Detecting grave sites from surface anomalies: A longitudinal study in an Australian woodland

Forensic investigations of single and mass graves often use surface anomalies, including changes to soil and vegetation conditions, to identify potential grave locations. Though numerous resources describe surface anomalies in grave detection, few studies formally investigate the rate at which the surface anomalies return to a natural state; hence, the period the grave is detectable to observers. Understanding these processes can provide guidance as to when ground searches will be an effective strategy for locating graves. We studied three experimental graves and control plots in woodland at the Australian Facility for Taphonomic Experimental Research (Sydney, Australia) to monitor the rate at which surface anomalies change following disturbance. After three years, vegetation cover on all grave sites and control plots had steadily increased but remained substantially less than undisturbed surroundings. Soil anomalies (depressions and cracking) were more pronounced at larger grave sites versus the smaller grave and controls, with leaf litterfall rendering smaller graves difficult to detect beyond 20 months. Similar results were observed in two concurrent burial studies, except where accelerated revegetation appeared to be influenced by mummified remains. Extreme weather events such as heatwaves and heavy rainfall may prolong the detection window for grave sites by hindering vegetation establishment. Observation of grave‐indicator vegetation, which exhibited abnormally strong growth 10 months after commencement, suggests that different surface anomalies may have different detection windows. Our findings are environment‐specific, but the concepts are applicable globally.

these processes can provide guidance as to when ground searches will be an effective strategy for locating graves. We studied three experimental graves and control plots in woodland at the Australian Facility for Taphonomic Experimental Research (Sydney, Australia) to monitor the rate at which surface anomalies change following disturbance. After three years, vegetation cover on all grave sites and control plots had steadily increased but remained substantially less than undisturbed surroundings.
Soil anomalies (depressions and cracking) were more pronounced at larger grave sites versus the smaller grave and controls, with leaf litterfall rendering smaller graves difficult to detect beyond 20 months. Similar results were observed in two concurrent burial studies, except where accelerated revegetation appeared to be influenced by mummified remains. Extreme weather events such as heatwaves and heavy rainfall may prolong the detection window for grave sites by hindering vegetation establishment. Observation of grave-indicator vegetation, which exhibited abnormally strong growth 10 months after commencement, suggests that different surface anomalies may have different detection windows. Our findings are environment-specific, but the concepts are applicable globally.

K E Y W O R D S
clandestine graves, forensic anthropology, forensic archaeology, forensic botany, grave detection, taphonomy

| INTRODUC TI ON
In criminal proceedings involving alleged homicide, the presence of a body is important evidence [1]. In cases where the body of the victim is suspected to have been concealed through burial, much effort goes into the search for potential clandestine grave locations.
As such, there is a wealth of literature that characterizes surface anomalies that can be used during a ground search to indicate potential areas of soil disturbance, hence possible grave locations [2][3][4]. These surface anomalies include changes to typical soil and vegetation characteristics that can be identified by experts, often from a multidisciplinary background [3]. The duration for which surface anomalies persist, and hence their relevance to search teams, can vary widely depending on environmental conditions [5][6][7].
Understanding the nature of how surface anomalies change over time may assist the planning of search strategies, either by providing specific features on which search teams should focus, or used more broadly, predicting whether a search is likely to be effective.
Soil anomalies can result from the mechanical process of digging and refilling a grave [8], as well as impacts to the soil surface as a body undergoes decomposition [2]. The mixing process of excavating and refilling a grave often results in an excess of soil at the surface (known as overburden). The overburden may be a mixture of surface soil and deeper soil horizons, hence may have a different appearance to undisturbed soil (e.g., color, texture, presence of stones) As a body within a grave progresses through the stages of decomposition, it may initially increase in volume due to the distension of the torso resulting from decomposition gases, then decrease in volume as organic material is slowly degraded and gases disperse. This results in an expansion and contraction of the soil within the grave, leading to distinct soil anomalies such as cracking and depressions [9]. In addition to variables such as available moisture, the extent to which these anomalies are presented is strongly influenced by soil characteristics such as texture (i.e., fraction of clays/sands) and structure (aggregations) [10,11]. For example, sandy soils typically show less evident cracking than clay-rich soils [12].
Vegetation anomalies can include differences in species composition, size, color, or phenology (e.g., flowering time) [13][14][15]. These anomalies may result generally from vegetation recolonization after ground disturbance, whether it be grave construction or any other soil disturbance, or specifically as a direct response to a grave environment. Recolonization after disturbance tends to be dominated by ruderal, early succession species, including grasses and annual forbs [14,15] which may be evident when compared to surrounding undisturbed vegetation. In addition, the recolonizing vegetation may be at a different size and phenological state, although this is seasonally influenced and may not be readily apparent [14]. The response of vegetation to a grave environment is conceptually well-described [2,13,16], but in practice the varied responses of vegetation can be challenging to interpret. Conceptually, a grave site represents a novel habitat in which vegetation responds differently to the surrounding environment. This may be due to available nutrient and water sources (i.e., decomposition products), and/or altered water infiltration due to modified soil properties or the inclusion of other products in the grave, such as plastic wrapping [16]. The resultant vegetation growth may be either accelerated or suppressed, being influenced by the depth of interment, soil properties, vegetation characteristics, and climate. The few controlled studies exploring this [3,14,15,17,18] have suggested vegetation changes may be subtle, but can persist over time. However, certain geographical contexts may result in more obvious changes: For example, native plants in many parts of Australia are well-adapted to nutrient poor soils. An influx of nutrients provided by decomposition products may therefore result in a modified plant community invaded by exotic or annual species that corresponds to the body location, as discussed in [14]. Several case studies have also identified grave locations based on certain vegetation indicators [19,20].
Despite the increasing body of knowledge and conceptual understanding of the relationship between vegetation and soil anomalies, there is a lack of empirical studies that can be used to provide specific and practical guidance to search organizations. Here, we report the results of how surface anomalies change over 36 months in a multiple grave experiment, conducted at the Australian Facility for Taphonomic Experimental Research (AFTER) near Sydney, Australia.
We monitored the change in various soil and vegetation anomalies with the aims: (a) to identify any vegetation specific to grave environments, (b) to identify the duration that soil and vegetation anomalies remain visible, and (c) to identify factors that may affect the visibility of graves in this environment. We supplemented our observations with opportunistic observations from concurrent grave sites located at the same research facility, being used for archeological and chemical studies.

| ME THODS
A parcel of bushland measuring 30 m × 30 m was chosen to construct six experimental plots: three graves and three controls. The three grave sites (GR1, GR3, and GR6) were prepared containing one, three, and six human cadavers, respectively, as part of a longterm single and mass grave anthropological study (reported in [9]; Table 1). The three control plots (GR2, GR4, and GR6) did not contain any cadavers but were of similar size to the respective graves. The larger graves and controls (GR3, GR4, GR5, and GR6) were dug to 1-1.4 m depth using excavating machinery to mimic the soil displacement and compaction that would occur in a mass grave scenario. The smaller grave and control (GR1 and GR2) were dug to 0.3 m depth

Highlights
• Buried cadavers were used to assess how soil and vegetation grave anomalies change through time.
• Revegetation was slow but potential plant indicators were evident after a lag of several months.
• Smaller graves were obscured beyond 20 months; larger graves were still evident at 36 months.
• This information can provide searchers with relevant information to identify potential graves.
using hand tools. In both cases, the overburden was retained adjacent to the graves and controls.
The study was conducted at AFTER, located 50 km northwest of Sydney (33.620 S, 150.677 E). This site is located in open eucalyptus woodland classified as Cumberland Dry Sclerophyll Forest, on sandy clay loam to gravelly sandy clay soils [21]. The soils are acidic (topsoil pH 5.5, subsoil pH 5.8) with a mean electrical conductivity of 80.9 μS/cm (topsoil) and 58.8 μS/cm (subsoil) (E.M.J. Schotsmans pers. comm.) The surface topsoil is thin and typically contains the bulk of the soil seed bank in eucalyptus woodlands [22,23], whereas the subsoil is rocky, with a higher clay content. Daily ambient temperature and rainfall data were collected with an Onset Hobo weather station (Bourne, MA, USA) during the study period and summarized to monthly average temperature and monthly total rainfall ( Figure 1). The light environment is generally consistent throughout the study site.
The usual procedure at AFTER is to transport cadavers directly from the anatomy laboratory, refrigerated and unembalmed, to the study site. However, due to the large number of donors required for this study, all cadavers were unembalmed, but frozen for preservation. Prior to commencement of the experiment, the cadavers were thawed for 24 h at room temperature, but were still below ambient temperature at the time of placement in the graves.
The experiment commenced in June 2016. The sites were monitored from June 2016 to July 2019, a period of 36 months. Initially, sites were monitored every 3 months; however, this frequency was reduced as the experiment progressed due to minimal variation over time.
The following surface anomalies were monitored on grave and control sites: vegetation cover (%), native and exotic plant species present, litter cover (%), and soil characteristics (e.g., amount of cracking and depression). At each monitoring event, we assessed the surrounding environment, including species composition, phenology, and ambient cover of vegetation and litter.
We supplemented our observations by comparing soil anomalies and vegetation regeneration at two concurrent studies being conducted at AFTER during the same period (named "Schotsmans study" and "Ueland study," Table 2). These observations were taken opportunistically and, although they represent snapshots in time rather than longitudinal data, their proximity and similar study periods provide relevant comparison.
The Schotsmans study used buried cadavers to mimic ancient burial practices [24]. The first part of this project commenced in  Physical details of all cadavers used in each study are presented as  For the graves (GR3, GR5) and controls (GR4, GR6) that were machine-excavated, deeper soil horizons had been moved to the surface during the grave excavation and filling process. These soils were different in color to surface soils, contained more stones, and presented an obvious indicator of disturbance, both at the grave sites and as overburden surrounding the grave location (see Figure 2).

| Soil anomalies
As a result of both depressions and changes in surface soil, pooling of water on the grave sites was evident following periods of high rainfall (see [9]). Water pooling was not observed in the surrounding environment naturally.
Litter cover increased at an approximately linear rate at all locations ( Figure 3A), reducing the amount of visible bare soil. At the conclusion of this experiment, mean litter cover on the grave and control plots (80% ± 2.9 SE; 85% ± 2.9 SE) was equivalent to the magnitude of litter cover present in the surrounding area.

| Vegetation anomalies
Although revegetation of grass species commenced 10 days after the experiment began, vegetation was slow to recolonize on both the grave and control sites. Following initial recolonization, vegetation cover fluctuated with favorable and unfavorable growth conditions ( Figure 3B; Table 4). Species were initially limited to native forbs and grasses (early succession species), but eventually included shrubs and tree seedlings. At the end of the observation period, the mean vegetation cover for grave and control sites was still low: 9 ± 5.4% and 14 ± 1.0% coverage respectively, compared to the surrounding vegetation which retained a cover of 60%-70% throughout the study period. The smaller, shallow graves initially had a faster rate of recolonization reaching a peak cover of 30% (grave) and 50% (control) after 10 months, following which vegetation cover declined as low rainfall and high temperatures persisted. Visually, the larger graves and controls remained readily identifiable after 30 months, whereas beyond 20 months, soil cracks and depressions of the smaller grave and control became obscured by litter and vegetation.
A total of 43 species were identified within the study area prior to commencement (Table 4) forbs Senecio madagascariensis and Conyza sp.) were observed on both the grave and control sites but not observed in the intact native vegetation. However, these species did not demonstrate unusual growth and did not typically persist beyond two monitoring periods. and 20% cover, respectively). The lack of vegetation on SCH1704 rendered the grave location clearly evident after 3 years, which also aligned with our experimental observations in single graves and controls (GR1 and GR2). The 2019 grave containing a mummified flexed cadaver (SCH1909) also showed high levels of vegetation regeneration (70% cover; Figure 7A) after only 10 months, compared to the non-mummified cadaver and the control plot (each 5% vegetation cover; Figure 7B,C).

| DISCUSS ION
The results of this study suggest that in a temperate Australian    A novel species that colonized disturbed areas but was not initially present. Cracking and depression of the soil are likely to be evident at grave locations. These markers are formed during active decomposition of a body, as bloating and subsequent degradation of the soft tissues changes the soil displacement in the grave [2]. Cracks and depression may be visible in some cases for extended periods of time.
However, this is likely to be dependent on the local soil type: in this study, the sandy clay loam soils readily form cracks under disturbance. In coarse-textured soils (e.g., sands), anomalies such as cracks and depressions may be less evident or may persist for a shorter period [12,25]. The size of the grave, number of bodies in the grave, and indeed the size of individuals within the grave (e.g., large adult male compared to a small child) will also impact the degree of soil anomalies present. At the site used in this study, the surface topsoil is thin and friable, whereas the subsoil is rocky, with a higher clay content. When the subsoil horizons were disrupted and moved to the surface through excavation, it formed a visibly different layer (more pale, hard, and stony) and was more susceptible to cracking and water pooling. Vegetation re-establishment was limited as the native seed bank was absent, and the clay surface was not conducive for seedling survival. Survivorship of seedlings is further restricted by pooling water during wet periods and hard clay crusts during dry periods.
As such, a fluctuation of vegetation cover and species richness was observed throughout the study as vegetation slowly established, then died off, or was suppressed during extreme weather events, that is, heatwaves, droughts, or flooding periods. A similar result was observed by Caccianiga and co-authors [15], where the fluctuating vegetation community was driven by the loss of annual species during hot, dry summer months. The nature and timing of these extreme weather events are therefore likely to impact the persistence of surface anomalies. This phenomenon was particularly evident at the larger, deeper grave sites with more subsoil at the surface.
The classic template of forensic botany is that vegetation, particularly early succession species, grows more readily on grave sites due to the increased nutrient and moisture supply [2,13] [26][27][28]. This phenomenon has been reported in a similar context to our study, albeit investigating revegetation and nutrient fluxes around surface deposition of kangaroo carcasses [29]. In our case, the abnormally strong growth habit of this vegetation indicator only became evident 10 months following grave construction and serves as a valuable reminder that vegetation anomalies may emerge later-and persist longer-than soil anomalies.
The persistence of vegetation anomalies is rarely reported, and however, differences between disturbed and control sites have been observed lasting more than two years [19] and beyond five years [17]. Seasonal timing of events may also play a factor, particularly in regions with distinct climatic variation. For example, in north-eastern North America, a grave excavated in autumn may still be devoid of vegetation after six months when spring vegetation flush commences. However, a grave excavated in spring at the same location may be almost completely revegetated several months later [14].  [30,31]. The placing of the cadavers in a flexed position may also contribute to observed differences.
While this unexpected observation warrants more targeted investigation into factors impacting decomposition rates, it also serves as a caution that despite our generally slow rate of revegetation, certain situations may encourage vegetation regeneration, resulting in a faster-than-expected obscuring of a grave site.
Throughout our study, we observed other surface anomalies not routinely highlighted in search strategies. Variation in insect activity, particularly ants, was observed throughout this study but was not systematically investigated. While invertebrates are frequently studied in association with decomposition (e.g., to provide estimates of time since death [32,33]), subterranean insect activity associated with grave sites are rarely studied. This observation highlights a future opportunity to further understand the taphonomic environment and provide further indications of where soil disturbance has occurred.
Likewise, there is still much to learn about how rates of belowground decomposition are related to environmental conditions and hence to how this is expressed at the surface. In mass graves, for example, decomposition rates can vary within the grave depending on the location of the cadavers: those near the interior tend to decompose slower than those closer to the grave periphery [34]. While temperature strongly influences surface decomposition rates [35], these effects may be moderated below ground and thus change our expectation of decomposition. In the Schotsmans study, the mean soil temperature at grave depth only varied approximately 5°C throughout the year (E.M.J. Schotsmans pers comm), compared to external temperature variations of approximately 15°C. Rainfall is likely linked to moisture conditions within graves, but there is surprisingly little empirical evidence to confirm this. Natural variation between cadavers is also present (e.g., size, age, composition) but given the paucity of data using human donors, we cannot yet quantify this variation.
The following anthropological component of these projects (excavation and recovery) will provide important within-grave temperature and relative humidity data, as well as information on decomposition states, to aid in our understanding of these elements.
When planning a search for a grave location, it is important to recognize conditions where surface anomalies are likely to be apparent. Certain factors that are likely to increase the visibility and persistence of surface anomalies, including larger disturbance areas (i.e., multiple bodies in a grave), fine-textured soils, shallow topsoil depth, distinct subsoil, extreme weather events such as heatwaves and wet periods, and distance from intact vegetation (i.e., revegetation sources). Vegetation indicators may not always correspond to the richer growth reported in the early literature and may be limited to subtle changes in species composition, or unexpected growth that may not be evident for several months after grave construction.
Although variation around surface anomalies can be complex, incorporating specialized knowledge provided by archeologists and botanists can be useful for informing search strategies and interpreting surface anomalies as potential graves.

ACK N OWLED G EM ENTS
We would like to acknowledge all donors to the UTS Body Donation Program, as well as UTS and AFTER staff and students who supported this research. We acknowledge the comments of the two anonymous reviewers who helped to improve this manuscript.