Diversity in the cork oak forests of the northern Straits of Gibraltar Region (Southern Spain)

Several components of diversity (ecological, taxonomic, chorological and morphological– functional) were studied in the woody vegetation of the cork oak forests in the northern Straits of Gibraltar Region, and their relationship to the two main environmental gradients (one mainly climatic—precipitation, fog—and another secondary related to the disturbance and hydric stress associated with hillslope orientation). An inventory of the woody species and their cover measure was carried out on 94 plots. The relationship between the components of diversity and the main environmental gradients was analysed by means of regression analysis. Decreases of species richness, Shannon diversity index, Western Mediterranean species cover, percentage of endemism and the share of the Herrera Syndrome I in diversity were related to the increase of the precipitation and the fog. However, the cover and percentage of species of the Circum-Mediterranean and Mediterranean-Eurosiberian chorological groups increase with the precipitation and the fog. Disturbance and hydric stress associated with hillslope orientation, have a negative effect on specific richness of the Mediterranean-Eurosiberian chorological group and taxonomic singularity, but its have a positive effect on the share of the Herrera Syndrome I in diversity. The number of edaphic endemisms is associated mainly with disturbance. In the cork forests does not exist an inverse relationship between the number of endemisms and floristic richness, as it happens in other Mediterranean areas. It has suggested that this pattern is due to effect of competitive exclusion associated with disturbance in vegetation on relatively fertile substrate.     

The gothic earthflow revisited: a chronosequence examination of colonization on a subalpine earthflow

I examine the process of plant community colonization through a chronosequence study of permanent plots spanning 45 years on a 70-year-old subalpine earthflow in south-western Colorado, USA.While local diversity and average density of vegetation remained stable throughout the study, the overall diversity increased through the early years of the study with little change over the past 20 years. A gradual shift in species composition occurred through time with species with good dispersal and stress tolerance abilities colonizing the site initially followed by more generalist species.After over 70 years the disturbed communities remained distinct from adjacent relatively undisturbed areas in diversity, density, and species composition. Soil translocation experiments suggest that it is the severe microclimate of the earthflow which is limiting further colonization of the site.     

Wind disturbance in remnant forest stands along the prairie-forest ecotone,Minnesota, USA

Strong winds are an important disturbance agent in northern Minnesota forests. On June 19, 1994, strong winds (>160 km h^(-1)) associated with a tornado damaged forested areas within the Rydell National Wildlife Refuge, situated in Polk County Minnesota along the prairie-forest boundary. Field sampling was conducted immediately following the storm to quantify the type and extent of damage in four different community types, and to project future composition based on the nature of the storm damage and current understory characteristics, including the impact of overbrowsing by deer. Basal area in six sampled remnant forest stands was reduced by 33.5%, although the damage was heterogeneous; basal area in one stand was reduced by 68.1%. The overall effect of the storm was the removal of early- successional species (primarily Populus tremuloides) in larger size classes. Trees situated at stand edges were not more susceptible to snapping or uprooting than interior trees. Projections of future stand composition indicate that wind disturbance, unlike other agents of disturbance such as fire, may accelerate succession on the Refuge, such that early-successional stands will assume a later-successional character, while Acer-Tilia stands should maintain their late-successional character. Overbrowsing and preferential foraging by deer may significantly alter stand recovery patterns.     

Arbuscular mycorrhizal infection in Cyperaceae and Gramineae from natural,disturbed and restored savannas in La Gran Sabana,Venezuela

 The mycorrhizae of a tropical savanna growing in oligotrophic and stony soils were compared with those of a disturbed area that had been reclaimed with introduced species and of an area that was disturbed but not revegetated. All were compared with natural regeneration in a savanna that had been disturbed 12 years previously. Arbuscular mycorrhizae (AM) were common in savannas. Cyperaceae species, which were codominant with Graminaea, showed high levels of infection frequency (45%) like the Gramineae (61%). Arbuscules observed in the Cyperaceae indicated functionality. There were few plants in disturbed, nonrevegetated sites, but those present had AM. Observations of roots from soil monoliths showed that AM were present in disturbed areas, but compared with natural, succesional and revegetated savanna had a lower infection frequency (48–59% vs 75%), lower intensity (10–15% vs 25%) and a lower percentage of arbuscules (0.7–0.8% vs 2.3%). The percentage of vesicles was also lower in succesional savanna than in natural savanna (1.6% vs 4.8%). The revegetated site had the highest percentage of vesicles (6.6%). Although a high frequency of mycorrhizal infection has been reestablished in disturbed areas, the intensity and structure of the infection suggests that mycorrhizal function has not been restored to the original levels. We hypothesize that neither plants nor fungi have adapted to the new edaphic conditions. Accepted: 10 July 1995     

Regeneration growth of the invasive clonal forb Rorippa austriaca (Brassicaceae) in relation to fertilization and interspecific competition

A special type of clonal growth, spread by lateral roots, ishypothesized to be a favourable trait of invasive, opportunistic plant speciesof disturbed habitats. We tested this hypothesis for the invasive forbRorippa austriaca (Brassicaceae). Regenerationfrom root fragments, subsequent vegetative spread and allocation patterns inrelation to varied nutrient supply and intensity and pattern of interspecificcompetition were analyzed in container experiments. Regeneration success fromroot fragments was 100% and clonal spread was rapid but vegetativeperformance was strongly reduced under unfertilized conditions and,particularly, when interspecific competition was present. While the ratio ofabove- to belowground bio-mass did not differ considerably betweentreatments, R. austriaca allocated a high amount ofresources to belowground growth resulting in low aboveground but highbelowground biomass at harvest time relative to the matrix vegetation.Differences in shoot number or biomass between simulated gaps and denselyvegetated quadrants in the containers were (relatively) weak.Reproductive effort was less reduced under low resource levels, and the clonesdid not set seed at all, irrespective of the treatment. Our results show thatclonal growth by lateral roots and plasticity in clonal growth patterns inR. austriaca promote both exploitation of gaps andnutrient-rich microsites and resistance to competitors. Such plasticity,combined with its ability to regenerate from widely-dispersed rootfragments, contribute to the ability of the species to invade and persistwithindisturbed and spatially heterogeneous habitats.     

Disturbance effects on herbaceous layer vegetation and soil nutrients in Populus forests of northern lower Michigan

Abstract. Recent disturbance models have identified changes in resource availability as factors that control plant community response. Soil nutrient resources typically are assumed to change following forest disturbance, usually with nutrient availablity increasing initially and subsequently decreasing through later stages of succession. We examined the effects of disturbance (clearcut harvesting with a brief recovery period) on soil organic matter, pH and extractable soil nutrients in successional aspen forests of northern lower Michigan to determine relationships of these variables to changes in herbaceous layer vegetation. Two site types were identified: drymesic (glacial outwash sands, low in organic matter) and mesic (calcareous clay till, high in organic matter). Extractable nutrient concentrations were 1.5 to 3 times higher in the A₁ horizon of mesic sites than those of dry-mesic sites. Soil pH and cations increased after disturbance on mesic sites, but not on dry-mesic sites. Patterns of change with disturbance were less pronounced in lower horizons on both site types. Herblayer species diversity increased after disturbance on mesic sites, but with decreases in the importance of shade-tolerant tree species and Maianthemum canadense. Species characteristic of open habitats (e.g. Pteridium aquilinum, Rubus spp., Fragaria virginiana, and Diervilla lonicera), increased in importance. Soil factors, species composition and diversity on dry-mesic sites changed little after disturbance, with Pteridium aquilinum and ericaceous species remaining dominant in both mature (55–82 yr) and disturbed (≤ 15 yr) stands. These results suggest that soil nutrient resources do not always change through secondary succession and that patterns of change can be distinctly site-dependent. Disturbance response patterns in the herbaceous layer of these aspen forests are also site-dependent.     

Relationships between change in fire frequency and mortality due to spruce budworm outbreak in the southeastern Canadian boreal forest

Abstract. We present a simple empirical model that allows an estimation of mortality due to spruce budworm (Choristoneura fumiferana) outbreak in relation to fire frequency and site characteristics. The occurrence of a recent spruce budworm outbreak around Lake Duparquet (48° 30’N, 79° 20’W, ca. 300 m a.s.l.) in northwestern Québec permitted a reconstruction of the stand composition before the outbreak, and also of the mortality of Abies balsamea due to the outbreak. The basal area of A. balsamea increases with time since fire in all site types but with increasing values for (1) rock and shallow till, via (2) till and mesic clay up to (3) hydric clay. Mortality (measured as percentage loss of basal area due to the outbreak) increases with time since fire but did not vary with site type. The increasing abundance of A. balsamea with time since fire is mainly responsible for this increase in mortality. Mortality for a specific basal area is, however, lower for the more recently burned stands suggesting a significant residual effect of time since fire. A landscape model integrating mortality due to the outbreak for stands of different age is developed. Both absolute and relative losses of basal area increased with the length of the fire cycles. According to this model, changes in fire cycle could explain a large portion of the spatio-temporal variations observed in outbreak mortality in the southeastern boreal forest of Canada.     

Patch size of forest openings and arthropod populations

Summary Five sizes of canopy openings (0.016 ha to 10 ha) were established in the Southern Appalachian Mountains in early 1982 to examine the initial patterns of plant and arthropod establishment across a size range of forest disturbances. Vegetation standing crop after the first growing season was considerably higher in large than small openings in apparent response to greater resource release (e.g., sunlight) in larger openings. Woody stump and root sprouts were the dominant mode of revegetation in each patch size. Forest dominants such as Quercus rubra, Q. prinus and Carya spp. were less important as sprouters in openings than several minor forest components (e.g., Robinia pseudo-acacia, Acer rubrum, Halesia carolina and Cornus florida). Arthropod abundance and community composition varied across the size range of forest openings. Arthropods from the surrounding forest readily utilized the smallest canopy openings (0.016 ha). All feeding guilds were well represented in these small openings and herbivore biomass and load (mg of herbivores/g of foliage) were much higher than in larger patches. In contrast, arthropod abundance and species richness were significantly lower in mid-size than smaller patches. The relatively sparse cover and high sunlight in mid-size openings may have promoted surface heat buildups or soil surface/litter moisture deficits which restricted arthropod entry from the surrounding forest. Arthropod abundance and species richness were higher in large than mid-size patches. The greater vegetation cover in larger openings may have minimized the deleterious effects on arthropod populations. However, the absence of population increases among these arthropod species maintained herbivore loads at very low levels in large patches. Our results suggest that arthropod abundance and diversity in sprout-dominated forest openings are highly dependent on the extent of environmental differences between patch and surrounding forest.