Vegetation history of mid- to western Ireland in the 2nd millennium a.d.; fresh evidence from tephra-dated palynological investigations

The historic Icelandic tephra layers, from Hekla in a.d. 1104 and Öræfajökull in a.d. 1362 that have been found in four peat profiles obtained from lowland and upland mid to western Irish bogs, provide the dating for high-resolution palynological investigations of regional land use over the last thousand years. Marginal agriculture is investigated through the study of an upland blanket peat and a lowland Atlantic blanket peat. At the lowland site, the landscape has been altered, primarily by removal of hazel scrub, while in the uplands, there has been little scrub woodland throughout the last millennium. Pastoral agriculture has a long, unbroken history at both sites, with a short period of arable agriculture, dated to the early 19th century, detected in the uplands. At the two lowland sites, changes in land use associated with medieval monastic and secular activity were similar but not synchronous. The a.d. 1362 tephra in one lowland profile provides high-resolution dating of the palynological evidence for agricultural collapse in the aftermath of the Black Death. The palynological evidence of late medieval woodland clearance is contrasted with the written record. The effects of 19th century population expansion on land use are considered. A synthesis of regional land use in Ireland during the last thousand years is presented.     

Long-term development of a cultural landscape: the origins and dynamics of lowland heathland in southern England

The lowland heathlands of southern England comprise ca. 14?% of the total area of this habitat in Europe yet their history is poorly understood. This paper presents the first detailed palaeoecological evidence (combining palynological, microscopic charcoal and radiocarbon data) relating to the origin and long-term dynamics of heathland vegetation in southern England. Valley peat sites, situated on the Lower Greensand Group (coarse-grained sandstones) at Conford (Hampshire) and Hurston Warren (West Sussex) have been investigated. The sequence from Conford indicates the unusually late survival of Pinus sylvestris (to as late as ca. 6050?cal. b.p.) in southern England. This is attributed to edaphic factors and, after ca. 7050?cal. b.p., to frequent fires. After intervening phases of dominance by deciduous woodland, heathland vegetation became established in the proximity of both sites in the Late Bronze Age (ca. 3000?cal. b.p.) with increases in indicators of grazing and burning demonstrating an association between the development of heathland and human activity. Thereafter, the pollen and charcoal records show that the vegetation remained in a dynamic state as the scale and nature of human activity varied through time. Major expansions in the extent of heathland occurred relatively recently; after ca. 1450?cal. b.p. at Hurston Warren and after ca. 850?cal. b.p. at Conford. A review of the palaeoecological evidence suggests that the most intense use and greatest coverage of heathland in southern England probably occurred during the medieval to post-medieval periods.     

Late Holocene land-use and vegetation dynamics in an upland karst region based on pollen and coprophilous fungal spore analyses: an example from the Burren, western Ireland

The late Holocene environmental history of two karstic uplands in the Burren, western Ireland is reconstructed. The palaeoecological investigations focus on species-rich, upland plant communities of high biogeographic interest that include Sesleria-dominated grasslands and heath communities with Dryas octopetala, Arctostaphylos uva-ursi and Empetrum nigrum. Short monoliths taken from shallow peats were pollen analytically investigated. Particular attention was paid to non-pollen palynomorphs, and especially coprophilous fungal spores as indicators of environmental change and pastoral activity at local level. The exposed north-west Burren uplands carried Pinus sylvestris-dominated woodland during the mid and late Bronze Age. The demise of pine on these uplands at ca. 600 B.C. is ascribed to human impact. Evidence is presented for increased pastoral activity in the uplands from early medieval times (ca. sixth century A.D.) onwards. Farming, involving intensive grazing of the uplands, attained greatest intensity during the late 18th and early 19th century, and resulted in more or less total clearance of Corylus scrub which, prior to that, was common in both the upland and lowland Burren. The potential of non-pollen palynomorphs and especially the coprophilous fungal spore record for elucidating traditional Burren farming practices, including winterage (a type of transhumance), is highlighted.     

Raised peat bog development and possible responses to environmental changes during the mid- to late-Holocene. Can the palaeoecological record be used to predict the nature and response of raised peat bogs to future climate change?

Palaeoecological analyses of raised peat bog deposits in northwest Europe show the naturalness, antiquity and robust response of these ecosystems to environmental changes from c. 7800 years ago to the present. A review of the techniques used to identify these long-term features is presented and the role of climate change, autogenic change processes and human disturbance is discussed. Millennial records of vegetation changes recorded in peat deposits demonstrate the response (often rapid) of raised peat bog vegetation to climatic changes during the mid-Holocene, Bronze Age/Iron Age transition and the Little Ice Age. Greenhouse warming scenarios exceed the reconstructed Holocene record of climatic changes (c. the last 11, 500 years), and bog-water tables may fall considerably. A combination of centennial palaeoecological analyses of bogs affected by human disturbance and experimental manipulations have been used as analogues for the potential response of raised peat bog vegetation to these changes. These show that possible greenhouse gas climate forcing scenarios may exceed the ability of Sphagnum- dominated raised peat bogs to respond to projected increases in summer temperature and decreases in summer precipitation. In combination with increasing N deposition, a loss of their Sphagnum-rich vegetation and increases in the abundance of vascular plants could occur on decadal timescales.     

Multi-profile fine-resolution palynological and micro-charcoal analyses at Esklets,North York Moors,UK, with special reference to the Mesolithic-Neolithic transition

Multi-proxy palaeoecological data from two peat profiles at Esklets on the North York Moors upland provide a record of vegetation changes for much of the Holocene. Possible vegetation disturbance in the late Mesolithic and activity in the Neolithic and Bronze Age are recognised. In both profiles fine resolution analyses have been applied to the period leading up to the mid-Holocene Elm Decline which in this upland has been dated to ca. 4,800 bp (uncalibrated ¹⁴C years). Disturbance impacts at the Esklets Elm Decline are low scale, but phases of woodland disturbance, which include cereal (Hordeum)-type pollen, occur in both profiles ca. 5,200 bp, some centuries before the Elm Decline on the North York Moors, but similar to dates for this key palynological horizon in nearby lowland areas. A protocol is presented for the separation of Hordeum (cultivated species) and Glyceria (wild grass) pollen. The Esklets sites record disturbances during the late Mesolithic-Neolithic transition. These pre-Elm Decline disturbance phases represent either early penetration of neolithic cultivator-pastoralists into this upland or the activities of final mesolithic foragers. No neolithic archaeological sites occur nearby, but a ‘Terminal Mesolithic’ flint site dominated by microlith ‘rod’ forms occurs close to the palaeoecological sites. Such rod sites are dated in northern England to the centuries leading up to 5,000 bp and so are contemporary with the disturbance phases that included Hordeum-type pollen at Esklets. The cultural context of these disturbance phases and the role of ‘rod’ microlith sites during the Mesolithic-Neolithic transition require further focused research to clarify all issues relating to this important period.     

Fossil insect evidence for the development of raised mire at Thorne Moors, near Doncaster

Fossil insects contained within a monolith of peat taken from Thorne Moors, to the northeast of Doncaster, UK, were studied. The changing entomofauna demonstrates clearly the development of raised mire from fen woodland. Many of the trees emerging from the peat have been severely burnt and the role of fire in the destruction of the woodland is considered. A number of species recovered from the lower peats are now extinct in Britain; others have extremely limited distributions in Europe today.     

Ecology of Racomitrium lanuginosum in British blanket mire - evidence from the palaeoecological record

Abstract

The moss Racomitrium lanuginosum (Hedw.) Brid. is widely distributed in the cool-oceanic north and west of the British Isles, where it is an important element in the vegetation of blanket mire. It has been described as occurring on the present-day surface of British blanket mire in two situations (i.e. where the local mire water table may be lowered, despite high atmospheric humidity), on the tops of tall hummocks and adjacent to areas of peat erosion, where it may be dominant in the vegetation. Accordingly, the occurrence of R. lanuginosum is widely perceived as indicative of drier mire conditions and/or mire degradation. In contrast, recent palaeoecological studies have documented the recurrence of R. lanuginosum in the absence of either hummock upgrowth or peat erosion. Such studies suggest that R. lanuginosum may also occur on blanket mire as a component of climatically-mediated mire development. This paper presents critical new data to document the decomposition of R. lanuginosum and describes the results of recent palaeoecological studies that have negated the exclusive role of R. lanuginosum in mire drying/degradation. We present evidence for the active role of R. lanuginosum in persistent blanket mire development, suggesting the moss may occur during periods of climate change towards increased wetness, after a sustained period of drier conditions. The results are of wider relevance in evidencing the sensitivity of oceanic blanket mire to past climate change.     

Long-term ecological study (palaeoecology) to chronicle habitat degradation and inform conservation ecology: an exemplar from the Brecon Beacons, South Wales

For ecosystems perceived as degraded, but for which the causal factors or timescales for the degradation are disputed or not known, long-term (palaeo-)ecological records may aid understanding and lead to more meaningful conservation approaches. To help ‘bridge the gap’ between (very) long-term ecology and contemporary ecology for practical application, there have been calls for working relationships to be established between palaeoecologists and conservation ecologists. One environment in which this has been attempted is blanket mire. Many blanket mires in Europe are degraded and contain few sphagna. In South Wales, almost all exhibit symptoms of degradation, with dominance by purple moor grass (Molinia caerulea) widespread. We used palaeoecological techniques on three peat profiles in the Brecon Beacons to investigate vegetation history of high-altitude blanket mire to help assess the relative contribution of various factors in mire degradation and to inform strategies for mire conservation and restoration management. We found that declines in sphagna preceded the rise to dominance of monocotyledons. Macrofossil records showed that although Molinia was already present on the Beacons before the start of the industrial revolution, its major rise to dominance in one profile was within the 20th Century, coincident with evidence for local fire. In another profile, it was out-competed by Eriophorum vaginatum after the start of the industrial revolution; there is circumstantial evidence to suggest that a reduction in burning contributed to the rise in E. vaginatum. Conservation management to reduce the current local dominance of both Eriophorum and Molinia is supported by the palaeoecological data, but severe erosion and hagging of peat will constrain practical methods for achieving this on the Beacons until the peat is stabilised. We suggest that palaeoecological techniques have wider applicability in conservation.     

Natural resources and agriculture during the Iron Age in Ostrobothnia,western Finland,investigated by pollen analysis

In Ostrobothnia, western Finland, the Viking period (A.D. 800–1050) in contrast to the rich Migration period (A.D. 400–550/600), is poor in archaeological finds. Archaeologists have interpreted this as indicating a break in settlement continuity. Palaeoecological investigations using pollen analyses and radiocarbon dating of peat cores from ten sites show that field cultivation and animal husbandry have taken place continuously throughout the entire Iron Age in Ostrobothnia. Slash-and-burn cultivation was not of importance in the studied area, but small-scale cereal cultivation occurred on permanent, tilled and manured fields. The Iron Age agriculture was largely dependent on animal husbandry and therefore was located close to the sea because the natural, highly productive shore meadows were an indispensable fodder resource. As a consequence of the progressive rapid change of the natural environment caused by the flat topography and land upheaval, the settlements were regularly relocated to keep pace with the westwards retreating sea. Settlement continuity in Iron Age coastal Ostrobothnia has to be looked upon in a regional rather than a local perspective because of the changing landscape. The results of this palaeoecological study, in which investigations were carried out in several parts of the region, demonstrate regional settlement continuity throughout the Iron Age.     

Four thousand years of plant exploitation in the Chad Basin of northeast Nigeria I: The archaeobotany of Kursakata

This paper discusses archaeobotanical remains from the settlement mound of Kursakata, Nigeria, comprising both charred and uncharred seeds and fruits as well as charcoal. In addition, impressions of plant tempering material in potsherds were analysed. The late Stone Age and Iron Age sequence at Kursakata is date from 1000 cal. B.C. to cal. A.D. 100. DomesticatedPennisetum (pearl millet), wild Paniceae and wild rice are the most common taxa. Kernels from tree fruits were regularly found including large numbers ofVitex simplicifolia—a tree which is absent from the area today. A distinct change in plant spectra can be observed between the late Stone Age and the Iron Age. Although domesticated pearl millet was already known at the beginning of the settlement sequence of Kursakata, it only gained greater economic importance during the Iron Age. Besides farming, pastoralism and fishing, gathering of wild plants always played a major role in the subsistence strategy of the inhabitants of Kursakata. The charcoal results show that firewood was mainly collected from woodlands on the clay plains, which must have been more diverse than today. The end of the late Stone Age in the Chad Basin was presumably accompanied by the onset of drier environmental conditions from ca. 800 cal. B.C. onwards.     

Vegetation changes and development of agriculture at „Kerfontaine“ (Sérent,Massif Armoricain,France)

As part of a new research programme, studies of peat bogs in Brittany are being carried out to trace the changes that have occurred in the course of time, and to determine the different regional stages related to the human activities of land clearance and agriculture. In this context, a palynological study of the Kerfontaine peat bog was undertaken to consider changes in local and regional vegetation dating from after about 7800 B.P. Local vegetation history from Neolithic times until the Middle Ages involved a succession of twelve pollen zones clearly related to variations in water level resulting from natural or anthropogenic influences. The dominant vegetation was alder carr, bog-myrtle mire and birch woods. Heath vegetation, which appeared at the end of the Iron Age, developed during the Gallo-Roman period, finally invading the entire bog in the Middle Ages. Regional vegetation history was characterised, among other things, by the presence of beech woods which developed after 3000 B.P. and then declined during the Middle Ages in conjunction with an increase of cultivation between the 8th and 11th centuries A.D., an indication of great activity during the Carolingian period. Human activity reached its peak around 1800 A.D. at the time of the first pine plantations.     

Fossil insects and ecosystem dynamics in wetlands: implications for biodiversity and conservation

We review the uses of fossil insects, particularly Coleoptera (beetles) and Chironomidae (non-biting midges) from ancient deposits to inform the study of wetland ecosystems and their ecological and restoration processes. In particular, we focus on two contrasting ecosystems, drawing upon research undertaken by us on British raised mire peats and shallow lake systems, one an essentially terrestrial ecosystem, the other aquatic, but in which wetland insects play an important and integral part. The study of raised mire peats suggests that faunal stability is a characteristic of these wetland systems, over what appear to be extensive periods of time (up to several millennia), whilst studies of shallow lake ecosystems over recent timescales indicates that faunal instability appears to be more common, usually driven by increasing eutrophication. Drawing upon a series of fossil Coleoptera records spanning several thousand years from Hatfield Moors, south Yorkshire, we reconstruct in some detail the mire’s ontogeny and fluctuations in site hydrology and vegetation cover, illustrating the intimate association between substrate, topography and peat development. A comparison between fossil and modern beetle populations indicates that the faunal characteristics of this mire and its adjacent neighbour, Thorne Moors, become established during the early phases of peat development, including its rare endemics, and that the faunal biodiversity on the sites today is dictated by complex site histories. The over-riding characteristic of these faunas is of stability over several thousand years, which has important implications for the restoration of degraded sites, especially those where refugial areas are limited. In contrast, analyses of fossil Chironomidae from shallow lakes allow researchers to track changes in limnological status and while attempts have been made to reconstruct changes in nutrient levels quantitatively, the chironomids respond indirectly to such changes, typically mediated through complex ecosystem dynamics such as changes in fish and/or macrophyte communities. These changes are illustrated via historic chironomid stratigraphies and diversity indices from a range of shallow lakes located across Britain: Slapton Ley, Frensham Great Pond, Fleet Pond, Kyre Pool and Barnes Loch. These sites have shown varying degrees of eutrophication over recent timescales which tends to be associated with a decline in chironomid diversity. While complex functional processes exist within these ecosystems, our evidence suggests that one of the key drivers in the loss of shallow lake chironomid diversity appears to be the loss of aquatic macrophytes. Overall, while chironomids do show a clear response to altered nutrient regimes, multi-proxy reconstructions are recommended for a clear interpretation of past change. We conclude that if we are to have a better understanding of biota at the ecosystem level we need to know more of the complex interactions between different insect groups as well as with other animal and plant communities. A palaeoecological approach is thus crucial in order to assess the role of insect groups in ecosystem processes, both in the recent past and over long time scales, and is essential for wetland managers and conservation organisations involved in long term management and restoration of wetland systems     

Agricultural practices in Arctic Norway during the first millennium b.c.

Palaeoecological investigations of mires suggest that agriculture was established north of the Arctic Circle in Norway during the late Bronze Age (1100?C500 b.c.) and Pre-Roman Iron Age (500?C1 b.c.). The lack of archaeobotanical and archaeological investigations has made it difficult to assess the nature of this early agricultural expansion into the Arctic in any detail. Here we present the first well documented archaeobotanical investigation from north Norway that covers this agricultural pioneer phase. Remains of charred seeds show that barley (Hordeum) was already being cultivated in the late Bronze Age, and that wheat (Triticum) was introduced in the Pre-Roman Iron Age. Large amounts of crowberry (Empetrum) seeds are also typical of the Pre-Roman Iron Age and were obviously an important food plant at the time, at least locally. Charcoal rich layers dated to the late Bronze Age suggest that the local birch forest was initially cleared away with the help of fire, possibly related to a slash-and-burn cultivation practice. Lithostratigraphic and pollen-analytical results indicate that the cultivation practice of the Pre-Roman Iron Age was a form of bush-fallow system with intensive soil re-working alternated with long periods of fallow.     

Effects of the Hekla 4 tephra on vegetation in Northwest Iceland

Vegetation plays a key role in preventing the remobilisation of tephra and aeolian activity following tephra fall. Recent volcanic eruptions in Iceland have highlighted the consequences of tephra fall for ecosystems and human health. Improved understanding of the mechanisms behind ecosystem recovery following tephra fall is particularly important for Iceland. Today?~42% of the country is classified as desert and unvegetated and sparsely vegetated areas are unable to trap tephra fall and prevent subsequent wind erosion. This paper presents palaeoenvironmental reconstructions before and after the Hekla 4 tephra from two lakes in Northwest Iceland, from within a woodland in the lowland, and in open woodland under stress at the highland margin. The c. 4,200 cal bp. Hekla 4 tephra is one of the most extensive Icelandic Holocene tephra layers and the eruption produced an estimated?~9 km³ of tephra. The palaeoecological reconstructions provide an insight into the responses of two relatively stable ecosystems to thick tephra deposits during a period of cooling climate. The understory vegetation in the lowland woodland was buried by the tephra, however Betula pubescens trees were not severely affected and the woodland recovered relatively quickly. In contrast, open woodland at the highland margin that was already at its ecological limit, shifted to dwarf shrub heath, a more resilient vegetation community in response to the tephra fall and cooling climate.     

The Palaeoenvironmental Impact of Prehistoric Settlement and Proto-Historic Urbanism: Tracing the Emergence of the Oppidum of Corent,Auvergne, France

Early human societies and their interactions with the natural world have been extensively explored in palaeoenvironmental studies across Central and Western Europe. Yet, despite an extensive body of scholarship, there is little consideration of the environmental impacts of proto-historic urbanisation. Typically palaeoenvironmental studies of Bronze and Iron Age societies discuss human impact in terms of woodland clearance, landscape openness and evidence for agriculture. Although these features are clearly key indicators of human settlement, and characterise Neolithic and early to Middle Bronze Age impacts at Corent, they do not appear to represent defining features of a protohistoric urban environment. The Late Iron Age Gallic Oppidum of Corent is remarkable for the paucity of evidence for agriculture and strong representation of apophytes associated with disturbance. Increased floristic diversity – a phenomenon also observed in more recent urban environments – was also noted. The same, although somewhat more pronounced, patterns are noted for the Late Bronze Age and hint at the possibility of a nascent urban area. High percentages of pollen from non-native trees such as Platanus, Castanea and Juglans in the late Bronze Age and Gallic period also suggest trade and cultural exchange, notably with the Mediterranean world. Indeed, these findings question the validity of applying Castanea and Juglans as absolute chronological markers of Romanisation. These results clearly indicate the value of local-scale palaeoecological studies and their potential for tracing the phases in the emergence of a proto-historic urban environment.     

Effects of nutrient loadings from catchments on Asajino mire, a small coastal ombrotrophic mire in northernmost Japan

The relationship between water chemistry and vegetation was studied in a coastal ombrotrophic mire in northern Hokkaido, Japan. The distributions of Sphagnum and Phragmites communities were separated clearly by the pH and ion concentration of the peat surface-pore water. The drainage ditches along the road across the center of the mire had a high pH and ion concentration, as did the peat water in the western part of the mire. It was found that fields used for livestock farming on a hill to the west of the mire leached materials into the mire through drainage ditches, surface runoff, and probably also through ground water, and thus influenced the water chemistry of the mire. Management of the water, including that in the catchment of the mire, should be introduced before biological buffering capacity against excess nutrient loading caused by human activity is exceeded and the mire loses its ombrotrophic status.     

Classification of boreal mires in Finland and Scandinavia: A review

Mires have been classified in northern Europe at two levels: (1) mire complexes are viewed as large landscape units with common features in hydrology, peat stratigraphy and general arrangement of surface patterns and of minerogenous vs. ombrogenous site conditions; (2) mire sites are considered as units of vegetation research and used in surveys for forestry and conservation. This paper reviews the development of site type classifications in Fennoscandia (Finland, Sweden, Norway), with a discussion on circumboreal classification and corresponding mire vegetation types in Canada. The scale of observation affects classifications: small plot size (0.25–1 m²) has been used in Scandinavia to make detailed analyses of ecological and microtopographical variation in mostly treeless mire ecosystems, while larger sampling areas (up to 100–400 m²) have been commonly employed in Finnish studies of forested peatlands. Besides conventional hierarchic classifications, boreal mires have been viewed as an open, multidimensional, non-hierarchic system which can be described and classified with factor, principal component or correspondence analyses. Fuzzy clustering is suggested as an alternative method of classification in mire studies where only selected environmental and vegetational parameters are measured or estimated.Nomenclature: Lid, J. (1987) Norsk, svensk, finsk flora (vascular plants). Corley et al. (1981) Journal of Bryology 11: 609–689 (bryophytes)     

Invader or native? — vegetation clearance in northern Britain during Romano-British time

Pollen diagrams from nine mire sites in the Hadrianic-Antonine frontier area have been constructed to assess the record of human impact on vegetation over the last 3,000 years. Of particular interest is the effect of the Roman invasion and occupation of northern Britain on vegetation, especially that related to the construction of the Hadrianic and Antonine walls, forts and roads. Pollen analysis was undertaken to investigate whether the impact was widespread across the frontier zone or was confined to the proximity of Roman walls and forts. The results of high-resolution pollen analysis, supported by radiocarbon dates, have demonstrated that there was little woodland clearance during the Bronze Age and that the first major and permanent clearance of vegetation at certain sites occurred during the Iron Age. This is followed by a second clearance relating to the Roman occupation. At Fozy Moss, Northumbria, minimal Iron Age clearance occurs and the first major clearance occurs at the time of the Roman occupation. The dramatic response of the grass pollen curves and the relatively low level of agricultural indicators is in accord with the archaeological evidence for the Roman impact being one of woodland clearance for military purposes rather than for settled agriculture. A contribution to the 8th IPC, Aix-en-Provence, Sept. 1992     

Recognition of peat-forming plant communities from their peat deposits in two south Swedish bog complexes

Peat samples, 3 015 from 103 boring points, on two mires (Åkhult mire, Store Mosse mire) south Sweden, have been subjected to macrofossil analysis. Based on plant remains, 9 peat groups were distinguished in the field. A further classification using phytosociological methods revealed 29 peat types. The affinities between the peat types were determined from TABORD classification and a Reciprocal Averaging ordination. The primary floristic differentiation is correlated with a gradient from treeless-to wooded stands, which coincides largely with the mire expanse-mire margin gradient. The poor-rich gradient seems to parallel the treeless-wooded gradient as well and may reflect the natural conditions in this mire before it was affected by man. The hummock-mud-bottom gradient is easy to distinguish in peat, formed by bog communities, but is not distinct in peat formed by fen communities and impossible to detect in peat dominated by wood remains. The amount of identifiable remains depends on the decomposition, which is determined by (1) the period of time the plant litter stays in the acrotelm, and (2) the nutrient status. The decomposition is greatest in fen-peat with abundant wood remains. This probably depends on a good supply of oxygen caused by greater horizontal water movements and better nutrient status.     

Change of peat mass during decomposition in mires of Poland and West Siberia

The paper presents the results of field experiment on peat decomposition in peat deposits of mires in West Siberia and Poland. Two principal factors determining the dynamics of peat mass are revealed: position in relief and contribution from the below ground organs of herbaceous plants. In a raised bog, an upland microlandscape, maximum values of peat mass loss were obtained in the upper layer of peat and minimum values in the lower layer. In all poor fen ecosystems which are lowland microlandscapes, similar values of peat mass losses at different depths were obtained. In mire ecosystems with a considerable proportion of grasses in the phytocenosis, the supply of peat in the upper half-meter layer is replenished by dead below ground organs of plants.