Widespread decline in winds delayed autumn foliar senescence over high latitudes

The high northern latitudes (>50°) experienced a pronounced surface stilling (i.e., decline in winds) with climate change. As a drying factor, the influences of changes in winds on the date of autumn foliar senescence (DFS) remain largely unknown and are potentially important as a mechanism explaining the interannual variability of autumn phenology. Using 183,448 phenological observations at 2,405 sites, long-term site-scale water vapor and carbon dioxide flux measurements, and 34 y of satellite greenness data, here we show that the decline in winds is significantly associated with extended DFS and could have a relative importance comparable with temperature and precipitation effects in contributing to the DFS trends. We further demonstrate that decline in winds reduces evapotranspiration, which results in less soil water losses and consequently more favorable growth conditions in late autumn. In addition, declining winds also lead to less leaf abscission damage which could delay leaf senescence and to a decreased cooling effect and therefore less frost damage. Our results are potentially useful for carbon flux modeling because an improved algorithm based on these findings projected overall widespread earlier DFS than currently expected by the end of this century, contributing potentially to a positive feedback to climate.

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.

An experimental assessment of the impact of drought and fire on western larch injury, mortality and recovery

Climate change is increasing drought and fire activity in many fire-prone regions including the western USA and circumpolar boreal forest. These changes highlight the need for improved understanding of how multiple disturbances impact trees in these regions. Recent studies linking fire behaviour to plant ecophysiology have improved understanding of how fire affects tree function and mortality but have not investigated interactions between drought stress and fire. In this study, Larix occidentalis saplings were subjected to different levels of water stress followed by low-intensity surface fires in a controlled laboratory setting. Post-fire mortality, recovery and growth were monitored for up to 1 year post fire. Generally, increased pre-fire water stress resulted in decreased post-fire stem diameter (up to 5% lower) and height (up to 19% lower) growth. However, severely water-stressed saplings whose foliage had senesced before the fires had lower 1-year mortality (14%) and significantly greater post-fire bud densities than moderately stressed saplings that did not senesce (86% mortality). The mortality patterns suggest that water-stressed western larch saplings exposed to low-intensity wildfires, or prescribed fires conducted as part of forest management activities, may exhibit lower mortality rates if stress-induced foliar senescence has occurred.

Correlations among Attributes of Senescence and Antioxidative Status of Leaf Discs during Epiphyllous Bud Differentiation in Kalanchoe pinnata Lam. (Pers.)

Leaf detachment is a common signal that triggers both the differentiation of dormant epiphyllous buds as well as the onset of foliar senescence in Kalanchoe pinnata Lam. (Pers.). The present study looked for any probable correlations among selected attributes of foliar senescence, e.g. soluble proteins, chlorophylls a and b (Chla+b), and membrane stability index (MSI), and the antioxidative status, e.g. phenolics, ferric reducing ability in plasma equivalence (FRAPeq), and membrane protection index (MPI), during epiphyllous bud differentiation. The experimental system comprised 0.75-cm leaf discs, with or without a dormant epiphyllous bud, cultured in vitro and exposed for ten days to continuous light or dark. A steady depletion of soluble proteins and Chla+b, and lowering of MSI in the leaf discs were observed, the decline being relatively faster and of higher magnitude in discs exposed to dark rather than to light. The pigment loss in discs with differentiating epiphyllous buds was greater and faster than in those lacking buds, a somewhat reverse situation was observed in case of soluble proteins. Simultaneously, a time-dependent decrease in the level of phenolics was also observed. Their content was found to be lower in discs exposed to dark as compared to light, pointing to a relationship with a higher rate of senescence-related degradative processes in the dark. The change in the content of Chla+b was found to be significantly correlated with the variation in the level of phenolics. The average FRAPeq after ten days was one half that of the initial level, which could be correlated with the decreasing levels of phenolics (intra-correlation) and maximally correlated with variations in Chla+b and protein contents (inter-correlation). Aqueous alcohol foliar extracts significantly (p < 0.05) protected membranes against peroxidative stress, although the pattern was not found to be in line with that of the phenolics content or FRAPeq. The diminishing Chla+b content was found to be maximally correlated with alterations in the membrane protection.