Department of Geography

In this special issue, we have tried to bridge studies of the Chinese state and of the Chinese city by employing the concepts of space production and territoriality. Three sets of analytical tools frame our questions: First, we use the concept of “urbanization of the local state” instead of “state-led urbanization” to capture the active role of urban processes as a formative force in social transformation and a definitive element in the making of the local state. Urban construction has become the key mechanism of local state building in the areas of public finance, territorial power consolidation, and local leaders’ political performance. Second, we expand the concept of the city to encompass the notion of territoriality, defined as spatial strategies to consolidate power in a given place and time and to secure autonomy. Territorial contestation is unusually intense when the premises of state authority are under-defined and local state jurisdictional boundaries shift frequently, as has been the case in China over the past thirty years. Third, we expand the analysis of territoriality from the realm of the state to that of society with the concept of “civic territoriality.” This concept refers to societal actors’ conscious cultivation and struggle to build territory for self protection and autonomy at the physical, socio-political, and discursive levels. Civic territoriality is central to societal actors’ cultivation of collective identities, to their framing of grievances and demands, and to their options and choice of collective actions. This framework helped to organize the seven contributions of this issue into the following three themes: Territorial Order and State Power, Territorialization of Capital, and Civic Territoriality.   Download PDF for full text of Introduction.

Tropical second-growth forests of the Amazon sequester large amounts of carbon and are important carbon sinks, contributing substantially to climate change mitigation, biodiversity conservation, and providing crucial ecosystem services. Deforestation due to selective logging and shifting cultivation is expanding second-growth forest areas in tropical forest regions, which if well managed, regenerate rapidly over time. Maximum forest canopy height is an important metric of biomass and carbon accumulation in second-growth forests and is strongly influenced by water availability. The water limitation hypothesis explains the positive influence of water availability on maximum tree heights and has been examined and demonstrated at a small-scale using field data, and at a global scale, with limited accuracy, using remote sensing data in tropical ecosystems. However, this hypothesis concerning maximum canopy height has not been much studied at regional and national scales for tropical second-growth forests. In this study, we leveraged NASA GEDI spaceborne lidar data across the Brazilian Amazon and derived second-growth forest relative height metrics for delineating the influence of water availability, second-growth forest age, and topographic elevation on maximum canopy height. Water availability was found to significantly influence the maximum canopy height of second-growth forest trees, of age range from 30 to 35 years, at elevations less than 500 m and maximum precipitation thresholds of 1500 mm. Our results indicate that changing precipitation patterns or increased drought conditions under different climate change regimes could impact forest structure, plant communities, ecosystem functioning, and carbon sequestration capabilities of tropical second-growth forests in the Amazon.

Abstract Previous studies have shown that the extratropical-to-tropical teleconnection exhibits seasonal dependence, but the underlying mechanisms remain unclear. In this study, we use an idealized framework to explore the teleconnection and its underlying dynamics. We impose extratropical thermal warming in an aquaplanet model coupled to a mixed layered ocean under two idealized control climates: a perpetual winter-like climate and a perpetual summer-like climate. The extratropical-to-tropical teleconnection is more pronounced in the perpetual summer-like case, primarily due to differences in eddy momentum flux behavior. The unstable environment in the summer-like climate leads to vertically extended warming, which suppresses baroclinicity effectively and results in a subtropical eddy momentum flux convergence anomaly. This in turn drives adjustments in the Hadley cell. Moreover, the summer-like climatological Hadley cell is in a regime that responds effectively to anomalous eddy momentum flux. The anomalous eddy momentum flux convergence weakens the Hadley cell, triggering air–sea interactions and bringing the extratropical influence to the deep tropics. The strong mean-state dependence of the extratropical-to-tropical teleconnection provides insight into the atmospheric response to more realistic extratropical forcings, such as volcano eruptions, ozone depletion, and aerosol emission.

Abstract: We explore the relative roles of Earth’s axial tilt (“tilt effect”) and orbital eccentricity (“distance effect”) in generating the seasonal cycle of tropical sea surface temperature (SST), decomposing the two contributions using simulations of an Earth system model varying the eccentricity and longitude of perihelion. Tropical SST seasonality is largely explained by the annual contribution from tilt, but with significant contributions from the semiannual contribution from tilt and annual contribution from distance, especially in regions where the tilt annual contribution is relatively small. Precessional changes to tropical SST seasonality are readily explained by the distance annual component whose amplitude increases linearly with eccentricity and whose phase changes linearly with the longitude of perihelion, while the tilt contributions remain essentially unchanged. As such, the annual cycle contribution from distance can become significant at high eccentricity (e > 0.05) and dominate the SST annual cycle in some regions of the tropics. The annual cycle tropical SST response to the distance effect consists of a tropics-wide warming peaking ∼2 months after perihelion, consistent with a direct thermodynamic effect and a dynamic contribution characterized by a cooling of the Pacific cold tongue peaking 5–6 months after perihelion. For current orbital conditions, the thermodynamic contribution acts to dampen the tropical SST seasonal cycle of the Northern Hemisphere from the tilt influence and amplify it in the Southern Hemisphere. The dynamic contribution acts to shift the Pacific cold tongue seasonal cycle arising from tilt to earlier in the season, by ∼1 month. Significance Statement: It is commonly assumed that climate seasonality is caused by Earth’s axial tilt. However, this is not necessarily the case for the tropics, where the annual variation of insolation from changes to the Earth–sun distance due to orbital eccentricity is significant. Here, we decompose the contributions from tilt and eccentricity on the seasonality of tropical sea surface temperature as simulated by an Earth system model. Orbital eccentricity drives an appreciable portion of tropical sea surface temperature seasonality even in today’s low-eccentricity orbit, modifying the seasonality driven by Earth’s axial tilt. At high orbital eccentricity (e > 0.05), the response to Earth–sun distance changes can dominate the annual cycle of sea surface temperature in some regions of the tropics.

The apparent respiratory quotient (ARQ) of tree stems, defined as the ratio of net stem CO2 efflux (ES_CO2) to net stem O2 influx (ES_O2), offers insights into the balance between local respiratory CO2 production and CO2 transported via the xylem. Traditional static chamber methods for measuring ARQ can introduce artifacts and obscure natural diurnal variations. Here, we employed an open flow-through stem chamber with ambient air coupled with cavity ring-down spectrometry, which uses the molecular properties of CO2 and O2 molecules to continuously measure ES_CO2, ES_O2, and ARQ, at the base of a California cherry tree (Prunus ilicifolia) during the 2024 growing season. Measurements across three stem chambers over 3–11-day periods revealed strong correlations between ES_CO2 and ES_O2 and mean ARQ values ranging from 1.3 to 2.9, far exceeding previous reports. Two distinct diurnal ARQ patterns were observed: daytime suppression with nighttime recovery, and a morning peak followed by gradual decline. Partitioning ES_CO2 into local respiration and xylem-transported CO2 indicated that the latter can dominate when ARQ exceeds 2.0. Furthermore, transported CO2 exhibited a higher temperature sensitivity than local respiration, with both processes showing declining temperature sensitivity above 20 °C. These findings underscore the need to differentiate stem CO2 flux components to improve our understanding of whole-tree carbon cycling.

Previous studies have noted the asymmetry in the annual cycle of zonal mean surface air temperature, defined as the difference in the lengths of warming and cooling periods. Pronounced north-south hemispheric differences in this asymmetry, by up to 40 days, were attributed to the eccentricity of Earth's orbit. However, we propose that the dominant factor comes from the difference in the land-sea fraction between hemispheres, because the asymmetry is strongly influenced by the annually varying heat capacity and land-sea interactions. The oceanic temperature annual cycle generally features a longer cooling period than warming due to the seasonal variation in ocean mixed layer depth, and exhibits the opposite situation when there is seasonal sea ice. Land-sea interactions impact the zonal mean temperature annual cycle by resulting in an earlier winter trough of the downstream oceanic temperature and delaying the summer peak in west coasts.

Abstract To predict ecological responses at broad environmental scales, grass species are commonly grouped into two broad functional types based on photosynthetic pathway. However, closely related species may have distinctive anatomical and physiological attributes that influence ecological responses, beyond those related to photosynthetic pathway alone. Hyperspectral leaf reflectance can provide an integrated measure of covarying leaf traits that may result from phylogenetic trait conservatism and/or environmental conditions. Understanding whether spectra‐trait relationships are lineage specific or reflect environmental variation across sites is necessary for using hyperspectral reflectance to predict plant responses to environmental changes across spatial scales. We measured hyperspectral leaf reflectance (400–2400 nm) and 12 structural, biochemical, and physiological leaf traits from five grass‐dominated sites spanning the Great Plains of North America. We assessed if variation in leaf reflectance spectra among grass species is explained more by evolutionary lineage (as captured by tribes or subfamilies), photosynthetic pathway (C 3 or C 4 ), or site differences. We then determined whether leaf spectra can be used to predict leaf traits within and across lineages. Our results using redundancy analysis ordination (RDA) show that grass tribe identity explained more variation in leaf spectra (adjusted R 2 = 0.12) than photosynthetic pathway, which explained little variation in leaf spectra (adjusted R 2 = 0.00). Furthermore, leaf reflectance from the same tribe across multiple sites was more similar than leaf reflectance from the same site across tribes (adjusted R 2 = 0.12 and 0.08, respectively). Across all sites and species, trait predictions based on spectra ranged considerably in predictive accuracies ( R 2 = 0.65 to <0.01), but R 2 was >0.80 for certain lineages and sites. The relationship between Vc max , a measure of photosynthetic capacity, and spectra was particularly promising. Chloridoideae, a lineage more common at drier sites, appears to have distinct spectra‐trait relationships compared with other lineages. Overall, our results show that evolutionary relatedness explains more variation in grass leaf spectra than photosynthetic pathway or site, but consideration of lineage‐ and site‐specific trait relationships is needed to interpret spectral variation across large environmental gradients.

Introduction: Understanding the mechanisms of tree mortality in tropical ecosystems remains challenging, in part due to the high diversity of tree species and the inherently stochastic nature of mortality. Plant functional traits offer a mechanistic link between plant physiology and performance, yet their ability to predict growth and mortality remains poorly understood. Given recent increases in tree mortality rates in the Amazon forest following extreme drought and wind events, we tested if lower wood density and acquisitive plant functional traits were associated with increased growth and mortality for common co-occurring trees in the Central Amazon. Methods: Seventeen trees of different species with similar sizes but a range in wood density (WD) and wood traits were felled, then assessed for 27 different individual functional parameters, including whole tree architecture, stem xylem anatomical and hydraulic traits and leaf traits. Traits of the individual trees were related to stand-level growth and mortality rates collected periodically over 30 years from nearby permanent inventory plots. Results: Higher wood density was associated with smaller leaf size, lower foliar base cations, lower stem water content and sapwood fraction, in agreement with the fast-slow plant economics spectrum. Lower wood density was associated with more acquisitive characteristics with greater hydraulic capacity and foliar nutrient concentrations, correlating with greater growth and mortality rates. Discussion: Our results show that lower wood density is part of a coordinated suite of traits linked to high resource acquisition, fast growth, and increased mortality risk, providing a functional framework for predicting species performance and forest vulnerability under future climate stress.

Previous studies have suggested that Arctic sea-ice loss can have a profound influence on atmospheric circulation far away from the Arctic. However, there is little scientific consensus on the features of these remote responses, with the opposite impacts reported. Here we present a multi-model analysis of the decadal climate response to Arctic sea-ice loss using state-of-the-art energy conserving methodologies to isolate the impacts of sea-ice decline. We observe weakening of the Aleutian Low and development of a geopotential ridge in the North Pacific, associated with drier winter conditions over the southwest United States. Over the Atlantic, a negative NAO-like response drives wetter winter conditions across the western Mediterranean. These decadal-scale impacts substantially differ from reported centennial-scale responses to Arctic sea-ice loss simulated using non-energy conserving methodologies. Factors such as the timescale of the response and methodologies used to isolate the impacts of disappearing sea-ice cover should be carefully considered when consolidating scientific understanding on the future impacts of changing Arctic.

Future climate presents conflicting implications for forest biomass. We evaluate how plant hydraulic traits, elevated CO₂ levels, warming, and changes in precipitation affect forest primary productivity, evapotranspiration, and the risk of hydraulic failure. We used a dynamic vegetation model with plant hydrodynamics (FATES-HYDRO) to simulate the stand-level responses to future climate changes in a wet tropical forest in Barro Colorado Island, Panama. We calibrated the model by selecting plant trait assemblages that performed well against observations. These assemblages were run with temperature and precipitation changes for two greenhouse gas emission scenarios (2086-2100: SSP2-45, SSP5-85) and two CO₂ levels (contemporary, anticipated). The risk of hydraulic failure is projected to increase from a contemporary rate of 5.7% to 10.1-11.3% under future climate scenarios, and, crucially, elevated CO₂ provided only slight amelioration. By contrast, elevated CO₂ mitigated GPP reductions. We attribute a greater variation in hydraulic failure risk to trait assemblages than to either CO₂ or climate. Our results project forests with both faster growth (through productivity increases) and higher mortality rates (through increasing rates of hydraulic failure) in the neo-tropics accompanied by certain trait plant assemblages becoming nonviable.