Emergy evaluation of reforestation alternatives in Puerto Rico

Abstract

Six alternative ways of reforesting degraded lands in Puerto Rico were evaluated using emergy (spelled with an 'm'). Emergy and its economic equivalent, emdollars, put the contributions of environmental work and human services on a comparable basis. This article shows the emergy method for evaluating forest contributions to public benefit and its use to select alternatives for reforestation. Emdollar values were compared for six scenarios for reforestation of degraded land in Puerto Rico: (1) the natural succession within or adjacent to mature forest; (2) reforestation from the spread of the exotic tree siris(Albizia lebbek); (3) reforestation with plantations of siris and mahogany for harvest; (4) reforestation by leaving plantations unharvested; (5) direct planting of seedlings of many species; and (6) starting patches of forest by massive transfer of topsoil, seed bank, and roots. After energy systems diagrams were made foreach reforestation alternative, data were assembled and evaluation tables prepared that estimated the emergy required for: (1) canopy closure and (2) developing species complexity if left unharvested. To explain the method, detailed calculations were included for one of the alternatives, exotic Albizia lebbek plantation on 11 yr harvest cycle. All alternatives generated net public benefit (emdollar yield ratios 4.2 to 24.3). The emdollar value of a closed canopy developed in 10 to 20 yr ranged from 20,000 to 48,000 em$/ha, whereas the economic costs were $1200 to $9700. For complex forest development in 25 to 60 yr, values ranged from 63,000 to 118,000 em$ /ha, much higher than economic costs of $4000 to $12,000/ha. Highest public benefit per dollar cost came from succession (24.7 em$/$) and exotic colonization (19.1 em$/$). Highest potential monetary returns were from exotic spread (15.1 $/$) and plantations (17.9 and 14.5 $/$). Stand quality after 60 yr, as measured by the transformity (emergy/energy), was largest in mahogany plantation (6.4 x 104 sej/J) and succession forest (3.9 x 104 sej/J).