The response of forest floor vegetation and tree regeneration to deer exclusion and disturbance in a riparian deciduous forest, central Japan

Published source details Nomiya H., Suzuki W., Kanazashi T., Shibata M., Tanaka H. & Nakashizuka T. (2002) The response of forest floor vegetation and tree regeneration to deer exclusion and disturbance in a riparian deciduous forest, central Japan. Plant Ecology, 164, 263-276.
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Summary

In many regions of Japan, Sika deer Cervus nippon have increased greatly in numbers in the past few decades; consequently increased browsing and grazing intensity is having serious effects on vegetation in some areas. A study was undertaken in riparian deciduous forest where deer had increased drastically and heavily affected the forest floor vegetation, in particular, reducing the population of dwarf bamboos. The study aimed to understand the effect of deer browsing on forest floor vegetation and tree regeneration by using exclosure experiments, which in turn would assist in conservation management policy.

Study area: The study area was an old-growth riparian forest, dominated by elm Ulmus davidiana, bordering Lake Chuzenji (36º45'N, 139º45'E), Tochigi Prefecture, Kanto Region, Honshū island, Japan. Periodic flooding had formed two habitat zones, 'lower terrace' (LT) and 'higher terrace' (HT) according to flooding frequency.

Prior to 1988, two dwarf bamboos, Sasa palmata and Sasamorpha borealis, dominated the ground flora of the HT. However, Sika deer increased and browsed the bamboos intensively, so that by 1993 at the start of the study, S.palmata had almost disappeared and S.borealis persisted only in small patches.

Experimental design: Experiments were undertaken in four different habitat types:

i) S.borealis patches under closed canopy;
ii) under closed canopy with no dwarf bamboo;
iii) open site (tree-fall gap) on higher terrace;
iv) open site (riparian gap) on lower terrace.

In November/December 1993, two sets of exclosure (wire mesh net, 4 x 4 m; 1.8 m height) and control (4 x 4 m area adjacent to the exclosure) areas were established at each site. Two, 1 × 3 m permanent quadrats (divided into three 1 × 1 m subquadrats) were demarked in each exclosure and control area. Light conditions at each site were estimated (by hemispherical photograph taken at 1 m above the ground inside and outside of the exclosure at two points in each site in June 1994), and the canopy openness was calculated using image analyses.

Vegetation surveys: Forest floor vegetation was surveyed in August 1994, 1995 and 1996. Vascular plants occurring within each subquadrat and their maximum height were recorded. Aboveground biomass was estimated by harvesting (clipping) two 0.25 mÂ² quadrats (avoiding the main research quadrats) inside and outside each exclosure each year. From June 1994 to November 1996 newly emerged seedlings were tagged and regularly checked for survivorship; height was measured at the end of every growing season (November).

Within the exclosures, aboveground biomass increased steadily but species diversity changed little. Dwarf bamboo which was recovering well in enclosures at one site strongly inhibited tree seedling recruitment. Outside the exclosures, seedlings of all tree species were badly damaged by deer browsing, especially the taller ones. A total of 142 species (38 woody, 78 herbaceous, 19 graminoid, 2 dwarf bamboos and 5 ferns) were recorded in the plots.

Palatability of species: Of 15 abundant species (1 woody, 6 herbaceous, 7 graminoid and 1 dwarf bamboo) with sufficient occurrence for analyses and an average height of at least 10 cm outside the exclosure, seven were classified as non-palatable; five Senecio nikoensis, Senecio nemorensis, Cynanchum caudatum, Hypericum ascyron and Aster ageratoides, dominated the forest floor outside the exclosures at CL and OP. Except for these non-palatable species, signs of deer browsing were observed on all other major species.

Aboveground biomass of forest floor vegetation: Aboveground biomass varied between sites and exclosures but was greatest in those where dwarf bamboo (S.borealis) was recovering; the bamboo was heavily browsed outside the exclosure where it continued to decrease. The aboveground biomass of non-palatable species outside of the exclosure was greater than that of the palatable species at CL and OP, but not at DB (as the bamboo suppressed the other species) or at LT, effected by a small flood in September 1994. The biomass of woody species inside the exclosures rapidly increased, while that outside remained the same or decreased.

Tree seedling survival: Seedling survival of Phellodendron amurense, Populus maximowiczii, Salix sachalinensis and Quercus crispula was significantly affected both by site and deer exclusion. Betula spp., P.amurense, Kalopanax pictus, Malus toringo and Q.crispula survived significantly better in the exclosures on the higher terrace. P.maximowiczii and S.sachalinensis survived better in exclosures on the lower terrace. Only seven seedlings over 10 cm tall were recorded in transects outside the exclosures, whilst 210 were recorded in quadrats within the exclosures.

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