SciCombinator

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Concept: Elevation

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Critical thermal limits are thought to be correlated with the elevational distribution of species living in tropical montane regions, but with upper limits being relatively invariant compared to lower limits. To test this hypothesis, we examined the variation of thermal physiological traits in a group of terrestrial breeding frogs (Craugastoridae) distributed along a tropical elevational gradient. We measured the critical thermal maximum (CT max; n = 22 species) and critical thermal minimum (CT min; n = 14 species) of frogs captured between the Amazon floodplain (250 m asl) and the high Andes (3,800 m asl). After inferring a multilocus species tree, we conducted a phylogenetically informed test of whether body size, body mass, and elevation contributed to the observed variation in CT max and CT min along the gradient. We also tested whether CT max and CT min exhibit different rates of change given that critical thermal limits (and their plasticity) may have evolved differently in response to different temperature constraints along the gradient. Variation of critical thermal traits was significantly correlated with species' elevational midpoint, their maximum and minimum elevations, as well as the maximum air temperature and the maximum operative temperature as measured across this gradient. Both thermal limits showed substantial variation, but CT min exhibited relatively faster rates of change than CT max, as observed in other taxa. Nonetheless, our findings call for caution in assuming inflexibility of upper thermal limits and underscore the value of collecting additional empirical data on species' thermal physiology across elevational gradients.

Concepts: Scientific method, Evolution, Biology, Temperature, Heat, Hypothesis, Maxima and minima, Elevation

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Elevational gradients of biodiversity have been widely investigated, and yet a clear interpretation of the biotic and abiotic factors that determine how species richness varies with elevation is still elusive. In mountainous landscapes, habitats at different elevations are characterized by different areal extent and connectivity properties, key drivers of biodiversity, as predicted by metacommunity theory. However, most previous studies directly correlated species richness to elevational gradients of potential drivers, thus neglecting the interplay between such gradients and the environmental matrix. Here, we investigate the role of geomorphology in shaping patterns of species richness. We develop a spatially explicit zero-sum metacommunity model where species have an elevation-dependent fitness and otherwise neutral traits. Results show that ecological dynamics over complex terrains lead to the null expectation of a hump-shaped elevational gradient of species richness, a pattern widely observed empirically. Local species richness is found to be related to the landscape elevational connectivity, as quantified by a newly proposed metric that applies tools of complex network theory to measure the closeness of a site to others with similar habitat. Our theoretical results suggest clear geomorphic controls on elevational gradients of species richness and support the use of the landscape elevational connectivity as a null model for the analysis of the distribution of biodiversity.

Concepts: Scientific method, Ecology, Gradient, Elevation, Topography, Complex network, Landscape, Geomorphology

27

Climate change is affecting high-altitude and high-latitude communities in significant ways. In the short growing season of subarctic habitats, it is essential that the timing and duration of phenological phases match favorable environmental conditions. We explored the time of the first appearance of flowers (first flowering day, FFD) and flowering duration across subarctic species composing different communities, from boreal forest to tundra, along an elevational gradient (600-800 m). The study was conducted on Mount Irony (856 m), North-East Canada (54°90'N, 67°16'W) during summer 2012. First, we quantified phylogenetic signal in FFD at different spatial scales. Second, we used phylogenetic comparative methods to explore the relationship between FFD, flowering duration, and elevation. We found that the phylogenetic signal for FFD was stronger at finer spatial scales and at lower elevations, indicating that closely related species tend to flower at similar times when the local environment is less harsh. The comparatively weaker phylogenetic signal at higher elevation may be indicative of convergent evolution for FFD. Flowering duration was correlated significantly with mean FFD, with later-flowering species having a longer flowering duration, but only at the lowest elevation. Our results indicate significant evolutionary conservatism in responses to phenological cues, but high phenotypic plasticity in flowering times. We suggest that phylogenetic relationships should be considered in the search for predictions and drivers of flowering time in comparative analyses, because species cannot be considered as statistically independent. Further, phenological drivers should be measured at spatial scales such that variation in flowering matches variation in environment.

Concepts: Time, Evolution, Biology, Phylogenetic tree, Phylogenetics, Phylogenetic comparative methods, Elevation, Evolutionary physiology

27

An investigation of terrestrial bryophyte species diversity and community structure along an altitudinal gradient from 2,001 to 4,221 m a.s.l. in Gongga Mountain in Sichuan, China was carried out in June 2010. Factors which might affect bryophyte species composition and diversity, including climate, elevation, slope, depth of litter, vegetation type, soil pH and soil Eh, were examined to understand the altitudinal feature of bryophyte distribution. A total of 14 representative elevations were chosen along an altitudinal gradient, with study sites at each elevation chosen according to habitat type (forests, grasslands) and accessibility. At each elevation, three 100 m × 2 m transects that are 50 m apart were set along the contour line, and three 50 cm × 50 cm quadrats were set along each transect at an interval of 30 m. Species diversity, cover, biomass, and thickness of terrestrial bryophytes were examined. A total of 165 species, including 42 liverworts and 123 mosses, are recorded in Gongga mountain. Ground bryophyte species richness does not show any clear elevation trend. The terrestrial bryophyte cover increases with elevation. The terrestrial bryophyte biomass and thickness display a clear humped relationship with the elevation, with the maximum around 3,758 m. At this altitude, biomass is 700.3 g m and the maximum thickness is 8 cm. Bryophyte distribution is primarily associated with the depth of litter, the air temperature and the precipitation. Further studies are necessary to include other epiphytes types and vascular vegetation in a larger altitudinal range.

Concepts: Plant, Elevation, Topography, Bryophyte, Moss, Species diversity, Marchantiophyta, Contour line

16

Seagrass meadows provide numerous ecosystem services and their rapid global loss may reduce human welfare as well as ecological integrity. In common with the other ‘blue carbon’ habitats (mangroves and tidal marshes) seagrasses are thought to provide coastal defence and encourage sediment stabilisation and surface elevation. A sophisticated understanding of sediment elevation dynamics in mangroves and tidal marshes has been gained by monitoring a wide range of different sites, located in varying hydrogeomorphological conditions over long periods. In contrast, similar evidence for seagrasses is sparse; the present study is a contribution towards filling this gap. Surface elevation change pins were deployed in four locations, Scotland, Kenya, Tanzania and Saudi Arabia, in both seagrass and unvegetated control plots in the low intertidal and shallow subtidal zone. The presence of seagrass had a highly significant, positive impact on surface elevation at all sites. Combined data from the current work and the literature show an average difference of 31 mm per year in elevation rates between vegetated and unvegetated areas, which emphasizes the important contribution of seagrass in facilitating sediment surface elevation and reducing erosion. This paper presents the first multi-site study for sediment surface elevation in seagrasses in different settings and species.

Concepts: Ecology, Coast, Saudi Arabia, Elevation, Intertidal zone, Salt marsh, Intertidal ecology, Seagrass

13

Mangrove wetlands provide ecosystem services for millions of people, most prominently by providing storm protection, food and fodder. Mangrove wetlands are also valuable ecosystems for promoting carbon © sequestration and storage. However, loss of mangrove wetlands and these ecosystem services are a global concern, prompting the restoration and creation of mangrove wetlands as a potential solution. Here, we investigate soil surface elevation change, and its components, in created mangrove wetlands over a 25 year developmental gradient. All created mangrove wetlands were exceeding current relative sea-level rise rates (2.6 mm yr(-1)), with surface elevation change of 4.2-11.0 mm yr(-1) compared with 1.5-7.2 mm yr(-1) for nearby reference mangroves. While mangrove wetlands store C persistently in roots/soils, storage capacity is most valuable if maintained with future sea-level rise. Through empirical modeling, we discovered that properly designed creation projects may not only yield enhanced C storage, but also can facilitate wetland persistence perennially under current rates of sea-level rise and, for most sites, for over a century with projected medium accelerations in sea-level rise (IPCC RCP 6.0). Only the fastest projected accelerations in sea-level rise (IPCC RCP 8.5) led to widespread submergence and potential loss of stored C for created mangrove wetlands before 2100.

Concepts: Biodiversity, Wetland, Ecosystem, Biome, Elevation, Ecosystem services, Biodiversity Action Plan, Mangrove

13

Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves. Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth. The Indo-Pacific region holds most of the world’s mangrove forests, but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers. This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise. Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments. We find that sediment availability can enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.

Concepts: Oceanography, Marsh, Wetland, Ecosystem, River, Sustainability, Elevation, Mangrove

8

Our ability to model global carbon fluxes depends on understanding how terrestrial carbon stocks respond to varying environmental conditions. Tropical forests contain the bulk of the biosphere’s carbon. However, there is a lack of consensus as to how gradients in environmental conditions affect tropical forest carbon. Papua New Guinea (PNG) lies within one of the largest areas of contiguous tropical forest and is characterized by environmental gradients driven by altitude; yet, the region has been grossly understudied. Here, we present the first field assessment of aboveground biomass (AGB) across three main forest types of PNG using 193 plots stratified across 3,100-m elevation gradient. Unexpectedly, AGB had no direct relationship to rainfall, temperature, soil, or topography. Instead, natural disturbances explained most variation in AGB. While large trees (diameter at breast height > 50 cm) drove altitudinal patterns of AGB, resulting in a major peak in AGB (2,200-3,100 m) and some of the most carbon-rich forests at these altitudes anywhere. Large trees were correlated to a set of climatic variables following a hump-shaped curve. The set of “optimal” climatic conditions found in montane cloud forests is similar to that of maritime temperate areas that harbor the largest trees in the world: high ratio of precipitation to evapotranspiration (2.8), moderate mean annual temperature (13.7°C), and low intra-annual temperature range (7.5°C). At extreme altitudes (2,800-3,100 m), where tree diversity elsewhere is usually low and large trees are generally rare or absent, specimens from 18 families had girths >70 cm diameter and maximum heights 20-41 m. These findings indicate that simple AGB-climate-edaphic models may not be suitable for estimating carbon storage in forests where optimal climate niches exist. Our study, conducted in a very remote area, suggests that tropical montane forests may contain greater AGB than previously thought and the importance of securing their future under a changing climate is therefore enhanced.

Concepts: Climate, Forest, Rainforest, Tropical rainforest, Montane, Elevation, Cloud forest, Tropical and subtropical moist broadleaf forests

8

Study Design Retrospective epidemiologic investigation. Objective To investigate the relationship between altitude and concussion rate in the National Football League (NFL). Because of the physiologic responses that occur during acclimatization to altitude, it was hypothesized that games played on fields at a higher altitude would have reduced concussion rates compared to games played on fields at a lower altitude. Background Recent research indicates that the elevation above sea level at which football games are played may be associated with the likelihood of a concussion in high school football athletes. Methods Data on incident concussions and athlete exposures for the first 16 weeks of the NFL 2012 and 2013 regular seasons were obtained from publicly available web-based sources and used to calculate competition concussion rates for each NFL stadium. Concussion rates were analyzed in relation to game elevation. Results During the first 16 weeks of the 2012 and 2013 NFL regular seasons, 300 concussions, involving 284 players, were reported (64.3 primary cases per 10 000 game exposures). The odds of a concussion were 30% lower when playing at a higher elevation (equal to or greater than 644 ft [196.3 m] above sea level) compared to a lower elevation (odds ratio = 0.70; 95% confidence interval: 0.53, 0.94). A multivariable generalized linear model controlling for season, week, and clustering of team at home and away confirmed these results, showing that the odds of at least 1 concussion were reduced by 32% in games played at higher elevation. Conclusion The results of this epidemiological investigation indicate that increased altitude was associated with a reduction in the odds of a sport-related concussion in NFL athletes. The reported relationship of concussion incidence and field elevation should be further investigated, and, if verified, further work will be needed to understand why that relationship exists. Level of Evidence Prognosis, level 2c. J Orthop Sports Phys Ther, Epub 28 January 2014. doi:10.2519/jospt.2014.5298.

Concepts: Epidemiology, American football, Elevation, Topography, Concussion, Altitude, National Football League, High school football

7

Climate and tectonics have complex feedback systems which are difficult to resolve and remain controversial. Here we propose a new climate-independent approach to constrain regional Andean surface uplift.87Sr/86Sr and143Nd/144Nd ratios of Quaternary frontal-arc lavas from the Andean Plateau are distinctly crustal (>0.705 and <0.5125, respectively) compared to non-plateau arc lavas, which we identify as a plateau discriminant. Strong linear correlations exist between smoothed elevation and87Sr/86Sr (R2 = 0.858, n = 17) and143Nd/144Nd (R2 = 0.919, n = 16) ratios of non-plateau arc lavas. These relationships are used to constrain 200 Myr of surface uplift history for the Western Cordillera (present elevation 4200 ± 516 m). Between 16 and 26°S, Miocene to recent arc lavas have comparable isotopic signatures, which we infer indicates that current elevations were attained in the Western Cordillera from 23 Ma. From 23-10 Ma, surface uplift gradually propagated southwards by ~400 km.

Concepts: Feedback, Isotope, Constraint, Andes, Elevation, Nuclide, The Western, Radiogenic