Tropical forests are among the most diverse plant communities on Earth and provide a variety of ecosystem services. They have therefore received widespread attention from policy makers and conservation organizations. Now a common assumption about them — that humid tropical forests are generally water saturated — is brought into question. An assessment of species distribution patterns across the Isthmus of Panama shows that drought plays a large role in structuring plant communities in these tropical forests. Changes in soil moisture availability due to climate change and habitat fragmentation are therefore likely to hit tropical species dramatically. Water availability directly shapes local and regional species distributions in tropical forests. This mechanistic understanding of the factors shaping species distributions is critical for improving vegetation–climate models used to project shifts in tropical forest composition, diversity and ecosystem function under changing precipitation patterns associated with past and future global climate change. Although patterns of tree species distributions along environmental gradients have been amply documented in tropical forests1,2,3,4,5,6,7, mechanisms causing these patterns are seldom known. Efforts to evaluate proposed mechanisms have been hampered by a lack of comparative data on species’ reactions to relevant axes of environmental variation1. Here we show that differential drought sensitivity shapes plant distributions in tropical forests at both regional and local scales. Our analyses are based on experimental field assessments of drought sensitivity of 48 species of trees and shrubs, and on their local and regional distributions within a network of 122 inventory sites spanning a rainfall gradient across the Isthmus of Panama. Our results suggest that niche differentiation with respect to soil water availability is a direct determinant of both local- and regional-scale distributions of tropical trees. Changes in soil moisture availability caused by global climate change and forest fragmentation are therefore likely to alter tropical species distributions, community composition and diversity.

Drought sensitivity shapes species distribution patterns in tropical forests.

http://www.umsl.edu/divisions/artscience/biology/files/pdfs/dave-clark/2002%20DrakeetalREM.pdf

Estimation of tropical forest structural characteristics using large-footprint lidar

During the mid-1900s, most of the island of Puerto Rico was deforested, but a shift in the economy from agriculture to small industry beginning in the 1950s resulted in the abandonment of agricultural lands and recovery of secondary forest. This unique history provides an excellent opportunity to study secondary forest succession and suggest strategies for tropical forest restoration. To determine the pattern of secondary succession, we describe the woody vegetation in 71 abandoned pastures and forest sites in four regions of Puerto Rico. The density, basal area, aboveground biomass, and species richness of the secondary forest sites were similar to those of the old growth forest sites (>80 yr) after approximately 40 years. The dominant species that colonized recently abandoned pastures occurred over a broad elevational range and are widespread in the neotropics. The species richness of Puerto Rican secondary forests recovered rapidly, but the species composition was quite different in comparison with old growth forest sites, suggesting that enrichment planting will be necessary to restore the original composition. Exotic species were some of the most abundant species in the secondary forest, but their long-term impact depended on life history characteristics of each species. These data demonstrate that one restoration strategy for tropical forest in abandoned pastures is simply to protect the areas from fire, and allow natural regeneration to produce secondary forest. This strategy will be most effective if remnant forest (i.e., seed sources) still exist in the landscape and soils have not been highly degraded. Patterns of forest recovery also suggest strategies for accelerating natural recovery by planting a suite of generalist species that are common in recently abandoned pastures in Puerto Rico and throughout much of the neotropics.

Forest Regeneration in a Chronosequence of Tropical Abandoned Pastures: Implications for Restoration Ecology

Aim

Greenhouse-induced warming and resulting shifts in climatic zones may exceed the migration capabilities of some species. We used fourteen combinations of General Circulation Models (GCMs) and Global Vegetation Models (GVMs) to investigate possible migration rates required under CO2-doubled climatic forcing.



Location

Global.



Methods

Migration distances were calculated between grid cells of future biome type x and nearest same-biome-type cells in the current climate. In `base-case' calculations, we assumed that 2 × CO2 climate forcing would occur in 100 years, we used ten biome types and we measured migration distances as straight-line distances ignoring water barriers and human development. In sensitivity analyses, we investigated different time periods of 2 × CO2 climate forcing, more narrowly defined biomes and barriers because of water bodies and human development.



Results

In the base-case calculations, average migration rates varied significantly according to the GVM used (BIOME3 vs. MAPSS), the age of the GCM (older- vs. newer-generation GCMs), and whether or not GCMs included sulphate cooling or CO2 fertilization effects. However, high migration rates (≥ 1000 m year−1) were relatively common in all models, consisting on average of 17% grid cells for BIOME3 and 21% for MAPSS. Migration rates were much higher in boreal and temperate biomes than in tropical biomes. Doubling of the time period of 2 × CO2 forcing reduced these areas of high migration rates to c. 12% of grid cells for both BIOME3 and MAPSS. However, to obtain migration rates in the Boreal biome that were similar in magnitude to those observed for spruce when it followed the retreating North American Glacier, a radical increase in the period of warming was required, from 100 to >1000 years. A reduction in biome area by an order of magnitude increased migration rates by one to three orders of magnitude, depending on the GVM. Large water bodies and human development had regionally important effects in increasing migration rates.



Main conclusions

In conclusion, evidence from coupled GCMs and GVMs suggests that global warming may require migration rates much faster than those observed during post-glacial times and hence has the potential to reduce biodiversity by selecting for highly mobile and opportunistic species. Several poorly understood factors that are expected to influence the magnitude of any such reduction are discussed, including intrinsic migrational capabilities, barriers to migration, the role of outlier populations in increasing migration rates, the role of climate in setting range limits and variation in species range sizes.

/pdf/estimated-migration-rates-under-scenarios-of-global-climate-2s4fjvk97u.pdf

Estimated migration rates under scenarios of global climate change

The abundance and diversity of lianas were examined along a tropical forest chronosequence at the Barro Colorado Nature Monument, Panama. Lianas M 0.5 cm diameter were sampled along transects in two replicated stands in second- ary (20, 40, 70 and 100 y after abandonment) and old-growth (>500 y) forests. Ordination of stands based on relative abundance, but not presence-absence, showed a significant separation of stands by age. Lianas were significantly more abundant and diverse (Fisher's α) in younger forests (20 and 40 y) than in older forests (70 and 100 y, and old-growth). The decline in liana abundance with stand age was offset by increased mean basal area per individual, resulting in a relatively constant total basal area and estimated biomass across stand age. The proportions of tendril climbers decreased and stem twiners increased over stand age. Decline in liana abundance and changes in liana composition may be related to changes in support and light availability. Although lianas are recognized as playing an important role in the early secondary succession of many tropical forests, these results have shown that their important contribution to total basal area and biomass can continue as the forest matures, even as the number of established lianas declines.

/pdf/density-and-diversity-of-lianas-along-a-chronosequence-in-a-5bn7mqvqdq.pdf

Density and diversity of lianas along a chronosequence in a central Panamanian lowland forest.

This long-awaited book would seem to mark the end of classical tropical botany and phytogeography as subjects for scholarly pursuits. Since the middle of the century, when the first edition of The Tropical Rain Forest appeared, the wet tropical lowlands of the world have become an industrial battleground and, today at the end of the "Second Millennium", the future of the remaining rainforests that have evolved over millions of years looks bleak. Indeed, the book may well become "a record of what the rainforest was like in the twentieth century", as stated on its first page. This elegiac declaration not only reflects world concern about its pending extinction, but also Professor Richards' increasingly personal involvement with rainforest conservation in his later years.

/pdf/the-tropical-rain-forest-an-ecological-study-z6cgm61wvf.pdf

Jiro Kikkawa

Papers

The Tropical Rain Forest: An Ecological Study