Heathland responses to nitrogen deposition : exploring the role of habitat management and soil biochemistry

Abstract

Elevated levels of nitrogen deposition are altering the normal functioning of seminatural ecosystems around the world. Of concern in Europe are the nitrogen driven changes occurring to (semi)-natural ecosystems such as obrotrophic bogs, upland moors and lowland heathland. Lowland heathland is a particularly vulnerable oligotrophic habitat, with a narrow climatic optimum that restricts the remaining fragmented patches to a small corner of highly populated northwestern Europe. Increased nitrogen availability reduces the competitiveness of characteristic heathland plants, such as Calluna vulgaris, that are adapted to situations where nitrogen is the limiting nutrient. Resulting changes such as accelerated productivity, increased herbivore and frost damage, and changes in the nutrient limitation status of heathlands can lead to invasion by nitrophilous graminoid species, but this process may be dependent on (1) the intensity and duration of elevated nitrogen inputs, (2) the relative availability of other nutrients and (3) conservation management intended to reduce overall nutrient levels. The present study investigated the effects of nitrogen deposition in N and P limited vegetation, and in heathlands around the UK that are experiencing anthropogenically enhanced background levels of nitrogen deposition. Physical and biochemical parameters of vegetation, litter and soil were used as indicators. A long-term fieldscale manipulation study at Thursley Common, Surrey was used to quantify the effect of nitrogen inputs to an N limited system and, in particular, determine how these effects were modified by a gradient of management intensity, and an unplanned wildfire. The process of vegetation recovery, following a cessation of nitrogen inputs was also investigated. The effects of nitrogen deposition to P limited heathland were investigated using heathland mesocosms. Two nationwide heathland surveys investigated how the effect of nitrogen deposition within a gradient of background deposition in England was modified by environmental factors, such as geology or soil type. The first survey examined the effect of stand age (and management) on responses to nitrogen deposition. The second survey focused on the physical and biochemical responses to nitrogen deposition in Calluna, litter, bryophytes and soil to quantify its impact on heathlands across England. The Thursley nitrogen manipulation investigation showed clear ongoing responses in rates of canopy development and shoot growth to nitrogen deposition, which were reduced by increasing levels of management intensity. Historical nitrogen inputs in the recovery experiment continued to significantly increase rates of Calluna canopy development 9 years after inputs had ceased. These effects were re-invigorated by the unplanned wildfire. The mesocosm experiment indicated that P limited heathlands are susceptible to the detrimental effects of nitrogen deposition, particularly, altering shoot phenology and increasing drought stress. Both the nationwide surveys indicated that, at a national scale, heathland vegetation and soil biochemical indices are responsive to background nitrogen deposition levels within (and exceeding) the critical load range, despite environmental influences, such as geology and soil type which were also found to have significant effects on these parameters.