Effects of cordgrass on saltmarsh fucoids: Reduced desiccation and light availability, but no changes in biomass

There is a nearly obligate co-occurrence of saltmarsh fucoids with cordgras s in eastern Canada. We examined the possibility that this might be a facil itative association in which Spartina alterniflora ameliorates an otherwise unsuitable soft sediment habitat producing favourable conditions for the d evelopment of large fucalean populations. Likely direct effects of cordgras s on fucoids are: (i) reduction of desiccation; (ii) reduction of light lev els through shading; and (iii) stabilisation of fucoid mats which appear "s tapled" in position by grass shoots. A likely indirect effect of Spartina o ccurs when drifting rafts of plant litter become trapped within the swards above the fucoid mats. We found that cordgrass canopy reduced light and deh ydration levels at the mud surface where fucoid mats occur. The reductions measured were certainly within ranges known to affect photosynthetic rates in fucoids. Nevertheless, there were no effects of manipulations of cordgra ss canopy or of litter rafts on either the biomasses of adult fucoids, or o n the growth of juvenile fucoids. Power analysis showed that the absence of significant results is not attributable primarily to inadequate replicatio n, but to very small experimental effect sizes. Reductions in light and des iccation by the cordgrass canopy have reverse effects on fucoid photosynthe sis (reduced light inhibits photosynthesis; reduced dehydration enhances ph otosynthesis). Hence, it is possible that some of the physical habitat modi fications produced by Spartina are not translated into effects on fucoid bi omass or growth. We propose that the stabilisation effects of cordgrass swa rds on fucoid mats may operas on much larger scales than the 1 m(2) plots u sed, and it is at these large scales that the facilitative mechanism might be detected. (C) 1999 Elsevier Science B.V. All rights reserved.