Tropical forests are global centres of both biodiversity and carbon storage. Many tropical countr... more Tropical forests are global centres of both biodiversity and carbon storage. Many tropical countries aspire to protect forest to fulfil biodiversity and climate mitigation policy targets, but the conservation strategies needed to achieve these two functions depend critically on the tropical forest diversity-carbon relationship and this remains largely unexplored. Attempts to assess and understand this relationship in tropical forest ecosystems have been hindered by the scarcity of inventories where carbon storage in aboveground biomass and species identifications have been simultaneously and robustly quantified. Here, we compile a unique pan-tropical dataset of 360 plots located in old-growth closed-canopy forest, surveyed using standardised methods, allowing a multi-scale evaluation of the relationship between carbon storage and tree diversity. We find strongly contrasting variation in diversity and carbon among continents. Thus, on average, African forests have high carbon storage...
Identifying physical and ecological boundaries that limit where species can occur is important fo... more Identifying physical and ecological boundaries that limit where species can occur is important for predicting how those species will respond to global change. The island of Borneo encompasses a wide range of habitats that support some of the highest richness on Earth, making it an ideal location for investigating ecological mechanisms underlying broad patterns of species distribution. We tested variation in richness and range-size in relation to edaphic specialization and vegetation zone boundaries using 3060 plant species from 193 families centered around the elevational gradient of Mt Kinabalu, Borneo. Across species, average range-size increased with elevation, consistent with Rapoport's rule. However, plants associated with ultramafic soil, which is low in nutrient and water availability and often has high concentrations of heavy metals, had larger range-sizes and greater richness than expected along the elevational gradient, as compared to a null model with randomization of edaphic association. In contrast, non-ultramafic species had smaller range-sizes and lower richness than expected. These results suggest that tolerance of resource limitation may be associated with wider range-sizes, whereas species intolerant of edaphic stress may have narrower range-sizes, possibly owing to more intense competition in favorable soil types. Using elevation as a predictor of average range-sizes, we found that piece-wise models with breakpoints at vegetation zone transitions explained species distributions better than models that did not incorporate ecological boundaries. The greatest relative increases in range-size with respect to elevation occurred mid-elevation, within the montane cloud forest vegetation zone. Expansion of average range-size across an area without physical boundaries may indicate a shift in ecological strategy and importance of biotic versus abiotic stressors. Our results indicate that elevational range-size patterns are structured by ecological constraints such as species' edaphic association, which may limit the ability of species to migrate up or down mountains in response to climate change.
Thermal sensitivity of tropical trees A key uncertainty in climate change models is the thermal s... more Thermal sensitivity of tropical trees A key uncertainty in climate change models is the thermal sensitivity of tropical forests and how this value might influence carbon fluxes. Sullivan et al. measured carbon stocks and fluxes in permanent forest plots distributed globally. This synthesis of plot networks across climatic and biogeographic gradients shows that forest thermal sensitivity is dominated by high daytime temperatures. This extreme condition depresses growth rates and shortens the time that carbon resides in the ecosystem by killing trees under hot, dry conditions. The effect of temperature is worse above 32°C, and a greater magnitude of climate change thus risks greater loss of tropical forest carbon stocks. Nevertheless, forest carbon stocks are likely to remain higher under moderate climate change if they are protected from direct impacts such as clearance, logging, or fires. Science , this issue p. 869
Annual vegetative periodicity is not well known in equatorial tropical rain forests except for ph... more Annual vegetative periodicity is not well known in equatorial tropical rain forests except for photoperiodically induced or El-Niño-drought induced synchronous flowering/fruiting. The lack of vegetative periodicity such as leaf flush and fall in these forests has been believed to reflect an "aseasonal" climate. In the present study, we show a distinct annual seasonality in canopy dynamics using a Fourier analysis with a statistical significance test on the long-term, fortnightly monitored dataset of leaf litterfall in nine Bornean evergreen tropical rain forests on Mount Kinabalu. Such periodicity occurs across altitudes and soil types in all years irrespective of the year-to-year climatic variability, suggesting that regional climatic factors rather than local edaphic and/or biotic conditions cause the precise 1-year periodicity. We examine climatic factors that have causative effects on the distinct 1-year periodicity using a newly developed spectrum convergent cross map...
Proceedings of the National Academy of Sciences of the United States of America, Feb 5, 2018
Knowledge about the biogeographic affinities of the world's tropical forests helps to better ... more Knowledge about the biogeographic affinities of the world's tropical forests helps to better understand regional differences in forest structure, diversity, composition, and dynamics. Such understanding will enable anticipation of region-specific responses to global environmental change. Modern phylogenies, in combination with broad coverage of species inventory data, now allow for global biogeographic analyses that take species evolutionary distance into account. Here we present a classification of the world's tropical forests based on their phylogenetic similarity. We identify five principal floristic regions and their floristic relationships: (i) Indo-Pacific, (ii) Subtropical, (iii) African, (iv) American, and (v) Dry forests. Our results do not support the traditional neo- versus paleotropical forest division but instead separate the combined American and African forests from their Indo-Pacific counterparts. We also find indications for the existence of a global dry for...
The original version of this Article contained an error in the third sentence of the abstract and... more The original version of this Article contained an error in the third sentence of the abstract and incorrectly read "Here, using long-term plot monitoring records of up to half a century, we find that intact forests in Borneo gained 0.43 Mg C ha-1 year-1 (95% CI 0.14-0.72, mean period 1988-2010) above-ground live biomass", rather than the correct "Here, using long-term plot monitoring records of up to half a century, we find that intact forests in Borneo gained 0.43 Mg C ha-1 year-1 (95% CI 0.14-0.72, mean period 1988-2010) in above-ground live biomass carbon". This has now been corrected in both the PDF and HTML versions of the Article.
The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, rele... more The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, released from the Kikai Caldera about 7300 years ago. The eruption was devastating and it is believed that primary succession and soil formation reinitiated across all altitudes at this point. We hypothesize that the concentrations of soil total phosphorus (P) and labile P fractions increase with increasing altitude because the soil formation has progressed less in upslope areas as a result of the cooler temperature and because of the ample P source of the volcanic ash. Conversely, we hypothesize that the concentration of soil inorganic nitrogen (N) decreases with increasing altitude. Available soil P and N would result in increasing P limitation downslope and increasing N limitation upslope, respectively. We studied soil P fractions and soil inorganic N, and P- and N-use efficiencies of the seven forests on Yakushima along an elevation gradient (170–1550 m a.s.l.). Contrary to our hypotheses, soil total P, labile soil P fractions, and inorganic N decreased with increasing altitude. The P- and N-use efficiencies of the forests were negatively correlated with the concentration of soil total active P (total P minus occluded P) and inorganic N, respectively. We suggest that progressive soil acidity and slower decomposition under cooler and wetter environments upslope must have dissolved the P contained in volcanic ash and accelerated P leaching. Forest ecosystems on Yakushima that show a distinct altitudinal zonation are, therefore, characterized by increasing P and N shortage with increasing altitude.
The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, rele... more The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, released from the Kikai Caldera about 7300 years ago. The eruption was devastating and it is believed that primary succession and soil formation reinitiated across all altitudes at this point. We hypothesize that the concentrations of soil total phosphorus (P) and labile P fractions increase with increasing altitude because the soil formation has progressed less in upslope areas as a result of the cooler temperature and because of the ample P source of the volcanic ash. Conversely, we hypothesize that the concentration of soil inorganic nitrogen (N) decreases with increasing altitude. Available soil P and N would result in increasing P limitation downslope and increasing N limitation upslope, respectively. We studied soil P fractions and soil inorganic N, and P- and N-use efficiencies of the seven forests on Yakushima along an elevation gradient (170–1550 m a.s.l.). Contrary to our hypotheses, soil total P, labile soil P fractions, and inorganic N decreased with increasing altitude. The P- and N-use efficiencies of the forests were negatively correlated with the concentration of soil total active P (total P minus occluded P) and inorganic N, respectively. We suggest that progressive soil acidity and slower decomposition under cooler and wetter environments upslope must have dissolved the P contained in volcanic ash and accelerated P leaching. Forest ecosystems on Yakushima that show a distinct altitudinal zonation are, therefore, characterized by increasing P and N shortage with increasing altitude.
In humid regions of the western Pacific, conifers and deciduous broadleaf trees dominate in colde... more In humid regions of the western Pacific, conifers and deciduous broadleaf trees dominate in colder climate while evergreen broadleaf trees dominate in warmer climate. There are two geographically discontinuous forest zones of conifer-dominated forests, i.e., boreal zone and temperate-to-tropical zone. Between them, there is a deciduous broadleaf forest zone with warm summer and cold winter (Kira’s warmth index, WI from 45 to 85 °C, and coldness index, CI −15 °C). Boreal conifer forests are distributed at high latitudes or altitudes in Japan (“subarctic or subalpine forest”). Temperate conifer forests occur in a narrow belt on the Pacific Ocean side in Japan and are more extensive and distributed to warmer climate at high altitudes in Taiwan and in temperate zone of New Zealand, where they are called “conifer-broadleaf” forests. These temperate conifer (or mixed) forests grade into tropical conifer forests at high altitudes in Southeast Asia and New Guinea. Thus, these forests may be collectively called “temperate-to-tropical conifer-mixed forests” in contrast to boreal conifer forest. It appears that relatively cool summer (or year-round low temperature on tropical mountains) limits the growth of evergreen broadleaf trees, while mild winter (or the absence of winter on tropical mountains) gives the competitive advantage to conifers over deciduous broadleaf trees, leading to conifer dominance in the temperate-to-tropical conifer-mixed forests.
Tropical forests are global centres of both biodiversity and carbon storage. Many tropical countr... more Tropical forests are global centres of both biodiversity and carbon storage. Many tropical countries aspire to protect forest to fulfil biodiversity and climate mitigation policy targets, but the conservation strategies needed to achieve these two functions depend critically on the tropical forest diversity-carbon relationship and this remains largely unexplored. Attempts to assess and understand this relationship in tropical forest ecosystems have been hindered by the scarcity of inventories where carbon storage in aboveground biomass and species identifications have been simultaneously and robustly quantified. Here, we compile a unique pan-tropical dataset of 360 plots located in old-growth closed-canopy forest, surveyed using standardised methods, allowing a multi-scale evaluation of the relationship between carbon storage and tree diversity. We find strongly contrasting variation in diversity and carbon among continents. Thus, on average, African forests have high carbon storage...
Identifying physical and ecological boundaries that limit where species can occur is important fo... more Identifying physical and ecological boundaries that limit where species can occur is important for predicting how those species will respond to global change. The island of Borneo encompasses a wide range of habitats that support some of the highest richness on Earth, making it an ideal location for investigating ecological mechanisms underlying broad patterns of species distribution. We tested variation in richness and range-size in relation to edaphic specialization and vegetation zone boundaries using 3060 plant species from 193 families centered around the elevational gradient of Mt Kinabalu, Borneo. Across species, average range-size increased with elevation, consistent with Rapoport's rule. However, plants associated with ultramafic soil, which is low in nutrient and water availability and often has high concentrations of heavy metals, had larger range-sizes and greater richness than expected along the elevational gradient, as compared to a null model with randomization of edaphic association. In contrast, non-ultramafic species had smaller range-sizes and lower richness than expected. These results suggest that tolerance of resource limitation may be associated with wider range-sizes, whereas species intolerant of edaphic stress may have narrower range-sizes, possibly owing to more intense competition in favorable soil types. Using elevation as a predictor of average range-sizes, we found that piece-wise models with breakpoints at vegetation zone transitions explained species distributions better than models that did not incorporate ecological boundaries. The greatest relative increases in range-size with respect to elevation occurred mid-elevation, within the montane cloud forest vegetation zone. Expansion of average range-size across an area without physical boundaries may indicate a shift in ecological strategy and importance of biotic versus abiotic stressors. Our results indicate that elevational range-size patterns are structured by ecological constraints such as species' edaphic association, which may limit the ability of species to migrate up or down mountains in response to climate change.
Thermal sensitivity of tropical trees A key uncertainty in climate change models is the thermal s... more Thermal sensitivity of tropical trees A key uncertainty in climate change models is the thermal sensitivity of tropical forests and how this value might influence carbon fluxes. Sullivan et al. measured carbon stocks and fluxes in permanent forest plots distributed globally. This synthesis of plot networks across climatic and biogeographic gradients shows that forest thermal sensitivity is dominated by high daytime temperatures. This extreme condition depresses growth rates and shortens the time that carbon resides in the ecosystem by killing trees under hot, dry conditions. The effect of temperature is worse above 32°C, and a greater magnitude of climate change thus risks greater loss of tropical forest carbon stocks. Nevertheless, forest carbon stocks are likely to remain higher under moderate climate change if they are protected from direct impacts such as clearance, logging, or fires. Science , this issue p. 869
Annual vegetative periodicity is not well known in equatorial tropical rain forests except for ph... more Annual vegetative periodicity is not well known in equatorial tropical rain forests except for photoperiodically induced or El-Niño-drought induced synchronous flowering/fruiting. The lack of vegetative periodicity such as leaf flush and fall in these forests has been believed to reflect an "aseasonal" climate. In the present study, we show a distinct annual seasonality in canopy dynamics using a Fourier analysis with a statistical significance test on the long-term, fortnightly monitored dataset of leaf litterfall in nine Bornean evergreen tropical rain forests on Mount Kinabalu. Such periodicity occurs across altitudes and soil types in all years irrespective of the year-to-year climatic variability, suggesting that regional climatic factors rather than local edaphic and/or biotic conditions cause the precise 1-year periodicity. We examine climatic factors that have causative effects on the distinct 1-year periodicity using a newly developed spectrum convergent cross map...
Proceedings of the National Academy of Sciences of the United States of America, Feb 5, 2018
Knowledge about the biogeographic affinities of the world's tropical forests helps to better ... more Knowledge about the biogeographic affinities of the world's tropical forests helps to better understand regional differences in forest structure, diversity, composition, and dynamics. Such understanding will enable anticipation of region-specific responses to global environmental change. Modern phylogenies, in combination with broad coverage of species inventory data, now allow for global biogeographic analyses that take species evolutionary distance into account. Here we present a classification of the world's tropical forests based on their phylogenetic similarity. We identify five principal floristic regions and their floristic relationships: (i) Indo-Pacific, (ii) Subtropical, (iii) African, (iv) American, and (v) Dry forests. Our results do not support the traditional neo- versus paleotropical forest division but instead separate the combined American and African forests from their Indo-Pacific counterparts. We also find indications for the existence of a global dry for...
The original version of this Article contained an error in the third sentence of the abstract and... more The original version of this Article contained an error in the third sentence of the abstract and incorrectly read "Here, using long-term plot monitoring records of up to half a century, we find that intact forests in Borneo gained 0.43 Mg C ha-1 year-1 (95% CI 0.14-0.72, mean period 1988-2010) above-ground live biomass", rather than the correct "Here, using long-term plot monitoring records of up to half a century, we find that intact forests in Borneo gained 0.43 Mg C ha-1 year-1 (95% CI 0.14-0.72, mean period 1988-2010) in above-ground live biomass carbon". This has now been corrected in both the PDF and HTML versions of the Article.
The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, rele... more The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, released from the Kikai Caldera about 7300 years ago. The eruption was devastating and it is believed that primary succession and soil formation reinitiated across all altitudes at this point. We hypothesize that the concentrations of soil total phosphorus (P) and labile P fractions increase with increasing altitude because the soil formation has progressed less in upslope areas as a result of the cooler temperature and because of the ample P source of the volcanic ash. Conversely, we hypothesize that the concentration of soil inorganic nitrogen (N) decreases with increasing altitude. Available soil P and N would result in increasing P limitation downslope and increasing N limitation upslope, respectively. We studied soil P fractions and soil inorganic N, and P- and N-use efficiencies of the seven forests on Yakushima along an elevation gradient (170–1550 m a.s.l.). Contrary to our hypotheses, soil total P, labile soil P fractions, and inorganic N decreased with increasing altitude. The P- and N-use efficiencies of the forests were negatively correlated with the concentration of soil total active P (total P minus occluded P) and inorganic N, respectively. We suggest that progressive soil acidity and slower decomposition under cooler and wetter environments upslope must have dissolved the P contained in volcanic ash and accelerated P leaching. Forest ecosystems on Yakushima that show a distinct altitudinal zonation are, therefore, characterized by increasing P and N shortage with increasing altitude.
The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, rele... more The vegetation on Yakushima Island, Japan, grows on soils derived from Akahoya volcanic ash, released from the Kikai Caldera about 7300 years ago. The eruption was devastating and it is believed that primary succession and soil formation reinitiated across all altitudes at this point. We hypothesize that the concentrations of soil total phosphorus (P) and labile P fractions increase with increasing altitude because the soil formation has progressed less in upslope areas as a result of the cooler temperature and because of the ample P source of the volcanic ash. Conversely, we hypothesize that the concentration of soil inorganic nitrogen (N) decreases with increasing altitude. Available soil P and N would result in increasing P limitation downslope and increasing N limitation upslope, respectively. We studied soil P fractions and soil inorganic N, and P- and N-use efficiencies of the seven forests on Yakushima along an elevation gradient (170–1550 m a.s.l.). Contrary to our hypotheses, soil total P, labile soil P fractions, and inorganic N decreased with increasing altitude. The P- and N-use efficiencies of the forests were negatively correlated with the concentration of soil total active P (total P minus occluded P) and inorganic N, respectively. We suggest that progressive soil acidity and slower decomposition under cooler and wetter environments upslope must have dissolved the P contained in volcanic ash and accelerated P leaching. Forest ecosystems on Yakushima that show a distinct altitudinal zonation are, therefore, characterized by increasing P and N shortage with increasing altitude.
In humid regions of the western Pacific, conifers and deciduous broadleaf trees dominate in colde... more In humid regions of the western Pacific, conifers and deciduous broadleaf trees dominate in colder climate while evergreen broadleaf trees dominate in warmer climate. There are two geographically discontinuous forest zones of conifer-dominated forests, i.e., boreal zone and temperate-to-tropical zone. Between them, there is a deciduous broadleaf forest zone with warm summer and cold winter (Kira’s warmth index, WI from 45 to 85 °C, and coldness index, CI −15 °C). Boreal conifer forests are distributed at high latitudes or altitudes in Japan (“subarctic or subalpine forest”). Temperate conifer forests occur in a narrow belt on the Pacific Ocean side in Japan and are more extensive and distributed to warmer climate at high altitudes in Taiwan and in temperate zone of New Zealand, where they are called “conifer-broadleaf” forests. These temperate conifer (or mixed) forests grade into tropical conifer forests at high altitudes in Southeast Asia and New Guinea. Thus, these forests may be collectively called “temperate-to-tropical conifer-mixed forests” in contrast to boreal conifer forest. It appears that relatively cool summer (or year-round low temperature on tropical mountains) limits the growth of evergreen broadleaf trees, while mild winter (or the absence of winter on tropical mountains) gives the competitive advantage to conifers over deciduous broadleaf trees, leading to conifer dominance in the temperate-to-tropical conifer-mixed forests.
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Papers by Shin-ichiro Aiba