Rüdiger Otto

Variation in species composition and vegetation structure of succulent scrub on Tenerife in relation to environmental variation

Abstract. On Tenerife, the occurrence of environmental gradients over short distances provides a unique opportunity to investigate the relationship between vegetation and environmental factors. In the semi-arid coastal region of Tenerife, floristic composition, species richness and vegetation structure of perennial plants have been studied in 67 locations covering the existing precipitation gradient. On the island as a whole, variation in species composition could be best explained by mean annual precipitation; at coastal sites, substrate age and soil characteristics also played a significant role. On the other hand, substrate chemistry and the type of eruptive material explained little of the floristic variation. Stand biomass was strongly correlated with mean annual precipitation and was, on the youngest lava flows studied, also affected by substrate age. The native stem succulent species made up the bulk of total biomass along the whole precipitation gradient. Disturbed and undisturbed sites differed significantly in stand biomass and cover. Species richness was correlated with precipitation and substrate age. Distribution of plant functional types was also related to the precipitation gradient. The relative abundance of hemicryptophytes and shrubs with non-hairy leaves increased with increasing precipitation whereas the ratio of shrubs with hairy/non-hairy leaves and succulent plants decreased. Some alien plants were quite frequent at disturbed sites but, on the whole, they contributed little to the species spectrum and to the stand biomass. Undisturbed sites remained almost free of introduced species not considering annuals.

Floristic and structural recovery of a laurel forest community after clear-cutting: A 60 years chronosequence on La Palma (Canary Islands)

We analyzed a post-clearcut chronosequence (0.5 to 60 years after harvesting) in the laurel forest of La Palma island (Canarian Archipelago) to determine the recovery of the stands with respect to species composition, richness, life strategies and structural parameters of the canopy. Multivariate analysis showed that exotic species, as well as annual ruderal species were confined to early-successional stages, while native perennials, typical of laurel forests, dominated the late-successional stages. Total species richness decreased significantly with time after clear-cutting. The relative fast recovery of understory native species may be due to low forest floor disturbance during harvesting. Shade-intolerant pioneer, pioneer-remnant and shade-tolerant late-successional species were the main life strategies of native tree species. Most structural parameters showed a continuous and monotonic increase (basal area, biomass) or decrease (density, percentage of photosynthetic biomass) during succession. Once clear-cutting, here performed with an interval of 8 years, is abandoned, the recovery of the laurel forest seems possible due to careful logging that protects the soil and a rapid asexual regeneration of native tree species, revealing this to be a sustainable management practice.RésuméOn a analysé une chronoséquence après coupe rase (0,5 à 60 ans après récolte) dans la forêt de lauracées de l’île de Palma (Archipel des Canaries) pour déterminer la reconstitution des peuplements pour ce qui concerne la composition spécifique, la richesse et les paramètres structuraux de la canopée. Une analyse multivariable a montré que les espèces exotiques aussi bien que les espèces rudérales étaient confinées aux premiers stades de la succession, tandis que les espèces naturelles pérennes typiques de la forêt de lauracées dominaient les derniers stades de la succession. La richesse spécifique totale a diminué significativement avec le temps après la coupe rase. La reconstitution relativement rapide des espèces naturelles du sous-bois peut être due à la faible perturbation de la surface du sol forestier au moment de la coupe rase. Les pionnières intolérantes à l’ombre, les pionnières rémanentes et les tolérantes à l’ombre des stades finaux de la succession constituaient les principales stratégies des espèces naturelles d’arbres. La plus grande partie des paramètres structuraux ont montré un accroissement continu et monotone (surface terrière, biomasse) ou décroissant (densité, pourcentage de la biomasse photosynthétique) pendant la succession. Autrefois réalisée ici avec un intervalle de 8 ans la coupe rase est abandonnée, la reconstitution de la forêt de lauracées semble possible grâce à une exploitation prudente des bois protégeant le sol et une régénération asexuée des espèces naturelles d’arbres, révélant que ceci est une pratique de gestion durable.

Bioclimatic and vegetation mapping of a topographically complex oceanic island applying different interpolation techniques

Different spatial interpolation techniques have been applied to construct objective bioclimatic maps of La Palma, Canary Islands. Interpolation of climatic data on this topographically complex island with strong elevation and climatic gradients represents a challenge. Furthermore, meteorological stations are not evenly distributed over the island, with few stations at high elevations. We carried out spatial interpolations of the compensated thermicity index (Itc) and the annual ombrothermic Index (Io), in order to obtain appropriate bioclimatic maps by using automatic interpolation procedures, and to establish their relation to potential vegetation units for constructing a climatophilous potential natural vegetation map (CPNV). For this purpose, we used five interpolation techniques implemented in a GIS: inverse distance weighting (IDW), ordinary kriging (OK), ordinary cokriging (OCK), multiple linear regression (MLR) and MLR followed by ordinary kriging of the regression residuals. Two topographic variables (elevation and aspect), derived from a high-resolution digital elevation model (DEM), were included in OCK and MLR. The accuracy of the interpolation techniques was examined by the results of the error statistics of test data derived from comparison of the predicted and measured values. Best results for both bioclimatic indices were obtained with the MLR method with interpolation of the residuals showing the highest R2 of the regression between observed and predicted values and lowest values of root mean square errors. MLR with correction of interpolated residuals is an attractive interpolation method for bioclimatic mapping on this oceanic island since it permits one to fully account for easily available geographic information but also takes into account local variation of climatic data.

Overview of Habitat History in Subtropical Oceanic Island Summit Ecosystems

Abstract Summit ecosystems of oceanic islands constitute one of the most ephemeral and isolated ecosystems existing, harboring specific features that confer on their biota an outstanding distinctness. Summits are short-lived entities, being the last ecosystems to be constructed during the growth of the new oceanic island, and the first to vanish due either to island subsidence, island erosion, or both. Whereas their geological emergence/disappearance is controlled by the volcanic/erosion activity, Pleistocene glaciations in the past million years, by forcing the altitudinal shift of the timberline, have also likely created or destroyed summit ecosystems, enabling the appearance of alpine ecosystems during glacial maxima where they were not present in interglacial periods and vice versa. On the other hand, summit ecosystems constitute islands within islands, being more isolated from climatically similar ecosystems than the coastlines of the islands containing them. Thus summit biota, frequently displaying a high endemicity, may originate either through dispersal from other close summit ecosystems during peak periods, or from the colonization of the summits and later evolution to the new conditions from mid-altitude species of the same island. Conversely, if peak periods are absent, the disappearance of summit ecosystems implies the extinction or extirpation of their constitutive species. Current summit species have likely occupied a much larger area during glacial periods. Thus the summits may be classified as climatic refuges. This is especially the case if glacial periods were associated with much drier conditions on oceanic islands as is the case on continents.

Road Edge Effect and Elevation Patterns of Native and Alien Plants on an Oceanic Island (Tenerife, Canary Islands)

We studied road edge effects on floristic composition and richness of alien and native plants on five zonal ecosystems, following a steep altitudinal gradient from arid coastal and mid-elevation scrublands, through laurel and pine forests, to summit scrub, on Tenerife (Canary Islands). We analyzed vegetation within transects running from the road edge to the core of natural habitats. Alien richness significantly decreased with distance to road edge for most ecosystems. Native richness also decreased with increasing road distance for the coastal scrub and pine forest, but increased for the thermophilous scrub. We found a decrease in both native and alien species richness with elevation. Our results suggest that road edge effects in alien plant invasion are stronger in native shrub communities at low elevations than in forests (laurel and Canary Island pine forest), where aliens were limited to a narrow road edge band. Detrended correspondence analyses showed that road edge plots were floristically very different from interior plots and that each ecosystem harboured a specific alien assemblage with few species present in more than one ecosystem, suggesting a marked species turnover of roadside alien species across altitudinal belts up to the pine forest. However, at high elevations, very few aliens invaded roadsides, probably due to harsh environmental conditions and still relatively low propagule pressure.

Regeneration niche of the Canarian juniper: the role of adults, shrubs and environmental conditions.

Abstract• Canarian Juniper woodlands, now very scarce, are rich in endemic and endangered plants. However, many aspects of juniper regeneration are almost unknown.• This paper relates occurrence and abundance of recruits of Juniperus turbinata ssp. canariensis to (1) small-scale soil characteristics; (2) vegetation cover; and (3) distance to conspecific adults in two contrasting juniper stands in the eastern mountains of Tenerife. We used non-parametric classification trees and generalised linear models (GLM) to evaluate the effect and importance of each explanatory variable on the occurrence of juniper saplings.• Sapling density, vitality and growth rate, as well as fruit production by adult trees, but neither cone density on the ground nor sapling size, varied significantly with respect to slope orientation, representing environmental stress. Within each stand, distance to nearest adult tree was the most important variable explaining the spatial distribution of juniper saplings and availability of seeds in cones. Additionally, saplings were positively associated with shrub cover at the microsite-level, but not with spiny shrub cover. Soil depth and rock cover had a weak negative effect on sapling establishment, but only at the south-facing site and in the open space microhabitat.• Results suggest that recruitment of Canarian juniper is facilitated by microhabitats offered by adults and shrubs. The key factors affecting recruitment are thought to be (1) favourable micro-environmental conditions and (2) high ambient seed availability. Browsing intensity in recent decades was very low. The presence of spiny shrubs did not favour juniper establishment. Facilitation therefore appears to result from amelioration of abiotic conditions rather than from protection against herbivory.

Distributional patterns of endemic, native and alien species along a roadside elevation gradient in Tenerife, Canary Islands

Invasion by alien plant species may be rapid and aggressive, causing erosion of local biodiversity. This is particularly true for islands, where natural and anthropogenic corridors promote the rapid spread of invasive plants. Although evidence shows that corridors may facilitate plant invasions, the question of how their importance in the spread of alien species varies along environmental gradients deserves more attention. Here, we addressed this issue by examining diversity patterns (species richness of endemic, native and alien species) along and across roads, along an elevation gradient from sea-level up to 2050 m a.s.l. in Tenerife (Canary Islands, Spain), at multiple spatial scales. Species richness was assessed using a multi-scale sampling design consisting of 59 T-transects of 150 m × 2 m, along three major roads each placed over the whole elevation gradient. Each transect was composed of three sections of five plots each: Section 1 was located on the road edges, Section 2 at intermediate distance, and Section 3 far from the road edge, the latter representing the “native community” less affected by road-specific disturbance. The effect of elevation and distance from roadsides was evaluated for the three groups of species (endemic, native and alien species), using parametric and non-parametric regression analyses as well as additive diversity partitioning. Differences among roads explained the majority of the variation in alien species richness and composition. Patterns in alien species richness were also affected by elevation, with a decline in richness with increasing elevation and no alien species recorded at high elevations. Elevation was the most important factor determining patterns in endemic and native species. These findings confirm that climate filtering reflected in varying patterns along elevational gradients is an important determinant of the richness of alien species (which are not adapted to high elevations), while anthropogenic pressures may explain the richness of alien species at low elevation.

Diversity and distribution of the last remnants of endemic juniper woodlands on Tenerife, Canary Islands

Identifying ecological requirements, species diversity patterns and distribution ranges of habitats of interest is an important task when developing conservation and restoration programs. The Canarian juniper woodland formed by Juniperus turbinata ssp. canariensis is listed as a priority habitat by the European Union. Although very common in the past, this vegetation type has suffered immense destruction and degradation over the last five centuries on the Canary Islands, especially on the largest most populated island of Tenerife. We evaluated the geographical distribution range of the last remnants of Canarian juniper woodlands on Tenerife and analyzed their ecological status, floristic composition and plant species diversity. Despite the degradation of the original vegetation, we still observed outstanding species diversity. Endemic species richness and number of typical habitat species were best predicted by summer rainfall, which seems to be the limiting factor for this habitat in the lower drier regions. Human disturbance has had a negative effect on endemic species richness but a positive effect on the distribution of alien plants, highlighting the potential threat to this habitat. Ecological characterization and floristic composition were most influenced by climatic factors related to the dichotomy of a humid windward and a drier leeward slope of the island and by altitude. However, vegetation structure and human disturbance also determined species composition. Environmental requirements indicated a circuminsular potential distribution of this habitat. Given the exceptional plant diversity, the scarcity of dense stands and the low protection status, immediate protection of the remaining stands and future restoration programs should be the priority for conservation strategies of this endemic vegetation type.

Spatiotemporal variation of a Pinus seed rain available for an endemic finch in an insular environment

A major goal of avian ecological research is to determine how distribution and abundance of preferred resources available influence population dynamics and contribute to understand life-history characteristics. Food is widely considered the ultimate factor influencing these traits. We studied, with seed traps, the spatiotemporal variability of Pinus canariensis seed rain during 2007–2008, as a means to explain why a post-dispersal seed predator of conservation concern, the endemic blue chaffinch Fringilla teydea, can adjust its annual life cycle with this variation in an insular environment. Generalized linear mixed models and geostatistical tools were used. Results highlight that temperature and relative humidity are important predictors of seed release rates. Additionally, a high temporal variation was detected in seed abundance (i.e., peaks of massive seed release during the summer months, intermediate values in the autumn, and minimum release rates in winter and spring). Finally, within-stand spatial variation in seed flux was surprisingly large with the most productive microsites receiving three to four times more seeds than the least productive ones. Pine seeds showed a high protein value and a low germination rate. Based on these findings, we suggest that the fortunes of the blue chaffinch should be intimately related to spatiotemporal annual P. canariensis seed crops, temperature acting as a proximate cue, and food availability as the ultimate factor. For the endangered blue chaffinch population on Gran Canaria, we recommend, until more data are available, improving the seed supply during the winter season, either artificially (feeders) or naturally (planting Myrica faya shrubs).

Effects of Wildfire on Endemic Breeding Birds in a Pinus canariensis Forest of Tenerife, Canary Islands

Abstract: Fire is a key ecological force in pine forests worldwide, and faunal responses to this disturbance have been a major topic of ecology, yet little is known for oceanic island environments. Using line transects we surveyed the bird community of a natural Pinus canariensis forest burned in the summer of 2007 on Tenerife, Canary Islands, Spain. Our goal was to identify important environmental variables and the thresholds that influenced the pattern of bird abundance at the community, foraging guild, and species-specific level. Models were constructed by means of regression trees and cross validation applying the 1-SE rule. Mixed results were observed and only 2 species were clearly affected by fire severity. Overall, total bird abundance, total species richness, and total bird diversity were positively influenced by low, very low, and moderate canopy fire severity, respectively, and the presence of either Erica or Myrica shrubs. Consequently, high canopy fire severity had an overall negative effect on bird community characteristics. Abundance of Myrica shrubs affected positively general ground foragers such as common blackbird (Turdus merula cabrerae) and Canary Islands chiffchaff (Phylloscopus canariensis), a canopy forager. The number of thin trees was important for canopy foragers such as goldcrest (Regulus regulus teneriffae) and African blue tit (Cyanistes teneriffae teneriffae), but also for a bark forager, the great spotted woodpecker (Dendrocopos major canariensis). European robin (Erithacus rubecula superbus) and the ubiquitous endemic blue chaffinch (Friugilla teydea teydea) were influenced by low and moderate canopy fire severity, respectively. We recommend that land managers incorporate these results into future post-fire management plans. Priority post-fire actions should be directed toward the protection and conservation of the endemic blue chaffinch, a species of conservation concern on the nearby island of Gran Canaria.