The role of habitat features in a primary succession RUI B. ELIAS & EDUARDO DIAS Elias, R.B. & E. Dias 2007. The role of habitat features in a primary succession. Arquipélago. Life and Marine Sciences 24: 1-10. In order to determine the role of habitat features in a primary succession on lava domes of Terceira Island (Azores) we addressed the following questions: (1) Is the rate of cover development related to environmental stress? (2) Do succession rates differ as a result of habitat differences? One transect, intercepting several habitats types (rocky hummocks, hollows and pits, small and large fissures), was established from the slope to the summit of a 247 yr old dome. Data on floristic composition, vegetation bioarea, structure, demography and soil nutrients were collected. Quantitative and qualitative similarities among habitats were also analyzed. Cover development and species accumulation are mainly dependent on habitat features. Habitat features play a critical role in determining the rate of succession by providing different environmental conditions that enable different rates of colonization and cover development. Since the slope’s surface is composed of hummocks, hollows and pits the low succession rates in these habitats are responsible for the lower rates of succession in this geomorphologic unit, whereas the presence of fissures in the dome’s summit accelerates its succession rate. Key words: Azores, Calluna vulgaris, cover development, dispersion, Juniperus brevifolia Rui B. Elias ([email protected]) & Eduardo Dias, Centro de Estudos do Clima, Meteorologia e Mudanças Globais, Departamento de Ciências Agrárias, Universidade dos Açores, Terra Chã, PT-9701-851 Angra do Heroísmo, Açores, Portugal. INTRODUCTION 2007). Sites that lack the conditions for microclimatic and substrate amelioration are not How vegetation recovers from disturbance is a opportune places for a plant to grow (Titus & del central ecological question, and the understanding Moral 1998). In fact, succession stages may relate of how volcanic landscapes recover informs us primarily to habitat differences related to the lava about fundamental ecological processes. Studies substrate. Of particular importance are the surface of primary succession are important despite the morphology and soil depth as well as the relatively small areas that are now undergoing microclimatic conditions (Tagawa et al. 1985; this process, largely confined to sand dunes, Marchese & Grillo 2000). The rate of plant cover recent volcanic deposits, receding glaciers, river development may be related to environmental point bars, land slides, some anthropogenic sites stress while the rate of species accumulation may and a few other sources of new soil (Aplet et al. depend on the degree of isolation (del Moral & 1998; Dlugosch & del Moral 1999; Marchese & Bliss 1993). Early colonization of isolated sites Grillo 2000; Mueller-Dombois 2000; Kamijo et may be stochastic and dispersal may affect al. 2002; Walker & del Moral 2003). species composition more strongly than The rates of primary succession can environmental factors or competitive interactions differ significantly among sites and in different (del Moral 1999). climatic regimes, but as conditions are In a previous work (Elias & Dias 2004) we ameliorated by weathering and nutrient have demonstrated that the primary succession on accumulation, the rate of succession gradually lava domes of Terceira Island occurs mainly accelerates (del Moral & Ellis 2004; del Moral through an increase in biomass and structural 1 complexity. It was possible to see examples of 15ºC. The study area is part of the NATURA late succession forests on large fissures, in the 2000 Site of Community Importance of Santa summit of young domes. It was also demonstrated Bárbara-Pico Alto and has an important set of that the succession rates were quite different endemic vegetation, dominated by Juniperus among the geomorphologic units (footslopes, forests and succession vegetation (Dias et al. slopes and summits) of the domes, resulting in 2004). different stages being simultaneously present on the dome. As a result we proposed the hypothesis DOME SURFACE MORPHOLOGY that the observed differences were determined by Lava (trachyte) domes result from the habitat features that influenced the establishment accumulation of viscous peralkaline salic lavas of species and the rate of cover development around a fissure. As a result these geologic forms during the colonization and early succession are round or oval, lack a crater and have very stages. Our goal is to clarify how habitat steep slopes (Self 1976). Lava domes are heterogeneity influences the primary succession composed of three geomorphologic units: on lava domes. For that purpose we will address footslope (near the undisturbed vegetation), slope the following questions: (1) Is the rate of cover (the steep lateral slopes of the dome) and summit development related to environmental stress? (2) (the upper part of the dome). The most recent Do succession rates differ as a result of habitat dome (object of the present study), formed 247 differences? years ago (Neves 1826; Drummond 1836), presents clear differences between geomorphologic units due to different succession MATERIAL AND METHODS rates. The decrease of the succession rates is reflected in the decrease in vegetation cover and STUDY AREA structural complexity from footslope, to summit, Terceira Island is located in the central group of to slope (Elias & Dias 2004). This means that, in Azorean Islands, it has an area of 382 km2 and the terms of succession, the slope of the dome is less highest mountain is the Santa Bárbara volcano developed than the summit, and these two with 1023 m a.s.l. The study area is located on the geomorphologic units are less developed than the western part of Terceira Island, on the eastern footslope. Since the vegetation of the footslope is slope of the Santa Bárbara volcano (38º44’N; well developed and has high similarity with the 27º17’W). After the formation of the caldera, this surrounding vegetation it was not sampled for this volcano experienced the emission of several study. trachyte domes and coulées both inside and on the The surface morphology of a dome is very western and eastern slopes of the caldera, in the irregular, with the presence of hummocks, last 15000 years. Subsequent deposits of pumice hollows and pits, on both the slope and summit covered some of these domes until 2080 years (Fig. 1). These habitats are present in the lateral ago. The formation of trachyte domes and coulées slopes and summit of the dome. The summit also occurred, in a non continuous manner, until 1761 includes small and large fissures that are a A.D. when the dome called «Mistérios Negros» consequence of the eruption’s final events. As the was formed (Zbyszewski et al. 1971; Self 1976; lava supply exhausted, large fissures developed Dias et al. 2004). This dome (situated at 630 m parallel to the dome perimeter, as the central part a.s.l.) was the object of the present study. drained out and collapsed. As the lava cools The climate is temperate maritime with a high contraction cracks (small fissures) also develop. level of humidity and rainfall which is both regular and well distributed throughout the year. FIELD DATA COLLECTION The annual precipitation is 4.109 l/m2 (at 600 m One 80 m long line intercept transect, was of altitude) and the average annual temperature is established from the slope to the summit of the 2 Fig. 1. Geomorphology of a lava dome showing two details of the surface (A and B) with the five habitat types identified: hummock, hollow, pit, small and large fissures. dome. The method was adapted from the transect DATA ANALYSIS method described by Mueller-Dombois & The collected data were separated by habitat type Ellenberg (1974). Along the transect several and the species bioarea was calculated. Previous habitat types were intercepted, each occupying studies showed that the best approach for the different lengths: hummocks - 38 m; hollows - study of Azorean natural vegetation was the use 29 m; pits - 4 m; small fissures - 4 m; large of the biovolume and bioarea (Dias 1996; Elias & fissures - 5 m. Canopy height and length, along Dias 2004). Assuming that the shape of the the intercept, were measured for each individual canopies (in lateral view) is similar to an ellipse, plant. Habitat type was also recorded. In the case it is possible to calculate the area occupied by the of grasses, small ferns, and macro bryophytes, canopies along the transect (giving the trunk a only the height and length were measured. Algae, symbolic bioarea value). lichens and bryophytes were not sampled with the exceptions of Sphagnum spp. and Polytrichum Thus the absolute bioarea (BAa) (m2) of commune. Nomenclature is in accordance with a given individual plant is given by (1): Rivas-Martínez et al. (2002). A profile of the dome, with each individual plant in its exact BAa = π × (CL/2) × (CH/2) (1) position, was made. Several soil samples were collected for each habitat type. where, BAa is the absolute bioarea, CL is 3 the canopy intercept length and CH is the canopy between habitats was evaluated by the height. For grasses, small ferns and macro Community Coefficient (CC) as defined by bryophytes the absolute bioarea (m2) is given by Sorensen (1948) and modified by Bornkamm (2): (1981). Quantitative similarity was determined by BAa = L × H (2) the Percent Similarity (PS) (Bornkamm 1981) but using relative bioarea instead of percent cover. where, BAa is the absolute bioarea Clearly, CC will be sensitive to changes in occupied by the individual (or set of individual species presence/absence, whereas PS will plants), L is its length intercept and H is its indicate changes in relative bioarea relations. height. In this case the shape of the individual Analysis of associations between soil features plants in lateral view was considered similar to a and vegetation data was conducted through rectangle.