Identification of fossil juniper seeds from Rancho La Brea (California, USA): drought and extirpation in the Late Pleistocene

Juniperus spp. are keystone shrubs in western North America and important climatic indicators in paleo-records. However, a lack of taxonomic resolution among fossil species limits our ability to track past environmental changes. Plant macrofossils at Rancho La Brea (RLB) allow for reconstructions of juniper occurrence to species across 60 000 yr. We use microscopy, image analysis, species distribution modeling (SDM), and radiocarbon dating to identify an unknown Juniperus species at RLB and put it into chronological context with fossil Juniperus californica at the site to infer past environmental conditions. We identify the unknown taxon as Juniperus scopulorum Sargent, 1897. The Pleistocene occurrence of this species in California expands its known distribution and documents its extirpation. Temporal ranges of the two fossil junipers alternate, revealing a pattern of differential climatic sensitivity throughout the end of the Pleistocene. Occurrence patterns suggest sensitivity to temperature, moisture availability, and the presence of two mega-droughts at c. 48-44.5 ka and c. 29.3-25.2 ka. Extirpation of both taxa by c. 13 ka is likely driven by climate, megafaunal extinction, and increasing fire. The extirpation of fossil junipers during these past climatic events demonstrates vulnerability of juniper species in the face of global change.

Shrub ecosystem structure in response to anthropogenic climate change: A global synthesis

Anthropogenic warming is predicted to alter ecological boundaries in energy-limited shrub ecosystems. Yet we still lack a sound understanding of the structural changes that shrub ecosystems are undergoing on a global scale and the factors driving them. To that end, here we collected studies of shrub dynamics from 227 sites worldwide to conduct a quantitative review, including the rate of advancing shrubline (their upslope shift), the rates of shrub cover and recruitment changes. Our results revealed that shrub expanded (e.g. shrubline shifts, shrub cover and recruitment increase) at the vast majority of sites (84 %); in contrast, they remained stable in 10 % of sites and descended at just 6 % of them. The mean global shift rate of shrubline was 1.22 m/year, being significantly faster in subarctic (> 60°N) than temperate (< 60°N) regions, and likewise more quickly in wet (total annual precipitation >400 mm) than dry (total annual precipitation <400 mm) areas; the annual change rates of shrub cover and recruitment increased by 0.89 % and 2.02 %. Shrubs communities have expanded rapidly in response to ongoing climate warming. The combination of autumn precipitation and winter temperature largely contributed to the general shift rates of shrubline, while the shrub cover and recruitment were mainly affected by summer temperature and precipitation in both spring and autumn. Furthermore, the site-specific pace of their expansion probably depends on a combination of local climatic and non-climatic drivers (such as fine-scale environmental conditions, disturbance, their interactions, and dispersal limitation). The increase of shrub distribution may alter the function and albedo of the ecosystems at high-latitude and -elevation regions, resulting in the feedback on climate.

Climate effect on the growth and hydraulic traits of two shrubs from the top of a Mediterranean mountain

Increasing mean global temperatures in conjunction with increases in the frequency and severity of drought events affect plant growth and physiology, particularly in more arid and mountainous ecosystems. Thus, it is imperative to understand the response of plant growth to climatic oscillations in these regions. This study used dendrochronological and wood anatomical traits of two shrub species growing over 1500 m.a.s.l. in the Serra da Estrela (Portugal), Juniperus communis and Cytisus oromediterraneus, to analyze their response to temperature and water availability parameters. Results showed an increase in shrub growth related to the increase over time of the mean minimum and maximum monthly temperature in Serra da Estrela. Warming seems to promote shrub growth because it lengthens the growing season, although J. communis responds mainly to spring maximum temperature while C. oromediterraneus is influenced by fall maximum temperature. Hydraulic traits of J. communis and C. oromediterraneus were negatively influenced by winter drought. Additionally, there were species-specific differences in response to changes in water availability. J. communis radial growth was significantly affected by spring drought conditions, while C. oromediterraneus radial growth was significantly affected by spring precipitation. C. oromediterraneus hydraulic traits were also significantly affected by drought conditions from the previous spring and fall. This study shed light on specific differences in the response to climate between two co-occurring shrub species in the top of an understudied Mediterranean mountain, which could have implications in the future distribution of woody species within this region.

Asymmetric effects of daytime and nighttime warming on alpine treeline recruitment

Trees at their upper range limits are highly sensitive to climate change, and thus alpine treelines worldwide have changed their recruitment patterns in response to climate warming. However, previous studies focused only on daily mean temperature, neglecting the asymmetric influences of daytime and nighttime warming on recruitments in alpine treelines. Here, based on the compiled dataset of tree recruitment series from 172 alpine treelines across the Northern Hemisphere, we quantified and compared the different effects of daytime and nighttime warming on treeline recruitment using four indices of temperature sensitivity, and assessed the responses of treeline recruitment to warming-induced drought stress. Our analyses demonstrated that even in different environmental regions, both daytime and nighttime warming could significantly promote treeline recruitment, and however, treeline recruitment was much more sensitive to nighttime warming than to daytime warming, which could be attributable to the presence of drought stress. The increasing drought stress primarily driven by daytime warming rather than by nighttime warming would likely constrain the responses of treeline recruitment to daytime warming. Our findings provided compelling evidence that nighttime warming rather than daytime warming could play a primary role in promoting the recruitment in alpine treelines, which was related to the daytime warming-induced drought stress. Thus, daytime and nighttime warming should be considered separately to improve future projections of global change impacts across alpine ecosystems.

Warming-induced tipping points of Arctic and alpine shrub recruitment

Shrub recruitment, a key component of vegetation dynamics beyond forests, is a highly sensitive indicator of climate and environmental change. Warming-induced tipping points in Arctic and alpine treeless ecosystems are, however, little understood. Here, we compare two long-term recruitment datasets of 2,770 shrubs from coastal East Greenland and from the Tibetan Plateau against atmospheric circulation patterns between 1871 and 2010 Common Era. Increasing rates of shrub recruitment since 1871 reached critical tipping points in the 1930s and 1960s on the Tibetan Plateau and in East Greenland, respectively. A recent decline in shrub recruitment in both datasets was likely related to warmer and drier climates, with a stronger May to July El Niño Southern Oscillation over the Tibetan Plateau and a stronger June to July Atlantic Multidecadal Oscillation over Greenland. Exceeding the thermal optimum of shrub recruitment, the recent warming trend may cause soil moisture deficit. Our findings suggest that changes in atmospheric circulation explain regional climate dynamics and associated response patterns in Arctic and alpine shrub communities, knowledge that should be considered to protect vulnerable high-elevation and high-latitude ecosystems from the cascading effects of anthropogenic warming.

Predicting the responses of subalpine forest landscape dynamics to climate change on the eastern Tibetan Plateau

Subalpine vegetation across the Tibetan Plateau is globally one of the most sensitive to climate change. However, the potential landscape-scale effects of climate change on subalpine forest dynamics remain largely unexplored. Here, we used a forest landscape model (LANDIS-II) coupled with a forest ecosystem process model (PnET-II) to simulate forest dynamics under future climate change in Jiuzhaigou National Nature Reserve in the eastern subalpine region of the Tibetan Plateau. We examined changes in the composition, distribution and aboveground biomass of cold temperate coniferous forests, temperate coniferous forests, deciduous broad-leaved forests and redwood forest under four climate change scenarios (RCP2.6, RCP4.5, RCP8.5 and the current climate) from 2016 to 2096. Our model predicts that by 2096, (i) cold temperate coniferous forests will expand and increase by 7.92%, 8.18%, 8.65% and 7.02% under current climate, RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively; (ii) distribution of forests as a whole shows upward elevational range shift, especially under RCP8.5 scenario and (iii) total aboveground biomass slowly increases at first and then decreases to 12%-16% of current distribution under RCPs. These results show that climate change can be expected to significantly influence forest composition, distribution and aboveground biomass in the subalpine forests of eastern Tibetan Plateau. This study is the first to simulate forest dynamics at the landscape scale in subalpine areas of the Tibetan Plateau, which provides an important step in developing more effective strategies of forest management for expected climate change, not only in China but also around the world.

Century-long record of polycyclic aromatic hydrocarbons from tree rings in the southeastern Tibetan Plateau

Limited studies have been carried out on the historical variations of atmospheric polycyclic aromatic hydrocarbons (PAHs), especially in remote regions of the world. In this study, century-long record of PAHs (1916-2018) were reconstructed from tree rings in the remote southeastern Tibetan Plateau (TP). The total concentrations of 15 PAHs varied from 27.5 to 6.05 × 10² ng/g dry weight (dw), with a mean value of 1.40 × 10² ng/g dw. Higher levels of PAHs were observed during World War Ⅱ and the Peaceful Liberation of Tibet, and increasing trends were observed starting from rapid industrialization in India. Both the isomer ratios and the positive matrix factorization model results indicated biomass and coal combustion were the dominant sources of PAHs. The carcinogenic risk of PAHs to local residents was assessed, which might have been negligible in most past periods and lower than in other regions of the world. Nevertheless, since the beginning of the 21st century, the cancer risk has been increasing year by year, indicating more actions are needed to reduce emissions of PAHs. This study provides an idea for reconstructing the pollution history of PAHs at the global scale.

Optimal temperature of vegetation productivity and its linkage with climate and elevation on the Tibetan Plateau

Vegetation productivity first increases and then decreases with temperature; and temperature corresponding to the maximum productivity is called optimal temperature (T_opt ). In this study, we used satellite derived near-infrared reflectance of vegetation (NIR_v ) data to map T_opt of vegetation productivity at the spatial resolution of 0.1° on the Tibetan Plateau (TP), one of most sensitive regions in the climate system. The average T_opt of non-forest vegetation on the TP is about 14.7°C, significantly lower than the T_opt value used in current ecosystem models. A remarkable geographical heterogeneity in T_opt is observed over the TP. Higher T_opt values generally appear in the north-eastern TP, while the south-western TP has relatively lower T_opt (<10°C), in line with the difference of climate conditions and topography across different regions. Spatially, T_opt tends to decrease by 0.41°C per 100 m increase in elevation, faster than the elevational elapse rate of growing season temperature, implying a potential CO₂ regulation of T_opt in addition to temperature acclimation. T_opt increases by 0.66°C for each 1°C of rising mean annual temperature as a result of vegetation acclimation to climate change. However, at least at the decadal scale, there is no significant change in T_opt between 2000s and 2010s, suggesting that the T_opt climate acclimation may not keep up with the warming rate. Finally, future (2091-2100) warming could be close to and even surpass T_opt on the TP under different RCP scenarios without considering potential climate acclimation. Our analyses imply that the temperature tipping point when the impact of future warming shifts from positive to negative on the TP is greatly overestimated by current vegetation models. Future research needs to include varying thermal and CO₂ acclimation effects on T_opt across different time scales in vegetation models.

Climate-Driven Plant Response and Resilience on the Tibetan Plateau in Space and Time: A Review

Climate change variation on a small scale may alter the underlying processes determining a pattern operating at large scale and vice versa. Plant response to climate change on individual plant levels on a fine scale tends to change population structure, community composition and ecosystem processes and functioning. Therefore, we reviewed the literature on plant response and resilience to climate change in space and time at different scales on the Tibetan Plateau. We report that spatiotemporal variation in temperature and precipitation dynamics drives the vegetation and ecosystem function on the Tibetan Plateau (TP), following the water-energy dynamics hypothesis. Increasing temperature with respect to time increased the net primary productivity (NPP) on most parts of the Tibetan Plateau, but the productivity dynamics on some parts were constrained by 0.3 °C decade-1 rising temperature. Moreover, we report that accelerating studies on plant community assemblage and their contribution to ecosystem functioning may help to identify the community response and resilience to climate extremes. Furthermore, records on species losses help to build the sustainable management plan for the entire Tibetan Plateau. We recommend that incorporating long-term temporal data with multiple factor analyses will be helpful to formulate the appropriate measures for a healthy ecosystem on the Tibetan Plateau.