ASSESSING THE PERFORMANCE OF POPULUS CASPICA AND POPULUS ALBA CUTTINGS UNDER DIFFERENT IRRIGATION INTERVALS

Understanding the function of poplar species under different irrigation regimes is critical for water resources and ecosystem sustainable management. This study was conducted in order to understand the performance (survival and height growth) of two poplar species (Populus alba L. and Populus caspica Bornm.) cuttings in two lengths (15 cm and 25 cm) under three irrigation treatments (7-day, 14-day and 21-day intervals). One trial was established using split-split-plot design with three replications. Every two weeks height and number of remaining cuttings were measured. Results showed the survival average at the end of periods for 7-day, 14-day and 21-day irrigation treatments were 83.33 %, 22.08 % and 0 %, respectively, for P. alba and P. caspica were 25.89 % and 44.39 %, respectively and for cuttings in 15 and 25 cm were 29.61 % and 40.66 % respectively. The results of two-way analysis of variance of the survival among cuttings indicated that the differences survival among cuttings were all marked under the four treatments and analysis of variance of the height growth indicated that except under the size treatment, the differences height growth among cuttings were all marked under the other three treatments.


INTRODUCTION
Poplar is an excellent candidate for short rotation coppice cultures, which rely on species that is characterized by fast growth and dynamic production of biomass (Payamnour et al, 2013). This species as an alternative source for wood production (Alimohamadi et al. 2012) have been planted in Iran for many years. Annual production of poplar plantations, according to available statistics, is more than triple the production of Caspian forests (the only commercial forests in Iran) while their area is less than 10 percent of Caspian forests area (Bozorgmehr et al. 2014). Poplar growth is highly dependent on the moisture content of the soil (Bagheri et al. 2012) and soil water can exert an important control on poplar growth (Dong et al. 2011, Hogg et al. 2013. Accordingly having a permanent resource of water is a primary need of poplar plantation. Since Iran is in the arid zone which receives less than a third of world average precipitation (Badripour, 2006), shortage of water resources is a major obstacle to successful poplar cultivation. In order to increase the productivity of poplar plantations in arid and semiarid lands, high efficiency irrigation strategies must be used. The efficiency of water use can be examined through assessment of poplar responses which differ from one species to another and from one age to other age. Various indices have been used to find the poplar responses to irrigation which consist of: a) growth traits such as diameter at breast height (Bagheri et al. 2012, Shock et al. 2005, basal area at breast height (O'Neill andAllen 2015, Voltas et al. 2006), total height (Bagheri et al. 2012, Shock et al. 2005, stem height (Bagheri et al. 2012), crown diameter (Bagheri et al. 2012), relative growth rate (Karačić and Weih 2006), volume (O'Neill and Allen 2015, Bagheri et al. 2012, BaoFang et al. 2007, Shock et al. 2005, biomass (Monclus et al. 2009, Karačić and Weih 2006), survival (O'Neill and Allen 2015, Saeidi and Azadfar 2009, Van den Driessche et al. 2003, radial growth (Giovannelli et al. 2002), root number (BaoFang et al. 2007); b) physiological characteristics such as foliar nutrition (Shock et al. 2005), mesophyll-to-stomatal conductance ratio (Rancourt et al. 2015), photosynthetic performance (Zhu et al. 2014;Saeidi andAzadfar, 2009, Fengjun et al. 2006 ), transpiration (Samuelson et al. 2007), leaf-specific hydraulic conductance (Samuelson et al. 2007), canopy stomatal conductance (Samuelson et al. 2007), stem water potential (AiHong et al. 2009); c) morphological traits such as leaf area (Monclus et al. 2009, BaoFang et al. 2007, Samuelson et al. 2007; d) phenology events such as bud-burst (Bagheri et al. 2012, Sera andPons 2013), leaf expansion (Bagheri et al. 2012), leaf abscission (Bagheri et al. 2012), flowering (Bagheri et al. 2012;Sera and Pons 2013) and e) anatomical characters such as wood anatomy (Cocozza et al. 2011).
In order to develop efficient water use strategies in poplar plantations, different irrigation regimes can be conducted. Irrigation regime which is determined by the rate of irrigation, irrigation frequency (intervals), and time of water applications to crops. Some research has been undertaken on high efficiency irrigation regime in poplar plantations. Zhu et al (2014) investigated the effects of changed irrigation (water-saving irrigation and flood irrigation) on two poplar species (P. euphratica and P. russkii) growing in arid ecosystems. Results showed that reduced water availability during water-saving irrigation had a moderate but not significant impact on the photosynthesis of the two poplar species. Sera and Pons (2013) analyzed the dynamics of poplars between 2002 and 2008 which comprised periods of water surplus and water scarcity. No difference was observed between periods of water scarcity and water surplus. Bagheri et al (2012) showed significant difference between the 4, 8 and the 12 day irrigation intervals in respect to growth parameters of P. euramericana, P. trichocarpa, P. alba, P. nigra and P. deltoids. Cocozza et al (2011) investigated the correlation between the main ring traits of young poplar clones (P × canadensis and P. deltoides) and irrigation regimes (irrigated with 70 mm of water every week and no-irrigated). Results showed P. deltoides has the potential to recover promptly after drought stress. AiHong et al (2010) in temperate desert zone analyzed the change of water potential of Populus euphratica Oliv. and P. Russkii Jabl under different irrigation volumes. Saeidi and Azadfar (2009) investigated the effect of drought and hydromorphy stresses on net photosynthesis rate and survival of P. nigra , P. deltoides and P. euramericana. Result showed P. deltoides and P. nigra had more resistance to drought stress than hydromorphic. Zomer et al (2007) irrigated their poplar plantations during April to June, when the atmospheric temperature is high with a low level of relative humidity. Fengjun et al (2006) applied four water treatments: wellwatered condition, slight water stress, moderate water stress, and severe water stress. The results revealed that the clones with higher long-term water use efficiency always had strong photosynthetic capacity and optimum root/shoot ratio. Shock et al (2005) applied five irrigation treatments for hybrid poplar consisted of three water application rates using micro sprinklers and two water application rates using drip tape during five years. Result showed drip irrigation with two tapes per tree row resulted in higher tree growth than micro sprinkler irrigation.
In Iran, most of the researches about efficient irrigation management have been done on agricultural crops. Furthermore, there is no research available to determine growth response of P. alba and P. caspica as a local and critically endangered species in Caspian forest of Iran (Falah et al 2011) under different irrigation treatments. Given the importance of Poplars and for optimum use of water resources in arid season, in this study we explored the growth response and survival of P. caspica and P. alba cuttings in two lengths to different irrigation intervals.

MATERIAL AND METHODS Site
Experiments were carried out in the Chamestan forest and rangeland research station located in Noor city (36° 25' N & 51° 55' E, 70 m a.s.l), Mazandaran province, Iran. Mean annual temperature and precipitation are 15.8 C° and 840 mm, respectively. Absolute minimum and maximum temperature are -8.5 C° and 36 C°, respectively. Average relative humidity is 78%.

Experiment design
Cuttings of P. caspica and P. alba in two lengths (15 and 25 cm) were established in a field trial, as a split-split-plot design with three replications. The species as factor A was assigned to whole plots, then the cutting length as factor B was assigned to split-plots within the applications of Factor A, and then split the experimental units used for factor B into sub-sub-plots to receive different irrigation intervals as factor C. For this purpose a total of 108 (three treatments of irrigation intervals, two treatments of cutting lengths, two kinds of species, three pots and three replications) plastic bags were put into a water container with 15 cm depth and 46 cm diameter ( Figure 1). The culture medium was a soil with 33 percent clay, 47 percent loam and 21 percent sand. Soil pH and electrical conductivity were 6.58 and 0.57 ds/m, respectively. Percentage of lime, organic carbon and total nitrogen were 1.19, 2.78 and 0.29, respectively and finally the amounts of phosphorus, potassium, calcium and magnesium were 23.78, 420, 175.5 and 37.5 ppm, respectively (Rouhi Moghadam, 2008). The cuttings were planted on February with one cutting in each pot. Weeding operation was done during the growing season. Three different irrigation intervals were applied: 7day intervals, 14-day intervals and 21-day intervals. The amount of irrigation water at each time was 2000 mL for per pot.

Measured objects and statistical analyses
In order to calculating the seedling height growth as an appropriate indicator for evaluating the effects of stress (Sadati et al. 2011) and survival under different treatments, every two weeks height and number of remaining cuttings were measured. Ten measurements were performed on April 27P th P , May 11P th P , May 25P th P , June 8P th P , June 22P th P , July 6P th P , July 20P th P , August 3P rd P , August 17P th P and August 31P th P . Data were evaluated by Kolmogorov-Smirnov test for assessing the normality, two-way analysis of variance for analyzing the effects of applied treatments on height growth and survival, Duncan's multiple range test for measuring the differences between irrigation and period treatments. Statistical analyses were conducted using SPSS 21.0. The percentage survival was zero of which three treatments were related to P. alba include the cuttings in 15 cm under 14 and 21-day irrigation intervals and the cuttings in 25 cm under 21-day irrigation intervals and two treatments were related to P. caspica include the cuttings in 15 cm under 21-day irrigation intervals and the cuttings in 25 cm under 21-day irrigation intervals.At the end of periods there was no zero percent survival in any 7-day irrigation treatments but all cuttings under 21-day intervals (all treatments related to two species and two sizes) had died that four deaths had occurred at the ninth period and the P. alba cuttings in 15 cm under 21-day irrigation intervals had died at the eighth period. The only treatment with 100 percent survival at the end of periods was related to P. caspica cuttings in 25 cm under 7-day irrigation intervals.

RESULTS AND DISCUSSION
The survival average at the end of periods for 7-day, 14-day and 21-day irrigation treatments were 83.33 %, 22.08 % and 0 %, respectively, for P. alba and P. caspica were 25.89 % and 44.39 %, respectively and for cuttings in 15 and 25 cm were 29.61 % and 40.66 % respectively.
The average height growth of P. alba and P. caspica were 7.44 cm and 9.62 cm. Furthermore, for 7-day, 14-day and 21-day irrigation intervals were 9.25 cm, 8.68 cm and 7.67 cm, respectively and the average height growth of cuttings in 15 cm and 25 cm were 8.50 cm and 8.56 cm (Figures 4 and Figure 5). The maximum height growth of four treatments occurred during the fourth period, for four others occurred during fifth period, two treatments during sixth period had maximum height growth and two treatments during first period had maximum height growth (Figure 4)  The results of two-way analysis of variance of the survival among cuttings (Table 1) indicated that the differences among cuttings were all marked under the four treatments (length, species, irrigation intervals and periods). Based on Duncan's multiple range test, irrigation intervals were classified in two separate groups: 21-day and 14-day irrigation treatments as first group and 7-day irrigation treatment as second group. Furthermore, periods based on multiple comparisons were divided into 3 groups ( Figure 6). The results of two-way analysis of variance of the height growth among cuttings (Table 2) indicated that except under the size treatment, the differences height growth among cuttings were all marked under the other three treatments (species, irrigation intervals and periods). Based on Duncan's multiple range test, irrigation intervals were classified in two separate groups: 21-day irrigation treatment as first group and 14-day and 7-day irrigation treatments as second group. Furthermore, periods based on multiple comparisons were divided into 5 groups (Figure 7).  Owing to limited water resources, irrigation interval is more important than irrigation volume in conventional and industrial poplar plantation systems (Bagheri et al. 2012). According to this our goal was to examine the more efficient irrigation regime for P. caspica and P. alba plantations. The results of applying three irrigation intervals during four months indicated that all cuttings under 21-day irrigation intervals after about three months died. Furthermore the only 22 percent of the cuttings under 14-day irrigation intervals survived at the end of period, but 83 percent of cuttings under 7-day irrigation intervals survived, therefore it might be concluded that these irrigation regimes (14-day and 21-day) are not appropriate for poplar wood production. This conclusion is comparable with the work of Bagheri et al (2012) that showed all the clones of P.euramericana, P.trichocarpa, P. alba, P. nigra and P. deltoides have intensive growth reduction at the 12-day interval irrigation in comparison 4-day and 8-day regime, but there was no difference between 4-day and 8-day irrigation intervals. Moreover Hara (2004) suggested weekly irrigation during the dry months for high intensity production of poplar. Chaghaii (2016) believed in areas where there is no groundwater the irrigation intervals may be reduced to 10-day and soil should be completely full of water, but on windy sites with sandy soil the irrigation intervals may be reduced to 7-5 day. Some researchers suggest that the irrigation intervals should not be considered the same and according to local conditions should be given more flexibility on warm days (Bagheri et al. 2012, Zomer et al. 2007). Figure 7: Grouping of periods based on mean height growth Unlike this fact that P. alba is known as high drought tolerance species (Edward et al. 1994) the present result showed the survival average at the end of periods and average height growth of P. alba cuttings are significantly less than P. caspica cuttings and therefore P. alba than P. caspica are more sensitive to longer intervals of irrigation, but Bagheri et al (2012) showed P. alba clones than other study poplar clones are less affected by drought conditions and have the same performance at three irrigation intervals (4, 8 and 12-day).
The results showed the survival of cuttings in 25 cm length are significantly more than cuttings in 25 cm length, although there was no significant difference between height growths of cuttings in two lengths. These results are confirmed by some studies that showed the longer the cuttings, the higher are the survival (Singh Thakur et al. 1995, Rossi, 1991. Since potential growth of poplar is highly dependent on the amount of applied irrigation (Shock et al. 2002) and soil moisture (Kharytonov et al. 2017) therefore the improvement of water use efficiency especially in arid areas with regard to limited water supply is a key objective to improve the sustainability of cultivated poplar (Rancourt et al. 2015).
Water-saving irrigation (Zhu et al. 2014) and increasing the intervals of irrigation (Bagheri et al. 2012) could be as economical practices for managing water use in such areas, As our goal was to examine the effect of different irrigation intervals on survival and height growth of two poplar species. Furthermore, management of irrigation intervals owing to water influence on pest management is very important. As the work of Tahriri Adabi et al (2013) showed high provided amount of water in 4 days interval in comparison to 8 and 12 days, make poplar species and clones more susceptible against pest. It is worth mentioning that today using treated wastewater for poplar irrigation is suggested as an alternative strategy for water supply problems in semi-arid to arid areas (Houda et al. 2016).

CONCLUSIONS
Our study revealed that 1) survival of study poplar was found to be dependent on irrigation intervals; 2) the best performance (survival and height growth) of study poplar was under 7-day irrigation interval; 3) P. caspica has relatively higher water-stress resistance than P. alba; 4) longer cuttings than shorter cuttings had higher survival.