Large‐diameter, tall‐stature, and big‐crown trees are the main stand structures of forests, generally contributing a large fraction of aboveground biomass, and hence play an important role in climate ...change mitigation strategies. Here, we hypothesized that the effects of large‐diameter, tall‐stature, and big‐crown trees overrule the effects of species richness and remaining trees attributes on aboveground biomass in tropical forests (i.e., we term the “big‐sized trees hypothesis”). Specifically, we assessed the importance of: (a) the “top 1% big‐sized trees effect” relative to species richness; (b) the “99% remaining trees effect” relative to species richness; and (c) the “top 1% big‐sized trees effect” relative to the “99% remaining trees effect” and species richness on aboveground biomass. Using environmental factor and forest inventory datasets from 712 tropical forest plots in Hainan Island of southern China, we tested several structural equation models for disentangling the relative effects of big‐sized trees, remaining trees attributes, and species richness on aboveground biomass, while considering for the full (indirect effects only) and partial (direct and indirect effects) mediation effects of climatic and soil conditions, as well as interactions between species richness and trees attributes. We found that top 1% big‐sized trees attributes strongly increased aboveground biomass (i.e., explained 55%–70% of the accounted variation) compared to species richness (2%–18%) and 99% remaining trees attributes (6%–10%). In addition, species richness increased aboveground biomass indirectly via increasing big‐sized trees but via decreasing remaining trees. Hence, we show that the “big‐sized trees effect” overrides the effects of remaining trees attributes and species richness on aboveground biomass in tropical forests. This study also indicates that big‐sized trees may be more susceptible to atmospheric drought. We argue that the effects of big‐sized trees on species richness and aboveground biomass should be tested for better understanding of the ecological mechanisms underlying forest functioning.
Large‐diameter, tall‐stature and big‐crown trees are the main stand structures of forests, generally contributing a large fraction of aboveground biomass, and hence, play an important role in climate change mitigation strategies. We show that the “big‐sized trees effect” overrides the effects of remaining trees attributes and species richness on aboveground biomass in tropical forests. This study also indicates that big‐sized trees may be more susceptible to atmospheric drought. We argue that the effects of big‐sized trees on species richness and aboveground biomass should be tested for better understanding of the ecological mechanisms underlying forest functioning.
ABSTRACT
Large old trees are some of the most iconic biota on earth and are integral parts of many terrestrial ecosystems including those in tropical, temperate and boreal forests, deserts, savannas, ...agro‐ecological areas, and urban environments. In this review, we provide new insights into the ecology, function, evolution and management of large old trees through broad cross‐disciplinary perspectives from literatures in plant physiology, growth and development, evolution, habitat value for fauna and flora, and conservation management. Our review reveals that the diameter, height and longevity of large old trees varies greatly on an inter‐specific basis, thereby creating serious challenges in defining large old trees and demanding an ecosystem‐ and species‐specific definition that will only rarely be readily transferable to other species or ecosystems. Such variation is also manifested by marked inter‐specific differences in the key attributes of large old trees (beyond diameter and height) such as the extent of buttressing, canopy architecture, the extent of bark micro‐environments and the prevalence of cavities. We found that large old trees play an extraordinary range of critical ecological roles including in hydrological regimes, nutrient cycles and numerous ecosystem processes. Large old trees strongly influence the spatial and temporal distribution and abundance of individuals of the same species and populations of numerous other plant and animal species. We suggest many key characteristics of large old trees such as extreme height, prolonged lifespans, and the presence of cavities – which confer competitive and evolutionary advantages in undisturbed environments – can render such trees highly susceptible to a range of human influences. Large old trees are vulnerable to threats ranging from droughts, fire, pests and pathogens, to logging, land clearing, landscape fragmentation and climate change. Tackling such diverse threats is challenging because they often interact and manifest in different ways in different ecosystems, demanding targeted species‐ or ecosystem‐specific responses. We argue that novel management actions will often be required to protect existing large old trees and ensure the recruitment of new cohorts of such trees. For example, fine‐scale tree‐level conservation such as buffering individual stems will be required in many environments such as in agricultural areas and urban environments. Landscape‐level approaches like protecting places where large old trees are most likely to occur will be needed. However, this brings challenges associated with likely changes in tree distributions associated with climate change, because long‐lived trees may presently exist in places unsuitable for the development of new cohorts of the same species. Appropriate future environmental domains for a species could exist in new locations where it has never previously occurred. The future distribution and persistence of large old trees may require controversial responses including assisted migration via seed or seedling establishment in new locales. However, the effectiveness of such approaches may be limited where key ecological features of large old trees (such as cavity presence) depend on other species such as termites, fungi and bacteria. Unless other species with similar ecological roles are present to fulfil these functions, these taxa might need to be moved concurrently with the target tree species.
Determining physiological mechanisms and thresholds for climate-driven tree die-off could help improve global predictions of future terrestrial carbon sinks. We directly tested for the lethal ...threshold in hydraulic failure – an inability to move water due to drought-induced xylem embolism – in a pine sapling experiment.
In a glasshouse experiment, we exposed loblolly pine (Pinus taeda) saplings (n = 83) to drought-induced water stress ranging from mild to lethal. Before rewatering to relieve drought stress, we measured native hydraulic conductivity and foliar color change. We monitored all measured individuals for survival or mortality.
We found a lethal threshold at 80% loss of hydraulic conductivity – a point of hydraulic failure beyond which it is more likely trees will die, than survive, and describe mortality risk across all levels of water stress. Foliar color changes lagged behind hydraulic failure – best predicting when trees had been dead for some time, rather than when they were dying.
Our direct measurement of native conductivity, while monitoring the same individuals for survival or mortality, quantifies a continuous probability of mortality risk from hydraulic failure. Predicting tree die-off events and understanding the mechanism involved requires knowledge not only of when trees are dead, but when they begin dying – having passed the point of no return.
Extreme heat as a result of climate change is already being felt in Los Angeles and will only increase throughout the century. Not all residents of Los Angeles feel these effects equally. Shaded ...areas provide valuable relief from the heat, but the shade provided by street trees has historically been concentrated in certain communities and excluded from others. To create a just future in the face of climate change, all communities must have the resources to maintain habitable conditions, including shade trees. Establishing the number of trees required to build an equitable tree canopy for the city requires another scarce resource: water. This paper analyzes the amount of water and associated impacts required to establish an equitable tree canopy in Los Angeles through a lens of distributive justice. I conclude that, from a water standpoint, the benefits of increased tree canopy outweigh the energy, financial, and supply costs needed to achieve a more equitable tree canopy.
Matthew Russell asks whether Earth's existing forestland has the space and resources necessary to enable 1 trillion additional trees to survive and thrive
Matthew Russell asks whether Earth's ...existing forestland has the space and resources necessary to enable 1 trillion additional trees to survive and thrive.
Conifer tree rings are generally composed of large, thin-walled cells of light earlywood followed by narrow, thick-walled cells of dense latewood. Yet, how wood formation processes and the associated ...kinetics create this typical pattern remains poorly understood.
We monitored tree-ring formation weekly over 3 yr in 45 trees of three conifer species in France. Data were used to model cell development kinetics, and to attribute the relative importance of the duration and rate of cell enlargement and cell wall deposition on tree-ring structure.
Cell enlargement duration contributed to 75% of changes in cell diameter along the tree rings. Remarkably, the amount of wall material per cell was quite constant along the rings. Consequently, and in contrast with widespread belief, changes in cell wall thickness were not principally attributed to the duration and rate of wall deposition (33%), but rather to the changes in cell size (67%). Cell enlargement duration, as the main driver of cell size and wall thickness, contributed to 56% of wood density variation along the rings.
This mechanistic framework now forms the basis for unraveling how environmental stresses trigger deviations (e.g. false rings) from the normal tree-ring structure.
The response of tree growth to a change in temperature may differ in predictable ways. Trees with conservative growth strategies may have little ability to respond to a changing climate. In addition, ...high latitude and altitude tree growth may be temperature-limited and thus benefit from some degree of warming, as opposed to warm-adapted species. Using data from 63 studies, we examined whether trees from different functional groups and thermal niches differed in their growth response to a change in growth temperature. We also investigated whether responses predicted for a change in growth temperature (both reduced and elevated) were similar for increased temperatures by repeating the analysis on the subset of raised temperature data to confirm the validity of our results for use in a climate-warming scenario. Using both the temperature-change response and the warming response, we found that elevated temperatures enhanced growth (measured as shoot height, stem diameter and biomass) in deciduous species more than in evergreen trees. Tropical species were indeed more susceptible to warming-induced growth declines than temperate or boreal trees in both analyses. More carbon may be available to allocate to growth at high temperatures because respiration acclimated more strongly than photosynthesis, increasing carbon assimilation but moderating carbon losses. Trees that developed at elevated temperatures did not simply accelerate growth but followed different developmental trajectories than unwarmed trees, allocating more biomass to leaves and less to roots and growing taller for a given stem diameter. While there were insufficient data to analyze trends for particular species, we generated equations to describe general trends in tree growth to temperature changes and to warming for use at large spatial scales or where data are lacking. We discuss the implications of these results in the context of a changing climate and highlight the areas of greatest uncertainty regarding temperature and tree growth where future research is needed.
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