In press in Ecological Applications 2002 Running head Reforestation with native trees in Panama 1 2 3 4 5 6 7 Responses of 20 native tree species to reforestation strategies for abandoned farmland in Panama 8 ELAINE HOOPER 1 2 3 4 RICHARD CONDIT 2 AND PIERRE LEGENDRE3 9 10 1 Department of Biology McGill University 1205 Dr Penfield Ave Montreal Quebec Canada 11 H3A 1B1 12 2 Center for Tropical Forest Science Smithsonian Tropical Research Institute Unit 0948 APO 13 AA 34002 0948 USA 14 3 D partment de Sciences Biologiques Universit de Montr al C P 6128 succursale Centre 15 ville Montr al Qu bec Canada H3C 3J7 16 17 18 19 20 4 Corresponding address Laboratoire de Pierre Legendre Department de Sciences Biologiques Universit de Montr al C P 6128 succursale Centre ville Montr al Qu bec Canada H3C 3J7 Hooper et al 2 1 Abstract Deforestation in the tropics often leads to unproductive agriculture and results in 2 abandoned degraded grasslands that tree species re colonize poorly To evaluate why forests do 3 not regenerate naturally and to identify potential species for use in reforestation of degraded 4 areas we planted 15 000 seeds of twenty native tree species varying in seed size and shade 5 tolerance in abandoned Panamanian farmland dominated by the exotic grass Saccharum 6 spontaneum To determine the effects of above and below ground constraints on tree seedling 7 germination survival and growth we utilized four mowing and shading treatments of the 8 Saccharum Shading the Saccharum effectively eliminated it whereas mowing led to increased 9 light above ground but did not reduce Saccharum growth rate Germination survival and 10 growth of tree seedlings approximately doubled in shade treatments compared to the unshaded 11 control but were lowest when the Saccharum was mown three times Fire significantly 12 decreased germination and survival 13 Some species did not follow these general trends though and we identified four species 14 groups that varied in their response to Saccharum competition Very small seeded light 15 demanding species performed poorly and we do not recommend their use in reforestation 16 because they tolerate neither above nor below ground constraints imposed by the Saccharum 17 Light demanding species with large seeds were limited by above ground constraints namely 18 shading Small seeded shade tolerant species were limited by below ground constraints imposed 19 by the Saccharum Large seeded moderately to highly shade tolerant species performed well in 20 the Saccharum we recommend a reforestation strategy which includes planting this last group 21 22 23 Keywords abandoned farmland natural regeneration Panama Saccharum spontaneum secondary succession tropical reforestation INTRODUCTION 24 Deforestation has left vast areas of primary humid tropical forest in a degraded state 25 Houghton et al 1991 Central America for example is now 10 abandoned agricultural land 26 Finegan 1992 Land rehabilitation through re establishment of native trees is becoming 27 increasingly attractive since native trees have been shown to improve soil conditions Hooper et al 3 1 significantly within four years on badly degraded tropical land Fisher 1995 Facilitating natural 2 tree regeneration may be an important management option but significant barriers to tree 3 regeneration must be overcome Lack of seed dispersal is often cited as a major limitation to 4 natural regeneration Guevara et al 1986 Janzen 1988 Nepstad et al 1990 Vieira et al 1994 5 Aide and Cavelier 1994 Aide et al 1995 da Silva et al 1996 and harsh microsite 6 characteristics including low nutrient status high irradiance high temperature and low soil 7 moisture during the dry season can also limit tree seedling survival Uhl et al 1988 Nepstad 8 1989 Nepstad et al 1990 Gerhardt 1993 9 These physiological stresses are worsened when abandoned lands are invaded by exotic 10 grasses that compete with tree seedlings for moisture and nutrients Nepstad 1989 and increase 11 the propensity for fire D Antonio and Vitousek 1992 In Panama abandoned sites in the Canal 12 area are invaded by the exotic invasive grass Saccharum spontaneum L ssp spontaneum 13 Saccharum attains an average height of 2 5 m and grows in dense impenetrable stands It is 14 drought adapted burns frequently and does not yield to weeding mulching fire or deep 15 plowing because of its deep rhizomes Panje 1970 It is listed as one of the most serious weeds 16 in Indonesia India Thailand the Philippines and Puerto Rico Holm et al 1979 17 Reforestation options in Saccharum grasslands have not been studied but there has been 18 considerable research on tree restoration of deforested tropical sites invaded by a similar grass 19 species Imperata cylindrica in South East Asia Nevertheless no feasible large scale method 20 has been found for restoring the original tree cover Planting indigenous tree seedlings directly 21 into the Imperata had limited success because of the physical difficulty of planting grass 22 competition allelopathy fire susceptibility as well as soil degradation and compaction 23 Kuusipalo et al 1995 Intensive deep plowing followed by planting of a fast growing exotic 24 tree crop to suppress grass and favour natural regeneration has been recommended Otsamo et al 25 1995 However the high cost of site preparation and planting seedlings for this type of 26 restoration program is economically unattractive for large scale application in tropical countries 27 Instead forest rehabilitation and management systems are needed that simultaneously accelerate Hooper et al 4 1 natural regeneration of species rich native forest while also providing economically and socially 2 valuable forest products Parrotta et al 1997 Our goal is to find a low cost alternative to 3 intensive mechanical site preparation and tree plantations initiated from nursery grown seedlings 4 for restoring productivity of Saccharum dominated grasslands in Panama 5 If seed source is a major limitation planting desired species would improve natural 6 regeneration Once trees are established they may act as regeneration nuclei by attracting 7 vertebrate seed dispersers Nepstad et al 1990 Lamb et al 1997 and providing favorable 8 germination and growth microhabitats Kellman 1985 Native tree species have been 9 underutilized in tropical reforestation projects despite their proven ability to grow successfully 10 on degraded pastures Gonzalez and Fisher 1994 Butterfield 1995 Butterfield and Espinoza 11
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