heterogeneity and texture as important factors in niche diversification in deserts, as did Danin, Orshan & Zohary (1975) in their survey of vegetation of the northern Negev and Olsvig-Whittaker, Shachak & Yair (1983) in their study of vegetation of a Negev Desert watershed. This is congruent with our findings, using both Whittaker and McAuliffe techniques. Soil heterogeneity, especially stoniness, contributes through its effects on the distribution of soil moisture (Noy-Meir 1973; Danin, Orshan & Zohary 1975; Olsvig-Whittaker, Shachak & Yair 1983). Thisis particularly evidentinthelarge wadis inMakhtesh Ramon, where dominance by different perennial species changes frequently. Stoniness appears to change considerably from year-to-year dueto the force of floods bearing stones and rocks to various parts of the wadis. This introduces considerable stochastic variation in plant community structure andbears further examination. This factor appears to be most likely to have impact on the dominance of communities where Lycium shawi and Ochradenus baccatus are present. These two species are effective colonizers, becoming dominant in areas where water flow during floods is most turbulent and disturbing the local vegetation.

Soil salinity is another important factor affecting desert vegetation, although not examined here. This is evidenced by presence of a specialized group of xero- halophytes, both annuals (mostly summer-growing Chenopodiaceae) and shrubs (Chenopodiaceae, Zygo- phyllaceae, Tamaricaceae) (Noy-Meir 1973). In Makh- tesh Ramon, these groups are well represented (e.g. Reaumuria hirtella, Tamarix nilotica, Zygophyllum dumosum), as they are in many deserts.

The difference between west and east sands appears to be due to the absence of loess in eastern Makhtesh Ramon. This factor clearly covaries with altitude (see interaction effect of soil texture and altitude, Fig. 3). The soils in the western part of the cirque have higher water capacity, favoring plants with shallow roots and rapid growth pulses, particularly ephemerals and annuals. In sandy or gravelly soils, where water capacity is lower (as in eastern Makhtesh Ramon) water tends to percolate deeper, shifting the advantage to perennials with deep root systems (Walter 1964; Noy-Meir 1970). These factors therefore readily explain the linear relationship between the number of annual plant species per 0.1 ha and altitude in Makhtesh Ramon.

Our emphasis has been on relating vegetation patterns to environmental factors. However, historical effects may also be important. A likely example of this is the


difference in the vegetation between the sand dunes in the Ardon and Machmal valleys. Although the two sand dunes are very similar in their relationships with environmental variables (Figs. 2 and 3), species composition differs drastically. There are twopossiblereasons forthis: 1. Ben-David (pers. comm.) suggests that Ardon valley was once isolated from the rest of the cirque by a wall of limestone on its western boundary, and constituted a cirque of its own. This barrier to dispersal was removed at least 10 000 years ago. It remains a moot point whether this is too short a time for plants to have colo- nized the Ardon valley from Machmal and vice versa. 2. A more prosaic reason is that the prevalent wind direction in the cirque is from west to east. This wind is indeed the reason for the formation of these sand dunes in the eastern edge of the cirque, because sand blowing across the cirque is trapped by the eastern wall of the cirque. Wind may enforce seed dispersal in an easterly direction, blowing seeds out of the cirque. This argument may be valid for wind-dispersed seeds (the major- ity of species), but is insufficient explanation for other dispersal mechanisms.

Another important historical effect on the vegetation in the Negev Desert is grazing of domestic stock. There has been continuous occupation of the central Negev since about 2000 B.C.E. (Evenari, Shanan & Tadmor 1982). Makhtesh Ramon is, as the boundary of the steppe and true desert (sensu Shmida 1985), south of the limit of agricultural activity, which is determined largely by rainfall (Zohary 1962). However, therearealargenumber of Stone Age sites, as well as a Nabatean (an agricultur- alist Arab people dominant in this area from 300 B.C.E. - 100 C.E.) caravan way-station. A road built by the Romans at the beginning of the Common Era to transport spices from the Far East traverses the cirque from north to south in the eastern part of the cirque. Although the cirque has not been grazed by domestic stock in more than 20 yr, it is likely that there were long periods of grazing by domestic stock prior to this. Many of the dominant species in the cirque (e.g. Artemisia herba- alba and Zygophyllum dumosum) are unpalatable or poisonous. We suggest tentatively that some of these patterns may be a result of a long grazing history that has removed palatable species.

Patterns of species richness and equitability

We found no evidence for a uni-modal relationship between species richness, equitability or annual species richness and altitude, the dominant environmental gradient. This contrasts with the results of Ghazanfar (1991), who found that maximum species richness occurred at intermediate elevations on a desert mountain in Oman. We suggest that a uni-modal relationship can not be

considered to be the norm, as Ghazanfar (199 1) sug- gested, but depends on local conditions. Indeed, in another study covering a wider geographical area in the Negev along an aridity gradient from 100 mm - 40 mm annual rainfall, we also found that there was a linear relationship between these variables and altitude (Ward & Olsvig-Whittaker in press).

Changes in desert vegetation with time

Lipkin (1971) conducted a survey of the vegetation of the southern Negev Desert, following the phytosociological approach of Braun-Blanquet. Although his work is not directly comparable with ours because of differences in sampling technique and analyses, he too found that there were strong interacting effects of altitude and parental material. He described the dominant plant species on the parental materials as follows: 1. Limestone-dolomite: Zygophyllum dumosum; 2. Sandstone: Anabasis articulata, less so Artemisia herba-alba; 3. Basalt: Gymnocarpos decandrum, Anabasis articulata; 4.Gypsum: Capparis ovata, Salsola spp.

According to Lipkin (1971), some perennial plant species were found widely diffused in the vegetation of the cirque. Gymnocarpos decandrum was common on most rock types in most areas. Anabasis articulata was less common but also widespread. Artemisia herba- ulba occurred nearly everywhere, but mostly on sandstones and on the higher southwest end of the cirque. Zygophyllum dumosum mostly occurred in the eastern, lower end of the cirque. Our data differ only in a few very minor features. Lipkin (1971) considered Gymno- carpos decandrum to be the most widespread dominant species in the cirque, while our data suggest that it is Anabasis articulata which is most widespread.

These differences may simply be due to sampling differences, because Lipkin (1971) used the Braun- Blanquet technique and subjectively chose sampling places based on their apparent representativeness of a particular ‘community’. Our plots were chosen using their positions along the gradsect to maximize sampling among a range of environmental variables, and data were collected using, we believe, the more rigorous Whittaker diversity sampling technique. These differences in sampling technique are also shown by the absence of Salsola spp. in our samples and their presence in Lipkin’s (1971) data. Salsola inermis was cer- tainly present on the gypsum substrate in the cirque during our study but was not present in our samples. Another difference between our two studies was that Anabasis setijera is commonly found on the basalt hills in our study. This species is a colonizing species with winged diaspores, commonly found on screes subjected


to perpetual landsliding and changes of the ground (Danin, Orshan & Zohary 1975).

The vegetation sampling technique used by Lipkin (1971) was essentially a phytosociological one, which prevented a detailed quantitative comparison with our data. We have established permanent plots in the sites of our Whittaker samples, which will facilitate more rigorous comparison and determination of changes in desert vegetation over time (these original data and information on location of sites have been deposited with the Israel Nature Reserves Authority, Jerusalem). This is particularly important because differences in site selection may affect the outcome of many multivariate anaiyses.

McAulijje versus Whittaker sampling

The low degree of congruence between samples using these two techniques is likely an effect of differences in the type of information collected. The McAuliffe technique provides data on the relative abundance of perennial species based on coverage. The Whittaker plots provide information on the relative abundance of all vascular plant species, based on frequency. These two kinds of data do not necessarily covary.

Despite the fact that the McAuliffe technique meas- ures only perennial species, we believe that it is a valuable tool in desert vegetation analysis. It is more rapid to perform (0.5-1 h per plot) than the Whittaker technique (2-6 h per plot) and gives quantitative abundance values for perennial vegetation that are readily usable. Management surveys are frequently constrained by time and season and are unable to do the amount of work necessary to collect an appropriate number of Whittaker plots. When sample size is constrained, McAuliffe plots are more likely to provide significant and interpretable results. These factors make this a potentially important technique for managers in desert habitats because vast areas can be surveyed quickly. If, as is often the case, managers require detailed knowl- edge of vegetation because of its importance for large herbivores, this technique is more suitable than the Whittaker technique because it enables rapid and accu- rate estimation of biomass. Biomass may be estimated by measuring canopy volume for each species, by measuring median canopy depth and multiplying by the square of median canopy diameter.

Although the abundance of annual vegetation may change significantly with grazing (e.g. Naveh & Whittaker 1979 or Olsvig-Whittaker et al. in press), there is considerable year-to-year variation in germina- tion of desert annuals which may overwhelm grazing effects in any given year (Loria & Noy-Meir 1979; Bowers 1987). Hence, monitoring of changes in perennial vegetation only might provide a more reliable index

of grazing impact than monitoring of the entire flora.

Acknowledgements. We thank Orit Skutelsky, Nen, Anat and Arthur Du Mosch, Rupert Nanni, Tamar Sinai, Shimrit Lahav, Roy Turkington andHannaFrank1 for assistance in the field. We are grateful to Yoav Avni for advice on geological matters and to Ya’akov Lipkin for providing us with a copy of his unpub- lished PhD thesis. We are most grateful to the staff of Rotem (Israel Plant Information Centre), particularly Hagar Leschner and Ori Fragman, for the identification of many of the species in this study. We received financial support from the Israel Nature Reserves Authority, Israel Ministry of Science and Teiiiiioiogy, aiid the Local Council of Mizpc Rzmon. This is publication no. 2 of the Ramon Science Center and no. 158 of the Mitrani Center for Desert Ecology.

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Received 3 December 1991; Revision received 11 September 1992;

Accepted 20 October 1992.

Vegetation-environment relationships in a Negev Desert erosion cirque

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