Quantifying the importance of day length in process-based models for the prediction of temperate spring flowering phenology.

Owing to climate change and frequent extreme weather events, changes in spring flowering phenology have been observed in temperate forests. The flowering time response to climate change is divergent among species and is difficult to predict due to the complexity of flowering mechanisms. To compare the effects of spring warming, winter chilling, and day length on spring flowering time, we evaluated eight process-based models (two types of forcing models, two types of chilling-forcing models, and four models with the effect of day length added to the aforementioned four models). We used flowering data of seven temperate species (Cornus officinalis, Rhododendron mucronulatum, Forsythia koreana, Prunus yedoensis, Rhododendron yedoense f. poukhanense, Rhododendron schlippenbachii, and Robinia pseudoacacia) observed in nine different arboretums in South Korea over 9 years. Generally, the forcing model performed better than the sequential chilling-forcing model, regardless of the species. The performance gap between the models was reduced when day length term was included in model, but the chilling-forcing model did not outperform the forcing model. The effect of day length on flowering time differed depending on the species. Prunus yedoensis, which had a particularly low warming sensitivity compared to other species, was more dependent on day length than other species. On the other hand, day length had little effect on the flowering time of Robinia pseudoacacia and Cornus officinalis, mostly found in the early successional stage. These findings imply that the effect of chilling on flowering time would be minor for the seven species inhabiting the warm-temperate forest, and the effect of day length on flowering time was species-specific and dependent on species' temperature (warming) sensitivity and life strategy. In the future warm climate, the flowering time of day length sensitive species would not advance significantly, which may result in a phenological mismatch and endanger their life.

Topography and forest diversity facets regulate overstory and understory aboveground biomass in a temperate forest of South Korea.

Diversity-biomass studies across (sub-)tropical forest strata have been explored, but our understanding on how multiple facets of forest diversity and abiotic factors regulate aboveground biomass across forest strata in temperate forests remains somehow unclear. Here, we conducted a comprehensive analysis of the effects and relative importance of taxonomic, phylogenetic and functional diversity, community-weighted mean (CWM) of trait values, coefficient of variation in individual tree diameter at breast height (CV DBH), and soil and topographic factors on aboveground biomass to select the diversity facets that could have the strongest response to abiotic factors across over- and understory and whole community in a temperate forest of South Korea. We used forest inventory, functional traits and environmental factors data from 259 plots to select the most important diversity facets with abiotic factors through multi-model inference tests, and we then used structural equation models. In the overstory, the most important regulators of aboveground biomass were topographic factor, species evenness, functional richness, and CV DBH. In the understory, the significant drivers of aboveground biomass were topographic factor, species evenness, phylogenetic species richness and CWM of height. In addition, diversity facets of overstory such as functional richness and CV DBH also had significant direct and/or indirect effects on understory aboveground biomass. Moreover, the diversity facets influencing aboveground biomass at the whole community were the combination of the multiple facets of forest diversity influencing aboveground biomass at each forest stratum. The role of functional dominance (CWM of height) seems to be negligible in the overstory but significant in the understory, indicating different diversity drivers as shown previously for a subtropical forest. Hence, our study suggests the urgent need of exploring diversity-biomass studies across forest strata in different forest ecosystems and types in order to provide more specific guidelines for the management of a specific natural forest.

Comparative assessment of net CO2 exchange across an urbanization gradient in Korea based on eddy covariance measurements.

BACKGROUND: It is important to quantify changes in CO2 sources and sinks with land use and land cover change. In the last several decades, carbon sources and sinks in East Asia have been altered by intensive land cover changes due to rapid economic growth and related urbanization. To understand impact of urbanization on carbon cycle in the monsoon Asia, we analyze net CO2 exchanges for various land cover types across an urbanization gradient in Korea covering high-rise high-density residential, suburban, cropland, and subtropical forest areas. RESULTS: Our analysis demonstrates that the urban residential and suburban areas are constant CO2 sources throughout the year (2.75 and 1.02 kg C m-2 year-1 at the urban and suburban sites), and the net CO2 emission indicate impacts of urban vegetation that responds to the seasonal progression of the monsoon. However, the total random uncertainties of measurement are much larger in the urban and suburban areas than at the nonurban sites, which can make it challenging to obtain accurate urban flux measurements. The cropland and forest sites are strong carbon sinks because of a double-cropping system and favorable climate conditions during the study period, respectively (- 0.73 and - 0.60 kg C m-2 year-1 at the cropland and forest sites, respectively). The urban area of high population density (15,000 persons km-2) shows a relatively weak CO2 emission rate per capita (0.7 t CO2 year-1 person-1), especially in winter because of a district heating system and smaller traffic volume. The suburban area shows larger net CO2 emissions per capita (4.9 t CO2 year-1 person-1) because of a high traffic volume, despite a smaller building fraction and population density (770 persons km-2). CONCLUSIONS: We show that in situ flux observation is challenging because of its larger random uncertainty and this larger uncertainty should be carefully considered in urban studies. Our findings indicate the important role of urban vegetation in the carbon balance and its interaction with the monsoon activity in East Asia. Urban planning in the monsoon Asia must consider interaction on change in the monsoon activity and urban structure and function for sustainable city in a changing climate.

Modification of the moving point test method for nighttime eddy CO2 flux filtering on hilly and complex terrains.

The measurement of carbon dioxide (CO2) fluxes using the eddy covariance technique is difficult in forests in complex terrain because of the horizontal advection of CO2 at night. This results the under- or overestimation of net ecosystem exchanges of CO2. We propose a technique for nighttime filtering (and correction) of CO2 fluxes to eliminate (and replace) those significantly affected by horizontal advection: the modified moving point test method. This was developed by merging the friction velocity filtering and van Gorsel methods. It is based on an approach using moving windows for time and friction velocity, allowing a nighttime CO2 flux correction that includes an assessment of CO2 drainage at midnight. We tested the method using datasets from two flux towers in forests in hilly and complex terrains, where the application of generic nighttime filtering methods is difficult because CO2 drainage is generated earlier than the time assumed by the generic methods. The method produced carbon budgets consistent with previous research results, while showing improved applicability. •We propose a nighttime CO2 flux filtering method for hilly and complex terrain that combines the friction velocity filtering and van Gorsel methods.•This method determines the friction velocity threshold and the significance of CO2 drainage at midnight based on an approach using moving windows for time and friction velocity.•The method produced consistent results and shows improved applicability.

Temporal Changes in Species, Phylogenetic, and Functional Diversity of Temperate Tree Communities: Insights From Assembly Patterns.

Species-based approaches to the analysis of changes in successional community assemblages are limited in the ability to reflect long-term evolutionary and functional trait responses of organisms to environment change. Recent advances in concepts and analyses of community phylogenetics and functional traits have improved the interpretation and understanding of community assembly processes. Here, we examined phylogenetic signals of four functional traits such as maximum height, leaf size, seed mass and wood density in woody plant species and temporal changes in species, phylogenetic, and functional diversity among forest strata (i.e., whole, overstory, and understory strata) at four forest long term ecological research sites in South Korea. A census of woody plant species was implemented in a 1-ha permanent plot of each study site every 5 years. We analyzed community structure and compositional turnover using twenty-five 20 × 20 m2 quadrat data converted from 1-ha plot data of each site. We found that phylogenetic signals for four functional traits were low but significant, indicating that phylogenetic diversity may be used as a crude surrogate measure of functional diversity. Temporal changes in alpha and beta components of the three diversity differed among forest strata and four study sites over time. This study also revealed that the temporal changes of phylogenetic and functional diversity for understory strata in a forest, which were consecutively damaged by typhoon, were more extreme and larger than those of understory strata in the other sites. Therefore, our study supports recent studies that plant community structures differ among forest strata and such differences of community structure among sites can be accelerated by disturbance. Although the role and relative importance of niche-based deterministic and neutral processes for the patterns of successional community structure differed among the study sites, we found niche-based deterministic processes are the dominant drivers in structuring plant community assembly regardless of forest age and disturbance in this study. From these results, our study suggests that contemporary forest ecosystems are composed of mosaics of plant communities that are formed by interactions among various processes.

Partitioning the regional and local drivers of phylogenetic and functional diversity along temperate elevational gradients on an East Asian peninsula.

Species-centric approaches to biodiversity in ecological research are limited in their ability to reflect the evolutionary history and functional diversity of community assembly. Recently, the introduction of alternative facets of biodiversity, such as phylogenetic and functional diversity, has shed light on this problem and improved our understanding of the processes underlying biodiversity patterns. Here, we investigated the phylogenetic and functional diversity patterns of α, ß and γ components in woody plant assemblages along regional and local elevational gradients in South Korea. Although the patterns of phylogenetic and functional diversity varied along regional and local elevational transects, the main drivers were partitioned into two categories: regional area or climate for phylogenetic diversity, depending on whether the transect was at a regional or local scale; and habitat heterogeneity for functional diversity, which was derived in elevational bands. Moreover, environmental distance was more important than was geographic distance for phylogenetic and functional ß diversity between paired elevational bands. These results support the hypothesis that niche-based deterministic processes such as environmental filtering and competitive exclusion are fundamental in structuring woody plant assemblages along temperate elevational gradients regardless of scale (regional vs. local) in our study areas.

Disentangling the local-scale drivers of taxonomic, phylogenetic and functional diversity in woody plant assemblages along elevational gradients in South Korea.

Recently, new alternative matrices of biodiversity such as phylogenetic and functional diversity as a complement to species diversity have provided new insights into the mechanisms of community assembly. In this study, we analyzed the phylogenetic signals of five functional traits and the relative contribution of environmental variables and distance matrices to the alpha and beta components of taxonomic, phylogenetic and functional diversity in woody plant assemblages along four local elevational transects on two different mountains. We observed low but significant phylogenetic signals of functional traits, which suggest that phylogenetic dispersion can provide a rough approximation of functional dispersion but not perfect correlations between phylogenetic and functional diversity. Taxonomic alpha diversity showed a monotonic decline with elevation, and climatic variables were the main drivers of this pattern along all studied transects. Furthermore, although the phylogenetic and functional alpha dispersions showed different elevational patterns including increase, decrease and no relationship, the underlying processes driving the patterns of both types of alpha dispersion could be explained by the gradients of climatic and habitat variables as well as biotic interactions such as competition. These results suggest that both alpha dispersion patterns may be significantly controlled by niche-based deterministic processes such as biotic interactions and environmental filtering in our study areas. Moreover, the beta diversity with geographical distances showed distance-decay relationships for all transects. Although the relative importance of the environmental and geographical distances for beta diversity varied across the three facets of diversity and the transects, we generally found that environmental distances were more important for the beta components of the three facets of diversity. However, we cannot discriminate the effects of both distances on the three facets of diversity. Therefore, our study suggests that niche-based deterministic processes, potentially combined with neutral processes such as dispersal limitation and demographic stochasticity, may influence patterns of woody plant assemblage turnover in our study areas.