Tropical Rainforest Canopy Tree Research Articles

Is vulnerability segmentation at the leaf-stem transition a drought resistance mechanism? A theoretical test with a trait-based model for Neotropical canopy tree species

Key messageLeaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stem. However, although it has been intensively investigated these past decades, the extent to which vulnerability segmentation promotes drought resistance is not well understood. Based on a trait-based model, this study theoretically supports that vulnerability segmentation enhances shoot desiccation time across 18 Neotropical tree species.ContextLeaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stems thereby preserving expensive organs such as branches or the trunk. Although vulnerability segmentation has been intensively investigated these past decades to test its consistency across species, the extent to which vulnerability segmentation promotes drought resistance is not well understood.AimsWe investigated the theoretical impact of the degree of vulnerability segmentation on shoot desiccation time estimated with a simple trait-based model.MethodsWe combined data from 18 tropical rainforest canopy tree species on embolism resistance of stem xylem (flow-centrifugation technique) and leaves (optical visualisation method). Measured water loss under minimum leaf and bark conductance, leaf and stem capacitance, and leaf-to-bark area ratio allowed us to calculate a theoretical shoot desiccation time (tcrit).ResultsLarge degrees of vulnerability segmentation strongly enhanced the theoretical shoot desiccation time, suggesting vulnerability segmentation to be an efficient drought resistance mechanism for half of the studied species. The difference between leaf and bark area, rather than the minimum leaf and bark conductance, determined the drastic reduction of total transpiration by segmentation during severe drought.ConclusionOur study strongly suggests that vulnerability segmentation is an important drought resistance mechanism that should be better taken into account when investigating plant drought resistance and modelling vegetation. We discuss future directions for improving model assumptions with empirical measures, such as changes in total shoot transpiration after leaf xylem embolism.

Sensitivity of stomatal conductance to vapor pressure deficit and its dependence on leaf water relations and wood anatomy in nine canopy tree species in a Malaysian wet tropical rainforest

Canopy tree species with a large mean xylem vessel diameter has a high sensitivity of stomatal conductance to vapor pressure deficit in a wet tropical rainforest in Malaysia. We investigated the diurnal variation of stomatal conductance to water vapor (gs) and leaf water relations in nine tropical rainforest canopy tree species in Malaysia to investigate the sensitivity of stomatal conductance to VPD and its dependence on leaf water relations. We focused on the effect of wood anatomical properties on the stomatal sensitivity to VPD. Negative correlations were obtained between leaf-to-air VPD (VPDleaf) and gs for all species, in which interspecific variation was obtained for the sensitivity of gs to VPDleaf with the range of VPDleaf being 1.2–5.9 kPa. The species with large mean xylem vessel diameter had high sensitivity of gs to VPDleaf. Although significant variations were obtained among the species for leaf water relations, such as relative water content at the turgor loss point (RWCtlp) and water potential at the turgor loss point (Ψtlp), these water relations were not related to the stomatal sensitivity to VPDleaf. In the wet tropical forest in the present study, VPDleaf was the main regulator of gs in the nine tree species. The high sensitivity of gs to VPDleaf in the trees with a large vessel area may contribute to the maintenance of an adequate leaf water status, e.g., high levels of Ψleaf, throughout the day, across a period of varying air temperature.

Influence of Seasonal Variations in Soil Water Availability on Gas Exchange of Tropical Canopy Trees

AbstractSeasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree‐based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions.

Microhabitat specialization of tropical rain-forest canopy trees in the Sovi Basin, Viti Levu, Fiji Islands

Abstract:Island biotas often have lower species diversity and less intense competition has been hypothesized as a result. This should result in lower habitat specificity compared with mainland habitats due to larger realized niches. We investigate microhabitat associations of canopy trees with regard to differences in topography on an oceanic island (Viti Levu, Fiji) using twenty 10 × 60-m plots. We find high tree-species diversity (112 species with dbh ≥ 10 cm in a total of 1.08 ha) and high endemism (c. 60%), compared with other islands in Western Polynesia. Our sample plots aggregate into three distinct groups that are mostly defined by micro-topography: (1) ridges and steep slopes (well-drained sites), (2) moderate slopes and ridge flats (moderate drainage), and (3) flats (poor drainage). Associations with microhabitat are found for more than 50% of the 41 most common species but only one species is apparently restricted to a single habitat. These findings are similar to other rain forests and demonstrate considerable niche differentiation among island rain-forest tree species.

The impact of consumption of fruit by vertebrate and invertebrate frugivores on the germination success of an Australian rainforest seed

Abstract The consumption of fruits by vertebrates and invertebrates can be both advantageous or detrimental to the survival of the seeds they contain. This study investigated the effect of fruit size and consumption of fruit pulp by rodents and beetles on the germination of the seeds of Acmena graveolens, a tropical rainforest canopy tree found in northern Australia. As fruit size increased, germination success and the amount of pulp remaining on the fruits was greater. When beetles were absent, germination success was highest when most of the pulp was removed by rodents, suggesting that they removed an inhibitor of germination. When beetles were present, germination success did not differ significantly across pulp categories, so beetles apparently enhanced germination in seeds with little pulp initially removed, possibly by further removal of fruit pulp. In this study, both rodents and beetles enhanced germination success of A. graveolens seeds by consumption of fruit pulp. Acting as facilitators of germination is a relatively unusual role for both these frugivores that are generally considered to act as seed predators or (in the case of rodents) dispersers.

Application of merged 1‐m and 4‐m resolution satellite data to research and management in tropical forests

Summary Until very recently there have been no digital data from satellites for studying events that occur at scales of 10–1000 m2 over large areas (100–100 000 ha). Many phenomena of interest to ecologists, such as impacts of selective logging on forest processes, occur over large extents but at local scales. Here we report results from a pilot project to evaluate through visual interpretation the potential of newly available 1‐m panchromatic and 4‐m multi‐spectral data from the IKONOS satellite, for studying forest structure, dynamics and logging impacts in logged and old‐growth tropical moist forest. The study area, the Mil Madeireira Itacoatiara Ltda site of Precious Woods Amazon, near Itacoatiara, Amazonas, Brazil, is managed using reduced‐impact logging practices to minimize environmental impacts, and thus represents a lower bound for logging impacts in tropical rain forests. The IKONOS image was georeferenced using uncorrected global positioning system (GPS) locations for 10 control trees whose crowns were clearly visible in the image. The root mean square error (RMSE) of the geometric transformation was 4 m, while the mean crown diameter of 50 randomly chosen trees in old‐growth forest was 9·4 m. The fact that the RMSE was less than half the average crown diameter implies that it will usually be possible to locate from the ground crowns that are distinct on the image, given sufficiently accurate GPS locations. IKONOS data are well suited for evaluating and monitoring logging impacts. Many impacts of logging were clearly observable in the image, including major and some minor roads, logging patios and larger logging gaps. Smaller extraction roads and logging gaps were not observable. Many individual trees were distinct on the IKONOS image, indicating that it is now feasible to conduct demographic studies of tropical rain forest canopy trees based on repeated satellite observations. Linking these remotely sensed data to ground data will require improved GPS positions, because it is currently difficult to obtain accurate GPS readings in tropical rain forest understoreys. Synthesis and applications. IKONOS 1‐m and 4‐m data were found to be useful for identifying individual trees as well as some logging management features in a tropical moist forest in central Amazonia. These data will have many applications for research and management of intervened and old‐growth tropical forests, including planning and assessment of logging activities, as well as monitoring adherence to certification criteria such as those of the Forest Stewardship Council. Rapid development of these applications will come from building on existing data on forest structure and function, and by fostering collaborations between remote sensing scientists, ecologists and natural resource managers.

Photoinhibition of Photosynthesis in Leaves of two Developing Stages of a Tropical Rainforest Canopy Tree, Pometia Tomentosa

Photoinhibition of Photosynthesis in Leaves of two Developing Stages of a Tropical Rainforest Canopy Tree, Pometia Tomentosa

Long-Term Survivorship and Crown Area Dynamics of Tropical Rain Forest Canopy Trees

Long-Term Survivorship and Crown Area Dynamics of Tropical Rain Forest Canopy Trees

ASSESSING THE GROWTH OF TROPICAL RAIN FOREST TREES: ISSUES FOR FOREST MODELING AND MANAGEMENT

Growth performance was assessed for a diverse suite of canopy and emergent tree species in a lowland neotropical rain forest (the La Selva Biological Station, northeastern Costa Rica). Species were evaluated based on annual diameter measurements of large samples of individuals in all post-seedling size classes, over a 12-yr period. The study species were seven non-pioneers (Minquartia guianensis, Lecythis ampla, Hymenolobium mesoamericanum, Simarouba amara, Dipteryx panamensis, Pithecellobium elegans, and Hyeronima alchorneoides) and two pioneers (Cecropia obtusifolia and C. insignis). For each species, inherent growth capacity through ontogeny was estimated as the mean of the five largest annual increments (from different individuals) in each juvenile and adult size class. At all tree sizes, species differed highly significantly in this measure. In the small sapling sizes (≤4-cm diameter), the two pioneers showed markedly higher maximum growth than all the non-pioneers, in line with current understanding. This growth advantage, however, was quickly lost with increasing tree size. For eight of the nine species, the ontogenetic patterns of maximal growth closely corresponded to species groupings previously based on juvenile microsites (forest stage, light levels). Average growth rates, however, which are the combined result of environmental and biotic factors, did not show this correspondence. The groups based on microsite occupancy and growth potential provide a starting point for treating the great numbers of species of tropical rain forest trees as many fewer functional types, a sine qua non for modeling these complex forests. The exceptional growth capacity of one of our focal non-pioneer species (Simarouba) suggests that it represents a hitherto unrecognized functional type among tropical rain forest trees. The data from this long-term study indicate that several assumptions of current forest-dynamics models are not applicable to trees of this biome. For most species (1) diameter growth during regeneration was not correlated with tree size; (2) adult sizes were not characterized by a constant maximal basal-area increment; and (3) growth capacity did not decline toward zero as trees approached maximum size. Evaluation of overall increment distributions revealed a broad adult size range of exclusively high growth in the emergent Hymenolobium; for two additional species, sectors of the adult size range showed an absence of very low or negative growth. At most sizes, however, all nine species displayed the full range of growth increments, from slightly negative to observed maxima. Describing the performance of tropical rain forest canopy trees will require similar analysis of growth data obtained from many individuals and intervals, and stratified across juvenile and adult life stages. More realistic forest-dynamics models will depend on such evaluations of many tree species, in diverse tropical rain forests. Such models are greatly needed, both for designing management for this biome and for predicting its responses to changing climate and atmospheric conditions.

Redefining the ecological niche of a tropical rain forest canopy tree species using airborne imagery: long-term crown dynamics of Toona ciliata

Past controlled growth experiments indicate that the seedling and sapling responses of the tropical rainforest canopy tree species Toona ciliata are most consistent with a light-demanding, early successional pioneer. This ecological niche assignment was tested in the mature stage of its life cycle after it achieves a position in the upper canopy. Mortality, survivorship and crown growth rates over the 18-y period 1976–1994 were measured using co-registered repeat airborne stereophotographic coverage of a representative forest stand in northeast Queensland, Australia, where T. ciliata had the fourth highest relative importance in a population of 46 co-occurring canopy tree species. The airborne re-inventory was conducted in a 3.6-ha sample area and limited to only canopy trees. The results were compared with a ground-based inventory of both canopy and subcanopy trees ≥10 cm dbh in a 0.5-ha permanent plot. Over the period 1976–1994, there was no mortality and no evidence of decline among T. ciliata conspecifics having crown areas >60 m2 and trunk diameters >30 cm. In the 3.6-ha airborne sample area, more than 85% of T. ciliata survivors experienced positive crown growth, in contrast to only 57% of the other co-occurring canopy trees. Toona ciliata's crown growth rates were highest in the 60–80-m2 crown size class. Upon reaching an upper canopy position, T. ciliata not only persisted as a dominant canopy tree species, but it also achieved some of the largest crown areas (>100 m2). Toona ciliata mortality in the ground-based plot involved mainly subcanopy trees of 10 – 30 cm dbh that had not yet assumed a canopy position and were not detectable in the aerial stereopairs. Both the crown and dbh growth rates of T. ciliata indicate enhanced vigour in the later stage of its life cycle. Its long-term survivorship and growth patterns are indicative of a persistent canopy tree species that fits the niche of a long-lived, shade-intolerant pioneer. These results demonstrate the usefulness of a long-term airborne database on the life-histories of canopy tree crowns for more clearly defining their ecological niches.

Mortality, Recruitment, and Growth Rates of Montane Tropical Rain Forest Canopy Trees on Mount Bellenden-Ker, Northeast Queensland, Australia

To better understand the physiognomy and dynamics of tropical forest canopy trees, the mortality, recruitment, and growth rates of trees -30 cm DBH were quantified on a montane tropical rain forest hilislope over the 10-yr period 1982-1992. The results indicate that the smaller stature of canopy trees growing on or near ridge-crests of montane rain forests is a consequence of higher turnover rates, not slower growth rates. The annual canopy tree turnover rate of approximately 1.7 percent in the upslope section of the sample area was two times greater than in the downslope section. The DBH growth increments in the upslope and downslope sections also were significantly different (P < 0.05), but it was the upslope trees that grew at the faster rate (upslope = 3.1 mm yr-1; downslope = 2.4 mm yr -). The mean DBH growth increment of 2.7 mm yr-1 for the entire plot is relatively high compared to other montane tropical rain forests. Interspecific variation among the 23 tree species also was notable, with mean DBH increments ranging from 0.4 to 7.8 mm yr-1. Ceratopetalum virchowii and Elaeocarpus ferruginiflorus were the most dominant species in 1982: C. virchowii increased its dominance over the 10-year period with a higher rate of recruitment than mortality and a change in relative importance from 22.7 percent in 1982 to 28.4 percent in 1992; E. ferruginiflorus, in contrast, experienced the most significant change of all the species examined, with a decrease in relative importance from 14.1 percent in 1982 to 5.3 percent in 1992 as a result of its high mortality, its slow growth rates, and its failure to recruit any individuals into the -30 cm DBH size-class; the local demise of this species may be part of a shifting floristic mosaic associated with natural disturbance events or perhaps part of an irreversible trend. It is recommended that more attention be directed to the growth patterns of co-occurring species and the life histories of individual trees to obtain a clearer sense for the long-term dynamics of montane tropical rain forest canopy tree populations.

Aboveground Adventitious Roots and Stemflow Chemistry of Ceratopetalum virchowii in an Australian Montane Tropical Rain Forest

Aboveground adventitious roots emerging from the stems and branches of the montane tropical rain forest canopy tree species, Ceratopetalum virchowii, do not require epiphyte mats to stimulate their growth. The adventitious roots grow in dense clumps on fully exposed aboveground woody surfaces of this species on Mount Bellenden Ker in northeast Queensland, Australia, where mean annual rainfall exceeds 6000 mm. While the soil at the study site is nutrient-poor, the nutrient content of the rainwater draining from the stems of C. virchowii is unusually high compared to the stemflow of other neighboring canopy tree species. Ceratopetalum virchowii is particularly prone to the leaching of Ca2+, Mg2+, K+, and Na+. The aboveground adventitious roots of C. virchowii, therefore, may serve the adaptive function of nutrient recovery before its nutrient-rich stemflow reaches the ground and becomes available to other plants. I hypothesize that the growth of the aboveground adventitious roots of C. virchowii is influenced by the high nutrient content of its stemflow and branchflow drainage, and by the moist condition of its woody surfaces which remain wetted for relatively long periods after rainfalls.

Genetics of Brazil nut (Bertholletia excelsa Humb. &amp; Bonpl.: Lecythidaceae)

Here we report the results of a mating system analysis of an Amazonian population of Bertholletia excelsa, a tropical rain forest canopy tree species. Using progeny data from 29 seed parents, two highly polymorphic isozymes were analyzed to derive single locus and multilocus estimates of outcrossing, based on a mixed mating model. The two single locus estimates were very similar, and both were somewhat smaller than the multilocus estimate, indicating the possibility that the populations are genetically structured. The multilocus outcrossing estimate (tm=0.85±0.03) reveals that outcrossing is prevalent, but that a significantly low level of inbreeding may be occurring. The high outcrossing rate indicates that even though dispersion of individuals is very low within populations of this tropical rain forest tree, pollen dispersal mechanisms appear to be adequate to enable crosses with a relatively large number of potential mates.

Infiltration‐excess caused by Stemflow in a cyclone‐prone tropical rainforest

AbstractTropical rainforest canopy trees that have large projected areas of upwardly inclined branches are capable of funnelling large volumes of rainwater down their trunks. During periods of prolonged heavy rainfall on Mount Bellenden Ker in northeast Queensland, Australia, stemflow volumes were found to be as much as two orders of magnitude greater than the volume of incident rainfall expected in a rain gauge occupying an area equal to the trunk basal area. Stemflow totals ranging from 6000 to 70000 litres were generated by individual trees from 7800 mm of rainfall over two successive wet seasons.The combination of high intensity rainfall and the funnelling effect results in significant quantities of infiltration‐excess at the ground surface. Stemflow fluxes as high as 31.4 cm3 min−1 per cm2 of basal area (i.e. the equivalent of 314 mm min−1) were recorded when rainfall intensity was only 2 mm min−1. The mean infiltration capacity of the topsoil was determined to be 6.2 mm min−1. The areas over which the stemflow would have had to spread in order to infiltrate were computed to be as much as 3 m2 around the bases of individual canopy trees. Approximations of the distances that the infiltration‐excess would have travelled away from the tree bases were calculated by assuming that the infiltration area either expands radially outward in the form of an annulus or extends straight downslope from the tree base.

Interception storage capacities of tropical rainforest canopy trees

Abstract The rainwater interception storage capacities of mature canopy trees in a tropical rainforest site in northeast Queensland, Australia, were approximated using a combination of field and laboratory measurements. The above-ground vegetative surfaces of five selected species (three flaky-barked; two smooth-barked) were saturated under laboratory conditions in order to establish their maximum interception storage capacities. Average leaf surface interception storages ranged from 112 to 161 ml m−2. The interception storages of bark ranged from 0.51 to 0.97 ml cm−3. These standardized interception storages were applied to estimates of leaf surface area and bark volume for 51 mature canopy trees representing the selected species in the field site. The average whole tree interception storage capacities of the five species ranged from 110 to 5281 per tree and 2.2 to 8.3 mm per unit projected crown area. The highly significant interspecific differences in interception storage capacity suggest that both floristic and demographic data are needed in order to accurately calculate a forest-wide interception storage capacity for species-rich tropical rainforest vegetation. Species with large woody surface areas and small projected crown areas are capable of storing the greatest depth equivalents of rainwater under heavy rainfall conditions. In the case of both the flaky-barked and the smooth-barked species, bark accounted for > 50% of the total interception storage capacity under still-air conditions, and > 80% under turbulent air conditions. The emphasis in past interception studies on the role of leaf surfaces in determining the interception storage capacity of a vegetative cover must be modified for tropical rainforests to include the storage capacity provided by the bark tissue on canopy trees.