Summary Information on the onset of the leaf senescence in temperate deciduous trees and comparisons on its assessment methods are limited, hampering our understanding of autumn dynamics.We compare five field proxies, five remote sensing proxies and two data analysis approaches to assess leaf senescence onset at one main beech stand, two stands of oak and birch and three ancillary stands of the same species in Belgium during 2017 and 2018.Across species and sites, onset of leaf senescence was not significantly different for the field proxies based on chlorophyll leaf content and canopy coloration, except for an advanced canopy coloration during the extremely dry and warm 2018. Two remote sensing indices provided results fully consistent with the field data. A significant lag emerged between leaf senescence onset and leaf fall, and when a threshold of 50% change in the seasonal variable under study (e.g. chlorophyll content) was used to derive the leaf senescence onset.Our results provide unprecedented information on the quality and applicability of different proxies to assess leaf senescence onset in temperate deciduous trees. In addition, a sound base is offered to select the most suited methods for the different disciplines that need this type of data.

Highlights Budburst variability correlated to previous autumn phenology for oak and beech. Budburst variability correlated to tree characteristics for birch. 66% of the inter-individual variability of budburst explained.

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.

Abstract. Severe droughts are expected to become more frequent and persistent. However, their effect on autumn leaf senescence, a key process for deciduous trees and ecosystem functioning, is currently unclear. We hypothesized that (I) severe drought advances the onset of autumn leaf senescence in temperate deciduous trees and (II) tree species show different dynamics of autumn leaf senescence under drought. We tested these hypotheses using a manipulative experiment on beech saplings and 3 years of monitoring mature beech, birch and oak trees in Belgium. The autumn leaf senescence was derived from the seasonal pattern of the chlorophyll content index and the loss of canopy greenness using generalized additive models and piecewise linear regressions. Drought and associated heat stress and increased atmospheric aridity did not affect the onset of autumn leaf senescence in both saplings and mature trees, even if the saplings showed a high mortality and the mature trees an advanced loss of canopy greenness. We did not observe major differences among species. To synthesize, the timing of autumn leaf senescence appears conservative across years and species and even independent of drought, heat and increased atmospheric aridity. Therefore, to study autumn senescence and avoid confusion among studies, seasonal chlorophyll dynamics and loss of canopy greenness should be considered separately.

We explored the timing of spring xylogenesis and its potential drivers in homogeneous mature forest stands in a temperate European region. Three species with contrasting leaf development dynamics and wood anatomy were studied: European beech, silver birch and pedunculate oak. Detailed phenological observations of xylogenesis and leaf phenology were performed from summer 2017 till spring 2018. Cambium reactivation (CR) occurred before the buds of oak and birch were swollen, whereas these two phenological phases were concurrent for beech. On the other hand, initial earlywood vessels were fully differentiated (FDIEV) after leaf unfolding for all three species. Timing of CR was correlated to average ring-width of the last 10 years (2017–2008), tree diameter, and, partially, with tree age. In addition, the timing of FDIEV was correlated to tree age and previous’ year autumn phenology i.e., timing of wood growth cessation and onset of leaf senescence. Multivariate models could explain up to 68% of the variability of CR and 55% of the variability of FDIEV. In addition to the “species” factor, the variability could be explained by ca. 30% by tree characteristics and previous’ years autumn phenology for both CR and FDIEV. These findings are important to better identify which factors (other than environment) can be driving the onset of the growing season and highlight the influence of tree growth characteristics and previous’ year phenology on spring wood phenology, wood formation and, potentially, forest production.

Intensively managed shade coffee plantations are expanding in SW Ethiopia, at the cost of the more natural coffee agroforestry systems. Here, we investigated consequences for the potential pollinator community of Arabica coffee (Coffea arabica L.) in its natural range. We surveyed coffee flower visitors at six different sites in the Jimma region in SW Ethiopia, and compared species richness and abundance between semi-natural coffee forests and shaded coffee plantations. Overall, we found six bee (Hymenoptera: Anthophila) and twenty fly species (Diptera: Brachycera) visiting C. arabica flowers. Species richness and overall abundance of flower visitors was significantly higher in the seminatural forests compared to the plantations. A significantly higher abundance of non-Apis bees and hoverflies (Syrphidae) visiting C. arabica flowers was observed in the semi-natural forest plots, but numbers for other Diptera and honeybees (Apis mellifera L.) did not differ significantly between the agroforestry systems. Our results show an impoverishment of the coffee flower visiting insect community in response to agricultural intensification. This suggests a functional shift of the coffee pollinator community and, hence, may influence the stability of the provided pollination ecosystem services and coffee yield in the long term. We did, however, not quantify pollination services in this study.

Research on wood phenology has mainly focused on reactivation of the cambium in spring. In this study we investigated if summer drought advances cessation of wood formation and if it has any influence on wood structure in late successional forest trees of the temperate zone. The end of xylogenesis was monitored between August and November in stands of European beech and pedunculate oak in Belgium for two consecutive years, 2017 and 2018, with the latter year having experienced an exceptional summer drought. Wood formation in oak was affected by the drought, with oak trees ceasing cambial activity and wood maturation about three weeks earlier in 2018 compared to 2017. Beech ceased wood formation before oak, but its wood phenology did not differ between years. Furthermore, between the two years, no significant difference was found in ring width, percentage of mature fibers in the late season, vessel size and density. In 2018, beech did show thinner fiber walls, while oak showed thicker walls. In this paper, we showed that summer drought can have an important impact on late season wood phenology xylem development. This will help to better understand forest ecosystems and improve forest models.

Abstract. Severe droughts are expected to become more frequent and persistent. However, their effect on autumn leaf senescence, a key process for deciduous trees and ecosystem functioning, is currently unclear. We hypothesized that (I) severe drought advances the onset of autumn leaf senescence in temperate deciduous trees and that (II) tree species show different dynamics of autumn leaf senescence under drought. We tested these hypotheses using a manipulative experiment on beech saplings and three years of monitoring mature beech, birch and oak trees in Belgium. The autumn leaf senescence was derived from the seasonal pattern of the chlorophyll content index and the loss of canopy greenness using generalized additive models and piece-wise linear regressions. Drought did not affect the onset of autumn leaf senescence in both saplings and mature trees, even if the saplings showed a high mortality and the mature trees a high leaf mortality (due to accelerated leaf senescence and early leaf abscission). We did not observe major differences among species. Synthesis: The timing of autumn leaf senescence appears conservative across years and species, and even independent on drought stress. Therefore, to study autumn senescence, seasonal chlorophyll dynamics and loss of canopy greenness should be considered separately.