Comparison of the growth rate of two 30 year old sitka spruce trees; the bottom tree is normal, whereas the top one is stunted. The cause of the slow growth rate is linked to acid rain pollution, the lowering of pH levels in rainfall due to sulphur dioxide emissions into the atmosphere. Increased acidification of the soil, ground water, rivers & lakes results in a shift towards acid- tolerant species & a consequent decrease in species diversity.