Is there tree senescence? The fecundity evidence

The importance of regeneration processes on forest biodiversity in old-growth forests in the Pacific Northwest

Forest diversity is the outcome of multiple species-specific processes and tolerances, from regeneration, growth, competition and mortality of trees. Predicting diversity thus requires a comprehensive understanding of those processes. Regeneration processes have traditionally been overlooked, due to high stochasticity and assumptions that recruitment is not limiting for forests. Thus, we investigated the importance of seed production and seedling survival on forest diversity in the Pacific Northwest (PNW) using a forest gap model (ForClim). Equations for regeneration processes were fit to empirical data and added into the model, followed by simulations where regeneration processes and parameter values varied. Adding regeneration processes into ForClim improved the simulation of species composition, compared to Forest Inventory Analysis data. We also found that seed production was not as important as seedling survival, and the time it took for seedlings to grow into saplings was a critical recruitment parameter for accurately capturing tree species diversity in PNW forest stands. However, our simulations considered historical climate only. Due to the sensitivity of seed production and seedling survival to weather, future climate change may alter seed production or seedling survival and future climate change simulations should include these regeneration processes to predict future forest dynamics in the PNW. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

Tree adaptive growth (TAG) model: a life-history theory-based analytical model for post-thinning forest stand dynamics

Background

Understanding stand dynamics is essential for predicting future wood supply and associated ecosystem services for sustainable forest management. The dynamics of natural stands can be characterized by age-dependent growth and yield models. However, dynamics in managed stands appear somewhat different from that of natural stands, especially with difficulties in explaining the phenomenon of post-thinning overcompensation, based upon some long-term observations. Though overcompensation is an ideal outcome for the forest sector, it had been largely treated as an outlier and thus ignored or dismissed as "out-of-the-ordinary".

Methodology

We developed a life history theory-based, state-dependent model of Tree Adaptive Growth (TAG) to investigate this phenomenon and verified that overcompensation should be a common outcome in post-thinning forest stands when the stand growth over time is sigmoid shaped. TAG posits that individual trees will invest proportionately more into growth following thinning because it is evolutionarily adaptive to do so.

Results

Our investigation of the model's behavior unearthed diverse stand growth patterns similar to that which is observed in the empirical datasets and predicted by a statistics-based Tree's Compensatory Growth (TreeCG) model.

Conclusion

A simple, theory-driven, analytical model, TAG, can reproduce the diverse growth patterns in post-thinning stands and thus assist addressing silviculture-related issues. The model can be applied to various jurisdictions even without detailed regional growth and yield relationships and is capable of incorporating the effects of other time sensitive factors like fertilization, pruning, and climate change.

Masting is uncommon in trees that depend on mutualist dispersers in the context of global climate and fertility gradients

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands.

Reproductive collapse in European beech results from declining pollination efficiency in large trees

Climate warming increases tree mortality which will require sufficient reproduction to ensure population viability. However, the response of tree reproduction to climate change remains poorly understood. Warming can reduce synchrony and interannual variability of seed production ("masting breakdown") which can increase seed predation and decrease pollination efficiency in trees. Here, using 40 years of observations of individual seed production in European beech (Fagus sylvatica), we showed that masting breakdown results in declining viable seed production over time, in contrast to the positive trend apparent in raw seed count data. Furthermore, tree size modulates the consequences of masting breakdown on viable seed production. While seed predation increased over time mainly in small trees, pollination efficiency disproportionately decreased in larger individuals. Consequently, fecundity declined over time across all size classes, but the overall effect was greatest in large trees. Our study showed that a fundamental biological relationship-correlation between tree size and viable seed production-has been reversed as the climate has warmed. That reversal has diverse consequences for forest dynamics; including for stand- and biogeographical-level dynamics of forest regeneration. The tree size effects suggest management options to increase forest resilience under changing climates.

Effects of invasive Rosa rugosa on Baltic coastal dune communities depend on dune age

Rosa rugosa Thunb. (Japanese Rose) is one of the most invasive species in Europe. It spreads spontaneously in coastal areas of western, central and northern Europe, posing a threat to dune habitats, including those indicated in the EU Habitats Directive as particularly valuable. R. rugosa has already been reported to displace native plants and alter soil properties. However, little is known about how these effects are mediated by the habitat context or the invader condition (health, ontogenetic stage). This study addressed that gap by examining vegetation and soil in 22 R. rugosa-invaded sites, half of which were in yellow dunes and the other half in grey dunes, i.e. two habitats representing the earlier and later stages of dune succession. The study was conducted on the Hel Peninsula (Poland’s Baltic coast). R. rugosa had a significant impact on dune vegetation, but the impact was strongly dependent on the habitat type. In the yellow dune sites, R. rugosa outcompeted most resident plant species, which translated into a strong decline in their total cover and richness. The invasion was almost not accompanied by changes in soil properties, suggesting that it affected the resident vegetation directly (through space takeover and shading). In the grey dunes, R. rugosa caused a shift in species composition, from that characteristic of open communities to that typical of forests. In this habitat, a significant increase in the soil organic layer thickness under R. rugosa was also observed, which means that both direct and indirect effects of the invasion on the vegetation should be assumed. Finally, a negative relationship was found between the total chlorophyll content in R. rugosa leaves and the parameters of resident plant communities, showing that the invasion effects can vary not only across habitats, but also with the condition of the invader. The results may have practical implications for managing R. rugosa invasions in coastal sand dune systems. Since R. rugosa accelerates grey dune succession, protecting this habitat may be more urgent and, at the same time, more complicated than protecting dunes at the earlier stages of development.

Climate-driven convergent evolution in riparian ecosystems on sky islands

Climate-induced evolution will determine population persistence in a changing world. However, finding natural systems in which to study these responses has been a barrier to estimating the impact of global change on a broad scale. We propose that isolated sky islands (SI) and adjacent mountain chains (MC) are natural laboratories for studying long-term and contemporary climatic pressures on natural populations. We used greenhouse common garden trees to test whether populations on SI exposed to hot and dry climates since the end of the Pleistocene have phenotypically diverged from populations on MC, and if SI populations have converged in these traits. We show: (1) populations of Populus angustifolia from SI have diverged from MC, and converged across SI, in reproductive and productivity traits, (2) these traits (cloning and aboveground biomass, respectively) are significantly correlated, suggesting a genetic linkage between them, and (3) the trait variation is driven by both natural selection and genetic drift. These shifts represent potentially beneficial phenotypes for population persistence in a changing world. These results suggest that the SI-MC comparison is a natural laboratory, as well as a predictive framework, for studying long-term responses to climate change across the globe.

Integrating gradient with scale in ecological and evolutionary studies

Gradient and scale are two key concepts in ecology and evolution that are closely related but inherently distinct. While scale commonly refers to the dimensional space of a specific ecological/evolutionary (eco-evo) issue, gradient measures the range of a given variable. Gradient and scale can jointly and interactively influence eco-evo patterns. Extensive previous research investigated how changing scales may affect the observation and interpretation of eco-evo patterns; however, relatively little attention has been paid to the role of changing gradients. Here, synthesizing recent research progress, we suggest that the role of scale in the emergence of ecological patterns should be evaluated in conjunction with considering the underlying environmental gradients. This is important because, in most studies, the range of the gradient is often part of its full potential range. The difference between sampled (partial) versus potential (full) environmental gradients may profoundly impact observed eco-evo patterns and alter scale-gradient relationships. Based on observations from both field and experimental studies, we illustrate the underlying features of gradients and how they may affect observed patterns, along with the linkages of these features to scales. Since sampled gradients often do not cover their full potential ranges, we discuss how the breadth and the starting and ending positions of key gradients may affect research design and data interpretation. We then outline potential approaches and related perspectives to better integrate gradient with scale in future studies.

Dynamic Species Distribution Modeling Reveals the Pivotal Role of Human-Mediated Long-Distance Dispersal in Plant Invasion

Plant invasions generate massive ecological and economic costs worldwide. Predicting their spatial dynamics is crucial to the design of effective management strategies and the prevention of invasions. Earlier studies highlighted the crucial role of long-distance dispersal in explaining the speed of many invasions. In addition, invasion speed depends highly on the duration of its lag phase, which may depend on the scaling of fecundity with age, especially for woody plants, even though empirical proof is still rare. Bayesian dynamic species distribution models enable the fitting of process-based models to partial and heterogeneous observations using a state-space modeling approach, thus offering a tool to test such hypotheses on past invasions over large spatial scales. We use such a model to explore the roles of long-distance dispersal and age-structured fecundity in the transient invasion dynamics of Plectranthus barbatus, a woody plant invader in South Africa. Our lattice-based model accounts for both short and human-mediated long-distance dispersal, as well as age-structured fecundity. We fitted our model on opportunistic occurrences, accounting for the spatio-temporal variations of the sampling effort and the variable detection rates across datasets. The Bayesian framework enables us to integrate a priori knowledge on demographic parameters and control identifiability issues. The model revealed a massive wave of spatial spread driven by human-mediated long-distance dispersal during the first decade and a subsequent drastic population growth, leading to a global equilibrium in the mid-1990s. Without long-distance dispersal, the maximum population would have been equivalent to 30% of the current equilibrium population. We further identified the reproductive maturity at three years old, which contributed to the lag phase before the final wave of population growth. Our results highlighted the importance of the early eradication of weedy horticultural alien plants around urban areas to hamper and delay the invasive spread.

Forest regeneration within Earth system models: current process representations and ways forward

Earth system models must predict forest responses to global change in order to simulate future global climate, hydrology, and ecosystem dynamics. These models are increasingly adopting vegetation demographic approaches that explicitly represent tree growth, mortality, and recruitment, enabling advances in the projection of forest vulnerability and resilience, as well as evaluation with field data. To date, simulation of regeneration processes has received far less attention than simulation of processes that affect growth and mortality, in spite of their critical role maintaining forest structure, facilitating turnover in forest composition over space and time, enabling recovery from disturbance, and regulating climate-driven range shifts. Our critical review of regeneration process representations within current Earth system vegetation demographic models reveals the need to improve parameter values and algorithms for reproductive allocation, dispersal, seed survival and germination, environmental filtering in the seedling layer, and tree regeneration strategies adapted to wind, fire, and anthropogenic disturbance regimes. These improvements require synthesis of existing data, specific field data-collection protocols, and novel model algorithms compatible with global-scale simulations. Vegetation demographic models offer the opportunity to more fully integrate ecological understanding into Earth system prediction; regeneration processes need to be a critical part of the effort.

The role of demographic compensation in stabilising marginal tree populations in North America

Demographic compensation-the opposing responses of vital rates along environmental gradients-potentially delays anticipated species' range contraction under climate change, but no consensus exists on its actual contribution. We calculated population growth rate (λ) and demographic compensation across the distributional ranges of 81 North American tree species and examined their responses to simulated warming and tree competition. We found that 43% of species showed stable population size at both northern and southern edges. Demographic compensation was detected in 25 species, yet 15 of them still showed a potential retraction from southern edges, indicating that compensation alone cannot maintain range stability. Simulated climatic warming caused larger decreases in λ for most species and weakened the effectiveness of demographic compensation in stabilising ranges. These findings suggest that climate stress may surpass the limited capacity of demographic compensation and pose a threat to the viability of North American tree populations.

How will global change affect plant reproduction? A framework for mast seeding trends

Forest ecology traditionally focuses on plant growth and survival, leaving seed production as a major demographic process lacking a framework for how it will be affected by global change. Understanding plant reproductive responses to changing climate is complicated by masting, the annually variable seed production synchronized within populations. Predicting trends in masting is crucial, because masting impacts seed predation and pollination enough to override simple trends in mean seed production. Proximate mechanisms of seed production patterns in perennial plants are gathered to identify processes through which masting may be affected by a changing environment. Predicting trends in masting will require understanding the mechanisms that cause predictable seed failure after high-seed years, and the stochastic mechanisms that synchronize individuals in high-seed years.

Spatiotemporal limitations in plant biology research

The way we currently capture biological processes in space and time often limits our understanding of plant development and stress responses, leading to an incomplete picture of plant life. Choosing the correct time frame for the study of every biological process, from seed germination to senescence or in plant stress responses, is essential, despite methodological limitations. A greater effort is needed in current plant biology studies to incorporate spatiotemporal approaches so that scientific knowledge meets the possibilities technological advances currently provide. From molecular, biochemical, and cellular approaches to (eco)physiological and population studies scaled up to the ecosystem level, there is an urgent need to link space and time using integrative and scalable data.

Old and ancient trees are life history lottery winners and vital evolutionary resources for long-term adaptive capacity

Trees can live for many centuries with sustained fecundity and death is largely stochastic. We use a neutral stochastic model to examine tree demographic patterns that emerge over time, across a range of population sizes and empirically observed mortality rates. A small proportion of trees (~1% at 1.5% mortality) are life-history 'lottery winners', achieving ages >10-20× the median age. Maximum age increases with bigger populations and lower mortality rates. One-quarter of trees (~24%) achieve ages that are three to four times greater than the median age. Three age classes (mature, old and ancient) contribute unique evolutionary diversity across complex environmental cycles. Ancient trees are an emergent property of forests that requires many centuries to generate. They radically change variance in generation time and population fitness, bridging centennial environmental cycles. These life-history 'lottery' winners are vital to long-term forest adaptive capacity and provide invaluable data about environmental history and individual longevity. Old and ancient trees cannot be replaced through restoration or regeneration for many centuries. They must be protected to preserve their invaluable diversity.

Distinct Community-Wide Responses to Forecasted Climate Change in Afrotropical Forests

More refined knowledge of how tropical forests respond to changes in the abiotic environment is necessary to mitigate climate change, maintain biodiversity, and preserve ecosystem services. To evaluate the unique response of diverse Afrotropical forest communities to disturbances in the abiotic environment, we employ country-wide tree species inventories, remotely sensed climate data, and future climate predictions collected from 104 1-ha plots in the central African country of Gabon. We predict a 3–8% decrease in Afrotropical forest species richness by the end of the century, in contrast to the 30–50% loss of plant diversity predicted to occur with equivalent warming in the Neotropics. This work reveals that forecasts of community species composition are not generalizable across regions, and more representative studies are needed in understudied diverse biomes. This study serves as an important counterpoint to work done in the Neotropics by providing contrasting predictions for Afrotropical forests with substantially different ecological, evolutionary, and anthropogenic histories.

North American tree migration paced by climate in the West, lagging in the East

Suitable habitats for forest trees may be shifting fast with recent climate change. Studies tracking the shift in suitable habitat for forests have been inconclusive, in part because responses in tree fecundity and seedling establishment can diverge. Analysis of both components at a continental scale reveals a poleward migration of northern species that is in progress now. Recruitment and fecundity both contribute to poleward spread in the West, while fecundity limits spread in the East, despite a fecundity hotspot in the Southeast. Fecundity limitation on population spread can confront conservation and management efforts with persistent disequilibrium between forest diversity and rapid climate change.

Tree fecundity and recruitment have not yet been quantified at scales needed to anticipate biogeographic shifts in response to climate change. By separating their responses, this study shows coherence across species and communities, offering the strongest support to date that migration is in progress with regional limitations on rates. The southeastern continent emerges as a fecundity hotspot, but it is situated south of population centers where high seed production could contribute to poleward population spread. By contrast, seedling success is highest in the West and North, serving to partially offset limited seed production near poleward frontiers. The evidence of fecundity and recruitment control on tree migration can inform conservation planning for the expected long-term disequilibrium between climate and forest distribution.

Joint effects of climate, tree size, and year on annual tree growth derived from tree-ring records of ten globally distributed forests

Tree rings provide an invaluable long-term record for understanding how climate and other drivers shape tree growth and forest productivity. However, conventional tree-ring analysis methods were not designed to simultaneously test effects of climate, tree size, and other drivers on individual growth. This has limited the potential to test ecologically relevant hypotheses on tree growth sensitivity to environmental drivers and their interactions with tree size. Here, we develop and apply a new method to simultaneously model nonlinear effects of primary climate drivers, reconstructed tree diameter at breast height (DBH), and calendar year in generalized least squares models that account for the temporal autocorrelation inherent to each individual tree's growth. We analyze data from 3811 trees representing 40 species at 10 globally distributed sites, showing that precipitation, temperature, DBH, and calendar year have additively, and often interactively, influenced annual growth over the past 120 years. Growth responses were predominantly positive to precipitation (usually over ≥3-month seasonal windows) and negative to temperature (usually maximum temperature, over ≤3-month seasonal windows), with concave-down responses in 63% of relationships. Climate sensitivity commonly varied with DBH (45% of cases tested), with larger trees usually more sensitive. Trends in ring width at small DBH were linked to the light environment under which trees established, but basal area or biomass increments consistently reached maxima at intermediate DBH. Accounting for climate and DBH, growth rate declined over time for 92% of species in secondary or disturbed stands, whereas growth trends were mixed in older forests. These trends were largely attributable to stand dynamics as cohorts and stands age, which remain challenging to disentangle from global change drivers. By providing a parsimonious approach for characterizing multiple interacting drivers of tree growth, our method reveals a more complete picture of the factors influencing growth than has previously been possible.

The ecology and evolution of synchronized reproduction in long-lived plants

Populations of many long-lived plants exhibit spatially synchronized seed production that varies extensively over time, so that seed production in some years is much higher than on average, while in others, it is much lower or absent. This phenomenon termed masting or mast seeding has important consequences for plant reproductive success, ecosystem dynamics and plant-human interactions. Inspired by recent advances in the field, this special issue presents a series of articles that advance the current understanding of the ecology and evolution of masting. To provide a broad overview, we reflect on the state-of-the-art of masting research in terms of underlying proximate mechanisms, ontogeny, adaptations, phylogeny and applications to conservation. While the mechanistic drivers and fitness consequences of masting have received most attention, the evolutionary history, ontogenetic trajectory and applications to plant-human interactions are poorly understood. With increased availability of long-term datasets across broader geographical and taxonomic scales, as well as advances in molecular approaches, we expect that many mysteries of masting will be solved soon. The increased understanding of this global phenomenon will provide the foundation for predictive modelling of seed crops, which will improve our ability to manage forests and agricultural fruit and nut crops in the Anthropocene. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Intermediate disturbances are a key driver of long-term tree demography across old-growth temperate forests

Disentangling the relative influence of background versus disturbance related mortality on forest demography is crucial for understanding long-term dynamics and predicting the influence of global change on forests. Quantifying the rates and drivers of tree demography requires direct observations of tree populations over multiple decades, yet such studies are rare in old-growth forest, particularly in the temperate zone of Europe. We use multi-decade (1980-2020) monitoring of permanent plots, including observations of mode of mortality and disturbance events, to quantify rates and drivers of tree demography across a network of old-growth remnants in temperate mountain forests of Slovenia. Annual rates of mortality and recruitment varied markedly among sites and over time; census intervals that captured intermediate severity canopy disturbances caused subtle peaks in annual mortality (e.g., >2%/year), while rates of background mortality in non-disturbed intervals averaged about 1%/year. Roughly half of the trees died from modes of mortality associated with disturbance (i.e., uprooting or snapped-alive). Results of a Bayesian multilevel model indicate that beech (Fagus sylvatica) had a higher likelihood of disturbance related mortality compared to fir (Abies alba), which mainly died standing, and there was a notable increase in the odds of disturbance mortality with increasing diameter for all species. Annual recruitment rates were consistently low at sites (<0.5%) that lacked evidence of disturbance, but often exceeded 3% on sites with higher levels of past canopy mortality. Recruitment was dominated by beech on sites with more diffuse background mortality, while the less shade tolerant maple (Acer pseudoplatanus) recruited following known disturbance events. Our study highlights the important role of stand-scale, partial canopy disturbance for long-term forest demography. These results suggest that subtle climate-driven changes in the regime of intermediate severity disturbances could have an important influence on future forest dynamics and warrant attention.