Impacts of Restored Patch Density and Distance from Natural Forests on Colonization Success

Abstract The reduction and fragmentation of forest habitats is expected to have profound effects on plant species diversity as a consequence of the decreased area and increased isolation of the remnant patches. To stop the ongoing process of forest fragmentation, much attention has been given recently to the restoration of forest habitat. The present study investigates restoration possibilities of recently established patches with respect to their geographical isolation. Because seed dispersal events over 100 m are considered to be of long distance, a threshold value of 100 m between recent and old woodland was chosen to define isolation. Total species richness, individual patch species richness, frequency distributions in species occurrences, and patch occupancy patterns of individual species were significantly different among isolated and nonisolated stands. In the short term no high species richness is to be expected in isolated stands. Establishing new forests adjacent to existing woodland ensures higher survival probabilities of existing populations. In the long term, however, the importance of long‐distance seed dispersal should not be underestimated because most species showed occasional long‐distance seed dispersal. A clear distinction should be made between populations colonizing adjacent patches and patches isolated from old woodland. The colonization of isolated stands may have important effects on the dynamics and diversity of forest networks, and more attention should be directed toward the genetic traits and viability of founding populations in isolated stands.     

Travelling to a former sea floor: colonization of forests by understorey plant species on land recently reclaimed from the sea

Questions: What are important forest characteristics determining colonization of forest patches by forest understorey species? Location: Planted forests on land recently reclaimed from the sea, the Netherlands. Methods: We related the distribution of forest specialist species in the understorey of 55 forests in Dutch IJsselmeer polders to the following forest characteristics: age, area, connectivity, distance to mainland (as a proxy for distance to seed source) and path density. We used species of the Fraxino‐Ulmetum association for the Netherlands as reference for species that could potentially occur in the study area. Results: Area and age of the surveyed forests explained a large part of the variation in overall species composition and species number of forest plant species. The importance of connectivity and distance to the mainland of forest habitats became apparent only at a more detailed level of dispersal groups and individual species. The importance of forest parameters differed between dispersal groups and also between individual species. After 60 years, 75% of the potential pool of wind‐dispersed species has reached the polders, whereas this was only 50% for species lacking specific adaptations to long‐distance dispersal. However, the average percentage of successful colonizing species present per forest was substantially lower, ranging from 15 to 37%. Conclusions: The data strongly suggest that the colonization process in polder forests is still in its initial phase, during which easily dispersed species dominate the vegetation. Colonization success of common species that lack adaptations to long‐distance dispersal is affected by spatial configuration of the forests, and most rare species that could potentially occur in these forests are still absent. Implications for conservation of rare species in fragmented landscapes are discussed.     

Forest species in an agricultural landscape in the Netherlands: Effects of habitat fragmentation

Abstract. For 312 forest patches on sandy soils in the Netherlands, effects of fragmentation are studied of forest habitat in the past on the present occurrence of forest plant species. Using regression techniques, the numbers of forest edge, interior, zoochorous and anemochorous species, as well as occurrence of 24 individual species were related to patch area and connectivity measures. Connectivity was defined as the amount of forest habitat around patches within three zones up to 1000 m. Plant categories were distinguished by habitat type and dispersal mechanism. The results showed that number of total species and number of species of all habitat and dispersal categories increased with area. The occurrence of ten individually studied species were also positively related to area. Most of them were interior species. The number of zoochorous species increased with increasing connectivity. Also occurrence of ten individually studied species were affected by connectivity. Interior zoochorous species showed the highest percentage of affected species. The relationship of interior, animal-dispersed plants to connectivity can be explained by the limited distances covered by their dispersal agents (forest birds and ants) in a non-forest habitat. Also, some anemochorous plants appeared to be affected by connectivity, especially those with heavy seeds and potentially short distance dispersal. As not all species within a certain dispersal or habitat category react similar to area or isolation, it is suggested that differences in underlying processes of fragmentation such as local extinction and colonization need more focus.     

Impact of dispersal on population growth: the role of inter-patch distance

Dispersal can play a key role in the dynamics of patchy populations through patch colonization, and generally this leads to distance-dependent colonization. Less recognised are the roles of dispersal and inter-patch distance on the growth of a population after colonization. We use a laboratory mite model system in which both juveniles and adults can disperse to explore the impact of dispersal, and particularly inter-patch distance, on population dynamics. We examine the dynamics of patches after colonization by manipulating the presence of a dispersal corridor to a source patch at two inter-patch distances. Consistent with many field studies, the results show colonization was slower in more distant patches. Following colonization, the effect of the dispersal corridor on dynamics was dependent on inter-patch distance. In patches near the source, the number of adults tended to increase at a faster rate, and juveniles at a slower rate when connected with a dispersal corridor. In contrast, adult numbers grew slower and juveniles tended to grow faster when connected with a corridor in more distant patches. In the long-term, equilibrium adult numbers were lower in patches connected to the source patch at both distances. These results are likely to be driven by the effects of inter-patch distance on dispersal mortality, and the effects of dispersal on patch abundance and within-patch competition. These results confirm that distance is important for patch colonization and also show that distance can affect population density after colonization. The effects of dispersal and distance on local dynamics could be important in the dynamics of patchy populations in increasingly fragmented landscapes.     

Scale-dependence and mechanisms of dispersal in freshwater zooplankton

Communities of organisms form as a result of both interspecific and abiotic interactions within local habitat patches and dispersal among patches in a region. Local processes are expected to play a dominant role when dispersal occurs much more often than extinction. We performed two field experiments to examine rates and mechanisms of dispersal in freshwater pond zooplankton communities. First, we tested the effect of distance from a source on the rate of colonization of artificial habitat by placing wading pools at 5, 10, 30 and 60 m from two natural fishless ponds and observing the succession of zooplankton. Seventy-eight percent of the species in the source ponds that were capable of living in the pools colonized at least once during the experiment. A new species was found in the pools on average once every four days, suggesting that colonization events occur on the order of days to weeks for many species. Colonization rates declined further from the source at one pond but not the other, and the effect of distance was relatively weak at both ponds. This suggests that many species disperse broadly over short distances. The second experiment tested the role of animal vectors for zooplankton dispersal by restricting access to the pools. Eight treatments were imposed that excluded potential animal vectors along a body size gradient from large mammals to small insects. While the treatments affected zooplankton colonization, many species invaded even when all animals larger than 1 mm were excluded. Animal vectors may therefore be less important for dispersal than wind. Our results suggest that zooplankton are highly effective dispersers over short distances, and can disperse via several mechanisms. Local interactions should therefore play a dominant role in structuring these communities at small regional scales.     

Hedgerows as corridors for woodland plants: a test on the Po Plain,northern Italy

The use of hedgerows as corridors for forest vascular species has been widely studied, but only in humid oceanic and continental climates; no replicated trials have ever been performed on corridor function. Given these premises, a study was done on the eastern Po Plain, in a transition area between the Temperate (Eurosiberian) and Mediterranean climatic zones, adopting the same sample shape and dimensions as a North-American study [Corbit et al. (1999) J Ecol 87:220–232]. The following research questions were posed: (1) how common are forest species in hedgerows? (2) do origin, isolation, distance from source, width and adjacent land-use factors correlate with the frequency of forest species? (3) are hedgerows corridors for forest species? To address these, three functional types of hedgerows, identified by comparing old aerial-photos, were sampled: remnant attached (n = 12) and remnant isolated (n = 6) with respect to the nearest woodland and regenerated attached (n = 4). If wooded patches were a source for hedgerows, then regenerated hedgerows should be more similar to an adjacent woodland than an isolated remnant. A 900 m² circular plot in woodlands and an adjacent 90-m transect along hedgerows were sampled for the presence and cover of all plant species, then 39 woodland taxa were selected. Significant differences between the three hedgerow types emerged in forest species richness, but not in cover. The forest species composition in both remnant and regenerated attached hedgerows showed a strong affinity with the adjacent stand, implying a dispersal process from woodland (source) to regenerated hedgerows (sink). A distance effect on forest species distribution clearly linked to a corridor function was found only in regenerated hedgerows, while in the remnant attached ones, even with a composition similar to that of the nearest woodland source, other additional factors cannot be ignored to explain the fine scale distribution of forest species. The cover of the most common ant-dispersed species showed a similar distance effect while vertebrate-dispersed ones did not show any significant trend with distance from woodlands. Habitat suitability for forest species was affected by width, especially in hedges wider than 12 m, but not by adjacent land use.     

Woody plant colonization in an experimentally fragmented landscape

The pattern of distribution and abundance of woody plants colonizing old fields is influenced by landscape spatial features, in particular, by the distance from the old field to propagule sources and the size of the habitat patches undergoing succession. Colonization is also influenced by species life history traits, such as dispersal mode, growth form, and fecundity. As part of a long-term project studying effects of habitat fragmentation on secondary succession at the prairie-forest ecotone, we have examined the colonization patterns of early-successional woody plants in an experimentally fragmented old field, with emphasis on the three woody species [Cornus drummondii C. A. Mey (rough-leaved dogwood), Ulmus rubra Muhl. (slippery elm), and Juniperus virginiana L. (red cedar)], which currently dominate the woody community on the site. The shapes of the colonization curve (proportion of colonized quadrats vs time) differed between C. drummondii and U. rubra. The rate of colonization by C. drummondii showed a pattern of acceleration after its initial colonization, consistent with rapid in situ recruitment from clonal growth and early seed production. By contrast, colonization by U. rubra fits a roughly linear pattern, consistent with recruitment only from external propagule sources. For both C. drummondii and U. rubra, density is currently greater in large patches than in small patches. No patch size difference was found for J. virginiana. The stern density of both C. drummondii and U. rubra exponentially decreased with distance to external propagule sources. The negative exponential pattern of U. rubra (wind-dispersed) with distance is sharper than that of C drummondii (bird-dispersed). Moreover, the amount of spatial variation in density explained by distance to source is greater on small patches. Our results highlight the importance of life history traits of colonizing species and spatial aspects of habitat during succession.     

Migration of herbaceous plant species across ancient–recent forest ecotones in central Belgium

1 We studied the migration of forest plant species using their percentage cover and frequency in 197 plots distributed over 26 transects across ecotones between ancient and recent deciduous forests in the Meerdaal forest complex in central Belgium. The recent forest stands varied in age between 36 and 132 years, and all occurred on silty, well-drained soils.
2 The total cover, number and diversity of field layer species did not differ significantly between ancient and recent forest stands.
3 The number and cover of the ancient forest plant species and of ant-dispersed species correlated positively with the age of the recent forest and negatively with both the duration of its former agricultural land use and the distance to the ancient forest. This implies a slow colonization of the recent forest stands by these species; all species were, however, able to migrate across the ecotones.
4 The cover of four species ( Anemone nemorosa , Lamium galeobdolon , Convallaria majalis and Polygonatum multiflorum ) declined along the transect, suggesting that they are limited by seed dispersal. Their colonization rates, calculated from the occurrence of the farthest individual, ranged from < 0.05 to 1.15 m year⁻¹ and for other measures from < 0.05 to 0.65 m year⁻¹. Anemone and Lamium appeared to colonize the recent forest by establishment of isolated individuals, while Polygonatum and Convallaria expanded populations from existing patches on the border between ancient and recent forest.
5 Several forest species were able to colonize the recent forest rapidly, where some of them even reached a higher abundance, due to the increased availability of colonization sites with a higher nutrient content and a thinner organic layer.     

Of niche differentiation,dispersal ability and historical legacies: what drives woody community assembly in recent Mediterranean forests?

Community assembly rules have been extensively studied, but its association with regional environmental variation and land use history remains largely unexplored. Land use history might be especially important in Mediterranean forests, considering their historical deforestation and recent afforestation. Using forest inventories and historical (1956) and recent (2000) land cover maps, we explored the following hypotheses: 1) woody species assembly is driven by environmental factors, but also by historical landscape attributes; 2) recent forests exhibit lower woody species richness than pre‐existing due to the existence of colonization credits; 3) these credits are modulated by species’ life‐forms and dispersal mechanisms. We examined the association of forest historical type (pre‐existing versus recent) with total species richness and that of diverse life‐forms and dispersal groups, also considering the effects of current environment and past landscape factors. When accounting for these effects, no significant differences in woody species richness were found between forest historical types except for vertebrate‐dispersed species. Species richness of this group was affected by the interaction of forest historical type with distance to coast and rainfall: vertebrate‐dispersed species richness increased with rainfall and distance to the coast in recent forests, while it was higher in dryer sites in pre‐existing forests. In addition, forest historical types showed differences in woody species composition associated to diverse environmental and past landscape factors. In view of these results we can conclude that: 1) community assembly in terms of species richness is fast enough to exhaust most colonization credit in recent Mediterranean forests except for vertebrate‐dispersed species; 2) for these species, colonization credit is affected by the interplay of forest history and a set of proxies of niche and landscape constraints of species dispersal and establishment; 3) woody species assemblage is mostly shaped by the species’ ecological niches in these forests.     

Colonization of secondary woodlands by Anemone nemorosa

Migration of the herb species Anemone nemorosa from older woodlands into adjacent, recently established deciduous woodlands on former arable land was studied. The wood anemone had colonized part of all studied recent woods, varying in age between 30 and 75 years. Cover of A. nemorosa in the recent woods decreased with increasing distance from the older woodland at all sites but one, indicating dispersal limitation during colonization. The advancing edge of most populations was characterized by negative logarithmic decrease in cover with establishment of isolated pioneer individuals and later gradual infill between pioneers. Migration rates were calculated by three methods. The mean migration rate of A. nemorosa based on observed maximum cover in the recent woods was 0.20 m year¹, 0.40 m year^-1 based on half maximum cover and 0.85 m year¹ based on the individual found farthest from the former woodland border. The calculated migration rates were consistently higher than the rate of possible rhizome growth. Seed dispersal and establishment is thus very important for colonization of new woodlands. Migration rates increased with tree canopy cover–especially cover of broad-leaved species with quickly decomposing litter–and with soil pH. Migration rates decreased with increasing grass cover in the field layer. These differences in migration rates may be due to increased micro-site availability for establishment at high canopy cover, low grass cover and high soil pH. Our results show that the wood anemone generally colonizes recent woods from nearby source populations. However, colonization proceeds relatively slowly and is limited by both seed dispersal and availability of suitable micro-sites. Gradients in abundance of A. nemorosa within secondary woods may be detectable for long periods of time and indicate the recent origin of a woodland.     

道路对林地景观连接度的影响——以巩义市为例

基于景观连接度原理,借用景观连接度指数,在地理信息系统支持下,探讨了巩义市山区林地景观在不同距离阈值下连接度的变化,定量分析了道路对林地景观连接度的影响。结果显示,随着景观距离阈值的增大,无论是否有道路,林地景观整体可能连通性指数值都表现为逐渐增大;对林地景观连接度起"非常高"和"高"作用的林地斑块数量都比较少,但占林地总面积比例较大,面积大的林地斑块在提高景观连接度中起的作用较大;道路的分割使得林地斑块重要值降低,就单一斑块而言,随着景观距离阈值的增大,分割成的小斑块的重要值降低程度在逐渐减小。     

Clonal diversity and genetic differentiation of <Emphasis Type="Italic">Maianthemum bifolium</Emphasis> among forest fragments of different age

Maianthemum bifolium Schmidt (May Lily) is a woodland species with low colonisation ability and high demands for seedling establishment conditions. To study the colonisation process, we analysed the clonal organisation and population structure of Maianthemum bifolium populations in a number of forest fragments using AFLP. A total of 129 genets were identified. Most (41/53) patches of M. bifolium in recent (less than 80 years old) and old (at least 175 years of age) parts of the woodlands contained single genets. Only one of the 12 patches with multiple genotypes showed intermingled growth. Besides seed dispersal also dispersal of vegetative material contributed to colonisation since five genotypes were found in more than one patch, with patches up to 51 m apart. Population differentiation between populations in different forests was significant (F_ST = 0.10) and indicated low gene flow. This level of differentiation was already found between populations in different parts of the same forest, just over 200 m apart. In fact, only two adjacent populations were not significantly differentiated, one population located in old forest whereas the other was established next to it on a former pasture re-forested after 1916. Limited gene flow is also consistent with the decrease of genetic similarities with physical distance, which relationship is highly significant but only for distances up to 400 m.     

Factors influencing vegetation gradients across ancient‐recent woodland borderlines in southern Sweden

Abstract. We studied gradients in field layer vegetation across ecotone‐type borderlines between 12 ancient woodlands and adjacent secondary deciduous woodlands on former arable land. The aim of the study was to determine how distance from the borderline influences species distributions as compared with soil factors and degree of canopy closure. Correspondence Analysis showed that distance from the borderline is closely related to the first ordination axis at all study sites. Canonical Correspondence Analysis with variation partitioning revealed that distance from the borderline was the single most important factor in explaining vegetation variation. In general, the results suggest the following order of decreasing importance: Distance from the borderline < Soil reaction < Soil nitrogen < Soil moisture < Canopy cover. However, the sum of soil variables, as estimated by weighted averages of Ellenberg indicator values for moisture, reaction and nitrogen, accounted for as much as ca. 50–70% of the total variation explained by environmental variables. Important gradients in field layer vegetation are due to a decrease in typical woodland species and an increase in other species with increasing distance from the ancient woodland. The results suggest dispersal limitation of woodland species as an important determinant of secondary forest succession. However, the importance of distance to species distributions decreases with increasing stand age as most woodland species gradually colonize the recent woodlands. After 70 yr, ca. 50 % of the woodland species present at a site showed complete colonization within 50 m from the ancient woodland border.     

Effect of proximity to ancient deciduous woodland on restoration of the field layer vegetation in a pine plantation

The influence of dominant tree species. soil conditions and distance ärom an adjacent ancient ouk-liernbeam woodland upon the floristic composition. species richness and spatial distributions of species in a pine wood planted on dry rendzina soil were studied in southern Poland. It was found that. in spite of a 52-yr long succession. the cover and composition of species in the pine wood were significantly different from that in the adjacent ancient woodland. Canonical correspondence analysis (CCA) and stepwise regression analyses showed that the distance to the ancient woodland had a significant influence on species distributions in the recent wood. The numbers of species from the Querco-Fagerea class. vegetatively reproducing species and myrrnochores decreased with this distance. whereas the numbers of tree species in the shrub and field layers and anemochores increased. The light intensity had a positive effect on the numbers of species in the shrub layer. non-woodland species and anemochores. The number of woodland species increased considerably with the cover of quickly decomposing litter and with thickness of the humus layer. The migration rate of many woodland species. calculated on occurrence of the farthest individuals was very' slow. varying from 0J8 m yr' to 0.38 m yr'. These results indicate that the availability of micro-sites suitable for seed germination and plant development determined the successful colonization of the recent wood to an extent comparable with the seed dispersal.     

Effects of landscape matrix on population connectivity of an arboreal mammal,Petaurus breviceps

Ongoing habitat loss and fragmentation is considered a threat to biodiversity as it can create small, isolated populations that are at increased risk of extinction. Tree‐dependent species are predicted to be highly sensitive to forest and woodland loss and fragmentation, but few studies have tested the influence of different types of landscape matrix on gene flow and population structure of arboreal species. Here, we examine the effects of landscape matrix on population structure of the sugar glider (Petaurus breviceps) in a fragmented landscape in southeastern South Australia. We collected 250 individuals across 12 native Eucalyptus forest remnants surrounded by cleared agricultural land or exotic Pinus radiata plantations and a large continuous eucalypt forest. Fifteen microsatellite loci were genotyped and analyzed to infer levels of population differentiation and dispersal. Genetic differentiation among most forest patches was evident. We found evidence for female philopatry and restricted dispersal distances for females relative to males, suggesting there is male‐biased dispersal. Among the environmental variables, spatial variables including geographic location, minimum distance to neighboring patch, and degree of isolation were the most important in explaining genetic variation. The permeability of a cleared agricultural matrix to dispersing gliders was significantly higher than that of a pine matrix, with the gliders dispersing shorter distances across the latter. Our results added to previous findings for other species of restricted dispersal and connectivity due to habitat fragmentation in the same region, providing valuable information for the development of strategies to improve the connectivity of populations in the future.     

Former land use affects the nitrogen and phosphorus concentrations and biomass of forest herbs

The colonization rates of understorey plants into forests growing on former agricultural land differ remarkably among species. Different dispersal and recruitment largely account for the contrasting colonization rates, but different effects of the soil legacies of former agricultural land use on plant performance may also play a role. Seven herbaceous forest species were sampled in paired post-agricultural and ancient forest stands to study whether land-use history has an effect on the aboveground nutrient concentrations (N, P and N:P ratios) and biomass of forest herbs and, if so, whether slow and fast colonizing species respond differently. Results showed that P concentrations were significantly affected by former land use with higher concentrations in the post-agricultural stands. N concentrations were unaffected and N:P ratios were significantly higher in the ancient stands. Nutrient concentrations varied considerably among species, but the variation was unrelated to their colonization capacity. Six out of the seven species had higher biomass in the post-agricultural stands relative to the ancient stands, and the degree to which the species increased biomass was positively related to their colonization capacity, i.e., the fast colonizing species showed the strongest increase. Such differential responses to past land use may contribute to the contrasting colonization capacity of forest plants. Land-use history thus affected both the nutrient concentrations and biomass of forest herbs, and only the biomass response was related to colonization capacity.     

Colonization of Sphagnum on land uplift islands in the Baltic Sea: time, area, distance and life history

Aim To test whether species richness of Sphagnum mosses on islands in a land uplift archipelago is related to island age, area or connectivity, and whether the frequency of different species can be predicted by their life history and autecology. Location The northern Stockholm archipelago in the Baltic Sea, east‐central Sweden, with a current land uplift rate of 4.4 mm year^?1. Methods We sampled 17 islands differing in area (0.55–55 ha), height (3.6–18 m, representing c. 800–4000 years of age) and distance from mainland (1.6–41 km). For each Sphagnum patch we measured area, height above sea level, horizontal distance from the shore and shading from vascular plants. Factors affecting island species richness, species frequency and habitats on the islands were tested by stepwise regressions. Species frequency was tested on nine life history and autecological variables, including estimated abundance and spore output on the mainland, habitat preference and distribution. Results We recorded 500 patches of 19 Sphagnum species, distributed in 83 rock pools on 14 islands. Island species richness correlated positively with island area and with degree of shelter by surrounding islands, while distance from the mainland, connectivity, height or age did not add to the model. Species frequency (number of colonized islands and rock pools) was mainly predicted by spore output on the mainland and by habitat preference (swamp forest species were more frequent than others), while spore size, for example, did not add to the model. Species differed in mean height above and horizontal distance from the shore, area of occupied rock pools and in the degree of shading of patches. The mean horizontal distance from the shore and the area of occupied rock pools correlated positively with the normal growth position above the water table among species. Spore capsules were found in only 2% of patches, mostly in the bisexual Sphagnum fimbriatum. Main conclusions The presence of Sphagnum in the Stockholm archipelago seems to be governed by regional spore production and habitat demands. Sphagnum does not appear to be dispersal limited at distances up to 40 km and time spans of centuries. Species with a high regional spore output have had a higher colonization rate, which, together with the rarity of spore capsules on the islands, indicate the mainland as a source for colonization rather than dispersal among islands. Swamp forest species seem more tolerant to the island conditions (summer droughts and some salt spray) than open mire species. The different distances from the sea occupied by the species indicate a slow, continuous succession and species replacement towards the island interior as islands are being uplifted and thus expand in area. This partly explains why larger islands harbour more species. Our results thus support some of the island biogeographical theories related to the species–area relationship.     

Landscape matrix modifies richness of plants and insects in grassland fragments

There is an increasing awareness that not only area and isolation, but also the characteristics of the landscape surrounding habitat patches influence population persistence and species diversity in fragmented landscapes. In this study, we examine the effects of grassland fragmentation and land use in the landscape matrix (on a 2 km scale) on species richness of plants, butterflies, bees and hoverflies. These organisms were studied in replicated remnant patches of different sizes and isolation, embedded in landscapes dominated either by forest, arable land or a mix of these. We found positive effects of patch area on species richness of the three insect taxa, but not of plants. Isolation had a negative effect only on hoverflies. Matrix type had contrasting effects on the studied taxa. Species richness of plants and butterflies was lowest in patches in landscapes dominated by arable land and highest in forest‐dominated landscapes. For hoverflies, the negative effect of small patch area was strongest in forest‐dominated landscapes, and there was a similar non‐significant trend for bees. Our study shows the importance of considering matrix characteristics when studying responses to habitat fragmentation. Differences in matrix response among organism groups probably impinge on differing mechanisms. A forest matrix is likely to provide additional resources for butterflies but either constitute a barrier to dispersal or deprive resources as compared to an arable matrix for hoverflies. Enhanced plant diversity in grassland patches embedded in forested landscapes can be explained by habitat generalists more easily invading these patches, or by an unpaid extinction debt in these landscapes.     

Vegetation and songbird response to land abandonment: from landscape to census plot

While intensification of human activities and its ecological effects in many natural areas have recently received much attention, land abandonment in marginal areas is still the largely ignored side of a process rooted in the same socioeconomic context. Decreasing human impact in marginal rural areas often triggers a recovery of seminatural vegetation. Over a period of 25 years, we studied the changes in landscape and vegetation structure that followed land abandonment in a traditional Mediterranean mosaic of crops, grasslands, shrublands and woodlands, and assessed their effects on songbird occurrence and distribution. We combined an analysis of vegetation changes based on aerial photo interpretation with an analysis of bird censuses from 1978, 1992 and 2003 at two spatial scales: landscape and census plot (respectively 2800 and 3 ha). The perceived temporal changes in the vegetation were scale dependent. At the landscape scale, open habitats tended to disappear and woodlands matured. The contrasts in vegetation structure that defined habitat patches at the onset of the study tended to disappear. There was an overall shift of the bird community in favour of woodland species. At the scale of the census plot, however, the colonization by woody vegetation of patches formerly characterized by a homogeneous grass cover increased the local diversity of the vegetation, at least temporarily. Of seven species dependent on open habitats, the occurrence rate of five species significantly decreased, whereas it increased for two species: woodlark (Lulula arborea) and melodious warbler (Hippolais polyglotta). This increase was linked to the transitional increase in local vegetation diversity. In patches originally dominated by woodlands, local vegetation diversity decreased as woody vegetation expanded into clearings. The occurrence rate significantly increased for seven species relying on closed woodlands, while it decreased for two woodland species. As most species of high conservation profile in the Mediterranean are tied to open or to heterogeneous transitional habitats, these trends raise questions concerning their persistence in the future.     

Differential colonization causing non-random forest plant community structure in a fragmented agricultural landscape

We investigated forest plant community structure of recent forest patches in a highly fragmented agricultural landscape, A nestedness analysis has been performed at The community level and at the individual species level. We tested the hypothesis that plant species composition showed a nested structure and whether this was generated by isolation and differential colonization. Alternative hypotheses formulated in the past, such as nested habitats and patch area dependent species relaxation, are verified. Isolation measures were calculated between clearly defined source and target patches. At the community level target patches are defined as forest patches < 35 yr old while older patches are considered as source patches. At the individual species level we distinguished between actually occupied and potentially suitable target patches based on a habitat space model and only occupied source patches are taken into account.
All results point out that the nested community pattern in recent forest patches is generated primarily by isolation and differential colonization. These findings are confirmed at the species level, since the degree of nestedness of the individual species is highly correlated with their isolation sensitivity.
Logistic regression analyses prove that many forest plant species are sensitive to isolation. The probability of occurrence of the isolation sensitive species drops almost to zero, when the nearest source patch is situated further than 200 m. Probabilities of colonization increase significantly when source patches are situated adjacent to target patches.