Australian Rainforest Tree Species Research Articles

Low-Temperature X-ray Microanalysis Sheds New Light on Mineral Nutrition Aspects of Insect Leaf Galling.

Manipulation of host plant physiology by leaf-galling insects is a multifaceted process. Among fundamental knowledge gaps surrounding this scientifically intriguing phenomenon is the appropriation of plant mineral nutrients and moisture for galling advantage. Small, soluble mineral ions and watery cell contents in dense gall tissues risk disruption during routine sample preparations. In this study, an X-ray microanalysis was applied to investigate gall mineral nutrition. Morphologically diverse leaf galls were sampled from three Australian rainforest tree species. Using cryo-analytical scanning electron microscopy, real-time X-ray analytical maps of cellular mineral nutrients and water were integrated with anatomical images of gall and leaf cross-sectional surfaces. A comparison of host-leaf and gall anatomies bore direct evidence of drastic changes to leaf cells through the galling process. Distinct "wet" and "dry" regions within galls were anatomically and/or chemically differentiated, suggesting specific functionality. "Wet" regions comprising hydrated cells including soft gall-cavity linings where larvae are known to feed contained soluble plant mineral nutrients, while C-rich "dry" tissues largely devoid of mineral nutrients likely contribute structural support. Mapping immobile nutrients such as Mn may provide a means of "matching" specific gall cell types to those in ungalled host-leaf tissues. The findings here provided otherwise inaccessible insights into leaf-gall mineral nutrition.

Delayed greening during leaf expansion under ambient and elevated CO2 in tropical tree seedlings

AbstractTropical tree species employ varying strategies in young leaves to minimise losses to herbivory. The young leaves of species with delayed greening are thought to be less visible to herbivores, but likely incur a cost to leaf‐level carbon gain via lower photosynthetic rates during leaf development. Increasing atmospheric CO2 concentrations may reduce the cost of delayed greening, and/or modify leaf expansion rates, altering the ecological interaction of species. In this study, we evaluated the effects of elevated CO2 concentrations on physiological responses of three Australian rainforest tree species, two with delayed greening in young leaves. Net photosynthesis rates were significantly lower in recently flushed leaves of species with delayed greening than in the species with normal greening. Yet, surprisingly, total nitrogen concentrations were higher in the former than in the latter. Intrinsic water‐use efficiency increased at a steeper rate during leaf development under elevated CO2 in all three species, irrespective of greening strategy. Leaf growth rate, in terms of area expansion, did not increase under elevated CO2 in any of the species. Although elevated CO2 did not boost the performance of the delayed greening species more than that of the normal greening species, we found higher nitrogen concentrations in their young leaves. This supports the intriguing possibility that delayed greening may have evolved in these species to protect young leaves that are especially rich in nitrogen.

Growth and physiological response of six Australian rainforest tree species to a light gradient

An understanding of growth and photosynthetic potential of subtropical rainforest species to variations in light environment can be useful for determining the sequence of species introductions in rainforest restoration projects and mixed species plantations. We examined the growth and physiology of six Australian subtropical rainforest tree species in a greenhouse consisting of three artificial light environments (10%, 30%, and 60% full sunlight). Morphological responses followed the typical sun-shade dichotomy, with early and late secondary species ( Elaeocarpus grandis, Flindersia brayleyana, Flindersia schottiana, and Gmelina leichhardtii) displaying higher relative growth rate (RGR) compared to mature stage species ( Cryptocarya erythroxylon and Heritiera trifoliolatum). Growth and photosynthetic performance of most species reached a maximum in 30–60% full sunlight. Physiological responses provided limited evidence of a distinct dichotomy between early and late successional species. E. grandis and F. brayleyana, provided a clear representation of early successional species, with marked increase in A max in high light and an ability to down regulate photosynthetic machinery in low light conditions. The remaining species ( F. schottiana, G. leichhardtii, and H. trifoliolatum) were better represented as falling along a shade-tolerant continuum, with limited ability to adjust physiologically to an increase or decrease in light, maintaining similar A max across all light environments. Results show that most species belong to a shade-tolerant constituency, with an ability to grow and persist across a wide range of light environments. The species offer a wide range of potential planting scenarios and silvicultural options, with ample potential to achieve rapid canopy closure and rainforest restoration goals.

Variability of Mn hyperaccumulation in the Australian rainforest tree Gossia bidwillii (Myrtaceae)

This study examines the heterogeneity of the Mn-hyperaccumulative trait in natural stands of the Australian rainforest tree species Gossia bidwillii (Myrtaceae). It is the only known Mn hyperaccumulator from Australia, and has an unusual spatial distribution of Mn in its leaves. G. bidwillii occurs naturally on a range of Mn-containing substrates including ultramafic soils. Leaf samples were collected from individual trees and four small stands, over a longitudinal range of ∼600 km. While no variation in the spatial distribution of foliar Mn was detected, considerable variation in Mn concentration was found. G. bidwillii was shown to accumulate Mn when growing on a variety of substrates, and dry weight (DW) foliar Mn concentrations of all trees sampled ranged between 2,740 and 27,470 μg g−1. The majority of samples exceeded 10,000 μg g−1, the threshold value for Mn hyperaccumulation. The overall frequency distribution of foliar Mn concentration was found to be bimodal, with a small outlier of extreme hyperaccumulators. Highest values were obtained from trees growing on a basaltic krasnozem clay, not ultramafic soil. Soil Mn concentrations were measured, and no relationship was found between foliar Mn concentrations and extractable Mn concentrations in host substrates. Some of the variation in the Mn-hyperaccumulative trait in G. bidwillii throughout its large natural distribution may reflect the unresolved taxonomy of this most widespread species in the genus Gossia. Ability to hyperaccumulate Mn may serve as an additional diagnostic tool for resolving this taxonomy.

Cyanogenic glycosides from the rare Australian endemic rainforest tree Clerodendrum grayi (Lamiaceae)

The cyanogenic diglycoside lucumin (( R)-mandelonitrile-β- d-primeveroside) and monoglucoside prunasin (( R)-mandelonitrile-β- d-glucoside) were isolated from the foliage of the rare Australian rainforest tree species Clerodendrum grayi (Lamiaceae). This is the first reported isolation of the diglycoside lucumin from vegetative tissue (foliage), and the first reported co-occurrence of lucumin and prunasin. Furthermore, unusually, the diglycoside lucumin was the most abundant cyanogen accounting for approximately 60% of total cyanide in a leaf tissue.

Do temperate rainforest trees have a greater ability to acclimate to changing temperatures than tropical rainforest trees?

• Photosynthetic responses to acclimation temperature were investigated in seedlings of eight Australian rainforest tree species. Australian rainforests extend over 33° of latitude, providing an opportunity to compare temperature responses of temperate and tropical species. • Net photosynthesis was measured in leaves developed under a constant (22°C:14°C) or fluctuating (17°C:9°C-27°C:19°C) day/night temperature regime. These leaves were then subjected to a series of constant temperature regimes and net photosynthesis was measured 14d after acclimation to each new regime. • Acclimation potential was not affected by the contrasting temperature regimes. The temperate species showed at least 80% of maximum net photosynthesis over a larger span of acclimation temperature than the tropical species. • The lack of an effect of the contrasting temperature regimes on acclimation potential may reflect either that adjustments were unnecessary for temperate species, which already have broad photosynthetic responses to temperature, and tropical species were incapable of adjustments, or that in general species respond to the mean temperature regime and not to the amount of fluctuation in the regime. The higher acclimation potential shown by the temperate species is consistent with the larger seasonal and day-to-day variation in temperature of the temperate climate compared with the tropical climate.

The dynamics of photosynthetic acclimation to changes in light quanlity and quality in three Australian rainforest tree species.

Photosynthetic acclimation was studied in seedlings of three subtropical rainforest species representing early (Omalanthus populifolius), middle (Duboisia myoporoides) and late (Acmena ingens) successional stages in forest development. Changes in the photosynthetic characteristics of pre-existing leaves were observed following the transfer of plants between deep shade (1-5% of photosynthetically active radiation (PAR), selectively filtered to produce a red/far-red (R/FR) ratio of 0.1) and open glasshouse (60% PAR and a R/FR ratio of 1.1-1.2), and vice versa. The extent and rate of response of the photosynthetic characteristics of each species to changes in light environment were recorded in this simulation of gap formation and canopy closure/overtopping. The light regimes to which plants were exposed produced significant levels of acclimation in all the photosynthetic parameters examined. Following transfer from high to low light, the light-saturated rate of photosynthesis was maintained near pre-transfer levels for 7 days, after which it decreased to levels which closely approximated those in leaves which had developed in low light. The decrease in photosynthetic capacity was associated with lower apparent quantum yields and stomatal conductances. Dark respiration was the parameter most sensitive to changes in light environment, and responded significantly during the first 4-7 days after transfer. Acclimation of photosynthetic capacity to increases in irradiance was significant in two of the three species studied, but was clearly limited in comparison with that of new leaves produced in the high light conditions. This limitation was most pronounced in the early-successional-stage species, O. populifolius. It is likely that structural characteristics of the leaves, imposed at the time of leaf expansion, are largely responsible for the limitations in photosynthetic acclimation to increases in irradiance.

The effect of light quantity and quality during development on the photosynthetic characteristics of six Australian rainforest tree species.

Seedlings of six subtropical rainforest tree species representing early (Omalanthus populifolius, Solanum aviculare), middle (Duboisia myoporoides, Euodia micrococca) and late (Acmena ingens, Argyrodendron actinophyllum) successional stages in forest development were grown in a glasshouse, under four levels of neutral shade (60%, 15%, 5%, 1% of photosynthetically active radiation (PAR) in incident sunlight) and three levels of selectively filtered shade (producing 15%, 5%, 1% of PAR). This design served to analyse the interactions between reduced photon flux density (PFD) and reduced red/far-red (R/FR) ratio in their effects on selected photosynthetic characteristics of each species. The light-saturated rate of photosynthesis was significantly influenced by growth irradiance in five of the six species, with all of these showing a non-linear decrease in maximum assimilation rate from 60% down to 1% PAR. The degree of acclimation to this range was not clearly related to the successional status of the species. Dark respiration was more sensitive to growth irradiance in the early- and mid-stage species than in the late-stage species. Although levels of dark respiration were clearly greater in leaves of early- and mid-stage species from the highest light levels, differences between successional groups were negligible at 1% PAR. Growth in filtered shade, typical of that beneath a closed canopy, resulted in lower photosynthetic capacities and quantum yields in those species which did respond. Although dark respiration rates were more sensitive to filtered shade in the early-stage than in the late-stage species, there was no evidence from other gas exchange characteristics to suggest that overall sensitivity to light quality (as characterised by the R/FR ratio) is greater in early successional-stage species.

Acclimation to a Change in Light Regime in Seedlings of Six Australian Rainforest Tree Species

Acclimation potential of seedlings of six non-pioneer, North Queensland rainforest tree species (Diploglottis diphyllostegia, Flindersia brayleyana, Dysoxylum schgneri, Prunus turnerana, Neisosperma poweri and Castanospora alphandii) were evaluated in three different light regimes. The seedlings were grown from seeds and raised in 37, 10 and 2.5% photosynthetically active radiation (PAR) of full sunlight in a shade house. Plants were harvested at 1, 4, and 15 months after germination. At 4 months, for each species, one-third of the seedlings in each light environment were transferred to each of the two other light environments. The remaining one-third stayed in the same light environment and formed the control. The control seedlings of all species showed a marked response to increasing PAR: relative growth rate (RGR) and total biomass were low in the 2.5% PAR level, increased in the 10% PAR level and were maximal in the 37% PAR level, except for Neisosperma. The seedlings in the 25% PAR level showed a typical shade plant morphology with a high leaf area ratio, low root-shoot ratio and low specific leaf weight. For seedlings of Dysoxylum and Prunus, a change in light regime from 37% PAR to 2.5% PAR resulted in negative relative growth rates. Eleven months after transfer, many growth characteristics still showed significant initial × final light environment interactions, an indication of incomplete adjustment. Acclimation to increasing light availability was faster than acclimation to decreasing light availability. On the basis of biomass allocation patterns (root-shoot ratio, leaf area ratio and specific leaf weight), the six species could not be differentiated under the three light regimes and their dynamics. However, using RGR as a relative measure of carbon economy, it was possible to differentiate the species in their acclimation ability to decreased but not to increased irradiance. It was concluded that, for non-pioneer species, acclimation potential may be more related to physiological than to morphological plasticity.

Relationships between leaf growth and holes caused by herbivores

There are two approaches to the estimation of herbivory in a canopy. One is to measure the absolute amount consumed by herbivores in terms of bites of leaf tissue removed and express defoliation as metres squared of leaf surface area (or kilogram of photosynthetic tissue). This approach is useful for examining the population dynamics and feeding behaviour of the insects or, in a sense, to approach herbivory from a herbivore's perspective. The second is to measure the proportion of leaf area missing, usually expressed as a percentage of total leaf surface area. The latter approach was used for long-term studies of herbivory in Australian rainforests (Lowman 1982, 1984, 1985). and is currently in use for similar studies on eucalypt canopies (Lowman & Heatwole 1987, unpubl. data). One of the most frequently asked questions regarding defoliation measurements expressed as percentage leaf area losses is: 'Does the proportion of area of defoliation stay the same as a leaf expands?' Many herbivores exhibit marked preference for young leaf tissue (Lowman & Box 1983; Selman & Lowman 1983; Wint 1983). A direct measurement (cm2) of holes in mature leaves, therefore. may be an overestimation of the actual amount ingested by herbivores that fed upon young, partially expanded leaves. A proportional estimate of defoliation will remain accurate throughout the life of a leaf, however, if leaf holes expand at the same rate as leaf area. This was tested on young leaves of one species of a northern temperate forest. Liriodendron tullpifera L. (albeit with deciduous, and unusual tulip-shaped leaves), and its holes expanded at a rate proportional to leaf growth (Reichle et al. 1973). In this study, a similar test was performed for a range of Australian rainforest tree species exhibiting different leaf shapes, textures, and growth habits.

Temporal and spatial variability in insect grazing of the canopies of five Australian rainforest tree species

AbstractHerbivory was measured monthly for 2 years on leaves of permanently marked replicate branches in the canopies of five Australian rainforest tree species. Variability in insect grazing activities was evident with respect to several factors: leafage — young leaves were preferred over older leaf tissue: height — leaves closer to ground level were more heavily grazed: light — shade leaves were preferred to sun leaves: time — grazing was intense during spring and summer months, and almost negligible during autumn and winter, but was cumulatively similar between the 2 years: space — grazing was extremely variable on small spatial scales such as between individual leaves and branches, but similar where hundreds of leaves were pooled on larger scales between individual canopies and among geographically different sites: tree abundance — grazing was heavier at sites where a tree species was common than where it was rare: host tree species. Long term observations resulted in higher but more accurate estimates of herbivory since it was possible to quantify losses of leaves totally eaten, an event not accounted for infield methods of discrete sampling whereby leaves are merely harvested and measured for area missing. Variability in herbivory is discussed in terms of plant‐insect phenologies, plant defences, successional status of tree species, and insect behaviour.

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species.

Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO2 partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO2 partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.