Experimental canopy removal and clearance experiments

Early experiments - effects on the kelp population

Kelp growth rates after clearance

Epiphyte growth after canopy removal

Conclusions

Early experiments - effects on the kelp population

Scotland

The earliest manipulative experiments on a kelp forest were carried out in 1936 on the west coast of Scotland by Kitching (1941). He removed canopy plants of L. hyperborea with shears and one year later a new canopy of dense plants, 1 m high, had formed. The old holdfasts of the cut plants had gone, showing that they would not survive, along with their own distinctive fauna. The new holdfasts were described as very tight and clean of epibiota. This early experiment shows a standard pattern of response - the forest has considerable potential to regenerate but, when it does so, the age and size structure and plant morphology may be altered with consequent effects on the rest of the ecosystem.

Isle of Man

In 1975 Kain carried out clearance experiments in the Isle of Man which showed that although the L. hyperborea forest could regenerate, there might be a temporary domination by other species, notably Sacchorhiza polyschides. Concrete blocks at 0.8 and 4.4 m below ELWS were cleared of attached algae. Different blocks were cleared in different years and at different times of the year so that the recolonisation patterns could be related to length of recovery period and responses to seasonal differences in recovery. Blocks at the shallow level cleared in August were re-colonised by L. hyperborea but, if cleared in November, February or June, there was initial replacement by S. polyschides, Desmarestia aculeata and Alaria esculenta (in different years). Whatever the replacing species, S. polyschides became dominant by the August following clearance of the blocks. However, L. hyperborea had always replaced the S. polyschides after 2 years and, the biomass of L. hyperborea on the blocks was equal to that measured in control areas after 3 years. Colonisation of the blocks at 4.4 m was more variable and it took longer for the L. hyperborea to re-establish. On the control blocks at this depth, S. polyschides and D. aculeata flourished if the block did not hold any plants of L. hyperborea. Where blocks at a depth of 1.3 m. were continually cleared at intervals, kelp dominance was lost and the blocks were populated by 41 different species of algae, with reds having maximum biomass in the winter, browns in the spring and greens in the late summer. When sterilised stones were placed in the experimental area they were not colonised by L. hyperborea except when the canopy plants were reproductive, although on cleaned but not sterilised blocks, new sporophytes grew at all seasons (presumably from microscopic sporophytes or from fertilisation of gametophytes).

Kelp growth rates after clearance

Isle of Man

In further work with cleared areas in the Isle of Man, Kain (1976a) investigated the growth rates of remaining kelp plants. At 0.8 m depth, growth was rapid after canopy removal, indicating the role of the canopy in limiting the light available for the growth of other strata of kelps in the forest. 1-3 year old plants in the cleared areas became larger than those in control areas as a consequence. However, after 3 years the biomass and frond area index of the experimental area was restored. At 4.4 m depth, recolonisation was haphazard and the growth rates of the plants were lower.

The manipulative experiments of Jones & Kain (1967) in which the local population of Echinus esculentus was removed showed the potential of urchins to inhibit the regeneration of a kelp biotope after harvesting (see section IV.B.3.).

Epiphyte growth after canopy removal

Harkin (1981) examined the effect of kelp canopy removal on the algal epiphytes of the kelp stipes. There was a rapid increase of red algal biomass in the first summer but brown algae were able to grow better in the first winter. This re-established the previous mixture of red and brown epiphytes. Two years after the removal of the canopy, the biomass of epiphytes had returned to a level similar to that measured before the canopy removal allowed an explosion of growth.

Conclusions

These experiments suggest that some semblance of a kelp forest, in terms of macroalgal biomass and of subsidiary algal species, may be regained within 3 years of canopy removal. However, the size of kelp plants and the age structure of the population in the re-grown forest is different from the untouched forest. Furthermore, the experiments do not directly mimic the effect of kelp harvesting. For example, Harkin's experiments deliberately left intact stipes that a kelp harvesting dredge would have removed. New stipes would not have developed the normal epiphyte flora so quickly. While this work might superficially suggest that 3-4 years is a suitable interval between harvesting to allow regrowth of the macroalgae, a much better picture of the effects of harvesting can be obtained by examining harvested grounds in Norway and Brittany.

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