内蒙古典型草原禾本科植硅体形态

运用地层中植硅体组合解释过去草原植被及气候变化的关键之一,是要了解研究区现代植硅体形态及表土植硅体组合与现代植被的关系。文中研究内蒙古典型草原禾本科植物根、茎、叶、芒以及种子等不同部位的植硅体,对其中的12种主要禾本科植物叶表皮短细胞硅酸体进行分类及统计。研究表明：内蒙古典型草原禾本科叶表皮短细胞硅酸体可分为8种特殊形态类型。C3植物早熟禾亚科的叶表皮短细胞硅酸体形态多样。几乎所有早熟禾亚科都能产生圆型硅酸体,以贝加尔针茅(85．5％)、大针茅(89．7％)、克氏针茅(90％)以及芨芨草(96．6％)中的圆型硅酸体含量最丰富。针茅哑铃型主要见于针茅植物叶表皮短细胞中,克氏针茅的针茅哑铃型含量相对较高。羊草中未见针茅哑铃型硅酸体。浴草、披缄草叶表皮短细胞硅酸体以齿型为主,分别含87．3％和57．2％,齿型在硬质早熟禾中也占一定比例。沙生冰草中的脊圆型占优势,含74．4％。C3植物早熟禾亚科的叶表皮短细胞产生的截锥型硅酸体含量较少。C4植物虎尾草亚科中的糙隐子草叶表皮短细胞硅酸体以黍哑铃型、简单哑铃型、鞍型为主;黍亚科狗尾草则以黍哑铃型占优势(82．9％)。     

东北地区芦苇植硅体形态的空间差异

笔者对东北地区12个样点的芦苇叶片的植硅体进行了分析,探讨了芦苇植硅体在空间上的分布规律,揭示芦苇植硅体的环境指示意义,为定量恢复古环境提供参考。实验结果表明:12个样点中芦苇植硅体基本类型相同,共有5种主要植硅体类型,即中鞍型、帽型、芦苇扇型、尖型和棒型;但芦苇植硅体的浓度随着纬度的变化而相应的发生变化。总体来说,纬度较低的样点芦苇植硅体的浓度较大,而纬度较高的样点芦苇植硅体的浓度相对较小;随着纬度的变化,芦苇的特征植硅体中鞍型的浓度也呈现波状变化,牡丹江以南的几个样点中鞍型植硅体浓度相对较大,9、10两个月牡丹江以北的几个样点中鞍型植硅体浓度迅速减少。芦苇扇型植硅体则呈现出纬度较高的样点植硅体浓度较大,纬度较低的样点植硅体浓度较小的特点,芦苇扇型植硅体的空间变化规律与扇型植硅体的不同。     

Definition of grassland biomes from phytoliths in West Africa

Aim In order to enhance the effectiveness of comparisons between modelled and empirical data for present and past vegetation, it is important to improve the characterization of tropical grass‐dominated biomes reconstructed from fossil tracers. This study presents a method for assigning phytolith assemblages to tropical grass‐dominated biomes, with the objective of offering a new tool for combining pollen and phytolith data in the reconstruction of tropical biomes. Location The West African latitudinal transect studied here extends from 12° N (southern Senegal) to 23° N (southern Mauritania), passing through the Guinean, Sudanian, Sahelian and Saharan bioclimatic zones. Methods Modern phytolith assemblages were extracted from 59 soil surface samples taken throughout the study area and allocated, a priori, to three current biomes: (1) desert C₄ grassland, (2) short grass savanna, and (3) tall grass savanna. Five out of nine phytolith types identified were used as predictors in a discriminant analysis (with calibration and validation steps) for assigning phytolith assemblages to biomes. In addition, 74 modern pollen spectra from the West African transect, acquired from the African Pollen Database ( http://medias.obs‐mip.fr/apd ), were processed by the biomization method. This mathematical procedure involves assigning palynological taxa to one or more plant functional types, which represent broad classes of plants. The plant functional types, in turn, are combined to define biomes following a specific set of algorithms and rules. The resulting maps of the phytolith biomes thus derived were compared with maps of pollen biomes and of contemporary ecosystem classes. Results In the calibration and validation steps, 91.5% and up to 83%, respectively, of the phytolith samples were assigned to the correct biome. The short grass savanna and tall grass savanna biomes were assigned with similar accuracy by both the phytolith and pollen biomization methods, but the phytolith method gave substantially superior results for the desert C₄ grassland biome, providing seven out of seven correct assignments, compared with just one out of four by pollen biomization. Comparisons between an existing ecosystem map and the maps created from phytolith estimation showed close correspondence for desert C₄ grassland, short grass savanna and tall grass savanna, the latter providing correct assignments in 88, 62 and 91% of cases, respectively. Main conclusions The phytolith discriminant analysis method presented here accurately estimates three C₄ grass‐dominated biomes that are widespread in West Africa. Complementarities between the phytolith method and pollen biomization are highlighted. Combining complementary phytolith and pollen data would provide more accurate assignments of C₄ grass‐dominated biomes than pollen biomization alone.     

Effects of different fencing regimes on community structure of degraded desert grasslands on Mu Us desert,China

Grazing is one of the major anthropogenic driving factors influencing community structure and ecological function of grasslands. Fencing has been proved to be one of the main measures for rehabilitating degraded grasslands in northwestern China. However, data from combined empirical studies on the effects of different management regimes in desert grasslands are lacking. So we selected long‐term fencing (fenced since 1991), mid‐term fencing and seasonal fencing (fenced since 2002), and adjacent free‐grazing grasslands to investigate vegetation and soil properties on southwest Mu Us desert. Our results showed that fencing increased plant cover, height, aboveground biomass (AGB) of different plant life‐form groups, Shannon–Wiener diversity index, Evenness index, Simpson index, total soil nitrogen, total soil phosphorus, and soil organic matter, but decreased plant density, species richness, Richness index, soil bulk density, water content, and pH. However, 22–24 years of long‐term complete fencing might cause redegradation of vegetation and soil nutrients, characterized by the reduction of some vegetation properties, biodiversity, total AGB, and some soil properties. Seasonal fencing with 11–13 year was more beneficial to vegetation restoration than that with completely fencing measures. Our study suggests that appropriate artificial disturbances, such as seasonal fencing (winter grazing and summer fencing), should be used after long‐term fencing in order to maintain grassland productivity and biodiversity. These findings will help to provide theoretical support for vegetation restoration and sustainable management in grassland under grazing prohibition at Mu Us desert.     

Phytolith assemblages in grasses native to central Argentina

BACKGROUND AND AIMS: Phytolith reference collections are a prerequisite for accurate interpretation of soil phytolith assemblages aimed at reconstructing past vegetation. In this study a phytolith reference collection has been developed for several grasses native to central Argentina: Poa ligularis, Piptochaetium napostaense, Stipa clarazii, Stipa tenuis, Stipa tenuissima, Stipa eriostachya, Stipa ambigua, Stipa brachychaeta, Pappophorum subbulbosum, Digitaria californica, Bothriochloa edwardsiana and Aristida subulata. METHODS: For each species, phytoliths present in the leaf blades were classified into 47 morphotypes, and their relative frequency determined by observing 300-400 phytoliths per sample (n = 5). Data were analyzed by complete linkage cluster analysis, using the Morisita Index as measure of association. KEY RESULTS: The results showed differentiation among phytolith assemblages at species level or at plant functional type level. Cluster analysis separated C3 from C4 species and palatable from non-palatable species. CONCLUSIONS: This study highlights the possibility of reconstructing past vegetation in central Argentina grasslands through the analysis of soil phytolith assemblages.     

The importance of environment vs. disturbance in the vegetation mosaic of Central Arizona

Abstract. The vegetation of central Arizona is a mosaic of four vegetation types: chaparral, chaparral grassland, woodland, and woodland grassland. We analysed ten environmental variables, three disturbance variables, and five disturbance indicators to answer the question: What is the relative importance of environment and disturbance in explaining the vegetation pattern of our study area? We found that chaparral, chaparral grassland, and woodland are differentiated primarily by environmental factors and have high stability in the landscape. In contrast, woodland grassland is differentiated primarily by disturbance and is likely an early‐successional stage of woodlands. Although other researchers have indicated that semi‐arid vegetation is generally unstable, the vegetation of central Arizona is composed of two systems: those with a more stable landscape position determined primarily by environmental factors and those with a less stable landscape position determined primarily by disturbance factors.     

Do soil phytoliths accurately represent plant communities in a temperate region? A case study of Northeast China

Modern soil phytoliths can potentially provide analogues for phytolith assemblages from archaeological and palaeoecological contexts. To assess the reliability of soil phytoliths for representing different plant communities, we analysed phytoliths in surface soils and parent plants at 65 sites representing five types of regional vegetation in Northeast China. The results demonstrated that surface soil phytolith assemblages could clearly differentiate samples from herbaceous and woody communities, and samples from Poaceae and non-Poaceae communities could be separated statistically. In addition, woody communities could be differentiated into a broadleaf-Poaceae community, a broadleaf-non-Poaceae community and a conifer and broadleaf-non-Poaceae community, except for some overlapping samples. Soil phytolith assemblages are thus able to differentiate regional vegetation types into different plant community types. In the present study, soil phytoliths represented about 30% of the phytoliths present in the aboveground vegetation. In addition, soil phytoliths from different communities reflected the aboveground vegetation with slightly different degrees of accuracy, and in addition different morphotypes exhibited different degrees of representational bias. Some morphotypes (e.g. rondel, elongate psilate, lanceolate) overrepresented the abundance of the associated plant taxa; morphotypes such as tracheid, conical epidermal, stomata and others under-represented the original plant richness; and other morphotypes, e.g. saddle, trapeziform sinuate, scutiform, were in good agreement with the numbers of plant taxa in the plot inventory. Thus, any quantitative palaeovegetation reconstruction using phytoliths should begin with the calibration of soil phytolith assemblages. We conclude that our findings provide improved phytolith analogues for different plant communities, with applications in palaeoenvironmental reconstruction, and they also provide additional insights into the mechanisms of phytolith production and deposition.     

Using Soil 13C to Detect the Historic Presence of Schizachyrium scoparium (Little Bluestem) Grasslands on Martha's Vineyard

Abstract We used differences in soil carbon δ¹³C values between forested sites and grasslands dominated by the C₄ grass Schizachyrium scoparium (little bluestem) to detect the presence of former grasslands in the historical landscape of the coastal sand plain of Martha's Vineyard, Massachusetts, U.S.A. Soil δ¹³C was measured at (1) sites with long‐term forest or grassland vegetation and (2) sites with known histories where forest vegetation invaded grassland and where forest converted to grassland. The δ¹³C of soil under long‐term grassland was –24.1‰ at 0 to 2 cm depth and –23.4‰ at 2 to 10 cm and was enriched by 3.4‰ and 2.8‰ compared with soil under long‐term forest. In forests that invaded grasslands dominated by S. scoparium, soil δ¹³C decreased as C derived from trees replaced C from S. scoparium. This decline occurred faster in surface soils and in the light soil organic matter fraction than in the mineral soil. In forests that converted to grasslands, soil δ¹³C increased and the rate of increase was similar in surface and mineral soil and in the different soil organic matter fractions. Rates of change indicated that soil δ¹³C could be used to detect changes in vegetation involving the presence or absence of S. scoparium during the last 150 years. Application of this model to a potential grassland restoration site on Martha's Vineyard where the landscape history was not known indicated that the site was previously unoccupied by S. scoparium during this time. The δ¹³C of surface mineral soil can be useful for detecting the presence of historic S. scoparium grasslands but only in the period well after European settlement of these coastal sand plain landscapes.     

Change in the vegetation mosaic of central Arizona USA between 1940 and 1989

The landscape of central Arizona U.S.A. is characterized by a patchy distribution of three major vegetation types: chaparral, woodland and grassland. Disturbance is common in the landscape, primarily livestock grazing, fire and conversion (i.e., removal of woody plants to increase forage for livestock). The purposes of this research were to determine changes in the landscape mosaic of central Arizona between 1940 and 1989 and to predict future changes. Using aerial photographs from 1940, 1968 and 1989 and digital overlays followed by transition matrix analysis, we found that chaparral and its adjacent grassland had changed less than woodland and its grassland. However, both had nearly equal projected stabilization times. Moreover, disturbance increased time for stabilization and some results were scale-dependent.     

全新世早期中国长江下游地区橡子和水稻的开发利用

最近发掘的浙江上山和小黄山遗址(11400—8000cal BP)是长江下游地区迄今发现最早的村落遗址。这两个遗址均出土有大量磨石, 而有机物遗存保存很少。本文对7件石器进行了淀粉粒、植硅体以及矿物残留物的初步分析, 以便检验这些方法在复原古代生计形态和器物功能研究中的可行性。鉴定出的淀粉粒包括橡子、根茎类、薏苡以及疑似菱角等, 但是没有发现水稻。在发现的水稻植硅体中, 来自茎杆和叶的密度较高, 而来自颖壳的密度较低。矿物分析表明残留物来自工具的使用以及周围的土壤基质。结果显示这些磨石曾经用于加工富含淀粉的植物和制陶用的羼和料。我们认为, 对富含淀粉并且适于长期贮存的植物特别是橡子的大量开发利用是全新世早期长江下游地区生计形态中采集策略的特征。本研究显示了淀粉粒和植硅体分析在古代生计重建中的巨大潜力。对这些遗址残留物的进一步研究将有助于我们更好地理解这一地区从采集经济到定居农耕经济的转变。     

Grazing‐induced changes in plant–soil feedback alter plant biomass allocation

Large vertebrate herbivores, as well as plant–soil feedback interactions are important drivers of plant performance, plant community composition and vegetation dynamics in terrestrial ecosystems. However, it is poorly understood whether and how large vertebrate herbivores and plant–soil feedback effects interact. Here, we study the response of grassland plant species to grazing‐induced legacy effects in the soil and we explore whether these plant responses can help us to understand long‐term vegetation dynamics in the field. In a greenhouse experiment we tested the response of four grassland plant species, Agrostis capillaris, Festuca rubra, Holcus lanatus and Rumex acetosa, to field‐conditioned soils from grazed and ungrazed grassland. We relate these responses to long‐term vegetation data from a grassland exclosure experiment in the field. In the greenhouse experiment, we found that total biomass production and biomass allocation to roots was higher in soils from grazed than from ungrazed plots. There were only few relationships between plant production in the greenhouse and the abundance of conspecifics in the field. Spatiotemporal patterns in plant community composition were more stable in grazed than ungrazed grassland plots, but were not related to plant–soil feedbacks effects and biomass allocation patterns. We conclude that grazing‐induced soil legacy effects mainly influenced plant biomass allocation patterns, but could not explain altered vegetation dynamics in grazed grasslands. Consequently, the direct effects of grazing on plant community composition (e.g. through modifying light competition or differences in grazing tolerance) appear to overrule indirect effects through changes in plant–soil feedback.     

Vegetation and fire history of a Chinese site in southern tropical Xishuangbanna derived from phytolith and charcoal records from Holocene sediments

Aim The aims of this paper are to reconstruct the vegetation and fire history over the past 2000 years in a well‐preserved rain‐forest area, to understand interactions between climate, fire, and vegetation, and to predict how rain forest responds to global warming and increased intensity of human activity. Location Xishuangbanna, south‐west China, 21–22° N, 101–102° E. Methods Phytolith (plant opal silica bodies) morphotypes, assemblages, and indices were used to reconstruct palaeovegetation and palaeoclimate changes in detail. Micro‐charcoal particles found in phytolith slides, together with burnt phytoliths and highly weathered bulliform cells, were employed to reconstruct a record of past fire occurrence. A survey of field sediments, lithology, and ¹⁴C dating were also employed. Results Phytoliths were divided into 11 groups and classified into 33 well‐described morphotypes according to their shape under light microscopy and their presumed anatomical origins and ecological significance. The phytolith assemblages were divided into six significant zones that reveal a complete history of vegetation changes corresponding to climate variation and fire occurrence. Phytolith assemblages and indices show that the palaeoclimate in the study area is characterized by the alternation of warm–wet and cool–dry conditions. Phytolith and charcoal records reveal that 12 fire episodes occurred. Comparison of burnt phytoliths with an aridity index (Iph) shows that fire episodes have a strong relationship with drought events. Main conclusions Our results indicate that fire occurrence in the tropical rain forest of Xishuangbanna is predominantly under the control of natural climate variability (drought events). Nearly every fire episode is coupled with a climatic event and has triggered vegetation composition changes marked by a pronounced expansion of grasses. This indicates that drought interacts with fire to exert a strong influence on the ecological dynamics of the rain forest. However, the impact of human activity in recent centuries is also significant. Our results are important for understanding the interactions between climate, fire, and vegetation, and for predicting how rain forest responds to global warming and increased human activity.     

Acute salt marsh dieback in the Mississippi River deltaic plain: a drought‐induced phenomenon?

Aims Extensive dieback of salt marsh dominated by the perennial grass Spartina alterniflora occurred throughout the Mississippi River deltaic plain during 2000. More than 100,000 ha were affected, with 43,000 ha severely damaged. The aim of this work was to determine if sudden dieback could have been caused by a coincident drought and to assess the significance of this event with respect to long‐term changes in coastal vegetation. Location Multiple dieback sites and reference sites were established along 150 km of shoreline in coastal Louisiana, USA. Methods Aerial and ground surveys were conducted from June 2000 to September 2001 to assess soil conditions and plant mortality and recovery. Results Dieback areas ranged in size from ～300 m²?5 km² in area with 50–100% mortality of plant shoots and rhizomes in affected zones. Co‐occurring species such as Avicennia germinans (black mangrove) and Juncus roemerianus (needlegrass rush) were unaffected. Historical records indicate that precipitation, river discharge, and mean sea level were unusually low during the previous year. Although the cause of dieback is currently unknown, plant and soil characteristics were consistent with temporary soil desiccation that may have reduced water availability, increased soil salinity, and/or caused soil acidification (via pyrite oxidation) and increased uptake of toxic metals such as Fe or Al. Plant recovery 15 months after dieback was variable (0–58% live cover), but recovering plants were vigorous and indicated no long‐lasting effects of the dieback agent. Main conclusions These findings have relevance for global change models of coastal ecosystems that predict vegetation responses based primarily on long‐term increases in sea level and submergence of marshes. Our results suggest that large‐scale changes in coastal vegetation may occur over a relatively short time span through climatic extremes acting in concert with sea‐level fluctuations and pre‐existing soil conditions.     

Restoration of Midwestern U.S. Savannas: One Size Does Not Fit All

Lowland savannas are a rare variant of Midwestern United States savanna occurring on alluvial soils, for which reference information is sparse. To evaluate the appropriateness of using upland savanna as a surrogate source of reference information for lowland savanna, we studied a pre‐Euro‐American lowland savanna using original U.S. Public Land Survey data and other historical records. Historical vegetation was reconstructed and compared among upland savannas, lowland savannas, and lowland forests; we also evaluated potential disturbance dynamics maintaining these systems. We found that all three communities were dominated by members of the genus Quercus but also had extensive representation by many other tree species, especially notable for savannas in this region. There were no clear size–density relationships for species in the genus Quercus, indicating that these historical savannas were not characterized exclusively by large, scattered oak trees but rather by trees of many oak species and nonoak species in a wide range of size classes. Both upland and lowland savannas also contained a substantial shrub component. We found no evidence that lowland savannas were maintained by flooding, although the uneven‐aged canopy structure suggested that periodic disturbance occurred. Restoration of lowland savanna in this region should include provisions for maintaining nonoak species and shrubs, with disturbance timed to maintain an uneven‐aged canopy structure. Although the appropriateness of historical data in the face of climate change may be questionable, in this region, a warmer climate may actually help promote the “oak parkland” that was present from 8,000 BP up to Euro‐American settlement.     

Soil seed bank dynamics of fire-prone wooded grassland, woodland and dry forest ecosystems in Ethiopia

In order to reveal the role of soil seed banks in vegetation recovery after fire in savanna, the spatial distribution and temporal changes in the soil seed banks of regularly burning savanna in Gambella, western Ethiopia, was studied. The seedling emergence technique was employed to determine the species composition and density of the soil seed bank of six sites ranging in fire severity from wooded grassland with frequent fires over woodland with intermediate fire frequency to forest with absence of fires. Species composition and density of seeds in the soil were compared between seasons, depths and sites with different types of standing vegetation. Fourteen plant species were recorded in the soil seed bank from the grassland and woodland sites and 6 from the dry forests; 60 % of the taxa in the soil seed bank were annuals and 40 % were perennials. The soil seed banks were largely dominated by graminoids and 48–97 % of the soil seed bank in the grasslands and woodlands was of a single grass species, Hyparrhenia confinis , which was absent from the dry forests. The soil seed pools ranged from less than 100 to 4700 seeds per m² depending upon the season. The soil seed bank of graminoids was nearly empty after the onset of the rainy season whereas seeds of broadleaved herbs and woody species able to germinate were still found after this time. Floristic composition, representation of life forms and density of seeds in soil did not correspond closely with that of the standing vegetation, but within graminoids there was a strong similarity between the soil seed bank and the standing vegetation. The current fire regime of Ethiopian savanna woodlands appears to maintain the dominance of graminoids over broadleaved herbs and woody plants both as seeds in the soil and in the standing vegetation.     

Grass water stress estimated from phytoliths in West Africa

Aim This study calibrates the relationship between phytolith indices, modern vegetation structure, and a climate parameter (AET/PET, i.e. the ratio of annual actual evapotranspiration to annual potential evapotranspiration), in order to present new proxies for long‐term Quaternary climate and vegetation changes, and model/data comparisons. Location Sixty‐two modern soil surface samples from West Africa (Mauritania and Senegal), collected along a latitudinal transect across four bioclimatic zones, were analysed. Methods Two phytolith indices are defined as normalized data: (1) humidity‐aridity index [Iph (%) = saddle vs. cross + dumbbell + saddle], and (2) water stress index [fan‐shaped index (Fs) (%) = fan‐shaped vs. sum of characteristic phytoliths]. Vegetation structures are delimited according to Iph and Fs boundaries. Bootstrapped regression methods are used for evaluating the strength of the relationship between the two phytolith indices and AET/PET. Additional modern phytolith assemblages, from Mexico, Cameroon and Tanzania are extracted in order to test the calibration established from the West African samples. Accuracy of the AET/PET phytolith proxy is compared with equivalent pollen proxy from the same area. Results Characterization of the grass cover is accurately made through Iph. A boundary of 20 ± 1.4% discriminates tall grass savannas from short grass savannas. Water stress and transpiration experienced by the grass cover can be estimated through Fs. AET/PET is accurately estimated from phytoliths by a transfer function: AET/PET = ?0.605 Fs ? 0.387 Iph + 0.272 (Iph – 20)² (r = 0.80 ± 0.04) in the application domain (AET/PET ranging from 0.1 ± 0.04 to 0.45 ± 0.04). Phytolith and pollen estimate with similar precision (r_pollen = 0.84 ± 0.04) the AET/PET in the studied area. Conclusions This study demonstrates that we can rely on the phytolith indices Iph and Fs to distinguish the different grasslands in tropical areas. Moreover, a new phytolith proxy of AET/PET, linked to water availability, is presented. We suggest from these results that combining phytolith and pollen proxies of AET/PET would help to constrain this climate parameter better, especially when phytolith assemblages are dominated by Panicoideae and Chloridoideae C₄‐grass phytoliths, are devoid of Pooideae C₃‐grass phytoliths, and occur with a few tropical ligneous woody dicotyledon phytoliths. As AET/PET is a bioclimatic indicator commonly used in vegetation models, such a combination would help to make model/data comparisons more efficient.     

Decadal plant composition changes in grazed native grassland

Most high value grasslands of south‐eastern Australia are restricted to small fragmented areas of public land. However, a significant proportion of native grassland is privately owned, managed with grazing rather than fire and is rarely monitored. Hence, a better understanding of grassland management and conservation on the private estate is required. Eco‐markets are policy instruments that incentivise private landholders to effectively manage and conserve native vegetation. However, short funding cycles restrict the capacity of monitoring programs to assess long‐term vegetation changes. In this study, 17 native grassland remnants on private land managed with livestock grazing were monitored before and during a 4‐year eco‐market and 6 years after it ended to determine how composition had changed over 10 years of monitoring. Numerous grassland attributes were either maintained or improved over the 10‐year period, which coincided with one of the most severe and long‐lasting droughts in recent history (i.e. the Millennium drought). In addition, several threatened species were identified as part of the program. A decline in exotic forb richness, native grass cover and native forb richness suggests there is also some impact of the Millennium drought on plant mortality as evidenced by altered litter dynamics. Inherent variability year‐to‐year for most measures of grassland attributes over the monitoring period indicates that climatic conditions have a strong influence on grassland dynamics. Our ability to determine the driver(s) of grassland composition was limited by monitoring program design. Future eco‐market monitoring programs should have adequate resources allocated to enable effective monitoring designs, that incorporate reference information and control sites, and should aim to be long‐term (i.e. >10 years). This will provide clearer insights into the drivers of grassland dynamics and allow for refinement of management options for conservation of this threatened community.     

Simulated dynamics of succession in a North American subtropical Prosopis savanna

Abstract. A transition matrix model was used to explore the dynamics, rate and potential extent of changes in landscape vegetation patterns on a southern Texas Prosopis savanna. Transitions between seven vegetation classes were determined for the periods 1941–1960 and 1960–1983 on aerial photographs of three sites. During these periods, the sites were heavily grazed by cattle and were fire-free. Vegetation states assessed in grids of 20 m x 20 m cells superimposed on photographs ranged from grass-dominated to woody plant-dominated. The 1941–1960 period (denoted DRY) was characterized by prolonged drought, whereas annual rainfall during the 1960–1983 period (denoted WET) was typically normal to above-normal. The 1941 landscape consisted of herbaceous zones (6% of cells), woodland (50% of cells) and savanna parkland (44% of cells with grass/woody plant mixtures). The woodland state was the most stable, with probabilities of no change being 0.970 and 0.873 in WET and DRY periods, respectively. The herbaceous state was least stable, with corresponding values of 0.074 and 0.353. Past and future landscape structure was modelled by randomly selecting DRY or WET transitions at 20 year time steps. The model was run under a series of rainfall scenarios where the probability of selecting the WET transition matrix (P[WET]) ranged from 0 (DRY always chosen) to 1 (WET always chosen). Historical records indicate P[WET] has approximated 0.3 to 0.4 in the region. The rate of succession to states of greater woody cover increased as P[WET] increased. Forward simulations based on P[WET] > 0.2 suggest the present landscape is unstable and will develop into a closed-canopy woodland within the next 180 years, assuming the processes operating between 1941 and 1983 continue (e.g. grazing by cattle and lack of fire). Reverse simulations concur with historical observations and projections derived from woody plant growth rates in other studies and suggest that 200 to 300 yr BP these landscapes contained a substantially greater proportion of cells dominated by grassland or grassland with scattered woody plants (43 to 74%) than was present in 1983 (19%). Based upon elapsed time between predicted past and future steady states, succession from open savanna to closed-canopy woodland may occur in ca. 400 to 500 yr for P(WET) ≥ 0.33. Arresting or reversing the projected trend may require changes in climate and/or changes in livestock grazing and land management practices. The approaches employed in this study illustrate how time series maps, aerial photographs and satellite imagery can be analyzed and used to interpret, project and reconstruct local and regional changes in ecosystem structure. Difficulties and limitations associated with the use of Markov chains to model succession are identified and discussed.     

Tailoring restoration interventions to the grassland‐savanna‐forest complex in central Brazil

Defining the reference system for restoration projects in regions characterized by complex vegetation mosaics is challenging. Here we use the Cerrado region of Brazil as an example of the importance of clearly defining multiple natural and anthropogenically altered states in grassland‐savanna‐forest mosaics. We define three main, natural vegetation types–grassland, savanna, and scleromorphic (cerradão) forest–to (1) distinguish between original and degraded states and (2) set appropriate targets for and guide restoration. We contend that the differences in Cerrado vegetation composition originally were driven by soil conditions and secondarily by fire frequency and precipitation patterns that differ from the core to the edge of the Cerrado region. Grasslands are found on the shallowest, least fertile soils and/or in waterlogged soils; scleromorphic forests are generally located on deeper, more fertile soils; and savannas occupy an intermediate position. In recent decades, this biophysical template has been overlain by a range of human land‐use intensities that strongly affect resilience, resulting in alternative anthropogenic states. For example, areas that were originally scleromorphic forest are likely to regenerate naturally following low‐ or medium‐intensity land use due to extensive resprouting of woody plants, whereas grassland restoration requires reintroduction of grass and forb species that do not tolerate soil disturbance and exotic grass competition. Planting trees into historic grasslands results in inappropriate restoration targets and often restoration failure. Correctly identifying original vegetation types is critical to most effectively allocate scarce restoration funding.