Monterey Bay Aquarium Research Institute
Marine Botany
Distichlis spicata

Salinity and Morphology

Distichlis from sandy shore (site G), Hopkins Marine Station (~3 inches tall)

Distichlis from grass field (site E), Hopkins Marine Station (~12 inches tall)

Salinity Tolerances:

Although a number of laboratory studies have reached different conclusions regarding the maximum tolerable salinity for the Distichlis plant, the United States government (in the form of the Forestry Service and Department of Agriculture) maintain that saltgrass plants can tolerate salinities ranging all the way from fresh water to uncut seawater (salinity approximately 35 psu) [1, 6, 10].

Regardless of its maximum theoretical salinity tolerances, in natural enviroments saltgrass will generally become the dominant plant cover in areas where the salt content of the soil is greater than 0.33 to 0.5 percent and can even survive in areas where the soil surface is completely encrusted with salt. [10]

Salinity and Morphology:

Research also indicates, interestingly, that Distichlis plants exhibit stronger and stronger tendencies towards dwarfed growth and morphologies as the salinity of their environments increases [10]. The characteristics of dwarfed growth include changes in stem growth / length, leaf thickness and overall shoot height [6, 14]. A mechanism for these differences in expression has been proposed by Kemp and Cunningham [6] - in all lighting conditions, although most noticibly in low light treatments, increased salinity appears to decrease net photosynthesis by way of forcing stomatal closure.

The tendency of high salinities to lead to dwarfed growth was tested qualitatively with five stands of Distichlis plants found at the Hopkins Marine Station (sites A, C, D, E, and G on the distribution map).

Observations at Hopkins Marine Station:

The author found that the expected morphological patterns were demonstrated clearly in a survey of five stands of Distichlis found at Hopkins Marine Station, three located between 10 and 50 feet from the level of mean high tide (sites A, D and G) and two located several hundred feet from the shore (sites C and E).

Morphologies were compared between these two sets of stands to determine if distance from the shore had an effect on the physical characteristics of the plants, the hypothesis being that plants closer to the shore would effectively be living in a higher salinity environment (due to the influence of seawater in their ground water sources) than those farther from the shore, and should demonstrate dwarfed characteristics when compared to plants living far from the shore in lower salinities.

Informal but detailed observations of these five stands of plants show that Distichlis plants living closer to the shore indeed exhibited the characteristics of dwarf growth described below.

Plants living on sandy shores close to the water exhibited very noticibly shorter stalks, both on average and in maximum observed height (plants in the close-to-shore set averaged approximately 4-6 inches in height, while those in the far-from-shore set averaged closer to 8-12 inches in height). The nearshore plants could be generally characterized as "short and squat," exhibiting smaller average heights, only a short distance between blades along the stem, broad blades aligned almost perpendicular to the stem and significant deposits of salt on their blades.

The second set of plants, taken from a grassy field (site E) and a mulched, landscaped plot (site C) exhibited much longer stalks, greater inter-blade distance, thin, curled blades angled close to the main stem and smaller salt deposits on the blades.

Observations of salt deposits on the blades of plants from these two sets specifically indicate that these morphological differences may in fact be due to differences in environmental salinity.

Blades from site A (nearshroe) and site E (far-from-shore)

Since laboratory research indicates that the amount of salt a plant extrudes through its blades and stem is directly related to the salinity of its environment [14], these blade observations imply that the salinity experienced by Distichlis plants is higher in the stands closer to the shore, and, again as suggested by previous research, that this may have caused the observed morphological differences between the stands discussed above.

Conclusions are of course difficult to reach from such a limited, qualitative study. Other environmental differences between sites are of course relevant to plant morphology, including differences in soil nutrients, shading, and competition between the observed sites. The author nevertheless strongly speculates that the observed differences are caused by environmental salinity differences, and this conclusion would be consistent with more than one published experimental finding, as noted above. More quantitative study, including direct measurements of water and soil salinities at each of the sites, would be necessary to determine if this is in fact the case for the stands of saltgrass found at Hopkins Marine Station.

Last updated March 2003, Justin Kitzes.

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