Ecology, 86(3), 2005, pp. 772±785 q 2005 by the Ecological Society of America ALIEN PLANTS IN TEMPERATE WEED COMMUNITIES: PREHISTORIC AND RECENT INVADERS OCCUPY DIFFERENT HABITATS PETR PYSÏ EK,1,2,5 VOJTEÏ CH JAROSÏÂõK,1,2 MILAN CHYTRY ,3 ZDENEÏ K KROPA CÏ ,4 LUBOMÂõR TICHY ,3 AND JAN WILD1 1Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 PruÊhonice, Czech Republic 2Department of Ecology, Faculty of Science, Charles University, VinicÏna 7, CZ-128 01 Praha 2, Czech Republic 3Department of Botany, Masaryk University, KotlaÂrÏska 2, CZ-611 37 Brno, Czech Republic 4SlavõÂkova 16, CZ-130 00 Praha 3, Czech Republic Abstract. Variables determining the number of native and alien plants on arable land in Central Europe are identi®ed. Species richness of 698 samples of weed ¯oras recorded in the Czech Republic in plots of a standard size of 100 m2 in 1955±2000 was studied in relation to altitudinally based ¯oristic region, soil type, type of cultivated crop, climatic variables, altitude, year of the record, crop cover and height, and human population density in the region. Vascular plant species were classi®ed into native and alien, the latter divided in archaeophytes, introduced before AD 1500, and neophytes, introduced after this date. The use of minimal adequate models in the analysis of covariance allowed determination of the net effects of mutually correlated environmental variables. Models for particular species groups explained 33±48% of variation in species numbers and 27±51% in propor- tions; however, explanatory variables affected native species, archaeophytes, and neophytes differently. The number and proportion of neophytes increased in 1955±2000, whereas the number of native species and archaeophytes declined (in archaeophytes more slowly in the warm than in the moderate to cool altitudinal ¯oristic region). In warm and dry regions and on dry soils, where most archaeophytes ®nd optimum conditions, fewer native species are able to persist in weed communities than in colder and wetter regions. Archaeophytes respond like neophytes to some variables (climate, seasonal development of crop) and alternatively like native species to other variables (increasing agricultural intensi®cation through time, human population density). Archaeophytes are common in old crops intro- duced with the beginning of agriculture (cereals), but are poorly represented in relatively recently introduced crops (rape, maize), where neophytes are most numerous. These patterns re¯ect the history of plant invasions in Central Europe. Neolithic agriculture, introduced from the Near East in the sixth millenium BC, brought archaeophytes with crops and, by creating intense and continuous propagule pressure and imposing new agricultural man- agement, facilitated their invasion. By contrast, the crops introduced during the past ®ve centuries and their speci®c agrotechnical management have supported spreading of other weed species, mainly invaders from overseas. Key words: agricultural management; archaeophytes; biological invasions; Central Europe; climate; crop characteristics; exotic species; invasion history; Neolithic agriculture; neophytes; soil type. INTRODUCTION cies, is an environment with speci®c disturbance re- gimes. Arable ®elds are not only disturbed with varying Studies on plant invasions address several funda- frequency, intensity, and predictability; they also have mental topics, one of them being the identi®cation of been directly created by disturbances associated with features that make some habitats more invasible than agriculture since the Neolithic Age (Ellenberg 1950, others (Crawley 1987, Williamson 1996). It long has 1988, Holzner and Immonen 1982, di Castri 1989). been recognized that human-made (anthropogenic) Such disturbances can be described in terms of crop habitats, especially in settlements, are prone to invasion management but are dif®cult to quantify, as they often by alien species, which is attributable to habitat het- are hidden behind the overwhelming effect of site con- erogeneity, frequent and diverse disturbances, and in- ditions (PysÏek and LepsÏ 1991, Dale et al. 1992, Salonen tensive propagule pressure typical of this environment 1993, ErvioÈ et al. 1994, Andersson and Milberg 1998, (Gilbert 1989, Kowarik 2003). Arable land, a human- Hallgren et al. 1999). Studies analyzing the determi- made habitat with a high representation of alien spe- nants of weed species richness are rare (Stevenson et al. 1997, Kleijn and Verbeek 2000, HyvoÈnen and Sa- lonen 2002), and hardly any have focused on detecting Manuscript received 22 December 2003; revised 2 June 2004; accepted 12 August 2004. Corresponding Editor: C. C. general patterns of alien species invasions. Labandeira. In another paper, using the same data set (PysÏek et 5 E-mail: [email protected] al. 2005), we detected regional patterns of overall plant 772 March 2005 ALIEN PLANTS ON TEMPERATE ARABLE LAND 773 species richness in temperate weed communities, and the whole study period, direct comparison of weed spe- determined the net effects of environmental variables cies richness and cover is possible within our data set. that are mutually correlated. Numbers of weed species Species cover in the ®eld was estimated using a were signi®cantly affected by altitudinal ¯oristic re- Domin 10-degree scale, which was transformed to per- gions and by the year of sampling. The differences in centages to provide input data for analyses (Westhoff diversity of the weed ¯ora were mainly attributable to and van der Maarel 1978). The sample plots are stored management, and partly to crop-speci®c agricultural in the Czech National Phytosociological Database practices, as well as to general intensi®cation of man- (Chytry and Rafajova 2003: No. 342001±342781). We agement of arable ®elds during the past decades. How- deleted 14 randomly selected plots from those localities ever, the species pool of weeds on Central European where more than one plot was sampled, in order to arable land consists of two groups of species, distinct avoid oversampling of some areas (PysÏek et al. 2005). with respect to their origin status (sensu PysÏek et al. The remaining 698 sample plots, used in the analyses, 2004a), speci®cally whether they are native or alien to were distributed throughout the country (PysÏek et al. the region. Among the latter, two groups are tradition- 2005). Records of cultivated crop plants were deleted ally distinguished in Central Europe. Archaeophytes from the species data set and crop type was used as an were introduced between the beginning of Neolithic environmental variable. agriculture and the European discovery of America, Vascular plant species were classi®ed into native and while species introduced after that date are termed neo- alien, the latter group further divided in archaeophytes phytes (Thellung 1905, Holub and JiraÂsek 1967, PysÏek and neophytes. The status of these species was taken et al. 2002a). The separation between natives and ar- from PysÏek et al. (2002b). chaeophytes is sometimes dif®cult and relies on a com- For each sample plot, the following variables were bination of palaeobotanical, archaeological, ecological, recorded: (1) number of native species (range 1±31), and historical evidence (Preston et al. 2002, PysÏek et archaeophytes (4±39), and neophytes (0±6); (2) per- al. 2004a). Given their different history in the target centage of native species (4.3±78.8%), archaeophytes region and suite of traits in which archaeophytes differ (18.2±95.7%), and neophytes (0.0±24.0%); (3) relative from native taxa (Klotz et al. 2002, PysÏek et al. 2003c, cover of native species (3±89%), archaeophytes (11± 2004b) it can be expected that their occurrence in weed 97%), and neophytes (0±47%), determined as the sum vegetation of present-day arable ®elds may not be driv- of covers of all species in each group compared with en by the same factors (Lososova et al. 2004). The aim the sum of covers of all species recorded in the sample of the present paper is to assess putative differences in plot; (4) crop cover (0±90%); (5) crop height (0±270 the environmental af®nities of native species, archaeo- cm); (6) crop type, with the following eight categories, phytes, and neophytes. representing also a speci®c type of management: ce- We employ a large data set of vegetation plots and reals 377 sample plots (following crops were distin- statistical analysis to determine the net effect of par- guished within this category: wheat 174, rye 110, bar- ticular explanatory variables. The present paper at- ley 65, oats 28), fodder 108 (legume±grass mixture 38, tempts to answer the following questions: (1) What are alfalfa 36, clover 29, other 5), root crops 90 (potato the principal variables determining the number and 64, beet 25, other 1), stubble (a ®eld with remains of cover of native species, archaeophytes, and neophytes crop after harvest) 55, rape (canola) 24, vegetable 19, on Central European arable land, and do they differ maize 18, and other crops 7 (¯ax, poppy, sun¯ower, among these groups? (2) What were the historical dy- millet); (7) year of the record (range 1955±2000); (8) namics of native, archaeophyte, and neophyte species season, derived from the date when the sample plot on arable land over the second half of the 20th century? was taken (mid-March to early October) and coded as the number of half-month periods from the beginning THE DATA of the year (e.g., 14 was a code for the second half of We used a data set of 712 vegetation plots from the July [Lososova et al. 2004]); altitude (range 145±950 Czech Republic, a country that represents a suitable m above sea level); (9) soil type, classi®ed into 9 cat- model for studies of diversity at a landscape scale be- egories, based on FAO-UNESCO (1988) classi®cation: cause of its variable geology and climate (NeuhaÈuslova cambisol (brown soil) 347 plots (further divided into et al. 2001). The survey was made by Z. KropaÂcÏin following subgroups: dystric 198, eutric 118, mollic 1955±2000, from March to October, in plots of a stan- 21, stagno-gleyic 10), luvisol 94, chernozem 80, cal- dard size of 100 m2.