The Vegetation of Wisconsin: An Ordination of Plant Communities

21. Weed communities

CHAPTER 21 Weed communItIes NATURE OF DISTURBED HABITATS All of the communities treated in previous chapters occlIr ill undisturbed habitats or else are subject to what might be termed natural disturbance as induced by fire, wind, wave action, or f1ooding_ They represent a relatively rare condition in the present pattern of land lIse_ The majority of \Visconsin's land surfaces have been greatly modified by man through a variety of artificial disturbances. The plant groupings which occupy these modified areas differ in many ways from the natural communities; these groupings are the subject of the present chapter. The degree and the type of artificial disturbance of the habitat are obviously subject to great variation and are difficult to classify readily. Adopting the terminology of Hamel and Dansereau (1949), they may be grouped into three divisions for the sake of convenience. If the disCopyrighted Material WEED COM;\ll!NITlFS 413 wrbance of the original community is incomplete and sporadic, the habitat is said to be "degraded." If the original community is destroyed and the destructive agent is repeatedly applied, the habitat is called "ruderal" if the area is not used for the production of economic crops and "cultivated" if the area is used for crop production. The degraded habitats include prairies and forests used for pastures, meadows and fens used for hayfields, and forests used for intensive lumber production. The ruderal sites are extremely varied and include roadsides, spoil banks, railroad yards, trackways, city dumps, stockyards, and other waste places. The cultivated areas are farm lands devoted to field crops, vegetable gardens, orchards, and other intensively used areas that are ontinarily plowed or cultivated at least once each year. In spite of the great variety of such disturbed habitats, they almost always have one feature in common, namely the presence of a soillaycr of some sort, either left over from the original vegetation or produced artificially by mechanical mixing as in fills and banks. This is the major difference between disturbed sites and native primary sites on rock, water, or sand. The open disturbed areas also resemble each other; usually they are exposed to full sun, have violent fluctuations of daily temperature, and show rapid changes in water supply. The degraded forest sites naturally have fewer pioneer characteristics and may differ only slightly from natural conditions in certain situations. Another feature that is frequently present in disturbed habitats is a lack of soil stability, which is expressed as settling movements or as erosional shiftings of the surface. Again, this character is less evident in degraded habitats, especially in pastures or mowing meadows. NATURE OF SECONDARY SUCCESSION When a ruderal or cultivated site or severely degraded community is relieved from the causal disturbing agent and allowed to develop by itself, a secondary succession is initiated. The pioneer species are adapted to the disturbed conditions and are largely of the plant type called weeds. In each kind of area the very first plants are mostly species which were present during the period of active disturbance. As the succession proceeds, other weeds invade the area, together with pioneer native species from nearby native communities. With time, the proportion of native species increases until eventually a mature community , characteristic of the region, is developed. The time interval between initial abandonment and final recovery may be very great. Drew (1942) made a thorough study of the revegetation of abandoned Copyrighted Material 414 LESSER COMMUNITIES cropland in Missouri. He reviewed most of the literature dealing with the process and found that 25 to 40 years were required for the establishment of the dominants of the native community under a variety of conditions in central and eastern United States. None of the investigators ever had observed areas where the complete community was restored , although several thought this might occur within 80 to 100 years. In Wisconsin, such studies were made by Thomson (1943) and Buss (1956) and were discussed in Chapter 14. They both found an interpolation of what might be called a prairie stage in the secondary succession from cropland to forest. This phenomenon is relatively common in Wisconsin, since widely distributed prairie remnants provide sources of propagules for the invasion of the abandoned areas and since many of the prairie plants are well adapted to grow in such sites. In the absence of an inoculum of prairie species, the forest plants may invade the area directly; the type will depend on the moisture relations...