Experimental restoration of disturbed cliff-edge forests in Bruce Peninsula National Park, Ontario, Canada

Summary

Restoration of degraded vegetation in parks is often complicated by the need to maintain public access. In this study, planting of native white cedar Thuja occidentalis seed and saplings was undertaken in an attempt to restore degraded cliff edge woodland heavily used by tourists in Bruce Peninsula National Park, Ontario, Canada, without necessitating reductions in visitor numbers.

Study area: The study was conducted along a 360 m long by 10 m wide section of Niagara Escarpment cliff edge that included two major tourist attractions, the 'Grotto' and 'Indian Head Cove', as well as less frequently visited parts of the shoreline within Bruce Peninsula National Park, Ontario, Canada.

Experimental design: To stratify differences in disturbance levels, the area was divided into 10, 36 x 10 m blocks and disturbance quantified within each by conducting visitor counts. Over a 2-hr period on three dates (21 July, 5 August and 1 September 1999), simultaneous counts were taken of the number of people in each block to provide a relative measure of visitation rate.

Each block was divided into 24, 1.5 m sections, and randomly assigned to three tree age planting classes and eight treatments. A planting site was selected within each section, either a soil-filled depression or rock crevice, reflecting the natural habitats of trees in the vicinity. Sites were either within a metre of the cliff edge or 10 m from it. At 10 m distance, visitors have created a dense network of small footpaths. Planting sites were selected in either disturbed (on paths) or undisturbed areas. For cliff edge sites disturbance was uniformly heavy. Each planting site was also categorized by canopy cover (shady, open or intermediate).

Planting material: Three age classes of white cedar Thuja occidentalis were planted: Eighty 10-year-old and 80 4-year-old container-grown saplings, and 1,400 seeds. All seed were collected in autumn 1996 from cliffs in southern Ontario. Some seed was pre-germinated on moist filter paper until the radicle started to protrude. The saplings, grown from seed, where kept in outdoor pots at the University of Guelph.

Experimental treatments: Eight treatments were applied to test the effects of ameliorating growing conditions by: adding soil, water, or both; marking saplings with brightly coloured signs; protecting them in cages; and varying the planting location with respect to disturbance and distance from the cliff edge:

1) edge (planted at cliff edge, no supplements)

2) edge + soil (planted at cliff edge with added soil)

3) edge + water (planted at cliff edge no added soil, watered weekly during the first growing season)

4) edge + soil + water (planted at cliff edge with added soil, watered weekly during the first growing season)

5) edge + soil + cage (planted at cliff edge with added soil and protective enclosure)

6) edge + soil + sign (planted at cliff edge with added soil and sign)

7) 10 m in, on path (planted 10 m from edge, on a path, with added soil)

8) 10 m in, not on path (planted 10 m from edge, undisturbed location, with added soil)

Soil addition at time of planting consisted of 500 mL of commercial potting mix for 4-year-old saplings and seeds, and 1L for 10-year-old saplings. Watering (conducted in the first growing season only) consisted of a once weekly soil soaking around sapling or seeds. Cages (poultry netting 80 cm high for 10-year-olds, 60 cm high for 4-year-olds, and 40 cm high for seeds) were open at the top. Signs were of yellow plastic, 15 cm in diameter, and inscribed 'Replanted tree—please do not disturb', placed by planting sites.

Planting: Each planting location received one sapling (in the last week of May 1997) or several seeds. Five pre-germinated seeds per site were planted during the first week of June 1997. When few seedlings emerged, another five germinated seeds were added in half of the blocks 2 weeks later. There was a third planting of 10 un-germinated seeds per site in April 1998. All saplings and seeds were watered once immediately after planting.

Measurements: Each planting site was visited weekly, biweekly and monthly in the first, second and third growing seasons, respectively, and at the beginning (May) and end (September) of the fourth growing season.

Damage and shoot growth were assessed on five (10-year-old) or three (4-year-old saplings) randomly selected branches and the main leader. These branches were tagged at the beginning of each growing season. At each visit, a score between 0 and 5 was assigned to each branch in three categories: breakage (representing damage caused by humans, most often trampling); herbivore damage; and browning (including yellowing or wilting) as a sign of general poor health. Shoot growth was measured by applying a small mark of nontoxic paint on the branch leader and measuring the distance from the mark to the growing tip. The angle of the main axis with respect to the horizontal plane and the presence of exposed roots were scored on a scale of 1–3.

Germination & survival of seeds: The first 1997 seed planting resulted in emergence of only five seedlings out of the 400 planted (1.25%). None survived past the second week. Emergence was 17.5% for the second planting in 1997, with seedlings evenly distributed over blocks and treatments. But all died within a few weeks except for one that survived until midsummer 1998. None emerged from the un-germinated seed planted in April 1998, although several natural seedlings were observed in the vicinity of the plots.

Sapling survival: Mortality of saplings of both ages decreased over time, with overall, 64%, 44%, 39% and 39% surviving the first, second, third and fourth years, respectively. There was no overall significant difference in survivorship between 4- and 10-year-old saplings, but there were differences among treatments. Survival was highest for those planted 10 m from the cliff edge in undisturbed locations (treatment 8); 95% of these survived the first year and 85% were alive at the end of the study. Lowest survival (35% after year 1, 20% at the end) was of saplings planted at the cliff edge with no supplements (treatment 1). All other treatments were intermediate, only the difference between the two extremes being significant. There were also significant survival differences among blocks. Survival was highest in block 1 (94% surviving the first year, 87.5% the fourth year). Block 1 was significantly different from blocks 9 and 10, 31% surviving year 1 with only 6.25% (i.e. one out of 16 planted) surviving to the study end.

Sapling fate: Of all saplings, 21% became well established and were healthy throughout the study. Another 21% alternated between health and decline but were alive at the study end. Fifteen percent failed to become established, and 9% died without obvious external causes after alternating periods of health and decline. Damage from dislodged or trampled cages probably contributed to the death of 5% of saplings. Those remaining died from causes such as uprooting, trampling or herbivory.

Over a third (37%) were partially or completely uprooted at some point, this being the most frequent external cause of death. Of these, 16% were completely uprooted and either died immediately or disappeared. Partial uprooting led to a eventual death of 8%, whilst 12% survived.

A quarter of all saplings were severely or repeatedly trampled at some point. For 7%, this was almost certainly the cause of death; 7% survived, and for 11% it was considered a contributing factor but not necessarily the only reason for death. Seven percent of saplings experienced serious herbivory, and was the most likely reason for the death of 3%, whilst 4% experiencing similar herbivory levels, survived.

Saplings were more likely to be successful with added water, added soil, or both, failure more likely in their absence. Establishment success was significantly lower at the cliff edge than 10 m from it. Away from the edge, establishment did not differ between paths and undisturbed locations.

Herbivore damage was rare at the cliff edge but much more common 10 m in: 22.5% of all saplings in treatments 7 and 8 experienced herbivory, as opposed to only 2.5% in the other six treatments.

Saplings planted in shady sites were much more likely to be healthy and much less likely to die soon without external causes than in open sites.

Damage to living saplings: Damage by breakage increased over the first summer and leveled off after the end of September. The pattern was similar in the second and third years, but was lower. Browning increased steadily, peaking in early autumn; in the second year, initial scores were higher in the spring and the seasonal increase was smaller. Herbivore damage was highest in the year of planting and was greatest in the spring and lowest in midsummer. Root exposure ratings increased over the first summer and remained constant thereafter.

Breakage scores at the end of the first growing season showed significant differences due to sapling age and age × treatment. Small saplings experienced greater breakage than larger ones in all treatments except the caged treatment, where the relationship was reversed. Caging significantly decreased breakage in small saplings compared with the controls. In contrast, caging increased breakage in large saplings, although not significantly.

Browning scores at the end of the first growing season were significantly higher in the 10-year-old saplings. There was also a significant treatment effect on browning but no interaction between age and treatment. There was a significant difference in browning between undisturbed (lower) and disturbed (higher) locations 10 m from the cliff edge and also between disturbed locations at the cliff edge versus 10 m away. Soil addition significantly decreased browning scores at the end of the first growing season. Watering, caging and signs had no significant effect.

The tendency of planted saplings to fall over during their first growing season was much greater for larger than for smaller ones and those without additional soil were the most likely to fall. Root exposure ratings at the end of the first growing season were twice that for 10-year-old as for 4-year-old saplings. The greatest root exposure was seen at the cliff edge, in the absence of supplements, and in blocks with high visitor density.

Growth rates of surviving saplings: Shoot growth was typically seasonal, being fastest in spring and a slowing by midsummer. Total yearly growth was similar among treatments, blocks and sapling ages for each of the 3 measurement years. Small and large trees differed in growth in the year of planting.. Growth rate during the first time interval in 1997 (the month after planting) was significantly faster in 4-year-old than in 10-year-old saplings; thereafter, growth rates were similar for both age groups, although 10-year-old saplings tended to grow slightly faster.

Visitor counts: The total number of visitors counted in the 10 blocks over the four observation periods ranged from 5 to 789. Block 10 (at the Grotto) received the highest numbers, followed by block 6 where the Bruce Trail first accesses the shoreline. The lowest visitor numbers were found in blocks 1–3.

Conclusions: No trees successfully established from seed. Younger (4-year-old) saplings showed faster initial establishment and growth, but overall, survival was the same for both age groups (39%). Added soil improved sapling health, and both soil and water slightly improved survival. Cages did not affect survival and growth but decreased damage to 4-year-old saplings and increased it for 10-year-olds. Signs had no effect on any measured variable. Saplings planted away from the cliff edge and away from paths had the greatest establishment success, 4-year survival and general health. Planting location, especially with respect to shade where survival was higher, was also important.

Relative visitor density had the largest effect on planting success, the results suggesting a threshold of visitor density above which restoration may be impossible. At this site, restoration of open woodland appears possible without total visitor exclusion but some restrictions on visitor numbers or activities are necessary. The planting of 4-year-old trees without supportive measures is suggested as the most cost-effective restoration technique at this locality.


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