Polytrichum strictum as a solution to frost heaving in disturbed ecosystems: a case study with milled peatlands

  • Published source details Groeneveld E.V.G. & (2005) Polytrichum strictum as a solution to frost heaving in disturbed ecosystems: a case study with milled peatlands. Restoration Ecology, 13, 74-82.


Frost heaving limits plant recolonization on harvested peatlands at higher latitudes. The pioneer moss Polytrichum strictum commonly occurs in many such areas. A field experiment was conducted to determine the effectiveness of P.strictum against frost heaving, a major disturbance at a former peat extraction site which was severally restricting plant re-colonisation, and to assess its suitability as a nurse-plant to facilitate growth of other plants.

Study area: The experimental site was at Premier St-Laurent located near Rivière-du-Loup, Québec, Canada. It was chosen as it had physical characteristics and sparse vegetation typical of vacuum-harvested peatland sites in Québec. Hundreds of uprooted seedlings were scattered over the peat indicating that frost heaving was a major factor limiting revegetation.

The site was last harvested 20 years prior to the study, but vegetation was sparse (about 10% cover of a few vascular plants, lichens and mosses; P.strictum was the dominant moss).

Treatments and experimental design: The experiment began in May 2000 and ended in October 2001. Six 1.5. x 1.5 m plots were marked out. All existing vegetation and debris was removed and surface smoothed with a garden rake (producing a surface like that of a freshly abandoned peat field). Three types of P.strictum cover were trialled:

i) moss carpets - designed to represent naturally occurring P.strictum;

ii) moss fragments - to test the effectiveness of a current restoration techniques against frost heaving. i.e. shredded moss from a healthy bog spread over the peat surface;

ii) no cover.

Straw protects moss from desiccation and has been shown to reduce frost heaving; the three treatments were tested with and without straw.

Treatment application: P.strictum moss was laid in 25 × 25 cm squares, nine squares planted together in each plot to resemble a natural moss carpet.

Moss fragments, 0.225 m²/2.25m² plot (as commonly used in North American peatland restoration projects). Straw was applied by hand at 3,000 kg/ha (also the rate used in restoration). Fine plastic netting was lightly laid over the straw to prevent it from blowing away and removed during summer 2001.

Phosphorus fertilizer (phosphate rock) was applied at a rate of 150 kg/ha (the amount currently recommended for restorations) to enhance the establishment of the moss and fir seedlings. Plots with P.strictum were hand watered with bog water during the first 2 weeks.

Estimation of frost heave: Balsam fir seedlings and wooden dowels were used to estimate frost heaving. In each plot, eight small firs were planted. Fir trees were chosen because seedlings were abundant on the site, easy to transplant and clearly susceptible to frost heave. In addition, 16 wooden dowels, were inserted 10 cm into the peat in each plot. To measure vertical displacement due to frost heaving, 2 m metal rods were driven into the corner of each plot. Two metal bars were clamped to the rods 10 cm above the peat surface. A third bar slid along the two clamped bars. A ruler was used to measure heave displacement on: 24 August 2000 (before any frost heaving); on 7 November 2000 (during frost heaving); and on 29 May 2001.

Environmental variables: Measurements were made of: soil temperature, soil moisture, depth to the thaw line (on 25 April 2001, a few days after the snow cover had melted, and 30 April 2001).

A carpet of P.strictum was the most effective means of reducing frost heaving throughout the season. There was no frost heaving in any straw treatments in the autumn, but by spring there was some heave in the bare peat covered with straw and fragments as the straw had partially decomposed over-winter. P.strictum, on the other hand, grew, enhancing its protective effect.

Wooden dowels and fir trees placed in a P.strictum carpet experienced almost no frost heaving (average heaving of dowels = 0.6 cm, average heaving of fir seedlings = 0.82 cm). Heaving was severe on bare peat, in some cases, dowels and fir trees heaved completely out. P.strictum fragments thinly spread on bare peat reduced but did not eliminate frost heaving. The P.strictum carpet, P.strictum fragments and straw mulch reduced frost heaving by reducing the number of freeze–thaw cycles, by slowing the rate of ground thaw in the spring, and by reducing the unfrozen water content of the peat during the spring thaw.

By the end of the the frost heaving period in spring 2001, over 50% of fir trees planted in treatments other than the P.strictum carpet had heaved so much that their roots were pushed completely or partially out of the peat. None of the trees planted in the carpet suffered root exposure. Most heaving was observed in the absence of P.strictum where 75% of the seedlings heaved so much that their roots were visible. Fir trees planted in straw heaved half the amount of fir trees planted without straw.

Once the snow cover had melted, the bare peat quickly thawed. The result was severe frost heaving and extensive plant damage in bare peat plots. With a moss or straw covering, thawing was slowed and seedlings were not at risk of frost heaving.

Conclusions: In this study it is shown that P.strictum can reduces frost heaving. Whilst straw is effective in reducing forst heave, it can easily blow away, and its effect is short term due to rapid decomposition. Straw mulch is considered necessary in conjunction with P.strictum because it takes at least two growing seasons for the moss to reach an appreciable size, and straw mulch enhances establishment.

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