Support for the elevational Rapoport's rule among seed plants in Nepal depends on biogeographical affinities and boundary effects

Species richness and turnover patterns for tropical and temperate plants on the elevation gradient of the eastern Himalayan Mountains

Understanding species’ elevational distributions in mountain ecosystems is needed under climate change, but remote biodiverse mountain areas may be poorly documented. National Forest Inventories (NFIs) offer a potential source of data. We used NFI records from Bhutan to ask three questions about elevational richness patterns of Himalayan woody plant species. First, does the mean elevation for all species differ from those species whose entire elevational distribution is recorded in the survey? Second, how does the elevation of maximum richness differ when combining species originating from temperate and tropical regions vs. analyzing them separately? And third, do the highest species turnover rates adjoin elevation zones of maximum species richness? We used 32,198 species records from 1685 forest plots along a 7570 m gradient to map species elevation ranges. Species whose entire range was documented were those whose lowest records are located above 400 m, while bare rock defined all species’ upper limits. We calculated species richness and turnover using 400 m elevation bands. Of 569 species, 79% of temperate and 61% of tropical species’ elevation ranges were fully sampled within the NFI data. Mean elevation of tree and shrub species differed significantly for temperate and tropical species. Maximum combined species richness is from 1300 to 1700 m (277 species), differing significantly from maximum tropical (900–1300 m, 169) and temperate species richness (2500–2900 m, 92). Temperate tree turnover rate was highest in the bands adjoining its maximum species richness (2500–2900 m). But turnover for tropical trees was highest several bands above their maximum species richness, where turnover and decrease in richness interact. Shrub species turnover patterns are similar, but rates were generally higher than for trees. Bhutan’s NFI records show that woody plant species are arrayed on the Himalaya in part according to floristic origins, and that combining temperate- and tropical-originating floras for gradient-based studies such as species richness and turnover obscures actual elevational patterns. In addition, species whose ranges extend below the Himalayan elevation gradient should be accounted for in future studies that correlate climate and environment factors with elevational species richness patterns.

Niche breadth and elevational range size: a comparative study on Middle-European Brassicaceae species

High-elevation species are predicted to have larger elevational ranges compared with species of lower elevations. The reasoning is that temperature variability is greater at higher elevation, selecting for wider niche breadth and more plastic genotypes. We used macroevolutionary comparisons involving 90 Brassicaceae species of the central Alps to test for associations among median elevation of occurrence, elevational range size and thermal variability over space and time on the one hand, and their associations with performance breadth or trait plasticity on the other hand. Performance breadth and trait plasticity were estimated by raising replicate plants per species under three temperature treatments (mild, recurrent frost, recurrent heat). Against prediction, we found that mid-elevation species had the largest elevational ranges, and their ranges were associated with increased spatial thermal variability. Nevertheless, variability in the thermal regime was positively associated neither with niche breadth nor with plasticity. Evidence for adaptive constraints was limited to a trade-off between acclimation-based increases in frost and heat resistance, and phylogenetic niche conservatism for median elevation of occurrence and temporal thermal variability. Results suggest that large elevational range size is associated with divergent adaptation within species, but not with more niche breadth or trait plasticity. This article is part of the theme issue 'Species' ranges in the face of changing environments (part I)'.

Temperature and soils predict the distribution of plant species along the Himalayan elevational gradient

Tropical montane systems are characterized by a high plant species diversity and complex environmental gradients. Climate warming may force species to track suitable climatic conditions and shift their distribution upward, which may be particularly problematic for species with narrow elevational ranges. To better understand the fate of montane plant species in the face of climate change, we evaluated a) which environmental factors best predict the distribution of 277 plant species along the Himalayan elevational gradient in Nepal, and b) whether species elevational ranges increase with increasing elevation. To this end, we developed ecological niche models using MaxEnt by combining species survey and presence data with 19 environmental predictors. Key environmental factors that best predicted the distribution of Himalayan plant species were mean annual temperature (for 54.5% of the species) followed by soil clay content (10.2%) and slope (9.4%). Although temperature is the best predictor, it is associated with many other covariates that may explain species distribution, such as irradiance and potential evapotranspiration. Species at both ends of the Himalayan elevational gradient had narrower elevational ranges than species in the middle. Our results suggest that with further global warming, most Himalayan plant species have to migrate upward, which is especially critical for upland species with narrow distribution ranges.

The species range-size patterns for vascular plants of Seorak Mountain (Korea): Relationship between group of life forms and phytogeography affinity along the elevational gradient

Research on species richness patterns and the advanced elevational Rapoport rule (ERR) has been widespread in recent years; however, there is a lack of such research for the temperate mountainous regions in northeast Asia. Here, we collected plant species from the Seorak Mountain in northeast Asia through field surveys. The species were divided into 11 groups according to the life-form types and phytogeography affinities of each species. The ERR was evaluated using Steven's method and by examining the species richness patterns of each group. The species richness patterns revealed a positive multimodal pattern along the elevation gradient, but phytogeography affinities (increasing trend) and life-form analysis (unimodal) exhibited different patterns. The elevation gradients (1,350 m for the mean elevation-range relationships), which are affected by the boundary effect and different life forms, did not consistently support the ERR. However, herbs as well as rare, endemic, and red list species showed consistent support for the ERR, which could be attributed to the influence by phytogeography affinities. Therefore, the results from Seorak Mountain showed that the ERR was not consistent for different plant life forms in the same area; however, phytogeography affinities could support and explain ERR.

Climate-driven elevational variation in range sizes of vascular plants in the central Himalayas: A supporting case for Rapoport's rule

A fundamental yet controversial topic in biogeography is how and why species range sizes vary along spatial gradients. To advance our understanding of these questions and to provide insights into biological conservation, we assessed elevational variations in the range sizes of vascular plants with different life forms and biogeographical affinities and explored the main drivers underlying these variations in the longest valley in China's Himalayas, the Gyirong Valley. Elevational range sizes of vascular plants were documented in 96 sampling plots along an elevational gradient ranging from 1,800 to 5,400 m above sea level. We assessed the elevational variations in range size by averaging the range sizes of all recorded species within each sampling plot. We then related the range size to climate, disturbance, and the mid-domain effect and explored the relative importance of these factors in explaining the range size variations using the Random Forest model. A total of 545 vascular plants were recorded in the sampling plots along the elevational gradient. Of these, 158, 387, 337, and 112 were woody, herbaceous, temperate, and tropical species, respectively. The range size of each group of vascular plants exhibited uniform increasing trends along the elevational gradient, which was consistent with the prediction of Rapoport's rule. Climate was the main driver of the increasing trends of vascular plant range sizes in the Gyirong Valley. The climate variability hypothesis and mean climate condition hypothesis could both explain the elevation-range size relationships. Our results reinforce the previous notion that Rapoport's rule applies to regions where the influence of climate is the most pronounced, and call for close attention to the impact of climate change to prevent species range contraction and even extinction due to global warming.

Drivers of elevational richness peaks, evaluated for trees in the east Himalaya

Along elevational gradients, species richness often peaks at intermediate elevations and not the base. Here we refine and test eight hypotheses to evaluate causes of a richness peak in trees of the eastern Himalaya. In the field, we enumerated trees in 50 plots of size 0.1 ha each at eight zones along an elevational gradient and compared richness patterns with interpolation of elevational ranges of species from a thorough review of literature, including floras from the plains of India. The maximum number of species peaks at similar elevations in the two data sets (at 500 m in the field sampling and between 500 m and 1,000 m in range interpolation); concordance between the methods implies that statistical artefacts are unlikely to explain the peak in the data. We reject most hypotheses (e.g., area, speciation rate, mixing of distinct floras). We find support for a model in which climate (actual evapotranspiration [AET] or its correlates) sets both the number of species and each species optimum, coupled with a geometric constraint. We consider that AET declines with elevation, but an abrupt change in the association of AET with geographical distance into the plains means that the location of highest AET, at the base of the mountain, receives range overlaps from fewer species than the location just above the base. We formalize this explanation with a mathematical model to show how this can generate the observed low-elevation richness peak.

The species richness pattern of vascular plants along a tropical elevational gradient and the test of elevational Rapoport's rule depend on different life-forms and phytogeographic affinities

The research about species richness pattern and elevational Rapoport's rule (ERR) have been carried out mostly in the temperate regions in the recent years and scarcely in the tropical mountains; meanwhile, it is unclear whether the ERR is consistent among different life-forms and phytogeographic affinities. Here, we compiled a database of plant species of Mount Kenya, a tropical mountain of East Africa, and divided these species into twelve groups depending on the life-form and phytogeographic affinity of each species. We inspected the species richness pattern of each group along the elevation gradient and also tested ERR of each group using Stevens' method. Our results showed that species richness of the total species showed a positively skewed (hump-shaped) pattern along the elevation gradient and different life-forms and phytogeographic affinities showed similar hump-shaped patterns as the total species. The average elevation range size of the total species and herbaceous species showed increasing patterns along the elevation gradient, while lycophytes and ferns, and woody species showed an obvious downward trend after peaking in the high elevation regions. We concluded that the widely distributed herbaceous species which also have broad elevation range sizes are more applicable to ERR, while the narrowly distributed woody species with small elevation range sizes occurring in the higher elevations could reverse ERR. Therefore, we concluded that the ERR is not consistent among different organisms in the same region.

Elevational patterns of the percentages of plant genera with tropical and temperate affinities in Nepal

Background

Geographical patterns of species diversity are one of the key topics in biogeography and ecology. The effects of biogeographical affinities on the elevational patterns of species diversity have attracted much attention recently, but the factors driving elevational patterns of the percentages of plants with tropical and temperate biogeographical affinities have not been adequately explored.

Methods

We first used univariate least squares regressions to evaluate the effects of each predictor on the elevational patterns of the percentages of plant genera with tropical and temperate affinities in Nepal. Then, the lowest corrected Akaike information criterion value was used to find the best-fit models for all possible combinations of the aforementioned predictors. We also conducted partial regression analysis to investigate the relative influences of each predictor in the best-fit model of the percentages of plant genera with tropical and temperate affinities.

Results

With the increase of elevation, the percentage of plant genera with tropical affinity significantly decreased, while that of plant genera with temperate affinity increased. The strongest predictor of the percentages of plant genera with tropical affinity in the examined area was the minimum temperature of the coldest month. For the elevational patterns of the percentages of plant genera with temperate affinity, the strongest predictor was the maximum temperature of the warmest month. Compared with mid-domain effects (MDE), climatic factors explained much more of the elevational variation of the percentages of plant genera with tropical and temperate affinities.

Discussion

The elevational patterns of the percentages of plant genera with tropical affinities and the factors driving them supported the revision of the freezing-tolerance hypothesis. That is, freezing may filter out plant genera with tropical affinity, resulting in the decrease of their percentages, with winter coldness playing a predominant role. Winter coldness may not only exert filtering effects on plant genera with tropical affinity, but may also regulate the interactions between plant genera with tropical and temperate affinities. The elevational patterns of tropical and temperate plant diversities, and those of their percentages, might be controlled by different factors or mechanisms. Freezing-tolerance and the interactions between plant genera with tropical and temperate affinities regulated by climatic factors played stronger roles than MDE in shaping the elevational patterns of the percentages of plant genera with tropical and temperate affinities in Nepal.