Rain Forest Tree Seedlings Research Articles

Organic nitrogen improves the water use of tropical tree seedlings cultivated for restoration plantings

Societal Impact StatementRestoring degraded tropical lands is important for biodiversity protection and human livelihoods. Newly planted tree seedlings are often challenged by drought brought about by climate change. Here, we explored how nitrogen sources used for plant growth affected the water use of tropical tree seedlings under water limitation. We found that the application of the amino acid arginine reduced water use in the studied seedlings under water limitation, compared with the conventional ammonium nitrate fertiliser. Alternative sources of nitrogen should be considered for tree seedling production in nurseries as this could enhance drought resilience traits and improve the survival of seedlings in restoration plantings.Summary Restoration via tree planting is impacted by climate change‐induced water scarcity. Nitrogen (N) supply modulates the morphology and physiology of plants and impacts water use. We compared the responses of rainforest tree seedlings Acacia mangium and Alphitonia petriei grown with inorganic N (Osmocote™, ammonium nitrate) or organic N (Argrow™, liquid arginine), hypothesising that organic N confers drought resilience by increasing water use efficiency (WUE). Seedlings were grown in a glasshouse for 12 weeks with organic or inorganic N in well‐watered conditions, and then half the seedlings were subjected to water limitation for a further 4 weeks. A. mangium grew equally well on all N sources, but water limitation reduced biomass production. In contrast, N sources but not water regimes influenced biomass production in A. petriei. Under water limitation, arginine‐supplied A. petriei had higher WUE and more depleted leaf δ13C than inorganic N‐supplied plants. Our results suggest that organic N in the form of arginine can regulate stomatal conductance in A. petriei to convey drought resilience in seedlings. The generality of these findings should be explored to evaluate if organic N is a feasible source for generating drought‐resilient seedlings for restoration plantings.

Does root respiration in Australian rainforest tree seedlings acclimate to experimental warming?

Plant respiration can acclimate to changing environmental conditions and vary between species as well as biome types, although belowground respiration responses to ongoing climate warming are not well understood. Understanding the thermal acclimation capacity of root respiration (Rroot) in relation to increasing temperatures is therefore critical in elucidating a key uncertainty in plant function in response to warming. However, the degree of temperature acclimation of Rroot in rainforest trees and how root chemical and morphological traits are related to acclimation is unknown. Here we investigated the extent to which respiration of fine roots (≤2mm) of four tropical and four warm-temperate rainforest tree seedlings differed in response to warmer growth temperatures (control and +6 °C), including temperature sensitivity (Q10) and the degree of acclimation of Rroot. Regardless of biome type, we found no consistent pattern in the short-term temperature responses of Rroot to elevated growth temperature: a significant reduction in the temperature response of Rroot to +6 °C treatment was only observed for a tropical species, Cryptocarya mackinnoniana, whereas the other seven species had either some stimulation or no alteration. Across species, Rroot was positively correlated with root tissue nitrogen concentration (mgg-1), while Q10 was positively correlated with root tissue density (gcm-3). Warming increased root tissue density by 20.8% but did not alter root nitrogen across species. We conclude that thermal acclimation capacity of Rroot to warming is species-specific and suggest that root tissue density is a useful predictor of Rroot and its thermal responses in rainforest tree seedlings.

Environmental impacts of abrasive blasting of transmission towers in protected areas

In Australia, and other parts of the world, tower infrastructure in electricity transmission networks are nearing the end of their asset life. In changing economic, political and regulatory environments Transmission Network Service Providers are implementing new approaches to asset management and reinvestment, such as refurbishment to extend the life of existing assets, instead of replacement. As part of these refurbishment efforts, abrasive blasting and recoating is being employed to remove corrosion and extend the life of steel electricity transmission towers. New controls and procedures have been developed to manage the most likely impacts associated with the abrasive blasting of transmission towers. However, little or no data have been available on the environmental impacts of abrasive blasting or the effectiveness of management procedures currently being used to mitigate potential adverse environmental impacts.We conducted an integrated study on the impacts of abrasive blasting, which brought together on-site research; modelling; and controlled laboratory trials. The study was undertaken during a transmission tower refurbishment project within the World Heritage listed Wet Tropics Region in Queensland, Australia. Measured metal deposition around towers due to blasting, was primarily as large particles (>PM10) at 12–30 m from the tower. Soil concentrations of metals were highest under towers, with a small number of samples showing elevated zinc at 12–30 m. The presence of spent abrasive media and dust on the geofabric material used under the towers and up to 15 m from the tower base, as part of control measures used to contain the abrasive products, indicates that deposition also occurs between 0 and 12 m from the tower.The potential impacts of the abrasive blasting technique on plants and invertebrates appear to be low. Five species of tropical rainforest tree seedlings exposed to abrasive blasting dust at worst-case levels had no negative impact on physiological performance or plant health. This research will assist Transmission Network Service Providers and other operators of corroded linear infrastructure to plan and implement mitigating management actions and procedures during abrasive blasting projects and assist regulators and the community to better understand the impacts of the practice.

Insect herbivory on seedlings of rainforest trees: Effects of density and distance of conspecific and heterospecific neighbors.

Natural enemies of plants such as insect herbivores can contribute to structuring and maintaining plant diversity in tropical forests. Most research in this area has focused on the role of specialized enemies and the extent to which herbivory on individual plant species is density‐dependent. Relatively few insect herbivores specialize on a single host plant species. Insect herbivores that feed on more than one plant species may link the regeneration dynamics of their host species through “apparent competition” or “apparent mutualism.” We investigated herbivory and survival of seedlings of two tropical tree species (Cordia alliodora and Cordia bicolor) in the forests of Barro Colorado Island (Panama). We used experiments and observations to assess seedling fate in relation to the presence of conspecifics and heterospecifics across a range of spatial scales. Herbivory significantly increased seedling mortality and was highest at high local densities of C. alliodora seedlings. There was also evidence that high local densities of C. alliodora increased herbivory on co‐occurring C. bicolor seedlings. Synthesis. The elevated rates of seedling herbivory at high densities of conspecifics documented in our study are consistent with the predictions of the Janzen–Connell hypothesis, which explains how so many plant species can coexist in tropical forests. Our data also highlight the possibility that herbivore‐mediated density‐dependence, facilitated by herbivores that feed on multiple plant species, can also occur across plant species. Enemy‐mediated indirect effects of this sort have the potential to structure plant communities.

Nutrient-uptake and -use efficiency in seedlings of rain-forest trees in New Caledonia: monodominants vs. subordinates and episodic vs. continuous regenerators

Abstract:The upper canopy of some rain forests in New Caledonia is dominated by single species. These monodominants are commonly secondary species, their dominance not persisting without disturbance. We tested whether dominance is associated with efficient uptake and use of nutrients (N, P and K), comparing between seedlings of monodominants (Nothofagus spp., Arillastrum gummiferum and Cerberiopsis candelabra) and 14 subordinates, grown in a nursery house. We also tested whether this trend applies more broadly to shade-intolerant trees that regenerate episodically (ER species) versus shade-tolerant trees that regenerate continuously (CR species). In the sun treatment, monodominants had higher photosynthetic nutrient-use efficiency and productivity for N and K, and uptake efficiency for N, P and K, than subordinates; ER species had higher photosynthetic nutrient-use efficiency for N, P and K, and uptake efficiency for N and P, than CR species. Uptake efficiency and productivity per nutrient mass were uncorrelated across species, yet Nothofagus spp., A. gummiferum and C. candelabra combined high levels of both traits for N, and Nothofagus spp. and A. gummiferum combined moderate to high levels for P, in sun-grown seedlings. This trait combination may contribute substantially to competitiveness and post-disturbance dominance on these nutrient-poor soils.

Light and nutrient effects on the growth and photosynthesis of two Bolivian rain forest tree seedlings

Light and nutrient effects on the growth and photosynthesis of two Bolivian rain forest tree seedlings

Growth and biomass allocation in seedlings of rain-forest trees in New Caledonia: monodominants vs. subordinates and episodic vs. continuous regenerators

Abstract:Some species-rich secondary forests in New Caledonia have a monodominant canopy. Here we investigate growth and biomass allocation traits that might explain single-species’ dominance of these post-disturbance stands, and their later decline in the absence of large-scale disturbance. Seedlings of 20 rain-forest trees were grown in two light treatments in a nursery house. In the sun treatment, monodominants grew faster (56.7 ± 1.4 mg g−1 wk−1) than subordinates (40.2 ± 2.6 mg g−1 wk−1). However, some episodically regenerating (ER) subordinates had high growth rates similar to those of monodominants. In the shade treatment, monodominants and subordinates had similar growth rates (33.7 ± 2.6 and 34.0 ± 1.9 mg g−1 wk−1 respectively). Notably, monodominants in both sun and shade treatments had lower root mass fraction (0.29 ± 0.02 and 0.27 ± 0.02 g g−1 respectively) than subordinates (0.39 ± 0.02 and 0.37 ± 0.02 g g−1). Fast growth in sunny conditions is probably imperative for these relatively shade-intolerant ER monodominants. In field conditions, high shoot mass fraction combined with efficient root performance may facilitate faster growth in monodominants competing with other ER species in sunlit sites. Slower growth in shade may contribute to loss of dominance over time in undisturbed forests.

Shade-tolerance of seedlings of rain-forest trees: monodominants vs. subordinates and episodic vs. continuous regenerators

Abstract:Several monodominant rain-forest trees in New Caledonia have population size structures suggesting establishment following large-scale disturbance, with eventual replacement by shade-tolerant species predicted in the absence of future disturbance. Links of dominance and population dynamics to leaf-level photosynthesis were investigated in seedlings of 20 tree species from these forests, grown in experimental sun and shade conditions. In particular, we tested whether episodically regenerating (ER) species, including monodominants, have higher assimilation rates at high irradiances and lower tolerance of shade than continuously regenerating species (CR). ER species had higher maximum net assimilation rates (Amax-area) in sun plants (9.6 ± 0.4 μmol m−2 s−1) than CR species (6.2 ± 0.3 μmol m−2 s−1) and high plasticity, typical of shade-intolerant species, but monodominant species did not differ from other ER species. CR species had leaf-level traits consistent with shade tolerance, including lower dark respiration rates (Rd-area = 0.47 ± 0.03μmol m−2 s−1; Rd-mass = 7 ± 1 nmol g−1 s−1) than ER species (Rd-area = 0.63 ± 0.06 μmol m−2 s−1; Rd-mass = 11 ± 2 nmol g−1 s−1) in shade plants. Hence leaf-level assimilation traits were largely consistent with regeneration patterns, but do not explain why some shade-intolerant species can achieve monodominance.

A cost-benefit analysis of acclimation to low irradiance in tropical rainforest tree seedlings: leaf life span and payback time for leaf deployment.

The maintenance in the long run of a positive carbon balance under very low irradiance is a prerequisite for survival of tree seedlings below the canopy or in small gaps in a tropical rainforest. To provide a quantitative basis for this assumption, experiments were carried out to determine whether construction cost (CC) and payback time for leaves and support structures, as well as leaf life span (i) differ among species and (ii) display an irradiance-elicited plasticity. Experiments were also conducted to determine whether leaf life span correlates to CC and payback time and is close to the optimal longevity derived from an optimization model. Saplings from 13 tropical tree species were grown under three levels of irradiance. Specific-CC was computed, as well as CC scaled to leaf area at the metamer level. Photosynthesis was recorded over the leaf life span. Payback time was derived from CC and a simple photosynthesis model. Specific-CC displayed only little interspecific variability and irradiance-elicited plasticity, in contrast to CC scaled to leaf area. Leaf life span ranged from 4 months to >26 months among species, and was longest in seedlings grown under lowest irradiance. It was always much longer than payback time, even under the lowest irradiance. Leaves were shed when their photosynthesis had reached very low values, in contrast to what was predicted by an optimality model. The species ranking for the different traits was stable across irradiance treatments. The two pioneer species always displayed the smallest CC, leaf life span, and payback time. All species displayed a similar large irradiance-elicited plasticity.

Herbivory of tropical rain forest tree seedlings correlates with future mortality

Tree seedlings in tropical rain forests are subject to both damage from natural enemies and intense interspecific competition. This leads to a trade-off in investment between defense and growth, and it is likely that tree species specialized to particular habitats tailor this balance to correspond with local resource availability. It has also been suggested that differential herbivore impacts among tree species may drive habitat segregation, favoring species adapted to particular resource conditions. In order to test these predictions, a reciprocal transplant experiment in Sabah, Malaysia, was established with seedlings of five species of Dipterocarpaceae. These were specialized to either alluvial (Hopea nervosa, Parashorea tomentella) or sandstone soils (Shorea multiflora, H. beccariana), or were locally absent (S. fallax). A total of 3000 seedlings were planted in paired gap and understory plots in five sites on alluvial and sandstone soils. Half of all seedlings were fertilized. Seedling growth and mortality were recorded in regular samples over 3.5 years, and rates of insect herbivore damage were estimated from censuses of foliar tissue loss on marked mature leaves and available young leaves. Greater herbivory rates on mature leaves had no measurable effects on seedling growth but were associated with a significantly increased likelihood of mortality during the following year. In contrast, new-leaf herbivory rates correlated with neither growth nor mortality. There were no indications of differential impacts of herbivory among the five species, nor between experimental treatments. Herbivory was not shown to influence segregation of species between soil types, although it may contribute toward differential survival among light habitats. Natural rates of damage were substantially lower than have been shown to influence tree seedling growth and mortality in previous manipulative studies.

The Influence of Soil Type on Rain Forest Insect Herbivore Communities

ABSTRACTInsect herbivores were collected from five species of dipterocarp tree seedling within a large‐scale reciprocal transplant experiment in Sabah, Malaysia, on alluvial and sandstone soils in both gap and understory plots. The aim was to determine whether the location and ecological specialization of seedlings influenced the herbivore communities found on and around them. Three major groups of folivores were collected: Coleoptera, Orthoptera, and larval Lepidoptera. Herbivory of all species was confirmed through laboratory trials. Herbivore abundance in the understory plots was extremely low relative to the gaps. Rank‐abundance curves were similar on both soil types, differing only within the Lepidoptera. Coleoptera and Orthoptera communities were numerically dominated by a small suite of species capable of feeding on all dipterocarp species tested, whereas lepidopteran communities had both greater species richness and diversity. When corrected for leaf area surveyed, the abundance of Coleoptera was similar on both soil types, while larval Lepidoptera were more abundant in sandstone plots and Orthoptera were more abundant in alluvial plots. Estimated species richness of all three taxa was greater in alluvial forest, but there were contrasting patterns in Simpson diversity and evenness between groups. Species richness of Lepidoptera was greatest on seedlings when grown in their native soil type, providing partial evidence for possible escape effects, although this was not matched by differences in folivore abundance. The link between herbivore communities and herbivory rates on rain forest tree seedlings is complex and is unlikely to be detected through simplistic measures of abundance, species richness, or diversity.

Direct seeding to restore rainforest species: Microsite effects on the early establishment and growth of rainforest tree seedlings on degraded land in the wet tropics of Australia

Reforestation in tropical areas is usually attempted by planting seedlings but, direct seeding (the artificial addition or sowing of seed) may be an alternative way of accelerating forest recovery and successional processes. This study investigated the effects of various sowing treatments (designed to create different microsite conditions for seed germination) and seed sizes on the early establishment and growth of directly sown rainforest tree species in a variety of experimental plots at three sites in the wet tropical region of north-east Queensland, Australia. The different sowing treatments were found to have significant effects on seedling establishment. Broadcast sowing treatments were ineffective and resulted in very poor seedling establishment and high seed wastage. Higher establishment rates occurred when seeds were buried. Seed size was found to be an important factor affecting establishment in relation to micro-site condition. In general, larger seeded species had higher establishment rates at all three sites than species of small and intermediate seed size, but only in sowing treatments where seeds were buried. Overall these results suggest that direct sowing of seed can be used as a tool to accelerate recolonisation of certain rainforest tree species on degraded tropical lands, but initial success will be dependent on the choice of sowing method and its suitability for the seed types selected. The results also indicate that the recruitment of naturally dispersed tree species at degraded sites is likely to be severely limited by the availability of suitable microsites for seed germination. Consequently the natural recovery of degraded sites via seed rain can be expected to be slow and unpredictable, particularly in areas where soil compaction has occurred.

Neighbourhood composition determines growth, architecture and herbivory in tropical rain forest tree seedlings

Summary Interspecific and intraspecific competition in seedling banks may be important determinants of regeneration success in some groups of canopy trees, e.g. dipterocarps in south‐east Asian tropical rain forests, and the outcomes of these competitive interactions may contribute to the maintenance of the high levels of plant diversity. We present the results of an experiment examining the effects of seedling bank diversity within canopy gaps on seedling growth, architecture and levels of insect herbivore defoliation over 18 months. Within each of six experimental canopy gaps, we planted two conspecific blocks of Shorea leprosula (Dipterocarpaceae) seedlings and two heterospecific blocks of S. leprosula seedlings amongst four other dipterocarp species with differing growth rates and palatability to herbivores. The growth allocation patterns of S. leprosula differed according to whether they were planted among conspecifics or heterospecifics. Although S. leprosula seedlings grew taller in conspecific stands, their above ground biomass did not differ between planting treatments, due to increased branching and leaf area in seedlings in the heterospecific stands. Shorea leprosula displayed the lowest levels of antiherbivore defences and sustained the greatest levels of defoliation of all species studied. Despite an increase of almost 60% in leaf damage in conspecific vs. heterospecific stands, neither biomass nor survival of S. leprosula seedlings differed between stands over the 18‐month growth period. The important effects of neighbourhood composition on the growth and herbivory of S. leprosula seedlings, suggest that diversity of seedling banks can be an important determinant of regeneration success and, thus, forest dynamics. Neighbourhood composition may have similar impacts on competitive outcomes and biomass allocation patterns in other ecological communities.

High-Light Stress does not Impair Biomass Accumulation of Sun-Acclimated Tropical Tree Seedlings (Calophyllum longifolium Willd. and Tectona grandis L. f.)

Studies with seedlings of tropical rainforest trees ( Calophyllum longifolium Willd.; Tectona grandis L. f.) were designed to test whether high-light stress affects photosynthetic performance and growth. Seedlings were cultivated in pots at a field site in Central Panama (9 degrees N) and separated into two groups: (1) plants exposed to full solar radiation; (2) plants subjected to automatic neutral shading (48 %) whenever visible irradiance surpassed 1000, 1200, or 1600 micromol photons m-2 s-1. After 2-4 months, chlorophyll fluorescence (Fv/Fm ratio), photosynthetic net CO2 uptake, pigment composition, alpha-tocopherol content of leaves, and plant biomass accumulation were measured. Fully sun-exposed, compared to periodically shaded plants, experienced substantial high-light stress around midday, indicated by photoinhibition of photosystem II and depressed net CO2 uptake. Higher contents of xanthophyll cycle pigments, lutein, and alpha-tocopherol showed an enhancement of photoprotection in fully sun-exposed plants. However, in all experiments, the maximum capacity of net CO2 uptake and plant dry mass did not differ significantly between the two treatments. Thus, in these experiments, high-light stress did not impair productivity of the seedlings studied. Obviously, the continuously sun-exposed plants were capable of fully compensating for any potential costs associated with photoinhibition and repair of photosystem II, reduced CO2 assimilation, and processes of high-light acclimation.

Morphological plasticity of shade‐tolerant tropical rainforest tree seedlings exposed to light changes

Summary Previous studies on tropical rainforest seedlings have demonstrated low physiological plasticity in shade‐tolerant species in response to light, but little is known about the morphological plasticity of such species. We examined morphological traits in seedlings of 10 shade‐tolerant tropical rainforest tree species grown in four combinations of high and low light (10 and 0·8% full daylight, respectively). Plasticity of each trait was quantified using coefficients of variation. Plant traits varied in their plasticity, and could broadly be ranked as follows: stem form > crown traits > root form > leaf form > biomass allocation. Acclimation to an increase in light was generally greater than acclimation to a decrease in light, but crown traits showed significant changes in the high–low treatment. The ranking of species according to plasticity varied depending on the trait examined. Mean plasticity across all light treatments showed no relationship with either dry mass or relative growth rate of dry mass (RGRM) across species. However, plasticity in stem mass fraction (fraction of stem to total dry mass) was negatively correlated with low‐light RGRM across species. Principal components analysis indicated clustering of species depending on trait plasticity. These results suggest different strategies of light response among shade‐tolerant tropical tree seedlings.

Growth and mortality in high and low light: trends among 15 shade‐tolerant tropical rain forest tree species

Summary Past work on tropical rain forest tree seedlings has been dominated by contrasts between strongly light‐demanding and strongly shade‐tolerant species. We examined patterns of growth and mortality among shade‐tolerant tree seedlings in response to light, and investigated the morphological and physiological correlates of high seedling growth and survival rates across species. Seedlings of 15 tree species from Australian tropical lowland forest were grown for up to 1 year in neutral‐density shadehouses at three light levels (10%, 0.8% and 0.2% full daylight). All species showed negligible mortality in the 10% and 0.8% shadehouses, but survival was significantly reduced in 0.2% daylight. Seedling survival rate in 0.2% daylight showed no significant relationship with either the dry mass of seed reserves (embryo plus endosperm), or relative growth rates in dry mass (RGRM) in 0.8% and 10% light. The RGRM values in 0.8% and 10% daylight were strongly positively correlated, and showed a strong negative correlation with the dry mass of seed reserves. Interspecific variation in low‐light RGRM was driven by unit leaf rate (rate of accumulation of dry mass per unit area of leaf), whereas interspecific variation in high‐light RGRM was most closely correlated with leaf area ratio (leaf area per total plant dry mass). Variation in seedling characters in response to light may have important implications for the coexistence of shade‐tolerant tropical tree species.

Soil dilution as a surrogate for root competition: effects on growth of seedlings of Australian tropical rainforest trees

Summary Previous studies on the combined effects of light and nutrient supply on the growth rates of plants have involved adding nutrients to soils to levels rarely (or never) met by the plants in nature. As a better test of the likely response of plants to root competition for nutrients, we grew for 1 year seedlings of six species of Australian rainforest tree on forest topsoil undiluted and on soil diluted 50/50, 25/75 and 5/95 with coarse sand. Seedlings were grown in tall tubes in 6 and 9% daylight, equivalent to gap‐edge and gap‐centre conditions. The six species varied in their light requirements for establishment and onward growth. One species grew significantly and markedly more quickly on soil diluted 50/50 with sand (Elaeocarpus grandis). Improved aeration is the most likely explanation. The extent of inhibition by soil dilution correlated positively with the absolute amount of new dry mass accreted in one year, whether the larger accretion resulted from a combination of high relative growth rate (RGR) and small embryo size, or moderate RGR and larger embryo size. Soil dilution generally reduced leaf mass fraction and leaf area ratio, but had no significant effect on specific leaf area. In all species the overall concentration of nitrogen in the whole plant declined with soil dilution; in the four species tested, N concentration in leaves declined, and it is likely that the reduction in growth arose partly from a reduced unit leaf rate. Most species showed increases in the concentrations of P, K, Ca and Mg with soil dilution, or no change, but in Diploglottis bracteata the concentrations of Ca and Mg, especially in the older leaves, declined markedly with soil dilution, and one of these cations could have been limiting growth. Extrapolation of the results to the field suggests that in single‐treefall gaps the fastest growing species will the most inhibited by root competition. The soil‐dilution technique is likely to be more appropriate than nutrient addition as an indicator of sensitivity to root competition on a given soil.

Effects of above- and Belowground Competition on Growth and Survival of Rain Forest Tree Seedlings

Because reducing aboveground competition for photosynthetically active radiation (PAR) increases growth and survival rates of tropical rain forest seedlings, belowground competition for nutrients is often assumed to be of little or no importance. We tested this assumption. We measured the growth, over one year, of seedlings of Aspidosperma carapanauba (shade tolerator) and Dinizia excelsa (light demander) transplanted into understory (high aboveground competition; 1% incoming photosynthetic photon flux density [PPFD]) and single treefall gap areas (lower aboveground competition; 6% PPFD in centers of gaps) without and with trenches cut around plots (high and low belowground competition). Over the wetter six months, location in gaps significantly increased relative height growth rates by 320% and 570%, and relative leaf production rates by 190% and 280%, while trenching significantly increased height by 330% and 520%, and leaf production by 170% and 260%, for Aspidosperma and Dinizia, respectively, with significant interactions between treatments. Trenching approximately doubled height growth in the understory. Leaf loss rates were unaffected by either treatment but were 3.7 times higher in Dinizia compared with Aspidosperma. Aspidosperma could persist in the understory, as leaf production and loss were balanced at 0.60% PPFD, whereas Dinizia could not, as it required 2.4% PPFD. Reducing either aboveground competition or belowground competition allowed Dinizia to persist as leaf production was increased above unchanging leaf loss rates. Gap and trenching treatments both significantly reduced mortality rates in Dinizia. Location in gaps significantly reduced mortality rates in Aspidosperma. Both species significantly increased allocation to leaves and significantly decreased allocation to roots in response to trenching. The increased growth in the wet season caused by trenching, and increased foliar nitrogen and phosphorus amount, showed that both species were probably competing with adult trees for nutrients as well as PAR. We deduced that phosphorus did not limit growth; however, we could not deduce which nutrient did limit growth. Given that 60–70% of the world's tropical rain forests occur on low-fertility oxisols, intense adult–juvenile competition both aboveground and belowground may be the norm, not the exception.

Differences in light energy utilisation and dissipation between dipterocarp rain forest tree seedlings.

The light environment within tropical rain forests varies considerably both spatially and temporally, and photon flux density (PFD) is considered to be an important factor determining the growth and survival of rain forest tree seedlings. In this paper we examine the ability of four ecologically contrasting dipterocarps (Dryobalanops lanceolata, Shorea leprosula, Hopea nervosa and Vatica oblongifolia) to utilise and dissipate light energy when grown in different light environments in lowland dipterocarp rain forest in the Danum Valley Conservation Area, Sabah, East Malaysia. Specifically we report (i) photosynthetic light response curves and associated fluorescence characteristics, including quantum yield (ΦPSII) and non-photochemical quenching (qN) and (ii) the extent to which photoinhibition occurs when plants grown in either high or low light are exposed to short bursts of high PFD. When grown in low light (artificial or forest shade) all four species had low light saturated rates of photosynthesis which were achieved at low PFDs. In addition, values of ΦPSII and qN were similar over a range of measurement PFDs. D. lanceolata and S. leprosula were also grown at high PFD and showed marked differences in their responses. S. leprosula demonstrated an ability to increase its rate of photosynthesis and there was a small increase in capacity to dissipate excess light energy non-photochemically at high PFDs. Partitioning of this qN into its fast, photo-protective (qE) and slow, photoinhibitory (qI) components indicated that there was an increase in qE quenching. In contrast, although D. lanceolata survived in the high light environment, greater rates of photosynthesis were not observed and the plants showed a greater capacity to dissipate energy non-photochemically. Partitioning of qN revealed that the majority of this increase was attributable to the slower relaxing phases.

Growth, biomass allocation and plant nitrogen concentration in Chilean temperate rainforest tree seedlings: effects of nutrient availability.

Seedlings of nine southern Chilean trees were grown at three nutrient supply rates, to examine the roles of growth rate, biomass distribution and nutrient use traits in determining species natural distributions on resource gradients. Relative growth rate (RGR) showed no overall relationship with species site requirements, although RGR of fertile-site species tended to be more responsive to nutrient supply. In the low-nutrient treatment, infertility-tolerant Fitzroya cupressoides showed a higher RGR rank than a fertility-demanding species (Laurelia philippiana) which outgrew it substantially at the highest supply rate. This reversal of RGR ranks was associated with divergent nutrient use responses: at high nutrient supply both spp. had similar plant nitrogen concentrations (PNC), whereas at the low supply rate Fitzroya's production of biomass per unit of assimilated N was twice that of Laurelia's. However, this pattern does not appear to serve as a general explanation of the respective distributions of the study species, as RGR ranks of most species were unaltered by nutrient supply. At low nutrient availability, no clear differences in shoot:root ratio (SRR) were apparent between poor-site and fertile-site species. However, at high nutrient availability, SRR was markedly higher in the latter, resulting from differences in biomass allocation to stems (not leaves). Leaf area ratios (LAR) were higher in fertile-site species than in those tolerant of low fertility, because of differences in specific leaf area rather than leaf weight ratio. Very high LAR at high nutrient supply was characteristic of most shade-tolerant angiosperms, but not of shade-tolerant conifers. Although PNC showed no overall differences between poor- and fertile-site species, sensitivity of PNC to external supply rate was greatest in two infertility-tolerant conifers. In contrast, the angiosperm Weinmannia trichosperma, although tolerant of low fertility, responded to increased nutrient supply with greatly increased RGR and little change in PNC. Results show little trait convergence between conifers and angiosperms in adaptation both to shade and to infertile soils; i.e. fitness of different taxa in a given environment may hinge on different trait combinations.