The Influence of Soil Salinity, Growth Form, and Leaf Moisture on-the Spectral Radiance of Spartina alterniflora Canopies

Natural populations of S. alterniflora, growing at apparent soil salinities of 12 to 45OIo0, exhibited a 12 percent range in leaf moisture content. Despite the limited range of moisture content, significant differences in canopy spectral radiance were correlated with changes in leaf and canopy moisture content. Highmarsh and ditchbank S. alterniflora showed a decrease in biomass and leaf moisture content and an increase in spectral radiance as soil salinity increased. gen limitation (Valiela and Teal, 1974), anaerobiosis (Linthurst, 1980), tidal subsidy (Odum and Fanning, 1973; Steever et al., 1976), interstitial soil salinity (Nestler, 1977), or a combination of these and others. There probably is no singular causal agent; however, salinity is consistently implicated as a major contributor. Teal (1962) identified four height forms or distinct marsh types supporting S . alterniflora in a Georgia salt marsh. For a New England marsh, Niering and Warren (1980) have also characterized several S . alterniflora growth forms. S . alterniflora is tallest and most productive on the creekbank, becoming progressively less productive as one moves up the elevational gradient toward the * Presently with the U.S. Army Corps of Engineers, Waterways Experiment Station, P.O. Box 631, Vicksburg, MS 39180. PHOTOGRAMMETRIC ENG NEERING A D REMOTE S NSING, Vol. 49, No. 1, January 1983, pp. 77-83. Besides changes in biomass or proportions of live versus dead biomass, an important consequence of increasing soil salinity is physiological changes in the S. alterniflora related to plant-soil water relations. Increasing soil salinity requires a lower water potential in plant tissues in order to maintain net water flux into the plant. Water uptake is essential for photosynthesis and the survival of the plant. The osmotic drought induced by soil salinity suggests that a leaf water content gradient may be an aboveground indicator of soil water potential. Spectral detection of leaf water status has been accomplished in the 1.3 to 2.7 pm region where infrared radiation is strongly absorbed by water (Allen and Richardson, 1968). Reflectivity from a leaf measured in the 1.3 to 2.7 p m region results from Fresnel reflections at internal and external interfaces (mainly cell wall-intercellular air 0099-1 112/83/4901-77$02.25/0 k3 1983 American Societv of PhotonrammeW PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1983 spaces) and absorption by water (Allen et al., 1969; Gausman et al., 1970). This phenomenon has been observed and documented by many authors (Gates et al., 1965; Allen et al., 1970; Knipling, 1970; Woolley, 1971; Gausman, 1974; Gausman et al., 1974; among others). Thomas et al. (1971) demonstrated a higher middle infrared reflectance from corn leaves as leaf water content decreased. Similar results from leaves of agricultural crops monitored over the growing season showed an increase of middle infrared reflectance as moisture content decreased (Sinclair et al., 1971; Gausman et al., 1973). Soil salinity has also been associated with increased reflectance of single leaves in the reflected infrared spectral region. Thomas et al. (1966) working with cotton found reflectance in the infrared to increase below about 80 percent relative turgidity. They also detected increased reflectance from cotton leaves exposed to high concentrations of chloride. Changes in leaf thickness and color were suggested as morphological leaf changes associated with the higher reflectance. Additional work by Gausman et al. (1969) with cotton suggested a decreased leaf water content as a function of higher salinity generally increased infrared reflectance. They attributed near infrared reflectance in the 0.75 to 1.3 pm region to changes in cellular arrangement within the leaf. Laboratory measurements of increased infrared reflectance as moisture content decreased were reversed when spectral signatures of field canopies were considered. Thomas et al. (1967) and Meyers et al. (1966) found nonsaline cotton to be more reflective than salt-affected cotton. This they attributed to smaller leaves, fewer leaves, and increased amounts of soil reflectivity, characteristic of the salt affected canopies. These observations for salt-affected cotton can be related to canopy variations in height forms of S. alterniflora. The present research was concerned with canopy structure changes in height forms of S. alterniflora and how spectral characteristics changed relative to growth form. Sampling of S. alterniflora canopies relative to a salinity gradient seemed to be a logical approach. The objectives of this study were to identify the vegetative components of the canopy most related to the observed spectral changes and to equate leaf water content, canopy water content, and spectral response to ambient soil salinity regimes. SITE DESCRIPTION-HIGHMARSH The S. alternijlora salt marshes selected for study were within the drainage basins of Canary Creek and Old Mill Creek in the vicinity of Lewes, Delaware. Brackish and saline marshes supporting S. alternijlora were sampled in both creeks. A total of four sampling stations were chosen, representing the range of soil salinities under which highmarsh S. alterniflora would normally be found growing in the field. SITE DESCRIPTION-DITCHBANK A mosquito ditch, colloquially known as Renegade Ditch, was selected for study from within the Old Mill Creek drainage. Renegade Ditch extended approximately 343 m from the mouth to the nodal point (the point from which tidal water ebbs . or floods from opposite directions) and supported an even gradation of creekbank S. alterniflora along its ubanks. Five stations were identified to describe the gradual reduction in plant height and aboveground biomass as one progressed up the ditch to the nodal point. The stations were not evenly spaced up the ditch. HARVESTING PROCEDURE The aboveground vegetation was harvested at each of five stations on Renegade Ditch on 30 August 1980 and at the four highmarsh areas on 12-15 September 1980. A 0.1 square metre circular ring was placed in a representative area at each station and all the plants within were clipped at soil level. Three replicate harvests were performed at each sampling station in both the highmarsh and ditchbank areas. In the laboratory, a representative subsample was drawn from the harvested vegetation. The plant material was sorted into live leaves, live stems, and dead material. The sorted plant parts and the unsorted residual were washed with tap water, dried at 60°C to a constant mass, and weighed to determine dry weight biomass. Each plant component was estimated for the whole sample based on the biomass proportions determined for the sorted subsample. MOISTURE CONTENT DETERMINATION Approximately 25 green healthy leaves were clipped from S. alterniflora plants directly adjacent to each harvested plot. The leaves were immediately bagged and put on ice. Within three hours, the leaves from each sample were wiped clean with Kimwipes and the fresh weight determined to the nearest O.lg. After drying at 60°C to a constant mass, the dry weight of the leaves was determined. The moisture content of the leaves was expressed as the percent moisture content based on the fresh weight of the leaf; PMC = (1-(DWIFW) x 100, where PMC was percent moisture content, and FW and DW were frish and dry weight biomass, respectively.