Post sugar cane succession in moist alluvial sites in Puerto Rico

Abstract

Sugar cane cultivation spanned over two centuries in Puerto Rico, at one time covering 14% of the island's total area. The most productive plantations were located in alluvial valleys that were deforested before there was an opportunity to record the species composition and structural development of the original native forests. This has led to speculation about the nature of these forests. With the abandonment of sugar cane cultivation in the twentieth century, sugar cane fields have been converted to pastures and a variety of secondary forests. One of these is a new forest type dominated by Spathodea campanulata, an invasive, wind-dispersed, and evergreen or nearly deciduous introduced species. We studied three stands of these forests at ages 25-40 years old on moist alluvial sites and found high basal area, intermediate tree density, high Complexity Index, and relatively low species richness. We recorded 19 tree species with dbh > 2.5 cm and 25 understory species equivalent to a total of 31 species. Of these, 11 species were introduced and 20 were native, including two endemic tree species. The large tree stratum (dbh > 10 cm) had high dominance of S. campanulata, 6 tree species, and a mean height of 12 m. Compared to large trees, small trees (> 2.5 <10 cm dbh) had higher species richness, and less dominance, while the understory had the highest species richness and lowest dominance. In spite of centuries of cultivation, forest soils were nutrient-rich, which allowed trees to attain large diameters, and produce nutrient-rich litter. The structural parameters of alluvial S. campanulata forests are comparable with those of remnant alluvial moist forests elsewhere on the north coast of Puerto Rico but the species composition is different. Periodic inundation of these forests appears to favor flood-tolerant species such as S. campanulata, and extends the period of domination of this species relative to its behavior in other geologic substrates in Puerto Rico. This process is exacerbated by long-term and extensive history of cultivation that reduced the species pool of seed sources for alluvial forests in the region. It will be difficult to restore the species complement of the original native forests because the disturbance regime of alluvial sites in Puerto Rico will continue to favor the presence, albeit reduced, of introduced species in the mature phases of these forests.