Tropical Conifer Research Articles

An enigmatic tropical conifer from the Early Cretaceous of Gondwana

An enigmatic tropical conifer from the Early Cretaceous of Gondwana

Anatomical characteristics of the leaning stem in Pinus merkusii seedling

Abstract. Safitri NM, Prastiwi FW, Salsabila S, Feriawan Y, Brilianto A, Nugroho WD. 2023. Anatomical characteristics of the leaning stem in Pinus merkusii seedling. Biodiversitas 24: 3796-3802. Pinus merkusii Jungh. & de Vriese is one of the tropical conifer species. The species formed reaction wood called compression wood (CW) when the stem grows in an abnormal position. CW in stem wood is frequently found to cause several problems for utilization in the use of its products. However, there are few studies on the anatomical characteristics of reaction wood in tropical conifer. This study aimed to observe the differences in anatomical characteristics between compression (CW) and opposite (OW) woods in leaning the stem of P. merkusii seedlings and their anatomical differences compared to normal wood (NW). Approx. 1-year-old P. merkusii seedlings from a nursery in Trenggalek, East Java, Indonesia, that were naturally bent or tilted were used in the study. Anatomical characteristics of wood, namely the dimension of tracheids, rays, resin canals, and the proportion of cells were observed in the study. The results showed that CW of P. merkusii seedlings formed a rounded tracheid shape and intercellular space. The differences between CW and OW in P. merkusii seedlings were significant in terms of height of ray cells, tracheid proportion, and resin canal proportion. However, tracheid length, tracheid diameter, tracheid wall thickness, resin canal diameter and frequency, rays frequency, and ray proportion were not significantly different compared to OW and NW.

Dendrochronology in the tropics using tree-rings of Pinus kesiya

The recent decade has witnessed considerable progress in the number of tree-ring studies using the tropical conifer taxa, Pinus kesiya. Several tree-ring networks have been established in less explored regions in Northeast India, Southwest China and Vietnam. The seasonal climate response of P. kesiya tree-rings has been examined and used to reconstruct temperature and soil moisture variability over the past century and augment the short instrumental records in South and Southeast Asia. In addition to standard approaches, the application of stable isotope, wood density, and blue intensity measurements indicates a significant development in P. kesiya studies. This review elaborates the future prospects of using multiple tree-ring parameters to establish discrete proxies besides tree-ring width. We recommend blue intensity as a cost-effective alternative to quantitative wood anatomy in tropical pines, and call for routine assessments of the temporal stability of climate-growth responses to identify and study potentially non-stationary climate signals. Efforts should be made towards developing extensive networks of long P. kesiya tree-ring chronologies to extend regional climate reconstructions.

Neontological and paleontological congruence in the evolution of Podocarpaceae (coniferales) reproductive morphology

IntroductionPodocarpaceae are a diverse, primarily tropical conifer family that commonly produce large leaves and highly reduced, fleshy seed cones bearing large seeds. These features may result from relatively recent adaptation to closed-canopy angiosperm forests and bird-mediated seed dispersal, although determining precisely when shifts in leaf and seed cone morphology occurred is difficult due to a sparse fossil record and relatively few surviving deep lineages.MethodsWe compare the fossil record of Podocarpaceae with results from ancestral state reconstruction methods and correlated character models using neontological data and a previously published molecular time-tree.ResultsAncestral state reconstructions suggest that small leaves, small seeds, and multi-seeded cones are ancestral in crown Podocarpaceae, with reduced cones bearing few seeds appearing in the Early Cretaceous and the correlated evolution of large leaves and large seeds occurring from the Late Cretaceous onwards. The exact timing of these shifts based on neontological data alone are poorly constrained, however, and estimates of leaf and seed size are imprecise.DiscussionThe fossil record is largely congruent with results based on the molecular time-tree, but provide important constraints on the range of leaf and seed sizes that were present in Cretaceous Podocarpaceae and the time by which changes in cone morphology and seed size likely occurred. We suggest in particular that reduced seed cones appeared in the Early Cretaceous and are linked to the contemporaneous diversification of small bodied avialans (birds), with shifts to larger seed sizes occurring after the Cretaceous in association with the spread of closed-canopy angiosperm forests.

Respiratory temperature responses of tropical conifers differ with leaf morphology

Abstract Photosynthetic traits suggest that shade tolerance may explain the contrasting success of two conifer taxa, Podocarpaceae and Pinaceae, in tropical forests. Needle‐leaved species from Pinus (Pinaceae) are generally absent from tropical forests, whereas Pinus krempfii, a flat‐leaved pine, and numerous flat‐leaved Podocarpaceae are abundant. Respiration (R) traits may provide additional insight into the drivers of the contrasting success of needle‐ and flat‐leaved conifers in tropical forests. We measured the short‐term respiratory temperature (RT) response between 10 and 50°C and foliar morphological traits of three needle‐ and seven flat‐leaved conifer species coexisting in a tropical montane forest in the Central Highlands of Vietnam containing notable conifer diversity. We fit a lognormal polynomial model to each RT curve and extracted the following three parameters: a (basal R), and b and c (together describing the shape of the response). Needle‐leaved species (Pinus kesiya, Pinus dalatensis and Dacrydium elatum) had higher rates of area‐based R at 25°C (R25‐area) as well as higher area‐based modelled basal respiration (a) than flat‐leaved species (P. krempfii, Podocarpus neriifolius, Dacrycarpus imbricatus, Nageia nana, Taxus wallichiana, Keteeleria evelyniana and Fokienia hodginsii). No significant differences were found between needle‐ and flat‐leaved species in mass‐based R25 (R25‐mass) or in the shape of the RT response (b and c); however, interspecific differences in R25‐mass, R at nighttime temperature extremes (R4.1 and R20.6) and leaf traits were apparent. Differences in R25‐area and a suggest that needle‐leaved foliage may be more energetically costly to maintain than flat‐leaved foliage, providing new insight and additional support for the hypothesis that shade tolerance is an important driver of Podocarpaceae success and Pinaceae absence in the majority of tropical forests. Interspecific differences in R25‐mass and leaf traits highlight that varying ecological strategies are employed by conifers to coexist and survive in the Central Highlands of Vietnam. Ultimately, these data further our understanding of current conifer biogeographical distributions and underscore the need for additional studies to elucidate the effects of extreme temperature events on the continued survival of conifers in this unique forest. A free Plain Language Summary can be found within the Supporting Information of this article.

Fortuna Forest Reserve, Panama

The Fortuna Forest Reserve and adjacent upland areas of the Palo Seco Reserve in western Panama support some of the most extensively studied lower and premontane tropical forests in the world. The forests of Fortuna are among the most diverse in Central America and are therefore of exceptional significance for the preservation of regional biodiversity. This volume brings together more than 50 years of research on the climate, geology, soils, and major plant groups of Fortuna. Spanning the Continental Divide at around 1,000 m above sea level, some parts of the reserve receive more than 6,000 mm of annual rainfall, although there is considerable variation in cloud cover and seasonality. Soil fertility also varies markedly, reflecting the complex regional volcanic geology. The resulting gradients of climate and fertility across the reserve shape the composition, structure, and diversity of plant communities. A network of 12 one-hectare plots at Fortuna contains more than 400 species of trees greater than 5 cm diameter at breast height and reveals extensive compositional turnover across the reserve. One tree species, Oreomunnea mexicana, forms monodominant stands in otherwise species-rich forests, while forests on extremely infertile soils are dominated by the canopy palm Colpothrinax aphanopetala and include the tropical conifer Podocarpus oleifolius. There are also almost 400 species of bryophytes, almost 300 species of ferns and lycophytes, 31 species of palms, 80 species of bromeliads, and more than 200 species of orchids. Many species of ectomycorrhizal fungi identified from fruiting bodies are new to science. Overall, results from Fortuna highlight the remarkable diversity of plants that occur in montane forests and the extent to which their communities are structured by gradients of climate and soil fertility. The chapters in this volume provide a foundation for further research and exploration in this fascinating region.

Geographical variations and correlation among some chemical and thermal properties of Almaciga (Agathis philippinensis Warb.) resins from selected commercial sites in the Philippines

Almaciga (Agathis philippinensis Warb.), a tropical conifer that is tapped for its resin commercially known as Manila copal, grows in many parts of the Philippines, but resin buyers prefer and pay a better price for resin from southern Palawan. The study was done to characterize almaciga resins obtained from commercial sites in Davao Oriental and Palawan (Brooke's Point and Marufinas) to explain the quality and price differences. Fresh and aged resin samples were subjected to wet chemical, thermal, and spectral analyses. Generally, the resin samples were found to be soluble in the more polar solvents although significant variations were observed for resins from various sites. Titrimetric determination revealed no significant variation in acid and saponification values, although the Brooke's Point resin had significantly lower unsaponifiable components. Predominant Fourier transform infrared spectroscopy (FTIR) absorption peaks for fresh resin include strong C=O stretch and weak = C–H stretch. Aged Palawan resins, which showed increased solubility in polar solvents, exhibited increased intensity of prominent FTIR peaks such as O–H stretch and C=O stretch. The weak peak at 1719 cm−1 seen in fresh Palawan resins was no longer observed in aged resins. DSC revealed the semi-crystalline nature of almaciga resin and melting temperatures similar to diterpenoid resin acids. This is attributed to the presence of increasing amounts of oxidized abietic acids as shown by gas chromatography-mass spectrometry (GCMS). For the Davao Oriental resins, significant amount of agatholic acid was shown, while resin acids of the abietane and pimarane type were conspicuously absent in the GC-MS spectrograms. The study found evidence that almaciga resins from Davao and Palawan differ in chemical composition and physical properties, which could explain the quality and price differences.

The growth responses of Araucaria angustifolia to climate are adjusted both spatially and temporally at its northern distribution limit

Temporal instability and spatial inconsistency in tree growth sensitivity to climate have been reported, but little attention has been given to tropical species. Araucaria angustifolia (Bertol.) Kuntze. (Araucariaceae) is a critically endangered South American subtropical and tropical conifer. It offers the potential to evaluate alterations in tree growth responses as it is climate-sensitive and presents a core zone of distribution in the Brazilian meridional highlands (with small disjunct patches occurring in its most southern and northern ranges). Previous dendrochronological studies in the central and most southern distribution range of A. angustifolia showed different climate-related growth responses, which raises the question of whether the growth responses of A. angustifolia to climate are consistent over space and stable over time at the species' northern distribution limit? We developed well-replicated tree-ring chronologies for A. angustifolia at the species' northern distribution limit, examined its growth dynamics, and evaluated climate-growth relationships. The growth patterns of A. angustifolia allowed the synchronization of its tree ring time-series and the construction of two tree-ring width chronologies (at Brejo da Lapa – BL and Vale das Araucárias – VA) of 55 years and 171 years respectively. The chronologies demonstrated stable and robust signal strengths. Climate-growth response analyses indicated significant positive correlations of the VA chronology with precipitation variables, and with temperature variables in the BL chronology. Different diametric growth patterns were detected between BL and VA. We observed consistent reductions of growth responses to the climate in recent decades. Forest structure and dynamics, disturbance regime, local environmental factors, and climate change could generate physiological alterations and explain variations in A. angustifolia’s growth responses and increments. The species showed different sensitivities to climatic factors both spatially and temporally, and different growth patterns at its northern distribution limit.

Time lags between crown and basal sap flows in tropical broadleaf and temperate conifer trees

Time lags between crown and basal sap flows in tropical broadleaf and temperate conifer trees

Using niche centrality within the scope of the nearly neutral theory of evolution to predict genetic diversity in a tropical conifer species‐pair

AbstractAimEstimating genetic diversity is key for understanging biogeographic and evolutionary processes. However, gathering genetic information is not feasible for all taxa or populations, particularly in the tropical regions. Identifying proxies for inferring such values has thus become essential. Here, we built on the niche centrality hypothesis (NCH; or central‐abundance hypothesis) and the nearly neutral theory of evolution (NNT) to identify some of such proxies using a montane tropical conifer species‐pair as model. The NCH predicts more genetic diversity under optimal ecological conditions, which should also allow for more efficient purifying selection, according to the NNT.LocationThe Transmexican Volcanic Belt, central Mexico.TaxaA fir species‐pair endemic to central Mexico, Abies flinckii and A.religiosa.MethodsWe estimated patterns of genetic diversity from nuclear SSRs (A, HE), and gene‐coding sequences (πS, πN), together with the efficacy of purifying selection, measured as πN/πS. After testing for niche overlap, we used several geographic and ecological proxies (i.e. longitude, latitude, elevation, estimated area and distance to the niche centroid in the present and in the LGM) to predict genetic diversity and πN/πS using general linear models.ResultsPopulations at the west of the Trans Mexican Volcanic Belt (TVB) had lower genetic diversity than populations in the east of this mountain chain. Both species had significant niche overlap. The principal predictors for neutral genetic diversity (HE, A and πS) were longitude and latitude, followed by the current distance to the niche centroid; the efficiency of purifying selection was mostly accounted for by the current distance to the niche centroid (which was also correlated with elevation). No correlation was observed between genetic diversity or πN/πS and current population area.Main conclusionsHistorical and ecological factors have to be taken into account for explaining the amounts of genetic diversity in mountain tropical species. Following the NTT, populations closer to the niche centroid are more efficient at eliminating slightly deleterious mutations than marginal stands, independently of their size or geographic location (longitude). Expanding the central‐abundance theory within the scope of the NTT might help reconciling conflicting views concerning the extent of its empirical support.

Why are tropical conifers disadvantaged in fertile soils? Comparison of Podocarpus guatemalensis with an angiosperm pioneer, Ficus insipida.

Conifers are, for the most part, competitively excluded from tropical rainforests by angiosperms. Where they do occur, conifers often occupy sites that are relatively infertile. To gain insight into the physiological mechanisms by which angiosperms outcompete conifers in more productive sites, we grew seedlings of a tropical conifer (Podocarpus guatemalensis Standley) and an angiosperm pioneer (Ficus insipida Willd.) with and without added nutrients, supplied in the form of a slow-release fertilizer. At the conclusion of the experiment, the dry mass of P. guatemalensis seedlings in fertilized soil was approximately twofold larger than that of seedlings in unfertilized soil; on the other hand, the dry mass of F. insipida seedlings in fertilized soil was ~20-fold larger than seedlings in unfertilized soil. The higher relative growth rate of F. insipida was associated with a larger leaf area ratio and a higher photosynthetic rate per unit leaf area. Higher overall photosynthetic rates in F. insipida were associated with an approximately fivefold larger stomatal conductance than in P. guatemalensis. We surmise that a higher whole-plant hydraulic conductance in the vessel bearing angiosperm F. insipida enabled higher leaf area ratio and higher stomatal conductance per unit leaf area than in the tracheid bearing P. guatemalensis, which enabled F. insipida to capitalize on increased photosynthetic capacity driven by higher nitrogen availability in fertilized soil.

A 16,000-year record of climate, vegetation and fire from Wallacean lowland tropical forests

The tropical forests of Sulawesi represent some of the most diverse, biogeographically significant ecosystems globally. However, long-term ecological data for the region are scarce, making it difficult to predict vegetation response to future climatic and anthropogenic drivers of change. This is problematic as gauging the acclimation thresholds of forests within tropical Asia has been identified as an IPCC policy imperative.This study uses palynological and geochemical analysis of sediments from a tectonic basin – Lake Lantoa – to reconstruct the hydroclimatic, fire, and tropical rainforest dynamics of south Sulawesi over 16,000 years, and reveal the response of lowland ultramafic forest to large-scale environmental change. Our results show that the Late Pleistocene was characterised by a drier, more seasonal climate and persistent fire in the landscape. These conditions supported a slightly open, Gymnostoma-rich, semi-seasonal forest, and constrained the available habitat range for hydrophilic tropical conifers. Increased effective moisture in the Holocene, interpreted from the onset of periodic lake stratification under deeper conditions, appears to have precluded forest burning, and facilitated the expansion of a lowland ultramafic rainforest and upland tropical coniferous forest. An increase in garden/secondary forest taxa and ground ferns from ∼2 cal kyrs BP implies increased human management of the landscape. While these data highlight the compositional dynamism of lowland forest in Wallacea to seasonality, fire, and reduced precipitation, they demonstrate biome-scale resilience to glacial-scale environmental change. This contrasts with interpretations of forest sensitivity to climatic change drawn from analysis of leaf wax isotopes from adjacent lake basins, highlighting the value of adopting multi-proxy, taxonomically high-resolution techniques for reconstructing floristic histories.

Growth of the tropical Pinus kesiya as influenced by climate and nutrient availability along an elevational gradient

Abstract Aims In contrast to temperate regions, the reasons for growth reductions of individual tree species along elevational gradients in tropical mountain ranges are poorly known, especially for tropical conifers. We aimed at testing whether climatic or edaphic conditions are responsible for the reduced growth of Pinus kesiya, a widely distributed pine species of southern and south-eastern Asia, at higher elevations. Methods We analysed the stem diameter increment and the isotope discrimination against 13C (△ 13C) in tree rings of P. kesiya along an elevational gradient of ~900 to ~2000 m a.s.l. in the mountain ranges of South-Central Vietnam, and related growth to △ 13C and to climatic and edaphic variables. Important findings We found no consistent correlation patterns between the basal area increment (BAI) of the trees and temperature or precipitation. In contrast, across the elevational gradient, we obtained significantly negative correlations of BAI with △ 13C and with the C/N ratios and the δ 15N signature of the upper mineral soil. BAI was positively correlated with the concentrations of plant-available phosphorus (Pa) and of “base” cations (calcium, magnesium, potassium) in the soil. We conclude that lower temperatures at higher elevations exert an indirect effect on tree growth by inducing higher C/N ratios and by reducing the rate of nitrogen (N) and P mineralization, which may be further hampered by lower concentrations of “base” cations (upon enhanced leaching by precipitation) and a negative feedback from low availability of mineralized N and P at higher elevations. Our results may be transferable to the uppermost growth limit of P. kesiya and to other montane regions of the species’ occurrence.

El papel del fuego en la regeneración de los bosques de coníferas

Fire is a forest disturbance agent whose effects vary according to the natural fire regime, underwhich the species that inhabit it evolved. The aim of this paper is to present an overview of therole of fire in the regeneration of conifer forests. First, we review fire regimes and their effects ontree regeneration. Then, we describe the main methods for reconstructing fire regimes andexplore montane tropical conifer forest fire regimes. Finally, the possible effect of timberharvesting on fire regimes is described. It has been suggested that this activity is currently thedisturbance agent that has the greatest impact on fires regimes worldwide. In this review article,we propose that understanding the role of fire in the regeneration of conifer forests is essential toprevent catastrophic fires and that the use of this element as a forest management tool cancontribute to the conservation of the integrity of these forests.

Blue intensity from a tropical conifer’s annual rings for climate reconstruction: An ecophysiological perspective

We developed Blue Intensity (BI) measurements from the crossdated ring sequences of Fokienia hodginsii (of the family Cupressaceae) from central Vietnam. BI has been utilized primarily as an indirect proxy measurement of latewood (LW) density of conifers (i.e., LWBI) from high latitude, temperature-limited boreal forests. As such, BI closely approximates maximum latewood density (MXD) measurements made from soft x-ray. The less commonly used earlywood (EW) BI (EWBI) represents the minimum density of EW and is influenced by the lighter pixels from the vacuoles or lumens of cells. The correlation of our BI measurements with climate, strongest for EWBI, rivals that for total ring width (RW), and we demonstrate that it can be successfully employed as an independent predictor for reconstruction models. EWBI exhibits robust spatial correlations with winter and spring land temperature, sea surface temperature (SST) over the regional domain of ENSO, and the Standardized Precipitation Evapotranspiration Index (SPEI) over Indochina. However, in order to mitigate the effects of color changes at the heartwood – sapwood boundary we calculated ΔBI (EWBI-LWBI), and it too exhibits a significant (p < 0.05), temporally stable response to prior autumn (Oct-Nov) rainfall and winter (December to April) dry season temperature. We interpret this response as reflecting a potential cavitation defense by reducing lumen diameter as a means to safeguard hydraulic conductivity in the stem, and to prevent the xylem from imploding due to negative pressure. This study has wide implications for the further use of BI from the global tropics, though it is unclear how many tropical tree species will be appropriate for use. It seems very likely that other wood anatomical measurements can be combined with BI and RW for climate reconstruction.

Conifer tracheids resolve conflicting structural requirements: Data, hypotheses, questions

The nature of conduction involves movement of a liquid (under tension or pressure) through a solid (cell walls necessary to direct the liquid and provide mechanical strength). The numerous consequences of the liquid/solid nature of the conductive interface in plants can be viewed as a series of conflicting requirements that are resolved by various mechanisms. For example, the types of mechanical strength conferred by thicker cell walls (latewood) run counter to optimal conduction (earlywood). Conflict resolution situations are examined with light microscopy and SEM to show in detail not merely conflicting requirements but the various types of resolution in various conifers. Abies is presented as exemplary of a cool temperate conifer with numerous aspects to earlywood/latewood structure. Tropical conifers (Araucaria) present different compromises; the riparian conifer Dacrydium guillauminii has only earlywood; the parasitic conifer Parasitaxus has only latewood. Particular conifers have only some of the features by which latewood differs from earlywood. Cell dimorphism is only one aspect of resolution of conflicting requirements; others include modifications in pit size, shape, and density; the nature of the pit membrane; the nature of the pit cavity, pit border and pit aperture; and surface relief (warty layer) of the tracheid wall. The invention of coniferous bordered pits involves a circular shape, so that tension on the margo strands is equal, and thus the pit can be closed. These factors and margo pore maximization necessitate expending a large amount of space to pits in earlywood, the strength of which is thereby lessened and must be compensated by greater wall strength in latewood. The paper concludes with a series of twenty features which represent resolutions of conflicting requirements in terms of anatomical structure. Wood physiological literature is integrated with the anatomical observations.

Two tropical conifers show strong growth and water-use efficiency responses to altered CO2 concentration.

Conifers dominated wet lowland tropical forests 100million years ago (MYA). With a few exceptions in the Podocarpaceae and Araucariaceae, conifers are now absent from this biome. This shift to angiosperm dominance also coincided with a large decline in atmospheric CO2 concentration (ca). We compared growth and physiological performance of two lowland tropical angiosperms and conifers at ca levels representing pre-industrial (280 ppm), ambient (400 ppm) and Eocene (800 ppm) conditions to explore how differences in ca affect the growth and water-use efficiency (WUE) of seedlings from these groups. Two conifers (Araucaria heterophylla and Podocarpus guatemalensis) and two angiosperm trees (Tabebuia rosea and Chrysophyllum cainito) were grown in climate-controlled glasshouses in Panama. Growth, photosynthetic rates, nutrient uptake, and nutrient use and water-use efficiencies were measured. Podocarpus seedlings showed a stronger (66 %) increase in relative growth rate with increasing ca relative to Araucaria (19 %) and the angiosperms (no growth enhancement). The response of Podocarpus is consistent with expectations for species with conservative growth traits and low mesophyll diffusion conductance. While previous work has shown limited stomatal response of conifers to ca, we found that the two conifers had significantly greater increases in leaf and whole-plant WUE than the angiosperms, reflecting increased photosynthetic rate and reduced stomatal conductance. Foliar nitrogen isotope ratios (δ15N) and soil nitrate concentrations indicated a preference in Podocarpus for ammonium over nitrate, which may impact nitrogen uptake relative to nitrate assimilators under high ca SIGNIFICANCE: Podocarps colonized tropical forests after angiosperms achieved dominance and are now restricted to infertile soils. Although limited to a single species, our data suggest that higher ca may have been favourable for podocarp colonization of tropical South America 60MYA, while plasticity in photosynthetic capacity and WUE may help account for their continued persistence under large changes in ca since the Eocene.

Size structure, growth and regeneration of tropical conifers along a soil gradient related to altitude and geological substrates on Mount Kinabalu, Borneo

Conifers often co-occur with angiosperm trees at high altitudes and on infertile soils in tropical rainforests. To explore the mechanism of conifer–angiosperm coexistence, we investigated size structure, growth rate and regeneration of conifers along soil gradient across altitudes and geological substrates. Tree measurements were conducted in ten plots (0.06–1 ha) with varied geology at four altitudes (700, 1700, 2700 and 3100 m) on Mount Kinabalu, Borneo. Conifer juveniles were counted and understory light conditions were analyzed with hemispherical photographs. Size structure of conifers showed inverse-J shape distributions on ultrabasic rocks at ≥1700 m but unimodal or sporadic distributions in other plots. Densities of conifer seedlings and saplings were generally greater on ultrabasic rocks than on non-ultrabasic substrates at comparable altitudes, which concurred with the pattern in understory light conditions. Growth rates over 4 (or 5) years of conifers at 4.8–25 cm diameter were higher than angiosperm trees only under well-lit conditions on non-ultrabasic substrates, but were always similar or higher on ultrabasic rocks even in shaded conditions. Conifers showed continuous regeneration on ultrabasic rocks at ≥1700 m. This suggested that, on infertile soils, competition with angiosperm trees was relaxed and well-lit understory facilitated recruitment of conifers.

Rock outcrops reduce temperature-induced stress for tropical conifer by decoupling regional climate in the semiarid environment.

We aimed to understand the effect of rock outcrops on the growth of Podocarpus lambertii within a microrefuge. Our hypothesis holds that the growth and survival of this species depend on the regional climate decoupling provided by rock outcrops. To test this hypothesis, we characterized the microclimate of (1) surrounding vegetation, (2) rock outcrop corridors, and (3) adjacencies. We assessed population structure by collecting data of specimen stem diameter and height. We also assessed differences between vegetation associated or not with outcrops using satellite imaging. For dendrochronological analyses, we sampled 42 individuals. Tree rings of 31 individuals were dated, and climate-growth relationships were tested. Rock outcrops produce a favorable microclimate by reducing average temperature by 4.9 °C and increasing average air humidity by 12 %. They also reduce the variability of atmospheric temperature by 42 % and air humidity by 20 % supporting a vegetation with higher leaf area index. Within this vegetation, specimen height was strongly constrained by the outcrop height. Although temperature and precipitation modulate this species growth, temperature-induced stress is the key limiting growth factor for this population of P. lambertii. We conclude that this species growth and survival depend on the presence of rock outcrops. These topography elements decouple regional climate in a favorable way for this species growth. However, these benefits are restricted to the areas sheltered by rock outcrops. Although this microrefuge supported P. lambertii growth so far, it is unclear whether this protection would be sufficient to withstand the stress of future climate changes.

A trade-off in stand size effects in the reproductive biology of a declining tropical conifer Callitris intratropica

Because wind pollination is inefficient over longer distances, plants dependent on it may suffer Allee effects (lower individual reproductive fitness with lower density). However, at higher density, individual reproductive fitness may suffer because of intraspecific competition. We investigate density-dependent effects, via stand size, on cone and seed production and seed germinability in a conifer endemic to tropical Australia. Callitris intratropica R.T. Baker & H.G. Smith is an obligate-seeding tree that often occurs in monodominant stands embedded within savannas and on the fringes of monsoon forests. We found that isolated trees (50–300 m from stands) were taller, of broader profile, and produced approximately twice the number of cones (~407 cones per tree) as those in large stands (~173 cones per tree), suggesting that monodominance generates intraspecific competition. The number of seeds per cone (27 seeds) was not related to stand size. However, a contrasting effect in which seed germinability was higher in large stands (~20 vs. <10 % in small stands) was approximately compensatory and consistent with an Allee effect of wind pollination. The net effect of an approximately even trade-off between cone production and seed germinability was that there was neither an Allee or density-dependent effect of stand size on fitness, measured as the number of germinable seeds per tree. Nevertheless, because the likelihood of cross-fertilisation declines with distance, the ability of C. intratropica to persist as very isolated individuals may be limited.