Ya-Jun Chen

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.

Above- and below-ground competition in high and low irradiance: tree seedling responses to a competing liana Byttneria grandifolia

In tropical forests, trees compete not only with other trees, but also with lianas, which may limit tree growth and regeneration. Liana effects may depend on the availability of above- and below-ground resources and differ between tree species. We conducted a shade house experiment to test the effect of light (4% and 35% full sun, using neutral-density screen) on the competitive interactions between seedlings of one liana (Byttneria grandifolia) and three tree species (two shade-tolerant trees, Litsea dilleniifolia and Pometia tomentosa, and one light-demanding tree, Bauhinia variegata) and to evaluate the contribution of both above- and below-ground competition. Trees were grown in four competition treatments with the liana: no competition, root competition, shoot competition and root and shoot competition. Light strongly affected leaf photosynthetic capacity (light-saturated photosynthetic rate, Pn), growth and most morphological traits of the tree species. Liana-induced competition resulted in reduced Pn, total leaf areas and relative growth rates (RGR) of the three tree species. The relative importance of above- and below-ground competition differed between the two light levels. In low light,RGR of the threetree species was reduced more strongly by shoot competition (23.1-28.7% reduction) than by root competition (5.3-26.4%). In high light, in contrast, root competitionratherthanshootcompetitiongreatlyreducedRGR.Lianacompetitionaffectedmostmorphologicaltraits (except for specific leaf area and leaf area ratio of Litsea and Pometia), and differentially altered patterns of biomass allocation in the tree seedlings. These findings suggest that competition from liana seedlings can greatly suppress growth in tree seedlings of both light-demanding and shade-tolerant species and those effects differ with competition type (below- and above-ground) and with irradiance.

Responses of two field-grown coffee species to drought and re-hydration

The gas exchange, parameters of chlorophyll fluorescence, contents of pigments, and activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), as well as lipid peroxidation were investigated in two field-grown coffee species, Coffea arabica and C. liberica, exposed to drought and re-hydration. Drought caused a more pronounced inhibition of net photosynthetic rate in C. liberica compared to C. arabica. The de-epoxidation of xanthophyll cycle pigments at midday estimated by leaf reflectance was much higher in C. arabica than in C. liberica, but no significant change was found in response to drought. Under moderate drought, the activities of SOD and APX increased significantly only in C. arabica. The maximum photochemical efficiency of photosystem 2, PS2 (Fv/Fm) at predawn did not change and there was no lipid peroxidation during this time. Under severe drought Fv/Fm decreased and initial fluorescence (F0) increased for both species, and SOD activity increased, APX activity remained relatively high, and malondialdehyde (MDA) accumulated in C. arabica, while APX decreased in C. liberica. The photosynthetic apparatus of C. arabica was completely recovered after 5 d of re-irrigation as indicated by the restoration of Fv/Fm to the control values. A lack of recovery upon rewatering of C. liberica indicated irreversible damage to PS2. Hence compared to C. liberica, C. arabica possesses a higher desiccation-induced antioxidative protection and higher portion of the total pigment pool used in photoprotection, which might aid alleviating photoinhibitory damage during desiccation and photosynthesis recovery when favourable conditions are restored.

Nutrient resorption is associated with leaf vein density and growth performance of dipterocarp tree species

Nutrient resorption is important for the nutrient budget of plants, but little is known about which plant traits mediate nutrient resorption, how resorption efficiency is associated with other leaf traits and whether nutrient resorption has an impact on plant growth. In this study, 17 dipterocarp tree species were compared in a common garden experiment. N and P resorption efficiencies were regressed against suites of traits associated with phloem transport capacity (i.e. leaf vein density; D-vein), leaf nutrient conservation traits (e.g. leaf mass per area; LMA) and species growth rate. Across the dipterocarp species studied, N resorption efficiency (percentage N resorbed) was positively correlated with D-vein and leaf thickness. N resorption efficiency was also correlated with D-vein after considering phylogenetic effects. N resorption proficiency (N remaining in senesced leaves) was negatively correlated with D-vein, LMA, leaf thickness and palisade and spongy mesophyll thickness. Senesced-leaf N concentration was still negatively correlated with LMA and leaf thickness after considering phylogenetic effects. N resorption efficiency was positively correlated with both height and diameter growth rates. After considering phylogenetic effect, N resorption efficiency was marginally correlated with diameter growth rate. Green-leaf N concentration was positively correlated with height growth rate after considering phylogenetic effect. P resorption efficiency and proficiency were not related to any of the leaf morphological and anatomical traits, or to species growth rates.Synthesis. These results indicate that higher phloem transport capacity of the dipterocarp species is positively correlated with greater N resorption efficiency and that N resorption proficiency is closely linked with leaf nutrient conservation traits. Growth rates of the dipterocarps are more likely governed by photosynthetic rates associated with green-leaf N concentration than N resorption rates per se. Although P is generally deficient in tropical soils, it appears that N rather than P availability is the key limiting factor for the growth of the dipterocarp species.

The Heterogeneity and Spatial Patterning of Structure and Physiology across the Leaf Surface in Giant Leaves of Alocasia macrorrhiza

Leaf physiology determines the carbon acquisition of the whole plant, but there can be considerable variation in physiology and carbon acquisition within individual leaves. Alocasia macrorrhiza (L.) Schott is an herbaceous species that can develop very large leaves of up to 1 m in length. However, little is known about the hydraulic and photosynthetic design of such giant leaves. Based on previous studies of smaller leaves, and on the greater surface area for trait variation in large leaves, we hypothesized that A. macrorrhiza leaves would exhibit significant heterogeneity in structure and function. We found evidence of reduced hydraulic supply and demand in the outer leaf regions; leaf mass per area, chlorophyll concentration, and guard cell length decreased, as did stomatal conductance, net photosynthetic rate and quantum efficiency of photosystem II. This heterogeneity in physiology was opposite to that expected from a thinner boundary layer at the leaf edge, which would have led to greater rates of gas exchange. Leaf temperature was 8.8°C higher in the outer than in the central region in the afternoon, consistent with reduced stomatal conductance and transpiration caused by a hydraulic limitation to the outer lamina. The reduced stomatal conductance in the outer regions would explain the observed homogeneous distribution of leaf water potential across the leaf surface. These findings indicate substantial heterogeneity in gas exchange across the leaf surface in large leaves, greater than that reported for smaller-leafed species, though the observed structural differences across the lamina were within the range reported for smaller-leafed species. Future work will determine whether the challenge of transporting water to the outer regions can limit leaf size for plants experiencing drought, and whether the heterogeneity of function across the leaf surface represents a particular disadvantage for large simple leaves that might explain their global rarity, even in resource-rich environments.

Weak co-ordination between vein and stomatal densities in 105 angiosperm tree species along altitudinal gradients in Southwest China

Leaf-level water balance, as revealed by a correlation between stomatal density (SD) and vein density (VD), has been reported in some plants. However, the generality of this correlation and how it may be affected by altitude changes are unclear. Here, we investigated whether this balance is maintained across tree species of diverse families along a large altitudinal gradient. We measured leaf area (LA), SD, stomata length (SL), and VD in 105 angiosperm species across two altitudinal ranges, 800-1400m above sea level (a.s.l.) in tropical montane forests (TMF) and 2000-2600m a.s.l. in subtropical montane forests (SMF) in Yunnan, South-west China. The average SD was independent of altitude in both regions. Similarly, the average VD within either SMF or TMF was also not significantly different. However, overall, TMF had significantly larger VD and LA but smaller SL than SMF. Vein density was positively correlated with SD across SMF species, with a weaker correlation for TMF species and all species combined. Stomatal length was negatively correlated with SD and VD across all species. Our results extend the leaf water balance theory to diverse angiosperm tree species, and indicate decoupled adaptation of SD and VD in these species along a large altitudinal gradient.

Are invasive plants more competitive than native conspecifics? Patterns vary with competitors.

Invasive plants are sometimes considered to be more competitive than their native conspecifics, according to the prediction that the invader reallocates resources from defense to growth due to liberation of natural enemies [‘Evolution of Increased Competitive Ability’ (EICA) hypothesis]. However, the differences in competitive ability may depend on the identity of competitors. In order to test the effects of competitors, Ageratina adenophora plants from both native and invasive ranges competed directly, and competed with native residents from both invasive (China) and native (Mexico) ranges respectively. Invasive A. adenophora plants were more competitive than their conspecifics from native populations when competing with natives from China (interspecific competition), but not when competing with natives from Mexico. Invasive A. adenophora plants also showed higher competitive ability when grown in high-density monoculture communities of plants from the same population (intrapopulation competition). In contrast, invasive A. adenophora plants showed lower competitive ability when competing with plants from native populations (intraspecific competition). Our results indicated that in the invasive range A. adenophora has evolved to effectively cope with co-occurring natives and high density environments, contributing to invasion success. Here, we showed the significant effects of competitors, which should be considered carefully when testing the EICA hypothesis.

Implications of the Ecophysiological Adaptation of Plants on Tropical Karst Habitats for the Ecological Restoration of Desertified Rocky Lands in Southern China

Karst lands are widely spread in China, particularly in Southwest China, which cover the largest continuous karst area with a unique landscape. Due to the human interference for a long time and the special substrate and hot and humid climate, the karst lands in this region have been severely degraded, the desertified rocky karst lands now totally cover the area of 1.2×105 km2 in southern China. This results in environmental deterioration and difficulty for the social and economical development. Karst lands in southern China are characterized with thin or little soil, low soil nutrients and low water holding capacity. We have done a series of research on the ecophysiological adaption of plants on a tropical karst mountain. In this article, we briefly synthesize our research and the key findings, and also some other research in order to provide reference information for the ecological restoration of desertified karst lands. Our studies have indicated that the co-existence of evergreen and deciduous trees and lianas is helpful for the maintenance of water balance of karst ecosystems. The evergreen trees are tolerant to water deficit and xylem cavitation, while deciduous trees shed leaves to reduce water use in the dry season. Most of karst woody plants have deep roots, while the lianas have the deepest roots in order to reach deeper water source belowground in dry season. Some plants such as Bombax ceiba Linn. and Caryota urens Linn. store water in their stems to support the necessary physiological processes in dry season. Most of the tropical karst plants are short of Zn and K nutrients according to the foliar nutrient analysis, this should be taken into account during the ecological restoration and agricultural practices. Lianas are able to use the localized water and nutrients and employ the abundant light source and spaces of desertified karst lands by means of their climbing stems. They often have high photosynthesis and strong regulation of water use, and are useful plant materials for the restoration of the desertified karst lands. An important oil liana, Plukenetia volubilis L, is now recommended for plantation by the Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, which is an ideal plant for the restoration of desertified karst lands. The ecological restoration of desertified karst lands could consider using different function groups of plants to construct a multiple-storey community.

Carbon Allocation and Water Relations of Lianas Versus Trees

Despite lianas being fundamental components of tropical and subtropical forest ecosystems throughout the world, the physiological characteristics of this growth form are not well known. Different behaviors at the seedling stage were until recently largely unnoticed. In one extreme of a continuum of adaptive traits, freestanding liana seedlings invest a large proportion of biomass in self-support tissue while on the other extreme support-seeker seedlings invest more resources in rapid elongation of slender stems with an efficient hydraulic conductive system. Adult lianas often have lower wood density and higher specific leaf area than trees and have most of their leaves deployed at the top of the canopy, experiencing high irradiance and transpirational demands, which requires effective regulation of water loss to avoid desiccation. Recent studies show that lianas have faster stomatal responses to increasing vapor pressure deficit (VPD) and exhibit stronger partial stomatal closure compared to trees. Strong stomatal control and efficient water transport help lianas maintain leaf water potential (Ψleaf) within a safe hydraulic range to avoid xylem dysfunction despite their low stem water storage capacity, which is achieved at a minimum cost in terms of carbon assimilation. Liana colonization of tree crowns can significantly reduce tree growth and transpiration with consequences for carbon and water economy at individual tree and ecosystem levels.