Journal of Arid Environments (2001) 48: 461}473doi:10.1006/jare.2000.0773, available online at http://www.idealibrary.com on

The effects of loess erosion on soil nutrients, plantdiversity and plant quality in Negev desert wadis

David Ward*-?, Kayla Feldman*A & Yoav Avni*B

*Ramon Science Center and ?Mitrani Department for Desert Ecology, JacobBlaustein Institute For Desert Research, Ben Gurion University of the Negev,

Sede Boqer 84990, IsraelBGeological Survey of Israel, Jerusalem, Israel

(Received 27 April 1999, accepted 6 December 2000, published electronically 21 May 2001)

Severe erosion, initiated by climatic changes during the Late Pleistocene-EarlyHolocene period and resultant declines in dust deposition, causes the formationof waterfalls during the winter floods in many wadi systems in the centralNegev desert of Israel. In some areas, erosion of the original loess substrate hasbeen complete, so that the underlying rock has been exposed. We examined theeffects of this erosion in four wadis in the central Negev desert on soilnutrients, plant community structure and plant quality. We predicted thaterosion has caused reductions in soil nutrients. Reductions in soil nutrientsshould result in reductions in plant cover. Furthermore, reduced soil nutrientavailability should cause reductions in the nutrient status and quality of theplants growing there. In addition to the loss of biodiversity that may result, thiserosion may result in economic hardship for the Bedouin peoples whose herdsdepend on these resources. In this study, there were significant negativeeffects of erosion on soil organic carbon, nitrate nitrogen and water-holding capacity, but not on soil phosphorus, conductivity or pH. Further-more, there was a negative effect of soil erosion on an overall measure ofsoil quality derived from a principal components analysis in three of the fourwadis we studied. Erosion resulted in an increase in plant species richness andsignificantly altered plant community structure in eroded areas of wadis.Increased plant species richness in eroded sites is consistent with the intermedi-ate disturbance hypothesis of plant community structure. Plants growing ineroded areas did not differ in two quality indices (nitrogen content anddigestibility), although plants typical of eroded areas had significantly lowerlevels of common digestion inhibitors (total polyphenols) and toxins (alkaloids)than plants from undisturbed sites. These last-mentioned results are contraryto our prediction and are consistent with the notion that plants growing indisturbed (e.g. eroded) sites maximize growth at the expense of investmentsin defense.

( 2001 Academic Press

Keywords: erosion; soil nutrients; Negev desert; plant diversity; organiccarbon; land degradation

?Corresponding author. Department of Nature Conservation, University of Stellenbosch, P. Bag X01,Matieland 7602, South Africa.ACurrent address: University of British Columbia, 6270 University Boulevard, Vancouver V6T 1Z4, Canada.

0140-1963/01/080461#13 $35.00/0 ( 2001 Academic Press

462 D. WARD ET AL.

Introduction

It has long been recognized that deserts are sensitive ecosystems (e.g. Evenari et al.,1982; Schlesinger et al., 1996). Desert regions are particularly susceptible to soil erosiondue to their low plant cover; plant cover reduces the impact of rain on the soil, therebyminimizing the erosive force of water (Bull, 1981). In time, such soil erosion also resultsin the loss of soil nutrients, particularly carbon and nitrogen (West, 1991; Mokwunye,1996), due to the restrictions of feedbacks in carbon and nitrogen cycles between plants,atmosphere and soil (Schlesinger et al., 1990).

Interest in the effects of anthropogenic disturbance on soil erosion in desertregions and their margins has intensified in recent years, particularly as a result of theUnited Nations Rio conference on desertification in 1992 (Schlesinger et al., 1990,1996). However, little attention has been paid to the effects of natural erosionprocesses in these regions. Desert loessal substrates are particularly sensitive to erosion(Schumm, 1969; Dendy & Bolton, 1976) and to desertification (Toy & Hadley, 1987).Erosion of loessal substrates is a widespread problem in the Negev desert of Israel (seee.g. Nir & Klein, 1974; Rozin & Schick, 1996 and one of great economic concernbecause these are the most important habitats for agriculture (Evenari et al., 1982).Conventionally, soil erosion is viewed as a problem caused by anthropogenic distur-bance (Crawford & Gosz, 1982; Schlesinger et al., 1990). However, in the Negev desert,this erosion problem is probably caused by long-term changes in the climate during thetransition from the wuK rm pluvial phase of the upper Pleistocene to the inter-pluvialclimate of the Holocene. This change in climate caused a reduction in the eolian loesssupplied to the Negev desert from North Africa (Horowitz, 1979; Gerson & Amit, 1987;Zilberman, 1993). It has been hypothesized that the main winds bringing loess to theNegev desert prior to the Holocene came across the Sahara, and as a consequence of thegreat fetch of the wind, brought as much loess to the desert as that which is washed awayeach winter during the annual floods (Horowitz, 1992). Moreover, because relativelyhigh humidity and vegetation cover are required for the trapping of loess in anenvironment, the greater loess deposition during the Upper Pleistocene indicates thatthere was higher precipitation in the Negev than there is today (Gerson & Amit, 1987).Thus, because of both the greater amount of loess reaching the Negev and the morefavorable conditions for loess accumulation during the Upper Pleistocene, there was nonet erosion during this period. Since the Holocene, the principal wind direction has beeneasterly across the Mediterranean Sea, resulting in considerably reduced deposition inthe Negev desert (Horowitz, 1979, 1992; Gerson & Amit, 1987). It should be noted thata far lesser amount of loess is brought to the Negev desert from Saudi Arabia by thiswind, known as the ‘hamsin’ wind. The meaning of ‘hamsin’ is ‘fifty’ in Arabic, denotingthe approximate number of days per year that this wind blows. Thus, as a result of boththe rarity of this wind and its shorter fetch than the westerly wind, the ‘hamsin’ is oflesser importance in terms of loess deposition in the Negev. The net result of the changein principal wind direction is that less loess reaches the desert to replace that lost in theannual floods. This results in net erosion, which is a natural process (Avni, 1998). TheNabatean people, who inhabited the desert between 200 BC and 100 AD, built tens ofthousands of terraces in the wadis to stop this erosion and to facilitate the use of rainfallrun-off for agriculture (Evenari et al., 1982). In the modern era, such activity hasstopped, and the Nabatean terraces have broken down in many places, allowing erosion tocontinue unabated. This natural situation may be exacerbated by man’s activities throughsuch agents as overgrazing, soil crust disturbance, and global climatic changes thatincrease ambient temperature and reduce precipitation and thereby reduce plant growth(Evenari et al., 1982; Crawford & Gosz, 1982; Schlesinger et al., 1996). These anthropo-genic factors increase the rate of soil removal by the winter floods (Rozin & Schick, 1996).

In this study, we examined the effects of erosion in wadis (ephemeralrivers), where most plant material in the Negev desert is concentrated (Ward &

THE EFFECTS OF LOESS EROSION ON SOIL NUTRIENTS 463

Olsvig-Whittaker, 1993). Previous work had shown that severe erosion causes theformation of waterfalls (ephemeral waterfalls of this type are also called ‘knickpoints’ or‘gully headcuts’ by some geomorphologists) during the winter, and at these pointserosion may continue until bedrock is ultimately reached. In the wadis we studied,research over 13 years has shown that these waterfalls may erode headward(i.e. upstream) up to a maximum rate of about 100 m per year (mean$S.E."10·6$4·66 m; Y. Avni, unpublished data). While the physical effects of this erosionin terms of loss of substrate are obvious, we wished to know whether this erosion alsocauses reduction in soil quality in the substrate that remains, and whether these lossesresult in changes in the diversity of plant communities and in the quality of these plants. Ifthis erosion causes reductions in soil nutrients, and concomitant reductions in plant coverand quality, this erosion may result in economic hardship for the Bedouin peoples whoseherds depend on these resources, in addition to the loss of biodiversity that may result.

Hypotheses and predictions

We tested the following predictions about the effects of soil erosion in Negev desertwadis:

(1) If erosion is negatively affecting the quality of the soil in the wadis, we expectlower soil nutrient values below the waterfall than above in each case. Also, if soilnutrient input from the adjoining hillsides and/or tributaries downstream serves torecharge nutrient losses at the erosion knickpoint, we should see a recovery in thesoil parameters with increasing distance from the waterfall as we progress down-stream.

(2) If erosion is negatively affecting the quality of the soil in the wadis, we expectlower values for plant cover and species richness and diversity below the waterfallthan above the waterfall in each wadi.

(3) We predict that erosion would cause a change in species composition below thewaterfalls such that true desert species (from the Saharo-Arabian biogeographiczone) predominate over Mediterranean and steppe-desert species (from theIrano-Turanian biogeographic zone) because the former species are betteradapted to conditions of low water availability and poor soil nutrient status thatoccur there.

(4) We predict that plants growing in conditions of low soil nutrients (in eroded areasbelow waterfalls) will have fewer nutrient resources to invest in plant quality, andwill therefore be of lower quality than plants growing in soils with greater soilnutrient levels (non-eroded areas above waterfalls).

(5) The resource availability hypothesis of plant defense (Coley et al., 1985) predictsthat plants growing in environments with low soil nutrient levels should invest inchemical defenses because any material lost to herbivory is extremely expensiveto replace because growth is limited under these conditions. Conversely, plantsgrowing under high nutrient conditions should invest in growth at the expense ofdefense because any material lost to herbivory is easy to replace via regrowth.Thus, we predict that plants in eroded areas below waterfalls should have higherlevels of defensive chemicals than plants growing above waterfalls.

Methods

Study sites

We chose four wadis along a north-south rainfall gradient in the central Negev desert,that ranges from about 180 mm annual rainfall in the north to about 85 mm in the south

Figure 1. Locations of erosion study sites in the central Negev.

464 D. WARD ET AL.

(see Evenari et al., 1982). These sites were (from north to south) near Ramat Hovav(Nahal Sekher; mean$S.E. annual headward erosion"25·7$11·81 m; n"8 years),near Yeroham (Nahal Revivim; mean$S.E. annual headward erosion"4·3$3·39 m;n"6 years), near Sede Boqer (Nahal Ziporim; mean$S.E. annual headward ero-sion"1·5$0·55 m; n"7 years) and near Borot Lotz (Nahal Elot; annual headwarderosion"0·2}1·70 m; n"2 years) (Fig. 1). Note that ‘nahal’ is the Hebrew word for anephemeral river used in official nomenclature, while ‘wadi’ is the Arabic equivalentwhich is in common usage. Wadis ranged in width from 5 to 40 m.

All four sites have a loess substrate. The substrate at Nahal Sekher differs fromthat of the other three sites in its higher percentage of sand in the loess and in thepresence of an underlying chalk base layer. Thus, this site has more porous soil than theothers, allowing nutrients and water to filter through. All the above-mentioned sites havebeen the subject of long-term geomorphological study by one of us (YA).

Soil quality

Soil samples were collected every 20 m above the waterfalls, at the waterfalls, and at 20,40 and 60 m below the erosion knickpoints (waterfalls) in each of the wadis. At each20-m point, we collected three soil samples from sites in the central channel andon either side of it and combined them (see Clinebell et al., 1995) because of thehigh small-scale spatial variability in desert soils (Schlesinger et al., 1996). Below thewaterfall, we collected samples from 0}200 m downstream and above the waterfallfrom 0}60 m upstream. Thus, we analysed four pooled samples above each waterfalland 11 pooled samples below each waterfall. Soil samples were collected once insummer about 1 month after the last flood of the year. Soil was collected at a depth of5}15 cm.

THE EFFECTS OF LOESS EROSION ON SOIL NUTRIENTS 465

We measured the following soil characteristics: conductivity, a measure of overall soilsalinity; pH, (both pH and conductivity were measured using conventional meters);water-holding capacity. This variable was measured by slowly adding distilled water toa 10-g aliquot of dry sand and re-weighing it at the point of saturation. Water-holdingcapacity is considered to be (Final mass!Initial mass)/Initial mass*100. Water-holdingcapacity provides a reliable indirect measure of physical soil structure because soil grainsize and porosity affect the ability of the soil to hold water (Foth & Turk, 1972).Note that the term ‘water-holding capacity’ as used here differs from ‘availablewater-holding capacity’ used by some soil scientists; the latter term represents thedifference between field capacity moisture content and the ‘wilting point’. Nitratenitrogen was measured by standard Kjeldahl techniques (Mulvaney, 1996). Totalphosphorus, measured following standard techniques (Kuo, 1996). Organic carbon,measured by the Walkley}Black method (Nelson & Sommers, 1996). Standard weight-loss-on-ignition techniques could not be used because of the high carbonate levels inthese desert soils. Organic carbon is a good measure of overall soil quality (Foth & Turk,1972; Nelson & Sommers, 1996). Moreover, organic matter is frequently highly posit-ively correlated with two of the most important soil nutrients, nitrogen and phophorus,in many soils. We also used a bioassay of plant growth, which is an effective andinexpensive method of determining soil quality (Olsvig}Whittaker & Morris, 1982). Weconducted bioassays in soil collected from each site as described above. Bioassays wereconducted at an exposed field site in the central region of our study. For each site, wehad five pots in each of which we planted 10 seeds of the experimental plant, radish(Raphanus sativus L.) of the Cherry Belle variety. This plant is an effective bioassaybecause it is able to grow in a wide variety of conditions, and therefore differences ingrowth of the plant are unlikely to be due to differences in preferences for specificnutrients (Olsvig-Whittaker & Morris, 1982). The soil was maintained at field capacity byaddition of water two or three times a day depending on the rate at which the surface layerdried out. After a period of 30 days, we recorded the total dry biomass of each plant.

Plant cover and species diversity

We used McAuliffe’s (1990) log-series survey method to survey perennial plantcover and species composition in a single plot above and below each waterfall in eachwadi (see Ward et al., 1993 for further elucidation of the utility of this technique). Plotshad a 12·9 m radius. We used a point-frequency frame to calculate percentage cover,species composition and average plant height for annual plants in these plots.

Plant quality

High digestibility and high leaf nitrogen content indicate high plant quality relevant tomammalian herbivores (Van Soest, 1994). We used the in vitro technique of Zacharias(1986) to measure digestibility, because it simulates the digestive processes of a rumi-nant and thus is a useful index of the quality of the plants to such animals. Leaf nitrogencontent was measured using conventional Kjeldahl apparatus (see e.g. Mulvaney,1996).

We tested the resource availability hypothesis of plant defense using two major groupsof plant defense chemicals:

(1) Polyphenols, which are major digestion inhibitors in herbivores (Herms & Mat-tson, 1992; Van Soest, 1994). Hence, high levels of total polyphenols indicate lowplant quality. We used the modified Prussian Blue technique of Hagerman(1995) to measure total polyphenols. Total polyphenols were related to a

466 D. WARD ET AL.

commercial tannic acid standard [an almost pure hydrolysable tannin (Mole& Waterman, 1987)]. Results are expressed in tannic acid equivalents (mgT.A.E./g dry leaf mass). We compared polyphenol concentrations of five domi-nant plant species above and below waterfalls at each of the waterfalls in our studysites. We extended this analysis by comparing total polyphenol values of the threeplant species most commonly found in eroded sites below waterfalls over theNegev (Lycium shawi, Pituranthos tortuosum, and Thymelea hirsuta) and those ofthree species representative of undisturbed sites above waterfalls in this region(Artemisia sieberi, Hammada scoparia, and Moricandia nitens).

(2) Alkaloids, which are compounds found in many plant species that are known tohave serious toxic effects on herbivores (see e.g. Keeler, 1975; Lindroth& Batzli, 1986; Harborne, 1993). We used a protocol from Lindroth & Batzli(1986) that measures alkaloid concentration based on a reaction with Dragen-dorf’s reagent. Quinine was used as a standard. Results are expressed in quinineequivalents (mg Q.E./g dry leaf mass). We compared total alkaloid values of thethree plant species most commonly found in eroded sites below waterfalls overthe Negev (L. shawi, P. tortuosum, and T. hirsuta) and those of three speciesrepresentative of undisturbed sites above waterfalls in this region (A. sieberi,H. scoparia, and M. nitens).

Statistical analyses

For comparisons of measurements taken above vs. below waterfalls, we used nestedANOVA, with above and below waterfalls as treatment levels of a factor, position. Allsamples within a site were nested within the site in these analyses. The continuousvariable, distance from waterfall, was treated as a covariate. A principal componentsanalysis of all soil quality variables was used to obtain an overall, combined measure ofsoil quality. We used a Detrended Correspondence Analysis (DECORANA) to deter-mine changes in plant community structure induced by erosion in the wadis. We useda s2 test to compare the proportion of Saharo-Arabian plant species above and belowwaterfalls.

Results

Soil quality

We found a number of changes in soil quality due to erosion, although there was noconsistent pattern to these changes in the individual soil quality variables that wemeasured (Table 1). Conductivity was low and changed little with distance from thewaterfall in all sites, except for Nahal Sekher where it fluctuated widely with no apparentpattern. There was no significant difference in conductivity above and below thewaterfall (nested ANOVA, F"0·202, p"0·819, error df."49). In Nahal Sekher,conductivity was higher below the waterfall than above it at all sample points (Table 1).The reason for the higher conductivity below the waterfall than above it at Nahal Sekhersite is that the high porosity of the soil allows salts to diffuse down through the soil tothe chalk layer, resulting in low conductivity above the waterfall. Where the soil is erodedbelow the waterfall, samples were taken just above the chalk layer where all the salts aretrapped; hence the higher conductivity values there. No apparent pattern of change inpH was detected at any site (nested ANOVA, F"0·218, p"0·805, error df."24).There was a significant decrease in water-holding capacity below waterfalls relative toabove them (nested ANOVA, F"7·708, p(0·001, error df."24). Organic carbonwas significantly lower below the waterfall than above it in all four sites (nested ANOVA,F"4·640, p"0·006, error df."24). Soil nitrate declined immediately below the

Table 1. Differences in soil quality and plant cover and plant species richness above and below waterfalls in the Negev desert at four wadi sites

N. Sekher N. Revivim N. Ziporim N. ElotVariable Above Below Above Below Above Below Above Below

Conductivity, mS 0·34$0·04 1·65$0·48 0·31$0·03 0·36$0·04 0·42$0·05 0·49$0·08 0·39$0·01 0·41$0·07pH 7·60$0·06 7·44$0·03 7·31$0·02 7·33$0·03 7·48$0·04 7·45$0·01 7·52$0·02 7·57$0·05H2O-holding capacity, % 27·0$1·5 25·4$1·0 30·2$0·3 27·4$0·9 25·4$1·4 21·3$0·4 26·3$1·3 22·1$0·4Nitrate, mg/kg 6·53$1·43 6·60$2·12 2·23$0·45 6·78$1·49 5·60$1·45 6·35$1·15 12·38$2·50 6·38$1·23Phosphorus, mg/kg 4·13$0·45 4·13$0·72 6·50$0·39 5·65$0·52 8·75$0·38 5·83$1·16 11·58$1·77 8·75$1·18Organic carbon, % 0·25$0·06 0·15$0·04 0·65$0·03 0·32$0·04 0·77$0·11 0·44$0·07 0·77$0·06 0·57$02 17Bioassay, g 0·006$0·001 0·009$0·002 0·010$0·003 0·013$0·001 0·011$0·002 0·013$0·003 0·009$0·006 0·021$0·005Plant cover, % 29·45 9·97 45·48 11·15 23·09 29·28 24·78 13·13Species richness 21 9 19 20 17 22 26 36

Bioassay"total dry mass (shoots and roots) of radishes grown in these soils.

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Figure 2. Total soil phosphorus in the four wadis. Revivim ( ); Ziporim ( ); Sekher( ); Elot ( ). Negative values on the x-axis indicate the mean value for sample points abovethe waterfall/while positive values indicate sample values below the waterfall.

468 D. WARD ET AL.

waterfall at Nahal Sekher but within 40 m rose to a level higher than that above thewaterfall, while phosphorus dropped immediately below the waterfall and then re-covered within 20 m (Fig. 2). In Nahal Revivim there was no obvious pattern ofresponse in nitrate and phosphorus (Fig. 2), while in Nahal Ziporim, nitrate increasedbelow the waterfall (contrary to our expectation) and phosphorus showed the predictedpattern of decline, recovering to its original level (i.e. above waterfall) within 60 m(Fig. 2). In Nahal Elot, phosphorus declined and then recovered within 60 m of thewaterfall (Fig. 2), while nitrate declined and stayed lower than its above waterfall value.Overall there was a significant difference in nitrate nitrogen above and below thewaterfall (nested ANOVA, F"2·794, p"0·049, error df."24), and no significantdifference in phosphorus above and below the waterfall (nested ANOVA,F"2·421, p"0·076, error df."24). There was no significant difference in totaldry biomass of radishes grown in soils from above and below the waterfalls (nestedANOVA, F"1·347, p"0·266, error df."24).

The principal components analysis of overall soil quality showed that sites abovewaterfalls had higher values on the first principal component axis in three out of fourwadis (Fig. 3). Similarly, sites above waterfalls had higher values on the second principalcomponent axis in three out of four wadis (Fig. 3). Thus, there is differentiation ofsites above and below waterfalls based on overall soil quality, consistent with prediction1. Furthermore, the PCA ordinated sites along a north-south gradient (consistent withthe rainfall gradient), with the northernmost site (Nahal Sekher) having the lowest valueon the first PC axis and the southernmost site (Nahal Elot) with the highest value. Thislast-mentioned result suggests that there is a gradient in soil quality in wadis from northto south in the central Negev desert.

Plant cover and species diversity

Total percentage cover was lower below the waterfall than above it in three of thefour wadis (Nahal Ziporim was the exception—Table 1). Contrary to our prediction

Figure 3. Principal components analysis of soil nutrient variables. First and second PC axes aredisplayed. Points indicate centroids of PC values for each site. Above ( ); below ( ).

THE EFFECTS OF LOESS EROSION ON SOIL NUTRIENTS 469

(prediction 2), plant species richness was higher below the waterfall than above it inthree out of four wadis (Table 1).

Plant communities were arranged along a north-south gradient, with the highestvalues on the first DC axis in Nahal Sekher and the lowest in Nahal Elot (Fig. 4). Also,in each case, sites below waterfalls had lower values on the first DC axis than sites abovewaterfalls (Fig. 4). Thus, in terms of overall community composition, erosion has hada significant effect. Moreover, erosion has an effect similar to that observedwith a reduction in rainfall because the site-scores for eroded sites are lower than fornon-eroded sites on the first DC axis, as is true of southern (drier) sites relative tonorthern (wetter) sites. The variance seen on the second DC axis indicates that sitesdiffer considerably in their response to erosion.

We found no significant difference in the percentage of Saharo-Arabian speciesabove and below waterfalls (s2

"0·250, p'0·95, df."3), contra prediction 3.Mean$S.E.% Saharo-Arabian species above the waterfalls was 25·950$5·363% andbelow the waterfalls was 24·575$10·316%.

Leaf quality

In three of four sites, plant nitrogen content was lower below the waterfall than above it.However, contra prediction 4, there were no significant differences in leaf nitrogenconcentration (mg N g!1 leaf dry mass) for the five dominant species between sitesabove and below waterfalls (nested ANOVA, F"0·994, p"0·429, error df."26;above/below waterfall nested within the four sites). Similarly, there were no significantdifferences in leaf digestibility values for dominant species between sites above andbelow waterfalls (nested ANOVA, F"0·173, p"0·950, error df."26).

There were no significant differences in total polyphenol values for the dominantspecies between sites above (28·685$4·402 mg g~1 T.A.E.) and below waterfalls

Figure 4. Detrended correspondence analysis of plant communities in the four wadis. Above( ); below ( ).

470 D. WARD ET AL.

(26·175$5·723 mg g~1 T.A.E.) (nested ANOVA, F"0·229, p"0·920, errordf."26). Contrary to prediction 5, total polyphenol concentrations of the three speciestypical of eroded areas were significantly lower (mean$S.E."19·013$3·530mg g~1 T.A.E.) than those of species typical of uneroded sites(mean$S.E."44·430$8·783 mg g~1 T.A.E.) (F"58·474, p(0·001, errordf."24).

Total alkaloid values of the three species typical of eroded areas were significantlylower (mean$S.E."0·085$0·009 mg g~1 Q.E.) than those of species typical of un-eroded sites (mean$S.E."0·096$0·004 mg g~1 Q.E.) (nested ANOVA, F"8·427,p(0·008, error df."24). This last-mentioned result, and that for total polyphenols, iscontrary to our prediction 5, and may reflect selection for plant defenses in long-livedplants in undisturbed sites and for fast growth at the expense of investments in defense inplants growing in disturbed sites.

Discussion

Soil quality

The results showing significant decreases in organic carbon, water-holding capacity,nitrate nitrogen and overall soil quality (as indicated by PCA—Fig. 3) on sites below vs.above waterfalls, indicate a significant negative impact of erosion. In all the wadisstudied, erosion in the center of the wadi is visually apparent (pers. obs.), yet itseffects were not always detectable in terms of changes in soil quality and did notshow consistent results across all wadis. We stress that these results indicate howgeomorphologically-apparent desertification (see e.g. Nir & Klein, 1974; Rozin& Schick, 1996) and changes in soil nutrient content are not necessarily congruent.Nonetheless, a decline in soil nutrients was recorded in some of the most important soilvariables, and thus is likely to significantly impact plant growth. Although the majorerosion is usually in the center of the wadi in a strip that is only 5}30 m wide, most plant

THE EFFECTS OF LOESS EROSION ON SOIL NUTRIENTS 471

biomass and species diversity in the central region of the Negev desert is concentratedthere (Ward & Olsvig-Whittaker, 1993). Moreover, the concentration of the watercurrent in the central erosion gully necessarily reduces the water flow to the adjoiningsides of the valley during the winter floods. As a consequence of this reduction in wateravailability, leaching of salts is reduced (Shalhevet & Bernstein, 1968; Dan et al., 1973,Dan & Yaalon, 1982; Dan & Koyumdjisky, 1987) and soil salinity increases on the sidesof the valley. Thus, even in the soil that remains uneroded, soil quality declines overtime.

The inconsistent results for erosion effects on soil nutrients may have beencaused by the wadis being in different stages of the erosion process, or by dif-ferences in inputs of soil nutrients from the adjoining hillsides at the different sites.It is clear to us that the most effective way to tease apart the various effects ofsoil erosion on soil nutrients is to apply fertilizer to the soil (see also Binkley & Vitousek,1989). We make the prediction that the addition of fertilizer will have greater effects(e.g. increased plant cover) on the plant communities below the waterfalls than thoseabove the waterfalls.

Plant community composition

The effects of erosion on plant community composition are clear, and are akin toeffects caused by a reduction in rainfall (Fig. 4). This result suggests that rainfalland soil nutrients may have similar effects on plant growth. The lack of significantresults in above-below waterfall comparisons of individual parameters of these plantcommunities, such as species diversity, may be due to the over-riding importance ofinter-site differences. Replication of these measurements with more wadis at similarpoints on the mean annual rainfall gradient may help to differentiate betweeninter-site and erosion effects on these parameters.

The increased species richness of below-waterfall plots relative to above-waterfallplots is consistent with the Intermediate Disturbance Hypothesis for plant speciesrichness (Grime, 1979). Following this hypothesis, stable environments have few spe-cies because selection favors a few species that are able to dominate the others. In highlydisturbed sites, only a very few species can withstand the extreme conditions. At sitesthat suffer intermediate disturbance, such as below waterfalls in the Negev, manynew niches are created by disturbance for plants to invade, resulting in higher speciesrichness there.

Plant quality

The lack of significant results in terms of plant quality indicates that the major effectof erosion is in overall plant community composition, but not in quality. This means thatrestoration work in these eroded systems can concentrate on monitoring changes inspecies composition alone, rather than examining quality as well, the former task beingfar easier than the latter.

The only significant effects that we found in this section of the study, namely thedifference in polyphenols and alkaloids between species typical of eroded andnon-eroded sites, is consistent with the general observation (e.g. Ward & Olsvig-Whittaker, 1993) that 6000 years of heavy grazing by domestic stock has causedselection for species that have adequate chemical defenses against herbivory. Thus,perennial species found in undisturbed situations have been selected for high levels ofchemical defenses. Contrastingly, species that occur in disturbed, eroded sites may havebeen selected for fast growth in order to get a foothold prior to subsequent floods, whichremove any species that is not adequately anchored to the substrate. We speculate that

472 D. WARD ET AL.

investment in fast growth occurs at the expense of investment in defense, as is predictedby the optimal defense theory of resource allocation (Herms & Mattson, 1992; De Jong,1995; Ward et al., 1997; Rohner & Ward, 1997).

In sum, the results of this study show that long-term, natural processes of erosion cancause significant declines in soil quality, and point to the necessity of differentiatingbetween natural and short-term anthropogenic causes of erosion because we have littlecontrol over the former process. Only once natural and anthropogenic sources of erosionhave been differentiated will it be possible to develop adequate erosion controlpolicies.

We thank Iris Schmidt and Iris Musli for technical assistance, and Ann Hagerman, DeniseDearing and Rick Lindroth for providing chemical protocols. This study was partially funded bya grant from the Office of Regional Development and by the Israeli Ministry of Science andthe Local Council of Mizpe Ramon. Y. Avni received partial financial support from the GeologicalSurvey of Israel during the period of this study. This is publication number 116 of the RamonScience Center and publication number 321 of the Mitrani Center for Desert Ecology.

References

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