Vegetation dynamics on abandoned terraces of Sicily: the course and driving forces of succession

In Mediterranean Europe, terrace landscapes can be found where 1) the orography is characterized by hills or mountains, 2) there is a high amount of stone outcrops and 3) the human need for agricultural production was quite high till the recent past. During the last century, many of these landscapes have been abandoned, due to their unsuitability for mechanized, intensive agriculture and due to the general land abandonment trend which arised in large parts of Europe. Since with land abandonment start secondary succession processes, the plant and animal communities linked to agroecosystems are replaced by other species which find their habitat in the developing succession stages. This process has been widely studied in Europe for some agroecosystems, but not for terraced ones. At a European level, few literature exists, in Italy even less and almost nothing is known about Sicilian terraces, which are the subject of the present study. In my Diploma Thesis, I described secondary succession processes on undisturbed terraces of Pantelleria Island (Sicilian Channel). However, in most areas of Sicily disturbances such as wildfires and grazing are frequent. So, in order to know which are the secondary succession processes of disturbed and undisturbed vegetation on Sicilian terraces, which was the first main aim of the present study, a wider research perspective had to be chosen. The results of this research project are presented in the first part of the present study, focusing on a description of the plant communities involved in secondary succession processes a) from a floristic and structural point of view and b) from a species diversity point of view. Moreover, the ecological interpretation of the collected vegetation data describes the links of plant communities, single plant species and plant species diversity to environmental factors and disturbance regime. In order to obtain these results, 129 vegetation relev\ue9s (sensu Braun-Blanquet) were made on abandoned terraces in five of the main terraced areas of Siciliy: 1) the Aeolian Islands, 2) Pantelleria Island, 3) Mt. Etna, 4) the Palermo Mts. and 5) the Hyblaean Plateau. The first three areas share volcanic substrates, while sample plots in the last two areas are characterized by limestones. Moreover, the sample plots differed in bioclimate, abandonment age and disturbance status. Bioclimatic thermotype and ombrotype were assessed using thematic maps. Abandonment age was checked through aerial photographs of the years 1955, 1968, 1987, 1992, 2000 and 2002. In this way, plots were attributed to six different abandonment age classes: Class 1 = still cultivated today; Class 2 = still cultivated in 2002, thus abandoned not more than 4 years ago; Class 3 = still cultivated in 1987, thus abandoned about 5-20 years ago; Class 4 = still cultivated in 1968, thus abandoned about 20-40 years ago; Class 5 = still cultivated in 1955, thus abandoned about 40-50 years ago; Class 6 = already abandoned in 1955, thus abandoned about more than 50 years ago. Four types of disturbance status were defined (i.e. presence/absence and nature of disturbance): 1) burnt, 2) grazed, 3) ploughing, thus still cultivated and 4) undisturbed plots. Only abandoned vineyards or grain crop fields were selected as sample plots, while tree crops were excluded, as it can be assumed that secondary succession processes are modified by tree presence in the old fields. Selected sample plots were always 50 m2-sized. The results of biodiversity evaluation by t-tests and ANOVA showed that vascular plant diversity is linked to disturbance regime and to abiotic factors (especially geological substrate). Especially grazing increases species richness, since animal activity creates several microsites within old fields which would not occure if animals were absent (bare soil, small-scale nutrient gradients for excrements). In these microsites, many therophyte communities can establish. Moreover, it was found that on limestone species richness is higher than on volcanic substrates. As a second step of data evaluation, it was tested if there is a link of certain plant species to fire or grazing disturbance, comparing their cover values using Kruskal-Wallis One Way ANOVA on ranks. Only two of the 13 tested species were confirmed as indicator species. On limestone, Ampelodesmos mauritanicus showed statistically significant higher cover values in relation to fire disturbance. In contrast, on volcanic substrates Cistus spp. become dominant when fire disturbance occurs. As a third step, vegetation relev\ue9s were analysed with DCA and TWINSPAN. The resulting 14 sample plot groups (= clusters) were then used for a better interpretation of plant communities and to check the dynamic relations between them. In addition, in order to describe vegetation physiognomy of every cluster the available structural data (dominance, cover percentage of perennial herbs and woody species) was used. From a floristic point of view, plant communities involved in secondary succession processes on Sicilian terraces are quite different between and within the five study areas. As DCA showed, this is mainly due to different substrate and bioclimatic conditions. Moreover, vegetation is strongly influenced by abandonment age and disturbance status, and each of them plays an opposite role in modelling its structure. If no disturbance biases succession, then plant communities evolve rather rapidly (30-50 years) to maquis communities. If fire does not occur frequently, it does not alter the structure of vegetation, since most part of Mediterranean shrubs and trees resprout more or less rapidly. If frequent fires occur, secondary succession is blocked, so that vegetation remains for decades in a \u201csteady state\u201d (Ampelodesmos mauritanicus grasslands in the limestone sites, Cistus spp. garrigue on Pantelleria and Brachypodium retusum grasslands on the Aeolian Islands). Where grazing is very intense, the steady states were found to consist of herbaceous plant communities with few spiny or poisonous woody species individuals. With low-intensity grazing, higher woody cover was recorded. The rapid secondary succession processes present on undisturbed terraces are mainly due to the terrace soil body, which is a noteworthy water reservoir in comparison to soils on non-terraced slopes, and to the terrace wall which may act as dispersal center and as a favouring microsite for woody species colonization. The second part of the present study focuses on the driving forces of secondary succession and, more in detail, on the colonization machnisms of old fields by woody species. In a first section, the existence of two of these mechanisms are checked: 1) the neighbourhood effect and 2) the safe-site effect. The first term refers to the hypothesis that old fields surrounded by old succession stages are subject to a more rapid invasion of woody plants than those ones surrounded by old fields of the same abandonment age. On the other hand, the term \u2018safe site effect\u2019 describes the hypothesis that an old field which is characterized by a high number of safe sites is colonized more rapidly by woody species than an old field with few safe sites. A safe site can be defined as the environmental conditions at the scale of an individual seed that promote seed survival, germination, and seedling establishment. The existence of these two effects is verified by data from 51 transect relev\ue9s, made up of 357 subplot relev\ue9s. The transects were made in target fields 1) with older neighbour (i.e. old succession stage characterized by maquis communities) present directly attached to the target field and 2) with older neighbour absent within a 100 m-distance. In each of the seven subplots (1x1m) of a transect, a simplified vegetation relev\ue9 was made, and all woody species individuals were counted, recording seedlings, saplings and adults separately. In addition, for every individual was recorded if it grew within the influence of a potential safe site or not. As potential safe sites were analysed the former crop plants of vine and the terrace wall base. Data evaluation by Kruskal-Wallis ANOVA and Mann-Whitney Rank Sum confirmed the existence of the neighbourhood effect and of the safe site effect. Moreover, it was shown that animals as dispersal vectors strongly influence these effects. For the neighbourhood effect, seed dispersal distance is the crucial point, since as a general rule it can be said that seed rain decreases with increasing distance from the source. On Pantelleria, only birds and rabbits were found to transport seeds of maquis species at a distance >100 m. For the safe site effect, the main underlying mechanisms are: 1) passive facilitation (i.e. animals tend to create heterogeneous seed rain patterns because they frequent certain microhabitats more often than others) and 2) active facilitation (i.e. the positive influence of an existing woody or herbaceous plant individual on the establishment or the growth of another one). Vine plants are safe sites for woody species because they serve as perching structures for birds and as a shelter for mammals and, thus, are characterized by an elevated seed rain than the open spaces of an old field. In addition, the vine\u2019s canopy and the man-made soil depression in which they were planted lower heat and drought stress and, thus, increase seed germination and seedling survival probability. Also the wall base is characterized by milder microclimatic conditions and, thus, enhances recruitment rates. Passive facilitation on wall bases is performed probably more by mammals than by birds. As an overall conclusion, it can be stated that the pace of secondary succession on Pantelleria is dispersal-limited since the safe sites in a terraced old field are not saturated. The second section of the analysis of the driving forces of succession describes the performance of establishment of Quercus ilex L. in different microsites of terraced old fields. This analysis was done by seed arrival manipulation through acorn sowing and aimed at answering the question if there are microsites in old fields which favour Holm Oak establishment in old fields more than other microsites. In November 2004, acorns were buried on a North-facing slope and on a South-facing slope in five different microsites: 1) under vine plants, 2) at wall bases, 3) under the canopies of isolated shrubs, 4) between small rock accumulations and 5) in open spaces (i.e. outside of any of the previously named microsites). In monthly checks, seedling emergence, survival, height and leaf number were recorded. Moreover, in April and July were measured air temperature and air humidity in the different microsites. Most seedlings emerged in April and May 2005; overall emergence rate was 52.4% of all buried acorns (n = 1,020). More seedlings emerged on the South-facing slope (S; 59.8%) than on the North-facing slope (N; 45.0%). Emergence was higher when acorns were buried under vine plants and at the wall base than in other microsites of the old fields. At the end of the experiment (September 2006), 45.3% of all emerged seedlings were still alive (29.2% on N, 58.9% on S). The rest died mostly due to drying up during summer drought of 2005 and 2006, but also due to seedling predation by rabbits and rodents. Survival was higher under vine plants and at the wall base than in the open spaces of the old fields. Again, the South-facing slope was more favourable than the North-facing slope. Height and number of leaves of the seedlings were higher under the vine plants than in other microsites (only on the North-facing slope). Between rock accumulations, seedlings were higher and had more leaves on S than on N. From literature, it is known that seed vitality, seed germination and seedling survival of Quercus ilex are favoured by shady, wet and fresh conditions. The temperature and air humidity measurements showed that at the wall base, under vine plants and under isolated shrubs environmental conditions are milder than in open spaces. Shrubs probably did not result to enhance seedling survival for elevated predation rates, as predators prefer shrubs for the shelter they provide. However, even if temperature and relative air humidity seem to play an important role for Quercus ilex seedling emergence and survival, they did not unambiguously explain the differences between the safe site types. A factor of major importance is probably soil moisture, as from literature it is known that Quercus ilex is sensitive to water-stress in its establishment phase. The outlined observations are valid even if emergence and survival were higher on S than on N. Contrarily to what was expected, the measurements showed that the S-slope was characterized by milder conditions than the N-slope. This fact may be explained by differences in mesoclimate, e.g. due to wind which is an important factor on Pantelleria Island. As a last part, the present study discusses what does the obtained results mean for terrace landscape conservation and biodiversity management. In abandoned areas, on the long term species richness can be conserved only by a periodic re-set and re-abandonment of the abandoned areas, creating and maintaining in this way a mosaic of cultivated fields, young, intermediate and old succession stages. The analysis of the neighbourhood and safe site effect allow to make some consideration on the spatial design of such a mosaic, and, more in detail, may answer the questions when to intervene and how to intervene. Some examples are presented how the results may be applied, with a crucial point being the acceleration factor of woody cover increase by older neighbours and safe sites. Moreover, for terraced areas where such a mosaic will not be applied, a proposal is made how it might be possible to preserve terrace structure even with abandonment: since terrace wall seem to be stable under woods, rapid and undisturbed secondary succession might be the key for terrace landscape conservation. Where dispersal centers are absent, afforestation by extensive Quercus ilex acorn sowing might be a solution