Divergent phenological and leaf gas exchange strategies of two competing tree species drive contrasting responses to drought at their altitudinal boundary

Process-Based Modeling of Phenology and Radial Growth in Pinus tabuliformis in Response to Climate Factors over a Cold and Semi-Arid Region

(1) Background: Climate change significantly impacts the phenology and dynamics of radial tree growth in alpine dryland forests. However, there remains a scarcity of reliable information on the physiological processes of tree growth and cambial phenology in response to long-term climate change in cold and semi-arid regions. (2) Methods: We employed the process-based Vaganov-Shashkin (VS) model to simulate the phenology and growth patterns of Chinese pine (Pinus tabuliformis) in the eastern Qilian Mountains, northeastern Tibetan Plateau. The model was informed by observed temperature and precipitation data to elucidate the relationships between climate factors and tree growth. (3) Results: The simulated tree-ring index closely aligned with the observed tree-ring chronology, validating the VS model's effectiveness in capturing the climatic influences on radial growth and cambial phenology of P. tabuliformis. The model outputs revealed that the average growing season spanned from mid-April to mid-October and experienced an extension post-1978 due to ongoing warming trends. However, it is important to note that an increase in the duration of the growing season did not necessarily result in a higher level of radial growth. (4) Conclusions: While the duration of the growing season was primarily determined by temperature, the growth rate was predominantly influenced by water conditions during the growing season, making it the most significant factor contributing to ring formation. Our study provides valuable insights into the potential mechanisms underlying tree growth responses to climate change in cold and semi-arid regions.

Microtopography mediates the climate-growth relationship and growth resilience to drought of Pinus tabulaeformis plantation in the hilly site

Understanding the factors affecting the growth of plantation forests can reduce the loss of economic and ecological values caused by plantation forest subhealth. Plantation forests are widely distributed in hilly areas with microtopographic features. Microtopography influences climatic factors associated with plant growth, during not only general time but also extreme events like droughts. However, little research has been conducted on the effects of microtopography on the plantation forest growth. In this paper, we selected Pinus tabulaeformis planted in a hilly site, and studied the effect of microtopography on the climate-growth relationship and drought response of a typical plantation in Northeast China using dendroecological methods. We found: 1) Between hill positions, temperature caused a climatic growth difference. Compared to the hilltop, the correlation of annual growth on the hillside with monthly temperature was more negative in July-August and less positive in January-April. 2) Between aspects, precipitation intensities caused a climatic growth difference. Compared to the sunny slope, the correlation of annual growth on the shady slope with monthly total precipitation below 10 mm/day was less positive (May-June) or more negative (March-April and July), while that with monthly total precipitation above 10 mm/day was more positive in most months.3) Drought response varied significantly based on hill position and aspect. There was no significant difference in resistance between hill positions, while recovery and resilience on the hilltop were greater than those on the hillside.Resistance, recovery, and resilience were all lower on the sunny slope than those on the shady slope. Overall, microtopography exists the effects on the growth of plantation forests, both in terms of climate-growth relationships in general climate and in response to drought when extreme events. Meanwhile, the climatic factors that caused the difference in growth of plantation forests between hill positions and aspects differed. The difference in growth between hill positions was caused by temperature, while that between aspects was caused by precipitation intensity. Drought response difference reflected the legacy effect of drought on plantation growth, which could lead to subsequent changes in climate-growth relationships. These findings demonstrate that strengthening the research of forest trees on microtopography is necessary for accurate carbon sink assessment and precise forest management.

Cutting tree rings into time slices: how intra-annual dynamics of wood formation help decipher the space-for-time conversion

Tree-ring anatomy, microdensity and isotope records provide valuable intra-annual information. However, extracting signals at that scale is challenged by the complexity of xylogenesis, where two major processes - cell enlargement and wall thickening - occur at different times and rates. We characterized the space-for-time association in the tree rings of three conifer species by examining the duration, overlapping, inter-tree synchronicity and interannual stability during cell enlargement and wall thickening across regular tree-ring sectors (portions of equal tangential width). The number of cells and cell differentiation rates determined the duration of sector formation, which augmented more rapidly throughout the ring for wall thickening than for enlargement. Increasing the number of sectors above c. 15 had a limited effect on improving time resolution because consecutive sector formation overlapped greatly in time, especially in narrow rings and during wall thickening. Increasing the number of sectors also resulted in lower synchronicity and stability of intermediate-sector enlargement, whereas all sectors showed high synchronicity and stability during wall thickening. Increasing the number of sectors had a stronger effect on enhancing time-series resolution for enlargement- than for wall-thickening-related traits, which would nevertheless produce more reliable intra-annual chronologies as a result of the more similar calendars across trees and years in wall thickening.

Holm oak death is accelerated but not sudden and expresses drought legacies

The increase in abiotic and biotic stress driven by global change threatens forest ecosystems and challenges understanding of mechanisms producing mortality. Phytophthora spp. like P. cinnamomi (PHYCI) are among the most lethal pathogens for many woody species including Quercus spp. Dynamics of biotic agents and their hosts are complex and influenced by climatic conditions. We analysed radial growth trends of dead and live adult Quercus ilex trees from agrosilvopastoral open woodlands under intense land-use. A pronounced warming trend since the 1980s has coincided in these woodlands with high oak mortality rates generally attributed to PHYCI. Yet, tree mortality and latency of the pathogen could be expressed at variable time spans, whereas, like in many other forests worldwide, tree death could also be explained by other factors like drought. PHYCI was isolated from roots of all dead oaks from one region. Trees were younger than generally believed and ages of dead trees ranged between 38 and 230 years. Growth of dead trees reached a tipping point in 1980 and 1990 coincident with two-year extraordinary droughts. These dates set the start of growth declines up to 30 years before tree death. Live trees did not exhibit any recent growth decline. Tree growth was highly sensitive to climatic variability associated with water stress and climate-growth relationships suggested phenological changes since the 1980s. Live and dead trees showed differences in their sensitivity to moisture availability and temperature. The sensitivity of growth to climate was partially related to site environmental conditions. Simulated gross and net primary productivity were higher in live sites with less atmospheric demand for water. Tree death was not sudden but a slow multiannual process as expressed by radial growth declines likely triggered by drought. Regardless of the causal agent or mechanism, the observed mortality affected trees exhibiting negative drought and land-use legacies.

Timeline of autumn phenology in temperate deciduous trees

Cessation of xylem formation or wood growth (CWG) and onset of foliar senescence (OFS) are key autumn phenological events in temperate deciduous trees. Their timing is fundamental for the development and survival of trees, ecosystem nutrient cycling and the seasonal exchange of matter and energy between the biosphere and atmosphere, and affects the impact and feedback of forests to global change. A large-scale experimental effort and improved observational methods have allowed us to compare the timing of CWG and OFS for different deciduous tree species in Western Europe, particularly in silver birch, a pioneer species, and European beech, a late-succession species, at stands of different latitudes, of different levels of site fertility, for 2 years with contrasting meteorological and drought conditions, i.e., the low moderately dry 2017 and the extremely dry 2018. Specifically, we tested whether foliar senescence started before, after or concurrently with CWG. Onset of foliar senescence and CWG occurred generally between late September and early November, with larger differences across species and sites for OFS. Foliar senescence started concurrently with CWG in most cases, except for the drier 2018 and, for beech, at the coldest site, where OFS occurred significantly later than CWG. The behavior of beech in Spain, the southern edge of its European distribution, was unclear, with no CWG, but very low wood growth at the time of OFS. Our study suggests that OFS is generally triggered by the same drivers of CWG or when wood growth decreases in late summer, indicating an overarching mechanism of sink limitation as a possible regulator of the timing of foliar senescence.

Water potential control of turgor-driven tracheid enlargement in Scots pine at its xeric distribution edge

The extent to which water availability can be used to predict the enlargement and final dimensions of xylem conduits remains an open issue. We reconstructed the time course of tracheid enlargement in Pinus sylvestris trees in central Spain by repeated measurements of tracheid diameter on microcores sampled weekly during a 2 yr period. We analyzed the role of water availability in these dynamics empirically through time-series correlation analysis and mechanistically by building a model that simulates daily tracheid enlargement rate and duration based on Lockhart's equation and water potential as the sole input. Tracheid enlargement followed a sigmoid-like time course, which varied intra- and interannually. Our empirical analysis showed that final tracheid diameter was strongly related to water availability during tracheid enlargement. The mechanistic model was calibrated and successfully validated (R²  = 0.92) against the observed tracheid enlargement time course. The model was also able to reproduce the seasonal variations of tracheid enlargement rate, duration and final diameter (R²  = 0.84-0.99). Our results support the hypothesis that tracheid enlargement and final dimensions can be modeled based on the direct effect of water potential on turgor-driven cell expansion. We argue that such a mechanism is consistent with other reported patterns of tracheid dimension variation.