Relative Susceptibility of Thirteen Pine Species to Sodium Chloride Spray

Materials and Methods

Species used were Japanese black pine (Pinus thunbergi.), ponderosa pine (Pinus ponderos.), Austrian pine (Pinus nigr.), southwestern white pine (Pinus strobiformi.), Swiss stone pine (Pinus cembr.), Japanese red pine (Pinus densiflor.), bristlecone pine (Pinus aristata), Japanese white pine (Pinus parviflora), Balkan pine (Pinus peuc.), red pine (Pinus resinos.), Scotch pine (Pinus sylvestri.), jack pine (Pinus banksian.), and eastern white pine (Pinus strobu.). Seedlings 31-46 cm tall of these species were grown in 4.7 L pots in a 2:2:1 (v/v/v) peat: perlite: soil mix, in a lath house. Seedlings were arranged in a split plot design in 2 blocks, with salt or water spray representing major plots and species representing minor plots, there were 24 seedlings of each species for each treatment (salt or water-spray). Plastic bags kept salt out of the soil mix, and plastic sheets separated salt water and plain water plots.

Plants were sprayed in late winter or early spring for 3 consecutive years. Some seedlings were sprayed with distilled water only as controls. In year one, the other seedlings were sprayed to run-off with distilled water either with or without 2% NaCI solution once daily from Monday through Friday for 4 weeks (of March 3, March 10, March 24, and March 31), then with 10% NaCI on April 14, 15, and 16. In year two, they were sprayed with 10% NaCI for 5 days per week for 2 weeks beginning March 25, and for 2 days in mid-April. In year three they were sprayed with a 20% NaCI solution for a total of 10 days during the weeks of March 8 and March 22. Foliar symptoms were evaluated by estimating the percent chlorosis and necrosis on the needles of each seedling; data on symptoms were recorded the first year 2 weeks, 4 weeks, and 1 7 weeks after spraying ended. Survival was recorded 17 weeks, 1, 2, and 2½ years after the final spray of the first year. New yearly growth was measured after the second, third, and fourth growing season.

Four weeks after completion of the first year’s spraying, needle samples were taken from needles throughout each seedling for determination of sodium and chloride levels. External chloride was removed by two 1 -min rinses in 150 ml of distilled, deionized water. Needles were then oven-dried (70°C for 48 hrs) and weighed.

Dried needles were ground to a 20-mesh size using a Wiley mill. Chloride was determined by the method of La Croix et a. (9), in which a specific ion electrode is used to determine the endpoint of AgNO₃ titration. Sodium was analyzed with an inductively coupled plasma spectrograph at the Research Extension Analytical Laboratory, OARDC, Wooster, Ohio.

Results

Survival significantly decreased for many species over time (Table 1). Survival of P. strobus, P. resinosa, P. aristat., and P. densiflor. decreased within 17 weeks after the first year’s spraying. After 2½ years, survival of only P. strobiformis, P. nigra, P. thunbergi., and P. ponderos. had not significantly decreased in response to the salt application. In contrast, survival of P. cembra, P. banksiana, P. strobus, P. peuce. and P. aristat. was less than 20 per cent of control seedlings’ survival.

The characteristic symptom of injury was browning of the needles that began at the tips and progressed towards the base. Significant differences in foliar symptoms among species appeared 2 weeks after the first salt application was made, and were quite large 15 weeks later (Table 1). P. strobu. showed the most symptoms, whereas P. thunbergii, P. ponderos. and P. nigr. showed the least symptoms. Other species showed pronounced injury, but still less than that of P. strobus. Pinus ponderos. showed the lowest and P. strobu. the highest concentrations of needle surface chloride, internal chloride, and internal sodium (Table 2). Pinus strobiformis, P. thunbergii. and P. cembr. also had low levels of needle surface chloride. Pinus ponderosa, P. cembra, P. strobiformis, P. nigra. and P. thunbergi. were low in internal chloride and internal sodium whereas P. strobus, P. sylvestris, P. peuce. and P. parviflor. were high. In a correlation analysis of data, we found that high levels of foliar sodium and chloride were significantly associated across species with less new growth, more symptoms, and low survival.

Height growth was most suppressed by salt in P. strobus, P. densiflora, P. bankslana. and P. cembra. and was least suppressed in P. nigra, P. ponderosa. and P. thunbergi. (Fig. 1). These latter species showed greater suppression of new growth the second year after spraying began than in the third and fourth years.

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Figure 1.

Yearly height growth of sodium chloride-treated pine seedlings (expressed as a per cent of water-treated trees). Years 2, 3, and 4 refer to the second, third, and fourth years after the first year of salt treatment.

Discussion

Salt tolerance (best survival, least foliar symptoms, and least growth reduction — i.e., Pinus thunbergii, P. ponderosa, P. nigra. accompanied low internal chloride and sodium. The highly susceptible species (with poor susceptible high foliar injury, and great growth reduction) were P. strobus, P. banksiana, P. cembra, P. peuce. and P. densiflora. The intermediates were P. strobiformis, P. aristata, P. parviflora, P. resinosa. and P. sylvestris.

In this study, Pinus ponderos. appeared very salt-tolerant; it deserves more testing wherever salt-spray occurs. P. strobiformi. was the most salt-tolerant 5-needled pine. It could replace P. strobu. on salt-spray sites.

Most species with low internal sodium and chloride levels tolerated salt spray. We (19) and others (6, 7) found this pattern also in earlier, shortterm salt studies of pines.

One species, P. cembra. shows anomalous results: low levels of needle surface chloride, internal chloride, and internal sodium, yet the most severely reduced growth and survival 3 years after treatments. Pinus cembr. may exclude most of these ions but be injured by extreme sensitivity to those few ions that do enter.

Needle thickness of P. nigra, P. ponderos., and P. thunbergi. may help exclude salt ions. Barrick et a.. (1) considered that differences in epicuticular waxes and needle surface characteristics explained greater salt tolerance in P. nigra (vs. P. strobus.. Logan (10) considered that the cuticle of P. nigr., twice the thickness of that of P. strobu., was responsible for its greater tolerance to salt spray.

The quantity and quality of surface waxes appear to play an important role in the foliar uptake of polar solutions in general (8, 13). Holloway (8) showed surface wax to be an effective hydrophobic barrier to liquids with high surface tension if the wax covers the entire epidermis. Plants containing plate-like, overlapping leaf waxes are less penetrable to polar solutions like salt water than those with vertically oriented rod-like waxes, which do not entirely cover the surface. Differences in wax structure and cuticle thickness may explain differences among pine species’ sensitivity to deicing salt, but further studies are needed. Tolerant species, which generally accumulated the lowest quantities of needle surface chloride in this study, may be able to present a better hydrophobic barrier to entry of salt solution.