Warming-induced phenological mismatch between trees and shrubs explains high-elevation forest expansion

Accelerated succession in Himalayan alpine treelines under climatic warming

Understanding how climate change influences succession is fundamental for predicting future forest composition. Warming is expected to accelerate species succession at their cold thermal ranges, such as alpine treelines. Here we examined how interactions and successional strategies of the early-successional birch (Betula utilis) and the late-successional fir (Abies spectabilis) affected treeline dynamics by combining plot data with an individual-based treeline model at treelines in the central Himalayas. Fir showed increasing recruitment and a higher upslope shift rate (0.11 ± 0.02 m yr-1) compared with birch (0.06 ± 0.03 m yr-1) over the past 200 years. Spatial analyses indicate strong interspecies competition when trees were young. Model outputs from various climatic scenarios indicate that fir will probably accelerate its upslope movement with warming, while birch recruitment will decline drastically, forming stable or even retreating treelines. Our findings point to accelerating successional dynamics with late-successional species rapidly outcompeting pioneer species, offering insight into future forest succession and its influences on ecosystem services.

Mycorrhizal status regulates plant phenological mismatch caused by warming

Mycorrhiza is an important functional feature of plants, which plays a vital role in regulating plant phenology in response to environmental changes. However, the effect of mycorrhiza on plant phenological asymmetry in response to climate changes is still rarely reported. Based on a global database of mycorrhizal statuses (obligately mycorrhizal, OM and facultatively mycorrhizal, FM) and phenology, we demonstrated that mycorrhizas reduce the phenological mismatches between above- and below-ground plant responses to climate warming under OM status. The mismatch of above- and below-ground growing season length of FM plants to warming was as high as 10.65 days, 9.1925 days and 12.36 days in total, herbaceous and woody plants, respectively. The mismatch of growing season length of OM plants was only 2.12 days, -0.61 days and 7.64 days among plant groups, which was much lower than that of FM plants. Correlation analysis indicated that OM plants stabilized plant phenology by regulating the relationship between the start of the growing season and the length of the growing season. Path analysis found that herbaceous plants and woody plants reduced phenological mismatches by stabilizing below-ground and above-ground phenology, respectively. In exploring the effects of mycorrhizal status on early- or late-season phenophases, we found that different mycorrhizal statuses affected the response of early- or late-season phenophase to warming. OM promoted the advance of early-season phenophase, and FM promoted the delay of late-season phenophase among different plant groups. In different regions, OM and FM promoted the advance of early-season phenophase in temperate and boreal regions, respectively. Our results indicate that mycorrhizal status mediates plant phenological response to warming, so the potential effects of mycorrhizal status should be considered when studying plant phenology changes.

Flowering in the Northern Hemisphere is delayed by frost after leaf-out

Late spring frosts, occurring after spring phenological events, pose a dire threat to tree growth and forest productivity. With climate warming, earlier spring phenological events have become increasingly common and led to plants experiencing more frequent and severe frost damage. However, the effect of late spring frosts after leaf-out on subsequent flowering phenology in woody species remains unknown. Utilizing 572,734 phenological records of 640 species at 5024 sites from four long-term and large-scale in situ phenological networks across the Northern Hemisphere, we show that late spring frosts following leaf-out significantly delay the onset of the subsequent flowering by approximately 6.0 days. Late-leafing species exhibit greater sensitivity to the frosts than early-leafing species, resulting in a longer delay of 2.5 days in flowering. Trees in warm regions and periods exhibit a more pronounced frost-induced flowering delay compared to those in cold regions and periods. A significant increase in the frequency of late spring frost occurrence is observed in recent decades. Our findings elucidate the intricate relationships among leaf-out, frost, and flowering but also emphasize that the sequential progression of phenological events, rather than individual phenological stages, should be considered when assessing the phenological responses to climate change.

Warming-driven increased synchrony of tree growth across the southernmost part of the Asian boreal forests

Climate change has profoundly affected the synchrony of tree growth at multiple scales, thereby altering the structure and function of forest ecosystems. The Asian boreal forests extend southward to the Greater Khingan Range in northeast China. Given the ecological importance and susceptibility to climate change, the impacts of warming on this marginal forest community have been extensively investigated. Nonetheless, how tree growth synchrony changes across this region remains less understood. Focusing on this knowledge gap, we compiled a contiguously-distributed tree-ring network, containing 18 sampling populations and 475 individual larch trees, to explore the changes in multiple-scale growth synchrony across this region. We found increasing growth synchrony at both the individual and population levels over the past decades. The increasing trend of the regional inter-population growth synchrony was well in line with the increasing temperature and PDSI. Furthermore, 11 of the 18 sampling populations showed significant increases in their intra-population growth synchrony. We further associated the sliding intra-population growth synchrony with local climates. Intra-population growth synchrony of 13 and 11 sampling populations were significantly positively correlated with local temperature, and negatively correlated with local PDSI, respectively, demonstrating the driving role of warming-induced drought on growth synchrony. The linear regression model quantifying this relationship suggested that an increase of 1 °C in annual mean temperature would drive the intra-population growth synchrony to increase by 0.047. As warming trends in the study area are projected to continue over this century, our study warns of the further consequences of the increasing growth synchrony may have on the functioning, resilience, and persistence of forests.

Density-dependent species interactions modulate alpine treeline shifts

Species interactions such as facilitation and competition play a crucial role in driving species range shifts. However, density dependence as a key feature of these processes has received little attention in both empirical and modelling studies. Herein, we used a novel, individual-based treeline model informed by rich in situ observations to quantify the contribution of density-dependent species interactions to alpine treeline dynamics, an iconic biome boundary recognized as an indicator of global warming. We found that competition and facilitation dominate in dense versus sparse vegetation scenarios respectively. The optimal balance between these two effects was identified at an intermediate vegetation thickness where the treeline elevation was the highest. Furthermore, treeline shift rates decreased sharply with vegetation thickness and the associated transition from positive to negative species interactions. We thus postulate that vegetation density must be considered when modelling species range dynamics to avoid inadequate predictions of its responses to climate warming.

Drought limits vegetation carbon sequestration by affecting photosynthetic capacity of semi-arid ecosystems on the Loess Plateau

Drought is the driver for ecosystem production in semi-arid areas. However, the response mechanism of ecosystem productivity to drought remains largely unknown. In particular, it is still unclear whether drought limits the production via photosynthetic capacity or phenological process. Herein, we assess the effects of maximum seasonal photosynthesis, growing season length, and climate on the annual gross primary productivity (GPP) in vegetation areas of the Loess Plateau using multi-source remote sensing and climate data from 2001 to 2021. We found that maximum seasonal photosynthesis rather than growing season length dominates annual GPP, with above 90 % of the study area showing significant and positive correlation. GPP and maximum seasonal photosynthesis were positively correlated with self-calibrating Palmer Drought Severity Index (scPDSI), standardized precipitation and evapotranspiration index (SPEI) in >95 % of the study area. Structural equation model demonstrated that both drought indices contributed to the annual GPP by promoting the maximum seasonal photosynthesis. Total annual precipitation had a positive and significant effect on two drought indices, whereas the effects of temperature and radiation were not significant. Evidence from wood formation data also confirmed that low precipitation inhibited long-term carbon sequestration by decreasing the maximum growth rate in forests. Our findings suggest that drought limits ecosystem carbon sequestration by inhibiting vegetation photosynthetic capacity rather than phenology, providing a support for assessing the future dynamics of the terrestrial carbon cycle and guiding landscape management in semi-arid ecosystems.

Global tree growth resilience to cold extremes following the Tambora volcanic eruption

Although the global climate is warming, external forcing driven by explosive volcanic eruptions may still cause abrupt cooling. The 1809 and 1815 Tambora eruptions caused lasting cold extremes worldwide, providing a unique lens that allows us to investigate the magnitude of global forest resilience to and recovery from volcanic cooling. Here, we show that growth resilience inferred from tree-ring data was severely impacted by cooling in high latitudes and elevations: the average tree growth decreased substantially (up to 31.8%), especially in larch forests, and regional-scale probabilities of severe growth reduction (below -2σ) increased up to 1390%. The influence of the eruptions extended longer (beyond the year 1824) in mid- than in high-latitudes, presumably due to the combined impacts of cold and drought stress. As Tambora-size eruptions statistically occur every 200-400 years, assessing their influences on ecosystems can help humankind mitigate adverse impacts on natural resources through improved management, especially in high latitude and elevation regions.