Woodland Indicator Species Research Articles

Ancient woodland indicator species: can old herbarium specimens supplement recent records to inform ecological management?

Old herbarium specimens have become increasingly well-recognised as a rich source of ecological baseline data. For long-continuity plant communities, such as ancient woodland, these records may be particularly important for present day ecological management. To evaluate this potential, searches for pre-1950 Ancient Woodland Indicator (AWI) herbarium specimens collected in East Gloucestershire, UK, were conducted using digital open access sources and the physical Royal Agricultural University herbarium. In total 305 specimens were retrieved from twelve herbaria, with small regional collections being particularly important sources. The earliest specimen dated to 1834. There was a significant association between old specimen availability and year of collection, due to a peak in the late-1800s and early-1900s. Over half of the AWI species for the region were represented, although some taxonomic bias was evident. To determine if old AWI specimens contributed any new location records, 246 unique specimens with detailed georeferences were mapped and compared to the locations of 1950-1999 and 2000-2021 biological records. One third of the pre-1950 specimens had not been recorded in the same locality since collection of the old specimen, indicating either a gap in recent records or floristic change. However, length of time since specimen collection was not a predictor of a 1950-2000 or 2000-2021 record in the same locality. Overall, it is highly recommended that policy-makers, land managers, and field surveyors consult old AWI herbarium records for ancient woodland identification, management, and restoration.

Can epiphytic lichens of remnant Atlantic oakwood trees in a planted ancient woodland site survive early stages of woodland restoration?

Key messageEpiphytic lichens of remnant Atlantic oakwood trees, enclosed within a recently planted conifer matrix, show ability to survive early stages of woodland restoration (conifer removal).ContextAtlantic oakwood, ancient semi-natural woodland (ASNW), supports important epiphytic lichens. Fragmented ASNW, historically in-filled with conifers, are now being restored to reflect ASNW tree and ground flora character. Concerns exist that sudden and total removal of the conifer matrix will be detrimental to the epiphyte diversity of remnant trees retained within the former plantation.AimsHere, we ask whether an unintended consequence of habitat restoration is the loss of epiphyte populations on remnant trees.MethodsDynamics of ground flora development were studied at one 50-ha site on the west coast of Scotland using indicator species occurrence and species traits. Change in cover of lichen species was determined and lichen vitality was assessed in two Lobaria species using chlorophyll fluorescence as a proxy. Assessments pre-, post- and nine years after conifer removal were made in plantation areas (containing remnant oak trees) and ASNW areas.ResultsRe-vegetation of the ground flora was predominantly by ASNW vegetation. Species richness and occurrence of native woodland indicator species increased and the community showed stronger competitor traits. Lichen vitality was initially reduced but recovered. Tests showed change in the abundance of key lichen species and lichen community diversity was non-significant despite the loss of four lichen species on remnant trees.ConclusionGround flora dynamics indicate site recovery was underway within eight years of restoration activities and epiphytic lichens although variable in response were in this study largely unaffected, this restoration approach could be appropriate for other Atlantic oakwoods where lichen conservation is an objective.

Initial plant community responses to hardwood control treatments in restoration of remnant longleaf pine (Pinus palustris) woodlands

Changes in land use over the past century have contributed to substantial losses of longleaf pine (Pinus palustris) woodlands in the southeastern USA and replacement with higher density, mixed pine and hardwood stands that suppress understory development and limit application of prescribed fire. To increase understanding of limiting factors and identify potential approaches for restoring longleaf pine woodlands, we studied initial availability of light and soil water and 2-year (2018–2019) plant community responses after controlling overstory hardwoods in five remnant longleaf pine woodlands having no evidence of previous agriculture at the Savannah River Site near Aiken, SC, USA. Seven hardwood control treatments and a non-treated check were compared in a randomized compete block experiment: cutting, cutting + shredding of logging residues, stem injection with imazapyr herbicide, cutting + basal spray with imazapyr herbicide, cutting + basal spray with triclopyr herbicide, cutting + directed foliar spray with a mixture of glyphosate and imazapyr herbicides, and cutting + broadcast foliar spray with the same herbicide mixture. In the year prior to hardwood cutting (2016), understory light availability averaged 23% of full sun. Throughout the year of treatment installation (2017), average soil water content (SWC) was below permanent wilting point (5.5% SWC) where overstory hardwoods were retained; whereas, it was above wilting point (7.5%) where they had been cut. Thus, combined effects of shade and root competition from overstory hardwoods probably limited cover of herbaceous species in the non-treated check. In the 2 years following treatment, hardwood survival averaged > 99%, 62%, 42% and < 1% for the non-treated check, cutting, shredding, and herbicide treatments, respectively. Relative to the non-treated check, herbaceous species richness was increased by shredding, stem injection, basal spray, or directed foliar treatments; herbaceous cover was increased by basal spray or directed foliar treatments; and woody cover was decreased by directed or broadcast foliar treatments. The directed foliar spray was the most effective treatment for achieving many of the desired understory characteristics of a longleaf pine woodland, including a diverse understory dominated by herbaceous vegetation capable of supporting periodic prescribed fires. Although the imazapyr basal spray treatment increased cover of remnant woodland indicator species, none of the treatments increased richness of this species group, suggesting that restoration of native species composition will be expedited by enrichment seedings or plantings.

Colonisation of farmland deciduous plantations by woodland ground flora

ABSTRACT Newly planted broadleaved woodlands under agri-environment schemes lack floral diversity. The dispersal range and colonisation rates of herbaceous ground flora in thirty newly planted broadleaved woodlands in Northern Ireland were investigated during spring and early summer (2017). The colonisation rates of 14 species conventionally recognised as Ancient Woodland Indicator Species (AWIS) and Woodland Indicator Species (WIS) together with Lesser Celandine were assessed and the maximum dispersal distances from potential source populations was measured. The study found that the nine selected AWIS colonise new habitats significantly more slowly (p < 0.001) than the four woodland study species and colonisation rates corresponded approximately to the classifications AWIS and WIS. The effect of twelve selected habitat variables on species colonisation rates revealed that greater species dispersal range was associated with the management of the sources both within and bordering to the newly planted broadleaved woodlands. Ensuring connectivity to mature deciduous woodlands with an integrated, riparian zone together with a mosaic of inter-connected, species-rich hedges increased species dispersal. These results suggest succession from grassland into woodland only begins with the planting of trees. Colonisation of woodland plant species should be a key objective of establishing newly planted broadleaved woodlands within farmland afforestation schemes and should be facilitated by active management practices.

The effect of hedgerow density on habitat quality distorts species-area relationships and the analysis of extinction debts in hedgerows

ContextHedgerows are highly important for maintaining the biodiversity in deforested landscapes. Especially for habitat specialists such as several forest plants they can provide important refuge habitats.ObjectivesThis study aims to examine whether there is an extinction debt for forest plants in hedgerows.MethodsIn a study area in Northern Germany that had lost 47% of the hedgerow network over the past 120 years, hedgerows were mapped for the presence of forest vascular plants. In a multi-model approach, we compared the explanatory power of present and historical landscape variables and habitat quality on diversity patterns.ResultsThe change in landscape configuration had no effect on the species richness of forest plants in hedgerows, i.e. there was no sign of an extinction debt. The best explanatory variable was the hedgerow width with more species found in wider hedgerows. This demonstrates the importance of including local habitat variables in the study of extinction debt. For ancient woodland indicator species models including both the landscape configuration and habitat variables were superior to simple models. The best models included the historical distance to the nearest forest, suggesting an extinction debt. Counterintuitively, a high density of hedgerows had a negative influence on species richness, most likely because hedgerows are narrower in areas with higher densities due to land-saving measures by farmers. There was also a negative correlation between hedgerow density and the hedgerow proximity to forests.ConclusionsThe effects of important covariates may obscure species-area relationships and undermine extinction debt analyses.

The effect of game management on the conservation value of woodland rides

In recent decades open areas, created by natural disturbance or human activity, have been lost from many woodlands. Consequently, species that rely on the unique abiotic conditions provided by these open areas, including species characteristic of ancient woodland, have become of conservation concern. In woodlands managed for driven pheasant shooting linear open spaces (rides) are maintained. However, previous research has shown that game management can have negative effects on woodland communities, particularly within the area of pheasant release pens. It may be that rides in game woods do not support the same species as open areas in other woods. We assessed the effect of game management on the structure and management of rides, on the ground flora and shrub communities, and on butterfly abundance of these rides. Rides in game woods were wider and more open. Although disturbance by vehicles was higher in game woods, overall habitat disturbance was higher in control woods. These differences in physical attributes, combined with the direct effects of pheasants (including soil nutrient input), explained the differences observed in the ground vegetation of rides in game woods. For example, there were more fast-growing ruderal species and less bare ground in game woods. Although the richness of shrub species was also affected by game management, the direction of the effect depended on region. Overall, game management did not have a consistently negative effect on species of conservation concern, with the abundance of butterflies and richness of ancient woodland indicator species unaffected by game management. We suggest that the effect of game management on ride communities is benign and that factors other than those directly examined in this study (such as deer browsing) also need to be considered by woodland managers.

Lack of herbaceous layer community recovery in postagricultural forests across three physiographic regions of New York

Agricultural land-use legacies remain persistent determinants of plant-community composition, despite extended periods of recovery in postagricultural forests of New York. We surveyed the composition of vascular plant species for 33 paired residual (unconverted) and secondary (postagricultural) forest sites across three physiographic regions in New York to assess the recovery of residual forest species in secondary successional stands. There were 169 vascular plant species recorded among the 33 study sites. The residual stands exhibited slightly greater mean (± SE) species richness (26.8 ± 1.7 species per site) than secondary-interior (21.4 ± 1.2, P = 0.001) and secondary-edge (24.1 ± 1.6, P = 0.02) zones. Plant communities differed across the three physiographic regions (P = 0.001) and by stand history within regions (P = 0.004). Of the vascular plant species occurring at our sites, 22 (13%) were indicators of residual forest stands and 27 (16%) were indicators of secondary, postagricultural forest stands. Within 10 to 25 m of the historic field-forest ecotones, the relative frequencies of many residual woodland indicator species fell to 0–20% of the maxima they attained in the residual stands. Ant-dispersed (myrmecochorous) taxa represented 23% of residual and 0% of secondary forest indicators and exhibited lower migration rates (P < 0.001) than plants possessing other diaspore dispersal modes (anemochory, zoochory, barochory). In addition to plant dispersal mode, we found pH and forest-floor depth to be important determinants of plant species. Our results indicate that impoverishment of plant communities in postagricultural forests represent a regionally pervasive phenomenon with important resource management implications in the eastern USA. Residual woodlands remain important source populations for recovery of herbaceous species in postagricultural landscapes. However, given the slow historic recovery rates that we and others have observed in postagricultural secondary forests, in addition to other contemporary factors potentially inhibiting dispersal and establishment (e.g., herbivory, pollinator disruptions, invasive species), it seems likely that active restoration efforts are needed to promote full recovery of diverse, native temperate-forest herb communities.

Effects of clear-felling versus gradual removal of conifer trees on the survival of understorey plants during the restoration of ancient woodlands

Plantations of exotic conifers on ancient woodland sites retain many features of high conservation value and in many countries such sites are being actively restored. Clear-felling of the conifer trees is often a straight-forward first step, but recent guidance on restoration advocates gradual removal of the plantation crop to protect vulnerable woodland specialist plants. To date there has been little empirical evidence to support this policy. One hundred and four stands in 39 woodland sites in the UK owned by the Woodland Trust from Cornwall to Invernesshire were surveyed in 2001 and again in 2009 to assess the survival of woodland flora following restoration by both clear-felling and gradual removal of the plantation crop. On average, there were over seven woodland specialist and ancient woodland indicator species in each of the stands surveyed. Loss of specialist woodland species – the most shade adapted component of the flora – over this time period was strongly correlated with the degree of canopy opening. In contrast, the loss of more light demanding plant species was not related to the intensity of tree removals. This study suggests that a careful assessment and prioritisation of conservation values is needed before restoration. Rapid removal of an exotic species as part of a restoration programme may have an adverse effect on subsequent succession.

Ancient woodland indicators signal the climate change risk for dispersal-limited species

The climate change risk to biodiversity operates alongside a range of anthropogenic pressures. These include habitat loss and fragmentation, which may prevent species from migrating between isolated habitat patches in order to track their suitable climate space. Predictive modelling has advanced in scope and complexity to integrate: (i) projected shifts in climate suitability, with (ii) spatial patterns of landscape habitat quality and rates of dispersal. This improved ecological realism is suited to data-rich model species, though its broader generalisation comes with accumulated uncertainties, e.g. incomplete knowledge of species response to variable habitat quality, parameterisation of dispersal kernels etc. This study adopts ancient woodland indicator species (lichen epiphytes) as a guild that couples relative simplicity with biological rigour. Subjectively-assigned indicator species were statistically tested against a binary habitat map of woodlands of known continuity (>250yr), and bioclimatic models were used to demonstrate trends in their increased/decreased environmental suitability under conditions of ‘no dispersal’. Given the expectation of rapid climate change on ecological time-scales, no dispersal for ancient woodland indicators becomes a plausible assumption. The risk to ancient woodland indicators is spatially structured (greater in a relative continental compared to an oceanic climatic zone), though regional differences are weakened by significant variation (within regions) in woodland extent. As a corollary, ancient woodland indicators that are sensitive to projected climate change scenarios may be excellent targets for monitoring climate change impacts for biodiversity at a site-scale, including the outcome of strategic habitat management (climate change adaptation) designed to offset risk for dispersal-limited species.

Identifying the trait syndromes of conservation indicator species: how distinct are British ancient woodland indicator plants from other woodland species?

AbstractQuestionAncient woodland indicator species (AWIs) are plant species which are thought to be restricted to areas of long‐continuity woodland habitat. In many cases, however, these species have been identified on the basis of personal, to some extent, subjective experience. Do the species proposed as AWIs according to these lists have traits in common, and how distinct is their trait profile from that of other woodland plant species?LocationUnited Kingdom.MethodsWe applied classification tree analysis to a plant trait database to assess the extent to which proposed AWI species can be clearly separated from other woodland plants based upon their traits. We contrasted AWI species with an objectively defined list of plants that are not considered to be AWIs but that have been commonly recorded in woodlands. We also investigate the effects of phylogeny and region specificity on species' proposed AWI status.ResultsThe results provide support for the distinctiveness of plant species thought to be associated with ancient woodland; they were found to be almost exclusively short, perennial species, usually with a high seed weight. Results also indicate that rarer AWIs have a more distinguishable trait profile than more common species. No link was found between phylogeny and AWI status.ConclusionsAWI species do have a distinguishable trait profile, despite their often partially subjective selection. The results of the classification tree analysis suggest that traits reflecting poor dispersal ability may be partly responsible for confining these species to ancient woodlands. This confirms other studies that emphasize their low ability to colonize secondary woodland sites and hence vulnerability to habitat conversion.

Mammalian herbivores as potential seed dispersal vectors in ancient woodland fragments

Europe's remaining ancient woodland is highly fragmented and many specialist woodland plants persist as isolated relictual populations. Their apparent dispersal limitation and failure to colonise more recently established secondary woodlands may reflect a loss of vectors and mechanisms for dispersal. This is in contrast to long‐distance dispersal events evidenced both by paleoecology and some contemporary observations. Although increasing populations of deer are recognised as important dispersers of plant seeds, particularly for species with no apparent dispersal mechanism, the potential for deer to disperse ancient woodland plants has not been studied previously in the UK, where remaining ancient woodland fragments are frequently surrounded by arable landscapes. Viable seed content of 616 faecal samples collected from a guild of mammalian herbivores; red deer Cervus elaphus, fallow deer Dama dama, roe deer Capreolus capreolus, reeves' muntjac Muntiacus reevesi and brown hare Lepus europaeus, over five months and from four coppiced ancient semi‐natural woodlands in eastern England, was assessed. Following cold stratification and sowing in a controlled greenhouse, 502 seedlings of 41 species germinated. Three species, constituting just 1.8% of the total individual seedlings recorded, were characteristic of woodland habitats, including only one ancient woodland indicator species, wood speedwell Veronica montana. Germinated plant species were instead characteristic of non‐woodland habitats including grassland, arable and ruderal communities. The four most abundant species were the widespread grass common bent Agrostis capillaris, the ruderal greater plantain Plantago major and the two crop species, wheat Triticum spp. and rape Brassica napus. Mammalian herbivory in these ancient woodland fragments provided few dispersal benefits for the woodland plant community. Instead, larger free‐ranging herbivores transported large volumes of propagules of ruderal agrarian and open‐habitat species from surrounding habitats into ancient woodland fragments.

The Analysis of the Forest Flora of the Strzyżowskie Foothills from the Perspective of Presence of Anthropogenic Species

The anthropogenic pressure of the forest communities of the Strzyżowskie Foothills (the Western Carpathians) was estimated on the basis of phytosociological materials. Carr communities are among the most threatened by invasive and alien species of the forest flora. The phenomenon of invasiveness of native species such as Calamagrostis epigejos or Carex brizoides was described. The highest number of ancient woodland indicator species was noted in the beech forest.

Aliens or natives: who are the ‘thugs’ in British woods?

The invasion of native habitats by exotic, or alien, plant species has received considerable attention recently from policy, research, and practical conservation management perspectives. However, a new hypothesis for species dynamics in Britain suggests that a small number of aggressive native plant species (termed ‘thugs’) may have an equal, or greater, impact on native species and habitats than exotic species. Here, we examine this hypothesis using multivariate techniques with field-layer cover data collected during a country-wide survey of British woodlands. Multivariate analysis of these data identified a north-south gradient on the first axis, and that 20 of the 25 National Vegetation Classification woodland types were sampled within the study. The most abundant field-layer species included three of the proposed native ‘thugs’, i.e. Rubus fruticosus, Pteridium aquilinum and Hedera helix in addition to the native woodland indicator species Mercurialis perennis. Variation partitioning was used to compare the relative importance of native field-layer ‘thug’ species with invading alien shrub and tree species relative to other environmental drivers. The variation in the field-layer data-set explained by the three native ‘thug’ species was significant, but they explained a relatively small proportion of the variation relative to other environmental variables (climate, soil, management factors etc.). They did, however, explain almost four times as much variation as the three alien species that were significantly correlated with field-layer species composition (Acer pseudoplatanus, Impatiens glandulifera, Rhododendron ponticum). The results of this analysis suggest that the field-layer of British woodlands is impacted as much by native ‘thug’ species, as it is from ‘aliens’. Concern about the impact of these native ‘thug’ species has been reported previously, but their impact has not previously been compared to the impact of invading aliens. It is hoped that this analysis will do two things, first to act as a sound baseline for assessing any changing balance that should occur in the future, and second, to prompt both ecologists and conservationists to develop woodland management policies based on sound science.

The Modification of Ancient Woodland Ground Flora at Arable Edges

The last five decades have witnessed an intensification of agriculture with a marked increase in the use of agrochemicals. This study investigated whether arable edges affect ancient woods and the extent of any modifications to the ground flora. Plant species were recorded from a total of 84 transects into the centre of the woods, of which 59 were taken from the arable–woodland interface and 25 were taken from non-arable land or woodland rides. Ellenberg indicator and CSR values were used to infer fertility and features of the environment. Results suggested that, after adjusting for light and moisture conditions, elevated nitrogen levels might occur in ancient woodlands to a distance of at least 100 m from arable farmland. Furthermore, the transects from the arable fields had a substantially altered herb-layer with respect to species composition, notably a greater percentage cover of ruderal and nitrophilous species, and a lower cover of stress tolerator species. Smaller woods contained a greater cover of competitor and nitrophilous species and a lower cover of stress tolerators and ancient woodland indicator species. We conclude that farming may be unwittingly modifying the ground flora of ancient woodlands adjacent to arable land and that small and irregularly shaped woods are most at risk. It is recommended that farmers apply agrochemicals in such a way that drift is minimised, and that large, valuable ancient woods be protected by a substantial buffer zone.

The effects of phosphorus nutrition and soil pH on the growth of some ancient woodland indicator plants and their interaction with competitor species

The forest under-storey herbs Anemone nemorosa, Lamiastrum galeobdolon and Veronica montana are generally considered indicator species of old, broadleaved woodland sites where the soil fertility is often low. In a glasshouse bioassay, however, all three species not only showed large positive growth responses to supplied P concentrations (0–10 mg L −1) solutions, but also tolerated high P concentrations (20–40 mg L −1), well above those normally found in their natural habitat. Plants responded by raising the concentrations of P in their shoot and root tissues and increasing their biomass, resulting in an increased P uptake. A shade-tolerant competitor species, Urtica dioica, also grew vigorously across the full range of P concentrations, restricting the growth of the woodland species. This emphasises the difficulty of establishing semi-natural woodland vegetation in the presence of competitor species, for example in situations where new woodlands are planted on fertile ex-agricultural soils containing large residual concentrations of P. The influence of soil pH on the growth and nutrient relations of A. nemorosa, L. galeobdolon, V. montana, Poa trivialisandU. dioicawas determined in a separate experiment using an ex-arable soil as the growing medium with pH levels adjusted from 7.4 to 5.8 and 4.3 respectively. Acidifying the soil enhanced growth, but reduced the concentrations of N, P and K in the leaves of all three woodland species, probably due to dilution of these minerals in the increased dry matter production. The competitor species (P. trivialis and U. dioica) responded in similar manner to the woodland indicator species. These results suggest that manipulating soil pH as a means of facilitating the establishment of woodland indicator species in new farm woods is unlikely, in the short term, to be effective where competitor species are present.

Effects of area and isolation of woodland patches on herbaceous plant species richness across Great Britain

Richness of Ancient Woodland Indicator plant species was analysed in 308 woodland patches that were surveyed during the Countryside Survey of Great Britain carried out in 1998. The Countryside Survey recorded vegetation plots and landscape structure in 569 stratified 1 km sample squares and developed a remotely-sensed land cover map of the UK. Using these datasets, we tested the hypothesis that Ancient Woodland Indicator species richness in woodland fragments was limited by patch area, shape and spatial isolation and that woodland patches located in the lowland region of Great Britain would respond differently than those in the upland region. The variation in Ancient Woodland Indicator species richness in the British lowlands (n = 218) was mainly explained by patch area and two measures of connectivity, the length of hedgerows and lines of trees in the 1 km square and the area of woodland within 500 m of the vegetation plot. By contrast, variation in Ancient Woodland Indicator species richness in the British uplands (n = 90) was related to Ellenberg scores of the vegetation communities sampled – a surrogate for habitat quality – and no significant effect of spatial structure was detected. It therefore appears that the degree of fragmentation of woodland in the British lowlands limits the distribution of Ancient Woodland Indicator species, while in the uplands, failed colonisation is a matter of habitat quality rather than a result of landscape structure.

Atlantic Hazelwoods – a neglected habitat?

Summary Atlantic Hazelwoods form a distinctive (yet apparently over-looked) habitat that appears to be unique to western Scotland and, to a lesser degree, western Ireland. The origins of this habitat appear to date from the early Holocene, and there is compelling evidence that some stands may have existed in unbroken succession for 10,000 years. Recognition of the potential relict status of some areas of hazel gradually emerged as a result of several decades of study of the lichen flora. The smooth bark of hazel in western Scotland is known to support a species-rich and specialised lichen community (the Graphidion), including rare, endemic and old woodland indicator species. However, the full expression of this community is found to occur only in certain stands: often coastal, with no (or very few) other trees or shrubs present, and with a long history, but with little evidence of intensive exploitation. Seeking further evidence to back up the relic status of these Atlantic Hazelwood stands proved elusive...

A study of species richness and diversity in seed banks and its use for the environmental mitigation of a proposed holiday village development in a coniferized woodland in south east England

A survey was carried out to determine the density and species composition of germinable seed in the surface soil layers of 30 plots within a coniferized lowland woodland in East Kent in order to establish the resources available for habitat creation in the event of some areas being modified during a proposed holiday village development. The selected plots included conifer plantations (up to 69 years old), broad-leaved plantations and the semi-natural broad-leaved edges that remain on parts of the perimeter of the site which were used as the control. A total of 13 682 seedlings emerged from the soil samples during the four-month germination trials. Fifty-two species were identified of which eight were ancient woodland indicator species for south-east England. The most abundant species represented in the seed banks were: Juncus effusus, Rubus fruticosus, Carex sylvatica, Betula pendula and Agrostis tenuis. Between-site comparisons of coniferous plots of different ages revealed a marked reduction in the seed species and seed density in plantations over 65 years old. Results of soil nutrient and texture analyses ruled out the likelihood of edaphic factors being responsible for between-site differences in seed bank composition. Seed species richness and diversity (Shannon-Wiener diversity index) were greatest in the semi-natural broad-leaved edges, but the diversity index used also showed that two replanted conifer sites had high values despite few species being present. The usefulness and limitations of diversity indices in the context of seed bank studies is discussed. From the results of the study, management proposals for the site have been put forward in order to maintain floristic diversity and mitigate the impact of the proposed development.