Root Regeneration of Tilia Cord at a Cultivars After Transplanting in Response to Root Exposure and Soil Moisture Levels

Materials and Methods

Three-year-old, budded, 250 cm trees of Tilia cordata cultivars ‘Glenleven’ and ‘Greenspire’ were selected for uniformity and dug bareroot in May, 1983. The trees were subjected to a drying period prior to transplanting by leaving the roots completely exposed in a room at 22°C and 50% relative humidity for 0, 12, 24, and 48 hours. The trees were then planted in 55 liter plastic bag pots containing a soil:peat:perlite mix (2:1:1 v/v).

Four cyclic moisture level treatments were maintained after planting for trees from each of the pre-plant exposure treatments. Trees were watered after they had reached the desired soil moisture level. The levels of post-plant soil moisture were 100%, 50%, and 40% available moisture. The fourth level was obtained by allowing the trees to visibly wilt before watering. Percent available moisture was measured using gypsum Bouyoucos blocks and a Bouyoucos moisture meter (Beckman Instruments, Cedar Grove, N.J.).

When necessary, the soil surface was covered with a removable plastic shield to prevent the entry of rainwater. The shield could be easily lifted to facilitate drying of the soil. The trees were staked to prevent wind shifting. For each of the 100%, 50%, and 40% treatments, the mean of four trees per treatment group was taken as the available soil moisture level for that treatment. The trees that were allowed to wilt before watering were monitored individually. Soil temperature was monitored by copper-constantan thermocouples placed at various positions in the soil.

Five shoots were randomly chosen at the midpoint of the canopy of each tree and measured over the course of the experiment. The experiment was terminated at leaf drop in the fall. The soil-root mass was removed from the bag and the soil was carefully washed from the roots. New root growth was distinguishable by its lighter colour and point of origin on the old root system. All new root growth was removed and the dry weight was recorded.

The experiment was a 2X4X4 factorial arrangement in a completely randomized design. There were two cultivars, four levels of pre-plant exposure, and four levels of post-plant soil moisture levels with four replicates per treatment combination for a total of 128 trees in the experiment. Mean separation was by LSD at the 5% level of significance.

Results and Discussion

Drying exposure periods applied between digging and planting had no effect on subsequent root growth. The only exception was the death of one tree early in the experiment that had been subjected to the longest exposure period. There was no difference between the two cultivars in response to the treatments applied, nor was there any significant interaction between the treatments.

Exposure of Sitka spruce and white pine seedlings to drying conditions for only two hours resulted in the death or damage of most of the plants (1, 6). The trees in our experiment were able to withstand the amount of exposure allowed prior to planting. Since the rate of root regeneration was not measured over time, it is not known whether exposure had any deleterious effect on the initial emergence of roots. At the time new root growth was measured, there was no evidence of reduced root regeneration as a result of exposure. It must be noted that air movement over the plants during exposure was minimal and sun exposure was nil. Actual drying conditions in the field are characterized by wind and/or sun induced dehydration either as a result of improper protection during transport or by natural weather conditions (3). Thus, caution must be used in extrapolating these results to actual field drying conditions.

The pre-plant exposure had a significant effect on new shoot growth (Figure 1), but the reduction in shoot growth did not correlate well with the increasing severity of the preplant treatment. The trees subjected to the shortest exposure (12 hours) had the least amount of new shoot growth overall. Initially, all of the exposure treatments lagged behind the unexposed trees in amount of new shoot growth. At the last measurement date, only the 12 hour exposure treatment was significantly different from the unexposed control. The initial lag of all the exposed treatments can be attributed to the stress induced by the exposure treatment. However, the apparent inability of the trees in the 12 hour exposure treatment to recover as successfully as the more severe exposure treatments cannot be readily explained. Previous research has shown that root exposure may reduce both shoot and root growth (1, 6).

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Figure 1.

Shoot elongation of Tilia cordata cultivars exposed for varying lengths of time prior to planting. Bars at each point represent the standard error (overlapping bars omitted for clarity), y — sum of five shoots per tree

Varying soil moisture levels applied after transplanting had a more pronounced effect on-new root growth (Table 1 and Figure 2). Decreased soil moisture was accompanied by decreased amounts of new root growth. These results agree with the work of previous authors who observed decreased root regeneration and growth with increased water stress (2, 4). Water stress indirectly reduces the production of carbohydrate by limiting the amount of water available to the shoot (9). However, shoot growth was not affected by the soil moisture levels applied after transplanting.

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Figure 2.

Root systems of trees grown In soil maintained at: (a) 100%, (b) 50%, (c) 40%, available soil moisture, and (d) allowed to visibly wilt before watering.

Cyclic watering was used to induce the levels of soil moisture. After watering, root growth proceeds until sufficient water is extracted from the soil by the tree, or lost through evaporation, to induce stress in the plant. Roots become suberized under the stress and growth after subsequent watering is slow to commence (10). At a constant osmotic potential of −6 bars and below, red oak seedlings did not regenerate new roots (4). The cyclic soil moisture treatments used in this experiment mimic more closely the natural water relations in the soil after transplanting than the artificial maintenance of a low osmotic potential in the soil. Trees are usually watered at transplanting and then periodically receive water either by natural precipitation or by irrigation.

Summary

Based on this study, root exposure during the digging-transplanting process is less critical than often assumed. Maximum root growth after transplanting was achieved with a treatment that maintained 100% available moisture in a well aerated container soil mix. In soils that are poorly drained, excessive watering could cause waterlogging and anaerobic conditions resulting in reduced root growth and possible death (10). The results of this experiment indicate that in free draining soils, maximum root growth of Tllia cordata cultivars can be achieved by minimizing the interval between waterings to maintain a high level of soil moisture.