Abstract
Urban plant habitats have become primary drivers of species interactions. They consist of managed vegetation and spontaneous assemblages of native, naturalized, ornamental garden escapes, and invasive species. Our objective was to define urban habitat analogs for a plant species of conservation interest, Matthiola crassifolia, which has persisted in varying abundance in the Mediterranean city of Beirut.
We adopted a stepwise method that integrates two vegetation assessments, floristics, and physiognomy. We placed seventy-eight quadrats (1m x 1m) in 12 study sites following a deliberate biased method to capture habitat diversity. In every quadrat, we performed taxonomic identification and recorded life form of each species. We pooled species that shared the same life form into categories and estimated area cover for each of these life forms. We performed TWINSPAN analysis on floristic data to identify species positively associated with M. crassifolia, and on life forms, to determine plant assemblages that promote optimal M. crassifolia representation. We then combined findings from both analyses to generate a description of urban habitat analogs suitable for M. crassifolia.
The results revealed that urban habitat analogs favorable to M. crassifolia include green spaces dominated by palms, low-lying succulents, or by shrubs with scale-like leaves. On the other hand, spaces dominated by turf grass, canopy trees, or vegetation that produces significant litter were not favorable to M. crassifolia’s persistence. Based on these findings, we generated a plant palette of native and non-native species to design urban habitat analogs favorable to the persistence of M. crassifolia.
Synthesis and applications The application of this method can inform planting designs that yield suitable habitats for plants of conservation interest. It can also guide landscape management plans that seek to create or modify green spaces to optimize growing conditions for species of conservation interest. Depending on sites, and based on the information generated by the stepwise method, designers and managers may decide to exclude life forms of native or non-native species that do not support the growth of a species of conservation interest, or they may create an artificial habitat that is conducive to its persistence.
Introduction
Ornamental, native, naturalized, garden escapes, and invasive plant species, grow in managed, partially managed or unmanaged artificial urban habitats. Native plant species of conservation interest can adapt to such disturbed urban conditions depending on their ruderal behaviors [1]. However, their persistence is unlikely as their fate depends on how these artificial habitats are conceived, designed, and managed. This becomes critical when the geographic distribution of a species lies within the boundaries of the city. Urban biodiversity strategies have proposed to transform artificial urban habitats into habitats suitable for native plant conservation [2]. One example of urban biodiversity strategy is the use of species-rich herbaceous communities to promote biodiversity in cities [3]. Another strategy, referred to as reconciliation ecology, proposes the conversion of spaces assigned to human activities into spaces that support the persistence of native species [4]. Identifying habitat analogs in this case is essential to guide reconciliation ecology strategy in cities [5]. Provided appropriate conservation targets, habitat analogs could dilute the distinction between disturbed and non-disturbed habitats as favorable sites for plant conservation [6, 7]. Collecting data to inform and guide urban biodiversity strategies is challenging because all currently available methods are intended for field studies in natural areas and do not always yield clear findings in urban contexts (Table 1).
For example, studies that used floristics to describe urban habitats found an over-representation of ruderal species and high taxonomic diversity between relatively close sites [18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. In contrast, physiognomic and structural vegetation description, developed to describe natural vegetation over large areas, may be a more useful tool for urban biodiversity than floristics. Physiognomy reflects predominance of life strategies adopted by different life forms and it is applicable in highly modified sites and at both macro- and micro-climate conditions [36, 18, 37].
In addition to field assessment challenges in cities, the success of plant conservation strategies is also highly influenced by social perception and preference and should take into consideration such requirements. For example, studies have shown that spontaneous ‘unmanaged’ vegetation may not appeal to residents as aesthetically pleasing nor is it perceived as acceptable ‘urban nature’ by decision-makers [38, 39, 40]. This is further complicated by the fact that plant selection and management, is driven by landscape architects and landscape contractors who have limited experience with native species, and do not have clear guidelines to contribute to biodiversity conservation in cities [41, 42].
The objective of this study was to define urban habitat analogs for a plant species of conservation interest, Matthiola crassifolia, which has persisted in varying abundance in the Mediterranean city of Beirut.
Materials and methods
Species of conservation interest and its distribution
Named after Pietro Andrea Mattioli, Matthiola R.Br. is a widespread genus of flowering plants represented by about 48 species ranging from annual, biennial and perennial, woody and herbaceous plants and sub-shrubs, many of which are heavily scented, and available in a wide range of colors for horticulture and floristry [43]. The genus Matthiola can be split into 12 distinct species-groups [44] with species distributed throughout Macaronesia, the Mediterranean basin, the Saharo-Sindian region and NE Africa–Asia, and it exhibiting two centers of taxonomic diversity in Turkey and the Irano-Turanian region.
There are four Matthiola species recorded in Lebanon, two of which are either national or regional endemics. The Species-Group OVATIFOLIA is represented by the regional endemic Matthiola damascena Boiss. The Species-Group LONGIPETALA is represented by Matthiola tricuspidata and Matthiola longipetala. Species-Group INCANA is represented by the national endemic Matthiola crassifolia Boiss. & Gaill which is restricted to a few locations along the highly urbanized Lebanese coast and is the subject of this study. M. crassifolia is a taxon of conservation interest as the species is recognized as an endemic of Lebanon. However, [44] has questioned the taxonomic status of the species proposing that it be considered subspecies of Matthiola sinuata. Even if future molecular analyses support this preference, the taxon will remain an endemic of Lebanon yet at the intra-specific level.
The most comprehensive record of the distribution and status of M. crassifolia prior to this study was by [45] who performed a systematic survey of the Lebanese coast and recorded the presence of the species in three out of five previously reported sites, Beirut and Byblos. Subsequent field investigations by [46] added Saida, Khaldeh and Amchit as localities for M. crassifolia. Our field survey to these localities confirmed the extinction of M. crassifolia in Saida and its continued presence in Khaldeh, Beirut and Byblos [47].
Study area
Beirut (33.8869° N, 35.5131° E), the capital of the Republic of Lebanon, is located on the eastern coast of the Mediterranean. Archeological evidence shows that humans have continuously occupied Beirut for the last 5000 years [48, 49]. Today, Beirut has one of the highest urban densities in the Middle East with an area roughly over 20 km2, population density is estimated at 21,000 people per sq. km [50 ,51]. The topography of the city includes two hills, Achrafieh (100 m elevation from the sea) and Mousseitbeh (80 m elevation from the sea) [52]. Paul Mouterde, who conducted floristic studies in Beirut in the 20th century, reported 1200 floral species including native, naturalized, ornamental garden escapes, and invasive species [53].
Ras Beirut, our study site, is defined by a 6 km long and 2 km wide cape [54]. Today, this area consists of densely populated neighborhoods interspersed with managed landscapes and zones with spontaneous naturalized vegetation occurring within geographically adjacent lots. Recent floristic studies of semi natural areas in Ras Beirut revealed low community similarity, patchy species distribution, and predominance of habitat non-specific species [55]. Green spaces in Ras Beirut fall under two broad categories; managed landscapes, dominated by exotic ornamental species planted in raised beds with reconstructed soil, and spontaneous landscapes where spontaneous floral communities survive along with naturalized garden escapees, in coastal cliffs, along the rocky water front, and in un-built/abandoned lots [56]. Following early botanical studies of semi natural areas in Beirut, the city has been subjected to extensive landscape transformation, and today it still harbors a significant remnant native vegetation. Based on these facts, Beirut can be considered as a type-three city that is likely to be carrying an extinction debt [57].
Field data collection
We used a deliberate biased method to select study locations and to lay out sampling quadrats [58]. We set a total of 78 quadrats in 12 sites. We placed quadrats, 1 m × 1 m, in anthropogenic habitats and in semi natural habitats that do not include shrubby vegetation [18]. We placed larger quadrats, 2 m × 2 m, in locations where shrubs are present [18]. As in Dinsdale, deliberate bias method consists of placing quadrats in areas judged representative of the selected location and for capturing the maximum observed variation [58, 59].
We made three modifications to the sampling technique to address site-specific issues; 1) When the boundary of a given plant community was not clearly defined due to site disturbance, we set quadrats within assumed boundaries of the community to capture plant diversity, 2) In cases where species had an ‘individualistic’ distribution pattern adding to the difficulty in conceiving boundaries [60], we increased the number of quadrats to capture the observed variation, 3) Since we do not know the dispersal distance of the target species, when a vegetation community harbored the target species we placed two quadrats; we set one to include the target species and placed the other quadrat in a location where the target species did not grow. In communities that did not harbor the target species, we set only one quadrat.
We divided each quadrat into a grid of 100 subunits to ensure speed of measurement and relative accuracy [61, 62, 63, 64]. In every quadrat, we determined percent cover using the 11-point Domin cover scale by visually assessing subunits as: fully covered, empty, and partially covered for each species and each life form [18]. Data obtained from all subunits within a quadrat was then added to determine Domin cover per quadrat.
Taxonomic and life form identification
We identified each plant specimen by consulting published floras, voucher specimens at the University Herbarium (Post Herbarium), and photographic floras [65, 53, 46]. All identified species were described by their life form according to Ellenberg and Mueller-Dombois amended to include bunched shoot arrangement in reptant hemicryptophytes which forms a partially decomposed thick mat and peat accumulation [10]. We then pooled species that shared the same life form under the one category and estimated area cover for each life form accordingly.
Analysis
Based on the 11-point Domin cover scale, we analyzed floristic data, species and percent species cover, using TWINSPAN [66]. Using the same tool, TWINSPAN, we analyzed the life form data, which included life-form categories and percent cover as relative abundance of each life form within each quadrat. In the TWINSPAN, the cut levels 0-3-4-5-6-8 were applied. The TWINSPAN groups were characterized by constancy-percentage, average cover and representation of target species. A matrix was created to find intersections between quadrat groups defined by classifying life form and floristic data sets. This process led to the identification of new quadrat groups that share similar life form and species composition. The full dataset can be found in [47].
Results
M. crassifolia is most widely distributed in Beirut; based on our field surveys its presence was confirmed in 73 sites of which only one site, Pigeon Rock (Site 17), is protected by law, and another site, the limestone cliff facing Pigeon Rock (Site 16), is almost inaccessible and may be considered de facto protected. The remaining 71 sites offer highly diverse habitats and are not protected [47]. In remnant semi-natural sites M. crassifolia is found in spiny Mediterranean heaths, screes, sea cliffs and rocky offshore islands, growing on both sandstone and limestone formations and on (stabilized) coastal sand dunes. In anthropogenic sites, it grows near open sewers, in abandoned dump sites, through cracks in concrete walls and asphalt, on heaps of gravel, in street medians and on two occasions, almost epiphytically, out of the trunks of date and fan palms. The species’ tendency to utilize modified habitats reflects its partial behavior as a ruderal [36]. Over the course of the study, M. crassifolia was lost in 20 sites to urban development including one site which harbored the largest clump counts. Only four of these sites were recolonized during the course of the study. The extent of this species in Beirut was reduced by 800 m as a conserquence of this loss which accounts for a decrease of 17% in the plant’s range in the city over a period four years [47].
We recorded the presence of 124 plant species belonging to 40 families and 107 genera in the 78 sampled quadrats [47]. Plant species co-occurring with M. crassifolia shown in Table 2 include 16% non-native species (Table 2).
Analysis of floristic data by TWINSPAN clustered the 78 quadrats into 17 quadrat groups labeled F-A to F-Q (Table 3). M. crassifolia had the highest constancy and abundance in groups F-D, F-G and F-I. In contrast, groups F-C, F-F, F-K, F-M, F-N, F-O, F-P and F-Q completely excluded this species.
The low community similarity, patchy species distribution, and predominance of habitat non-specific species reported by [55] in their study of the floristics of the Lebanese coast was confirmed in this study. High floristic variability between and within different sites resulted in a large number of groups (58.8%) consisting of no more than two quadrats. Only one group (F-E) consisted of a large number of quadrats and represented a perceptible community of sparse vegetation on sandstone outcrops. Other groups were not site specific, but included quadrats exposed to similar disturbance; for example, in group G the nine quadrats were sampled from street medians and side walks and consisted of a combination of evergreen exotic ornamental species such as Agave americana, A. attenuata and Lampranthus multiradiatus. Similarly, F-T included quadrats characterized by a high representation of graminoids Cyperus rotundus and Cynodon dactylon which often grow in gardens and street medians under and around evergreen ornamentals such the shrub Pittosporum tobira and the creeping herbaceous forb Sphagneticola trilobata.
One problem we encountered with florsitics based TWINSPAN analysis is that many groups did not represent actual communities i.e. plant species found in an area are unique and capable of coexisting as distinct, recognizable units that are repeated regularly in response to biotic and environmental variations [67, 68, 69,70,71]. For example, group F-E, which included about 28% of sampled quadrats, consisted of several distinct vegetation assemblages that occur in different habitats, both semi-natural and anthropogenic, and the target species, a stress-tolerant ruderal, was the only common indicator species between these assemblages.
Life form description of plant species yielded 55 different life forms (Table 4).
Results revealed that more than half of all recorded species were therophytes with a total of 64 autotrophic therophyte and two heterotrophic annual vascular parasites. The high representation of therophytes reflects high disturbance of study sites [36]. Fig 1 presents the life-form spectrum defining the associates of M. crassifolia. Chamaephytes constituted the most prominent perennial life form and included 24 species. Over half of all chamaephytes were either regional or national endemics and only three were not native. Phanerophytes were represented by 14 species, 10 of which were not native. Perennials characterized by a periodic shoot reduction were represented by 15 hemicryptophytes and six geophytes.
Classification of data according to life form in 11 quadrat groups. M. crassifolia was highly represented in two of these groups a percent cover greater than 75% in 81-100% of quadrats within these groups (Table 5). Examples of life forms in these groups include, unbranched dwarf palm like trees (Phaner08), typical and tall evergreen dwarf-shrubs (Chamae03 & Chamae04), low reptant evergreen succulents (Chamae14), tall drought-deciduous hemicryptophytes (Hemicr01) and small reptant evergreen hemicryptophytes (Hemicr03) were common. Ornamental examples of these life forms include Agave and Yucca species (Phaner08), cultivated Sea Lavender species (Chamae03 and Chamae04), and Lampranthus multiradiatus (Chamae13).
Five groups excluded the target species and the dominant life form in these groups was mostly phanerophytes. These include mesophyllous large evergreen trees with spherical crown restricted to their upper half (Phaner01), mesophyllous normal-sized evergreen shrubs with spherical crown extending to near their base (Phaner04), microphyllous normal-sized evergreen shrubs with spherical crown extending to near their base (Phaner03), and mesophyllous tall deciduous shrub with spherical crown extending to near the base of the shrub (Phaner07). Ornamental examples of these life forms include various shade trees (Phaner01), and shrubs used as hedges such as Pittosporum tobira (Phaner04 and Phaner03).
Other groups that excluded the target species consisted mostly of typical evergreen reptant herbaceous chamaephytes (Chamae12). Ornamental plant species belonging to this life form and similar life forms include turfgrass species and the Singapore Daisy, Sphagneticola trilobata.
In Table 6 below, we integrated floristic and life-form classification results along into a single matrix by identifying common quadrats intersecting both classifications. Using this stepwise approach we generated a new set of quadrat groups which included quadrats that shared similar life form and species composition. To assess the relevance of these newly generated groups to M. crassifolia prevalence, we calculated constancy and abundance of M. crassifolia within each group. This stepwise approach generated 30 quadrat groups, 8 which were highly favorable to M. crassifolia, and 12 which excluded it. We then proceeded to describe life form and species prevalent in these groups.
Quadrat groups of high representation of target species formed by the intersection of both floristic and life form data classifications are listed in Table 7. The intersections that resulted in quadrat groups with the highest representation of the target species belonged to 4 out of 11 quadrat groups that were derived from the classification of the life form data set (L-C, L-D, L-E and L-F) and 4 out of 17 quadrat groups that were derived from the classification of the floristic data set (F-A, F-D, F-G and F-I).
Quadrat groups that excluded Matthiola crassifolia formed by the intersection of both floristic and life form data classifications are listed in Table 8. The intersections that resulted in quadrat groups with the highest representation of the target species belonged to 8 out of 11 quadrat groups that were derived from the classification of the life form data set and 11 out of 17 quadrat groups that were derived from the classification of the floristic data set.
Discussion
Floristic surveys are one of two main vegetation description methods used to obtain baseline data on native species of conservation interest and to generate community classification schemes and structure patterns that vary predictably in response to external factors such as environmental stress and disturbance (Table 1). Floristic method uses taxonomic identification and species abundance to describe vegetation. From the perspective of floristics, plant species found in an area are unique and capable of coexisting as distinct, recognizable units that are repeated regularly in response to biotic and environmental variations [67, 68, 69, 70, 71]. The other method, physiognomy, describes vegetation according to external morphology, life form, stratification, and size of each species. There is a consensus that physiognomic and physiological characteristics of plants, including species life-history strategies and population biology, are also important descriptors of vegetation communities [72, 36, 73, 74, 75, 76, 77, 78, 79]. Using either of these methods is a basic step necessary for understanding optimal plant habitats for species of conservation interest [18]. For example, plant communities were characterized to determine suitable habitats for rare species, e.g., [58] and for ecologically and economically important species [80]. Such studies, however, are mostly conducted in natural habitats, and in many instances, deliberately exclude disturbed areas from sampling [80]. In cities, plant habitats are disturbed, and vegetation communities often remain at early successional stages due to this disturbance. Mediterranean cities, where plant diversity and endemism are high, can offer a prospective refuge regardless of whether urban plant habitats are semi-natural or anthropogenic [81]. Plant diversity in Mediterranean cities has been assessed through floristic surveys in Greece, Italy and Spain [82, 83, 84, 85, 86, 87, 88]. The impact of Mediterranean cities on this diversity can be estimated as an extinction debt explained by the city’s current proportion of urban native vegetation and its historical development [57].
One aspect of urban vegetation that challenges field data analysis is the abundance of ruderal plant species which benefit from the absence of interspecific competition that would otherwise occur in later successional stages and colonize bare and disturbed land [36]. By spreading from nearby semi-natural vegetation, ruderals contribute to high variability in urban plant diversity, even between close sites, limiting the value of vegetation description using floristic methods [18]. Some of these ruderal species may be distantly related to agricultural weeds and others to plant species found across transportation networks [24]. The similarity in the infrastructure of a city may explain homogeneity of these urban ruderal species, which out-compete sown species [89]. For example, a 30-year green roof study concluded that spontaneous colonization should be accepted and considered as a design factor; and regional plant communities could serve as a model for seed recruitment and installations [89]. Ruderals are also populating green walls in cities [90]. The peculiarity of our study is that, not only is data analysis influenced by ruderals but the species of conservation interest M. crassifolia also behaves as a ruderal. Considering the diversity of habitats the species of conservation interest occupies, it was not possible to resolve this lack of location specificity with floristic assessments, which in turn did not allow us to develop an understanding of urban habitat analogs. Instead, the number of quadrat groups generated by the floristic analysis was large, and some of these clusters did not represent actual plant community assemblages.
Classifying life form data by including percent cover for each category helped specify which life forms and their respective abundance were positively or negatively associated with M. crassifolia. Our findings are in line with Kent [18], who emphasized that physiognomy might be more useful as a tool than floristics in highly modified habitats at different scales due to the responses of plant species to macro- and micro-climate conditions. Life history and life form are stronger predictors of underlying population processes than native status and can help explain allelopathic potentials [91, 92].
By using a stepwise approach which combines the two methods, floristics and physiognomy, we were able to minimize the masking effect of ruderal species and to identify life form similarities within distinct vegetation assemblages. In the last decade, researchers have combined life form and floristic vegetation description methods to overcome difficulties in analyzing data in disturbed habitats. For example, Vestergaard [93] generated quadrat groups based on floristic data through TWINSPAN and then described the life-form spectra in each to investigate the relationship between plant diversity and artificial dune development processes. Although similar to our methodology, Vestergaard did not use this combined methodology to define habitat analogs for target plant species. In 2014, a new vegetation classification approach that relies on both physiognomy and floristics over large areas was published under the name EcoVeg [19]. Our approach, however, differs from EcoVeg in that we first mathematically classify physiognomic data and later sort the classifications according to a specific floristic trend. In addition, we base our study on field data collected from small urban habitat sites while EcoVeg uses map data and is meant to classify vegetation over large natural landscapes. On the other hand, our stepwise approach can be integrated as a potential field verification tool with a recent technique proposed by [94] “light detection and ranging (LiDAR) data and model selection techniques.” LiDAR was developed to facilitate the management of urban vegetation for biodiversity conservation by determining potential locations for habitat analogs in cities through the relationship between the extent and vertical structure of urban vegetation.
The information we generated using a stepwise approach integrating floristics and physiognomy, may serve as blueprints for planting designs; it offers a plant selection palette that is not restrictive and does not enforce a native only policy. The urban habitat analogs that we identified include green spaces dominated by palms, low-lying succulents, or shrubs with scale-like leaves. In contrast, the species does not seem to persist in green spaces dominated by turf grass, canopy trees, or vegetation that produces a significant litter.
Furthermore, since knowledge of a target species’ preferred physiognomies includes an understanding of its position in the vertical stratification of its ecological community [18], we were able to identify additional habitats suitable for the introduction of M. crassifolia. Our findings revealed that the species could also thrive as part of the low shrub layer under taller nanophyllous shrubs like the Shaggy sparrow-wort, Thymalea hirsuta, in the understory of tuft-trees like the fan palm, Washingtonia robusta, and within groves of the giant reed, Arundo donax. Species belonging to these life forms, or similar ones, dominate many sites in Beirut including street medians and could serve as favorable habitats for M. crassifolia. Our findings also show that some exotic invasive species impacted M. crassifolia positively. M. crassifolia grew in sites dominated by Carpobrotus edulis, a potentially invasive in Lebanon, planted at the edge of pedestrian paths. Pedestrians avoided stepping onto these areas, maybe due to their appreciation of C. edulis as an evergreen ground cover [95]. As a result, this plant assemblage protected M. crassifolia and allowed C. edulis to spread constrained by water availability. Removal of invasive plant species should be determined based on its impact on endemic and rare vegetation present in a given region, and eradication should focus on those invasive species that compete with endemic species in general and those of conservation interest especially [96]. Huenneke and Thomson [97] suggest criteria for determining whether such species pose problems for specific rare native taxa and indicated the possibility that some species may be beneficial to endemics. Equipped with the findings above, landscape designers, architects, and managers can better reconcile between desired conservation targets and, socio-behavioral, and aesthetic outcomes by including M. crassifolia in an aesthetically pleasing setting. They can design urban habitat analogs that promote the persistence of M. crassifolia by excluding from the plant palette native or non-native species belonging to life forms associated with its low representation as reported in this study. Alternatively, they can design an urban habitat analog using a vegetation architecture conducive to the persistence of M. crassifolia. In the case established green spaces, they can manage the space to become suitable for M. crassifolia by selectively removing species with a life form that is incompatible or that restricts its abundance. In some situations, horticultural techniques, such as pruning, can modify the micro environment without changing species existing on site, to create suitable urban habitat analogs; for example, improving light conditions in cases where species of conservation interest is shade intolerant.
Conclusion
Given the rate of expansion of urban landscapes [98, 99, 100, 101], increasing species’ site area in a city is highly desired [4]. Our findings can serve as guidance on how to create or modify, through landscape planting designs, suitable habitats for species of conservation interest. By understanding the physiognomy and structure, and environmental conditions in which a species occurs, green areas may be designed to suit the requirements of a target species while established areas may be surveyed for candidate sites suited for the introduction of a target species. Our stepwise approach offers a detailed field assessment tool for urban plant habitat analog characterization.
Acknowledgements
This paper is derived from the dissertation submitted by M. Itani in partial fulfillment of the requirements for the MSc. degree at the American University of Beirut. We thank Drs. R. Zurayk, N. Farajalla, and K. Knio for their inputs throughout the study. We thank K. Mohamed, A. Jammool, S. El Masri, O. El Tal, R. Atallah and N. Halabi for their in field data collection.
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