Ecosystem dynamics of a subtropical floodplain forest

Abstract

The carbon,p hosphorus,a nd waterc ycleso f a subtropicalf loodplainf orest, and related ecosystemc haracteristics, were studied. Located at 750 m elevation in Puerto Rico (latitude 18? N) the foresth ad 27 tree species, 3059 stems/ha,a basal area of 42.4 m2/ha,m aximum heighto f 17 m, and a leaf area index of 3.3. Palm (Prestoea montana) dominated the forest,a nd, with two other species,a ccountedf or6 8% oft hed ominance.T hroughfall,st emflow,in terception,ru noff,tr anspiration, and evapotranspirationa ccounted for 81.6, 9.8, 8.6, 77.7, 13.7, and 22.3% of annual rainfall( 3725 mm) respectively.T he stand carbon storagew as 28.77 kg/M2,d istributeda s follows: aboveground 35.4%, vegetation 44.2%, soil to 1 m depth 55%, palms 10.9%. Vegetation biomass was partitioned as follows: leaves 9.8% (75% are palm leaves), wood 68%, and roots 21.8%. Net aboveground primary carbon productivity(N PP) was 876 g-m-2 g.m-2* d-' (palm leaves 32%, other leaves 39%, and wood 10%). Half-lives of decaying material were 188, 306, 462, and 576 d, respectively, forp alm leaves still attached to the parentt ree,d icotyledonousl eaves, palm leaves on the ground,a nd palm trunks.T otal organic carbon concentrationsi n stream water increased with increasings treamd ischarge( from2 g/m3t o 30 g/m3).W atershede xporto f carbon was 50 g-m-2 yr-' (including 12 g m-2 yr-I in the formo f leaf litter).M ean P concentrationi n palm leaves (1.18 mg/g) was twice that in dicotyledonousl eaves (0.64 mg/g).C ompared to a rainfallp hosphorusi nput to the watershedo f 63 mg m-2 yr-', leachingf romt he canopy was high (167 mg.m-2 yr-'), as was the loss of P fromt he watershed( 611 mg m-2 yr-'). Phosphorus-conservingm echanismsi ncluded a highr ate of retranslocationin palms (504 mg m-2 yr-'). In spite of these mechanisms,t herew as a net P loss fromt he watershedt hat ranged from 136 to 544 mg iM-2 yr-'. Periodic flooding,p oor soil aeration, intensivey ear-roundr ainfall,a nd low atmospherics aturationv apor pressured eficitsa re believed to be the main drivingf orces of the floodplain forest,w hich exhibits many characteristicst ypical of lowland rain forestsa nd floodplainw etlands.R ates of NPP, litterfall,an d biomass turnover( residence timeo f 14-17 yr)a re fastert hane xpectedf ort hec limaticc onditions,w hereasr ateso f wood production and storageo f organicm atteri n the vegetationa nd soil profilea re lowert han expectedf ort he climate.