Modelling dynamics including recruitment, growth and mortality for sustainable management in uneven-aged mixed-species rainforests

Maina Kariuki

Changes in species abundance and richness, stand structure, and species responses to habitat characteristics including disturbance intensity, were investigated in 28 permanent sample plots (PSP) covering a total area of about 6.2 hectares of subtropical rainforests in north-east New South Wales, Australia. The disturbance that occurred over 36 years previously varied from unlogged (controls), through single-tree selection (light), moderate selection and repeated single-tree selection to intensive (heavy) logging. Multivariate and univariate approaches were used to investigate changes in various plant groups, including all vascular plants, regenerating species (trees and understorey species > 1.3 m in height and < 10 cm diameter at 1.3 m above the ground level (dbh)), juvenile trees (regenerating species capable of attaining at least 10 cm dbh), and trees ¡Ý 10 cm dbh. In addition, dynamic parameters (recruitment, growth and mortality) were estimated using hierarchical multilevel modelling and parameter estimates used to simulate behaviour of the subtropical rainforest tree species in unlogged and logged stands, both within and beyond the range of available data. Chronological post-disturbance responses and changes in species abundance and richness as well as stand structure for trees ¡Ý 10 cm dbh were investigated using 20 PSP in a bid to better understand rainforest post-disturbance regeneration. The results portrayed minor and gradual ecological changes in the undisturbed controls analogous with natural forest dynamics where the changes were not significant. In logged sites, initial gradual changes were followed with more rapid and significant changes. In both unlogged and logged stands, the changes proceeded through three clearly identifiable stages. The first two stages lasted between 5 and 15 years each. In the initial stage, the number of individuals decreased in species with high frequency mainly in the shade tolerant species, with little or no change in species richness. In the second stage, species abundance and richness changed due to localised species turnover, replacement and losses with very little recruitment, and the numbers of both species and individuals declined to minima. In the third stage, due to recruitment beyond the 10 cm dbh, the net loss of species and trees ¡Ý 10 cm dbh were halted and reversed, and species diversity and abundance began to return to pre-disturbance levels. Species abundance and diversity for both shade tolerant and intolerant species in less intensively logged sites (single-tree selection logging) have recovered to levels comparable with that observed in the controls, but more intensively logged sites have not recovered to similar levels. Increased logging intensity was associated with increased and decreased densities in the shade intolerant and ¨Ctolerant species respectively. It was evident that stand structure in logged sites had yet to recover to levels comparable with that of unlogged controls. It appears the restoration of floristic diversity to levels similar to that of intact primary forest takes considerably less time than structural recovery. Changes in regeneration patterns in a subtropical rainforest in north-east New South Wales were investigated for a 13-year period during the 3rd and 4th decades following repeated single-tree selection logging. Multivariate and univariate analyses results showed that there were no significant differences in floristic assemblages within and between censuses; however, two contrasting trends of changes in plant groups were detected. In trees ¡Ý 10 cm dbh, the stem density and species richness increased in shade tolerant group, and while stem density increased in the shade intolerant group, species richness decreased during the study period. Amongst smaller sized species including trees (< 10 cm dbh), a general decrease in species richness was observed along with significant changes in stem densities (P = 0.03) with the number of individuals in the shade tolerant species increasing, while that of both shade intolerant and vine species decreased. Excluding the vines and understorey species from the broader regenerating species group, revealed a decrease in species richness in juvenile canopy tree, and a significant change in densities (P = 0.004) with the number of individuals in shade tolerant increasing, while that of shade intolerant trees decreased. A comparison between the canopy trees ¡Ý 10 cm dbh and juvenile canopy trees group showed that these groups were tending towards similar floristic assemblages. These results suggest gradual replacement of shade intolerant by shade tolerant species as stands were tending toward later stages of recovery within the regeneration. The results also show that the inclusion of regenerating species in long-term studies is both complementary to the larger plant component and more revealing of both trends and changes. Species-specific estimates of shade tolerance and size structure at maturity derived from observations on 23 plots were used to intuitively group 277 vascular plant species into 3 main groups including full floristic (all together), shade tolerant and intolerant groups. The shade tolerant and intolerant groups were further grouped into smaller plants groups depending on the development stage and maximum size at maturity. These groups included regenerating species, juvenile trees, and trees ¡Ý 10 cm dbh. Multivariate analyses of these nominated groups revealed that floristic assemblages were significantly associated with environmental gradients based on simple site characteristics. At the landscape (larger) scale, the floristic assemblages were significantly different between low (200-400), mid (400-700) and high altitude (over 700 meters above sea level). In addition, abundance of shade tolerant species including Doryphora sassafras Endl, Orites excelsa R.Br and Caldcluvia paniculosa (F.Muell) Hoogland was positively correlated with the altitudinal gradient. At the smaller (local) scale, intensively logged sites where large gaps were created had recovered their species richness, but effects of past logging were evident in trees ¡Ý 10 cm dbh, especially trees greater than 50 cm dbh where species abundance in shade tolerant decreased significantly. Less intensively logged sites at mid altitude where a few stems were removed had recovered their species richness in respect to trees ¡Ý 10 cm dbh, but the small gaps created may have healed quickly for appreciable regeneration response to occur, as juvenile trees component was significantly different to that of the (unlogged) controls. The abundance of shade intolerant and juvenile shade tolerant tree species increased in concert with levels of disturbance and abundance of shade intolerant species such as Duboisia myoporoides R.Br, Acacia melanoxylon R.Br, Rubus moorei F.Muell and R. hilli F.Muell was positively correlated with the disturbance gradient. Increase in number of individuals of certain tree species was positively correlated with both disturbance intensity and topography (mid slope through lower slope to creek/gully), but negatively correlated with aspect (NE - NW). We concluded that logged sites are yet to recover their pre-logging stand structure and rainforest tree species were found in sites where favourable recruitment and growth conditions were the encountered. These results show that logged sites had not yet regenerated to the pre-logging stand structure. They also show that habitat characteristics including history of disturbance and topography can influence the floristic assemblages in the sub-tropical rainforests at both the small (local) and large (geographic) scales. This supports the environmental control model that states ¡°species are found at sites where they encounter favourable living conditions¡±. Site characteristics as surrogate for some of these favourable living conditions were identified as useful potential variables to investigate the rainforest dynamic parameters (growth, recruitment and mortality) in both logged and unlogged stands. A quantitative model was developed using over 3 decades of data to describe and simulate the dynamics including recruitment, growth and mortality in unlogged stands and others subjected to different silvicultural regimes in uneven-aged mixed-species subtropical rainforests of north-eastern New South Wales. Hierarchical multilevel regression analyses including Poisson, Binomial and multinomial logit regression were used to estimate the rainforest dynamic parameters based on the assumption that trees perform differently in space and time, thus there are variations at both the plot and tree by measurement levels. Variations at the tree level required the botanical identity of trees to species level, and then the species-specific size at maturity and shade tolerance were used to classify species into 5 groups, each consisting of species with similar ecological characteristics. These groups were labelled as emergent and shade tolerant main canopy, shade tolerant mid canopy, shade tolerant understoreys, moderate shade tolerant and persistent, and shade intolerant pioneer tree species. Significant variables at the plot level including site characteristics such as topography (elevation, slope and aspect), and past disturbance were used as explanatory variables in species group models. The final model is as a classical matrix management-oriented model with an ecological touch and maximum size-dependent parameters of ingrowth and outgrowth. The model provides a tool to simulate stand performance after logging and to assess silvicultural prescriptions before they are applied in these types of forests. The simulations indicate that full recovery following a logging intensity where 47% of the overstorey basal area is removed with a checkerboard of logged and unlogged patches (group selection) on a 120-year cycle could enable sustainable timber production without compromising the ecological integrity in these forests. Following single-tree selection (33%), recovery takes about 150 years, and more intensive harvesting practices where 50-78% overstorey basal area is removed may take 180-220 years to recover. Pre-harvest climber cutting coupled with poisoning of non-timber species followed by intensive logging of merchantable trees would allow logging on a 300-year cycle. Shorter logging cycles may lead to changes in the forest structure and floristic composition where the overall species density is low with higher density of shade intolerant species.

Modelling dynamics including recruitment, growth and mortality for sustainable management in uneven-aged mixed-species rainforests

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Modelling dynamics including recruitment, growth and mortality for sustainable management in uneven-aged mixed-species rainforests

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