Christopher Still | Oregon State University (original) (raw)

Papers by Christopher Still

Science advances, Mar 1, 2016

The spread of C4 grasses in the late Neogene is one of the most important ecological transitions ... more The spread of C4 grasses in the late Neogene is one of the most important ecological transitions of the Cenozoic, but the primary driver of this global expansion is widely debated. We use the stable carbon isotopic composition (δ(13)C) of bison and mammoth tissues as a proxy for the relative abundance of C3 and C4 vegetation in their grazing habitat to determine climatic and atmospheric CO2 controls on C4 grass distributions from the Last Glacial Maximum (LGM) to the present. We predict the spatial variability of grass δ(13)C in North America using a mean of three different methods of classification and regression tree (CART) machine learning techniques and nine climatic variables. We show that growing season precipitation and temperature are the strongest predictors of all single climate variables. We apply this CART analysis to high-resolution gridded climate data and Coupled Model Intercomparison Project (CMIP5) mean paleoclimate model outputs to produce predictive isotope landsc...

Agu Fall Meeting Abstracts, Dec 1, 2006

While spatial and temporal variations in atmospheric CO2 and its carbon isotope composition have ... more While spatial and temporal variations in atmospheric CO2 and its carbon isotope composition have received considerable attention from the carbon cycle community, much less is known about the delta18O of atmospheric CO2 (delta18O-atm). Because delta18O-atm represents a fundamental coupling between the carbon and water cycles, there is great interest in using it to diagnose underlying variations in each cycle. While

The objective of this study was to understand how fog water inputs influence biogeochemical cycli... more The objective of this study was to understand how fog water inputs influence biogeochemical cycling and ecosystem function in coastal California forests. Specifically, we investigated whether additional fog-derived summertime moisture (1) stimulates summer photosynthesis and growth of Bishop pine trees; and (2) influences soil carbon cycling processes, including root respiration, microbial decomposition and biomass dynamics, and soil nutrient availability.

Computers and Electronics in Agriculture

ABSTRACT The downward excursion in delta18O of atmospheric CO2 observed during the 1990s and the ... more ABSTRACT The downward excursion in delta18O of atmospheric CO2 observed during the 1990s and the large interannnual variability characteristic of this isotopologue are not understood. We hypothesize that these variations in delta18O of atmospheric CO2 may be linked to global-scale variations in cloud cover and its influence on biosphere-atmosphere CO18O exchanges. Recent work has demonstrated the influence of clouds on canopy photosynthesis through increases in the diffuse radiation fraction and relative humidity, combined with decreases in leaf temperature. In concert, these alterations tend to increase canopy photosynthesis, which should also increase CO18O fluxes. However, photosynthetic CO18O fluxes also depend on the delta18O of leafwater, and enhanced cloudiness should decrease the delta18O of leafwater by enhancing relative humidity. Thus, the net impact of differing cloud cover on biosphere-atmosphere CO18O exchanges is difficult to predict. To capture these contrasting effects, we employed a comprehensive ecosystem isotope model (ISOLSM) in the southern great plains region of Oklahoma and Kansas. This region is particularly amenable for such a study because of the density of cloud property and radiation measurements. The region contains natural and agricultural ecosystems representing a variety of photosynthetic pathways and growth forms, including tallgrass prairie pastures, broadleaf forests, and crops. To drive the model across the entire region, we used Mesonet meteorological data collected at 120 stations in 2004, as well as precipitation delta18O values from the National Atmospheric Deposition Program network. LAI profiles from 2004 were derived from MODIS data. Our results suggest a large impact of clouds on photosynthetic CO2 and CO18O fluxes across this region. In an unstressed broadleaf deciduous forest (LAI=6.3), three sequential midsummer days with contrasting cloud cover illustrate this impact. Julian Day 222 is sunny, JD 223 is partly cloudy, and JD 224 is very cloudy. Increasing cloud cover over these days drives increases in relative humidity, the diffuse PAR fraction, and the fraction of canopy photosynthesis from shade leaves. Despite the lower total PAR on JD 223, total canopy photosynthesis is increased over the sunny day, driven by a large increase in shade leaf photosynthesis following the increase in diffuse PAR. Although canopy photosynthesis is higher on the partly cloudy day than the sunny day, the photosynthetic isoflux is lower due to the less enriched leafwater. The very cloudy day (JD 224) is predicted to have a negative isotopic discrimination and thus photosynthetic isoflux due to the lack of leafwater enrichment throughout the daytime. In contrast, discrimination against 13CO2 is predicted to increase slightly over this period, illustrating the myriad impacts that clouds have on biosphere-atmosphere isotope exchanges. Our results also suggest that these effects depend strongly on LAI and photosynthetic pathway (C3 or C4) of the vegetation, as well as on the effective water content of canopy leaves and its influence on leafwater delta18O.

We collected water samples in August, 2003 from a cloud forest in Monteverde, Costa Rica. This fo... more We collected water samples in August, 2003 from a cloud forest in Monteverde, Costa Rica. This forest is inundated frequently by orographic clouds formed during rapid uplift of trade winds along the Cordillera de Tilaran. Climate observations at Monteverde suggest an overall warming along with intensification of the January-April dry season when convective storms are minimal and orographic cloudwater inputs

Background/Question/Methods One of the key environmental factors controlling microbial activity i... more Background/Question/Methods One of the key environmental factors controlling microbial activity is moisture. Cool, wet winters separated by long, dry summers present some the most challenging conditions for microbial survival and growth. In coastal bishop pine forests on California’s Channel Islands, previous work has shown that fog can provide an important moisture source sustaining tree growth during the dry summers in this Mediterranean climate. However, little is known about how moisture inputs from fog affect soil microbial dynamics in these ecosystems. Starting in May 2008, we measured soil moisture, microbial biomass carbon and nitrogen, and soil inorganic nitrogen, root/microbial respiration, and dissolved organic carbon and nitrogen under tree canopies, and in open spaces, in two bishop pine stands on Santa Cruz Island, California. Measurements were made roughly every other month, and are ongoing. Results/Conclusions Soil water content was consistently higher under tree can...

International Journal of Climatology, 2014

Studies of regional drought history will be helpful for understanding current hydroclimate variab... more Studies of regional drought history will be helpful for understanding current hydroclimate variability with global warming as well as predicting future hydroclimate shifts. Long-term tree-ring records are scarce in the western Qilian Mountains of northwestern China, which is also the western boundary of the natural distribution of Qilian Juniper (Juniperus przewalskii Kom.) in this area. Here, we present an 850-year (AD 1161-2010) reconstruction of May-July self-calibrating Palmer Drought Severity Index (scPDSI) in the western Qilian Mountains that is based on two nearly millennia of ring-width chronologies derived from long-lived Qilian Juniper trees. The reconstruction suggests a relatively dry phase from the 15th century to the 18th century during the Little Ice Age (LIA) and a relatively wet period over the past two centuries. This reconstruction is consistent with other tree-ring-based hydroclimatic reconstructions from the northeastern Tibetan Plateau, as well as the corresponding century-scale solar activity during the LIA. The three mega-drought periods (AD 1260s-1340s, 1430s-1540s and 1640s-1740s) recorded by the tree-ring series also correspond to the Wolf, Spörer and Maunder solar activity minimum periods. Results of the multi-tape method analysis and wavelet analysis further confirmed the relationship between regional hydroclimate variability and solar activity forcing.

ABSTRACT Moisture and moisture pulses drive soil respiration dynamics in semi-arid, arid and Medi... more ABSTRACT Moisture and moisture pulses drive soil respiration dynamics in semi-arid, arid and Mediterranean ecosystems. This presentation will focus on the response of soil respiration, and its separate root and microbial sources, to seasonal and episodic changes in soil moisture in a Bishop pine (Pinus muricata) forest on the California Channel Islands, USA. This ecosystem is unique in that summertime water inputs are derived exclusively from many small fog-drip events, whereas winter inputs are dominated by fewer, larger rain events. Using automated chamber measurements in combination with radiocarbon (14C) source partitioning techniques, we demonstrate that heterotrophic and autotrophic respiration responded asynchronously to moisture inputs. Overall, autotrophic respiration dominated fluxes, contributing 50-76 % to soil respiration over the entire year. However, heterotrophic respiration was more dynamic, responding immediately to both summer- and winter-time moisture pulses, and in some cases comprising up to 90 % of the pulse response. In contrast, autotrophic respiration increases lagged larger moisture events, and in general, responded more gradually over seasonal timescales. Additionally, by applying time series analysis, we investigate the temporal correlations among soil respiration, pine physiological activity (assessed by sap flow measurements), temperature, and moisture inputs at multiple timescales.

BioScience, 2014

Historical ecology is becoming an important focus in conservation biology and offers a promising ... more Historical ecology is becoming an important focus in conservation biology and offers a promising tool to help guide ecosystem management. Here, we integrate data from multiple disciplines to illuminate the past, present, and future of biodiversity on California's Channel Islands, an archipelago that has undergone a wide range of land-use and ecological changes. Our analysis spans approximately 20,000 years, from before human occupation and through Native American hunter-gatherers, commercial ranchers and fishers, the US military, and other land managers. We demonstrate how long-term, interdisciplinary research provides insight into conservation decisions, such as setting ecosystem restoration goals, preserving rare and endemic taxa, and reducing the impacts of climate change on natural and cultural resources. We illustrate the importance of historical perspectives for understanding modern patterns and ecological change and present an approach that can be applied generally in conservation management planning.

Global Ecology and Biogeography, 2014

Aim C 4 photosynthesis is an adaptation that maintains efficient carbon assimilation in warm and ... more Aim C 4 photosynthesis is an adaptation that maintains efficient carbon assimilation in warm and low-CO 2 conditions. Due to the importance of C 4 grasses for carbon and surface energy fluxes numerous models have been proposed to describe their spatial distribution and forecast responses to climate change. These models often rely on broad climate predictors (e.g., temperature and precipitation) but fail to integrate other ecologically relevant factors, such as disturbance and competition, which may modify realized C 3 /C 4 grass distributions. We evaluate the contribution of ecological factors, in addition to climate predictors, to C 3 /C 4 grass distributions across multiple biogeographic regions of North America in a multi-source database of >40,000 vegetation plots. Location Conterminous United States of America (USA). Methods We identified a comprehensive pool of physiological-climatic models in the literature and used information theoretic criteria to select a primary physiological predictor of C 3 and C 4 grasses. Subsequently, the climate model was combined with ecological predictors using a multiple regression framework and tested within eight regions within the USA. Results Surprisingly, grass-dominated communities across the USA exist largely in a C 3 or C 4 48 dominated state. Transitions between C 3 /C 4 dominance were best explained by models that integrated temperature and precipitation with ecological factors that varied according to region. For some regions, such as Eastern Temperate Forests, local, ecological factors were comparable in strength to broad climate predictors of C 3 /C 4 abundance. Main conclusion Local, ecological factors modify C 3 /C 4 grass responses to broad-scale climatic drivers in ways that manifest at regional scales. In Eastern Temperate Forests, for example, C 4 grass abundances are maintained below climatic expectations where tree cover creates light

Global Ecology and Biogeography, 2014

Aim Numerous studies have documented ecological sorting of C3 and C4 grasses along air temperatur... more Aim Numerous studies have documented ecological sorting of C3 and C4 grasses along air temperature gradients. However, phylogenetically structured analyses suggest that closely related C3 and C4 grasses (in the same PACMAD clade: Panicoideae, Arundinoideae, Chloridoideae, Micrairoideae, Aristidoideae and Danthonioideae) occur in environments with similar air temperature, challenging our understanding of how the photosynthetic pathway influences grass biogeography. To better understand thermal differences between C3 and C4 grass lineages, we analysed the surface radiative temperature as an alternative measure that is more closely aligned with plant microclimate. Location Hawaiian Islands, USA. Methods We used the MODIS land surface temperature (LST) product, a satellite-based measurement of radiative temperature. We compared LST with mean annual air temperature (MAT) for locations where C3 and C4 grass species were collected. We also utilized other satellite products, like MODIS tree cover, as a proxy for relative habitat openness where these grasses occur. Results Comparisons of C3 lineages [in BEP (Bambusoideae, Ehrhartoideae and Pooideae) and PACMAD clades] and C4 lineages (PACMAD clade only) illustrate the differing thermal environments for each group. C4 taxa are found in the environments with the highest MAT, followed by C3 PACMAD species. By contrast, C3 PACMAD species are found in the environments with the coolest LST, and the LST values for C3 BEP species are substantially higher than their MAT values. The difference in LST between C3 and C4 PACMADs is larger than the difference in MAT between these groups. Main conclusions Though LST has been used infrequently in ecology and biogeography, it is intimately related to water and energy balance and ecosystem structure, and should more accurately capture plant temperatures and microclimates than MAT. Our results provide support for a pronounced thermal difference in the environments of closely related C3 and C4 grass taxa, and show that C3 PACMADs occur in the coolest and least variable thermal environments, probably due to the greater tree cover of these habitats.

Global Change Biology, 2013

A number of studies have demonstrated the ecological sorting of C 3 and C 4 grasses along tempera... more A number of studies have demonstrated the ecological sorting of C 3 and C 4 grasses along temperature and moisture gradients. However, previous studies of C 3 and C 4 grass biogeography have often inadvertently compared species in different and relatively unrelated lineages, which are associated with different environmental settings and distinct adaptive traits. Such confounded comparisons of C 3 and C 4 grasses may bias our understanding of ecological sorting imposed strictly by photosynthetic pathway. Here, we used MaxEnt species distribution modeling in combination with satellite data to understand the functional diversity of C 3 and C 4 grasses by comparing both large clades and closely related sister taxa. Similar to previous work, we found that C 4 grasses showed a preference for regions with higher temperatures and lower precipitation compared with grasses using the C 3 pathway. However, air temperature differences were smaller (2°C vs. 4°C) and precipitation and % tree cover differences were larger (1783 mm vs. 755 mm, 21.3% vs. 7.7%, respectively) when comparing C 3 and C 4 grasses within the same clade vs. comparing all C 4 and all C 3 grasses (i.e., ignoring phylogenetic structure). These results were due to important differences in the environmental preferences of C 3 BEP and PACMAD clades (the two main grass clades). Winter precipitation was found to be more important for understanding the distribution and environmental niche of C 3 PACMADs in comparison with both C 3 BEPs and C 4 taxa, for which temperature was much more important. Results comparing closely related C 3-C 4 sister taxa supported the patterns derived from our modeling of the larger clade groupings. Our findings, which are novel in comparing the distribution and niches of clades, demonstrate that the evolutionary history of taxa is important for understanding the functional diversity of C 3 and C 4 grasses, and should have implications for how grasslands will respond to global change.

Science advances, Mar 1, 2016

The spread of C4 grasses in the late Neogene is one of the most important ecological transitions ... more The spread of C4 grasses in the late Neogene is one of the most important ecological transitions of the Cenozoic, but the primary driver of this global expansion is widely debated. We use the stable carbon isotopic composition (δ(13)C) of bison and mammoth tissues as a proxy for the relative abundance of C3 and C4 vegetation in their grazing habitat to determine climatic and atmospheric CO2 controls on C4 grass distributions from the Last Glacial Maximum (LGM) to the present. We predict the spatial variability of grass δ(13)C in North America using a mean of three different methods of classification and regression tree (CART) machine learning techniques and nine climatic variables. We show that growing season precipitation and temperature are the strongest predictors of all single climate variables. We apply this CART analysis to high-resolution gridded climate data and Coupled Model Intercomparison Project (CMIP5) mean paleoclimate model outputs to produce predictive isotope landsc...

Agu Fall Meeting Abstracts, Dec 1, 2006

While spatial and temporal variations in atmospheric CO2 and its carbon isotope composition have ... more While spatial and temporal variations in atmospheric CO2 and its carbon isotope composition have received considerable attention from the carbon cycle community, much less is known about the delta18O of atmospheric CO2 (delta18O-atm). Because delta18O-atm represents a fundamental coupling between the carbon and water cycles, there is great interest in using it to diagnose underlying variations in each cycle. While

The objective of this study was to understand how fog water inputs influence biogeochemical cycli... more The objective of this study was to understand how fog water inputs influence biogeochemical cycling and ecosystem function in coastal California forests. Specifically, we investigated whether additional fog-derived summertime moisture (1) stimulates summer photosynthesis and growth of Bishop pine trees; and (2) influences soil carbon cycling processes, including root respiration, microbial decomposition and biomass dynamics, and soil nutrient availability.

Computers and Electronics in Agriculture

ABSTRACT The downward excursion in delta18O of atmospheric CO2 observed during the 1990s and the ... more ABSTRACT The downward excursion in delta18O of atmospheric CO2 observed during the 1990s and the large interannnual variability characteristic of this isotopologue are not understood. We hypothesize that these variations in delta18O of atmospheric CO2 may be linked to global-scale variations in cloud cover and its influence on biosphere-atmosphere CO18O exchanges. Recent work has demonstrated the influence of clouds on canopy photosynthesis through increases in the diffuse radiation fraction and relative humidity, combined with decreases in leaf temperature. In concert, these alterations tend to increase canopy photosynthesis, which should also increase CO18O fluxes. However, photosynthetic CO18O fluxes also depend on the delta18O of leafwater, and enhanced cloudiness should decrease the delta18O of leafwater by enhancing relative humidity. Thus, the net impact of differing cloud cover on biosphere-atmosphere CO18O exchanges is difficult to predict. To capture these contrasting effects, we employed a comprehensive ecosystem isotope model (ISOLSM) in the southern great plains region of Oklahoma and Kansas. This region is particularly amenable for such a study because of the density of cloud property and radiation measurements. The region contains natural and agricultural ecosystems representing a variety of photosynthetic pathways and growth forms, including tallgrass prairie pastures, broadleaf forests, and crops. To drive the model across the entire region, we used Mesonet meteorological data collected at 120 stations in 2004, as well as precipitation delta18O values from the National Atmospheric Deposition Program network. LAI profiles from 2004 were derived from MODIS data. Our results suggest a large impact of clouds on photosynthetic CO2 and CO18O fluxes across this region. In an unstressed broadleaf deciduous forest (LAI=6.3), three sequential midsummer days with contrasting cloud cover illustrate this impact. Julian Day 222 is sunny, JD 223 is partly cloudy, and JD 224 is very cloudy. Increasing cloud cover over these days drives increases in relative humidity, the diffuse PAR fraction, and the fraction of canopy photosynthesis from shade leaves. Despite the lower total PAR on JD 223, total canopy photosynthesis is increased over the sunny day, driven by a large increase in shade leaf photosynthesis following the increase in diffuse PAR. Although canopy photosynthesis is higher on the partly cloudy day than the sunny day, the photosynthetic isoflux is lower due to the less enriched leafwater. The very cloudy day (JD 224) is predicted to have a negative isotopic discrimination and thus photosynthetic isoflux due to the lack of leafwater enrichment throughout the daytime. In contrast, discrimination against 13CO2 is predicted to increase slightly over this period, illustrating the myriad impacts that clouds have on biosphere-atmosphere isotope exchanges. Our results also suggest that these effects depend strongly on LAI and photosynthetic pathway (C3 or C4) of the vegetation, as well as on the effective water content of canopy leaves and its influence on leafwater delta18O.

We collected water samples in August, 2003 from a cloud forest in Monteverde, Costa Rica. This fo... more We collected water samples in August, 2003 from a cloud forest in Monteverde, Costa Rica. This forest is inundated frequently by orographic clouds formed during rapid uplift of trade winds along the Cordillera de Tilaran. Climate observations at Monteverde suggest an overall warming along with intensification of the January-April dry season when convective storms are minimal and orographic cloudwater inputs

Background/Question/Methods One of the key environmental factors controlling microbial activity i... more Background/Question/Methods One of the key environmental factors controlling microbial activity is moisture. Cool, wet winters separated by long, dry summers present some the most challenging conditions for microbial survival and growth. In coastal bishop pine forests on California’s Channel Islands, previous work has shown that fog can provide an important moisture source sustaining tree growth during the dry summers in this Mediterranean climate. However, little is known about how moisture inputs from fog affect soil microbial dynamics in these ecosystems. Starting in May 2008, we measured soil moisture, microbial biomass carbon and nitrogen, and soil inorganic nitrogen, root/microbial respiration, and dissolved organic carbon and nitrogen under tree canopies, and in open spaces, in two bishop pine stands on Santa Cruz Island, California. Measurements were made roughly every other month, and are ongoing. Results/Conclusions Soil water content was consistently higher under tree can...

International Journal of Climatology, 2014

Studies of regional drought history will be helpful for understanding current hydroclimate variab... more Studies of regional drought history will be helpful for understanding current hydroclimate variability with global warming as well as predicting future hydroclimate shifts. Long-term tree-ring records are scarce in the western Qilian Mountains of northwestern China, which is also the western boundary of the natural distribution of Qilian Juniper (Juniperus przewalskii Kom.) in this area. Here, we present an 850-year (AD 1161-2010) reconstruction of May-July self-calibrating Palmer Drought Severity Index (scPDSI) in the western Qilian Mountains that is based on two nearly millennia of ring-width chronologies derived from long-lived Qilian Juniper trees. The reconstruction suggests a relatively dry phase from the 15th century to the 18th century during the Little Ice Age (LIA) and a relatively wet period over the past two centuries. This reconstruction is consistent with other tree-ring-based hydroclimatic reconstructions from the northeastern Tibetan Plateau, as well as the corresponding century-scale solar activity during the LIA. The three mega-drought periods (AD 1260s-1340s, 1430s-1540s and 1640s-1740s) recorded by the tree-ring series also correspond to the Wolf, Spörer and Maunder solar activity minimum periods. Results of the multi-tape method analysis and wavelet analysis further confirmed the relationship between regional hydroclimate variability and solar activity forcing.

ABSTRACT Moisture and moisture pulses drive soil respiration dynamics in semi-arid, arid and Medi... more ABSTRACT Moisture and moisture pulses drive soil respiration dynamics in semi-arid, arid and Mediterranean ecosystems. This presentation will focus on the response of soil respiration, and its separate root and microbial sources, to seasonal and episodic changes in soil moisture in a Bishop pine (Pinus muricata) forest on the California Channel Islands, USA. This ecosystem is unique in that summertime water inputs are derived exclusively from many small fog-drip events, whereas winter inputs are dominated by fewer, larger rain events. Using automated chamber measurements in combination with radiocarbon (14C) source partitioning techniques, we demonstrate that heterotrophic and autotrophic respiration responded asynchronously to moisture inputs. Overall, autotrophic respiration dominated fluxes, contributing 50-76 % to soil respiration over the entire year. However, heterotrophic respiration was more dynamic, responding immediately to both summer- and winter-time moisture pulses, and in some cases comprising up to 90 % of the pulse response. In contrast, autotrophic respiration increases lagged larger moisture events, and in general, responded more gradually over seasonal timescales. Additionally, by applying time series analysis, we investigate the temporal correlations among soil respiration, pine physiological activity (assessed by sap flow measurements), temperature, and moisture inputs at multiple timescales.

BioScience, 2014

Historical ecology is becoming an important focus in conservation biology and offers a promising ... more Historical ecology is becoming an important focus in conservation biology and offers a promising tool to help guide ecosystem management. Here, we integrate data from multiple disciplines to illuminate the past, present, and future of biodiversity on California's Channel Islands, an archipelago that has undergone a wide range of land-use and ecological changes. Our analysis spans approximately 20,000 years, from before human occupation and through Native American hunter-gatherers, commercial ranchers and fishers, the US military, and other land managers. We demonstrate how long-term, interdisciplinary research provides insight into conservation decisions, such as setting ecosystem restoration goals, preserving rare and endemic taxa, and reducing the impacts of climate change on natural and cultural resources. We illustrate the importance of historical perspectives for understanding modern patterns and ecological change and present an approach that can be applied generally in conservation management planning.

Global Ecology and Biogeography, 2014

Aim C 4 photosynthesis is an adaptation that maintains efficient carbon assimilation in warm and ... more Aim C 4 photosynthesis is an adaptation that maintains efficient carbon assimilation in warm and low-CO 2 conditions. Due to the importance of C 4 grasses for carbon and surface energy fluxes numerous models have been proposed to describe their spatial distribution and forecast responses to climate change. These models often rely on broad climate predictors (e.g., temperature and precipitation) but fail to integrate other ecologically relevant factors, such as disturbance and competition, which may modify realized C 3 /C 4 grass distributions. We evaluate the contribution of ecological factors, in addition to climate predictors, to C 3 /C 4 grass distributions across multiple biogeographic regions of North America in a multi-source database of >40,000 vegetation plots. Location Conterminous United States of America (USA). Methods We identified a comprehensive pool of physiological-climatic models in the literature and used information theoretic criteria to select a primary physiological predictor of C 3 and C 4 grasses. Subsequently, the climate model was combined with ecological predictors using a multiple regression framework and tested within eight regions within the USA. Results Surprisingly, grass-dominated communities across the USA exist largely in a C 3 or C 4 48 dominated state. Transitions between C 3 /C 4 dominance were best explained by models that integrated temperature and precipitation with ecological factors that varied according to region. For some regions, such as Eastern Temperate Forests, local, ecological factors were comparable in strength to broad climate predictors of C 3 /C 4 abundance. Main conclusion Local, ecological factors modify C 3 /C 4 grass responses to broad-scale climatic drivers in ways that manifest at regional scales. In Eastern Temperate Forests, for example, C 4 grass abundances are maintained below climatic expectations where tree cover creates light

Global Ecology and Biogeography, 2014

Aim Numerous studies have documented ecological sorting of C3 and C4 grasses along air temperatur... more Aim Numerous studies have documented ecological sorting of C3 and C4 grasses along air temperature gradients. However, phylogenetically structured analyses suggest that closely related C3 and C4 grasses (in the same PACMAD clade: Panicoideae, Arundinoideae, Chloridoideae, Micrairoideae, Aristidoideae and Danthonioideae) occur in environments with similar air temperature, challenging our understanding of how the photosynthetic pathway influences grass biogeography. To better understand thermal differences between C3 and C4 grass lineages, we analysed the surface radiative temperature as an alternative measure that is more closely aligned with plant microclimate. Location Hawaiian Islands, USA. Methods We used the MODIS land surface temperature (LST) product, a satellite-based measurement of radiative temperature. We compared LST with mean annual air temperature (MAT) for locations where C3 and C4 grass species were collected. We also utilized other satellite products, like MODIS tree cover, as a proxy for relative habitat openness where these grasses occur. Results Comparisons of C3 lineages [in BEP (Bambusoideae, Ehrhartoideae and Pooideae) and PACMAD clades] and C4 lineages (PACMAD clade only) illustrate the differing thermal environments for each group. C4 taxa are found in the environments with the highest MAT, followed by C3 PACMAD species. By contrast, C3 PACMAD species are found in the environments with the coolest LST, and the LST values for C3 BEP species are substantially higher than their MAT values. The difference in LST between C3 and C4 PACMADs is larger than the difference in MAT between these groups. Main conclusions Though LST has been used infrequently in ecology and biogeography, it is intimately related to water and energy balance and ecosystem structure, and should more accurately capture plant temperatures and microclimates than MAT. Our results provide support for a pronounced thermal difference in the environments of closely related C3 and C4 grass taxa, and show that C3 PACMADs occur in the coolest and least variable thermal environments, probably due to the greater tree cover of these habitats.

Global Change Biology, 2013

A number of studies have demonstrated the ecological sorting of C 3 and C 4 grasses along tempera... more A number of studies have demonstrated the ecological sorting of C 3 and C 4 grasses along temperature and moisture gradients. However, previous studies of C 3 and C 4 grass biogeography have often inadvertently compared species in different and relatively unrelated lineages, which are associated with different environmental settings and distinct adaptive traits. Such confounded comparisons of C 3 and C 4 grasses may bias our understanding of ecological sorting imposed strictly by photosynthetic pathway. Here, we used MaxEnt species distribution modeling in combination with satellite data to understand the functional diversity of C 3 and C 4 grasses by comparing both large clades and closely related sister taxa. Similar to previous work, we found that C 4 grasses showed a preference for regions with higher temperatures and lower precipitation compared with grasses using the C 3 pathway. However, air temperature differences were smaller (2°C vs. 4°C) and precipitation and % tree cover differences were larger (1783 mm vs. 755 mm, 21.3% vs. 7.7%, respectively) when comparing C 3 and C 4 grasses within the same clade vs. comparing all C 4 and all C 3 grasses (i.e., ignoring phylogenetic structure). These results were due to important differences in the environmental preferences of C 3 BEP and PACMAD clades (the two main grass clades). Winter precipitation was found to be more important for understanding the distribution and environmental niche of C 3 PACMADs in comparison with both C 3 BEPs and C 4 taxa, for which temperature was much more important. Results comparing closely related C 3-C 4 sister taxa supported the patterns derived from our modeling of the larger clade groupings. Our findings, which are novel in comparing the distribution and niches of clades, demonstrate that the evolutionary history of taxa is important for understanding the functional diversity of C 3 and C 4 grasses, and should have implications for how grasslands will respond to global change.