Abstract
One meter (3 ft.) tall whips of Fraxinus pennsylvanica ‘Emerald’ (pubescent), Fraxinus pennsylvanica (glabrous), Malus ‘Klehm’s Improved Bechtel’ (pubescent) and Malus “White Angel (glabrous) were used to evaluate the role of pubescence in plant water relations. All plants received the same watering regime and drying cycle. The Fraxinus showed a trend in which the pubescent cultivar maintained less trees had less negątive predawn water potintials. A comparison of four mulch types (shredded bark, fine bark, wood chips and medium grade gravel) to bare soil controls, showed that all mulch types resulted in less evaporational water loss. The shredded bark and medium grade gravel providing the greatest reduction in evaporational water losses. The pubescent Fraxinus cultivar ‘Emerald’ and a 4 cm (1.6 in.) layer of shredded bark mulch resulted in the least negative water potentials.
In order for plants to survive a prolonged water stress, they must be able to prevent or reduce water loss in certain tissues. Hinckley et al. (4) stated that leaf adaptations appear to be a key in the success of a species in a water-stressed environment. Common adaptations include increases in cutinization, thickness of both the mesophyll layers and the entire leaf, and leaf pubescence. LoGullo, Salleo and Rosso (6) reported that plants growing in arid habitats tend to possess leaves that have more pubescence than similar or related plants growing in mesic habitats.
Pubescence allows a plant to maintain carbon exchange under arid conditions, helps the plant avoid lethally high leaf temperatures, and reduces daily water loss by maintaining a saturated boundary layer. All of these factors will extend the plant’s growth for a longer period of time (7).
The application of a mulch layer will also improve the water status, of landscape plants. A mulch is any material applied to the soil to moderate or increase soil temperature, conserve soil moisture, or to suppress weed growth (3). Einert et al. (2) reported in a study comparing various mulch materials with bare soil that the soil moisture percentages in mulched plots were approximately twice as high as the bare soil plots, the summer soil temperatures were reduced by eight to 13 degrees (F), and the average time required to remove weeds was reduced by two-thirds. Mulches also reduce the impact of water droplets hitting the soil surface, thereby reducing soil erosion and crusting. This reduction in soil crusting increases water penetration into the soil.
Current recommendations call for the application of an organic mulch layer which is 4 to 10 cm (1.6-4.0 in.) deep (5). A layer of mulch deeper than this may reduce oxygen and water penetration into the soil (1), and this will ultimately reduce plant and root growth. The primary objectives of this research were to evaluate: 1) the role of leaf pubescence in increasing tolerance in selected woody ornamentals to imposed water stress, and 2) the effect of mulch type and depth on reducing soil evaporative losses.
Materials and Methods
Experiment 1
The first experiment contrasted glabrous leafed Fraxinus pennsylvanica with pubescent F. pennsylvanica ‘Emerald, and glabrous leafed Malus ‘White Angel with pubescent M. ‘Klehm’s Improved Bechtel’. All bareroot whips were pruned to .9 m (3.0 ft) above the graft union and the roots were standardized to a 50 cm (20 in) diameter circle. Each was then planted in a 38 L (10 gal.) black plastic container in a 1:1:1 (by volume) perlite, peat, sandy loam, soil medium.
Six replicates of each tree type were given one of three treatments: one 7 day dry period (Group A), two seven day dry periods separated by a rewatering (Group B), or no dry periods (Group C) (Table 1).
Three treatment groups with six cultivar replicates each of F. pennsylvanica, F. pennsylvanica‘Emerald’, Malus’V\IK and Malus KIB’.
Using a Sholander-type bomb, baseline predawn water potentials were taken on a single fully expanded leaf from each plant at the beginning of the experiment. After each of the 7 day dry periods was over, predawn water potential readings were again taken. To determine leaf hair density, two leaves were chosen randomly, one near the top and one near the middle, from each tree of the three treatments. A clear plastic sheet with a 10 cm by 10 cm (4 in by 4 in) grid was laid over each leaf and two different counts of leaf hairs were taken. All of the readings within a cultivar were then averaged.
A completely randomized design was used with 6 replicates/treatment/cultivar. The greenhouse was maintained at 21 °C day/16°C night temperatures. The data were analyzed through factorial analysis using the PROC-GLM procedure of SAS (SAS Inst. Inc., 1985, Cary, NC).
Experiment 2
In the second experiment, sixty-four 20 L (5.2 gal) black plastic containers were filled to a height of 30 cm (12 in) with a sandy loam soil then covered with either a 3.8 cm (1.5 in) or 7.6 cm (3.0 in) layer of hardwood chips, shredded pine bark, fine pine bark, medium grade gravel or were left uncovered. All of the experimental units were saturated with water and, when gravitational flow had ceased, initial pot weights were taken. Pot weights were taken during the following 5 days to determine the daily water loss.
A completely randomized design was used with 8 replicates/treatment. The greenhouse was maintained at 21 °C day/16°C night temperatures. The final pot weights were subjected to a t test for significance at the 5% level using the MINITAB statistical package.
Experiment 3
The Fraxinus trees from the first experiment were standardized to a height 3 m above the bud union. Four trees from each cultivar were given one of the following treatments: shredded bark (4 cm depth), medium grade gravel (4 cm depth), a fused non-woven polypropylene geotextile (DeWill Weed Barrier), or no mulch treatment. A class A evaporation pan apparatus was used to determine water evaporated over a given time period. A preliminary test was run on non-mulched trees to determine that 2 cm (0.8 in) of water evaporated from the pan before a critical predawn leaf water potential threshold of -1.0 MPa was reached.
Baseline predawn water potentials were taken, and were repeated when 1 cm (0.4 in) and 2 cm of water had evaporated from the pan. All experimental units were thoroughly rewatered for two days and then a second dry down began. Predawns were taken at the onset, after 1 cm evaporated, and after 2 cm evaporated.
A completely randomized design was used with four replicates/treatment. The greenhouse was maintained at 21 °C day/16°C night. All of the trees were destructively harvested and the leaves run through a Leaf Area Meter (LiCor Model 3100). Analysis of variance was run on the mean leaf area of each tree. The data from the predawn and daily water potentials were analyzed through factorial analysis using the PROC-GLM procedure of SAS (SAS, Inst. Inc. Cary, NC).
Results and Discussion
Experiment 1.
An examination of average leaf hairs/mm2 revealed that F. pennsylvanica had zero hairs, F. p.‘Emerald’ had 281; M. ‘White Angel’ had 21 hairs and M. ‘Klehm’s Improved Bechtel’ had 163. There were significantly different leaf hair densities within each genus (Table 2).
Leaf hair density for Fraxinus pennsylvanica, F. p.‘Emerald’, Malus‘WA’ and Malus‘KIB’.
The results presented here are from the two consecutive dry periods (Group B). The pubescent F. p. ‘Emerald’ displayed a non-significant trend toward less negative predawn water potential at the end of both dry downs compared to the glabrous F. pennsylvanica during the same dry periods (Figure 1). The pubescent M. ‘KIB’ maintained less negative, and therefore less stressed, predawn leaf water potential at the end of both dry periods compared to the glabrous M. ‘WA’ (Figure 2).
Average predawn water potential for Fraxinus pennsylvanica and F.p. ‘Emerald’ showing the results of no drought stress (wk 0), one week of imposed drought stress (wk 1) and two consecutive weeks of imposed drought stress (wk 2).
Average predawn water potential for Malus ‘White Angel and Malus ‘Klehms’s Improved Bechtel’ showing the results of no drought stress, one week of imposed drought stress and two consecutive weeks of imposed drought stress.
When contrasting the predawn water potentials of the two pubescent cultivars, the Malus KIB’ maintained less negative predawns than did the F. p. ‘Emerald’. The ‘KIB’ cultivar, with little more than half the leaf hairdensity of ’Emerald’, reached a most negative water potential of -.60 MPa (Figure 3a). The ‘Emerald’ cultivar reached an average water potential of -1.96MPa after completing the same two week period of imposed drought (Figure 3b). These results suggest that, although leaf pubescence is one factor that can increase a plant’s ability to withstand imposed drought stress, other factors, including total leaf area and tree architecture must also be considered.
Average weekly predawn water potential of pubescent Fraxinus ‘Emerald’ (3a) and Malus ‘KIB’ (3b) cultivars over two week treatment period, (week 0- no drought stress; week 1- one week of drought stress week 2-two consecutive weeks of drought stress).
Experiment 2
The results from the mulching experiment showed that after a five day period there were no significant differences based on depth within each mulch treatment. However, the mulches did reduce evaporational water loss on average of 27, 34, 43 and 50% relative to the bare soil control (Table 3). The gravel and shredded bark results were similar for both application depths, and the wood chips and fine bark results were similar at both depths. The gravel and shredded bark conferred the greatest savings. The application of mulch layer of 3.8 cm resulted in the same benefits as a 7.6 cm layer. This suggests that the application of a 3.8 cm layer is not only more economical, but may also help to alleviate problems associated with over-mulching.
Percent evaporative water loss saving by treatment and depth compared to control for shredded bark, gravel, wood chips and fine bark at depths of 3.8 cm and 7.6 cm, in 20 L containers.
Experiments
The results from the third experiment, which combined tree type and mulch type, further supported the results from the first two experiments. Across all four treatments, the pubescent F. p.‘Emerald’ trees maintained significantly less negative predawn water potential (by an average of -.16MPa) than the glabrous Fraxinus pennsylvanica after the first induced drought. Across both the species and cultivar, of the two particulate mulches — shredded bark and medium grade gravel — the shredded bark was associated with the least negative predawn water potential (Figure 4a and 4b).
Water potentials of Fraxinus pennsylvanica (4a) and F.p. ‘Emerald’ (4b) across four mulch treatments after 9 days dry cycle. Readings taken at predawn, 11 :00 a.m., 2:00 p.m. and 5:00 p.m.
At the end of the second consecutive dry down period, there was no difference in predawn water potential between the Fraxinus pennsylvanica and F. p. ‘Emerald’. However, within the two groups there was a difference between the gravel and shredded bark mulches. The shredded bark mulch was again associated with the least negative water potential for both tree types (Figure 5a and 5b).
Predawn water potentials of Fraxinus pennsylvanica (5a) and F. p. ‘Emerald’ (5b) across all mulch treatments on July 8, July 11 and July 15,1990.
Conclusions
Creating landscapes using low water input and high water efficiency is becoming increasingly important to nurserymen and landscapers as many regions in the nation face drought-like conditions. This research suggests that species with the morphological characteristic of pubescence are able to maintain more favorable water status when compared to glabrous members of the same species. As well, the application of a 3.8 cm (1.5 in) mulch layer resulted in reduced evaporational water loss ranging from 27-50% relative to a bare soil control; and a 3.8 cm layer yielded essentially the same results as a 7.6 cm (3.0 in) layer, suggesting that the 3.8 cm application would be more economical. Appropriate species selection and the application of a suitable mulch are only two ways to help create a water efficient landscape. This can result in a reduction in stress-related mortality of trees, help nurserymen and landscapers in drought-stricken areas to comply with water restrictions, and increase consumer satisfaction with plant performance.
- © 1992, International Society of Arboriculture. All rights reserved.
In this issue
Jump to section
Related Articles
Cited By...
- No citing articles found.