Patterns of vegetation expansion during dune stabilization at the decadal scale

There is a global stabilization of coastal dunes, during which, the trajectory of vegetation expansion was rarely studied and, thus, still remains unclear. In this study, patterns and driving factors of vegetation expansion during dune stabilization were examined in three coastal dunefields in Victoria, Australia. Results show that the trajectory of vegetation expansion is fundamentally determined by dunefield topography. At the dunefield scale, vegetation colonization can take place at different parts in dunefield, including (1) the landward margins, where the ecosystem is frequently dominated by woody species and the soil is more mature with higher nutrients and water content; (2) the interdune depressions, where is usually dominated by lower elevations and shelters; and (3) the seaward deflation basins and plains, where vegetation shows a more discontinuous and patchy distribution. At the individual dune scale, vegetation usually expands vertically from the dune toe towards the upper dune slope at both the lee side and stoss face of dunes/dune ridges. The deflation basins are more likely to be free of vegetation establishment. Pre‐existing vegetation also plays a critical role in dune re‐vegetation, and subsequent vegetation expansion takes place around the vegetation patches, more usually towards the downwind direction, probably because it can provide seeds for subsequent vegetation spread. Once established, it can also change the micro‐climates by providing shelter, increasing the surface roughness and changing the wind regimes. More importantly, existing vegetation can form a ‘fertility island’ with higher nutrients, organic matter and water‐holding capacity. Conceptual models depicting the trajectory of vegetation expansion were also generalized. This research provides an original perspective and innovative insight into the patterns and driving factors of vegetation expansion during dune stabilization, under the background of global coastal ‘greening’, which could be helpful to identify opportunities for management interventions.

The spatial distribution of vegetation on dunes that are being stabilized can be highly variable on the local scale because of the heterogeneity of the vegetation community and abiotic factors (e.g., the landforms, topographic features and wind regime) in the dunefield (Zonneveld, 1999).Moreover, a dual interaction exists in the dunefield, that vegetation colonization can reshape dune morphology, and in turn, dune morphology and topography can determine vegetation succession patterns (Hesp et al., 2011).Morphologic transformations induced by vegetation growth have been investigated by field, remote sensing and modelling studies (Baas & Nield, 2007;Xu et al., 2015;Yizhaq et al., 2013), whereas it is still rarely known that how vegetation establishment and growth are regulated by dunefield topography and morphology, especially in the widespread trend of coastal dune stabilization driven by revegetation.
Coastal dunes in Victoria, southeast Australia, have been widely stabilized by vegetation expansion since the mid-20th century.However, the spatial pathways of vegetation expansion during dune stabilization remain unclear.In this study, three dunefields with representative landform features (barchans/barchanoid ridges, transverse dunes, dome dunes, parabolic dunes and interdune depressions) were selected from the Victorian coast to explore the decadal spatial changes in vegetation expansion.They have experienced similar coastal management (e.g., natural asset protection by Parks Victoria) by the state authorities over the past century, providing an opportunity to understand the dune vegetation dynamics with identical boundary conditions.This paper aims to identify (1) the patterns of vegetation expansion during dune stabilization and (2) the main factors controlling vegetation expansion within the dunefield.This research will use real-world observations from high-resolution aerial photography and satellite images to demonstrate how vegetation colonized the dunefield at the decadal scale.To the best of our knowledge, this research provides a unique and innovative perspective to examine the interaction of vegetation and other abiotic factors during dune stabilization.In addition, a better understanding of vegetation expansion in time and space in coastal dunefields could be critical for maintaining natural functioning systems and predicting/understanding long-term change.

| STUDY AREA AND CHANGES IN DUNE VEGETATION COVER
Three dunefields at Discovery Bay, Yanakie Isthmus and Cape Howe were selected to examine the patterns of vegetation expansion during dune stabilization (Figure 1).All of the dunefields are featured with maritime temperate climate (Cfb), with warm, dry summers and cool, wet winters (Köppen-Geiger climate classification) (Peel et al., 2007).
The mean annual temperature and precipitation for each dunefield are 13.9 C and 822 mm, 14.2 C and 1080 mm and 15.2 C and 919 mm, respectively, from west to east (Figure 2).Wind speed also varies and has a dominant southwest-to-west direction with seasonal changes (Figure 1) (Australian Government Bureau of Meteorology, BOM).

| Discovery Bay dunefield
The Discovery Bay dune system at the west state boarder is the most extensive and widely drifting dunefields in Victoria, with an extension towards inland of about 3 km (Figure 1c).Dunes adjacent to the beach are bare and very drifting, with scattered areas of marram grass and scrubby vegetation.Sand is featured by high PH (>9) and low nutrients without any organic matter, although zones of marram grass roots may be seen at intervals.The midland dunes are less shifting, covered with a dry scrub, such as coast wattle (Acacia sophorae), muntries (Kunzea spp.) and heaths (Leucopogon spp.).Topsoils here have a slight darkening as a result of organic matter accumulation.Dunes at the inland margin are covered originally by a tall dry scrub of coastal tea-tree (Melaleuca pubescens), and she-oak (C.stricta), which has probably been stabilized for about 1000 years.This has resulted in the accumulation of much humus and organic matter in the topsoil (© State of Victoria [Agriculture Victoria], 1996-2023).
Drifting sand had been a problem at Discovery Bay since the beginning of the 20th century.Dune stabilization through marram grass planting, brush matting and sand trap fencing had been undertaken in areas where lakes and grazing land were in danger of being engulfed by advancing drifts during 1960s-1980s (Heyligers, 1981;Sharp & Arnold, 1982).Most of the dune system has been included into the Discovery Bay Coastal Park since 1979 (Parks Victoria, 2004).Since then, livestock has been withdrawn, the number of rabbits has been controlled, and the use of recreational vehicles (buggy) in the dunefields has been restricted (Bird, 1993;Sharp & Arnold, 1982).
The selected dunefield at Discovery Bay has an area of 2939 ha (Table S1).It contains a range of morphological types from blowouts at the seaward margin, large bare and mobile barchanoid ridges and transverse dunes in the central area, and older partially vegetated parabolic dunes formed probably in the Pleistocene at the most landward boundary (Figure 1c) (Heyligers, 1981).

| Yanakie Isthmus dunefield
Yanakie Isthmus is located in the northwest of Wilsons Promontory National Park.The Isthmus has been used for cattle grazing since the 1850s, featured by extensive coastal grasslands and grassy woodlands (Morgan et al., 2018).Marram grass was planted near the cattle ranch to combat soil erosion in some small areas, although the exact location and area are hard to know (Garnet et al., 2009).The isthmus was added to the Wilsons Promontory National Park in 1969 (Parks Victoria, 2002).The regular burning, which grazers had used for about a century for stock feeding, was ceased in the early 1972, and fires are only used for fuel reduction and ecological burning in small areas since 1987 (Bennett, 1994;Parks Victoria, 2002).Cattle grazing has been banned on the Isthmus since 1992 (Bennett, 1994).Rabbit numbers on the Isthmus appear to be trending downwards, resulting from warren ripping, baiting and trapping, and recurring outbreaks of myxomatosis (Parks Victoria, 2002).Significant changes in the vegetation structure have occurred on the Isthmus since the 1980s, when large areas of the coastal grassy woodlands are replaced by dense stands of Coast Tea-tree shrubs (Bennett, 1994).
The selected Yanakie dunefield has an area of 2080 ha (Table S1).
It is a transgressive dunefield, capped by various dune types, including hummocky dunes, barchan dunes and parabolic dunes (Figure 1d).
There are mainly two large unvegetated dunefields here, the Big Drift in the central area and the Little Drift at the east along the Shallow Inlet (Figure 1d).

| Cape Howe dunefield
Cape Howe at the remote coast of far-east Victoria has been included by Croajingolong National Park since 1979(National Parks Service, 1996).It is a designated wilderness zone that can only be accessed on foot, and the area is home to ancient forests, pristine inlets, giant sand dunes and abundant wildlife (National Parks Service, 1996) (Figure 2a).Dunes at Discovery Bay and Cape Howe were bare and mobile with a lower vegetation cover (19% and 39%, respectively) in the 1960s and 1940s.By 2020, they were largely stabilized by vegetation expansion with a cover of 52% and 59%, respectively (Figure 2a; Table S1).The dune at Yanakie was partially to well vegetated with a vegetation cover of 69% in 1965, which was mostly stabilized by vegetation with a cover of 85% by 2020 (Figure 2a; Table S1).
Increase in vegetation cover and dune stability in these dunefields is driven by both climatic changes and coastal management.For example, the increasing temperature and decreasing windiness (shown by RDP) can prompt vegetation colonization and growth by prolonging the growing season and by reducing aeolian activities (Figure 2b-g) (Jackson et al., 2019;Levin et al., 2017;Xu et al., 2015).The devegetation at Discovery Bay from 2006 to 2019 may be linked to the relatively dry condition in the recent decades, especially given that less rainwater was received here than that in Yanakie and Cape Howe (Figure 2c,e,g).Coastal management, such as dune stabilization programs (e.g., marram grass planting and fencing) and protection of national/coastal parks (e.g., cessation of stock grazing and regular burning, recreation management and soil conservation), can also prompt vegetation colonization and expansion in a direct and indirect way within three dunefields.

| METHODS
Historical aerial photography    S1).Images were georeferenced and classified following the method used in Gao et al. (2022).In brief, all images were georeferenced to the GDA1994 datum, with a total root-mean-square error of <2 m for each image.Maximum likelihood classification was performed, by considering two land types-bare sand and vegetated area were defined, and classification accuracy was higher than 92% for all images (Table S1).Area of vegetated dune surface was then calculated.Spatial expansion of vegetation over the past few decades was then analysed using the post-classification comparison method.The topography of the dunefield was analysed with the help of the high-resolution DEM (1-m horizontal resolution and 0.1-m vertical resolution) derived from an airborne Light Detection and Ranging (LiDAR) image.All analysis was conducted in ArcGIS v.10.7.
Vegetation expansion took place in 1992-2006, resulting from new establishment of plants along the seaward plains and lowlands, as well as a further expansion of established patches of vegetation (Figure 3e).The last period (2006-2019) saw a slight decline in vegetation cover by 3% (Figure 3f; Table S1).
Overall, the dunefield of Discovery Bay was largely bare in 1965, whereas it was more stabilized and fragmented, which resulted from large vegetation expansion by 2019 (Figures 3 and 4).Vegetation expansion mainly took place at the back dune at the landward side (e.g., D2 and D4 in Figure 4), the interdune depressions at the central area (e.g., D7 in Figure 4) and the seaward plains and lowlands (e.g., D9-10 in Figure 4) in 1966-2019.Temporal expansion of vegetation showed a band-like pattern, probably determined by the topographic characteristics within the dunefield (Figure 4).

| Yanakie
The dunefield of Yanakie was largely vegetated with a vegetation cover of 69% in 1965 (Table S1).Large bare sand mainly occurred at the Big Drift (Y1) in the west central and the Little Drift (Y2) in the east, with a total bare sand area of 912 ha (Figure 5a; Table S1).Overall, vegetation expanded between 1965 and1975 (by 11% shown in red colour in Figure 6) and 1983-2010 (by 5% shown in green colour in Figure 6).Large vegetation expansion mainly took place at the seaward deflation basins and plains, the Big Drift dunes and the Little Drift dunes (Figure 6).

| Cape Howe
The dunefield of Cape Howe was largely bare and mobile in 1941, with a total vegetation cover of 39% (Figure 7a).In 1941, vegetation cover was mainly concentrated in the woody forest in the hinterland, the open deflation plain at the west (H1) and the enclosed interdune depressions within the central dune (H2).There was a small interdune depression at the east (H3), but only sparse plants existed there in 1941 (Figure 7a).In the following decades, vegetation expanded from these vegetated locations towards surrounding environment, mainly towards the downwind/easterly direction (Figures 7 and 8).Vegetation also encroached from the inland forest to the dunefield, but this is to a less extent compared to vegetation expansion at the deflation basin/plains and interdune depressions (Figure 8).Along with the expansion of vegetation, by 2020, the whole dunefield was more segmented, and some individual dunes were more isolated compared with the earlier status in 1941 (Figures 7 and 8).

| Topography of the dunefield
Vegetation plays an important role in stabilizing coastal dunes by modifying aeolian process (e.g., sand transport, deposition and erosion) (Yousefi Lalimi et al., 2017).In turn, dune topography fundamentally influences the distribution and morphology of aeolian landforms via the creation of space for sediment deposition, the redistribution of water content and the modification of surface wind flow and at both landform (individual topographic dune forms) and whole-system scales (Hay et al., 2021;Yousefi Lalimi et al., 2017).
At the meso-scale (the scale of whole dune system), the topography is a clear control on the character of aeolian sediment accumulation and vegetation distribution in dunefields, mainly by determining wind regime and soil characteristics (e.g., soil water content and nutrients).For example, at Discovery Bay, vegetation showed a scattered and patchy distribution at the seaside and a mature woodland at the land margin, which resulted from the drifting sand with less organic matter in the bare dune at the seaside and higher soil nutrients in stabilized dune at the land margin (Figures 3 and 4).The hydrodynamic regime is also largely controlled by topographic characteristics, such as the extent and depth of the flooding in the rainy season (Dwyer et al., 2021;Martínez et al., 1997;Mountney & Russell, 2009).Lower elevation areas such as interdune depressions are more sheltered from wind erosion and are more subject to inundation or waterlogging during rainy days because of the infiltration of rainwater and higher groundwater levels at these areas, which can be accessible to plants (Hernández-Cordero et al., 2017;Hesp et al., 2011;Martínez et al., 1997;Miot da Silva et al., 2013).Therefore, the early vegetation establishment is more likely to occur at lower dune areas because of favourable growth conditions (Figures 3, 5 and 7).
At the finer scale (landform scale, individual dunes/ridges), dune topography can affect vegetation growth by determining local environmental conditions (Yousefi Lalimi et al., 2017).For example, a previous study showed that different patterns of lee-side airflows were exhibited after the wind pass over foredunes with different geometries (e.g., width, height and shape) (Lynch et al., 2010).Plants more favourably develop on the landward side of the foredune crest, as a result of the sheltering effect sharply changing local environmental conditions (Yousefi Lalimi et al., 2017).Topographic variations can significantly alter the vegetation patterns in water-limited ecosystems, mainly through changing the spatial redistribution of infiltration around plants and plant patches (McGrath et al., 2012).More detailed observations of Yanakie dunefield based on historical Google Earth images showed how vegetation initiated at the lower locations within a decade.Lower spots can retain rainwater, forming small water ponds.Vegetation patches were established at these lower spots within a few years (Figure 9).When intense, long-duration inundation events occur, it can cause vegetation to die because of the anaerobic condition (Martínez et al., 1997).
Vegetation is less likely to establish within the small bare deflation basins, such as the deflations at D2 in Discovery Bay and H4 in Cape Howe, probably because of the wind swirls and strong erosion within the basins, which can cause floor deflation (Figures 3, 7 and S3) (Hesp & Walker, 2012).

| Pre-existing vegetation
The presence of pre-existing vegetation can act as a nucleus for the development of vegetation patches and could have played an important role in subsequent vegetation expansion in Victoria.This could be because of the seed dispersal and the effect of the 'fertility island' around the established vegetation (Feagin et al., 2015;Xu et al., 2015).Additionally, the existence of the vegetation can generate a micro-environment around the vegetation.It can increase surface roughness and reduce wind erosion and provide shelter for the downwind area (Feagin et al., 2015;Hassanizade & Jafari, 2021).This could probably explain why vegetation spreads faster downwind (Figures 4,8 and 10).
Roots of some vegetation can fix nitrogen, whereas the litter mass (e.g., fallen foliage and dead plants) can add more nutrients and organic matter (including humic material), increasing water-holding capacity and particle cohesion of the soil (Feagin et al., 2009(Feagin et al., , 2015;;Kidron, 2010;Muñoz Vallés et al., 2011).All these processes can  (Feagin et al., 2009(Feagin et al., , 2015)).In Victoria, coastal dunes are dominated by woody species at the landward side, such as Coast Banksia (Banksia integrifolia ssp.integrifolia), Coastal Wattle (Acacia longifolia ssp.sophorae), Bangalay (Eucalyptus botryoides), Seaberry Saltbush (Rhagodia candolleana ssp.candolleana) and Coast Beard-heath (Leucopogon parviflorus) (Department of Sustainability and Environment, 2004).The soil at the landward side is more mature with higher nutrients and water-holding capacity compared with that at the seaside, because of the under-canopy microclimate (Kidron, 2010).For example, at Discovery Bay, soils at the seaward margin are unconsolidated, highly alkaline (pH > 9.5 at some time) sands showing no darkening from organic material; whereas the upper layer soils at the landward areas are darker because of the accumulation of organic matter (Gibbons & Downes, 1964).Therefore, vegetation expansion here could be related to the mature soils.In addition, the mature woody forest at the inland area can also provide substantial seed dispersal there, and this area is also less exposed to prevailing wind (Figures 3 and   7).More detailed examples of vegetation impact can be found at Cape Howe dunefield, where after the initial establishment, vegetation cover became denser at the current location, whereas new vegetation runners spread at the front edge (downwind) of the vegetated area (Figure 10).
Vertically, the presence of vegetation also has an impact on the rainwater runoff on the dune surface, through both the aboveground mass and the belowground root system (Feagin et al., 2015;White, 1971).At a dune face, the vegetated surface has a higher vertical penetrating rate, because the aboveground mass (roughness element) can slow down runoff, whereas the below-ground root network can increase the infiltration process, resulting in elevated soil moisture in the vegetated area (Weems & Monger, 2012;White, 1971).After the initial vegetation establishment at the dune toe (e.g., from the interdune depression), it can colonize the bare sand towards the upslope with the effect of 'fertility island' and increased moisture from the existing vegetation line (White, 1971).This can explain the banded pattern of vegetation expansion from the bottom towards the upper slope of the dune ridge as observed in the field (e.g., Figure 8).
The migration of the dune ridge can engulf existing vegetation in front of the dune ridges, as observed at the dunefield of Cape Howe (Figure S2).Exploring this process in more detail was outside the scope of this research as its primary aim was to focus on how vegetation expansion takes place throughout dune stabilization in the field.However, this would be an interesting avenue for future research.The presence of vegetation can increase the roughness of the dune surface, and the increase in surface roughness can, in turn, facilitate vegetation establishment (Heathfield & Walker, 2011).The common roughness elements at the coast and dunefield include debris from the sea (e.g., drifting wood and dead grasses), broken shell pieces and small rocks (García-Mora et al., 2001;Heathfield & Walker, 2011;Kennedy & Woods, 2012).Their existence can modify airflow and change the micro-environment and can also trap windblown sand in the backshore and form shadow areas for vegetation seedling and germination (Eamer & Walker, 2010;Hesp, 1981;Walker & Barrie, 2006).This is common at the seaward deflation basins and plains, where a lot of debris can occur, following the colonization of pioneer species and formation of incipient dunes (Eamer & Walker, 2010;Hesp, 1981Hesp, , 2002;;Walker & Barrie, 2006).Vegetation colonization and expansion at the seaside of Discovery Bay could be impacted by the roughness elements (e.g., previous dead marram grass roots) that occurred at the coast (Figure 11).

| Patterns of vegetation expansion during dune stabilization
Vegetation expansion within dunefield is mainly regulated by the topography of the dunefield, pre-existing vegetation and the roughness of the dune surface.The patterns and drivers of vegetation expansion during dune stabilization can be summarized by the conceptual modes as follows (Figure 12).At the dunefield scale, topography fundamentally influences the characteristics of landforms and vegetation distributions, and vegetation expansion can take place at different parts within the dunefield, such as (1) the back dune area at the landward side, (2) the interdune depressions and (3) the deflation basins and plains at the seaward side (Figure 12a).At the landward margins, vegetation is usually dominated by woody species (such as Banksia integrifolia).Vegetation expansion took place from the landward woodland towards the dunefield, because of the mature soil, shelter from the wind and substantial seed dispersal from existing vegetation (under canopy effect) (Figures 4, 6 and 8) (Feagin et al., 2009(Feagin et al., , 2015;;Kidron, 2010).At the interdune depressions, vegetation initiation usually occurs at the area with lower elevations with easier access to water, followed by a further expansion towards the surrounding environment from the current locations favoured by the 'fertilizer effect' of pre-existing vegetation (see D5-8 in Figure 3; H1-3 in Figure 7) (Feagin et al., 2015;Xu et al., 2015).This can result in strip-like vegetation zones if the interdune depressions are long and narrow basins (D5-8 in Figure 3).Otherwise, large vegetation patches will be formed if the interdune depressions are open and low-relief plains, like what was observed at Yanakie and Cape Howe (Y3-5 in Figure 5; H1-3 in Figure 7) (Xu et al., 2015).Once the interdune depressions were stabilized, vegetation expansion will continue to colonize the lower stoss face of dune ridges at the downwind side because of the wind sheltering, 'fertilizer island' effect and improved soil moisture provided by existing vegetation, although expansion towards the lee side of dune ridge (e.g., precipitation) at the upwind side can also occur (Figures 10   and S3) (Xu et al., 2015).Downwind migration of the dune ridges can take place if it is mobile; in this case, vegetation at the interdune depression can be engulfed, like what was observed at Cape Howe dunefield (Figures 7 and S2 to show how fast dune can migrate downwind).Vegetation colonization at the seaward side is in a relatively random way, and newly established vegetation could present a discontinuous and patchy distribution along the coast before it spreads towards the dunefield, which probably depends on the occurrence of seeds, surface roughness or wind/wave erosion (e.g., Figures 3 and 4).
At the individual dune scale, vegetation showed a vertical expansion from dune toe to the upper slope of dune face.With the soil moisture and shelters, vegetation establishment initiated from the lower slope of the dune face, and the established vegetation can change soil property (e.g., elevated nutrients and water-holding capacity) and surface roughness, which can boost consequent vegetation colonization around, usually towards in an up-slope direction (White, 1971).Take barchan dunes/barchanoid dune ridges within Cape Howe dunefield for example, vegetation expanded from the dune toe towards the upper slope at the stoss face, resulting in banded and more contour-paralleled vegetation patterns (e.g., Figure 8).Patterns of vegetation expansion are difficult to be observed at the slip face of rapidly migrating barchan/barchanoid dunes at Cape Howe (Figure S2).F I G U R E 1 2 Conceptual modes to show the patterns of vegetation expansion on dunefield.(a) Horizontally (at dunefield scale), vegetation established and expanded from the landward margin because of the mature soil, shelter and existed forest there; vegetation initiation and expansion at the interdune depression is mainly driven by the lower elevation with easier water access and the shelter; vegetation at the backshore is likely related to the surface roughness and the seeds on the beach.(b) Vertically (at landform scale), vegetation expanded from the lower to upper slope of dune face.Initial vegetation established at the lower slope can improve soil nutrients (known as the 'fertilizer island' effect) for later vegetation colonization.In addition, the existence of vegetation can alter soil moisture.Bare upper slope has a decreased rainwater penetration (because of surface run-off), resulting in a lower soil moisture, whereas the vegetated lower slope has a higher soil moisture because the above-and below-ground biomass can trap more rainwater with a higher penetration efficiency.Fertilized soil and higher moisture can drive the vegetation line advance towards the upper slope gradually (White, 1971).[Color figure can be viewed at wileyonlinelibrary.com] . The selected dunefield (525 ha) is dominated by barchanoid and transverse dunes, with a dune-blocked lake at the west and large area of enclosed depression in the central area (Figure 1e).The active dune F I G U R E 1 The study sites are located in Southeast Australia (a) along the coast of Victoria (b).The dunefields outlined in red in (c-e) have a varying cover of vegetation today.The dunefields have a southwest-to-west prevailing wind (the wind roses show the direction of the wind from) (f-h).Wind speed is measured in m/s and is shown by the side scale bars.[Color figure can be viewed at wileyonlinelibrary.com] ridges mainly oriented north-west to south-east, illustrating the effect of the prevailing southwest wind.The entire sand body was connected along the landward margin by a massive precipitation ridge in early time (e.g., in the 1940s), whereas dune ridges have been separated following dune stabilization by revegetation in recent years.2.2 | Changes and potential drivers of vegetation cover All of the dunefields showed an increase in vegetation cover (by 10%-33%) and dune stabilization between 1941 and 2020 F I G U R E 2 Decadal changes in vegetation cover (a) and meteorological data (b-g) in three coastal dunefields in Victoria, Australia.Long-term average annual temperature and rainfall were calculated and shown for each weather station.[Color figure can be viewed at wileyonlinelibrary.com] from the Victorian Department of Environment, Land, Water and Planning (DELWP) and recent F I G U R E 3 Vegetation expansion is shown by aerial images (a-f) and morphological features are shown by DEM (g) (elevation is shown in meters relative to mean sea level) of the dunefield at Discovery Bay.D1 shows the Swan Lake northwest of the dunefield; D2 shows examples of parallel parabolic dunes/arms; D3-4 present examples of the landward boundary of dunefield with higher elevations; D5-8 present some examples of deflation basins in the interdune with lower elevations; D9-10 show the seaward edge of dunefield with lower elevations, where vegetation patches existed along the coast.[Color figure can be viewed at wileyonlinelibrary.com] aerial images (2006-2020) from Google Earth were used to explore the spatio-temporal pattern of vegetation expansion in three selected coastal dunefields in Victoria (Table dunes with sparse vegetation in 1966, and banded vegetation was observed in 1975 resulting from vegetation expansion along the parabolic arms.The west interdune depression (D7), where with sparse vegetation in 1966, was colonized by a continuous vegetation band in 1975.More vegetation expansion was also observed at the east interdune depression (D8) in 1975.In the following years The Big Drift dunes were also largely vegetated (by 11%) in1965-  1975, resulting from the expansion of established vegetation in the interdune depressions (Y3-5 in Figure5a).This resulted in a more fragmented Big Drift with some isolated bare sand at the south and F I G U R E 4 Temporal change of vegetation expansion in the dunefield at Discovery Bay.The legend shows vegetated areas in the intervening years and the white area means no change in vegetation cover.[Color figure can be viewed at wileyonlinelibrary.com] east (Figure 5b).Vegetation also largely stabilized the foredunes (Y6) and blowouts (Y7) at the seaward side in 1975 (Figure 5b).Vegetation cover did not change much over 1975-1983, with a cover of about 80% (Figure 5b-c).A further vegetation expansion by 5% took place in 1983-2010, mainly within the deflation basins of Big Drift and from the west Little Drift, resulting in a more segmented Big Drift bare sand (Figure 5c-d).For example, one of the large bare sand patches (Y8) in 1983 was separated by vegetation into three smaller bare sand patches in 2010, and other bare dunes (Y9) decreased in size or were fully stabilized (Figure 5c-d).Little changes in vegetation were observed from 2010 to 2019 (Figure 5e).

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I G U R E 5 Vegetation change is shown by aerial images (a-e) and the morphological features are shown by DEM (f) (elevation is shown in meters) of Yanakie dunefield.Y1: the Big Drift; Y2: the Little Drift; Y3-5: examples of interdune depressions within the Big Drift; Y6: foredune; Y7: Blowout which is a passage for mobile sand to the inland dunefield; Y8: a large mobile dune was separated into three smaller dunes along with vegetation encroachment; Y9: examples of small mobile dunes which were stabilized by vegetation.[Color figure can be viewed at wileyonlinelibrary.com]

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I G U R E 6 Temporal change of vegetation expansion in Yanakie dunefield.The legend shows areas vegetated in the intervening years and the white area means no change.[Color figure can be viewed at wileyonlinelibrary.com]

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I G U R E 7 Vegetation expansion is shown by aerial images (a-f) and the morphological features are shown by DEM (g) (elevation is shown in meters) of Cape Howe dunefield.H1: open deflation plains at the west; H2-3: enclosed interdune depressions within the dunefield; H4-7: examples showing that previous active dunes become more segmented and isolated because of vegetation expansion.The displacement of interdune depressions and dunes indicates that the dunes are migrating towards the NE.[Color figure can be viewed at wileyonlinelibrary.com] reduce wind erosion and increase soil fertility around the established vegetation, which can facilitate vegetation growth nearby over the long term

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I G U R E 8 Temporal change of vegetation expansion at Cape Howe dunefield.[Color figure can be viewed at wileyonlinelibrary.com]F I G U R E 9 Examples of vegetation initiation at spots with a lower elevation at Yanakie dunefield, indicating the impact of the topography of the dunefield on vegetation establishment.[Color figure can be viewed at wileyonlinelibrary.com] 5.1.3| The roughness of the surface

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I G U R E 1 0 Examples show vegetation spread towards the downwind direction by increasing the roughness of the surface, indicating the impact of preexisting vegetation.[Color figure can be viewed at wileyonlinelibrary.com] However, a similar vertical vegetation expansion from the bottom to the upper slope can be assumed and observed for relatively stable F I G U R E 1 1 Examples illustrate the impact of the roughness of the dune surface on vegetation colonization.[Color figure can be viewed at wileyonlinelibrary.com] precipitation ridges at Yanakie and Cape Howe dunefields, driven by the 'fertilizer island' effect and the improved water-holding capacity provided by existing vegetation (Figures 12b and S3).
important role in stabilizing bare and mobile dune surfaces, and complex patterns of vegetation expansion were observed within coastal dunefield in Victoria.At the dunefield scale, vegetation expansion usually occurs at (1) the landward margins with woody forest, because of the mature soil, shelter from the wind and substantial seed dispersal from pre-existing vegetation (under canopy effect); (2) interdune depressions within the dunefield, because of water access determined by the topography and the effect of 'fertility island' provided by the pre-existing vegetation; and (3) the seaward deflation basins and plains, possibly influenced by the roughness elements and seeds trapped by wrack at the high tide mark.At the individual dune scale, vegetation showed a vertical expansion from dune toe to the upper slope at dune faces, resulting in a banded and counter-paralleled vegetation pattern.The vertical expansion of vegetation is also regulated by 'fertilizer island' effect and improved water-holding capacity of the soil provided by the aboveand below-ground biomass, which can favour vegetation colonizing further bare sand at the upper slope.It is also found that the deflation basins of parabolic dunes are more likely to be free of vegetation, probably because of the strong wind erosion in these areas because they are more bowl/saucer-shaped depressions.Patterns of vegetation colonization and expansion in the dunefields resulted from the interactions of multiple factors.It is evident that the topography of the dunefield, pre-existing vegetation and the roughness elements in the field played an important role in the pattern formation of vegetation colonization and expansion in the dunefield along the Victorian coast.