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Socio‐ecological resilience is the capacity of a system to adapt to changing ecological and social disturbances. Its assessment is extremely important to integrate long‐term management of ecological and social features of natural ecosystems. This is especially true for Sacred Natural Sites, such as sacred forests and groves, where it can reveal the influence of social processes in ecosystem recovery or degradation. Using tree ages determined through dendrochronology and tree population size‐class distributions collected in five sacred forests in Epirus (NW Greece), we explore spatial and temporal dynamics of resilience in a socio‐ecological system, identifying which cultural and social elements characterize resilience in space and time. Our main results show that over past centuries sacred forests in Epirus underwent periods of varying tree establishment rate, depending on the intensity of human activities and historical disturbance events. We also identified strong evidence of the role of the social component (i.e. the church and associated cultural praxis) in determining the spatial extent of the forests' current recovery phase, and thus the overall resilience of the system. Policy implications. Appreciation of the ways sacred forests' ecological resilience is linked to changing socio‐cultural praxis over both temporal and spatial scales is crucial for guiding conservation and restoration strategies. We argue that greater attention should be paid to the role of the social component of socio‐ecological systems and specifically for sacred natural sites that provide both a nucleus of established forest habitat and the conditions necessary for forest recovery and restoration. Read the free Plain Language Summary for this article on the Journal blog. Περίληψη Η κοινωνικο‐οικολογική ανθεκτικότητα είναι η ικανότητα ενός συστήματος να προσαρμόζεται σε μεταβαλλόμενες οικολογικές και κοινωνικές διαταραχές. Η αξιολόγησή της είναι εξαιρετικά σημαντική καθώς ενσωματώνει τη μακροπρόθεσμη διαχείριση οικολογικών και κοινωνικών χαρακτηριστικών των φυσικών οικοσυστημάτων. Αυτό ισχύει ιδιαίτερα για τους Ιερούς Φυσικούς Τόπους, όπως τα ιερά δάση και άλση, όπου μπορεί να αποκαλυφθεί ευκολότερα η επίδραση των κοινωνικών διαδικασιών στην ανάκαμψη ή την υποβάθμιση των συγκεκριμένων οικοσυστημάτων. Χρησιμοποιώντας τις ηλικίες των δέντρων που προσδιορίστηκαν μέσω της δενδροχρονολόγησης και κατανομές κλάσεων του μεγέθους των πληθυσμών τους που συλλέχθηκαν σε πέντε ιερά δάση στην Ήπειρο (ΒΔ Ελλάδα), διερευνούμε τη χωρική και χρονική δυναμική της ανθεκτικότητας σε ένα ιδιαίτερο κοινωνικο‐οικολογικό σύστημα, προσδιορίζοντας ποια πολιτιστικά και κοινωνικά στοιχεία επηρεάζουν την ανθεκτικότητά τους στον χώρο και στον χρόνο. Τα κύρια αποτελέσματά μας δείχνουν ότι κατά τη διάρκεια των τελευταίων αιώνων, τα ιερά δάση στην Ήπειρο πέρασαν περιόδους διαφορετικού ρυθμού εγκατάστασης δέντρων, ανάλογα με την ένταση της ανθρώπινης δραστηριότητας τοπικά, καθώς και ιστορικών συμβάντων διαταραχής αλλά και εγκατάλειψης. Εντοπίσαμε επίσης ισχυρές ενδείξεις για τον ρόλο κοινωνικών παραγόντων (όπως π.χ. της Εκκλησίας, και σχετικών πολιτιστικών πρακτικών) στον καθορισμό της χωρικής έκτασης και της σημερινής φάσης ανάκαμψης του δάσους, και συνεπώς της συνολικής ανθεκτικότητας του συστήματος. Δυνητικές εφαρμογές για τη δασική διαχείριση. Η εκτίμηση των τρόπων σύνδεσης της οικολογικής ανθεκτικότητας των ιερών δασών στις μεταβαλλόμενες κοινωνικο‐πολιτιστικές πρακτικές, τόσο σε χρονική όσο και σε χωρική κλίμακα, είναι ζωτικής σημασίας για τη χάραξη στρατηγικών διατήρησης και αποκατάστασης. Υποστηρίζουμε ότι πρέπει να δοθεί μεγαλύτερη προσοχή στον ρόλο της κοινωνικής συνιστώσας των κοινωνικο‐οικολογικών συστημάτων, ιδιαίτερα για συστήματα όπως οι Ιεροί Φυσικοί Τόποι, που μπορούν να λειτουργήσουν ως πυρήνες εξάπλωσης δασικών ενδιαιτημάτων, αλλά και να προσφέρουν τις απαραίτητες συνθήκες για την ανάκαμψη και αποκατάσταση δασών. Read the free Plain Language Summary for this article on the Journal blog.

Ecological communities exhibit pervasive patterns and interrelationships between size, abundance, and the availability of resources. We use scaling ideas to develop a unified, model-independent framework for understanding the distribution of tree sizes, their energy use, and spatial distribution in tropical forests. We demonstrate that the scaling of the tree crown at the individual level drives the forest structure when resources are fully used. Our predictions match perfectly with the scaling behavior of an exactly solvable self-similar model of a forest and are in good accord with empirical data. The range, over which pure power law behavior is observed, depends on the available amount of resources. The scaling framework can be used for assessing the effects of natural and anthropogenic disturbances on ecosystem structure and functionality.

• Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die-back in eucalypt forests. The role of hydraulic failure and tree size on canopy die-back in three eucalypt tree species during this drought was examined. • Wemeasured pre-dawn and midday leaf water potential (Ψleaf), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought-induced xylem embolism. Tree size and tree health was also surveyed. • Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78–100%). This is in contrast to trees with partial canopy die-back (30–70% canopy die-back: 72–78% native embolism), or relatively healthy trees (little evidence of canopy die-back: 25–31% native embolism). Midday Ψleaf was significantly more negative in trees exhibiting partial canopy die-back (−2.7 to −6.3 MPa), compared with relatively healthy trees (−2.1 to −4.5 MPa). In two of the species the majority of individuals showing complete canopy die-back were in the small size classes. • Our results indicate that hydraulic failure is strongly associated with canopy die-back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk.

The traditional paradigm is that plant growth at high latitudes is generally nitrogen (N) limited, whereas phosphorus (P) limitation occurs at low latitudes. However, this latitudinal pattern of nutrient limitation is not empirically tested and the underlying mechanisms are far from clear. Here we performed a coordinated experiment of N and/or P addition at three forest sites in China, a subtropical forest, a warm‐temperate forest and a cold‐temperate forest. By measuring relative growth rate (RGR) and leaf nutrient traits among different tree size groups, we assessed how they vary with nutrient addition and tree sizes and uncovered the likely mechanisms underlying these observed responses. Our results revealed that P addition enhanced the RGR of small trees (DBH < 15 cm) by 41% in subtropical forest and 114% in warm‐temperate forest, but reduced it by 57% in cold‐temperate forest. Moreover, small tree RGR increased linearly with soil available P at subtropical and warm‐temperate sites, while it followed a quadratic relationship with soil available N:P ratio at a cold‐temperate site. N addition only affected small tree RGR at the cold‐temperate site. In contrast, the RGR of large trees (DBH > 15 cm) was not impacted by any nutrient addition treatment or soil nutrient variations at any site. Leaf P concentration and resorption efficiency in both small and large trees mostly showed a linear response to soil available P at subtropical and warm‐temperate sites, while leaf N:P ratio in small trees elevated linearly with soil available N:P ratio at a cold‐temperate site. Overall, this study presents robust experimental evidence that growth in small trees, not large trees, is primarily limited by P in subtropical and warm‐temperate forests, but is co‐limited by N and P in cold‐temperate forests. This size‐dependent nutrient limitation highlights the importance of considering tree size classes when assessing nutrient limitation in forest. A plain language summary is available for this article. Plain Language Summary

Climate-induced tree mortality became a global phenomenon during the last century and it is expected to increase in many regions in the future along with a further increase in the frequency of drought and heat events. However, tree mortality at the ecosystem level remains challenging to quantify since long-term, tree-individual, reliable observations are scarce. Here, we present a unique data set of monitoring records from 276 permanent plots located in 95 forest stands across Switzerland, which include five major European tree species (Norway spruce, Scots pine, silver fir, European beech, and sessile and common oak) and cover a time span of over one century (1898-2013), with inventory periods of 5-10 years. The long-term average annual mortality rate of the investigated forest stands was 1.5%. In general, species-specific annual mortality rates did not consistently increase over the last decades, except for Scots pine forests at lower altitudes, which exhibited a clear increase of mortality since the 1960s. Temporal trends of tree mortality varied also depending on diameter at breast height (DBH), with large trees generally experiencing an increase in mortality, while mortality of small trees tended to decrease. Normalized mortality rates were remarkably similar between species and a modest, but a consistent and steady increasing trend was apparent throughout the study period. Mixed effects models revealed that gradually changing stand parameters (stand basal area and stand age) had the strongest impact on mortality rates, modulated by climate, which had increasing importance during the last decades. Hereby, recent climatic changes had highly variable effects on tree mortality rates, depending on the species in combination with abiotic and biotic stand and site conditions. This suggests that forest species composition and species ranges may change under future climate conditions. Our data set highlights the complexity of forest dynamical processes such as long-term, gradual changes of forest structure, demography and species composition, which together with climate determine mortality rates.

In tropical rainforests, tree size and number density are influenced by disturbance history, soil, topography, climate, and biological factors that are difficult to predict without detailed and widespread forest inventory data. Here, we quantify tree size–frequency distributions over an old-growth wet tropical forest at the La Selva Biological Station in Costa Rica by using an individual tree crown (ITC) algorithm on airborne lidar measurements. The ITC provided tree height, crown area, the number of trees >10 m height and, predicted tree diameter, and aboveground biomass from field allometry. The number density showed strong agreement with field observations at the plot- (97.4%; 3% bias) and tree-height-classes level (97.4%; 3% bias). The lidar trees size spectra of tree diameter and height closely follow the distributions measured on the ground but showed less agreement with crown area observations. The model to convert lidar-derived tree height and crown area to tree diameter produced unbiased (0.8%) estimates of plot-level basal area and with low uncertainty (6%). Predictions on basal area for tree height classes were also unbiased (1.3%) but with larger uncertainties (22%). The biomass estimates had no significant bias at the plot- and tree-height-classes level (−5.2% and 2.1%). Our ITC method provides a powerful tool for tree- to landscape-level tropical forest inventory and biomass estimation by overcoming the limitations of lidar area-based approaches that require local calibration using a large number of inventory plots.

Wood structure differs widely among tree species and species with faster growth, higher mortality and larger maximum size have been reported to have fewer but larger vessels and higher hydraulic conductivity (Kh). However, previous studies compiled data from various sources, often failed to control tree size and rarely controlled variation in other traits. We measured wood density, tree size and vessel traits for 325 species from a wet forest in Panama, and compared wood and leaf traits to demographic traits using species-level data and phylogenetically independent contrasts. Wood traits showed strong phylogenetic signal whereas pairwise relationships between traits were mostly phylogenetically independent. Trees with larger vessels had a lower fraction of the cross-sectional area occupied by vessel lumina, suggesting that the hydraulic efficiency of large vessels permits trees to dedicate a larger proportion of the wood to functions other than water transport. Vessel traits were more strongly correlated with the size of individual trees than with maximal size of a species. When individual tree size was included in models, Kh scaled positively with maximal size and was the best predictor for both diameter and biomass growth rates, but was unrelated to mortality.

• Tree size shapes forest carbon dynamics and determines how trees interact with their environment, including a changing climate. Here, we conduct the first global analysis of among-site differences in how aboveground biomass stocks and fluxes are distributed with tree size. • We analyzed repeat tree censuses from 25 large-scale (4–52 ha) forest plots spanning a broad climatic range over five continents to characterize how aboveground biomass, woody productivity, and woody mortality vary with tree diameter. We examined how the median, dispersion, and skewness of these size-related distributions vary with mean annual temperature and precipitation. • In warmer forests, aboveground biomass, woody productivity, and woody mortality were more broadly distributed with respect to tree size. In warmer and wetter forests, aboveground biomass and woody productivity were more right skewed, with a long tail towards large trees. Small trees (1–10 cm diameter) contributed more to productivity and mortality than to biomass, highlighting the importance of including these trees in analyses of forest dynamics. • Our findings provide an improved characterization of climate-driven forest differences in the size structure of aboveground biomass and dynamics of that biomass, as well as refined benchmarks for capturing climate influences in vegetation demographic models.

This article is a Commentary on Piponiot et al. (2022), 234: 1664–1677.

Drought disproportionately affects larger trees in tropical forests, but implications for forest composition and carbon (C) cycling in relation to dry season intensity remain poorly understood. In order to characterize how C cycling is shaped by tree size and drought adaptations and how these patterns relate to spatial and temporal variation in water deficit, we analyze data from three forest dynamics plots spanning a moisture gradient in Panama that have experienced El Niño droughts. At all sites, aboveground C cycle contributions peaked below 50-cm stem diameter, with stems ≥ 50 cm accounting for on average 59% of live aboveground biomass, 45% of woody productivity and 49% of woody mortality. The dominance of drought-avoidance strategies increased interactively with stem diameter and dry season intensity. Although size-related C cycle contributions did not vary systematically across the moisture gradient under non-drought conditions, woody mortality of larger trees was disproportionately elevated under El Niño drought stress. Thus, large (> 50 cm) stems, which strongly mediate but do not necessarily dominate C cycling, have drought adaptations that compensate for their more challenging hydraulic environment, particularly in drier climates. However, these adaptations do not fully buffer the effects of severe drought, and increased large tree mortality dominates ecosystem-level drought responses.