Scrub vegetation in northern Guam
Scrub and grass fields are disturbed communities dominated by introduced herbaceous and woody vegetation. Human disturbance is causing many parts of Guam to convert from their natural state into areas characterized by scrub vegetation. Many previously forested parts of Guam are now open fields or mowed areas. Places once covered by canopy are now wide open areas such as parks, fields, military areas, and neglected fields of scrub and grass. The soils in such areas are often shallow and well drained, with little organic matter left on the surface. Scrub fields are the ideal habitat for introduced weeds and various non-native woody plants to take hold. The first to become established are various rapidly spreading grasses, such as Paspalum, Panicum, Dichanthium, Sorghum, and Saccharum.
The most notorious invasive plant that thrives in scrub areas is tangantangan (Leucaena leucocephala). After being seeded on the island by U.S. military following WWII, it out-competed many native plants and covered large parts of the island. Similarly aggressive srub plants are chain-of-love (Antigonon leptopus) and dodder (Cuscuta campestris), both of which are vines that block the sunlight and compete for water with native plants. Finally, there is agalondi (Vitex parviflora), which is a non-native tree but now one of the most dominant trees on island.
Because of colonization by scrub vegetation, open fields are a successional community that usually transitions to secondary forest of non-native trees. Nevertheless, open fields can be stable for long periods of time, especially when the level of disturbance is high. Grasses such as elephant grass (Pennisetum purpureum) and wild sugarcane (Saccharum spontaneum) can form almost homogeneous and relatively persistent stands on limestone substrate. The edges of open fields are lined by low-lying ferns, such as Nephrolepis hirsutula (as can be seen along roads in Dededo and Yigo). Another zone-forming shrubby species found at the edge of fields and roads is alum (Melanolepis multiglandulosa).
Ultimately, scrub trees, notably tangantangan and molave tree (Vitex parviflora), and vines, especially Mexican creeper (Antigonon leptopus) and dodder (Cuscuta campestris), colonize open spaces and block the sunlight needed by smaller plants. The result is a scrub forest largely composed of introduced species. Many of them are now dominant vegetation on the island and include pi’ot (Ximenia americana), candle bush (Senna alata), beggarticks (Bidens alba), kamachili (Pithecellobium dulce), Siam weed or masiksik (Chromolaena odorata), guava (Psidium guajava), and indigo (Indigofera suffruticosa).
Text above is slightly modified from
"Environments of Guam" by Danko Taborosi, David R. Burdick, Claudine M. Camacho, Frank Camacho, published by BessPress, 2013.
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