OBN International Nitrogen deposition and raised bog restoration

Dr. André J.M. Jansen Federation of Forest Support Groups

Dutch Nitrogen Tour 26 November 2015 Characteristics of an Atlantic raised bog • Is being dominated by ombrotrophic peat forming plant communities; • Raises itself above the regional groundwater table given a specific climate as a consequence of: • the hydrological properties of the peat • the plant species growing on the bog The ideal raised bog

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Four zones: • Centre: very wet, lakes, hollows and lawns, lw hummocks; • Slope: hummocks and hollows perpendicular on the groundwater flow direction; • Edge: high hummocks of Eriophorum vaginatum, birch carr, gullies; • Lagg: ground water fed small-sedge vegetation In raised bogs water is crucial

• Enough recharge: precipitation; • Limited horizontal and vertical discharge • In an active (living) raised bog the peat forming vegetation and the peat itself are the most important regula- tors of the discharge; • Paradox: elevated and wet!

Peat mosses (Sphagna)

• Sphagnum species have the required hydrologic properties to fullfill this paradox; • Dependent of a high and stable groundwater table. • Construct on the longer term a system of related processes through which a raised bog maintains (under continuously changing weather- and climatic conditions). Self-regulation raised bogs due to • The inhomogenity of the peat body in an active raised bog; • Two layers with totally different hydrological properties: acrotelm and catotelm.

Two totally different layers Acrotelm: Catotelm: • Living Sphagna, • Much more (hollows & hummocks) and a compressed; few herb and grass • Much lower species; permeability & • Little degraded storage capacity . organic material originating from peat mosses; • Circa 30-70 cm; • A high storage capacity (many and big pores); • A high permeability. No active raised bog without an acrotelm • Permeability of the acrotem is characterized by a steep vertical gradient: the permeability drops with a factor 100,000 within 40 -50 cm • Due to this layering gradient a raised bog has high and stable groundwater table, despite its higher elevation, and also during lengthy dry periods

Raised bog restoration means restoration of acrotem conditions From raised bog landscape to bog remnant

Drawings: A.P. Grootjans From bog remnant to raised bog landscape of the 21th century The restoration approach : • Retention of precipitation water by: • the infilling of ditches and canals; • the construction of a large network of dams and dikes; • tree and shrub removal • Recovery of superficial lateral water flow via reconstruction of the former dome-shape of the body creating a system of terraces; • Reduction of horizontal and vertical water losses by the construction of large hydrological buffer zones on agricultural fields that formerly belonged to the bog expanse. Hydrological buffers required for restoration

Drawings: A.P. Grootjans Buffers and dams -1

• The impossibility to recover the original functional bog structure of fine-patterned and combined stagnation & storage on landscape scale; • Spatial division of stagnation & storage by the construction of boulder clay dams and buffer basins; • Boulder clay dams tend to an improved stagnation in the bog remant  fewer water loss

Buffers and dams - 2 • More effective water storage in the buffer basins than in the desiccated peat remnant, as in the extended former bog bodies. • As the extended former bog bodies, basins tend to: • lesser differences in water table between bog remnant and its surroundings; • a raised water table on the lowest locations, resulting into less discharge form the peat body to this lower locations; • increased evaporation in the vicinity reduced evapotranspiration form the bog vegetation better Sphagnum growth Sphagnum growth & C-limitation

(after Patberg et al. 2013). Sphagnum growth & C-limitation

• Too low CO2 concentrations Iimit the growth of submerged Sphagnum species: • deeply inundated areas over black peat without input of CO2 from inflowing surface or groundwater; • after rewetting, the strongly humified black peat does not produce sufficient methane and carbon dioxide to allow the formation of floating mats of Sphagnum; • Moving water usually has high concentrations of CO2, (particularly in case of inflow of (very) soft water)  improve superficial lateral water movement Raised bogs and atmospheric N- deposition • Critical load = 5-10 kg N/ha/y; • Present deposition Bargerveen = 20-40 kg N /ha/yr (Velders et al. 2013); • N deposition: not any longer a N limited vegetation, but one limited by P; • Strong increase in herbs, grasses and birches at N deposition from 15-20 kg N/ha/yr; • Sphagnum fallax & S. fimbriatum profit form the canceling of P-limitation, by input of birch leaves (more than double of atmospheric P-deposition, even by low birch densities).

Is raised bog restoration possible at high N-deposition levels?

Bobbink e.a. 2011, Limpens, 2009, Van Duinen et al., 2001 Effect N-deposition on the relation between peat mosses & phanerogams: increased interception dry deposition High atmospheric N deposition +

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Sphagnum N-filter saturated Sphagnum growth

- light competition - +

+ N availability rooting zone

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+ decomposition & Growth herbs, grasses & birches mineralization + interception Entomofauna + precipitation & transpiration Desiccation! + How to retain an open vegetation / re-open a dense vegetation?

The keys to raised bog restoration

• Recovery of acrotelm conditions; • Re-activate hydrological processes characteristic of raised bogs: superficial lateral water flow & limited downward vertical discharge (< 40 mm/yr); • Optimisation of the growth conditions of Sphagna: high & stable water tables, gradual superficial water flow, carbon and light; • Restoration of the inflow of (base-poor or base- rich) groundwater in de lagg zones; • Reduction of atmospheric N-deposition.

However …….. • Extra management by grazing is contrary to the aim of the Habitat Directive to conserve and recover NATURAL habitats; • Intensification of the management has negative impacts, for instance Saxicola rubetra decreased first in the areas which are being grazed; • Hydrological restoration is a prerequisite for raised bog habitats. It results in a netto deterioration when hydrological restoration measures have to compensate the damage of prolonged too high deposition; • Even under optimal hydrological conditions intensified manage- ment is required due to a too high deposition. However, the very wet conditions will hamper or frustrate regular management activities (sawing, grazing and mowing). Thus, the restoration strategy to mitigate too high emissions frustrates the required management as a consequence of too high N deposition; • It deteriorates the possibilities to keep extra cattle to reduce the impact of N-deposition. Thank you for your attention!