Fine Root Hydraulic Conductance Is Related to Post-transplant Recovery of Two Quercus Tree Species

In our study, we investigated whether root hydraulic conductance is related to post-transplant recovery. We used two Quercus species that differ in their transplant ability, Q. bicolor and Q. macrocarpa. Q. bicolor easily survives transplanting, whereas Q. macrocarpa often does not. We compared root hydraulic conductance after transplanting between control (without root pruning) and root-pruned, 1-year-old, small-caliper trees. We also examined the effects of transplant timing on post-transplant recovery of large-caliper trees. Hydraulic conductance in fine roots was correlated with recovery of the two Quercus species after transplanting. Six months after transplanting, small-caliper Q. bicolor trees had similar specific hydraulic conductance (KS) in fine roots compared with the KS before root-pruning, whereas fine root KS in small-caliper Q. macrocarpa trees decreased. Lower pre-dawn and midday xylem water potential in root-pruned Q. macrocarpa 6 weeks after transplanting indicates that root-pruned Q. macrocarpa experienced transplanting-induced water stress. For large-caliper trees, all Q. macrocarpa trees exhibited typical symptoms of transplant shock regardless of transplant timing, which was the result of higher vulnerability to mild water stress compared with Q. bicolor, resulting in a large reduction in fine root KS. Fine root KS in springtransplanted Q. bicolor trees was much higher than that in fall-transplanted trees, implying spring transplanting is optimal for Q. bicolor. Other intrinsic characteristics of the species should be considered in the future when making better decisions on transplant timing such as xylem anatomy, carbon storage, rhizosphere conditions, and plant growth. Survivorship of high-quality landscape field-grown trees is a particular challenge as a result of differences in post-transplant recovery between species. During bare-root tree transplanting, a major part of the root system is severed, the tree is held in storage, and then replanted into a new location. Not surprisingly, the loss of a large proportion of biomass usually results in major physiological changes within the tree until an adequate root system is rebuilt. Meanwhile, poor root–soil contact resulting from the loss of a majority of fine roots (the sites of water and nutrient uptake) often results in water stress in newly transplanted trees (Grossnickle, 1988). This large loss of plant biomass coupled with exposure to dry conditions is referred to as ‘‘transplant shock,’’ in which a plant shows less shoot growth, smaller ‘‘scorched’’ new leaves, and a general lack of vigor (Watson and Himelick, 1983). Root hydraulic conductance describes the ability of roots to take up water from a growing medium and transport the water to other parts of a tree. Water stress during transplant shock greatly disrupts normal water transport capacity of a tree. Under drought, reduced xylem water potential may cause embolisms, air-filled xylem conduits, leading to a reduction in hydraulic conductance [K (the mass flow rate through the segment divided by the pressure difference)] (Schultz and Matthews, 1988; Sperry and Saliendra, 1994) and thus transpiration (Sperry and Pockman, 1993). Root K, as a major component of whole-tree hydraulic architecture (Cruiziat et al., 2002), has been neglected in previous studies despite its reduction at transplanting and its probable influence on post-transplant recovery. Hydraulic conductance of the entire root system of Corylus colurna was reported to be reduced significantly after transplanting (Harris and Bassuk, 1995). Regarding trees, transpiration pulls water from soil to leaves and to the atmosphere and creates a variable gradient of water potential throughout the tree. Water potential distribution along the root system is thus very dependent on root architecture such as fine root and coarse root branching (Cruiziat et al., 2002). This makes it reasonable to assume fine roots and coarse roots may have different hydraulic responses during transplant shock and may play different roles in post-transplant recovery. Transplant timing is important in post-transplant recovery. Spring and fall are usually considered to be the appropriate times for temperate tree transplanting, but the question about which season is optimal is highly disputable. The major advantage of fall transplanting is to allow root regeneration before shoot growth in the spring and provide more time for roots to acclimate to the new soil environment, whereas spring provides ample soil moisture and allows transplanted trees to avoid cold weather (Richardson-Calfee et al., 2004). However, species vary in their survival as a function of transplant seasons (Harris et al., 2002). Fine root traits, including physiology and morphology, largely determine maximum potential growth rate of tree seedlings (Comas et al., 2002). By understanding the physiological basis of root behavior during fall vs. spring transplanting, better decisions regarding transplant timing can be made. Tree transplant size may affect post-transplant recovery. Although large-caliper trees are often more desired to produce an immediately mature landscape, it was often found that largecaliper trees have a slower growth rate than small-caliper trees Received for publication 17 July 2014. Accepted for publication 10 Sept. 2014. We thank Pat MacRae for help in transporting large-caliper trees from the nursery to Bluegrass Lane research field in Ithaca, NY. Corresponding author. E-mail: [email protected]. J. AMER. SOC. HORT. SCI. 139(6):649–656. 2014. 649 (Gilman et al., 1998). After root pruning, the artificial imbalance in the proportion of roots to shoots reduces vigor for a period of time with small-caliper trees often returning to the original shoot-to-root ratio sooner and thus having higher survival rates (Watson, 1985). However, there are some factors that may affect the rate of post-transplant recovery of smalland large-caliper trees that when accounted for can increase large-caliper tree recovery to a similar rate as small-caliper trees. For example, genetic variation between smalland largecaliper trees has often been overlooked: the more vigorous trees tend to be harvested at smaller caliper sizes in nursery production practices because they are the first to reach salable size and thus the last ones harvested from a nursery block are usually slower-growing large-caliper trees, which may be genetically inferior to the earlier harvests (Struve, 2009; Struve et al., 2000). In this study, we used two Quercus species that differ in their transplant ability, Q. bicolor and Q. macrocarpa. Although these species are closely related, Q. bicolor easily survives transplanting, whereas Q. macrocarpa often does not (Buckstrup and Bassuk, 2009). We transplanted both smalland large-caliper trees of the two species and examined K before and after transplanting in fine roots, coarse roots, and the entire root system of the trees. Additionally, we assessed how transplant timing affected post-transplant recovery in large-caliper trees. The objective of this study was to determine whether root K is related to post-transplant recovery of the two Quercus species and whether tree size and transplant timing may affect transplant recovery. Materials and Methods SMALL-CALIPER TREE MATERIAL AND EXPERIMENTAL DESIGN. Twelve bare-root 1-year-old Q. bicolor and 12 Q. macrocarpa trees (8 mm caliper; Lawyer Nursery, Plains, MT) were planted in #3 containers (11.3 L; Hydrofarm, Fairless Hills, PA) containing Pro Mix soilless media (70% peatmoss and 25% perlite soilless media; Premier Tech, Mississauga, Ontario, Canada) in Spring 2012 and grown in an unheated polyhouse at Cornell University in Ithaca, NY (lat. 42.48 N, long. 76.47 W, elevation 335 m). The temperature, humidity, and light intensity in the greenhouse were similar to seasonal growing conditions. In Mar. 2013, the trees were brought into the greenhouse where the temperature was maintained at 24 C during the day and 18 C during the night for the whole experimental time. The trees were well-watered daily from Mar. 2013 to Oct. 2013 and fertilized with 15N–2.2P–12.5K water-soluble fertilizer (Peters Excel 15-5-15 CalMag; Scotts Co., Marysville, OH) twice per week for 1 month before the experiment was initiated in Apr. 2013. In Apr. 2013, the trees were brought into the laboratory. For each Quercus species, six trees were randomly selected as controls (no root pruning treatment) and the other six trees of each species were randomly selected to undergo a simulated transplant treatment. The treatment trees were removed from their containers, pruned 80% of the entire root system from the bottom and sides of the root ball, and allowed to dry on the laboratory bench covered by a thin layer of the soilless media they were growing in for 3 d before returning them to their original containers with the former soilless media. The laboratory environmental conditions were maintained at 20 C, 30% relative humidity, and a photoperiod of 12/12 h (light/dark). The treatment trees were returned to the greenhouse immediately after the treatment with control trees. Locations of the trees within the greenhouse were randomly assigned. Tree locations were re-randomized every 2 weeks to minimize location effects. ROOT HYDRAULIC CONDUCTANCE MEASUREMENT. Before the 3-d drying period in Apr. 2013, three fine root branches were randomly collected from all control and treatment trees for hydraulic conductance measurements using a hydraulic conductance flow meter [HCFM (Gen 3; Dynamax, Houston, TX)]. The length of the fine root branch was 20 cm, and the diameter of the highest order roots (fourth order roots; Pregitzer et al., 2002) on the branch was 1.5 to 2.0 mm. Immediately before measurement, the end of the branch was re-cut off under water with a sharp blade resulting in the branch that was 15 cm long. Hydraulic conductance in fine root branches was measured with the transient measurement mode, which rapidly increased the applied pressure and simultaneously measured the corresponding flow (Tyree et al., 1995). Degassed deionized water was forced through the root branches under increasing pressure until the pressure reached 500 kPa. The instantaneous flow and pressure were recorded every 2 s. Hydraulic conductance (kg s kPa) was calculated from the slope of linear regression between the pressure and flow. The diameter of each fourthorder root was measured using a digital caliper to calculate specific hydraulic conductance (kg s m kPa), K divided by cross-sectional area of the root. Root pruning was conducted on the treatment trees immediately after the K measurement. For control trees, K was measured at the same time as root pruned trees and the trees were immediately placed back into the containers with old soilless media. Stem diameter of all trees was measured using a digital caliper 5 cm above the root collar. All of the trees were harvested in Oct. 2013. Hydraulic conductance was measured again on three fine root branches as described previously and on entire root systems for each tree using the HCFM. Shoots (including stem and leaves) were cut off 5 cm above the root collar. Stem diameter of all trees was measured again with a digital caliper at the cutting end, and stem diameter growth between Apr. 2013 and Oct. 2013 was calculated. To measure K of the entire root system, the root system was left in the soil and the root stump was immediately re-cut under water to avoid cavitation. The remaining root stump was connected to HCFM and K of the entire root system was measured as described previously as fine roots. All leaves were removed from stems and scanned using a leaf area meter (LI-3100; LI-COR, Lincoln, NE) to determine total leaf area of each tree. Leaf area was used to calculate leaf area-specific hydraulic conductance [KL (kg s m kPa)], total root system K divided by total leaf area, of root systems. VULNERABILITY CURVES. Three trees per species from the non-root-pruned treatment were sampled to determine vulnerability curves using the centrifuge technique described in Alder et al. (1997). In brief, an unbranched stem segment (generally at the base of the tree), 20 cm in length, was cut from each tree and re-cut to 14 cm under water before measurement. The stem segment was first flushed with degassed deionized water for 30 min at 350 kPa to remove native embolisms. The maximum hydraulic conductivity [kmax (kg m s MPa)] of the stem segment was measured using the gravity method described in Sperry et al. (1988). Gravity-induced flow of deionized water containing 20 mM KCl was applied to the segment with a pressure of 6 to 7 kPa. The solution was degassed before 650 J. AMER. SOC. HORT. SCI. 139(6):649–656. 2014. use by agitating it vigorously with a magnetic stirrer for 45 min under vacuum (Sperry and Tyree, 1990). For each measurement, hydraulic conductivity [k (kg m s MPa)] was calculated every 10 s as the mass flow rate of solution through the stem segment divided by the pressure gradient along the segment together with a CV of the previous 10 readings. When the CV was less than 0.3%, we averaged the last three readings as the conductivity for the stem segment. The measurement time for one stem segment was 15 to 20 min. The stem segment was then spun in a centrifuge (RC5G Plus; Thermo Fisher Scientific, Waltham, MA) with a custom-built centrifuge rotor to generate a given negative xylem pressure. The xylem pressure generated was –0.5, –1.0, –1.5, –2, –3, and –4 MPa, which was adjusted by varying the rotational velocity (Alder et al., 1997). Stem k was decreased with increasing negative xylem pressure. The percentage loss of hydraulic conductivity (PLC) was calculated as: PLC = 100 3 kmax k ð Þ kmax = The curve of PLC vs. xylem pressure was then plotted. Each PLC curve was fitted with a second-order polynomial model. The polynomial models were used to calculate the pressure potential at 50% loss of conductivity (P50) for each species separately. SOIL-TO-LEAF HYDRAULIC CONDUCTANCE MEASUREMENT. Six weeks after transplanting, pre-dawn [Ypre (MPa)] and midday leaf xylem water potential [Ymid (MPa)] was measured on the trees growing in the greenhouse using a pressure chamber (3005F01; Soilmoisture Equipment Corp., Santa Barbara, CA). Immediately before measuring Ymid, midday transpiration rate [E (mmol s m)] was measured on the same leaf using a portable photosynthesis system (CIRAS-2; PP Systems, Amesbury, MA). The measurements were conducted on two fully developed leaves of each tree once every 6 weeks until Sept. 2013. Soil-to-leaf K [KP (mmol s m MPa)] was then calculated as: