In press in Ecological Applications, 2002Running head: Reforestation with native trees in Panama12345Responses of 20 native tree species to reforestation strategies for6abandoned farmland in Panama78ELAINE HOOPER,1,2,3,4 RICHARD CONDIT,2 AND PIERRE LEGENDRE39101Department of Biology, McGill University, 1205 Dr. Penfield Ave, Montreal, Quebec, Canada11H3A 1B1122Center for Tropical Forest Science, Smithsonian Tropical Research Institute, Unit 0948, APO13AA 34002-0948 USA143Départment de Sciences Biologiques, Université de Montréal, C.P. 6128, succursale Centre-15ville, Montréal, Québec, Canada H3C 3J71617___________________184Corresponding address: Laboratoire de Pierre Legendre, Department de Sciences Biologiques,19Université de Montréal, C.P. 6128, succursale Centre-ville, Montréal, Québec, Canada H3C 3J720Hooper et al. 2Abstract. Deforestation in the tropics often leads to unproductive agriculture, and results in1abandoned, degraded grasslands that tree species re-colonize poorly. To evaluate why forests do2not regenerate naturally, and to identify potential species for use in reforestation of degraded3areas, we planted 15,000 seeds of twenty native tree species varying in seed size and shade-4tolerance, in abandoned Panamanian farmland dominated by the exotic grass, Saccharum5spontaneum. To determine the effects of above- and below-ground constraints on tree seedling6germination, survival, and growth, we utilized four mowing and shading treatments of the7Saccharum. Shading the Saccharum effectively eliminated it, whereas mowing led to increased8light above-ground, but did not reduce Saccharum growth rate. Germination, survival, and9growth of tree seedlings approximately doubled in shade treatments, compared to the unshaded10control, but were lowest when the Saccharum was mown three times. Fire significantly11decreased germination and survival.12Some species did not follow these general trends, though, and we identified four species13groups that varied in their response to Saccharum competition. Very small-seeded, light14demanding species performed poorly, and we do not recommend their use in reforestation15because they tolerate neither above- nor below-ground constraints imposed by the Saccharum.16Light-demanding species with large seeds were limited by above-ground constraints, namely17shading. Small-seeded, shade-tolerant species were limited by below-ground constraints imposed18by the Saccharum. Large-seeded, moderately to highly shade-tolerant species performed well in19the Saccharum; we recommend a reforestation strategy which includes planting this last group.20Keywords: abandoned farmland; natural regeneration; Panama; Saccharum spontaneum;21secondary succession; tropical reforestation.22INTRODUCTION23Deforestation has left vast areas of primary humid tropical forest in a degraded state24(Houghton et al. 1991). Central America, for example, is now 10% abandoned agricultural land25(Finegan 1992). Land rehabilitation through re-establishment of native trees is becoming26increasingly attractive, since native trees have been shown to improve soil conditions27Hooper et al. 3significantly within four years on badly degraded tropical land (Fisher 1995). Facilitating natural1tree regeneration may be an important management option, but significant barriers to tree2regeneration must be overcome. Lack of seed dispersal is often cited as a major limitation to3natural regeneration (Guevara et al. 1986, Janzen 1988, Nepstad et al. 1990, Vieira et al. 1994,4Aide and Cavelier 1994, Aide et al. 1995, da Silva et al. 1996), and harsh microsite5characteristics including low nutrient status, high irradiance, high temperature, and low soil6moisture during the dry season can also limit tree seedling survival (Uhl et al. 1988, Nepstad71989, Nepstad et al. 1990, Gerhardt 1993).8These physiological stresses are worsened when abandoned lands are invaded by exotic9grasses that compete with tree seedlings for moisture and nutrients (Nepstad 1989) and increase10the propensity for fire (D’Antonio and Vitousek 1992). In Panama, abandoned sites in the Canal11area are invaded by the exotic, invasive grass, Saccharum spontaneum L. ssp. spontaneum.12Saccharum attains an average height of 2.5 m and grows in dense, impenetrable stands. It is13drought-adapted, burns frequently, and does not yield to weeding, mulching, fire, or deep14plowing because of its deep rhizomes (Panje 1970). It is listed as one of the most serious weeds15in Indonesia, India, Thailand, the Philippines, and Puerto Rico (Holm et al. 1979).16Reforestation options in Saccharum grasslands have not been studied, but there has been17considerable research on tree restoration of deforested tropical sites invaded by a similar grass18species (Imperata cylindrica) in South-East Asia. Nevertheless, no feasible large-scale method19has been found for restoring the original tree cover. Planting indigenous tree seedlings directly20into the Imperata had limited success because of the physical difficulty of planting, grass21competition, allelopathy, fire susceptibility, as well as soil degradation and compaction22(Kuusipalo et al. 1995). Intensive deep plowing followed by planting of a fast-growing exotic23tree crop to suppress grass and favour natural regeneration has been recommended (Otsamo et al.241995). However, the high cost of site preparation and planting seedlings for this type of25restoration program is economically unattractive for large-scale application in tropical countries.26Instead, forest rehabilitation and management systems are needed that simultaneously accelerate27Hooper et al. 4natural regeneration of species-rich native forest while also providing economically and socially1valuable forest products (Parrotta et al. 1997). Our goal is to find a low-cost alternative to2intensive mechanical site preparation and tree plantations initiated from nursery-grown seedlings3for restoring productivity of Saccharum-dominated grasslands in Panama.4If seed source is a major limitation, planting desired species would improve natural5regeneration. Once trees are established, they may act as regeneration nuclei by attracting6vertebrate seed dispersers (Nepstad et al. 1990, Lamb et al. 1997) and providing favorable7germination and growth microhabitats (Kellman 1985). Native tree species have been8underutilized in tropical reforestation projects, despite their proven ability to grow successfully9on degraded pastures (Gonzalez and Fisher
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