In view of the extensive loss and disturbance of rainforests over the last two centuries, understanding the ecology of 'secondary' regeneration is now vital for the conservation of rainforest species and ecosystems in many parts of the world, and for sustaining ecosystem services on which millions of people depend. So far, our understanding of natural or managed regeneration after rainforest clearance has been limited by brief timescales of observation, and by lack of knowledge of functional ecology and spatial processes including seed dispersal.
This research combines studies of plant phylogeny, trait inter-relations and seed dispersal, as a basis for understanding processes of natural regeneration in intact rainforests and secondary rainforests on abandoned pastures. It identifies four key dimensions of ecological variation among rainforest plants, each represented by a spectrum of correlated traits of plant life-history and ecophysiology, and highlights the importance of seed dispersal in shaping the distribution and ecological diversity of regeneration in secondary rainforests in Australia's Wet Tropics.
Spatial data on natural regeneration comes from surveys of all plants (from 4th leaf seedlings to canopy trees) on transects from intact rainforest into adjacent, secondary rainforests at Mungalli on the Atherton Tablelands, mapping 13,690 plants, from 207 species, 153 genera and 68 families. A time-scaled phylogeny was estimated for 195 angiosperm species, to understand the evolutionary relationships among species, and to inform analyses of trait inter-relations and community structure. Bayesian relaxed clock methods were employed to simultaneously estimate tree topology, rates of sequence evolution, and divergence times. A series of analyses based on new trnL-F sequence data and 37 temporal calibrations explored the consequences of alternative temporal calibrations, tree priors, and models of sequence evolution; and resulted in the first well-supported and highly resolved phylogeny for this flora. The phylogeny encompasses 11% of Wet Tropics species (20% of genera, 46% of families), and divergence times from several of the earliest angiosperm divergences, through to recent speciation within Ficus and Psychotria. Combined with data on the traits of living species, the set of posterior 'time trees' enables analysis of correlated evolution in plant functional traits, while integrating phylogenetic uncertainty.
To identify key dimensions of trait variation I examined the trade-offs and interdependencies among traits of seeds, seedlings and adult plants, to quantify their changes over the plant life-cycle, and relations to shade tolerance as estimated by minimum light environments for seedling establishment and reproduction. Stem tissue density, leaf density and leaf strength emerged as key traits linked to shade tolerance at seedling and adult lifestages, reflecting fundamental trade-offs between allocations to physiological activity vs. structure and defences that are needed for survival and realised growth in shaded environments. The range of shifts observed in light environments from seedling to reproductive stages shows that shade tolerance is not a single, unchanging axis of variation across species, and depends not only on light and shade, but also on responses to other factors in a plant's changing environment.
Spatial analyses of regeneration show strong shifts in functional diversity and the representation of plant traits and dispersal mechanisms with increasing distance from primary rainforest sources, and through strata from seedlings to canopy trees. Dramatic changes were observed in the absolute and relative densities of plants dispersed by different spectra of birds and mammals, and this reflects not only variation in dispersal distances, but also the covariation of fruit and seed traits with features of other lifestages, especially the length of time to first reproduction. The role of seed dispersal was made visible by taking a functional perspective on plants and their dispersers' foraging and movement behaviours, to reveal important variation within widely-applied classes such as 'wind, bird or mammal' dispersal. Several shifts in dispersal spectra and relationships with other traits were only visible when classes of animal dispersal distinguished between frugivores that forage within forest canopies and move seeds over shorter distances, in contrast to frugivores that forage and move widely through the landscape.
Secondary forests close to intact 'source' rainforests can develop moderately complex structure and species composition within decades. However, dispersal distances and times to reach maturity limit the colonisation abilities of many rainforest plants, and dramatic declines in phylogenetic and functional diversity occur even 80 m into a closed canopy forest after 50 years of regeneration. It would take many more decades to converge on the ecological diversity of primary forests across a range of seed and plant traits. This highlights the importance of conserving animal seed dispersers in the landscape, and the scope for accelerating regeneration through time-phased direct seeding or planting of functionally diverse species beneath a pioneer canopy.