Vegetation succession among and within structural layers following wildfire in managed forests

Question: How does vegetation develop during the initial period following severe wildfire in managed forests? Location: Southwestern Oregon, USA. Methods: In severely burned plantations, dynamics of (1) shrub, herbaceous, and cryptogam richness; (2) cover; (3) topographic, overstory, and site influences were characterized on two contrasting aspects 2 to 4 years following fire. Analysis of variance was used to examine change in structural layer richness and cover over time. Non‐metric multidimensional scaling, multi‐response permutation procedure, and indicator species analysis were used to evaluate changes in community composition over time. Results: Vegetation established rapidly following wildfire in burned plantations, following an initial floristics model of succession among structural layers. Succession within structural layers followed a combination of initial and relay floristic models. Succession occurred simultaneously within and among structural layers following wildfire, but at different rates and with different drivers. Stochastic (fire severity and site history) and deterministic (species life history traits, topography, and pre‐disturbance plant community) factors determined starting points of succession. Multiple successional trajectories were evident in early succession. Conclusions: Mixed conifer forests are resilient to interacting effects of natural and human‐caused disturbances. Predicting the development of vegetation communities following disturbances requires an understanding of the various successional components, such as succession among and within structural layers, and the fire regime. Succession among and within structural layers can follow different successional models and trajectories, occurs at different rates, and is affected by multiple interacting factors.     

Species-area relations in a New Zealand tussock grassland,with implications for nature reserve design and for community structure

Abstract. A tussock grassland, in Blackrock Reserve, New Zealand, was sampled thoroughly at scales ranging from 0.01 m x 0.01 m to 20 m x 20 m, to investigate species-area relations of relevance to plant community structure, and to offer a pointer to reserve design. In total, 96 species were found. Of the native vascular species among these, 20% were new records for the reserve. For the total and the native vascular flora the observed points fell midway between the Arrhenius and Gleason fitted curves. Cryptogams fitted the Arrhenius model well, with z close to the 0.26 value expected for isolates under Preston's Canonical hypothesis. Extrapolation of the Arrhenius curve to the whole of the New Zealand uplands gave values far too large; the Gleason curve gave values much too low. A General Root model is introduced. It fits the observed species richnesses at various quadrat sizes considerably better than previous models. When biogeographic limitations on species richness are included, extrapolation of the General Root curve accurately predicts the size of the upland flora. This fit, the lack of an asymptote, and the rarity structure, are compatible with a random and individualistic model of community structure. However, there are differences in species-area relation between vascular and cryptogamic plants, which cautions against expecting any universal type of community structure. Extrapolations using the General Root model suggest that if the ideal is a 10 km x 10 km reserve, a reserve one tenth that size would contain 81% of the native species in that ideal, and the present Blackrock Reserve contains 67%.     

Vertical concentration gradients of heavy metals in Cladonia lichens across different parts of thalli

Certain lichens of the genus Cladonia are effective heavy-metal-tolerant colonisers of strongly contaminated and disturbed sites. Among them, Cladonia cariosa, Cladonia pyxidata and Cladonia rei are the major components of specific cryptogamic assemblages proven to be bioindicators of soil pollution. This study examines the bioaccumulation capacity and heavy metal accumulation pattern of these species in the context of element concentration levels in various parts of their thalli at various vertical distances from the ground. The content of Zn, Pb, Cd, As and Cu in primary squamules, lower and upper parts of secondary thalli (podetia), and fruiting bodies (apothecia), as well as the corresponding substrate, was analysed using the AAS method. The substrate turned out to be the main source of heavy metals in the examined Cladonia lichens. Element accumulation in particular parts of thalli greatly depends on metal enrichment in the immediate vicinity while Cu/Zn ratios for both substrate and lichen samples were very low and comparable within the species. Concentration levels in thalli usually decrease significantly with distance from the substrate. The exception is copper, which content was frequently higher in apothecia than in the upper parts of podetia. Low bioaccumulation factors calculated for the examined Cladonia specimens classified these lichens as weak accumulators of heavy metals. Even given an extremely high level of contaminants in the substrate, the upper parts of thalli are not greatly affected. Consequently, fruticose and erect growth form, in combination with low accumulation capacity and a remarkable decrease in metal content along a vertical gradient, may be an important attribute of Cladonia lichens in the colonisation of a highly contaminated substrate. The content of elements differs significantly between particular parts of Cladonia thalli; this should be taken into account whenever burdens of heavy metals are used as indicators in biomonitoring studies.     

Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis–Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.     

Testing reliability of short-term responses to predict longer-term responses of bryophytes and lichens to environmental change

Environmental changes are predicted to have severe and rapid impacts on polar and alpine regions. At high latitudes/altitudes, cryptogams such as bryophytes and lichens are of great importance in terms of biomass, carbon/nutrient cycling, cover and ecosystem functioning. This seven-year factorial experiment examined the effects of fertilizing and experimental warming on bryophyte and lichen abundance in an alpine meadow and a heath community in subarctic Sweden. The aim was to determine whether short-term responses (five years) are good predictors of longer-term responses (seven years). Fertilizing and warming had significant negative effects on total and relative abundance of bryophytes and lichens, with the largest and most rapid decline caused by fertilizing and combined fertilizing and warming. Bryophytes decreased most in the alpine meadow community, which was bryophyte-dominated, and lichens decreased most in the heath community, which was lichen-dominated. This was surprising, as the most diverse group in each community was expected to be most resistant to perturbation. Warming alone had a delayed negative impact. Of the 16 species included in statistical analyses, seven were significantly negatively affected. Overall, the impacts of simulated warming on bryophytes and lichens as a whole and on individual species differed in time and magnitude between treatments and plant communities (meadow and heath). This will likely cause changes in the dominance structures over time. These results underscore the importance of longer-term studies to improve the quality of data used in climate change models, as models based on short-term data are poor predictors of long-term responses of bryophytes and lichens.     

Goose grazing influences the fine-scale structure of a bryophyte community in arctic wetlands

Herbivores can shape plant communities, especially in the Arctic. We tested the role of geese for structuring bryophyte communities at fine spatial scales in the arctic tundra by excluding them from 4 × 4 m areas. We surveyed the presence and absence of bryophyte species in quadrats (10 × 10 cm) divided into 25 cells outside and inside these exclosures, after 5 and 11 years of treatment. Species richness per cell (4 cm²) was higher in the presence of geese, especially after 11 years of treatment, while geese had little effect on richness at larger scales (i.e. quadrat and whole exclosure). The slope of the species–area relationship within quadrats was consequently shallower outside exclosures. Our results further suggest that the community outside the exclosures was more variable in space and time than that inside the exclosures. We conclude that goose foraging activity promotes the coexistence of bryophyte species at the centimetre scale.     

Differential recovery of above‐ and below‐ground rich fen vegetation following fertilization

Abstract. For seven years we studied the recovery of vegetation in a Belgian P limited rich fen (Caricion davallianae), which had been fertilized with nitrogen (200 g.m^?2) and phosphorus (50 g.m^?2) in 1992. The vegetation in this fen has low above‐ground biomass production (< 100 g m^?2) due to the strong P limitation. Above‐ground biomass was harvested from 1992 to 1998 and P and N concentrations measured. In 1998, below‐ground biomass was also harvested. The response to fertilization differed markedly between below‐ and above‐ground compartments. Above‐ground, P was the single most important factor that continued to stimulate growth 7 yr after fertilization. Below‐ground, N tended to decrease live root biomass and increase dead root biomass and seemed to have a toxic effect on the roots. In the combined NP treatment the stimulating effect of P (an increase of live root biomass) was countered by N. The 1998 soil analysis showed no difference in soil P in the plots. Thus, Fe hydroxides are not capable of retaining P in competition with fen vegetation uptake. However, higher capture of P in root Fe coatings from N plots may partially explain this negative N effect. The results suggest that N root toxicity will be amplified in strongly P limited habitats but that its persistence will be less influenced by P availability. This mechanism may be a competitive advantage for N₂ fixing species that grow in strongly P limited wetlands.     

How does continuity in grassland management after ploughing affect plant community patterns?

Ley cultivation by ploughing and fertilising constitute a severe impact on plant community patterns in semi-natural subalpine communities. Since the 1950ties, such cultivation have been an important part of the agroecosystem in subalpine areas in Norway, and regularly practised. Today, several former leys are left for grazing management. Plant species diversity and heterogeneity was studied in subalpine grasslands in Valdres, southern Norway (ca. 950 m a.s.l.) at different successional phases after ploughing. Fourteen sites (100 m²) varying from recently ploughed grasslands (3–5 years at phase 1) to unploughed sites were used to construct a temporal gradient by a space for time substitution. Small-scale species diversity (0.25m²) of vascular plants increased continuously with the time since last ploughing, while the variation in gamma-diversity was minor among late successional phases 3–5 (>23–28 years, 32–39 years and unploughed sites) after an evident increase from phase 1–3. The number of exclusive species peaked within unploughed sites (10), and included perennial herbs (Antennaria alpina, Hieracium auricula, Viola canina), spore-producing species (Botrychium lunaria, Selaginella selaginoides) and woody life-forms. Species diversity of cryptogams was low at both a small and large scale within successional phases 1 and 2 (<12 years). Small scale cryptogam diversity peaked at the unploughed sites, while gamma diversity peaked at phase 3 (23–28 years). This intermediate phase 3 also had the highest number of exclusive species (12) which exceeded unploughed sites (8). A coarse spatial structure of cryptogams at phase 3 was indicated by a low mean of species distribution. The soil of uncultivated grasslands (phase 5) was, at the end of the growing season (mid August), quite base-rich with a higher content for all measured nutrients as compared to intermediate phases of continuity (phase 3 and 4) The exception was nitrate which was not detectable from phase 3 to 5, and available phosphate which peaked at an intermediate phase of succession (phase 3 and 4). Heterogeneity of vascular plants reached a minimum value at the intermediate phase (3) while cryptogam heterogeneity peaked at this phase. Differences in heterogeneity cycles among these species groups implies that structuring processes act on different temporal scales for vascular plants as compered to cryptogams along the successional gradient in grasslands after ploughing. The importance of continuity in management for plant biodiversity in subalpine grasslands is finally discussed from a conservation point of view.