Regeneration dynamics of natural forest species within a stand of the invasive alien Acacia mearnsii along the Buffeljagsrivier, Swellendam, South Africa

[摘要] ENGLISH ABSTRACT: Several studies have shown that stands of invasive alien plants in the natural forestenvironment can facilitate the rehabilitation and recovery of such forests and challenged thegeneral and global perception that such invasive species threaten the biodiversity andfunctioning of natural vegetation systems. The aim of this study was to develop anunderstanding of the dynamics of the spread and establishment of natural forest species in alarge stand of the invasive alien plant species Acacia mearnsii (Black wattle) along theBuffeljagsrivier, Western Cape, South Africa. Several patches of Moist forest, Dry forest andRiparian forest occur along the Buffeljagsrivier, above the Buffeljagsrivier dam. The stand ofBlack wattle consists of 90 ha for a distance of 3.12 km. The main objective was pursuedthrough four specific objectives: (i) to map and assess the patterns in the distribution, size andspecies composition of the natural forest clusters within the Black wattle stand; (ii) todetermine the relationship between natural forest clusters establishing within the Black wattlestand and the neighboring natural forest patches as potential seed sources for the developingforest clusters; (iii) to determine the subsequent spread of natural forest species from thedeveloping forest clusters into the rest of the Black wattle stand; and (iv) to synthesize theinformation on the dynamics of the spread and establishment of natural forest species into theBlack wattle stand as a basis for developing general guidelines for the conversion of invasivealien plant stands in the forest environment towards regrowing natural forest.In total, 329 clusters of natural forest species were GPS recorded and mapped (Arc-GIS) inthree zones (Proximal, Intermediate and Distant in relation to the forest patches) within theBlack wattle stand: 266 small clusters (one to three reproductively mature trees), 36 medium sizedclusters (four to nine trees) and 27 large clusters (more than 10 trees). Large clusterswere abundant in the zone close to the natural forest patches and the number of small clustersincreased with increasing distance from the forest patches. A total of 28 species of 20families were recorded. Natural forest species are therefore able to establish within a Blackwattle stand.The relationship between natural forest clusters establishing within the Black wattle stand andthe natural forest patches as potential seed sources was studied by sampling the standcomposition along transects through the stands. A total of 55 rectangular plots (20 m x 10 m,200 m2) were sampled across forest patches and forest clusters. Hierarchical clusteringanalysis, using number of stems of a species per plot, identified three main groups and 10sub-groups. All the sampled forest clusters were included in four of the five Riparian forestsub-groups. Most Moist and Dry forest species were absent from the forest clusters. The threemain forest types differed in their general characteristics and site conditions, and this wassupported by the ordination analyses: aspect, slope and canopy closure. The developing forestclusters within the Black wattle stand related more to the Riparian forest in terms of similarvery gentle south-westerly slope and mean stem diameter. This suggested Riparian forest tobe the primary seed source of the establishing forest clusters within the Black wattle stand.However, the large-sized stems of common species were not significantly different betweenRiparian forest patches and forest clusters, suggesting that large-sized stems in the forestclusters could be part of remnant forest patches, which could act as local seed sources.Detailed evaluation of species importance values and stem diameter distributions showed thatsome important Moist and Dry forest species are present in the forest clusters. The conclusionwas that every type of forest patch contributes to a greater or lesser degree to thedevelopment of forest clusters within the Black wattle stand.Seven large clusters were selected to sample the regeneration of natural forest species within18 m from the forest cluster boundary. Two species lists were generated; one of species fromadjacent natural forest patches, and another from 59 forest clusters of all sizes sampledthroughout the wattle stand. The results indicated that (1) Mature trees of well-establishedforest clusters were the main seed sources for the cluster expansion in all directions; (2)Three different patterns were observed in terms of the distance of expansion of regenerationfrom the clusters: a decrease in regeneration with increasing distance from the cluster margin;increasing regeneration with increasing distance from the cluster; and no distinct pattern witha lack of regeneration of the dominant species of a forest cluster. The 40 species recordedwithin the Buffeljagsrivier site include a wide range of fruit and seed characteristics. Fourmain groups of woody species were identified, based on their presence/absence in forestpatches and forest clusters. The presence/absence of most species can be explained in termsof their fruit/seed characteristics and dispersal mechanisms. The majority of recorded woody species were most likely dispersed by birds and mammal, particularly Rameron pigeons andbaboons.In conclusion, a conceptual framework was developed to guide the rehabilitation of stands oflight demanding invasive stands in the forest environment. Several topics for further researchwere identified.

[发布机构] Stellenbosch University