Effects of air pollution and soil acidification stress on Ontario's hardwood forests.

摘要

Forests in southern and central Ontario are exposed to some of the highest levels of air pollution (S deposition, N deposition, O3 concentrations) in Canada, and as such, are at risk of injury and altered ecosystem function. Sulphur deposition has decreased substantially over the last 30 years in Ontario but critical loads of acidity for forest soils continue to be exceeded across large areas due to continued high levels of S and N deposition. Concerns over declining sugar maple forests and the potential role of air pollution lead to the establishment of a long-term hardwood forest monitoring program in 1986 by Ontario's Ministry of Environment.; In this thesis, changes in soil and foliar chemistry were assessed at two sampling periods and potential relationships between ecosystem drivers (air pollution, climate, soil acidity) and ecosystem responses were explored using data from 35 of these long-term plots. Specifically, soil chemistry, sugar maple foliar chemistry and crown condition were compared using data collected in 2005 and archived samples collected in 1986 from 17 Ontario Forest Biomonitoring Network (OFBN) plots. Relationships between potential ecosystem drivers and potential indicators of ecosystem responses (foliar chemistry, Decline Index, lichen species richness, ground vegetation species richness and diversity, CWD, LFH mass, LFH S, N and C/N content) were examined at 35 OFBN plots spanning gradients of air pollution, climate and soil acidity.; Soil pH and exchangeable Ca, Mg and Na declined significantly in both A- and B-horizons while exchangeable K declined in only the B-horizon over the 19-year period between 1986 and 2005. Foliar Al, foliar Ca and foliar S concentrations also declined significantly between 1986 and 2005 but no change in N was found, consistent with the lack of change in N deposition. Despite an increase in soil acidity and reduced foliar Ca, tree health (expressed as a Decline Index) improved significantly between 1986 and 2005. Foliar N concentrations were positively related to modeled N deposition with the highest concentrations occurring in the southern part of the region.; Epiphytic foliose lichen species richness was negatively related to air pollution (S deposition, N deposition, 03 AOT40). Only five foliose lichen species were found at deposition levels above 10 kg S/ha/yr and 11 kg N/ha/yr, suggesting that epiphytic foliose lichen species richness is sensitive to current levels of air pollution in southern Ontario's hardwood forests.; Foliar Ca, foliar Mg and foliar Mn were related to soil acidity and exchangeable base cation concentrations in the soil. Soil Ca levels below 1 meq/100g corresponded with a sharp decline in foliar Ca, while soil Mg levels below 0.5 meq/100g corresponded with a similar decline in foliar Mg levels. In contrast, Mn concentrations in foliage increased dramatically in soils having pH values below 4.5. Presently, most of the sites have sufficient Ca and Mg in foliage and Mn levels are below reported critical thresholds, although if the soil acidification observed here continues, foliar chemistry may also shift towards a less favourable combination of nutrient levels.; Crown condition (Decline Index) was not related to indices of climate, air pollution, or soil acidity. However, in the late 1980s southern Ontario experienced repeated drought episodes and during that time poor crown condition was related to low foliar Ca concentrations and soil % base saturation. This relationship did not persist throughout the 19-year period, however, and average Decline Index (1986-2005) was not related to foliar Ca and % base saturation. Forest floor S and N concentrations were not related to S or N deposition, but instead, were positively related to precipitation and mineral soil pH and negatively related to temperature. Thus, S and N content in the forest floor---a traditional indicator of N saturation---is driven more by climate and/or soil acidity rather than acid dep