Precipitation and diversity

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Diversity is clearly correlated with climatic stability, and the plateau in Amazonian diversity from 1°N to 5°S falls squarely on the area with no predictable dry season, and the lowest variability in climate over time (Figures 10.5b, c). Both local and regional diversity in all data sets analyzed showed this trend. Diversity may also appear anomalously low in areas with a shorter dry season as calculated through the number of months that average below 100 mm, especially if the timing of rainfall is unpredictable and that area is subject to episodic super-annual drought that may not change average monthly figures in a systematic way. The correlation of diversity with seasonality and climate variability across latitude is seen clearly in western Amazonian plots, with the close correspondence between tree alpha diversity and climate standing in stark contrast to the patterns of vertebrate species diversity in western Amazonia, which remains nearly constant to ~14°S (Mares, 1992; Stotz et al., 1996; Symula et al., 2003). In addition to the latitudinal pattern, the longitudinal decrease in tree diversity in Amazonia also correlates well with both seasonality and climate variability. These data suggest that species are limited by physiological tolerances to drought, with areas of constant, wet climate decreasing water-use efficiency constraints and allowing species to occupy understory light environments that would be unprofitable in dry environments (Pitman et al., 2002). Comparisons of forests along climatic stability and rainfall gradients support this hypothesis, with species diversity of forests in wet areas increasing disproportionately rapidly among understory taxa, and with certain families being much more diverse in wet areas (Gentry and Emmons, 1987; Pitman et al., 2002). Givnish (1999) hypothesized that stable climate would increase distance-and density-dependence caused by moisture-loving plant pathogens. Though this complementary hypothesis remains untested, it would not necessarily predict the observed increase in specific plant families, or in understory plants. Indeed, it is unclear whether Janzen-Connell effects are stronger in the tropics than in the temperate zone (HilleRisLambers et al., 2002).

Previous results have shown both absolute amount of precipitation and average dry-season length to be related to diversity (Gentry, 1988; Clinebell et al., 1995; Ter

Tropical Rainforest Seasons Shows BothGamma Diversity Tropical Rainforest

Figure 10.7. Changes in dry-season length, precipitation variability, tree alpha diversity, and vascular plant gamma diversity with latitude for western Amazonian forests. Dry-season length and precipitation variability are lowest ~2°N to ~5°S. Tree alpha diversity shows a plateau from the equator to ~5°S. Gamma diversity, measured for the southern hemisphere, peaks at ~4°S with a slow decline south to ~11°S, with a steep decline farther south.

Elevation (m)

Figure 10.7. Changes in dry-season length, precipitation variability, tree alpha diversity, and vascular plant gamma diversity with latitude for western Amazonian forests. Dry-season length and precipitation variability are lowest ~2°N to ~5°S. Tree alpha diversity shows a plateau from the equator to ~5°S. Gamma diversity, measured for the southern hemisphere, peaks at ~4°S with a slow decline south to ~11°S, with a steep decline farther south.

Steege et al., 2003). Gentry (1988) presented evidence that showed diversity to be strongly correlated with absolute rainfall. He noted, however, that it was likely seasonality, as evidenced by the strength of the dry season, which was the true cause of the increase in diversity, rather than precipitation per se. Clinebell et al. (1995; and others subsequently) demonstrated an inverse correlation between diversity and the length of the dry season, taken as the number of months with average rainfall below either 65 mm or 100 mm.

Because rainfall in the Amazon is variable on multiple timescales, precipitation stability may be a better indicator of moisture regime as it affects plants over their life-cycles. Places with high average monthly rainfall but subject to episodic drought may cause decreases in diversity belied by measures of climate means. Areas with the same "average" dry-season length can have a wide range of variability, from a very consistent dry season from year to year, to one that is highly variable among years (Figures 10.3a-d).

The main oceanic climate forcings on Amazonian forests—El Nino/Southern Oscillation and the North Atlantic Oscillation/South Atlantic Oscillation dipole—do not affect Amazonian forests equally (Pezzi and Cavalcanti, 2001). For example, the warm phase of ENSO causes drought in the northeastern Amazon and Bolivia, and increased raininess in the western Amazon in Ecuador and northern Peru and central and southern Brazil. Central and southern Peru and the central Amazon are transition areas for ENSO correlations, and can have either positive or negative rainfall anomalies depending on the event. The main effect of oceanic forcings on Amazonian precipitation is to change the length of the rainy season while not changing the daily rain rate (Marengo et al., 2001). Because of this, the metric "dry-season length" varies across the Amazon. In areas of low variability the mean is representative of the long-term trend, while in areas of high variability the mean will over- or under-estimate dry-season length in any one year. Whether the variability affects forest structure and species composition awaits further study.

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