An integrated study of forested catchments in Ireland.

Abstract

Forestry is an important component of the Irish landscape and many of our river systems rise in or pass through forested catchments. Because of the scarcity of objective and broad scale data from Ireland, a national research group AQUAFOR, has been established to investigate, in a multidisciplinary way, the interactions between forestry and aquatic ecosystems. There are three study areas in the west (Connemara), east (Wicklow), and south (Munster) of the country. Preliminary results of these studies are presented in this paper. Marine ions, sodium, chloride, magnesium and sulphate dominate the atmospheric input to all catchments, particularly in the West. Dry deposition of these ions can also be high, giving the western sites extreme concentrations on occasions. Concentrations of these ions are increased by the passage of rainfall through the canopy of forests, thus increasing the concentrations of most in the soil waters. Stream water studies in a number of forested catchments have shown that acid episodes, with pH values frequently less than 4.0, can occur in the east and west on granite and other soft water catchments. Concomitant with these changes in pH, values of labile monomeric aluminium increased dramatically during low pH events and often exceeded 0.2mg/l, the concentration thought to be toxic to salmonids. In contrast, pH values in the better buffered catchments in southern Ireland did not drop below pH 5.0, irrespective of the land use and similarly aluminium levels were generally low (less than O.lmgJ-l total aluminium). From these preliminary results therefore, only a proportion of the country as a whole may be expected to be vulnerable to acidity problems associated with plantation forestry. Biological data supports the general conclusions from the chemical studies. Macro invertebrate communities varied with chemistry across all sites in all three regions. Diversity tended to be lower in afforested sites on sensitive geologies in the east and west whereas in the south, macro invertebrate abundance reflected stream water chemistry per rather than the type of land-use, with most sites having the acid-sensitive mayflies. Fish distributions in Wicklow were found to be associated with changes in altitude and the amount of afforestation in the catchment with nine sites of the nineteen fished being fish less. Similarly only two of the eleven suitable sites in the West had no fish . In the South, fish densities tended to be higher than elsewhere and in the few sites where fish were absent , physical features (waterfalls) rather than hydrochemistry and catchment land-use were more likely to be the cause. Fish densities here did show a decreasing trend with amount of catchment afforestation although fish condition appeared to be unaffected by land-use. Preliminary results from hydrological studies in the South indicate that the traditional methods (i.e. catchment characteristics/unit hydro graph) of stream flow computation are unreliable. This may be due to the forest land-use not being accommodated in the traditional methods.