Javad Eshaghi Rad?Seyed Rostam Mousavi Mirkala

Irrigation effects on diameter growth of 2-year-old Paulownia tomentosa saplings

Javad Eshaghi Rad?Seyed Rostam Mousavi Mirkala

Species of Paulownia are fast-growing trees that are planted in many countries in green spaces and agroforestry systems.We studied the influence ofirrigation on the diametergrowth of2-year-old Paulownia tomentosa saplings planted in 3 m 9 3 m grids at Nazlou Campus of Urmia University,Iran.Completely randomized experimental design was used with four irrigation amount,viz. 10,15,20 and 25 L,and three irrigation intervals,viz.3-and 6-days,in 8 replications.During the growth season,we measured collar diameters of allsaplings atthe end of each month.Diameter growth declined with reduced irrigation amount and increased irrigation interval.The lowest average diameter growth(9 mm)was recorded for 10 L water at 6-day intervals and maximum average diameter growth(28.7 mm)was recorded for 25 L volume at3-day intervals.The latter treatment resulted in significantly greater diameter growth than any other treatment(except 20 L at 3-day intervals).More than 80%of diameter growth was achieved in the first 3 months.Therefore,it is essential to irrigate with 25 L of water at 3-day intervals during the first three growth months to achieve maximum diameter growth for 2-year-old P.tomentosa saplings.

Paulownia tomentosaDiameter growthIrrigation intervalIran

Introduction

The decline in forest resources in developing countries is due to resource depletion and,in developed countries,due to the conversion of forests from industrial production to other uses such as recreation(Ashori 2006).Increasing demand forwood and shortage of wood supplies has led to renewed focus on use of fast-growing species(Swamy et al.2006;Mishra et al.2010).

Paulownia attracts increasing attention in many parts of the world.The value of Paulownia for forestry(Krikorian 1988)and afforestation of mining sites(Carpenter and Smith 1979)has been proven.Under natural conditions a 10-year old Paulownia tree measures 30–40 cm diameter at breast height(dbh)and contains a timber volume of 0.3–0.5 m3(Yadav et al.2013).In southeast USA,Paulownia tomentosa has been cultivated(Graves and Stringer 1989;Kays et al.1998)but producers experienced problems caused by poor growth and survival of Paulownia in heavy clay soils(Johnson et al.2003).P.tomentosa absorbs heavy metals such as zinc from soils(Azzarello etal.2012).P.tomentosa can be planted in areas where the minimum winter temperature is-20C(Clatterbuck and Hodges 2004).P.tomentosa and P.fortunei were planted in some Australian states and yielded good vegetative results (Borough 1991).Ayan et al.(2006)recorded 8.2 mm total diameter growth for 1-year-old P.tomentosa saplings in Kastamonu-Turkey.At Inwood Hill Park in New York County,P.tomentosa saplings with more than 30 cm dbh were found mainly in ridge top communities(Fitzgerald and Loeb 2008).Johnson et al.(2003)investigated the stability of P.tomentosa in Virginia,USA for 7 years, during which sapling survival was 27%,and diametrical growth rate for 2-year-old saplings was 17 mm(Johnson et al.2003).Aboveground growth of P.tomentosaseedlings is typically slow during the firstyear and a shift in emphasis from belowground to aboveground biomass accumulation occurs between the first and second year (Longbrake et al.2001).Even the growth rate of this species was higher than the growth rate of P.fortunei (24 mm)that had been planted in Mazandaran province (Riahifar et al.2009).Nevertheless,Bergmann(2003) measured the average diametrical growth of 2-year-old saplings of P.elongate to be 44 mm in the North Carolina state of America which showed a better condition.

Yin and He(1997)evaluated the temporal and spatial influences of integrated planting of Paulownia from the economic point of view and concluded that crop yields were reduced by increasing the density of Paulownia, while total economic value of timber and non-timber products of Paulownia,and the value of crops in integrated planting were 50–100%higherthan the economic value of crops alone in the controlarea.

In Iran,P.tomentosa was planted about50 years ago in the Forestand Range Research Station of Nowshahr where the results were notacceptable due to planting in undrained moist soil(Jazireyi 2003).However,adequate supply of waterisessentialto plantgrowth and the size ofindividuals of a given species can vary several fold in response to variation in moisture level(Bargali and Tewari 2004). Considering the shortage of rainfall and lack of water resources in the Mediterranean region,it is necessary to determine the amount and timing of irrigation needed for proper growth of P.tomentosa to avoid waterlogging.We assessed the influence of irrigation amount and interval on diameter growth of 2-year-old P.tomentosa saplings.

Materials and methods

The study was conducted in Nazlou campus of Urmia University,Iran.The study site covered 900 m2(square and flat)at 1,280 m a.s.l.Saplings were planted at 3 m 9 3 m intervals.During the last 62 years (1950–2012),average annual rainfall was 341 mm,maximum rainfall was 115.7 mm in winter,and minimum rainfall was 13 mm in summer.The average annual temperature of the area was 11.2C and the coldestmonth of the year was February with an average minimum temperature of-1.8C.The warmestmonth of the year was July with an average maximum temperature of 23.9C.

The soilof the area was calcareous with low acidity and was poor in terms of organic matter content.Soil concentrations of nitrogen and phosphorus were low.

In orderto examine the diametergrowth of P.tomentosa saplings,a factorial experiment with a completely randomized design was used in which the examined treatments were as follows:(1)irrigation amount treatment at four levels of 10(i10),15(i15),20(i20)and 25(i25)L;(2) irrigation interval of 3(p3)and 6(p6)days.

Eight treatments were tested.All treatments were replicated 8 times and the totalnumber of saplings was 64.Saplings were irrigated manually using containers.Atthe end of each month during the growing season,collardiameterwas measured for all trees(accuracy of 0.1 mm)and diameter growth rate was calculated by treatmentand month by subtracting collardiametersin consecutive months.To evaluate the trend in growth,diameter growth rates were calculated for 2-,3-,4-and 5-month(total)periods.

Data were analyzed using SPSS software version 18. Means were compared using ANOVA and Tukey’s multiple comparisons.

Results

Mean diameter growth in the first month was consistently greater in treatments p3i20(5.3 mm)and p3i25(5.4 mm), and consistently lower in p6i10(1.7 mm)and p6i15 (2.2 mm)(Fig.1a)but those treatments had statistically similarmean diametergrowth.Diametergrowth in the first month did not vary significantly by irrigation amount but more frequent irrigation yielded significantly greater diameter growth(Table 1).

Diameter growth for different treatments was similar in the second,third and fourth months(Fig.1b,c,d).Maximum mean diameter growth was recorded for p3i25 treatment(12.2 mm)in the second month.However,the overall rate of diameter growth during the third and fourth months declined gradually for all treatments.Minimum diameter growth(less than 1.7 mm)was obtained for p3i10,p3i15,p6i10,p6i15 in the fourth month.Highest mean diameter growth of treatments was in the order: p3i25[p3i20[p6i25[p6i20 and the remaining treatments had the lowest growth rates.Growth for treatment p3i25 was significantly greater than for p3i10,p3i15, p6i10,p6i15 and p6i20.All other treatments yielded statistically similar growth rates.Irrigation amount and interval had significantly independent and interactive effects on diameter growth of saplings in the second and fourth month(Table 1).

Diametergrowth in the fifth month for p6i10,p6i15 and p6i20 was zero and it was1 mm for the other treatments (Fig.1e).Also itwas higher in treatments p3i20 and p3i25. The shorter irrigation interval yielded significantly greater diameter growth of saplings(Table 1).

The trends in diameter growth of P.tomentosa saplings fordifferenttreatments during the growth season are shown in Fig.2.In general,average cumulative 2-month diameter growth for p3i20,p3i25,p620 and p6i25 was greater than for p3i10,p3i15,p6i10 and p6i15(Fig.2a).The highestmean diameter growth was recorded for p3i25(16.7 mm) and was significantly greater than for p3i10,p3i15,p6i10 and p6i15(less than 5.5 mm).Growth increments for treatments p3i20 and p6i25 were similar.

Fig.1 Means diameter growth of Paulownia tomentosa saplings in the first(a),second(b),third(c),fourth(d),fifth(e)month by treatment:irrigation amounts of 10(i10),15(i15),20(i20),and 25 L (i25),and irrigation intervals of 3-days(p3)and 6-days(p6).Different letters indicate significant differences between treatments

Table 1 Analysis of variance of diameter growth of Paulownia tomentosa saplings separated by different months

The increase trend in the 3-month diametricalgrowth of saplings for differenttreatments was similar to the 2-month diametrical growth(Fig.2b).The maximum 3-month diametrical growth rate(cumulatively)was observed in p3i25 (23 mm)which was significantly higher than in p3i10, p3i15,p6i10 and p6i15 treatments(less than 11 mm).

Mean 4-month cumulative diameter growth was significantly greater for treatments p3i20(21 mm)and p3i25 (27.7 mm)than for p6i10 and p6i15(less than 10 mm) (Fig.2c).The Highest mean 5-month cumulative diameter growth of saplings(Fig.2d)was recorded for p3i25 (28.7 mm),which wassignificantly greaterthan forallother treatments.Itwas followed by p3i20(21.7 mm),which was significantly greaterthan for p6i10,p6i15(minimum mean totaldiametergrowth10 mm).Mean totaldiametergrowth for p3i10 and p3i15 was similar to thatfor p6i20 and p6i25 (11–15 mm).Generally,the irrigation regime factors had significantly independentand interactive effects on 2-month (A),3-month(B),4-month(C),and total(D)diameter growth(cumulative)of P.tomentosa saplings(Table 2).

Discussion

Diameter growth rates of P.tomentosa saplings with different treatments increased until the second month andgradually declined in the third,fourth,and fifth months: average diameter growth was lowest in the fifth month at the end of the growth season.Overall,more than 60%of the diameter growth of all saplings took place in the first 2 months(Fig.2a)and more than 80%during the first 3 months(Fig.2b).Thus,irrigation amount and interval during the first3 months were more importantthan during other months.Maximum irrigation amount of 25 L and shortest irrigation interval of 3 days enhanced diameter growth by up to 199%.Similar results were reported by Bargali and Bargali(1997).These results could have resulted from the following reasons.First,Paulownia is moisture-demanding species(Jazireyi 2003).Second, reduced rainfalland increased airtemperatures atthe study site during June,July and August probably resulted in increased evapotranspiration and reduced availability of soil moisture.Llano-Sotelo et al.(2010)reported that P. tomentosa was somewhat tolerant to drought and more drought resistant than P.fortune,therefore more suitable for plantation in semi-arid conditions.At low irrigation amountand long irrigation intervals,the moisture required for optimal photosynthetic activity of Paulownia saplings was notavailable.This led to reduced diameter growth of saplings.Seeley(1990)demonstrated that drought stress reduced cytokinin and auxin level,and led to reduced growth.Of the eight treatments in this study,maximum mean 3-month diameter growth for saplings resulted from irrigation with 25 L at 3-day intervals(23 mm)and

minimum growth resulted from irrigation with 10 L of water at 6-day intervals(7.7 mm).Irrigation intervals had significant independent and interactive effects with irrigation amount on sapling diameter growth.Maximum average growth was recorded for irrigation with 25 L of water at 3-day intervals(28.7 mm).Diameter growth rate for 2-year-old saplings of P.tomentosa in Virginia,USA was 17 mm(Johnson etal.2003)which was 68.8%less than in this study.

Table 2 Analysis of variance of cumulative diameter growth of P.tomentosa saplings by month

Fig.2 Mean of 2-month(a),3-month(b),4-month(c),and 5-month (d)diameter growth(cumulative)of P.tomentosa saplings by treatment:irrigation amounts of 10(i10),15(i15),20(i20),and 25 L (i25),and irrigation intervals of3-days(p3)and 6-days(p6).Different letters indicate significant differences between treatments

Conclusion

Diametricalgrowth of 2-year-old saplings of P.tomentosa was increased by the reduction of irrigation interval and enhancementofirrigation amountduring the earliermonths ofthe growing season.More than 80%ofgrowth took place in the first 3 months with greatest irrigation amount and shortestirrigation intervalbutirrigation amountand interval were less important factors in the months 4 and 5,which showed that both variables could not considerably affect diametricalgrowth of2-year-old saplings of P.tomentosa at the end of the growing season.Therefore,we recommend thatthe 25-L volume and 3-day intervalbe used in the first 3 months of the growth season to achieve the highest diameter growth of 2-year-old saplings of this species.We also recommend thatan amountof15-L at3-day period be applied in the fourth month and 10-L irrigation with 3-day intervalbe applied in the fifth month.

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29 July 2013/Accepted:16 October 2013/Published online:8 January 2015

The online version is available at http://www.springerlink.com

Corresponding editor:Yu Lei

Department of Forestry,Faculty of Natural Resources,Urmia University,Sero Blvd,P.O.Box 165,Urmia,Iran

e-mail:Javad.Eshaghi@yahoo.com

S.R.M.Mirkala

e-mail:Rostammousavi@yahoo.com