Tropical Elevational Gradient Research Articles

Altitudinal variation in leaf mass per unit area, leaf tissue density and foliar nitrogen and phosphorus content along an Amazon-Andes gradient in Peru

Background: Leaf traits are important in determining the capacity for a plant to acquire carbon, but few data are available for montane cloud forests in the Andes. Aims: To investigate the changes in leaf traits along a large altitudinal transect (220–3600 m) from lowland to montane cloud forest in Peru. Methods: We determined leaf mass per area (LMA, g m−2), leaf tissue density (L d, g cm−3), and foliar nitrogen (N) and phosphorus (P) content, both on a mass (N m and P m, %) and area (N a and P a, g m−2) basis for the most abundant species locally. Results: LMA increased with altitude (62.8–169.4 g m−2), though overall, LMA was lower than in comparable tropical elevation gradients. N m declined significantly with altitude (2.39–1.25 %, P < 0.05), but N m contents were higher than in comparable studies. The relatively high N m and low LMA values are consistent with published global leaf trait datasets. No altitudinal trend for P m was found; rather, our data highlighted the spatial variability in P m (and P a) within and among sites at different elevations. Foliar N:P ratios did not show a trend with altitude and did not indicate N limitation except at 3000 m altitude. Conclusions: Though leaf traits showed altitudinal trends similar to other studies, contrary to the general hypothesis, our data suggest that the tropical montane forests presented here are not N limited.

Elevational distribution and zonation of tropical pteridophyte assemblages in Costa Rica

Abstract The use of species turnover rates to identify discontinuities in vegetation assemblages along environmental gradients is hampered by the difficulty of the application of significance tests. We used a simulation procedure to detect abrupt transitions in fern species assemblages along a tropical elevational gradient to detect elevational zonations between 100 and 3400 m in a species-rich tropical forest in Costa Rica. Significant species turnover values were only observed at 300, 1000, and 3200 m. These boundaries in vegetation assemblages were caused by different lifeforms within the pteridophyte species set: epiphytes attained high turnover values at low, terrestrials at high elevations. Accordingly, the mean elevational ranges of epiphytes and terrestrials differed clearly at high elevations, where those of terrestrial species showed a sharp decline above ca. 2500 m, but those of epiphytes kept more or less constant. The boundaries of significant range limit accumulations coincide with shifts in the environmental settings, namely humidity conditions and forest structure. The location of the observed boundaries in turn influences the elevational amplitudes of species restricted to the resulting elevational zones, determining the elevational distribution of range amplitudes. Between 1000 and 3200 m no significant boundaries were observed, although it has been argued that the lower cloud condensation layer and the frost line are the prime drivers of elevational zonations.

Body size and host range co‐determine the altitudinal distribution of Neotropical tephritid flies

ABSTRACTAim We addressed the following questions: (1) Does tephritid body size tend to increase in species found at higher elevations, as predicted by Bergmann's rule? (2) Do tephritids conform to Rapoport's rule, so that species found at higher elevations tend to have broader altitudinal ranges? (3) More generally, how do body size and host range jointly affect the patterns of altitudinal distribution among Neotropical tephritid flies?Location The Mantiqueira mountain range, south‐eastern Brazil, at sites ranging from c. 700 to 2500 m a.s.l.Methods At each site we collected flower heads of all Asteraceae species to rear out endophagous immatures (from January to June in 1998 and 1999). We used structural equation models (SEM) to evaluate jointly the relationships between body size, host range and altitudinal distribution (range and mid‐point).Results Neotropical tephritid body size showed a negative relationship with altitudinal distribution. SE modelling showed no significant direct effect of body size on altitudinal range; however, it had significant indirect negative effects through host range and altitudinal mid‐point. The SE model was a good predictor of observed correlations and accounted for 84% of the variation in tephritid altitudinal range.Main conclusions The altitudinal range of flower‐head‐feeding tephritids is related to host range and is indirectly affected by body size via host range and altitudinal mid‐point. As predicted by Rapoport's rule, tephritids that occur at higher elevations also present wider altitudinal ranges. Bergmann's rule does not apply to Neotropical tephritids along a tropical elevational gradient, but rather its converse was found. Body size may determine host range by imposing a restriction upon large individuals using small flower heads. Host species turnover along the altitudinal gradient may be the main factor explaining the strong relationship between host range and insect elevational distribution.

The role of environment and mid‐domain effect on moth species richness along a tropical elevational gradient

ABSTRACTAim The biodiversity of geometrid moths (Lepidoptera) along a complete tropical elevational gradient was studied for the first time. The patterns are described, and the role of geometric constraints and environmental factors is explored.Location The study was carried out along the Barva Transect (10° N, 84° W), a complete elevational gradient ranging from 40 to 2730 m a.s.l. in Braulio Carrillo National Park, Costa Rica, and adjacent areas.Methods Moths were sampled manually in 2003 and 2004 at 12 rain forest sites using light ‘towers’, each with two 15 W ultraviolet fluorescent tubes. We used abundance‐based rarefaction, statistical estimation of true richness (Chao 1), geographically interpolated observed richness and Fisher's alpha as measures of local diversity.Results A total of 13,765 specimens representing 739 species were analysed. All four measures showed a hump‐shaped pattern with maxima between 500 and 2100 m elevation. The two subfamilies showed richness and diversity maxima at either lower (Ennominae) or higher (Larentiinae) elevation than Geometridae as a whole. Among the four environmental factors tested, relative humidity yielded the highest correlation over the transect with the rarefaction‐based richness estimates as well as with estimated true species richness of Geometridae as a whole and of Larentiinae, while rainfall explained the greatest variation of Ennominae richness. The elevational pattern of moth richness was discordant with both temperature and with tree species richness. A combination of all environmental factors in a stepwise multiple regression produced high values of r2 in Geometridae. The potential effects of geometric constraints (mid‐domain effect, MDE) were investigated by comparing them with observed, interpolated richness. Overall, models fitted very well for Geometridae as a whole and for Ennominae, but less well for Larentiinae. Small‐ranged species showed stronger deviations from model predictions than large‐ranged species, and differed strikingly between the two subfamilies, suggesting that environmental factors play a more pronounced role for small‐ranged species. We hypothesize that small‐ranged species (at least of the Ennominae) may tend to be host specialists, whereas large‐ranged species tend to be polyphagous. Based on interpolated ranges, mean elevational range for these moths was larger with increasing elevation, in accordance with Rapoport's elevational rule, although sampling effects may have exaggerated this pattern. The underlying mechanism remains unknown because Rapoport's ‘rescue’ hypothesis could not explain the observed pattern.Conclusions The results clearly show that moth diversity shows a hump‐shaped pattern. However, remarkable variation exists with regard to taxon and range size. Both environmental and geometric factors are likely to contribute to the observed patterns.

The mid‐domain effect applied to elevational gradients: species richness of small mammals in Costa Rica

AbstractAim The objective of this study was to comprehensively document and examine the alpha and gamma patterns of species richness in non‐volant, small mammals (rodents, shrews and mouse opossums) along a tropical elevational gradient. These data were used to determine the support for existing hypotheses of species richness encompassing mid‐domain null models, as well as climatic, and community overlap hypotheses.Location Field studies were conducted along a Caribbean slope of the Río Peñas Blancas watershed in the north‐eastern region of Costa Rica between 750 and 1850 m at 10 sampling sites.Methods Species richness and abundances of small mammals were surveyed for four seasons including three temporal replicates at each of five elevational sites: late wet season (2000), early wet season (2001), and dry season (2002), and one spatial replicate at five different sites within the same elevations during the late wet season (2001). Species richness at elevations below 700 m was compiled from specimen records from 23 US national and international collections. Predictions of a null model based solely on geometric constraints were examined using a Monte Carlo simulation program, Mid‐Domain Null.Results In 16,900 trap nights, 1561 individuals from 16 species were captured. Both alpha and gamma species richness peaked at mid‐elevation between 1000 and 1300 m, with richness declining both at higher and lower elevations. Most of the empirical curves of species richness occur within 95% prediction curves of the mid‐domain model, although deviations from the null model exist. Regression of the empirical richness on the null model predictions explained nearly half of the variation observed (r2 = 0.45, P = 0.002).Main conclusions The geometric constraints of montane topography appear to influence the diversity pattern of small mammals, although climatic conditions including an intermediate rainfall and temperature regime, and distance from the persistent cloud cap also are correlated with the pattern of species richness. The predictions of productivity, and community overlap hypotheses are not supported with the empirical data.

Variations in Belowground Carbon Storage and Soil CO2 Flux Rates along a Wet Tropical Climate Gradient1

We used a humid tropical elevation gradient to examine the relationships among climate, edaphic conditions, belowground carbon storage, and soil respiration rates. We also compared open and closed canopy sites to increase the range of microclimate conditions sampled along the gradient, and determine the effects of canopy openings on C and P storage, and C dynamics. Total soil C, the light C fraction, and all of the component fractions of the P pool were significantly related to soil moisture, and all but total soil C were also significantly related to temperature. Both labile and recalcitrant soil P fractions were negatively correlated with the light C fraction, while the dilute HCl-extractable P pool, generally thought of as intermediate in availability, was positively correlated with light C, suggesting that P may play an important role in C cycling within these systems. Total fine root biomass was greatest at 1000 m elevation and lowest at 150 m, and was strongly and positively correlated with soil moisture content. Soil respiration rates were significantly and negatively correlated with fine root biomass and the light C fraction. In forested sites, soil respiration rates were strongly and negatively correlated with total belowground C pools (soils 1 roots 1 forest floor). Belowground C pools did not follow the expected increasing trend with decreases in temperature along the gradient. Our results indicated that in humid tropical forests, the relationships among soil C and nutrient pools, soil respiration rates, and climate are complex. We suggest that frequent and prolonged anaerobic events could be important features of these environments that may explain the observed trends.

Avian distribution patterns along a Sonoran Desert bajada

Avian population density, foraging behavior, nest site selection and other aspects of vegetation use were studied at four points along a vegetation gradient at Organ Pipe Cactus National Monument, Arizona, U.S. A., to elucidate causes of species distributions. The vegetation gradient, a result of edaphic change along a bajada (coalesced alluvial fans), ranged from a diverse subtree-large cacti-shrub community on gravelly soils at the top of the bajada to a simple low shrub community on fine sandy loam at the bottom of the bajada. Twenty-three species of birds bred in the diverse vegetation at the top of the bajada. Proceeding down the bajada, bird species dropped out of the community as vegetation architecture and composition became more simple, with only two species breeding all along the gradient. Most species distributions were determined by requirements for specific vegetation components for nesting and foraging. Competition and abiotic factors seem to have played only a minor role, if any, in determining distributions. Causes for species distributions along this bajada are similar to those affecting avian distribution along vegetation gradients on temperate mountain slopes but differ from those along tropical elevational gradients.