Are lianas more drought-tolerant than trees? A test for the role of hydraulic architecture and other stem and leaf traits

Linking physiology, epidemiology, and demography: Understanding how lianas outcompete trees in a changing world

Extending and safeguarding tropical forest ecosystems is critical for combating climate change and biodiversity loss. One of its constituents, lianas, is spreading and increasing in abundance on a global scale. This is particularly concerning as lianas negatively impact forests' carbon fluxes, dynamics, and overall resilience, potentially exacerbating both crises. While possibly linked to climate-change-induced atmospheric CO2 elevation and drought intensification, the reasons behind their increasing abundance remain elusive. Prior research shows distinct physiological differences between lianas and trees, but it is unclear whether these differences confer a demographic advantage to lianas with climate change. Guided by extensive datasets collected in Panamanian tropical forests, we developed a tractable model integrating physiology, demography, and epidemiology. Our findings suggest that CO2 fertilization, a climate change factor promoting forest productivity, gives lianas a demographic advantage. Conversely, factors such as extreme drought generally cause a decrease in liana prevalence. Such a decline in liana prevalence is expected from a physiological point of view because lianas have drought-sensitive traits. However, our analysis underscores the importance of not exclusively relying on physiological processes, as interactions with demographic mechanisms (i.e., the forest structure) can contrast these expectations, causing an increase in lianas with drought. Similarly, our results emphasize that identical physiological responses between lianas and trees still lead to liana increase. Even if lianas exhibit collinear but weaker responses in their performance compared to trees, a temporary liana prevalence increase might manifest driven by the faster response time of lianas imposed by their distinct life-history strategies than trees.

Lianas in tropical dry seasonal forests have a high hydraulic efficiency but not always a higher embolism resistance than lianas in rainforests

BACKGROUND AND AIMS

Lianas have higher relative abundance and biomass in drier seasonal forests than in rainforests, but whether this difference is associated with their hydraulic strategies is unclear. Here, we investigate whether lianas of seasonally dry forests are safer and more efficient in water transport than rainforest lianas, explaining patterns of liana abundance.

METHODS

We measured hydraulic traits on five pairs of congeneric lianas of the tribe Bignonieae in two contrasting forest sites: the wet 'Dense Ombrophilous Forest' in Central Amazonia (~2 dry months) and the drier 'Semideciduous Seasonal Forest' in the inland Atlantic Forest (~6 dry months). We also gathered a broader database, including 197 trees and 58 liana species from different tropical forests, to compare hydraulic safety between habits and forest types.

KEY RESULTS

Bignonieae lianas from both forests had high and similar hydraulic efficiency but exhibited variability in resistance to embolism across forest types when phylogenetic relationships were taken into account. Three genera had higher hydraulic safety in the seasonal forest than in the rainforest, but species across both forests had similar positive hydraulic safety margins despite lower predawn water potential values of seasonal forest lianas. We did not find the safety-efficiency trade-off. Merging our results with previously published data revealed a high variability of resistance to embolism in both trees and lianas, independent of forest types.

CONCLUSIONS

The high hydraulic efficiency of lianas detected here probably favours their rapid growth across tropical forests, but differences in hydraulic safety highlight that some species are highly vulnerable and may rely on other mechanisms to cope with drought. Future research on the lethal dehydration threshold and the connection between hydraulic resistance strategies and liana abundance could offer further insights into tropical forest dynamics under climatic threats.

Vegetative phenologies of lianas and trees in two Neotropical forests with contrasting rainfall regimes

Among tropical forests, lianas are predicted to have a growth advantage over trees during seasonal drought, with substantial implications for tree and forest dynamics. We tested the hypotheses that lianas maintain higher water status than trees during seasonal drought and that lianas maximize leaf cover to match high, dry-season light conditions, while trees are more limited by moisture availability during the dry season. We monitored the seasonal dynamics of predawn and midday leaf water potentials and leaf phenology for branches of 16 liana and 16 tree species in the canopies of two lowland tropical forests with contrasting rainfall regimes in Panama. In a wet, weakly seasonal forest, lianas maintained higher water balance than trees and maximized their leaf cover during dry-season conditions, when light availability was high, while trees experienced drought stress. In a drier, strongly seasonal forest, lianas and trees displayed similar dry season reductions in leaf cover following strong decreases in soil water availability. Greater soil moisture availability and a higher capacity to maintain water status allow lianas to maintain the turgor potentials that are critical for plant growth in a wet and weakly seasonal forest but not in a dry and strongly seasonal forest.

Climate and hydraulic traits interact to set thresholds for liana viability

Lianas, or woody vines, and trees dominate the canopy of tropical forests and comprise the majority of tropical aboveground carbon storage. These growth forms respond differently to contemporary variation in climate and resource availability, but their responses to future climate change are poorly understood because there are very few predictive ecosystem models representing lianas. We compile a database of liana functional traits (846 species) and use it to parameterize a mechanistic model of liana-tree competition. The substantial difference between liana and tree hydraulic conductivity represents a critical source of inter-growth form variation. Here, we show that lianas are many times more sensitive to drying atmospheric conditions than trees as a result of this trait difference. Further, we use our competition model and projections of tropical hydroclimate based on Representative Concentration Pathway 4.5 to show that lianas are more susceptible to reaching a hydraulic threshold for viability by 2100.

JA, Sanchez‐Azofeifa GA, De Deurwaerder HPT, Krishna Moorthy SM, Schnitzer SA, Marvin DC, Longo M, Liu C, Broadbent EN, Almeyda Zambrano AM, Muller‐Landau HC, Detto M, Verbeeck H. Liana optical traits increase tropical forest albedo and reduce ecosystem productivity

Lianas are a key growth form in tropical forests. Their lack of self-supporting tissues and their vertical position on top of the canopy make them strong competitors of resources. A few pioneer studies have shown that liana optical traits differ on average from those of colocated trees. Those trait discrepancies were hypothesized to be responsible for the competitive advantage of lianas over trees. Yet, in the absence of reliable modelling tools, it is impossible to unravel their impact on the forest energy balance, light competition, and on the liana success in Neotropical forests. To bridge this gap, we performed a meta-analysis of the literature to gather all published liana leaf optical spectra, as well as all canopy spectra measured over different levels of liana infestation. We then used a Bayesian data assimilation framework applied to two radiative transfer models (RTMs) covering the leaf and canopy scales to derive tropical tree and liana trait distributions, which finally informed a full dynamic vegetation model. According to the RTMs inversion, lianas grew thinner, more horizontal leaves with lower pigment concentrations. Those traits made the lianas very efficient at light interception and significantly modified the forest energy balance and its carbon cycle. While forest albedo increased by 14% in the shortwave, light availability was reduced in the understorey (-30% of the PAR radiation) and soil temperature decreased by 0.5°C. Those liana-specific traits were also responsible for a significant reduction of tree (-19%) and ecosystem (-7%) gross primary productivity (GPP) while lianas benefited from them (their GPP increased by +27%). This study provides a novel mechanistic explanation to the increase in liana abundance, new evidence of the impact of lianas on forest functioning, and paves the way for the evaluation of the large-scale impacts of lianas on forest biogeochemical cycles.

Differentiation in stem and leaf traits among sympatric lianas, scandent shrubs and trees in a subalpine cold temperate forest

The scandent shrub plant form is a variant of liana that has upright and self-supporting stems when young but later becomes a climber. We aimed to explore the associations of stem and leaf traits among sympatric lianas, scandent shrubs and trees, and the effects of growth form and leaf habit on variation in stem or leaf traits. We measured 16 functional traits related to stem xylem anatomy, leaf morphology and nutrient stoichiometry in eight liana, eight scandent shrub and 21 tree species co-occurring in a subalpine cold temperate forest at an elevation of 2600-3200 m in Southwest China. Overall, lianas, scandent shrubs and trees were ordered along a fast-slow continuum of stem and leaf functional traits, with some traits overlapping. We found a consistent pattern of lianas > scandent shrubs > trees for hydraulically weighted vessel diameter, maximum vessel diameter and theoretical hydraulic conductivity. Vessel density and sapwood density showed a pattern of lianas = scandent shrubs < trees, and lianas < scandent shrubs = trees, respectively. Lianas had significantly higher specific leaf area and lower carbon concentration than co-occurring trees, with scandent shrubs showing intermediate values that overlapped with lianas and trees. The differentiation among lianas, scandent shrubs and trees was mainly explained by variation in stem traits. Additionally, deciduous lianas were positioned at the fast end of the trait spectrum, and evergreen trees at the slow end of the spectrum. Our results showed for the first time clear differentiation in stem and leaf traits among sympatric liana, scandent shrub and tree species in a subalpine cold temperate forest. This work will contribute to understanding the mechanisms responsible for variation in ecological strategies of different growth forms of woody plants.

Increasing axial parenchyma fraction in the Malagasy Magnoliids facilitated the co-optimisation of hydraulic efficiency and safety

The evolution of angiosperms was accompanied by the segregation and specialisation of their xylem tissues. This study aimed to determine whether the fraction and arrangement of parenchyma tissue influence the hydraulic efficiency-safety trade-off in the basal angiosperms. We examined xylem anatomical structure and hydraulic functioning of 28 woody species of Magnoliids in a tropical rainforest of Madagascar and reported, for the first time, quantitative measurements that support the relationship between vessel-to-xylem parenchyma connectivity and the hydraulic efficiency-safety trade-off. We also introduced a new measurement - the distance of species from the trade-off limit - to quantify the co-optimisation of hydraulic efficiency and safety. Although the basal angiosperms in this study had low hydraulic conductivity and safety, species with higher axial parenchyma fraction (APf) had significantly higher hydraulic conductivity. Hydraulic efficiency-safety optimisation was accompanied by higher APf and vessel-to-axial parenchyma connectivity. Conversely, species exhibiting high ray parenchyma fraction and high vessel-to-ray connectivity had lower K_s and were further away from the hydraulic trade-off limit line. Our results provide evidence that axial parenchyma fraction and paratracheal arrangement are associated with both enhanced hydraulic efficiency and safety.

Unraveling the relative role of light and water competition between lianas and trees in tropical forests: A vegetation model analysis

Despite their low contribution to forest carbon stocks, lianas (woody vines) play an important role in the carbon dynamics of tropical forests. As structural parasites, they hinder tree survival, growth and fecundity; hence, they negatively impact net ecosystem productivity and long-term carbon sequestration.Competition (for water and light) drives various forest processes and depends on the local abundance of resources over time. However, evaluating the relative role of resource availability on the interactions between lianas and trees from empirical observations is particularly challenging. Previous approaches have used labour-intensive and ecosystem-scale manipulation experiments, which are infeasible in most situations.We propose to circumvent this challenge by evaluating the uncertainty of water and light capture processes of a process-based vegetation model (ED2) including the liana growth form. We further developed the liana plant functional type in ED2 to mechanistically simulate water uptake and transport from roots to leaves, and start the model from prescribed initial conditions. We then used the PEcAn bioinformatics platform to constrain liana parameters and run uncertainty analyses.Baseline runs successfully reproduced ecosystem gas exchange fluxes (gross primary productivity and latent heat) and forest structural features (leaf area index, aboveground biomass) in two sites (Barro Colorado Island, Panama and Paracou, French Guiana) characterized by different rainfall regimes and levels of liana abundance.Model uncertainty analyses revealed that water limitation was the factor driving the competition between trees and lianas at the drier site (BCI), and during the relatively short dry season of the wetter site (Paracou). In young patches, light competition dominated in Paracou but alternated with water competition between the wet and the dry season on BCI according to the model simulations.The modelling workflow also identified key liana traits (photosynthetic quantum efficiency, stomatal regulation parameters, allometric relationships) and processes (water use, respiration, climbing) driving the model uncertainty. They should be considered as priorities for future data acquisition and model development to improve predictions of the carbon dynamics of liana-infested forests. Synthesis. Competition for water plays a larger role in the interaction between lianas and trees than previously hypothesized, as demonstrated by simulations from a process-based vegetation model.

Can Functional Traits Explain Plant Coexistence? A Case Study with Tropical Lianas and Trees

Organisms are adapted to their environment through a suite of anatomical, morphological, and physiological traits. These functional traits are commonly thought to determine an organism’s tolerance to environmental conditions. However, the differences in functional traits among co-occurring species, and whether trait differences mediate competition and coexistence is still poorly understood. Here we review studies comparing functional traits in two co-occurring tropical woody plant guilds, lianas and trees, to understand whether competing plant guilds differ in functional traits and how these differences may help to explain tropical woody plant coexistence. We examined 36 separate studies that compared a total of 140 different functional traits of co-occurring lianas and trees. We conducted a meta-analysis for ten of these functional traits, those that were present in at least five studies. We found that the mean trait value between lianas and trees differed significantly in four of the ten functional traits. Lianas differed from trees mainly in functional traits related to a faster resource acquisition life history strategy. However, the lack of difference in the remaining six functional traits indicates that lianas are not restricted to the fast end of the plant life–history continuum. Differences in functional traits between lianas and trees suggest these plant guilds may coexist in tropical forests by specializing in different life–history strategies, but there is still a significant overlap in the life–history strategies between these two competing guilds.

Costs and benefits of gas inside wood and its relationship with anatomical traits: a contrast between trees and lianas

Gas inside wood plays an important role in plant functioning, but there has been no study examining the adaptive nature of gas inside wood across plants differing in biomechanical demands. Using a comparative approach, we measured gas volumetric content, xylem's anatomical traits and wood density of 15 tree and 16 liana species, to test whether gas content varies between these plant types strongly differing in their biomechanical demands. We asked (i) whether trees and lianas differ in gas content and (ii) how anatomical traits and wood density are related to gas content. Lianas had significantly less gas content in their branches compared with tree species. In tree species, gas content scaled positively with fiber, vessel and xylem cross-sectional area and fiber and vessel diameter, and negatively with dry-mass density. When pooling trees and lianas together, fiber cross-sectional area was the strongest predictor of gas content, with higher xylem cross-sectional area of fiber associated with higher gas content. In addition, we showed, through a simple analytical model, that gas inside wood increases the minimum branch diameter needed to prevent rupture, and this effect was stronger on trees compared with lianas. Our results support the view that gas inside wood plays an important role in the evolution of biomechanical functioning in different plant forms. Gas inside wood may also play an important role in physiological activities such as water transport, storage, photosynthesis and respiration, but it is still unknown whether these roles are or are not secondary to the mechanical support.

Fully exposed canopy tree and liana branches in a tropical forest differ in mechanical traits but are similar in hydraulic traits

Large lianas and trees in the forest canopy are challenged by hydraulic and mechanical failures and need to balance hydraulic conductivity, hydraulic safety and mechanical safety. Our study integrates these functions in canopy branches to understand the performance of canopy trees and lianas, and their difference. We sampled and measured branches from 22 species at a canopy crane in the tropical forest at Xishuangbanna, SW China. We quantified the hydraulic conductivity from the xylem-specific hydraulic conductivity (KS), hydraulic safety from the cavitation resistance (P50) and mechanical safety from the modulus of rupture (MOR) to evaluate trade-offs and differences between lianas and trees. We also measured a number of anatomical features that may influence these three functional traits. Our results suggest the following: trade-offs between hydraulic conductivity, hydraulic safety and mechanical safety are weak or absent; liana branches better resist external mechanical forces (higher MOR) than tree branches; and liana and tree branches were similar in hydraulic performance (KS and P50). The anatomical features underlying KS, P50 and MOR may differ between lianas and trees. We conclude that canopy branches of lianas and trees diverged in mechanical design due to fundamental differences in wood formation, but converged in hydraulic design.

Are hydraulic patterns of lianas different from trees? New insights from Hedera helix

Lianas form long and flexible but disproportionately narrow stems, and thus require particular strategies to maintain the integrity of xylem water transport and ensure supply to large crown areas. The hydraulic architecture of lianas and the respective within-plant coordination of transport efficiency and safety, and the underlying anatomical variations in xylem, are largely unexplored. We analysed Hedera helix, a liana widespread in European temperate forests, with respect to hydraulic and xylem anatomical variations between the main stem and branches, between juvenile and adult life phases, and along the vertical axis. Main stems were significantly less embolism resistant but exhibited a higher hydraulic conductivity than branches. In branches, the cell turgor loss point of leaves decreased, while the embolism resistance and conductivity of xylem, as well as conduit diameters, increased with height. High water-transport capacities allow ivy to compensate for the small cross-section of stems, while the limited resistance to drought-induced xylem dysfunction of the main stem is probably linked to conservative stomatal regulation. Pronounced differences in xylem anatomy, hydraulic efficiency, and safety between the main stem and branches and along the vertical axis are surprisingly similar to those of self-supporting plants, and indicate that the coordination of carbon and water economies requires similar internal adjustments in tall plants.

The hydraulic efficiency-safety trade-off differs between lianas and trees

Hydraulic traits are important for woody plant functioning and distribution. Associations among hydraulic traits, other leaf and stem traits, and species’ performance are relatively well understood for trees, but remain poorly studied for lianas. We evaluated the coordination among hydraulic efficiency (i.e., maximum hydraulic conductivity), hydraulic safety (i.e., cavitation resistance), a suite of eight morphological and physiological traits, and species’ abundances for saplings of 24 liana species and 27 tree species in wet tropical forests in Panama. Trees showed a strong trade‐off between hydraulic efficiency and hydraulic safety, whereas efficiency and safety were decoupled in lianas. Hydraulic efficiency was strongly and similarly correlated with acquisitive traits for lianas and trees (e.g., positively with gas exchange rates and negatively with wood density). Hydraulic safety, however, showed no correlations with other traits in lianas, but with several in trees (e.g., positively with leaf dry matter content and wood density and negatively with gas exchange rates), indicating that in lianas hydraulic efficiency is an anchor trait because it is correlated with many other traits, while in trees both efficiency and safety are anchor traits. Traits related to shade tolerance (e.g., low specific leaf area and high wood density) were associated with high local tree sapling abundance, but not with liana abundance. Our results suggest that different, yet unknown mechanisms determine hydraulic safety and local‐scale abundance for lianas compared to trees. For trees, the trade‐off between efficiency and safety will provide less possibilities for ecological strategies. For lianas, however, the uncoupling of efficiency and safety could allow them to have high hydraulic efficiency, and hence high growth rates, without compromising resistance to cavitation under drought, thus allowing them to thrive and outperform trees under drier conditions.

Liana and tree below-ground water competition-evidence for water resource partitioning during the dry season

To date, reasons for the increase in liana abundance and biomass in the Neotropics are still unclear. One proposed hypothesis suggests that lianas, in comparison with trees, are more adaptable to drought conditions. Moreover, previous studies have assumed that lianas have a deeper root system, which provides access to deeper soil layers, thereby making them less susceptible to drought stress. The dual stable water isotope approach (δ18O and δ2H) enables below-ground vegetation competition for water to be studied. Based on the occurrence of a natural gradient in soil water isotopic signatures, with enriched signatures in shallow soil relative to deep soil, the origin of vegetation water sources can be derived. Our study was performed on canopy trees and lianas reaching canopy level in tropical forests of French Guiana. Our results show liana xylem water isotopic signatures to be enriched in heavy isotopes in comparison with those from trees, indicating differences in water source depths and a more superficial root activity for lianas during the dry season. This enables them to efficiently capture dry season precipitation. Our study does not support the liana deep root water extraction hypothesis. Additionally, we provide new insights into water competition between tropical canopy lianas and trees. Results suggest that this competition is mitigated during the dry season due to water resource partitioning.

Tree differences in primary and secondary growth drive convergent scaling in leaf area to sapwood area across Europe

Trees scale leaf (A_L ) and xylem (A_X ) areas to couple leaf transpiration and carbon gain with xylem water transport. Some species are known to acclimate in A_L  : A_X balance in response to climate conditions, but whether trees of different species acclimate in A_L  : A_X in similar ways over their entire (continental) distributions is unknown. We analyzed the species and climate effects on the scaling of A_L vs A_X in branches of conifers (Pinus sylvestris, Picea abies) and broadleaved (Betula pendula, Populus tremula) sampled across a continental wide transect in Europe. Along the branch axis, A_L and A_X change in equal proportion (isometric scaling: b ˜ 1) as for trees. Branches of similar length converged in the scaling of A_L vs A_X with an exponent of b = 0.58 across European climates irrespective of species. Branches of slow-growing trees from Northern and Southern regions preferentially allocated into new leaf rather than xylem area, with older xylem rings contributing to maintaining total xylem conductivity. In conclusion, trees in contrasting climates adjust their functional balance between water transport and leaf transpiration by maintaining biomass allocation to leaves, and adjusting their growth rate and xylem production to maintain xylem conductance.

Tropical dry forest trees and lianas differ in leaf economic spectrum traits but have overlapping water-use strategies

Tree species in tropical dry forests employ a wide range of strategies to cope with seasonal drought, including regulation of hydraulic function. However, it is uncertain if co-occurring lianas also possess a diversity of strategies. For a taxonomically diverse group of 14 tree and 7 liana species, we measured morphological and hydraulic functional traits during an unusual drought and under non-drought conditions to determine (i) if trees have different water-use strategies than lianas and (ii) if relationships among these traits can be used to better understand how tree and liana species regulate diurnal leaf water potential (Ψdiurnal). In this Costa Rican tropical dry forest, lianas and trees had overlapping water-use strategies, but differed in many leaf economic spectrum traits. Specifically, we found that both lianas and trees employed a diversity of Ψdiurnal regulation strategies, which did not differ statistically. However, lianas and trees did significantly differ in terms of certain traits including leaf area, specific leaf area, petiole length, wood vessel diameter and xylem vessel density. All liana and tree species we measured fell along a continuum of isohydric (partial) to anisohydric (strict or extreme) Ψdiurnal regulation strategies, and leaf area, petiole length, stomatal conductance and wood vessel diameter correlated with these strategies. These findings contribute to a trait-based understanding of how plants regulate Ψdiurnal under both drought stress and sufficient water availability, and underscore that lianas and trees employ a similarly wide range of Ψdiurnal regulation strategies, despite having vastly different growth forms.

Different hydraulic traits of woody plants from tropical forests with contrasting soil water availability

In southwestern China, tropical karst forests (KF) and non-karst rain forests (NKF) have different species composition and forest structure owing to contrasting soil water availability, but with a few species that occur in both forests. Plant hydraulic traits are important for understanding the species' distribution patterns in these two forest types, but related studies are rare. In this study, we investigated hydraulic conductivity, vulnerability to drought-induced cavitation and wood anatomy of 23 abundant and typical woody species from a KF and a neighboring NKF, as well as two Bauhinia liana species common to both forests. We found that the KF species tended to have higher sapwood density, smaller vessel diameter, lower specific hydraulic conductivity (ks) and leaf to sapwood area ratio, and were more resistant to cavitation than NKF species. Across the 23 species distinctly occurring in either KF or NKF, there was a significant tradeoff between hydraulic efficiency and safety, which might be an underlying mechanism for distributions of these species across the two forests. Interestingly, by possessing rather large and long vessels, the two Bauhinia liana species had extremely high ks but were also high resistance to cavitation (escaping hydraulic tradeoff). This might be partially due to their distinctly dimorphic vessels, but contribute to their wide occurrence in both forests.

Trade-offs between water transport capacity and drought resistance in neotropical canopy liana and tree species

In tropical forest canopies, it is critical for upper shoots to efficiently provide water to leaves for physiological function while safely preventing loss of hydraulic conductivity due to cavitation during periods of soil water deficit or high evaporative demand. We compared hydraulic physiology of upper canopy trees and lianas in a seasonally dry tropical forest to test whether trade-offs between safety and efficiency of water transport shape differences in hydraulic function between these two major tropical woody growth forms. We found that lianas showed greater maximum stem-specific hydraulic conductivity than trees, but lost hydraulic conductivity at less negative water potentials than trees, resulting in a negative correlation and trade-off between safety and efficiency of water transport. Lianas also exhibited greater diurnal changes in leaf water potential than trees. The magnitude of diurnal water potential change was negatively correlated with sapwood capacitance, indicating that lianas are highly reliant on conducting capability to maintain leaf water status, whereas trees relied more on stored water in stems to maintain leaf water status. Leaf nitrogen concentration was related to maximum leaf-specific hydraulic conductivity only for lianas suggesting that greater water transport capacity is more tied to leaf processes in lianas compared to trees. Our results are consistent with a trade-off between safety and efficiency of water transport and may have implications for increasing liana abundance in neotropical forests.

Liana effects on biomass dynamics strengthen during secondary forest succession

Secondary forests are important carbon sinks, but their biomass dynamics vary markedly within and across landscapes. The biotic and abiotic drivers of this variation are still not well understood. We tested the effects of soil resource availability and competition by lianas on the biomass dynamics of young secondary tropical forests in Panama and assessed the extent to which liana effects were mediated by soil resource availability. Over a five-year period, growth, mortality, and recruitment of woody plants of ≥1 cm diameter were monitored in 84 plots in 3-30-year-old secondary forests across the Agua Salud site in central Panama. Biomass dynamics and the effects of lianas and soil resources were examined using (generalized) linear mixed-effect models and a model averaging approach. There was strong spatial and temporal variation in liana biomass within and across the plots. The relative biomass of lianas had a strong negative effect on overall tree growth, growth of understory trees decreased with soil fertility and dry season soil water content, and the effect of lianas on tree mortality varied with soil fertility. Tree recruitment was not associated with any of the predictor variables. Our model indicates that tree biomass growth across our landscape was reduced with 22% due to competition with lianas, and that the effect of lianas increased during succession, from 19% after five years to 32% after 30 years. The projected liana-induced growth reduction after 60 years was 47%, which was consistent with data from a nearby site. Our study shows that the observed liana proliferation across tropical forests may reduce the sequestration and storage of carbon in young secondary forests, with important implications for the carbon balance of tropical forest landscapes and consequently for global climate change. Our study highlights the need to incorporate lianas and soil variables in research on the biomass dynamics of secondary forest across tropical landscapes, and the need for well-replicated longitudinal studies to cover landscape-level variability in the relevant abiotic and biotic components.

Time lags between crown and basal sap flows in tropical lianas and co-occurring trees

Water storage in the stems of woody plants contributes to their responses to short-term water shortages. To estimate the contribution of water storage to the daily water budget of trees, time lags of sap flow between different positions of trunk are used as a proxy of stem water storage. In lianas, another large group of woody species, it has rarely been studied whether stored water functions in their daily water use, despite their increasing roles in the carbon and water dynamics of tropical forests caused by their increasing abundance. We hypothesized that lianas would exhibit large time lags due to their extremely long stems, wide vessels and large volume of parenchyma in the stem. We examined time lags in sap flow, diel changes of stem volumetric water content (VWC) and biophysical properties of sapwood of 19 lianas and 26 co-occurring trees from 27 species in 4 forests (karst, tropical seasonal, flood plain and savanna) during a wet season. The plants varied in height/length from <5 to >60 m. The results showed that lianas had significantly higher saturated water content (SWC) and much lower wood density than trees. Seven of 19 liana individuals had no time lags; in contrast, only 3 of 26 tree individuals had no time lags. In general, lianas had shorter time lags than trees in our data set, but this difference was not significant for our most conservative analyses. Across trees and lianas, time lag duration increased with diurnal maximum changeable VWC but was independent of the body size, path length, wood density and SWC. The results suggest that in most lianas, internal stem water storage contributes little to daily water budget, while trees may rely more on stored water in the stem.

Short and Long-Term Soil Moisture Effects of Liana Removal in a Seasonally Moist Tropical Forest

Lianas (woody vines) are particularly abundant in tropical forests, and their abundance is increasing in the neotropics. Lianas can compete intensely with trees for above- and belowground resources, including water. As tropical forests experience longer and more intense dry seasons, competition for water is likely to intensify. However, we lack an understanding of how liana abundance affects soil moisture and hence competition with trees for water in tropical forests. To address this critical knowledge gap, we conducted a large-scale liana removal experiment in a seasonal tropical moist forest in central Panama. We monitored shallow and deep soil moisture over the course of three years to assess the effects of lianas in eight 0.64 ha removal plots and eight control plots. Liana removal caused short-term effects in surface soils. Surface soils (10 cm depth) in removal plots dried more slowly during dry periods and accumulated water more slowly after rainfall events. These effects disappeared within four months of the removal treatment. In deeper soils (40 cm depth), liana removal resulted in a multi-year trend towards 5–25% higher soil moisture during the dry seasons with the largest significant effects occurring in the dry season of the third year following treatment. Liana removal did not affect surface soil temperature. Multiple and mutually occurring mechanisms may be responsible for the effects of liana removal on soil moisture, including competition with trees, and altered microclimate, and soil structure. These results indicate that lianas influence hydrologic processes, which may affect tree community dynamics and forest carbon cycling.

Are functional traits a good predictor of global change impacts on tree species abundance dynamics in a subtropical forest?

Significant changes in the composition of tree species have been observed in various forests worldwide. We hypothesised that these changes might result from variable sensitivities of species to global change, and species sensitivities might be quantified, using functional traits. Employing long-term (1978-2010) species abundance data of 48 tree species from a permanent subtropical forest plot, where multiple global change factors have been observed, including soil drying, we examined the relationships between temporal trends in abundance and suits of functional traits. We found that species with high photosynthesis rates, leaf phosphorus and nitrogen concentrations, specific leaf area, hydraulic conductivity, turgor loss point and predawn leaf water potential had increased in abundance, while species with opposite trait patterns had decreased. Our results demonstrate that functional traits underlie tree species abundance dynamics in response to drought stress, thus linking traits to compositional shifts in this subtropical forest under global changes.

Water-use advantage for lianas over trees in tropical seasonal forests

Lianas exhibit peak abundance in tropical forests with strong seasonal droughts, the eco-physiological mechanisms associated with lianas coping with water deficits are poorly understood. We examined soil water partitioning, sap flow, and canopy eco-physiological properties for 99 individuals of 15 liana and 34 co-occurring tree species in three tropical forests that differed in soil water availability. In the dry season, lianas used a higher proportion of deep soil water in the karst forest (KF; an area with severe seasonal soil water deficit (SSWD)) and in the tropical seasonal forest (TSF, moderate SSWD), permitting them to maintain a comparable leaf water status than trees in the TSF or a better status than trees in the KF. Lianas exhibited strong stomatal control to maximize carbon fixation while minimizing dry season water loss. During the dry period, lianas significantly decreased water consumption in the TSF and the KF. Additionally, lianas had a much higher maximum photosynthetic rates and sap flux density in the wet season and a lower proportional decline in photosynthesis in the dry season compared with those of trees. Our results indicated that access to deep soil water and strong physiological adjustments in the dry season together with active wet-season photosynthesis may explain the high abundance of lianas in seasonally dry forests.

Species divergence and phylogenetic variation of ecophysiological traits in lianas and trees

The climbing habit is an evolutionary key innovation in plants because it is associated with enhanced clade diversification. We tested whether patterns of species divergence and variation of three ecophysiological traits that are fundamental for plant adaptation to light environments (maximum photosynthetic rate [A_max], dark respiration rate [R_d], and specific leaf area [SLA]) are consistent with this key innovation. Using data reported from four tropical forests and three temperate forests, we compared phylogenetic distance among species as well as the evolutionary rate, phylogenetic distance and phylogenetic signal of those traits in lianas and trees. Estimates of evolutionary rates showed that R_d evolved faster in lianas, while SLA evolved faster in trees. The mean phylogenetic distance was 1.2 times greater among liana species than among tree species. Likewise, estimates of phylogenetic distance indicated that lianas were less related than by chance alone (phylogenetic evenness across 63 species), and trees were more related than expected by chance (phylogenetic clustering across 71 species). Lianas showed evenness for R_d, while trees showed phylogenetic clustering for this trait. In contrast, for SLA, lianas exhibited phylogenetic clustering and trees showed phylogenetic evenness. Lianas and trees showed patterns of ecophysiological trait variation among species that were independent of phylogenetic relatedness. We found support for the expected pattern of greater species divergence in lianas, but did not find consistent patterns regarding ecophysiological trait evolution and divergence. R_d followed the species-level pattern, i.e., greater divergence/evolution in lianas compared to trees, while the opposite occurred for SLA and no pattern was detected for A_max. R_d may have driven lianas' divergence across forest environments, and might contribute to diversification in climber clades.

Different biomechanical design and ecophysiological strategies in juveniles of two liana species with contrasting growth habit

• Premise of the study: Lianas constitute a major functional type in tropical zones. While some liana species start climbing immediately after germination (shade-avoidance), others have a long self-supporting phase (shade-tolerance). The morphophysiological characteristics of these two growth habits are unknown.• Methods: We quantified growth traits, biomass allocation, mechanics, anatomy, and hydraulics for saplings of Ventilago calyculata (an immediate obligate climber) and Ziziphus attopensis (having a long self-supporting phase), both in the family Rhamnaceae. The mechanics, anatomy, and hydraulics for the mature individuals of the two species were also evaluated.• Key results: In the juvenile stage, V. calyculata had a higher slenderness ratio, height growth rate, and photosynthetic rate but similar biomass growth rate compared with Z. attopensis. In contrast, Z. attopensis had a higher leaf area growth rate, specific leaf area, and leaf mass fraction. Ziziphus attopensis had stiffer, but less conductive stems than V. calyculata. Stem rigidity of saplings decreased from base to apex in Z. attopensis, but increased in V. calyculata. Both species had similar resistance to xylem embolism. However, the leaves of V. calyculata were able to resist greater water deficits. At the mature stage, wider and longer vessels emerged in the xylem, and both species increased stem specific conductivity and drought resistance in stems and leaves. Ventilago calyculata had significantly higher specific conductivity and was more drought tolerant than Z. attopensis.• Conclusions: The two lianas differed significantly in growth, biomass allocation, anatomy, mechanics, ecophysiology, and hydraulic properties in line with their growth habits and shade adaptation strategies.

Phenotypic correlates of the lianescent growth form: a review

BACKGROUND

As proposed by Darwin, climbers have been assumed to allocate a smaller fraction of biomass to support organs in comparison with self-supporting plants. They have also been hypothesized to possess a set of traits associated with fast growth, resource uptake and high productivity.

SCOPE

In this review, these hypotheses are evaluated by assembling and synthesizing published and unpublished data sets from across the globe concerning resource allocation, growth rates and traits of leaves, stems and roots of climbers and self-supporting species.

CONCLUSIONS

The majority of studies offer little support for the smaller allocation of biomass to stems or greater relative growth rates in climbers; however, these results are based on small sized (<1 kg) plants. Simulations based on allometric biomass equations demonstrate, however, that larger lianas allocate a greater fraction of above-ground biomass to leaves (and therefore less biomass to stems) compared with similar sized trees. A survey of leaf traits of lianas revealed their lower average leaf mass per area (LMA), higher N and P concentration and a slightly higher mass-based photosynthetic rate, as well as a lower concentration of phenolic-based compounds than in woody self-supporting species, consistent with the specialization of lianas towards the fast metabolism/rapid turnover end of the global trait spectra. Liana stems have an efficient hydraulic design and unique mechanical features, while roots appear to penetrate deeper soil levels than in trees and are often able to generate hydraulic pressure. Much remains to be learned, however, about these and other functional specializations of their axial organs and the associated trade-offs. Developmental switches between self-supporting, searcher and climbing shoots within the same individual are a promising field of comparative studies on trait association in lianas. Finally, some of the vast trait variability within lianas may be reduced when species with different climbing mechanisms are considered separately, and when phylogenetic conservatism is accounted for.