Frost controls spring phenology of juvenile Smith fir along elevational gradients on the southeastern Tibetan Plateau

Increased productivity of temperate vegetation in the preceding year drives early spring phenology in the subsequent year in northern China

Under global warming, rising temperature have advanced spring phenology in recent decades. However, the internal physiological mechanisms driving changes in spring phenology still remain poorly understood. Here, we investigated the effects of temperate vegetation gross primary productivity (GPP) during the preceding year on spring phenology of the subsequent year based on the start of growing season (SOS) extracted from NDVI datasets between 1982 and 2015. We found that the preceding year's GPP had an effect on the subsequent year's SOS, equivalent to 33 %-50 % of effect of the preseason's mean temperature. Specifically, in the temperate and semi-humid or humid conditions, the preceding year's GPP had a stronger effect on SOS than in boreal or semi-arid conditions. In addition, the SOS of the dwarf vegetation, with less transport pressure and higher carbon concentrations, was more sensitive to the preceding year's GPP than that of tall forests. We found the effects of the preceding year's GPP on SOS varied with space and vegetation types. Therefore, the physiological mechanism should be considered in future spring phenology model separately according to space and vegetation types, to improve the accuracy of future phenology and then global carbon sequestration predictions.

Detection and attribution of the start of the growing season changes in the Northern Hemisphere

Global climate change has led to significant changes in land surface phenology. At present, research on the factors influencing the start of the growing season (SOS) mainly focuses on single factor effects, such as temperature and precipitation, ignoring the combined action of multiple factors. The impact of multiple factors on the spatial and temporal patterns of the SOS in the Northern Hemisphere is not clear, and it is necessary to combine multiple factors to quantify the degrees of influence of different factors on the SOS. Based on the GIMMS3g NDVI dataset, CRU climate data and other factor data, we used geographic detector model, random forest regression model, multiple linear regression, partial correlation analysis and Sen + Mann-Kendall trend analysis to explore the variation of the SOS in the Northern Hemisphere to reveal the main driving factors and impact threshold of 17 influencing factors on the SOS. The results showed that (1) during the past 34 years (1982-2015), the SOS in Europe and Asia mainly showed an advancing trend, whereas the SOS in North America mainly showed a delaying trend. (2) The SOS was mainly controlled by frost frequency, temperature and humidity. Increasing frost frequency inhibited the advancement of the SOS, and increasing temperature and humidity promoted the advancement of the SOS. (3) There were thresholds for the influences of the driving factors on the SOS. Outside the threshold ranges, the response mechanism of the SOS to driving factors changed. The results are important for understanding the response of the SOS to global climate change.

Decoupling the response of vegetation dynamics to asymmetric warming over the Qinghai-Tibet plateau from 2001 to 2020

Global land surface air temperature data show that in the past 50 years, the rate of nighttime warming has been much faster than that of daytime, with the minimum daily temperature (Tmin) increasing about 40% faster than the maximum daily temperature (Tmax), resulting in a decreased diurnal temperature difference. The Qinghai-Tibet Plateau (QTP) is known as the "roof of the world", where temperatures have risen twice as fast as the global average warming rate in the last few decades. The factors affecting vegetation growth on the QTP are complex and still not fully understood to some extent. Previous studies paid less attention to the explanations of the complicated interactions and pathways between elements that influence vegetation growth, such as climate (especially asymmetric warming) and topography. In this study, we characterized the spatial and temporal trends of vegetation coverage and investigated the response of vegetation dynamics to asymmetric warming and topography in the QTP during 2001-2020 using trend analysis, partial correlation analysis, and partial least squares structural equation model (PLS-SEM) analysis. We found that from 2001 to 2020, the entire QTP demonstrated a greening trend in the growing season (April to October) at a rate of 0.0006/a (p < 0.05). The spatial distribution pattern of partial correlation between NDVI and Tmax differed from that of NDVI and Tmin. PLS-SEM results indicated that asymmetric warming (both Tmax and Tmin) had a consistent effect on vegetation development by directly promoting greening in the QTP, with NDVI values being more sensitive to Tmin, while topographic factors, especially elevation, mainly played an indirect role in influencing vegetation growth by affecting climate change. This study offers new insights into how vegetation responds to asymmetric warming and references for local ecological preservation.

Local adaptation shapes functional traits and resource allocation in black spruce

Climate change is rapidly altering weather patterns, resulting in shifts in climatic zones. The survival of trees in specific locations depends on their functional traits. Local populations exhibit trait adaptations that ensure their survival and accomplishment of growth and reproduction processes during the growing season. Studying these traits offers valuable insights into species responses to present and future environmental conditions, aiding the implementation of measures to ensure forest resilience and productivity. This study investigates the variability in functional traits among five black spruce (Picea mariana (Mill.) B.S.P.) provenances originating from a latitudinal gradient along the boreal forest, and planted in a common garden in Quebec, Canada. We examined differences in bud phenology, growth performance, lifetime first reproduction, and the impact of a late-frost event on tree growth and phenological adjustments. The findings revealed that trees from northern sites exhibit earlier budbreak, lower growth increments, and reach reproductive maturity earlier than those from southern sites. Late-frost damage affected growth performance, but no phenological adjustment was observed in the successive year. Local adaptation in the functional traits may lead to maladaptation of black spruce under future climate conditions or serve as a potent evolutionary force promoting rapid adaptation under changing environmental conditions.

Non-linear modelling reveals a predominant moisture limit on juniper growth across the southern Tibetan Plateau

BACKGROUND AND AIMS

Tree growth in plateau forests is critically limited by harsh climatic conditions. Many mathematical statistical methods have been used to identify the relationships between tree growth and climatic factors, but there is still uncertainty regarding the relative importance of these factors across different regions. We tested major climatic limits at 30 sites to provide insights into the main climatic limits for juniper trees (Juniperus tibetica Kom.) across the southern Tibetan Plateau.

METHODS

We analysed the linear and non-linear relationships between tree growth and climatic factors using Pearson correlation statistics and a process-based forward Vaganov-Shashkin-Lite (VS-Lite) model, respectively. These relationships were used to identify the strength of the influence of different climatic factors throughout the species' growing season and to identify the main climatic factors limiting tree growth.

KEY RESULTS

Growth of juniper trees began in April and ended in October in the study area. The radial growth of juniper trees was limited by soil moisture throughout the summer (June-August) of the current year at 24 sampling sites and was limited by temperature at the other six sites on the southern Tibetan Plateau.

CONCLUSIONS

Soil moisture limited juniper growth at the majority of sites. Temperature in the current summer limited the growth of juniper trees at a few sampling sites in the western part of the study area. Local climate conditions may contribute to different limiting factors in the growth response of trees on the southern Tibetan Plateau. These findings may contribute to our understanding of divergent forest dynamics and to sustainable forest management under future climate scenarios.

Spatial Difference between Temperature and Snowfall Driven Spring Phenology of Alpine Grassland Land Surface Based on Process-Based Modeling on the Qinghai–Tibet Plateau

As a sensitive indicator for climate change, the spring phenology of alpine grassland on the Qinghai–Tibet Plateau (QTP) has received extensive concern over past decade. It has been demonstrated that temperature and precipitation/snowfall play an important role in driving the green-up in alpine grassland. However, the spatial differences in the temperature and snowfall driven mechanism of alpine grassland green-up onset are still not clear. This manuscript establishes a set of process-based models to investigate the climate variables driving spring phenology and their spatial differences. Specifically, using 500 m three-day composite MODIS NDVI datasets from 2000 to 2015, we first estimated the land surface green-up onset (LSGO) of alpine grassland in the QTP. Further, combining with daily air temperature and precipitation datasets from 2000 to 2015, we built up process-based models for LSGO in 86 meteorological stations in the QTP. The optimum models of the stations separating climate drivers spatially suggest that LSGO in grassland is: (1) controlled by temperature in the north, west and south of the QTP, where the precipitation during late winter and spring is less than 20 mm; (2) driven by the combination of temperature and precipitation in the middle, east and southwest regions with higher precipitation and (3) more likely controlled by both temperature and precipitation in snowfall dominant regions, since the snow-melting process has negative effects on the air temperature. The result dictates that snowfall and rainfall should be concerned separately in the improvement of the spring phenology model of the alpine grassland ecosystem.

Distinct Responses of Leaf Traits to Environment and Phylogeny Between Herbaceous and Woody Angiosperm Species in China

Leaf traits play key roles in plant resource acquisition and ecosystem processes; however, whether the effects of environment and phylogeny on leaf traits differ between herbaceous and woody species remains unclear. To address this, in this study, we collected data for five key leaf traits from 1,819 angiosperm species across 530 sites in China. The leaf traits included specific leaf area, leaf dry matter content, leaf area, leaf N concentration, and leaf P concentration, all of which are closely related to trade-offs between resource uptake and leaf construction. We quantified the relative contributions of environment variables and phylogeny to leaf trait variation for all species, as well as for herbaceous and woody species separately. We found that environmental factors explained most of the variation (44.4-65.5%) in leaf traits (compared with 3.9-23.3% for phylogeny). Climate variability and seasonality variables, in particular, mean temperature of the warmest and coldest seasons of a year (MTWM/MTWQ and MTCM/MTCQ) and mean precipitation in the wettest and driest seasons of a year (MPWM/MPWQ and MPDM/MPDQ), were more important drivers of leaf trait variation than mean annual temperature (MAT) and mean annual precipitation (MAP). Furthermore, the responses of leaf traits to environment variables and phylogeny differed between herbaceous and woody species. Our study demonstrated the different effects of environment variables and phylogeny on leaf traits among different plant growth forms, which is expected to advance the understanding of plant adaptive strategies and trait evolution under different environmental conditions.

Nighttime warming alleviates the incidence of juniper forest growth decline on the Tibetan Plateau

Recent warming over the Tibetan Plateau (TP) is approximately twice the global-mean surface temperature increase and poses a threat to the healthy growth of forests. Although many studies have focused on whether recent climate warming has caused forest growth decline on the TP, it remains unclear how asymmetric warming, that is faster increasing nighttime temperature than daytime, impacts forest growth decline. We explored this question by using a ring-width index series from 1489 juniper trees (Juniperus prezwalskii and J. tibetica) at 50 sites on the TP. We calculated the percentage of trees with growth decline (PTD) to reconstruct historical forest growth decline and employed a piecewise structural equation meta-model (pSEM) and linear mixed model (LMM) to explore influencing factors. We found that the PTD has decreased since the late 19th century, with an abrupt decreasing trend since the 1980s. Results of the pSEM show that winter minimum temperature has a stronger indirect negative effect on the variation in PTD (β = -0.24, p < 0.05) compared to that of the weak indirect positive effect of summer maximum temperature (β = 0.16, p < 0.05). The results of LMM show that the variation in PTD is directly negatively (p < 0.001) affected by both winter minimum temperature and summer total precipitation, but the former has a greater independent contribution than the latter (with 17.7% vs 2.5% of variances independently explained, respectively). These results suggest that increased winter minimum temperature substantially mitigates the growth decline in juniper forests on the TP. As the minimum temperature generally occurs at night, we conclude that the asymmetric increase in nighttime temperature has decreased the incidence of juniper forest growth decline on the TP under climate warming. This alleviating effect of nighttime warming is likely due to reduced low-temperature constraints and reduced damage to tree growth.

Growing-season frost is a better predictor of tree growth than mean annual temperature in boreal mixedwood forest plantations

Increase in frost damage to trees due to earlier spring dehardening could outweigh the expected increase in forest productivity caused by climate warming. We quantified the impact of growing-season frosts on the performance of three spruce species (white, black, and Norway spruce) and various seed sources with different frost tolerance in two plantations, established on both sides of the eastern Canadian boreal-temperate forest ecotone. The objectives of this study were to determine (a) if spruce species and seed sources planted in sites far from their natural provenance would be less adapted to local site conditions, leading to increased frost damage and reduced height growth; (b) at which height above the ground growing-season frosts ceased to damage apical meristems; and (c) if height growth was best predicted by extreme climatic events (growing-season frosts) or by mean annual or summer temperature. At each site and for all spruce species and seed sources, we cross-sectioned spruce trees at different heights above the ground. Tree rings were cross-dated and screened for frost rings, which were then given a severity score based on cellular damage. Frost severity reduced height growth of all spruce species and provenances at both sites. Height growth of the non-native Norway spruce was the most reduced by frost severity and was the smallest species at both sites. Frost caused the highest growth reduction in white spruce at the boreal mixedwood site and had the least effect on black spruce at both sites. For all spruce species, height growth was affected up to 2 m above the ground. Model selection based on corrected Akaike's information criteria (AICc) identified that minimum temperature in May was by far the best climate variable predicting tree growth (AICc weight = 1), highlighting the importance of considering extreme climatic events, which are likely to increase in the future.

Explaining the exceptional 4270 m high elevation limit of an evergreen oak in the south-eastern Himalayas

Unlike the well-understood alpine treeline, the upper range limits of tree taxa that do not reach the alpine treeline are largely unexplained. In this study, we explored the causes of the exceptionally high elevation (4270 m) occurrence of broad-leaved evergreen oaks (Quercus pannosa) in the south-eastern Himalayas. We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limit of this oak species. Linked to leaf phenology, we analyzed freezing resistance and assessed minimum crown temperature for the past 65 years. We also examined potential carbon limitation at the range limit of this species. Last season buds and leaves operated at a safety margin of 5.5 and 11 K in mid-winter. Once fully dehardened early in July, last season foliage is damaged at -5.9 and new foliage at -4.6 °C. Bud break is timed for late June to early July when low temperature extremes historically were never below -3.0 °C. The monsoon regime ensures a long remaining season (149 days), thus compensating for the late onset of shoot growth. Compared with a site at 3450 m, specific leaf area is reduced, foliar non-structural carbohydrate concentrations are similar and the δ13C signal is higher, jointly suggesting that carbon limitation is unlikely at the range limit of this species. We also show that these oaks enter the growing season with fully intact (not embolized) xylem. We conclude that the interaction between phenology and freezing tolerance results in safe flushing, while still facilitating shoot maturation before winter. These factors jointly determine the upper range limit of this oak species. Our study illuminates an exceptional case of broad-leaved evergreen tree performance near the treeline, and by exploring a suite of traits, we can underpin the central role of flushing phenology in such a stressful environment.

The Interactive Effects of Chilling, Photoperiod, and Forcing Temperature on Flowering Phenology of Temperate Woody Plants

The effects of winter chilling, spring forcing temperature, and photoperiod on spring phenology are well known for many European and North American species, but the environmental cues that regulate the spring phenology of East Asian species have not yet been thoroughly investigated. Here, we conducted a growth chamber experiment to test the effects of chilling (controlled by different lengths of exposure to natural chilling conditions), forcing temperature (12, 15, or 18°C) and photoperiod (14 or 10 h) on first flowering date (FFD) of six woody species (three shrubs and three trees) native to East Asia. The three-way analysis of variance (ANOVA) separately for each species showed that the effects of chilling and forcing temperature were significant for almost all species (P < 0.05). Averaged over all chilling and photoperiod treatments, the number of days until FFD decreased by 2.3-36.1 days when the forcing temperature increased by 3°C. More chilling days reduced the time to FFD by 0.7-26 days, when averaged over forcing and photoperiod treatments. A longer photoperiod could advance the FFD by 1.0-5.6 days, on average, but its effect was only significant for two species (including one tree and one shrub). The effects of forcing temperature and photoperiod interacted with chilling for half of the studied species, being stronger in the low chilling than high chilling treatment. These results could be explained by the theory and model of growing degree-days (GDD). Increased exposure to chilling coupled to a longer photoperiod reduced the GDD requirement for FFD, especially when plants grew under low chilling conditions. However, shrubs (except Viburnum dilatatum) had lower chilling and heat requirements than trees, suggesting that, by leafing out sooner, they engage in a more opportunistic life strategy to maximize their growing season, especially before canopy closure from trees' foliage. Our results confirmed the varying effects of these three cues on the flowering phenology of woody species native to East Asia. In future climate change scenarios, spring warming is likely to advance the spring phenology of those woody species, although the reduced chilling and shorter photoperiod may partly offset this spring warming effect.

The onset of xylogenesis in Smith fir is not related to outer bark thickness

PREMISE

The resumption of stem growth varies across the ontogenetic development of trees. Compared with younger trees, older ones have thicker outer bark with a temperature-insulating effect that could potentially prevent the stem from warming in the spring. However, the question of whether xylogenesis in old trees is influenced by the thick bark still remains unresolved.

METHODS

We investigated the onset of xylogenesis across the ontogenetic development of Smith fir (Abies georgei var. smithii) trees in the Sygera Mountains, southeastern Tibetan Plateau. The outer bark of older trees was also removed. Xylogenesis was monitored in microcores we collected every 3 days during May and June in 2017.

RESULTS

Xylogenesis began in late May in young (<50 yr) and mature (50-100 yr) trees, 1 week earlier than in adult (>100-150 yr) and old (>150-200 yr) trees. Older (>200 yr) trees had the latest onset of xylogenesis, 2 weeks after young trees. The resumption of xylogenesis was similar between the control and bark-removed trees.

CONCLUSIONS

Growth resumption was delayed in older and bigger trees. Outer bark did not affect the onset of xylogenesis, which indicated that the delayed resumption of growth during the lifespan of trees could be more related to endogenous factors than to an insulating effect of the thick bark of older individuals.