Expl Agric. (1983), volume 19, pp. 187-198

Printed in Great Britain

EFFECTS OF IRRIGATION ON BIOMASS PRODUCTIONoF 32 PROSOPIS (MESqUITE) ACCESSIONS

.By PETER FELKERt, G. H. CANNELLf ,J.F.OSBORN$,P. R. CLARK$ and P. NASH+

tCaesar Kleberg Wildlife Research Institute, Texas A€sf Uniaersity,Kingsuille, Texas 78363 and tDepartment of Soil and EnaironmentalSciences, Uniuersity of Califumia, Riaerside, Califomia 92521, USA

(Accepted 6 August 1982)

SUMMARY

Thirty-two Prosopis accessions were studied and biomass production (dry matter) determinedfot 27 of them, which were irrigated when the soil moisture tension reached 60, 200, or500 kPa. Three seasons after transplanting (i.e. after 2th years) the trees were harvested,weighed, and sub-sampled for moisture content determinations. Little difference was obsenredin productivity among irrigation treatments. A 2O-fold range in biomass productivity occurredamong accessiors; Prosopis chilensis (0009) from Argentina gave the greatest production of13.4 t ha-r a-r. The water use efficiency of US rangeland accessions ranged from 2300-2600kg HrO kg dry matterr. P. chilensis (0009) in the driest irrigation treatment had a water useefficiency of 345 kg HrO kg dry matter-t.

The firewood shortage in the tropical semi-arid regions of the world is a majorreason for the ever-increasing pace of deforestation and desertification (Anon.,1980). Leguminous trees of the genus Prosoprs (mesquite) are well adapted toheat and drought stresses, and have potential for fuelwood production, forageproduction, and increasing the fertility of soils through dinitrogen fixation insemi-arid regions (Felker, 1979). Some Prosopz's species fix dinitrogen and growwell at extreme salinity (Felker et a|.,1981b).' An earlier study on Prosopis reported methods for establishing seedlings inthe glasshouse, field planting, and insect and weed control measures (Felkeret a1.,1981a). The present study included a comparison of 32 Prosopis acces-

sions (single tree collections) that received 7.4 cm of water by flood or basinirrigation when water potentials at 30 cm reached 60 kPa (- 0.6 bar, measuredby tensiometers),200 kPa (-2.0 bar) or 500 kPa (-5.0 bar, resistance blocks),providing wet, medium and dry treatments, respectively. The yields of biomass,estimated as stem volumes, ranged from 0.4 to 88 cm3 per tree at the end ofthe first season. The most productive accessions were from Argentina while theleast productive were from the rangelands of southern Arizona, New Mexicoand west Texas. The trial was continued for two further years, at the end ofwhich all trees were harvested and weighed. The diameters of their stems weremeasured at the ends of the second and third years and sample trees werechipped (homogenized) and sub-sampled for moisture content determinations,so that the yields of oven-dry matter could be estimated. Regression equations

0014-4797 18210000-0891 $01.00 O 1983 Cambridge University Press

188 PETER FELKER, et al.

were developed to estimate the oven-dry biomass at the end of each of thethree seasons' growth. The dry biomass measured at the end of the third sea-

son's growth was then compared with dry weight estimates from the regressionequations.

METHODS

The soil type, origin of the germplasm, planting procedures, irrigation system,and experimental design were described in detail in an earlier publication(Felker et al., 1981a). Six entries, recorded as P. spp. in the earlier paper, wereassigned to taxa (0001, 0013, 0032,0137,0025 and 0138) (Table 6). Fiveaccessions, P. pallida (0041 and 0140), P. juffiora (0044) and P. africana(0040 and 0045) were killed by * abnormally severe frost (- 5"C) during thefirst winter, so that the final harvest was of 27 accessions only. One of the P.

pallida (0140) trees resprouted from the stump and was included in the analysisafter the second season but not after the third even though it survived the en-

suing winter.Each water regime was applied in one of three randomized blocks, each

containing one plot of 12 trees of each of the 32 (later 27) accessions. Watertreatments were not replicated and their effects are therefore wholly confoun-ded with block differences. Valid estimates of error can however be made forwaterxaccession interactions. The 12 trees in each plot were arranged in a

4xZ aray at a 1.2 m x 1.2 m spacing. Each plot was surrounded by an earthenberm. Three-month-old plants, originally transplanted into the plots on 28 June1978, were harvested in November 1980. No fertilizer was applied to any ofthe plots.

Riverside, California has a Mediterranean type winter rainfall climate. Thereis no precipitation from mid-May to mid-October, so that effects of irrigationtreatments are to be expected. The winter rainfalls were 420 mm in L978-79and 370 mm in 1979-80. Water needs were determined independently for each

plot in each moisture regime, using tensiometers and resistance blocks as in1978. Soil water potentials were measured at 45 cm in 1979 and 60 cm in 1980in the hope of making the measurements more sensitive as the roots extendedin depth. Each time the readings indicated the predetermined water potential,7.4 cm of water were applied by flood irrigation, applying water separately toeach plot through its own valve and pipe opening. The amount of water appliedincreased from year to year in the wet and medium treatments as the canopiesexpanded. No irrigation was applied to the dry treatment during 1978 because

the soils in that treatment did not dry out to 500 kPa at 30 cm. Because irriga-tion had no effect during the second season, the dry plot was not irrigatedduring the third season. The amounts of water applied during the three summersare given in Table 1. The accession which made most growth, P. chilensis(0009), received a total irrigation over all three seasons of 1980, 1650 and510 mm in the wet, medium, and dry treatments, respectively.

Effects of irrigation on biomass production of Prosopis

Table 1. Ranges in water application (**)tWet (60 kPa) Medium (200 kPa)

189

7 5-225I 00-9003 00- I 000

f An additional 420 mm rainfall occurred in the 1978-79 winter and 370 mm in the 1979-80 winter. The 60, 200 and 500 kPa soil moisture levels were measured at 300,450 and

600 mm soil depths in 1978, 1979 and 1980, respectively.

BIOMASS ESTIMATION

In November 1978 and 1979 the basal diameters of the stems of all trees were

measured, but no trees were harvested. In November 1980, 2.5 years from the

original transplanting, all trees were harvested at ground level with a chain saw

and the basal stem diameters measurcd. The trees were weighed with pre-

calibrated spring scales of 14, 45, or 90 kg capacity to allow two significantfigures throughout the range of tree sizes.

Dry matter contents were determined by sub-sampling chipped trees of each

of the accessions. A whole-tree drum-chipper, donated by the Asplundh TreeExpert Co., was used to chip one of the 10 outside and one of the two inside

trees from each plot of 12 (3 x a) trees, so that one-sixth of all trees (162trees) were chipped. Three samples (500 to 1000 g each) of chips from each

tree were weighed within 15 minutes of chipping in the field and dried to con-

stant weight at 65oC. Because of mechanical difficulties, only trees in themedium irrigation treatment were chipped within 0.5 h after they had been

weighcd fresh, and the dry matter contents determined on these trees were

therefore applied to all moisture treatments. This was regarded as acceptable

because the differences among the mean dry matter contents in the varioustreatments were only 4.4To.

In the previous paper (Felker et al., 1981a) stem volume estimates were

used to estimate total biomass, since stem volumes were reported to be the pre-

ferred variable for biomass cstimation for use with linear regressions. However,

the most recent work comparing basal diameter and fresh weight in mesquite

(Felker et a1.,1982a) indicates that log-log expressions yield closer correla-

tions than linear regressions. Furthermore, the residual errors were not decreased

when height x basal diameter was used in the correlations rather than basal dia-

meter alone, so height measurements were discontinued after the autumn 1978

measurements.The data for biomass in the third season, derived directly from harvested

trees, were compared with the dry biomass estimates from first and second

seasons by means of regression equations. Fresh weights were only determinedin this study at the end of the third season. Theoretically, fresh weights were

determined on 972 trees (27 x 12 x 3) but only 935 trees were harvested because

of lack of initial planting stock or mortality during this trial. To determine the

Year

19 78t97919 80

0-7 51 00-580300-1 200

Dry (500 kPa)

0I 00-580

0

190 PETER FELKER, et al.

general relations between biomass over all three seasons, harvested freshweights, dry matter coefficients, and stem diameter data from 212 first seasonand 205 second season trees of Prosopis, grown in a plantation in another study(Felker et al., in preparation), were included in the total set of data. The regres-sion equation developed from the combination of the three data sets was: logledry weight (kg) = 2.1905 logls stem diameter (.-) - 0.9811. This highly signi-ficant relation, based on 1352 pairs of observations, had an 12 of 0.83 and anF value of 6681. Thus, dry biomass could be estimated for each of three sea-sons from the regression equation, and actual and estimated dry biomass couldbe compared at the end of the third season. To compare mean biomass produc-tion for the three irrigation treatments a Student-Newman-Keuls procedure wasuscd because populations among the treatments were unbalanced for reasonsdescribed earlier.

RESULTS

Dry matter contentThe differences between accessions in dry matter content at the end of the

third year (range 53 to 60%) and the standard deviations (range 0.0 to 4.8%)were surprisingly small (Table 2). The winter-deciduous North American nativespecies, P. glandulosa var. torreyana (0001), P. uelutina, P.sp.(0074) andp.sp. (0080) had slightly larger dry matter contents (60-61%) than the nearlyevergreen P. alba selections (53-57%), perhaps because the former had littlefoliage at the time of harvest.

Yields of accessionsThe average production of dry biomass per tree (estimated by regressions on

basal diameters) ranged from 0.02 to 0.29 kg tree-1 (15-fold differences) afterthe first ycar (Table 3), from 0.17 to 8.82 kg (2O-fold) after the second year(Table 4), and from 0.37 to 7.49 kg (20-fold) after the third year (Table 5).Measured production (Table 6) after the third year ranged from 0.4 to 9.2 kgtree-l (23-fold differences). P. chilensrs (0009) had the largest estimated bio-mass at theendof thethree seasons (0.3,3.8 and 7.5 kgtree-l), and the greatestmeasured biomass (9.2 kg tree-l) at the end of the third season. p. qlba (0099)had thc second largest estimated biomass in all three seasons (0.9,3.1 and 6.3kg tree-l) and yielded 6.8 kg at the end of the third, when it was outyielded byP. articulata (0016) (7.8 kg tree-l). Both in estimated and measured produc-tion, the shrubby prostrateaccessions 0074,0080, and 0028 from NewMexico,Arizona, and west Texas were among the least productive under all water treat-ments, and in all seasons. P. kuntzei, the least productive accession (only 0.4kg tree-l at final harvest) has no true leaves when mature. This species is photo-synthetically-active only in its stems, and might be expected to grow slowly,but it is of interest because its specific gravity (1.25-1.30; Burkart, 1976) islarge. Surprisingly, when grown in the glasshouse nursery under continuous

a

Effects of irrigation on biomass production of Prosopis 191

Table 2. Dry matter coefficients of third season Prosopis trees

Species

P. albaP. albaP. albaP. albaP. albaP. articulataP, chilensisP. kuntzeiP. laeaigataP. nigraP. n'igraP. nigraP. nigraP. ruscifoliaP. tamarugoP. uelutinaP. aeluti,naP. aelutinaP. glandulosa var. torreyanaP. glandulosa van glandulosaP. tpp.P. spp.P. spp.P. spp.P. albaP. albaP. alba

Accessionnumber

0098003 700390t320L3400160009013001 1400360034003 80133013100420020003 2

0025000 I00280080007 4010801r6001301370138

Dry weight:fresh weight

(%)

Mean and standarddeviationt

56 t 1.253 t 2,557 t 2.053 t 5.555 x 2.254 t L.656 t 1.760 t 4.358 t 2.556 x 2.26l t 0.055 t 2.656 t L.258 t 1.255 t 1.257 t I.258 t 1.46O t 2.460 t 1.063 t 0.261 t 0.860 t 0.053 t 1.453 t 0.653 t 0.653 t 3.157 t 2.4

Mean 56.6

f Values are for trees harvested in the medium inigation treatment as described in the text.Dry matter contents are means of three sub-samples from each of two homogenized(chipped) trees.

fluorescent light, P. kuntzei increased in height more rapidly than any otheraccession. The accession with the smallest production in the dry plot was P.

ruscifolia, which is reputed to be a serious weed in Argentina (Burkart, 1976).

Variations between seasonsThe estimated biomass per tree increased over l0-fold from the first to the

second season and by 2-3-fold from the second to the third. This is not sur-prising, since full leaf cover was not achieved until either late in the secondor early in the third season. Over the three seasons P. articulata (0016), P.

tamarugo (0042) and P. uelutina (0025) improved in biomass productivityranking while P. glandulosa var. torueyana (0001) declined in rank. Estimateddry weights at the end of the third season were smaller than the actual weightsof the more productive accessions, whereas the two values were similar in thesmaller ones. This justifies the use of data for first and second season trees from

192

Table 3.

Species

P. ch'ilensisP. albaP. albaP. pallidaP. pallidaP. glandulosa

var. totrey anaP. albaP. albaP. albaP. albaP. nigraP. articulataP. julifloraP. albaP. nigraP. spp.P. aelutinaP. uelutinaP. nigraP. aelutina

Accessionnumber

000900390013004 I0140

000 1

013 2003 7or37009801330016004401380038010800200025003 6003 201 1601340r 14003401310080

00280042007 4013000450040

PETER FELKER , et &1.

First season predicted dry biomass production (kS tree-r)

Biomass (kg tree-t )Irrigation treatment

spp.albalaeaigatanigraruscifoliaspp.glandulosa

var. glandulosatamarugospp.kuntzeiafricanaafricana

60 kPa 200 kPa

0.19 0.240.43 0.190. I 6 0.140.23 0.1 7

0.17 0.14

0.12 0.150.17 0.160.10 0.130.19 0.070.0 7 0. 100.13 0.060.13 0.140.15 0.130.1 I 0.1 I0.09 0. 130.14 0.070.11 0.060.08 0.090.12 0.0 7

0.06 0.1 1

0.0 7 0.060.0 7 0.080.04 0.060.05 0.040.04 0.040.05 0.03

0.03 0.020.03 0.030.03 0.030.02 0.020.01 0.010.01 0.01

0. 1 1 2y 0.09 2x

500 kPa

0.430.250.300. 190.20

0.1 7

0.100.200.150.230.1 7

0.090.090.130.120.060.090.090.0 7

0.090.100.080.0 7

0.060.050.04

0.050.040.030.030.010.01

0.L22y

Average

0.290.290.200.200.17

0.150. l40. 140.140.130.120.r20.120.120.1 1

0.090.090.090.090.090.080.080.060.050.040.04

0.030.030.030.020.010.01

0.1 1Meant

f Means followed by the same letter are not significantly different at P=0.05. The regression equationlogro dry matter= 2,1905 logro diameter - 0.981I was used to estimate the dry biomass yield per tree.

the other Prosopis field trials and strengthens the validity of the biomass com-

parisons among younger trees.

Effects of irrigation treatmentsThe medium irrigation treatment, in which water was applied when the soil

moisture at 30, 45 or 60 cm depths reached 200 kPa (- 2 bar), proved to be

the most productive overall water regime (although not statistically significant)in the second and third season's estimated growth, and in the final measured

growth. Surprisingly, in the first season, the wet and dry were better than themedium treatment. Irrigation had no effect on either the estimated or actualweights of P. chilensrs (0009) in the third season, even though no water was

,

Effects of irrigation on biomass production o/Prosopis

Table 4. Second season pred.icted dry biomass production (kg tree-t)

Biomass (kg tree-r )Irrigation treatment

193

Species

P. chilensisP. albaP. albaP. albaP. albaP. alb aP. albaP. glandulosa

vat. totTeyanaP. n'igraP. albaP. articulataP. spp.P. aelutinaP. nigraP. aelutinaP. spp.P. albaP. aelutinaP. nigraP. laeaigataP. pallidaP. spp.P. tamarugoP. tpp.P. tpp-P. ru,sci,foliaP. nf,graP. kuntzeiP. julifloraP. pallidaP. africanaP. africana

Meant

Accessionnumber

0009003900130t37013 20098013 8

000 r01 33003 700160t0800200038002501 l60t340032003601140140002800420080007 4013 I003401300044004 I00450040

60 kPa 200 kPa 500 kPa

4.553.282.51l.g70.7 61.891.79

2.171.431.7 2

l.3l0.931.341.231.290.9 20.910.7 30.7 4l.a20.000.610.7 30.3 g

0.440.220.39o.2L0.000.000.000.00

l.33xy

Average

3.823.102.292.271.911.871.80

1.7 2

1.621.611.601.541.27l.l9l.l4l.0l1.000.960.840.690.590.580.550.440.420.420.350.170.000.000.000.00

1.34

2.873.092.042.492.04l.l41..80

4.0+2.942.312.352.932.821.80

1.25 t.7 3l.l I 2.371.10 2.061.7 0 I .801.7 L 1.991.37 1.100.8 2 1.501.24 0.900.71 1.400.95 L.L20.7 4 L.4l1.00 0.780.42 0.630.59 0.000.42 0.7 20.41 0.510.44 0.490.21 0.610.46 0.560.27 0.380.09 0. 190.00 0.000.00 0.000.00 0.000.00 0.00

1.19y 1.52x

f Means followed by the same letter ale not significantly different at P=0.05. Averages are of all trees.Where mortality caused uneven sample sizes between treatments the average is not equal to the mean ofthe three irrigation treatments.

applied to the dry plot in the first season (other than 100 mm at transplanting),or in the third season, and only 500 mm were applied in the second season. Inthis treatment, the total water received by P. chilensrr (0009), from irrigationplus rainfall between planting in June 1978 to harvest in November 1980, was

1393 mm.

Estimates of production per hectareThe biomass production for these plots was expressed in dry kg tree-l

because data from small plots of 12 trees on a 4 x 3 array cannot be directlyextrapolated to kg hal due to border effects. Nevertheless, it is useful to arriveat order of magnitude estimates in kg ha 1. In the 4 x 3 anay, only two trees in

t3

194

Species

P. chilensisP. albaP. albaP. articulataP. albaP. albaP. nigraP. albaP. albaP. spp.P. albaP. glandulosa

var. totreyanaP. uelutinaP. albaP. n'igraP. aelutinaP. tpp.P. laeaigataP. nigraP. aelutinaP. spp.P. tamaragoP. spp.P. tpp.P. nigraP. ru,scifoliaP. kuntzeiP. julifloraP. pallidaP. pallidaP. africanaP. africana

Meanf

Accessionnumber

000900390098001 6013 7001 30133013201380r08003 7

000100250134003800200l 1600140036003 200280042007 4008000340131013000400140004100450040

PETER FELKER, et aI.

Table 5. Third season predicted dry biornass production (kS tree-t)

Biomass (kg tree-r )Irrigation treatment

60 kPa 200 kPa

6.967.444.095.405.545.333.7 |4.245.295.494.04

3.243.082.983.323.942.152.032.992.391.421.661.06l.3l0.931.0 I0.230.000.000.000.000.00

3.40y

8.155.556.736.226.195.615.557.004.245.604.7 6

4.213.924.093.7 02.253.l g

4.072.894.092.221.431.82l.691.361.800.500.000.000.000.000.00

4.03x

500 kPa

7.345.856.444.7 04.l g4.514.022.073.43l.7g3.99

5.1I3.463.282.892.833.7 22.401.951.441.391.881.241.030.950.390.390.000.000.000.000.00

3.05y

Average

7.496.295.585.445.335.1 I4.454.444.324.264,25

4. 1g3.453.453.303.003.002.872.642.641.691.661.371.341.051.050.370.000.000.000.000.00

3.49

f Means followed by the same letter are not significantly different at P= 0.05.

each plot were completely surrounded by other similar trees. The averagedirectly-measured dry biomass of the two inner P. chilensis trees was 6.0 and6.2 kg in the medium and dry tre atments, respectively, about 66% of the averageof outer trees in the plot. This is a larger border effect than was observed in theCalifornia Imperial valley in a field study of P. chilensis (0009) (in a 5 x 5 arrayon 1.5 m x 1.5 m spacings) where theinnerninetreeshadTS%of thebiomassof the outer trees. In an analysis of small plot work with Populus, Zavitkovski(1981) found that the border effect could extend into the sixth row on closely-spaced (0.3 m x 0.3 m) plantings, but it did not extend past the outer row in1.2 m x 1.2 m plantings. As these Prosopis plants were grown at a 1.2 m x1.2 m spacing we believe that extrapolation of the data for the two innertrees to biomass production in kg ha-l is reasonable. At this spacing there were

Effects of irrigation on biomass production of Ptosopis

Table 6. Third, season measured d,ry biomass production (kg tree-r)t

Biomass (kg tree-r )

Irrigation treatment

195

Species

P. chilensisP. art'iculataP. albaP. albaP. nigraP. albaP. albaP. nigraP. albaP. aelutinaP. glandulosa

vat. totteyanaP. ni,graP. albaP. albaP. tpp.P. tpp.P. albaP. laeaigataP. aelutinaP. aelutinaP. spp.P. tamarugoP. nf,graP. tpp.P. spp.P. ruscifoliaP. kuntzeiP. julifloraP. pallidaP. pallidaP. africanaP. africana

Mean$

Accessionnumber

000900160039013701330098003 7003600130025

000100380t3401320r 160l 080l 380l 14002000320028004200340080007 40l3l0l 3000440140004100450040

60 kPa 200 kPa

9.97,99.69.04.43.84.66.76.25.5

8.89.04.97.16.55.76.24.15.34.7

8.96.45.93.74.76.14.23.53.34.5

9.27.86.86.75.25.15.04.94.94.9

t Dry biomass was calculated from the measured fresh biomass and the dryTable l.f Means followed by the same letter are not significantly different at P = 0.05.

5.1 4.94.3 4.84.4 4.41.0 4.35.0 4.11.2 4.03.5 3.93.2 3.22.7 3.01.0 2.51.8 1.7

1.8 1.7

1.2 t.70.80 1.40.85 1.20.30 1.00.46 0.400.00 0.000.00 0.000.00 0.000.00 0.000.00 0.00

3.31y 4.04

matter coefficients listed in

6718 trees ha-r, indicating a yield of about 40,300 kg dry matter htr or 13.4 thtr a t. Similar calculations for P. articulata suggest a dry biomass yield of4g,7O0 and,27,500 kg ha-l, corresponding to 16.6 and 9'2 t ha-t a-r, on the

medium and dry irrigation plots, respectively.With a total water input of 1393 mm (irrigation plus rain) the apparen,t

water use efficiency of i. chilensis (0009) was 345 kg H2O kg dry matter-l.Apparent is used to qualify water use efficiency since the trees may have

ucquired water from deep in the profile. Neutron probe data indicated little*ut.r within 4.2 m of the surface but the trees might have obtained water from

much deeper horizons. Similar calculations with the southwestern range species

(accession numbers 0080 and 0074) produced water use efficiency values larger

500 kPa Average

4.2 5.33.6 6.63.9 4.85.1 6.73.2 4.35.6 5.54.8 3.42.4 3.84.1 2.32.0 4.51.5 2.O

1.6 1.71.6 2.21.4 2.10.88 I .80.94 2.00.31 0.540.00 0.000.00 0.000.00 0.000.00 0.000.00 0.00

4.27x 4.53x

196 PETER FELKEr., et al.

than 2000 kg H2O kg dry weight-l. Whatever the source of water, the produc-tion of 13.4 t dry matter ha-l a-l with less than 500 mm per year is remark-able.

DISCUSSION

Prod.uction and. possible yield,s

The ranges in biomass production observed here were similar to those esti-mated from stem volume measurements in an earlier study (Felker et al.,1981a). The annual production of the best accession predicted in that study,4 t ha-r a-1, was considerably exceeded in this one, where the productivity ofP. chilensts (0009) averaged 13.4 tha-r tl with no more than 460 mm of watereach year. This yield of biomass compares well with those of rye and maize(9.6 and 18.8 t ha I a-1) managed for optimal biomass production in moremesic environments in closely-spaced plantings (100,000 plants ha t) and with250 kg htr a I nitrogen (Crookston et al., L978). It is also several-fold greaterthan the biomass production of unmanaged semi-arid grasslands or desert eco-systems (Webb et aI., 1978) with comparable water inputs. In field trials in theCalifornia Imperial Valley similar dry matter production (29 t ha-r in 2 years,I4.5 t ha-r a 1) has been obtained for P. chilens'is and P. alba (0039) (Felkeret aI., in preparation).

The water use efficiency of 345 kg H2O kg dry matter-r calculated for P.

chilensis (0009) in this study is larger than efficiencies of 1671 and 4800 kgH2O kg dry matter-t (DM) measured by McGinnies and Amold (1939) andDwyer and DeGarmo (1970) for Arizona and New Mexico mesquite, respectively.Accessions from New Mexico (i.e. 007a) and Arizona (i.e. 0080) used in thisstudy had water use efficiencies of 2270 and 2645 kg H2O kg DM-r, which is ingood agreement with the same two earlier studies. Clearly, dry matter produc-tivity and water use efficiency in Prosopls is dependent on the genetic com-position of the selection under investigation.

The water use efficiency of 345 kg H2O kg DM-l for P. chilenszs (0009) is

considerably greater than domesticated legumes, which range from 500 to900 kg H2O kg DM-t (Briggs and Shantz, l9I4; Ludlow and Wilson, 1972) andcompares favourably with the C-4 grasses, maize and sorghum, which havewater use efficiencies of 240-3Lb and 223-360 kg H2O kg DM-r, respectively(Briggs and Shantz, 1914).

Prosopis pod production occurred in the second and third season and wasgreatest in the dry irrigation treatment (Felker et al., in preparation). Thegreatest pod production occurred in the southern Anzona accessions but in nocase did the total pod plus wood and leaf biomass of these accessions exceed60% of the P. chilensis leaf plus woody biomass.

Many of the slow-growing accessions (such as 007 4 and 0080) were prostrate,multi-stemmed and quite thorny. In contrast, some of the P. alba and P.

chilensis accessions were nearly erect and thornless. Results of a cold hardiness

Effects of irrigation on biomo,ss production of Prosopis 197

screening trial at 1500 m elevation indicated that shrubby prostrate accessionswere more cold tolerant than P. chilensis (0009) or P. alba (0039) but less thanthe nearly single-stemmed, more upright accessions 0025 and 0001 (Felker efal., I982b). North American accessions were identified in the cold hardinesstesting that were nearly erect and had greater cold tolerance and biomass pro-ductivity than either 0001 or 0025. Thus, with the possible exception of abun-dant pod-producing genetic characters in accession 0080, the slower-growingNorth American rangeland mesquites 0074,0080 and 0028 hold little potentialfor direct use or breeding stock.

The truly tropical P. pallida and P. juliflora accessions from Hawaii and Sene-

gal, West Africa, respectively, appear promising where temperatures never fallbelow -1.5oC. In contrast, both P. africana accessions appear to be very poorlyadapted to arid climates even in frost-free regions. Some of the Prosopis speciesexamined here have other exceptionally promising physiological traits. Forexample, P. articulata (0016), P. pallida (0041) and P. tamarugo (317) grewrapidly on a nitrogen-free nutrient solution equivalent in salinity to 50% sea-

water (Felker et a1.,1981b).In conclusion, thornless Prosopis accessions with very good water use effi-

ciency and biomass productivity have been identified that are promising forwoody biofuel production on arid lands short of water.

Acknowledgements. The whole-tree chipper was graciously donated by theAsplundh Tree Expert Company. The excellent advice and technical assistanceprovided by Paul Moore and UC-Riverside Agricultural Operations Staff, andthe financial support of the US Department of Energy Grant No. ET-78-G-01-307 4, are gratefully acknowledged.

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