Quantifying the deciduousness of tropical forest canopies under varying climates

Deciduousness is an important functional attribute of tropical trees, refle cting climatic conditions. Precisely quantifying and mapping deciduousness in tropical forests will be necessary for calibrating remote sensing images which attempt to assess canopy properties such as carbon cycling, producti vity, or chlorophyll content. We thus set out to assess the degree of canop y deciduousness in three moist, semi-deciduous tropical forests in central Panama. One site is a 6-ha research plot near the Atlantic coast of Panama, where rainfall is 2830 mm/yr. The second site is a 50-ha plot on Barro Col orado Island, near the center of the isthmus of Panama, where rainfall is 2 570 mm/yr, and the final site is a 4-ha plot near the Pacific coast of Pana ma, where rainfall is 2060 mm/yr. At each site, a random sample of trees fr om all canopy species (those with individuals greater than or equal to 30 c m DBH) were visited and scored for deciduousness three times during the 199 7 dry season. The estimated peak fraction of deciduous individuals in the c anopy at the wetter site was 4.8%, at the intermediate site, 6.3%, and at t he drier site, 24.3%. The estimated fraction of crown area deciduous peaked at 3.6%, 9.7%, and 19.1% at the wet, medium, and dry sites respectively. T he percentage of canopy species that was deciduous -14%, 28%, and 41%-was m uch higher than the percentage of deciduous individuals, because not all in dividuals of deciduous species were deciduous. During the 1999 dry season, every individual of all the deciduous species was visited at the two drier sites, and the total number of deciduous trees observed closely matched the estimated numbers based on the smaller 1997 samples.