S. R. Pezeshki

Flooding and saltwater intrusion: potential effects on survival and productivity of wetland forests along the U.S. Gulf Coast

Abstract The predicted global warming trend and resultant sea-level rise will increase inundation and salinity along coastal regions worldwide. As water level increases, saltwater encroaches farther inland in many coastal areas. Responses of Gulf of Mexico coastal tree species to increased flooding and saltwater intrusion is evaluated. A review of investigations which address changes in morphological characteristics and carbon assimilation rates under short-term flooding and/or salinity conditions is presented. The combination of flooding and salinity causes foliage damage and substantial reductions in carbon assimilation. The morphological response and reductions in gas exchange rates are closely associated with increases in salt levels. Exposure to salt concentrations greater than 50 mol m−3 (3 ppt) causes some leaf burning and decline in carbon assimilation rates of up to 84% in seedlings of some species. Results indicated that increased flooding and saltwater intrusion, a problem facing U.S. Golf Coast bottomland forests, can cause drastic reduction in net carbon assimilation, leaf burning and seedling mortality. The stress at sublethal levels can lead to weaker seedlings and, consequently, reduced survival rates, and the potential for long-term habitat changes thus limiting the existing natural range of these forests.

Accretion and canal impacts in a rapidly subsiding wetland. I.137Cs and210Pb techniques

The influence of canals on vertical marsh accretion, including mineral sediment and organic matter accumulation, was evaluated at three locations along the Louisiana coast representing different geographic regions. The isotopes210Pb and157Cs were used to determine vertical accretion along transects representing a canal and a control site. Rapid rates of vertical accretion were measured at all sites and ranged from 0.47 cm yr−1 to 0.90 cm yr−1. Results indicated that there was no measurable effect of canals on marsh accretionary processes. In general, greater variation in vertical accretion, including mineral sediment deposition and organic matter accumulation, was observed between geographical regions than between canal and control sites within a region. Statistical analysis of data suggest that any difference between canal and control site would be less than 0.20 cm yr−1. Such a change in marsh surface-water level relationships as a result of any canal influence on marsh accretionary processes would be less than reported eustatic sea-level rise for the Gulf of Mexico. Results suggest that any change in the marsh surface-water level relationship could be the influence of canals on local hydrology, resulting in increased water level rather than any appreciable reduction in accretionary processes. Such changes in hydrology under certain conditions could stress vegetation, resulting in marsh deterioration.

An oxidation-reduction buffer for evaluating the physiological response of plants to root oxygen stress

Abstract Zea mays and Spartina patens were grown in nutrient solution containing either an oxidized (+4) or a reduced (+3) form of titanium citrate. Low oxidation-reduction conditions in the nutrient solution as a result of titanium (+3) citrate reduced photosynthetic activity of Zea mays. Photosynthetic activity of flood-tolerant S. patens was initially reduced by the addition of titanium (+3) citrate but subsequently increased, indicating the existence of adaptation mechanisms in S. patens. Titanium citrate was non-toxic since titanium (+4) citrate (oxidized form) added to rooting medium resulted in no reduction in photosynthetic activity of either species. Titanium (+3) citrate may be an excellent non-toxic oxidation-reduction buffering system for evaluating wetland plant response to root oxygen stress.

A comparative study of the response of Taxodium distichum and Nyssa aquatica seedlings to soil anaerobiosis and salinity

Abstract Taxodium distichum L. and Nyssa aquatica L. seedlings were subjected to anaerobiosis at soil redox potentials of - 160 mV, and salinity concentration of 51 mol m−3, under greenhouse and laboratory conditions. The subsequent carbon assimilation and growth responses to anaerobiosis, salinity, and combined stresses were evaluated. Anaerobiosis alone resulted in a greater reduction in carbon assimilation rates in T. distichum than in N. aquatica. Under combined stresses, carbon assimilation rates decreased 46% in T. distichum and 24% in N. aquatica. Based on height growth, both species appear to have same level of sensitivity (height growth was reduced by 56% in T. distichum and by 54% in N. aquatica). Results also indicated that, in general, both species are salt-sensitive. Thus, seedling survival and regeneration will be adversely affected in those coastal forests where saltwater intrusion occurs frequently.

Response of baldcypress (Taxodium distichum L. var.Distichum) to increases in flooding salinity in Louisiana’s Mississippi River deltaic plain

Baldcypress (Taxodium distichum L. var.distichum) plants were subjected to flooding with tap water and salt water with salinity ranging from two to seven parts per thousand (ppt) under controlled environment conditions. Imposition of flooding and salinity stresses was designed to simulate the increase in submergence and salinity level which Louisiana’s extensive cypress swamps are currently experiencing due to rapid subsidence. The effect of flooding and salt water intrusion on stomatal behavior and net photosynthesis were measured. Stomatal conductance was reduced between 40% and 65% and net photosynthesis declined between 51% and 70% in response to flooding and increases in salinity within three days of flooding and salt application. Both responses were rapid, occurred shortly after treatment began, and Jasted throughout experiment. However, stomatal conductance recovered within three weeks after flooding was initiated. Net photosynthesis recovery was up to 75% of pre-flood levels within three weeks. Increase in salinity, however, caused more extensive impact and greater reductions of stomatal conductance, and net photosynthesis. Stomatal conductance and net photosynthesis did not recover when salinity exceeded three ppt. Results presented suggest that baldcypress swamps of the Gulf Coast area will be adversely affected if salt water intrusion is not contained or if the predicted global increase in sea level occurs.

Organic matter fluxes and marsh stability in a rapidly submerging estuarine marsh

We studied organic matter cycling in two Gulf Coast tidal, nonsaline marsh sites where subsidence causes marine intrusion and rapid submergence, which mimics increased sea-level rise. The sites experienced equally rapid submergence but different degrees of marine intrusion. Vegetation was hummocked and much of the marsh lacked rooted vegetation. Aboveground standing crop and production, as measured by sequential harvesting, were low relative to other Gulf CoastSpartina patens marshes. Soil bulk density was lower than reported for healthyS. alterniflora growth but that may be unimportant at the current, moderate sulfate levels. Belowground production, as measured by sequential harvesting, was extremely fast within hummocks, but much of the marsh received little or no belowground inputs. Aboveground production was slower at the more saline site (681 g m−2 yr−1) than at the less saline site (1,252 g m−2 yr−1). Belowground production over the entire marsh surface averaged 1,401 g m−2 yr−1 at the less saline site and 585 g m−2 yr−1 at the more saline site. Respiration, as measured by CO2 emissions in the field and corrected for CH4 emissions, was slower at the less saline site (956 g m−2 yr−1) than at the more saline site (1,438 g m−2 yr−1), reflecting greater contributions byS. alterniflora at the more saline site which is known to decompose more rapidly thanS. patens. Burial of organic matter was faster at the less saline site (796 g m−2 yr−1) than at the more saline site (434 g m−2, yr−1), likely in response to faster production and slower decomposition at the less saline site. Thus vertical accretion was faster at the less saline site (1.3 cm yr−1) than at the more saline site (0.85 cm yr−1); slower vertical accretion increased flooding at the more saline site. More organic matter was available for export at the less saline site (1,377 g m−2 yr−1) than at the more saline site (98 g m−2 yr−1). These data indicated that organic matter production decreased and burial increased in response to greenhouse-like conditions brought on by subsidence. *** DIRECT SUPPORT *** A01BY069 00016

The influence of soil oxygen deficiency on alcohol dehydrogenase activity, root porosity, ethylene production and photosynthesis in Spartina patens

Abstract Laboratory experiments evaluated root-shoot responses of Spartina patens (Ait) Muhl. to changes in soil redox potential ( Eh ). Three levels of soil redox potential, +460, +230 and −110 mV were imposed in microcosms where plants were grown. Leaf chlorophyll content, gas exchange, root aerenchyma formation, alcohol dehydrogenase (ADH) activity in the roots and ethylene production of leaf and roots in response to the redox treatments were measured. Root responses to hypoxia included a significantly increased porosity and greater ADH activity in hypoxic roots as compared to roots of control (aerated) plants. Ethylene production was significantly greater in leaves and roots of plants under hypoxic treatment compared to control plants. Leaf chlorophyll content was not affected by the treatments; however, stomatal conductance and net carbon assimilation were reduced significantly in response to hypoxia, by 46 and 18%, respectively. Results show a close relationship between root hypoxia, increase in ADH activity, ethylene production and aerenchyma tissue development in S. patens . The enhanced ADH activity and ethylene production found in plants subjected to hypoxia support the postulate that these metabolites have adaptive significance for plants under hypoxic conditions.

Root cortex structure and metabolic responses of Spartina patens to soil redox conditions

Abstract Under controlled rhizotron oxidation-reduction conditions, roots of Spartina patens (Ait) Muhl. were subjected to low soil redox potentials. Root elongation was inhibited by root hypoxia resulting from low soil oxidation-reduction and there was a substantial change in root cortex structure. Studies on root anatomy indicated that low soil oxidation-reduction resulted in a dramatically changed cellular structure in the cortex but not in the vascular cylinder. Alcohol dehydrogenase activity in the roots was increased by approximately three times that of control plants, an indication of acceleration in fermentation. Results suggest that in coastal marshes, S. patens requires periods of drainage or less reducing soil conditions during the growing season for root penetration into otherwise anaerobic zones of the soil-root profile.

Influence of soil oxidation-reduction potential and salinity on nutrition, N-15 uptake, and growth ofSpartina patens

Nutrition, photosynthesis, and growth responses ofSpartina patens to various intensities of sediment reduction (redox potentials, Eh) ranging between — 115 mV to +475 mV and/or salinity of < 1 and 6 ppt were evaluated under controiled environmental conditions. Reduction in soil Eh to — 115 mV seemed to have little effect on net photosynthesis, but 6 ppt salinity combined with low Eh significantly decreased net photosynthesis, indicating the adverse effects of combined low Eh and salinity on gas exchange functioning. The treatments showed striking differences in concentration of nutrients in the plants. In each Eh treatment, N content in the plants was low under 6 ppt salinity compared to the < 1 ppt treatment. N content and N-15 concentration in the plant also decreased as the soil Eh decreased. Data indicated that uptake of N was inhibited by low Eh due to reduced ability of roots to take up nitrogen and/or poor physical growth of roots. Plant Fe and Mn content increased considerably at low Eh treatments. However, salinity did not show consistent influence on Fe and Mn content of tissue. Concentration of K, Ca, Mg, Zn, and Cu in plant tissue was not influenced by Eh or salinity. Root dry weight was significantly decreased in response to lowering of Eh. Within each Eh treatment, root dry weight was not influenced by salinity except in the aerated (high Eh) treatment. This finding indicates that under salinity concentrations tested, soil hypoxia is the dominant factor controlling nutrient uptake and growth ofSpartina patens. Results suggest that in submerged coastal environments, such as the Mississipppi River deltaic plain, increased flooding and soil redox conditions seem to be the primary stress factors affecting productivity ofSpartina patens unless salinity levels substantially exceed the present levels. Another primary reason for vegetation stress is the reduced uptake of nitrogen as a result of reduction in soil Eh.

Response of Spartina patens to increasing levels of salinity in rapidly subsiding marshes of the Mississippi River Deltaic Plain

Spartina patens (Ait.) Muhl. plants were collected from a Mississippi River deltaic plain brackish marsh located along the Louisiana Gulf Coast. The plants were subjected to salinity levels of 4 to 22 ppt under controlled environment conditions. Salt water instrusion is currently occurring in much of Louisianas extensive S. patens brackish marshes due to rapid coastal subsidence and that impact is simulated in this study. Specifically, the influence of salt water influx on the stomatal behaviour and net photosynthesis of S. patens was studied. Stomatal conductance was reduced 54% and net photosynthesis 43% as soil salinity was increased from 0 to 22 ppt. Both responses were rapid (within 24 h) and persisted throughout the entire experiment. Salt excretion from both leaf surfaces was noted within three to five days after salt application. Results presented suggest that salt water intrusion is contributing to the rapid rate of marsh deterioration occurring in coastal Louisiana.