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.