- Allometry of Prosopis glandulosa along a topographic gradient in the Chihuahuan Desert - 111

Journal of Vegetation Science 14: 111-120, 2003© IAVS; Opulus Press Uppsala.

Abstract. The allometric relationships of trees in temperate andtropical forests are relatively well known, but not those ofwoody shrubs or transitional (shrub/tree) life forms. We ex-plored the transition of Prosopis glandulosa var. torreyanafrom tree to shrub along a semi-arid topographic sequencecomprising of six landforms (hillslope, footslope, upper andlower bajada, playa and dune) with varying soil texture andwater availability. In each landform, we measured P. glandulosashoot pre-dawn water potentials (Y) in one ‘dry’ and one ‘wet’year. We also measured plant height, widest basal stem diam-eter, crown area and number of basal branches. Total basal stemarea was calculated. We used simple (Model II linear regres-sion) and expanded (incorporating an asymptote to height orcrown area) allometry models to compare height with widestbasal stem diameter and crown area with total basal stem area.There were significant correlations between maximum plantsize and inter-year Y means suggesting that soil water availabil-ity decreased down the topographical sequence. The heightasymptote was statistically significant in all landforms andlower toward finer-textured soils. On the other hand, crown areawas a linear function of total basal stem area and was also sitespecific. Our results suggest that more basal branches are pro-duced as plant height decreases in more stressful environments,effectively increasing crown area with a minimum investmentin supporting tissues. The polymorphic characteristics ofProsopis may partly explain their occurrence in many arid andsemi-arid environments.

Keywords: Geomorphology; Phenotypic plasticity; Plant ar-chitecture; Polymorphic; Shrub; Tree.

Nomenclature: Correll & Johnston (1970); for families: Anon.(1998).

Allometry of Prosopis glandulosa var. torreyanaalong a topographic gradient in the Chihuahuan desert

Martínez, Armando J. & López-Portillo, Jorge*

Instituto de Ecología, Apartado Postal 63, Xalapa 91070 Veracruz, Mexico;*Corresponding author; Fax +5228187809; E-mail lopez-p@ecologia.edu.mx

Introduction

The architecture of plants (trees in particular) hasbeen explored through models which consider each speciesisolated from its surroundings. Under these conditions wecan infer the genetic potential in development expressedby the vigour, arrangement and re-iteration of plant or-gans from one or more apical meristems (Hallé et al.1978; Tomlinson 1983). However, actual plant growthform results from the development of an architecturalmodel against a specific ecological background (Fisher1984, 1986; Raven 1986; Tomlinson 1987; Steeves &Sussex 1989). Environmental pressures within this back-ground may be both abiotic (e.g. resource scarcity orover-exposure) and biotic (e.g. competition for resources,herbivory). In this context, plasticity in growth form as aresponse to environmental pressures may have an adap-tive value and, potentially, a genetic basis (Fisher &Hibbs 1982; Schlicting 1986; Mopper et al. 1991).

A common approach to explain plant architecture is torelate plant size traits to trade-offs between resourcegain and structural efficiency (Whittaker & Woodwell1968; Kohyama 1987; Niklas 1994; Farnsworth &Niklas 1995; Sterck & Bongers 1998). The studies thatrelate tree allometry to environmental variation arebased mainly on interspecific comparisons in tropicaland temperate forests, where light may be a resource inshort supply due to intense canopy competition (Clausen& Maycock 1995; Aiba & Kohyama 1996; King 1996;Thomas 1996; Bullock 2000). Few have consideredintraspecific responses or environmental gradients(Mohler et al. 1978; King 1996). These studies arebased on analyses of the different plant structural compo-nents such as crown area, height (linear or asymptotic)and stem diameter. While these size traits suffice todescribe single-stemmed trees, they may not be enoughto describe woody shrubs which, by definition, are plantswith multiple basal stems (Wilson 1995).

Some woody plants may develop as either tree orshrub; they are considered ‘intermediate’ (Bullock 2000)or polymorphic. We found that 71 families in the flora ofTexas (Correll & Johnston 1970) and 69 families in the

112 Martínez, A.J. & López-Portillo, J.

Sonoran Desert (Shreve & Wiggins 1964), have woodyspecies. This accounts for 40% and 50% of total numberof families (174 vs 138) in the Texan and Sonoran desertfloras, respectively. For dicotyledons, the proportions ofpolymorphic plants are 3.4% of a total of 3664 species inTexas and 1.5% of a total of 6 634 species in the SonoranDesert. The Fabaceae (Caesalpinaceae, Fabaceae andMimosaceae of Shreve & Wiggins, 1964) is the mostdiverse family in both sites, and ranks third (after Ro-saceae and Fagaceae ) and first in the number of poly-morphic species in the aforementioned floras. Finally, ifwe only consider woody dicotyledons, the proportions ofpolymorphic plants are, in the same geographic order,19.3% and 16.5%, similar to those of trees (25.2% vs13.2% ). Such phenotypic plasticity may be of adaptivevalue: plants able to assume both forms may have a widedistribution range, shrub form frequency increasing withenvironmental stress (Rundel 1991; Wilson 1995). Anideal case to test such an assumption is Prosopis glandulosavar. torreyana (Fabaceae) (honey mesquite), which canbe found as a single-stemmed ‘tree’ or a multi-stemmed‘shrub’ (Fig. 1).

We only know of one study on shrub allometry(Whittaker & Woodwell 1968) and ours is the first on theallometry of polymorphic woody plants. The evidence

for asymptotic height has been demonstrated for temper-ate and tropical forests (Aiba & Kohyama 1996; Thomas1996; Bullock 2000). Trees and shrubs in desert environ-ments are of low stature; there are no reports on theirscaling relations. We hypothesized that plants would betaller and have less basal stems with increasing wateravailability. Thus, the objective of this paper was to explorethe allometric relationship between Prosopis height andstem diameter and its variation along a semi-arid topo-graphic gradient of soil texture and water availability. Withthe same objective in mind, we explored the relationshipbetween crown area, also a potentially asymptotic size trait(Thomas 1996), and total basal stem area.

Methods

Study site

Fieldwork was carried out in the Mapimí BiosphereReserve, Durango, Mexico (26∞29' to 26∞52' N and 103∞32'to 103∞ 58' W; 1110-1450 m a.s.l.). This area is locatedwithin the southern Chihuahuan desert, in an endorreicbasin system known as the Bolsón de Mapimí (Bartolino1988). The climate is semi-arid, with a mean annualtemperature of 20.8∞C, a seasonal variation of 16.2∞C anda mean daily range of 20∞C. The mean number of dayswith freezing temperatures which occur mainly in De-cember and January is 37 (range: 11-58). Long-termyearly mean rainfall is 264 mm, 77% distributed assummer showers of short duration (Cornet 1988).

Landforms and soils along the topographic sequence

We selected six sites with the aid of aerial photo-graphs and a topographic map (INEGI Mapimí E14-16,1:250000). They were located along an 8.4-km longtopographic transect that extends in a NNW direction,from the slope of Cerro de San Ignacio (26∞41' N, 103∞44'W, 1200m a.s.l.) to a semi-mobile parabolic dune area(26∞88 45'N, 103∞44' W, 1070m a.s.l.). Six landforms canbe distinguished within this sequence (Fig. 2):

1. Hillslope, with relief slopes steeper than 25%; very thin calcaricregosols over rocky colluvia and zero salinity;2. Footslope at 1170m a.s.l., similar to the former but slopes as gentle as8%, deeper soils and zero salinity;3. Upper Bajada at 1165m a.s.l., hapli-calcaric regosols with slopes <6%; medium and fine elements and moderate (< 2 mS.cm–1) salinity;4. Lower Bajada at 1150 m a.s.l.; gentle slopes (0.5-2%); yermosols orxerosols over alluvia of medium and fine texture and moderate (< 2mS.cm–1) salinity;5. Playa at 1130 m a.s.l.; very gentle slopes (0.5%), yermosols overalluvia of fine texture and a high degree of salinity (3-19 mS.cm–1);6. Dunes at 1125m a.s.l., 10-15 m high and up to 80 m wide ¥ 300 m long

and sandy regosols of moderate (< 2 mS.cm–1) salinity (Delhoume 1988).

Fig. 1. (a) Tree form and (b) shrub form representing the rangeof physiognomic plasticity of Prosopis glandulosa var.torreyana at the Bolsón de Mapimí, Durango, Mexico, in theChihuahuan desert.

- Allometry of Prosopis glandulosa along a topographic gradient in the Chihuahuan Desert - 113

As in other semi-arid areas a gradient of soil texture,and probably water infiltration and availability, can befound along this topographic sequence (Noy-Meir 1973).Such a gradient results from two processes of erosion,transport and deposition (Breimer 1985; Blair &McPherson 1994; Lancaster & Nickling 1994). In thefirst (colluvial and alluvial) process, colluvia (rockyand fine sediments) at the hillslope are transportedover the footslope and fine (silt and clay) sedimentstravel further down along upper and lower bajadas,overlying marls of marine Tertiary origin (Bartolino1988). Bajadas are the coalescing alluvial fans of thehill watersheds in desert environments, with steeperslopes in the higher section. In the same way, finersediments deposit in playas, which are shallow ephem-eral lakes (basins) where sediment laden water re-mains until it evaporates (Breimer 1988; Delhoume1988; Wondzell et al. 1990). Finer texture in playasoils limits water infiltration and therefore root devel-opment into deeper horizons (Lunt et al. 1973).

In the second (aeolian) process, sandy sedimentsare wind transported from the margins of a shallowlake (Laguna de Palomas, ca. 26∞48' N, 103∞54' W;Bartolino 1988) or dry watercourses and form longitu-dinal dunes. Since sandy-textured substrates can haveless density than clayey-textured ones, there is deeperpenetration of water and oxygen and a lower resistanceto root growth (Danin 1991). In this sense, the dunesare not part of the main topographic sequence (MTS),but we include them in this study due to their impor-tance as habitat for Prosopis.

Plant communities

Vegetation is a xerophytic scrub (Rzedowski 1978) orChihuahuan desert scrub (Brown 1982). Besides Prosopis,other common species are Fouquieria splendens(Fouquieriaceae), Flourensia cernua (Asteraceae), Opuntiarastrera (Cactaceae), Agave asperrima (Agavaceae),Hilaria mutica (Poaceae), Castela texana (Simaroubaceae)and Larrea tridentata (Zygophyllaceae) in the upper partsof the topographical sequence, and Dalea scoparia(Fabaceae), Yucca elata (Liliaceae), Ephedra trifurca(Ephedraceae), Acacia greggii (Fabaceae) and Zinniaacerosa (Asteraceae) in the dune zone (Montaña & Breimer1988). There is a lower diversity of perennial species in thelower bajada, with H. mutica and O. rastrera ascodominants with Prosopis. Prosopis was the only woodyperennial species in the playas we sampled.

Study plant

Prosopis glandulosa is a frequent or dominant winterdeciduous species in most plant communities within theChihuahuan desert, its natural distribution encompassessouthern United States and northern Mexico (Simpson &Solbrig 1977; Rzedowski 1988). It is a facultativephreatophyte that produces both lateral and deep rootsand can occupy a wide range of soil depths (Heitschmidtet al. 1988; Ansley et al. 1990; Cuomo 1990). Theglandulosa variety is considered as a pest in grasslandsoverexploited by cattle in the southern United States(Fisher 1977; Archer et al. 1988; Archer 1989). Our studysubject is the torreyana variety, which is common andwidely distributed in the arid and semi-arid zones of NEMexico and SE USA (Burkart & Simpson 1977;Rzedowski 1988). In common with other species in thegenus e.g. Prosopis velutina (Stromberg et al. 1992,1993), it reaches its maximum height when growingabove relatively shallow aquifers, but is also present inareas where aquifers are lacking or very deep. Prosopisglandulosa refoliates, produces shoots and flowers threemonths before summer (Simpson & Solbrig 1977; López-Portillo et al. 1993), which is the season with the highestrainfall probability at our study site (Cornet 1988). Thereare few published studies concerning Prosopis phenol-ogy along gradients of soil texture or water availability(Lee & Felker 1992), but in our study site we haveobserved that refoliation and flowering starts earlier inareas where there is more water availability, such as alongephemeral water courses or around small man-made damsused by cattle.

Fig. 2. Idealized section of the topographic gradient at theBolsón de Mapimí, Durango, Mexico, in the Chihuahuandesert. Arrows indicate the direction of increase in differentphysical traits along landforms. Landforms are: HS, Hillslope;FS, Footslope; UB, Upper Bajada; LB, Lower Bajada; PL,Playa and DU, Dune.

114 Martínez, A.J. & López-Portillo, J.

Pre-dawn water potentials

As an estimate of water availability among landformswe measured pre-dawn water potential (Y) of Prosopisshoots with a pressure chamber (PMS Instruments,Corvallis, Oregon) in April 1999 and March 2001, whenProsopis leaves were fully expanded. We have no indica-tion of available groundwater or aquifers which may beused by plants along this topographical sequence. Fur-thermore, there has never been intensive use of water forirrigation and local ranchers depend on deep (> 30m)water wells, ca. 20km away from our study area, tomaintain cattle. Therefore, we assumed that Prosopisdepends on soil water during spring regrowth. On the firstdate (April 1999, Y1999), we sampled two shoots of eachof ten similarly sized trees per landform on two consecu-tive pre-dawn periods (23.00-06.00). This was after the1998 El Niño Southern Oscillation, and was the seventhconsecutive year below the long-term yearly rainfall mean(211, 240, 138, 206, 205 and 122 mm from 1992 to 1998vs the long-term of 264 mm). We also measured pre-dawn water potentials on a second date (March 2001,Y2001), a year after rainfall was greater than the long-termyearly mean (293 mm vs 264 mm). On this occasion, wemeasured one shoot of each of ten similarly sized plantsand in two topographical sequences since we wanted tomaximize Y range among trees and replicate landforms,but with a similar sampling effort. We calculated themean Y from the values of both years (1999-2001, Yy) asan indicator of soil water availability.

We used nested ANOVA to compare among landformswith respect to pre-dawn water potentials. In the firstcase, the model was Y1999 = landform + tree [landform](nesting factor within brackets). In the second case, wheretwo sites per landform were considered, the model was:Y 2001 = site + landform[site] and trees were considered asreplicates for every landform. Post-hoc comparisons ofmeans for pre-dawn water potentials were done withTukey HSD for equal sample sizes (Zar 1987).

Census of individuals and plant traits

We placed five 100m ¥ 20m transects (long axisperpendicular to the slope) giving a total of 1 ha in eachlandform, except in the lower bajada, where 0.8 ha wassufficient due to high P. glandulosa density, and at thehillslope, where we added one transect (0.2 ha) due to lowP. glandulosa abundance. The maximum distance be-tween transects was 200m at the hillslope and playa, and50m in the other landforms, depending on the area theyoccupied. For every individual within each transect wemeasured height, basal stem diameter (the widest if theindividual was multi-stemmed), the minimum (Dmin) andmaximum (Dmax) diameters of the canopy and counted

the number of basal branches. The crown area (C) wasestimated as an ellipse (C = 0.25p.Dmin.Dmax). We alsomultiplied the basal diameter of each individual by itsnumber of basal branches to obtain an index of thesupporting capacity of foliage mass that can be betterrelated to C. This index may overestimate the actual totalbasal area of the plant, but enables us to compare amonglandforms.

Pre-dawn water potentials vs maximum plant size

We correlated pre-dawn water potentials with maxi-mum plant size attained in each landform. Maximumplant size was considered an indicator of the optimaldimensions attained by Prosopis at each landform, andpre-dawn water potentials as an indicator of water stressto these plants. To do so, we used a subset of the tenhighest values (Yymax) for each plant trait in everylandform. We conducted normality tests previous to cor-relations and, depending on the outcome of these tests oron the type of variable, carried out either Pearson orSpearman rank correlation analyses (Zar 1987) betweenpre-dawn water potentials (Y1999, Y2001 and the inter-yearmean Yy) and maximum plant size for each trait. Eachcorrelation was calculated with six data pairs, one perlandform.

Simple and expanded allometry

We used simple (log y = a + blog x) and expandedallometry equations to explore the significance of archi-tectural plasticity (height vs. diameter, and crown area vstotal basal stem area) and its relationship with the topo-graphic sequence. Also, for simple allometry, the scalingcoefficient and scaling exponent were calculated by meansof Model II (reduced major axis, RMA) regression analy-sis of log-transformed data. The scaling exponent (theslope of the RMA regression: aRMA) was calculated as theslope obtained by ordinary least squares (aOLS) dividedby the regression coefficient (LaBarbera 1989; Niklas1994). The scaling coefficient (the Y1-intercept of theRMA regression: bRMA) was calculated following Niklas(1994, p. 331). We also selected aRMA over aOLS becausethe correlation coefficients may vary among landformsdue to sampling error and thus a standardized option ismore robust for comparison.

The expanded allometry equation we used was that ofAiba & Kohyama (1996):

1/H = 1/ADh + 1/H* (1)

where H = plant height (m) and D = basal stem diameter(cm); A = the initial slope (m/cm), h (a dimensionlessshape constant) and H* = upper limit to plant height (m)are regression constants. We considered three criteria to

- Allometry of Prosopis glandulosa along a topographic gradient in the Chihuahuan Desert - 115

choose between models: no trend in residuals, H* π 0, anda significant correlation coefficient between H* and maxi-mum H. The same function was used for crown area C vstotal basal stem area T:

1/C= 1/ATc + 1/C* (2)

In this case, the variables of the model are C = crowncover (m2) and T = total basal stem area (m2); the con-stants are A (m2/m2), c (dimensionless) and C* (m2, theupper limit to crown cover). The criteria to choose be-tween models was similar to that above.

We used SigmaPlot 5.05 to estimate the constants ofthese non-linear regressions, and to obtain the necessaryinformation for significance tests and confidence inter-vals to compare among landforms. We tested for differ-ences among landforms in both linear and non-linearparameters, including scaling exponents, with the aid of95% confidence intervals.

Results

Pre-dawn water potentials and maximum plant size

Plant pre-dawn water potentials (Y) in 1999 weregenerally more negative than in 2001 (Fig. 3). There weresignificant differences among landforms in 1999 (F5,54 =41.6; P < 0.0001); multiple comparisons indicated that Yvalues were more negative in the lower bajada and morepositive at the hillslope and dune (Fig. 3). We found nosignificant differences in Y-values between sites in2001(F1,10= 0.1; P = 0.9), but there were significantdifferences in landforms within sites (F10,108 = 56.7; P <0.0001). In this case, multiple comparisons indicated thatY-values were more positive at the hillslope and theupper and lower bajadas, and more negative in the playa(Fig. 3). The between-year ranges in Y were greater atthe intermediate landforms of the topographic sequence(footslope, upper bajada and lower bajada), and lowerat the other sites (Fig. 3). Inter-year means (Yy) werehigher at the hillslope, lower at the playa and intermedi-ate in the remaining landforms (Fig. 3).

For variables related to pre-dawn water potential andplant traits from the six landforms which were normallydistributed, we calculated Pearson correlations (r); fornumber of branches, a Poisson-distributed variable, weused Spearman rank correlation (rs). We found no signifi-cant correlations between Y1999 or Y2001 and maximumheight, basal diameter, crown area, and number ofbranches. Significant correlations (P < 0.04; n = 6) werefound for inter-year pre-dawn water potential means (Y y)against height (r = 0.88) and basal stem diameter (r =0.83), but not against crown area (r = 0.60; P = 0.20) ornumber of branches (rs= –0.29; P = 0.58).

Diameter-height allometry

The simple allometric model was significant for thesix landforms (r2 range: 0.18 to 0.73, P < 0.001), but therewas a quadratic trend in the residuals in upper and lowerbajada and in the general data set. Using a Model IIregression to estimate H (Hest) for the maximum observedvalue of D at each landform, we found that the linear modelis not a good predictor, since the correlation coefficientbetween Hest and Hmax was non-significant (r = –0.61; n = 6;P = 0.198).

The expanded allometry model was also significantfor the six landforms (P < 0.0001), and the proportion ofthe total variability in the data accounted by this model(16-82%) was slightly lower than that of the simpleallometric model in four out of the six cases, a differenceof 1 to 9%. We found no trends in the residuals derivedfrom any of the expanded allometry models correspond-ing to the six landforms. There was great data dispersionin height for a given basal stem diameter (Fig. 4a) whichexplains the ample confidence intervals in some of threenon-linear regression coefficients (Fig. 4 b-d). The insetin Fig. 4a shows the models for the two extreme landformsalong the main topographic sequence. There were nosignificant differences among landforms in the A coeffi-cients (Fig. 4b). The significantly different pair of hcoefficients (Fig. 4c) corresponded to the landforms clos-est to the extremes of the topographical sequence, as wasthe case for estimated maximum height (H*), whichtended to decrease along such sequence (Fig. 4d). Asopposed to the linear model, the non-linear linear modelwas a good predictor, since the correlation coefficientbetween H* and Hmax was significant (r = 0.84, n = 6, P =0.04). This result suggests that height is asymptotic andtends to vary with the landform.

Fig. 3. Prosopis glandulosa var. torreyana pre-dawn waterpotentials (Y, MPa) in six landforms along a topographicsemi-arid sequence in 1999 (●), 2001 ( ) and the inter-yearaverage (▲). Vertical lines indicate one standard error. Forlandforms see Fig. 2.

116 Martínez, A.J. & López-Portillo, J.

Crown area-total basal stem area allometry

The percentage of the total variability in the dataaccounted by the simple allometry model ranged from34 to 85%, for the non-linear model it ranged from 22 to62%. In this case, the correlation between Cest and Cmaxwas significant (r = 0.84, n = 6, P = 0.04), but thecorrelation between C* and Cmax (r = 0.43, n = 6, P =0.42) was not. There was some dispersion in the data,but no curvilinear trend (Fig. 5a), as we confirmed bythe distribution of residuals of the linear models for eachlandform (not shown). These elements justify choosing

linear over non-linear regression, and imply that crowncover is not asymptotic.

The Model II regression lines corresponding to theextreme landforms (hillslope and playa) are representedas an inset in Fig. 5a. The scaling exponent (Fig. 5b)tended to increase, and the scaling coefficient (Fig. 5c)tended to decrease, along the main topographical se-quence. The consequence of the inverse correlationbetween scaling exponent and scaling coefficient can beseen in the inset of Fig. 5a. This inset shows that morecrown area is reached for the same basal area at thehillslope up to the intersection of the two regressionlines at ca. 0.01m2 total basal stem area. After thatintersection, more crown area is reached for the sametotal basal stem area at the playa, probably because it isdivided into more basal stems: the number of basalstems (mean ± confidence interval) increased from thehillslope (3.1±2.3) to the playa (7.7±0.1) and dunes(11.2±3.0).

Fig. 4. (a) Relationship between height and stem basal diameterof Prosopis glandulosa var. torreyana in six landforms along asemi-arid topographic sequence. Inset shows fitted lines of theexpanded allometric equation (1/H = 1/ADh + 1/H*) for the twoextreme landforms. (b) - (d): Changes of A, h and H* coeffi-cients, respectively, of the expanded allometric equation in sixlandforms along the main topographic sequence (symbols joinedby dotted line) and dunes. Vertical lines are 95% confidenceintervals. For landforms see Fig. 2.

Fig. 5. (a) Relationship between crown area and total basal stemarea of Prosopis glandulosa var. torreyana in six landformsalong a semi-arid topographic sequence. Inset shows fitted linesof the simple allometric equation (log y = log a + b log x) for thetwo extreme landforms. (b) - (c): scaling coefficients (aRMA =slope) and exponent (bRMA = intercept) obtained by reducedmajor axis regression in six landforms along the main topo-graphic sequence (symbols joined by dotted line) and dunes.Vertical lines are 95% confidence intervals. For landforms seeFig. 2.

- Allometry of Prosopis glandulosa along a topographic gradient in the Chihuahuan Desert - 117

Discussion

The results highlight the relevance of the polymor-phic growth strategy in a soil-texture and water avail-ability gradient. Shrubs with multiple stems could beviewed as many ‘small trees’ packed in space, with aslender diameter bound to have a limited height. Ifisolated, each ‘tree’ would have a limited crown area; itis then that the ability to produce many basal stemsbecomes relevant, each one adding to the total crownarea of the whole plant. To support these assertions wewill discuss the relationship between soil texture andwater availability in the topographic sequence, Prosopisglandulosa allometries and finally, the adaptive signifi-cance of the polymorphic life-form strategy.

Water potentials, rainfall, and soil texture

Year-to-year soil and plant water potentials varydepending on rainfall, water infiltration and storage, andas a consequence the relationship between rainfall andwater potentials does not seem to be simple (Miller et al.2001), as we have found in our study site. Studies fromthe area and from other arid and semi-arid zones (Breimer1985; Delhoume 1988; Blair & McPherson 1994) showthat available and stored water are associated to a soiltexture gradient: more water infiltrates in coarse than infine soils, where water not locally lost as run-off mayremain in the top 30cm and evaporate or be accessible toplants for relatively short periods, a phenomenon termedthe ‘inverse texture effect’ (Noy-Meir 1973).

Several dry years, e.g. 1992-1999 in our study site,failed to replace water loss by evapotranspiration andresulted in lower plant water potentials. We were able todocument two extreme cases: a dry spell, which endedwith an El Niño year and a subsequent rainy year. Withthese results, we suggest that the lack of correlationbetween plant size and pre-dawn water potentials inindividual years on one hand, and the significant correla-tion among two plant size attributes (height and basaldiameter) and inter-year pre-dawn water potential on theother hand, would indicate that plant dimensions resultfrom ‘averaging’ water availability over many years.

The lack of a shallow water aquifer as has beenreported for riparian habitats (Stromberg et al. 1993) isevidenced by large inter-year differences in, or verynegative, pre-dawn water potentials down the main topo-graphic sequence. Of particular interest is the largecontrast between years in pre-dawn water potentials(very negative in the dry year and very positive in thewet year) at the lower bajadas, where up to 1.5 ¥ therainfall can infiltrate due to the dense vegetation stripescovered by Prosopis and grasses (Cornet et al. 1992).Such a contrast indicates that plants deplete the water

store in a relatively short period, and therefore morenegative water potentials develop in time. Pre-dawnwater potentials in the playas were similarly low be-tween the measuring dates, but this is probably as aconsequence of their much higher salt content relative tothe other landforms (Noy-Meir 1973; Delhoume 1988;Breimer 1988). Finally, the minimum inter-year differ-ence in the relatively higher pre-dawn water potentialsfrom Prosopis in hillslopes and dunes suggests thatsome rainfall water infiltrates and remains as a long-term store available at deeper horizons (Reynolds et al.1999; Cuomo et al. 1992). This dynamic landscapescenario may be fully encompassed by plants that, likeProsopis, produce both shallow and deep roots and takewater from upper and lower soil horizons (Heitschmidtet al. 1988; Ansley et al. 1990).

Prosopis glandulosa allometries

We found a limit in height as the plants increased ingirth, as described for other ecosystems (Aiba &Kohyama 1996; Thomas 1996; Bullock 2000). Theinitial slope (A) of the asymptotic equation was similaramong landforms, and the shape parameter (h) tended toincrease, while estimated maximum height (H*) tendedto decrease, down the main topographic sequence. Ourvalues of A were lower than those reported by Aiba &Kohyama (1996) and the mean ± s.e. was 0.77 ± 0.17 vs.1.71 ± 0.06. Since A is the initial slope, the biologicalinterpretation would be that, during ontogeny, moreheight was achieved per unit diameter in the warm-temperate forest tree species studied by Aiba & Kohyama(1996) than in the semi-arid polymorphic species westudied. In this context, A would be life-form specific,while h and height limit H* would be site specific.

The scaling relationship between crown area andtotal basal stem area was also site specific. However, incontrast to the trend in H*, the scaling exponent (aRMA= the slope) tended to increase and the scaling coeffi-cient (bRMA = the Y1-intercept) tended to decrease to-ward fine-textured soils. In biological terms, a highercrown cover per unit total basal stem area will be ini-tially found in landforms with coarse-grain substrates(e.g. hillslope) rather than in landforms with finer sub-strates (e.g. playa) but that (as indicated by the intersec-tion of the two allometric lines in Fig. 5a) this relation-ship will change as total basal stem area increases:crown cover will then be higher in clayey than in rockysubstrates. Since there is a positive correlation betweenplant height and crown width (Niklas 1994; Bullock2000), we would expect lower crown areas in the smallerProsopis individuals from the playa when compared totaller individuals on the hillslope, but this was not thecase due to multiple basal branches.

118 Martínez, A.J. & López-Portillo, J.

Adaptive significance of the polymorphic life form

Desert environments are typically open areas withlow vegetation cover except where water availability ishigh, as occurs along watercourses, where there is ashallow aquifer, or where water running as sheet flowadds to local rainfall water, as in vegetation stripes(Cornet et al. 1992; López-Portillo & Montaña 1999).Since investment in height implies an increasing alloca-tion to supporting tissues, Givnish (1995) predicted that“tall plants may be unable to survive in unproductiveenvironments”. This seems true within species such asProsopis: height is lower in the sites with finer soiltexture and lower long-term water availability. How-ever, there is another characteristic in this plant that maycontribute towards a higher carbon gain given the spe-cific constraints within the semi-arid area we studied:when of low height, it expands its crown area by in-creasing the number of basal branches. Wilson (1995)proposed two hypotheses on the adaptive nature of theshrub form and thus of multiple basal branches: (1)stems have relatively low cost and are expendable, andmay die under stress without compromising plant sur-vival and (2) that ‘adaptations for surviving high stressimposed the shrub form’. Our field observations indi-cate that both hypotheses are complementary, but wefound few dead branches in most of the Prosopis indi-viduals we measured. An additional advantage of pro-ducing more basal branches is to increase the photosyn-thetic area and decrease water evaporation by shadinggreater soil surfaces (Francis 1994).

There is a well-established positive relationship be-tween diversity of desert plant species or growth formsand coarse soil textures (Noy-Meir 1973; Bowers &Lowe 1986; Ezcurra et al. 1987), but ours is the firstreport on the association between landforms and plantmorphology of a particular species. As stated by Bowers& Lowe (1986), species that occur at the lowest posi-tions within a topographic gradient have broad physi-ological tolerances. Prosopis has broad tolerances, ascan be inferred from the fact it is found in most plantassociations of the Chihuahuan desert (Burkart &Simpson 1977; Rzedowski 1988).

Plant size traits are related to light capture, watertransport, mechanical support and resistance to dynamicloading (Givnish 1995). Space and light are limitingresources in multi-species communities, such as tem-perate and tropical forests. In their successional proc-esses beginning from open land, shrubs precede treesand succeed herbs, and are thus considered as the inter-mediate woody growth forms. There are transitionalforms in temperate forests (‘tree-like shrub’ and ‘shrub-like tree’, Küppers 1989), in dry tropical forests (Bul-lock 2000), and in other plant communities (e.g. Ohwi

1965; Croat 1978; Gillison & Walker 1981). In relationto plant morphology, tree-like forms would have a com-petitive edge in crowded environments (i.e. along wa-tercourses), where light availability is the limiting re-source due to competition between canopies, while shrub-like forms would have higher crown cover, productivityand reproductive success in areas under higher environ-mental stress, where vegetation cover is sparse.

How general is the relationship between polymor-phic plants and distribution range? There are six tree/shrub species in our study site, but none are as frequentas Prosopis. However, we would expect them to have asimilar pattern (i.e. low height and more basal branchesin stressful sites) to that described for our study plant.Another leguminous polymorphic species, Acaciafarnesiana, occupies many grasslands in the semi-desertareas south of the Mapimí (Gentry 1957) and is probablyanother good example of the relationship between plantpolymorphism and distribution range. It is also verycommon in slopes, foothills, and basins, and alternatesin dominance with Prosopis glandulosa var. torreyanaor P. juliflora, especially in the semi-arid highlands ofMexico (Gómez et al. 1970; A. Martínez & J. López-Portillo pers. observ.). It would be interesting to test ifother polymorphic plant species in arid and non-aridcommunities have similar responses to environmentalstress, including those produced in the ecotones betweenpreserved and disturbed vegetation communities.

Acknowledgements. This study is in partial fulfillment of thePh.D. dissertation of A. Martínez at the Posgrado de Ecologíay Manejo de Recursos Naturales, Instituto de Ecología, A.C.with the support of a CONACyT grant. Field work was sup-ported by projects 902-16 (Instituto de Ecología, A.C.),Departamento de Ecología, and CONACyT 903-23 granted toL. Hernández. A. Eben, J.B. Fisher, M. Martínez, J.F. Ornelas,C. Lara, A. Priego and V. Rico-Gray, V. Sosa and two anony-mous referees provided useful comments and criticisms. Z.Durán, A. Mottet, G. Gordillo, D. Pérez, G. Beucher and A.Herrera provided assistance in the field and A. Piña withdigital processing of plant illustrations. We also appreciate thelogistical support of the crew at the Laboratorio del Desierto inthe Mapimí Biosphere Reserve.

- Allometry of Prosopis glandulosa along a topographic gradient in the Chihuahuan Desert - 119

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Received 27 November 2001;Revision received 2 October 2002;

Accepted 9 October 2002.Coordinating Editor: E. Ezcurra.