Variation in Leaves and Roots of Sugar Maple and its Allies: A Key to Improve Performance of Trees in the Landscape?

William R. Graves and Rolston St. Hilaire
Department of Horticulture, Iowa State University, Ames, IA 50011-1100

Abstract. Cultivars of sugar maple (Acer saccharum Marsh.) are selected based mainly on aesthetic features and production considerations. Less attention is focused on the provenance of the cultivar. Sugar maple and taxa allied to it occur over a broad geographic area of North America in which the environment varies. We have been testing the hypothesis that leaves and roots of these taxa vary with provenance, and that provenance differences can be used to select and develop trees that will perform comparatively well in managed landscapes where drought is common. Initial steps in this long-term effort have shown some foliar traits related to drought resistance differ predictably based on origin within a transect of the eastern United States where annual precipitation tends to decrease from east to west. Related studies revealed that certain inherent foliar differences among provenances are stable over time and are consistent with differences found among plants from selected provenances that were cultured in a common environment. Plants from these provenances used as rootstock differed in initial compatibility with a sugar maple cultivar as scion, yet scions of successful grafts developed similarly well. Differences among rootstock-scion combinations in foliar gas exchange suggest a more rigorous examination of rootstock effects on drought resistance is justified. We propose that selecting maples from native populations that express foliar traits of drought resistance and grafting cultivars on rootstocks that impart resilience during drought are testable strategies for improving sugar maple and its allied taxa for use in managed landscapes.
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We are grateful for funding from the Iowa Nursery and Landscape Association, the Landscape Plant Development Center, and the J. Frank Schmidt Family Charitable Trust. Thanks to the Uihlein Sugar Maple Field Station, New York, for supplying seed, and to Mark Kroggel for technical help.
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The long-term success of sugar maples (Acer saccharum Marsh.) planted along streets and in other managed landscapes often is poor. Trees that survive often are marred by foliar scorch, marginal necrosis, and tatter. These symptoms suggest the affected leaves were unable to sustain a favorable water balance during periods of drought and/or high evaporative demand. Drought often is exacerbated in managed landscapes by the small volume of soil available for root expansion and by the runoff of precipitation on impermeable surfaces. Although some cultivars appear to have better resilience to drought than others, we continue to need selections of ornamental hard maples with superior resistance to drought stress.
At least two systematic strategies may exist to guide the selection process. First, we might attempt to link known variations in the environment where sugar maples occur naturally with differences in traits of the trees that have evolved under those conditions. Sugar maple and its allied taxa in the Saccharina Pax. section of the Aceraceae family occur over a broad geographic area of North America. For simplicity, we will refer to this group collectively as hard maples. Mean annual precipitation is among the environmental differences that exist where these taxa occur in nature. Might hard maples native to regions where annual precipitation is relatively low and droughts are common in the summer be adapted to low soil moisture and/or high evaporative demand in ways their allied taxa in wetter climates are not? We might answer this question systematically by attempting to develop statistical models to predict how traits associated with drought resistance vary across a geographic area where precipitation changes predictably. Foliar traits associated with drought resistance would be a logical place to begin such modeling work because most water lost by a tree transpires through leaves.
A second strategy to guide the selection process might be focused on identifying root systems that are unusually effective at preventing the occurrence of leaf water deficits. Mechanisms of superior effectiveness include the development of a large surface area capable of water uptake, an unusually high hydraulic conductance, and production of chemical signals to evoke changes in stomatal aperture, root resistance, or other traits that govern water status. Use of specific rootstocks that might impart such a benefit probably would not require major changes in production practices because cultivars of sugar maple commonly are propagated by budding. We have heard that producers of sugar maple in certain regions of the United States bud cultivars on local forms of hard maple. But, to our knowledge, experiments have not been done to test whether rootstocks of hard maple taxa from distinct geographic and ecological niches influence sugar maple scions in ways that might affect resistance to drought.
This report is a summary of research related to these two strategies. One of our objectives was to model how various foliar traits likely to influence leaf water status differ among trees of hard maple native near the 43°N latitude in the central and eastern United States. That work was led by Rolston St. Hilaire as part of a doctoral dissertation he completed at Iowa State University in 1998, and additional details can be found in the literature (St. Hilaire and Graves, 1999). A second objective is to compare leaf water relations of a sugar maple cultivar budded on hard maple rootstocks from different geographic areas. Our intent is to present the methods and selected results in general terms that broadly represent key findings without great detail. Please feel free to contact the senior author for any additional information you might desire.

Materials and Methods
Foliar traits. Leaf morphology and anatomy of hard maples indigenous near 43°N latitude were examined in 1995 and 1996. Leaves exposed to direct solar radiation were sampled from up to 20 trees indigenous at each of 42 sites. We used regression analysis and other statistical methods to test for relationships between foliar traits and the longitude of origin from 70° to 94°W longitude, an area that extends from southern Maine west to central Iowa. Physical and climatic information about the sites where trees were sampled is available (St. Hilaire and Graves, 1999). We collected leaves in Iowa. Cooperators in other states used the same methods during the same time periods and mailed samples to Iowa for us to analyze.
Rootstock effects. We collected samaras in two provenances, eastern and central Iowa, during the autumn of 1995. We received cold-stratified samaras in early 1996 from New York and Vermont. These half-siblings were considered the eastern United States provenance. During the same winter we received seed of Caddo sugar maples. Seedlings derived from these seeds were grown in a greenhouse at Iowa State University in individual containers beginning in early 1996. Bud wood of 'Flax Mill Majesty' sugar maple was inserted into dormant seedling rootstocks from all provenances during February, 1999. The percentages of unions that were successful varied among rootstock provenances and half-sibling groups and were highest for rootstocks from the eastern United States. Successful unions were grown in the greenhouse during 1999, overwintered elsewhere, and returned to the greenhouse in early 2000. After a flush of new shoot growth developed, eight visually uniform unions of each of the four rootstock provenances were selected. These were randomly assigned to two irrigation treatments, control and drought, and randomly arranged in the greenhouse. Controls were irrigated to container capacity every other day. Irrigations were withheld from plants in the drought treatment during three 12-day periods separated by 9-day recovery periods when the root medium was kept moist. Gas exchange of the youngest fully expanded leaf was measured in each drought cycle by using a LI-COR 6400 Photosynthesis System. The moisture content of the rooting medium was estimated each time a measure of gas exchange was made, and water potential of the leaf sampled for gas exchange was determined on day 12 of each cycle. After the third cycle, leaf surface area and dry weights of leaves, entire shoots, and roots of all plants were determined.

Results and Discussion
Foliar traits. Morphology of all leaves from east of 75.84°W and from 92.98°W and further west was consistent with sugar maple and black maple, respectively. Leaves with intermediate morphologies were found between these two longitudes. Leaves from 90° to 94°W had the highest surface area (St. Hilaire and Graves, 1999). Up to 800 trichomes/cm2 were present on the abaxial surface of laminae from west of 85°W. Laminae from further east were glabrous or had fewer than 200 trichomes/cm2 (St. Hilaire and Graves, 1999). Laminae from western habitats also had relatively high stomatal frequency, and stomatal apertures of laminae west of 91°W were particularly narrow (St. Hilaire and Graves, 1999). Longitude did not affect specific weight and thickness of laminae, which averaged 5.5 mg·cm-2 and 90 µm, respectively. This conflicts with a previous report that seedlings of sugar maple had lower specific leaf weights than seedlings of black maple from further east (Graves, 1994). Principal component analysis of foliar traits revealed two clusters. A large group was dominated by data from trees in New England but also contained data from trees as far west as about 93°W longitude. Data obtained from leaves removed from trees further west were clustered separately (St. Hilaire and Graves, 1999).
Rootstock effects. Withholding irrigation caused root-zone moisture to decrease to 10% or less by volume among plants in the drought treatment. Data on moisture obtained on the last day (day 12) of each drought cycle showed root-zone moisture of controls was 27% or more. This analysis also showed no rootstock-by-irrigation treatment interaction, which indicates that plants exposed to drought dried similarly regardless of rootstock. Photosynthesis rates were plotted against moisture content of the rooting medium and against leaf water potential. Separate plots were made for each rootstock treatment. Rates of photosynthesis were related best to root-zone moisture by quadratic regression functions. The slope of those functions at low (20% or lower) soil moisture showed the drought-induced decrease in photosynthesis of 'Flax Mill Majesty' scion tended to be relatively low when on roots of Caddo maple or sugar maple from central Iowa. Photosynthetic rates also tended to be higher at low leaf water potential among unions involving roots of sugar maple from central Iowa. As expected, drought stunted the development of both root and shoot systems. Of more interest were tests for effects of rootstock provenance and tests for interactions between rootstock and irrigation treatments. No such effects were found for measures of scion development. A marginal effect of provenance existed for root dry weight due to large rootstocks from eastern Iowa. There were no differences found in ratios of root mass to leaf mass or area, however.

Conclusions
Selecting maples from native populations that express foliar traits of drought resistance and grafting cultivars on rootstocks that impart resilience during drought are testable strategies for improving sugar maple and its allied taxa for use in managed landscapes. Gradual, continuous, and predictable changes in leaf morphology of indigenous hard maples exist along a gradient in longitude near the 43°N latitude in the eastern and central United States. Trichome and stomatal frequencies are among the foliar traits for which the most change with longitude was found; both of these traits might influence tree drought resistance. Results of principal component analysis suggest trees west of 93°W longitude are distinctive, and we propose that evaluating aesthetically superior individuals from west of 93°W longitude is warranted. Any conclusions regarding the rootstock study must be tempered by the fact that only 32 plants were studied. Yet the occurrence of rootstock-based differences in foliar gas exchange during drought among this small sample suggests that further experiments with more plants is warranted.

Literature Cited
Graves, W.R. 1994. Seedling development of sugar maple and black maple irrigated at various frequencies. HortScience 29:1292-1294.

St. Hilaire, R. and W.R. Graves. 1999. Foliar traits of sugar maples and black maples near 43 °N latitude in the eastern and central United States. Journal of the American Society for Horticultural Science 124:605-611.