Drought effects on carbon cycling The response of forest ecosystems to drought is increasingly important in the context of a warming climate. Anderegg et al. studied a tree-ring database of 1338 forest sites from around the globe. They found that forests exhibit a drought “legacy effect” with 3 to 4 years' reduced growth following drought. During this postdrought delay, forests will be less able to act as a sink for carbon. Incorporating forest legacy effects into Earth system models will provide more accurate predictions of the effects of drought on the global carbon cycle. Science , this issue p. 528

Wood formation consumes around 15% of the anthropogenic CO2 emissions per year and plays a critical role in long-term sequestration of carbon on Earth. However, the exogenous factors driving wood formation onset and the underlying cellular mechanisms are still poorly understood and quantified, and this hampers an effective assessment of terrestrial forest productivity and carbon budget under global warming. Here, we used an extensive collection of unique datasets of weekly xylem tissue formation (wood formation) from 21 coniferous species across the Northern Hemisphere (latitudes 23 to 67°N) to present a quantitative demonstration that the onset of wood formation in Northern Hemisphere conifers is primarily driven by photoperiod and mean annual temperature (MAT), and only secondarily by spring forcing, winter chilling, and moisture availability. Photoperiod interacts with MAT and plays the dominant role in regulating the onset of secondary meristem growth, contrary to its as-yet-unquantified role in affecting the springtime phenology of primary meristems. The unique relationships between exogenous factors and wood formation could help to predict how forest ecosystems respond and adapt to climate warming and could provide a better understanding of the feedback occurring between vegetation and climate that is mediated by phenology. Our study quantifies the role of major environmental drivers for incorporation into state-of-the-art Earth system models (ESMs), thereby providing an improved assessment of long-term and high-resolution observations of biogeochemical cycles across terrestrial biomes.

Future seasonal dynamics of wood formation in hyperarid environments are still unclear. Although temperature-driven extension of the growing season and increased forest productivity are expected for boreal and temperate biomes under global warming, a similar trend remains questionable in water-limited regions. We monitored cambial activity in a montane stand of ponderosa pine (Pinus ponderosa) from the Mojave Desert for 2 consecutive years (2015-2016) showing opposite-sign anomalies between warm- and cold-season precipitation. After the wet winter/spring of 2016, xylogenesis started 2 months earlier compared to 2015, characterized by abundant monsoonal (July-August) rainfall and hyperarid spring. Tree size did not influence the onset and ending of wood formation, highlighting a predominant climatic control over xylem phenological processes. Moisture conditions in the previous month, in particular soil water content and dew point, were the main drivers of cambial phenology. Latewood formation started roughly at the same time in both years; however, monsoonal precipitation triggered the formation of more false rings and density fluctuations in 2015. Because of uncertainties in future precipitation patterns simulated by global change models for the Southwestern United States, the dependency of P. ponderosa on seasonal moisture implies a greater conservation challenge than for species that respond mostly to temperature conditions.

Summary Old‐growth studies commonly emphasize structural and age conditions, selecting proxy indicators of long‐term ecological processes. Transition dynamics from mature to old‐growth status reveal how natural legacies are progressively accumulated in forests after major disturbances, including human ones. In late‐successional, multi‐aged forests, the chronosequential ranking of developmental stages is a difficult task, as stand age provides little information, and time since last stand‐replacing disturbance cannot be easily determined. Canopy age features, disturbance/suppression history and growth trajectories were reconstructed from ring‐width series of canopy trees in a network of 19 old‐growth and managed European beech forests in the eastern Alps and central Apennines. A set of tree‐ring metrics able to describe the intensity and time distribution of biological and ecological processes (e.g. understorey suppression, canopy accession age) were used to describe the advancement of old‐growth status and compared to established metrics of forest structure. Tree‐ring metrics were site dependent, as biogeoclimate affects turnover rates and constrains the onset and recovery rate of old‐growth attributes. Under the same environmental conditions (high‐mountain, limestone‐bedrock beech forests), values of the best indicators (number/duration of growth suppression phases; synchronicity of first release; maximum and range of canopy tree age; canopy accession age of the slowest‐growing trees) increased monotonically (two to five times) from managed to secondary and primary old‐growth forests. Trees in well‐conserved primary old‐growth forests experienced several and long suppressions, showing the highest complexity in recruitment history, canopy accession and growth trajectories. The best tree‐ring metrics, condensed in a Naturalness Score to provide a synthetic functional ranking of forests, varied coherently with structural complexity, which represented stand dynamics more closely than biomass‐related metrics. Synthesis and applications. We propose a synthetic ranking of forest functional naturalness based on the ecological processes experienced by trees. This ranking helps to overcome the limitations associated with the use of arbitrary size‐ or age‐related thresholds of old‐growth status and provides a functional approach to establish chronosequences in ecological studies. The quantitative description of complex processes underpinning the unique biological and ecological features (e.g. extreme tree longevity) found in primary old‐growth forests enhances their irreplaceable value in nature conservation. The proposed framework of tree‐ring indicators describes functional traits tightly related to forest naturalness and may thus become a tool to identify and protect old‐growth forests, benchmark the impact of silvicultural practices, prescribe targets or evaluate the effectiveness of restoration programmes.

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Key message Anatomical features of Pinus flexilis under warmer and drier conditions along an altitudinal transect revealed a shorter growing season and shifts in the timing of wood formation. Abstract Future climate change driven by greenhouse warming is expected to increase both frequency and severity of drought events and heat waves. Possible consequences for forest ecosystems include changes in foundation species and extended die-off phenomena. We investigated tree growth under the set of biotic and abiotic conditions, and their interactions, that are expected in a drier and warmer world using mountain observatories designed to capture elevation gradients in the Great Basin of North America. Stem cambial activity, wood anatomy, and radial growth of limber pine (Pinus flexilis) were examined at two different elevations using automated dendrometers and repeated histological microcores in 2013-2014. Mean annual temperature was 3.7°cooler at the higher site, which received 170 mm year -1 of precipitation more than the lower site. Mean air temperature thresholds for xylogenesis computed using logistic regression were 7.7 and 12.0°C at the higher and lower site, respectively. No differences in the onset date of cambial activity were found under such naturally contrasted conditions, with the global change analog provided by the lower site. Growing season was shortened by increasing drought stress at the lower site, thereby reducing xylem production. Stem expansion was only detectable by automated dendrometers at the higher site. Using elevation to simulate climatic changes and their realized ecosystem feedbacks, it was possible to express tree responses in terms of xylem phenology and anatomical adaptations.

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In the framework of the Project of National Interest (PRIN) "Climate change and forests -Dendroecological and ecophysiological responses