Effect of Irrigation Frequency on Tree Seedling Production
Department of Forestry and Natural Environment Management, Technologiko Ekpedeftiko Irdyma Anatolikis Makedonias and Thrakis (EMaTTech) Drama 66100, Greece
Mediterranean ecosystems are of particular ecological importance, but they are facing restoration problems, mainly due to their prevailing semi-arid climate. The increased predicted temperatures associated with climate change pose greater obstacles to restoration efforts. This chapter aims to report on species responses, specifically, Quercus pubescens, under different irrigation frequencies and address their ability to successfully regenerate sites through research through tools like the root growth potential (RGP). The hypothesis was that seedlings that experience reduced irrigation frequency have a greater RGP. Based on the experimental procedure, under field conditions, seedlings were exposed to five irrigation frequencies and by the end of the month were evaluated based on characteristics (morphological and physiological). Continually, the seedling pre-exposed to the irrigation treatments were placed in an RGP growth chamber under regular irrigation and were evaluated at the end of the month. The results indicated that for the species of Quercus pubescens one watering per week was adequate to retain its normal growth while it increased their RGP ability. The water potential and the leaf characteristics were good indicators on the growth status of the seedlings. Overall, the study provided substantial insights on preconditioning seedlings to increase their potential transplanting success while saving water; a substantial advantage for semi-arid ecosystems.
Mediterranean ecosystems are of high importance, mainly due to their increased biodiversity levels. However, these ecosystems are subjected to frequent and intense disturbances that relate to natural phenomena such as fires and floods. Based on research on climatic alterations induced by the predicted increased temperatures will further enhance the severity and frequency of these phenomena (Xoplaki et al., 2005; Lionello et al., 2006; IPCC, 2013). Further, anthropogenic interference associated with practices such as land-use alterations and overgrazing also induces further biodiversity loss (Thirgood, 1981).
Consequently, due to the importance of these ecosystems, their immediate restoration after a disturbance event is apriority for the forest nurseries. However, the prevailing semi-dry climatic conditions (Lionello et al., 2006; Mendoza et al., 2009; Rudel, 2007; Scarascia-Mugnozza et al., 2000) reduce restoration success. Prevailing drought growth conditions that the plants experience after transplanting, particularly during hot summers, are a determining factor for their growth and survival (Maranon et al., 2004; Salvador et al., 1999).
Therefore, the quality of the produced seedlings by the forest nurseries is a priority, since better-equipped seedlings favor the success of the restoration efforts. Research has indicated that seedlings with larger root systems are better equipped to survive under adverse outdoor growth conditions, especially under reduced water availability. This relates to the increased ability of the seedlings for water and nutrients exploitation that enhances the transplanting success (Puertolas et al., 2003; Villar-Salvador et al., 2008).
Prior to seedling transplant, preconditioning nursery practices reduce transplanting shock (van den Driessche, 1992; Kozlowski and Pallardy, 2002). The preconditioning involves the exposure of the seedlings to a water stress period by reducing the irrigation frequency/quantity. This triggers signals that increase biomass allocation to the roots resulting to seedlings with greater ability to produce new root system; a characteristic that helps seedlings to cope and overcome the transplant stress usually associated with water deficit conditions (Iakovoglou and Halivopoulos, 2016).
Proper irrigation frequency that suits each species provides substantial benefits. It enables the production of qualitatively better-equipped seedlings (e.g., larger root systems) that result in the more successful restoration of highly disturbed sites. It also results in greater water saving that is of substantial value for semi-arid areas where water availability is limited particularly during diy summer periods. Therefore, the associated cost of producing seedlings is smaller, since proper irrigation allows the production of a better quality of planting material (greater root potential) with a reduced amount of water.
Irrigation protocols combined with research tools that help assess the quality of the produced seedlings enables to determine the ideal irrigation frequency for each species. Such tools help in evaluation of the RGP that relates to the root dynamics in developing new roots for further nutrients and water exploitation (Mattsson, 1986; Ritchie and Tanaka, 1990). The evaluation of root growth potential has been used to assess the transplanting ability for conifer species in the North America (Jenkinson, 1980; Larsen et al., 1986). Research for the species of Fraxinus excelsior L. (O'Reilly et al., 2002) and Quercus spp. (Wilson and Jacobs, 2006) have shown that greater RGP was directly associated with the increased viability of plants under field conditions. Based on the so far cited literature, no research has been conducted on the combined effect of irrigation frequency and the RGP ability of the seedlings. Preliminary research has been carried out for Pinus halepensis Mill, where the most stressed plants had the greatest growth potential (Syropli et al., 2014).
One of the native forest species that thrives in the Mediterranean Greek ecosystems is Quercus pubescens Willd. (pubescent oak). This species has strategies and survival mechanisms associated with the ability to develop vigorous root systems (Dillaway et al., 2007; Mendoza et al., 2009). It is a highly valuable timber species that also provides food and shelter for many animals. Therefore, it is one of the main species that is highly used by forest Greek nurseries for reforestation purposes.
This study aimed to provide insights on the abilities of Quercus pubescens seedlings to produce new roots as evaluated by the RGP under irrigation frequencies. The hypothesis was that seedlings that have experienced reduced irrigation frequency will have a greater ability to produce new roots (greater RGP). Ultimately, the irrigation frequency that provides the best RGP provides better quality seedlings for reforestation efforts that promise greater transplanting success. The finding of this research will help assess the quality of the produced plant material prior to transplant with tools like the RGP for the most successful restoration.
MATERIAL AND METHODS
The experimental seedlings were Quercus pubescens that were grown for three years under field nurseiy conditions with daily irrigation and weekly fertilization frequency during the summer months at the Forest nursery of
Chalkidona, Greece (41°52'17" E, 45°167'05" N). The seedlings were grown in containers of 55 cm x 60 cm x 160 cm (QuickPot QP 24T/16) with a standard nursery soil mixture of 1-part of perlite and 3-parts of peat.
Before the experiment, there was an acclimation period for the seedlings where they were exposed to daily irrigation and no fertilization for a month. The experiment took place during August that is considered the hottest summer month for Greece and was composed of two experimental parts. The first experimental part of the irrigation treatments was tested under field conditions, while at the second experimental part the pretreated seedlings were placed in controlled growth chamber conditions of regular irrigation and evaluated for their RGP.
12.2.1 FIRST EXPERIMENTAL PART
After the acclimation period, the seedlings were placed under outdoor shed conditions to fully control the irrigation of the seedlings from unexpected rainfall events. Further, the seedlings were treated with five irrigation frequencies: watering three times every week (31/1 w), watering twice eveiy week (2I/lw), watering once every week (11/lw), watering once every two weeks (ll/2w) and watering once every four weeks (ll/4w), for a month. For each irrigation treatment, five seedlings were randomly harvested for evaluation. Further, for the second experimental part, five pretreated seedlings were also used.
12.2.2 SECOND EXPERIMENTAL PART
Seedlings that experienced the five irrigation treatments (first experimental part) were transferred in a growth chamber (Fig. 12.1 A) designed for estimating the RGP (Mattsson, 1986). The chamber retained controlled growth conditions with 18 h of light duration (altered with 6 h of dark) under high- pressure sodium lamps (SON T 400 W, General Electric) and controlled air and water bath temperatures of 20 ± 2 °C.
The seedlings were transplanted in stainless steel trays filled with a volumetric soil mixture of 1-part peat and 1-part sand that enabled the development of a new root system (Fig. 12. IB). Continuously, the stainless trays were merged in the water bath of 20 ± 2 °C. The seedlings were grown under the controlled growth conditions for one more month. They were watered three tunes a week with the excess water being pumped out of the stainless containers with a vacuumed-pipe to avoid anoxic growth conditions.
At the end of the month, the seedlings were harvested. Particularly, the newly developed roots were carefully washed and removed to estimate the ability of each species to grow new roots (Fig. 12.1C).
FIGURE 12.1 (A) Presentation of the growth chamber designed to estimate the Root Growth Potential (RGP). (B) Indication of the soil mixture placed in the stainless-steel trays where the seedlings were grown in the growth chamber. (C) The development of the new roots (RGP) for the treatment of one-irrigation per week (11/lw).
12.2.3 STUDIED VARIABLES
By the end of the month for each experimental part, five seedlings were harvested and evaluated. Specifically, the height and the diameter of the root collar were measured through the use of a digital caliber. The Portable Leaf Area meter (LI-3000C) was used to evaluate the leaf area. The number of leaves, the number of branches, and the number of lateral roots were also determined. The Specific Leaf Area (SLA = leaf area/leaf diy weight) was also estimated (Gamier et al., 2001; Wilson et al., 1999). Further, the dry weights were measured by a digital analytical balance after placing the samples in an oven for three days at the 80 ± 2 °C. Specifically, for each seedling, the dry weight of the leaves, the stem, the branches, the tap root, and the lateral roots were evaluated. Also, the total above-ground seedling dry weight parts (diy weight of leaves + stem + branches) was estimated. The total dry weights of the below ground seedling parts were estimated as the partitioning of the dry weight of the tap and lateral roots, while the total diy weight of the seedlings was the partitioning to the above and below seedling dry weight parts. The root-to-shoot ration was also estimated. Also, the soil moisture content (Theta Meter type HH1) and the water potential (WP4 Dew Potential Meter) of the seedlings were evaluated prior to seedling harvesting. For the “second experimental part,” in addition to the above variables, the
RGP as defined by the dry weight of new roots and their maximum root length were also evaluated.
The statistical analysis of the data was conducted by using the ANOVA with the SPSS® statistical software, version 15.0 (SPSS, 2006) with the data been tested for normality and homogeneity. The mean differences were tested with the Tuckey’s multiple range tests at significance levels ofp< 0.05.