Structure, Productivity, and Transpiration of a Subtropical Dry Forest in Puerto Rico

Plants in the Guainica Forest (lat. 180 N) grew under the following climatic conditions: annual temperature, 25.8'C; annual rainfall, 929 mm; annual pan evaporation, 2074 mm; actual evapotranspiration, 722 mm; yearly runoff, 205 mm; and mean solar radiation, 4366 kcal/m2/day. The climate was seasonal with most of the runoff between September and October. Soils normally exhibited water deficits during 10 months of the year. Shallow soils with frequent limestone outcropping compounded the water-supply problem of plants. Four upland plant associations were studied. The scrub forest had a basal area (BA) of 4.2 m2/ha and a complexity index (CI) of 2 calculated for trees with dbh > 5 cm. Its shallow soils were very sandy, or rocky. Succulent plants were common, and the canopy was open. The deciduous forest had a BA of 8 m2/ha and a CI of 20. Its soils were deeper, and comprised of 64 percent sand and 33 percent silt. A taller and denser canopy developed in this forest. A mahogany plantation grew in semi-evergreen forest soils. There, soils were as deep as 1 m and rich in clay and silt. An ecotonal forest (between deciduous and semi-evergreen) had a BA of 10.7 m2/ha and a CI of 20. The canopy of the ecotonal forest was relatively closed with a leaf area index (LAI) that varied seasonally between 1.2 and 4.2. Plant photosynthesis, respiration, transpiration, litter fall, and loose litter standing crop responded to moisture availability. Net daytime pho:osynthesis increased linearly with soil moisture availability (r = 0.76) and averaged 2.0 g/m2 leaf area/day for non-succulent plants. Nighttime respiration rates increased with decreasing soil moisture availability (r=0.65) and averaged 0.36 g/m2 leaf area/day for non-succulent plants and 0.1 g/m2 photosynthetic area/day for succulent plants. Tree trunks respired during the dry season at a rate of 2.64 g/m2 surface area/day. Soil respiration was 4.74 g/m2/day. Transpiration of succulent plants was 64 g H20/m2 photosynthetic area/day, while for non-succulent plants it was 576 g H20/m2 leaf area/day. Water-use efficiency was 6.2 for succulent plants and 1.8 for non-succulent plants. Succulent plants also exhibited nighttime or 24 hr C-uptake and water loss during early morning or late afternoon. Leaf fall and standing crop of loose litter increased with decreasing soil moisture availability. Mean litter-fall production for the scrub, deciduous forest, and mahogany plantation in g/m2/day was: 0.28, 0.79. and 1.48, respectively. Mean litter fall and mean standing crop of loose litter for the forest as a whole were 0.66 g/m2/day and 715 g/m2, respectively. Mean annual litter turnover was 0.34. Gross primary productivity averaged 6 g/m2/day and correlated linearly with soil moisture availability (r = 0.87). It is concluded that soil moisture availability (a function of shallow soils and low rainfall) is the factor that determines forest productivity, growth characteristics, water loss, and physiognomy. THE GUANICA FOREST is located on the southwestern coast of Puerto Rico (approx. 180 N lat.). The forest is underlain by limestones of Miocene and Oligocene age which surface in some places, and become virtually the only substrate for plant growth at elevations below 80 m (Monroe 1976). The 3998 ha forest became a Commonwealth Forest in 1917 and has been protected from farming or grazing for more than 40 years although post-sized timber has been extracted periodically. Ewel and Whitmore (1973) classified the area in the Subtropical Dry Forest Life Zone, a water-limited life zone. The vegetation of this region has been described by a number of authors (Gleason and Cook 1927, Wadsworth 1950, Williams 1967, Little and Wadsworth 1964, Kepler and Kepler 1973, and Little et al. 1974). Quantitative descriptions of vegetation, however, are lacking. Gleason and Cook (1927) described seven plant associations for the south coast of Puerto Rico and discussed their successional relationships. They noted the presence of a bank of epiphytic plants in the Guanica Forest at an elevation of 80 m. This elevation coincides roughly with the transition between the scrubby cactus forest growing at lower elevations and the continuous canopy forest of the uplands. In a generalized vegetation map of Puerto Rico, Little and Wadsworth (1964) referred to the Guainica Forest as a Dry Limestone Forest. This study was undertaken as a necessary prerequisite to the development and implementation of a management plan for the Guainica Forest. In this paper, we present the most important findings of a three-year ( 1974-1976) research effort that included mapping and quantitative descriptions of plant associations, monitoring of litter fall and climatic factors, determination of rates of C02 and H20 gas exchange, and modeling of ecosystem processes. Results of computer simulations of ecosystem processes are not presented in this report but were reported in Gonzalez-Liboy et al. (1976) and may be obtained from K. Dugger. 1Current address: Department of Botany, University of Florida, Gainesville, Florida 32611, U.S.A. 2The editor apologises to the authors for having set all their rate expressions, such as 4366 Kcal/m2. day, in the traditional form 4366 Kcal/m2/day. 278 BIOTROPICA 10(4): 278-291 1978 This content downloaded from 157.55.39.17 on Fri, 02 Sep 2016 05:41:35 UTC All use subject to http://about.jstor.org/terms