The bracken effect is the dominance of this invasive and potentially harmful plant in woodlands, where it can suppress biodiversity and reduce habitat quality. Its spread is often attributed to global climatic changes and favourable conditions for its development such as abundant light and dry soils. It can have a profound impact on wildlife and landscapes with its dense shady canopy and thick litter covering the ground and inhibiting new growth from competing plants. It is also known to release allelopathic chemicals which can prevent or inhibit the establishment of other vegetation and cause problems for livestock and humans in terms of tick infestations.
Precipitation, temperature and relative humidity were constantly monitored on-site during the monitoring period (Fig. 1d). A Pronamic Rain-O-Matic small rain gauge (Ringkoebing, Denmark) was used to record rainfall amount and duration. Temperature and humidity were recorded with two ONSET HOBO external sensor data loggers (Bourne, USA).
Biomass was measured on the surface at a frequency of two per week during the growing season and was sampled from the fronds at each point in time (Fig. 2a). The biomass of a plot was estimated from the sum of the aboveground frond and rachis biomass. The biomass was measured until the middle of July, at which point a maximum value was reached and then the monitoring was brought to a stop due to a tilted bracken canopy caused by a heavy storm.
PTA concentrations in the soil pore water increased significantly during the growing season and decreased after senescence. It is important to note that the concentrations of phosphate and potassium (PTA and PTB) are not always identical in the soil pore water; a portion of the total PTA content of the rhizomes is released during decomposition, while the remainder is stored within the vegetative tissues, or is leached by precipitation through the stomata.
The analysis of stomatal conductance indicated that the PTA concentrations in the soil pore waters depend on the volume and intensity of the precipitation event as well as the stage of the rhizome. In addition, frost may play an important role as it degrades soluble compounds in the litter layer by lysis of cells (Fig. 4).
The control of bracken is a complex issue, with many different approaches being employed. Some methods are based on removing the plant entirely, although this can cause other habitats to decline in the process (such as a loss of rare ferns including adder’s tongue and killarney). Other methods include the use of herbicides such as Asulam or glyphosate. These should be applied when fronds are fully unfurled to ensure that the chemical is absorbed by the plants. However, removal should be accompanied by the establishment of alternative flora to provide wildlife with the habitats they require. Otherwise, bracken will recolonise the area as soon as conditions are right. The shady canopy, allelopathic effects and toxic chemical emissions of the plant can all be overcome with the correct control measures.