Dai Koide

Null model approaches to evaluating the relative role of different assembly processes in shaping ecological communities

Various local processes simultaneously shape ecological assemblages. β-diversity is a useful metric for inferring the underlying mechanisms of community assembly. However, β-diversity is not independent of γ-diversity, which may mask the local mechanisms that govern community processes across regions. Recent approaches that rely on an abundance-based null model could solve this sampling issue. However, if abundance varies widely across a region, the relative roles of deterministic and stochastic processes may be substantially misestimated. Furthermore, there is additional uncertainty as to whether null models used to correct γ-dependence in β-diversity should be independent of the observed patterns of species abundance distributions or whether the models should reflect these patterns. Here, we aim to test what null models with various constraints imply about the underlying processes shaping β-diversity. First, we found that an abundance-driven sampling effect could substantially influence the calculation of γ-corrected β-diversity. Second, we found that the null models that preserve the species abundance patterns could better reflect empirical patterns of spatial organization of individuals. The different implications generated from different applications of the null model approach therefore suggest that there are still frontiers regarding how local processes that shape species assemblages should be quantified. Carefully exploring each facet within different assembly processes is important.

Disentangling relationships between plant diversity and decomposition processes under forest restoration

Summary nBiodiversity has been elucidated to be one of the major factors sustaining ecosystem functioning. The vast majority of studies showing a relationship between biodiversity and ecosystem functioning have come from experiments, and this knowledge has not yet been applied to most real-world cases of conservation and management. This is especially true in forest ecosystems, characterized by the dominance of long-lived organisms (trees) and high levels of structural complexity and environmental heterogeneity. nTo apply biodiversity–function relationships to actual forest management, there are several issues to be considered. These include employing a cross-taxon perspective, as some functions (e.g. soil biogeochemical processes) cannot be maintained by a narrow set of organisms, as is usually the case with experimental systems. More specifically, although the interaction between above- and below-ground diversity is important for many functions in forests, there are few studies that evaluated the roles of diversity in both subsystems in a manner that could be informative in practice. nTo evaluate the roles of above- and below-ground diversity to support natural soil ecosystem functions, we conducted a decomposition experiment in the northern forests of Japan, which are currently under restoration management. The restoration area consists of mosaics of different vegetation types by various revegetation activities and establishment of ungulate exclosures. Using structural equationxa0modelling and linear mixed-effects models, we assessed direct and indirect pathways from diversity to functions by focusing on both of taxonomic and functional diversity indices. To put our findings into practice, we utilized a trait-based approach, which provides a link between the functional consequences of human influences and ecosystem structure. nWe found little direct effects of tree diversity on below-ground functions such as decomposition rate and litter stabilization. However, once the diversities of understorey herbaceous plants and soil fungi were considered as a possible mediating explanatory factor, we found a significant effect of tree diversity to indirectly support these functions by supporting these other types of biodiversity. Particularly, we found that the models based on functional trait diversity, rather than on taxonomic species richness, best explained the variation in below-ground processes. nSynthesis and applications. Forest restoration in the northern forests of Japan has had no explicit objective to restore soil functions. Nevertheless, afforestation, and the associated increase in tree diversity as a measure of forest restoration, was, although often unintentionally, proven effective for the maintenance of multiple ecosystem functions, such as soil biogeochemical processes. This finding suggests a great potential for management to make local tree assemblages functionally dissimilar and diverse for the sake of supporting and enhancing fundamental ecosystem functions in forests.

Differential processes underlying the roadside distributions of native and alien plant assemblages

Although biological invasion often alters ecosystem properties and community composition at different scales, considerable uncertainty still exists regarding the underlying mechanisms that regulate the spread of alien species into new habitats. An alien invasion is generally achieved through multiple processes from multiple sources; this type of invasion often prevents us from understanding of the dispersal mechanisms. Here, we aim to disentangle the processes of alien invasion by focusing on a single migration source. We surveyed the distribution of alien and native plant species in Shiretoko National Park, located in northern Japan. We measured the coverage of each species and the environmental and spatial factors in 362 quadrats established along roadsides. We found 101 native species and 35 alien species (γ-diversity) throughout the quadrats. The local species richness (α-diversity) was higher for the alien species (6.1 species) than for the native species (3.2 species). There was a significant negative correlation in α-diversity between native and alien species. Moreover, the α-diversity and distance from the nearest town (migration source) showed a negative relationship for alien assemblages while the native assemblages showed the opposite trend. These results suggest that the alien species are expanding their distribution outward from the town, resulting in a decrease in the α-diversity of native species in localities close to the migration source. Overall, our results emphasize that roadsides could unintentionally act as corridors for alien species, even in protected areas. Careful consideration is thus needed for utilizing these human-created habitats even though they were designed for conservation and management purposes.

Land abandonment and changes in snow cover period accelerate range expansions of sika deer

Abstract Ongoing climate change and land‐use change have the potential to substantially alter the distribution of large herbivores. This may result in drastic changes in ecosystems by changing plant–herbivore interactions. Here, we developed a model explaining sika deer persistence and colonization between 25 years in terms of neighborhood occupancy and habitat suitability. We used climatic, land‐use, and topographic variables to calculate the habitat suitability and evaluated the contributions of the variables to past range changes of sika deer. We used this model to predict the changes in the range of sika deer over the next 100 years under four scenario groups with the combination of land‐use change and climate change. Our results showed that both climate change and land‐use change had affected the range of sika deer in the past 25 years. Habitat suitability increased in northern or mountainous regions, which account for 71.6% of Japan, in line with a decrease in the snow cover period. Habitat suitability decreased in suburban areas, which account for 28.4% of Japan, corresponding to land‐use changes related to urbanization. In the next 100 years, the decrease in snow cover period and the increase in land abandonment were predicted to accelerate the range expansion of sika deer. Comparison of these two driving factors revealed that climate change will contribute more to range expansion, particularly from the 2070s onward. In scenarios that assumed the influence of both climate change and land‐use change, the total sika deer range increased by between +4.6% and +11.9% from the baseline scenario. Climate change and land‐use change will require additional efforts for future management of sika deer, particularly in the long term.

Projecting spatiotemporal changes in suitable climate conditions to regenerate trees using niche differences between adult and juvenile trees

Assessing suitable climate conditions to regenerate trees over a large area is of great importance to investigate potential impacts of climate change. In this study, we developed a size-based species distribution model (SBSDM) to assess spatiotemporal changes in the tree regeneration niche separately from the growth niche in adults. Siebold’s beech (Fagus crenata) was selected as the target species. We projected (1) areas where adult and juvenile potential habitats (PHs) overlapped, (2)only-adult PHs, (3) only-juvenile PHs, and (4) non-habitats for 2080–2099 using the SBSDM, a distribution dataset from the Phytosociological Relevé Database of Japan, and a future climatic dataset from 24 general circulation models (GCMs). We also projected juvenile PHs for all decades between 2011 and 2099 using four representative GCMs to assess potential lost decades of the regeneration niche. The SBSDM provided sufficient projections of adult and juvenile tree distributions as well as their niche differences under the current climate. Overlapping areas and only-adult PHs were projected to decrease by the end of this century. An increase in only-juvenile PHs was projected to occur in snowy regions, with juvenile PHs starting to decrease in warm and less snowy regions. Furthermore, juvenile PHs are expected to decrease widely around 2060 as well as at the end of this century due to considerable rapid warming around those times. We conclude that regeneration of F. crenata will start to decline in 2060, but snowy conditions will postpone the timing of the regeneration loss, causing an increase in only-juvenile PHs.

An upward elevational shift of native and non-native vascular plants over 40 years on the island of Hawai'i

Questions nHave vascular plants shifted in elevation along a tropical mountain gradient on Mauna Loa, Hawaii over the past 40 years? If so, do native and non-native species differ in their shifts of mean elevation range, upper elevational limit, or lower elevational limit? n nLocation nHawaii Volcanoes National Park, along the southeast slope of Mauna Loa on the island of Hawaii. n nMethods nWe resurveyed an elevational belt transect of vegetation plots that was first surveyed in the early 1970s. Differences between 1970 and 2010 were calculated for 69 plant species’ mean elevation, upper elevational limit, lower elevational limit, and elevational range. Statistical significance of differences was assessed by Monte Carlo permutation tests. n nResults nAveraging across all 69 species, a statistically significant upward shift of 65.2 m was detected in mean elevation. Both upper and lower elevational limits shifted significantly upward by 31.6 m and 90.5 m, respectively. Elevational range did not change significantly. For the 49 native species, the upper elevational limit did not change, but the lower elevational limit shifted significantly upward by 94.1 m. In contrast, the 20 non-native species displayed a different pattern of a significant upward shift in both their upper and lower elevational limits, by 126.4 m and 81.6 m, respectively. Native species seemed to display a contracting elevational range of 101.2 m, but this was not significant (p = 0.053), while non-native species maintained a stable elevational range (p = 0.365). n nConclusions nVascular plants have shifted upward in elevation on the tropical island of Hawaii, but the shifting trend differs between native and non-native species. Non-native species, which were often found at lower elevations, are in an upward niche expansion phase. Native species show trends toward range contraction due to lack of expansion in upper elevational limit, probably due drought stress correlating with a trade-wind inversion (TWI) that severely limits rainfall at higher elevations. As more non-native species reach the TWI elevation, their upward expansion may also become more limited. n nThis article is protected by copyright. All rights reserved.

Temporal changes in the relationship between tree-ring growth and net primary production in northern Japan: a novel approach to the estimation of seasonal photosynthate allocation to the stem

Tree-ring (TR) observations provide important data on long-term forest dynamics and their underlying ecophysiological mechanisms. To elucidate the seasonal link between photosynthetic carbon acquisition and TR growth, we analyzed the correlation between observed TR data (carbon sink) and model-estimated net primary production (NPP; carbon source). Temporal trends of the TR–NPP correlation over the last century were also analyzed to identify influences of past climate changes. We used TR data from Picea glehnii at seven sites on Hokkaido Island, Japan, which were obtained from the International Tree-Ring Data Bank. At each site, NPP was estimated using the Vegetation Integrative Simulator for Trace gases model, which was driven by long-term (1900–2010) meteorological data. Site-mean tree-ring width index (TRWI) chronologies were analyzed to reveal any relationship with the current or previous year’s annual or monthly NPP. We found moderate to strong correlations between TRWIs and model-estimated monthly NPP from April to June, especially in June of the current year, but no clear spatial trend was observed. During the twentieth century, the TRWI–NPP correlation increased for February, March, April, and July NPP of the current year and for October NPP of the previous year. Ecophysiologically, the period from April to June corresponds to the season when tree cambial cells are formed in the study area. Our findings suggest that photosynthate produced during this cambial growth season is allocated to stem growth and that this source allocation season has become longer due to past environmental changes.

Different trends in phylogenetic and functional structure of plant communities along an elevation gradient

The study of diversity gradients due to elevation dates back to the foundation of biogeography and ecology. Although elevation-driven patterns of plant diversity have been reported for centuries, uncertainty still exists about the assembly rules that drive these patterns. In this study, we revealed the causal factor of community assemblies for the diversity of tree and herb species along an elevation. To this end, we applied an integrated method using both functional traits and phylogeny, called the mean pairwise functional-phylogenetic distance, to understand the assembly rules for woody and herbaceous species communities along an elevation gradient. At higher elevation sites, woody and herbaceous communities were comprised of species having similar traits. The phylogenetic trends for woody species were consistent with the functional trends; closely related species co-occurred more frequently than expected at higher elevations. Phylogenetic trends for herb species were opposite to the functional trends; species with similar traits but having a random phylogenetic distribution co-occurred at higher elevations. We suggest that the community assembly rules for woody and herb species vary with elevation; and functional constraints due to environmental filtering at higher elevation act as assembly rules along gradients in both woody and herbaceous communities, even though their phylogenetic backgrounds differ.

Potential impact of climate change on canopy tree species composition of cool-temperate forests in Japan using a multivariate classification tree model

Climate change will likely change the species composition or abundance of plant communities, and it is important to anticipate these changes to develop climate change adaptation policies. We chose beech (Fagus crenata Blume) and its competitive tree species as target species to evaluate potential turnover in forest types under climate change using a multivariate classification tree model. To construct the model, geographical presence/absence data for nine target species were used as multivariate response variables, with five climatic factors were used as predictor variables. Current and future distribution probabilities for the target species were calculated, and the 15 dominant forest types were subjectively classified in approximately 1-km2 grid cells within the area of the current beech forest distribution. All 16,398 grid cells of the beech-dominant forest type (FCR-QCR) were projected to be replaced in the future by five Quercus crispula-dominant types (59% of FCR-QCR grid cells), four Q. serrata types (22%), two Q. salicina types (8%), or two Abies firma types (0.1%). The FCR-QCR type remained unchanged (stable) in only 11.4% of grid cells; these were mainly distributed at high elevations in snowy areas on the Sea of Japan side of the country. In contrast, vulnerable habitats (future probability of beech occurrence less than 1.0%) were found at low elevations on both the Sea of Japan and the Pacific Ocean sides. Northwards or upwards range expansions or increases of Quercus spp., in particular, need to be carefully monitored.