Ai-Ying Wang

Stem hydraulic traits and leaf water-stress tolerance are co-ordinated with the leaf phenology of angiosperm trees in an Asian tropical dry karst forest

BACKGROUND AND AIMSnThe co-occurring of evergreen and deciduous angiosperm trees in Asian tropical dry forests on karst substrates suggests the existence of different water-use strategies among species. In this study it is hypothesized that the co-occurring evergreen and deciduous trees differ in stem hydraulic traits and leaf water relationships, and there will be correlated evolution in drought tolerance between leaves and stems.nnnMETHODSnA comparison was made of stem hydraulic conductivity, vulnerability curves, wood anatomy, leaf life span, leaf pressure-volume characteristics and photosynthetic capacity of six evergreen and six deciduous tree species co-occurring in a tropical dry karst forest in south-west China. The correlated evolution of leaf and stem traits was examined using both traditional and phylogenetic independent contrasts correlations.nnnKEY RESULTSnIt was found that the deciduous trees had higher stem hydraulic efficiency, greater hydraulically weighted vessel diameter (D(h)) and higher mass-based photosynthetic rate (A(m)); while the evergreen species had greater xylem-cavitation resistance, lower leaf turgor-loss point water potential (π(0)) and higher bulk modulus of elasticity. There were evolutionary correlations between leaf life span and stem hydraulic efficiency, A(m), and dry season π(0). Xylem-cavitation resistance was evolutionarily correlated with stem hydraulic efficiency, D(h), as well as dry season π(0). Both wood density and leaf density were closely correlated with leaf water-stress tolerance and A(m).nnnCONCLUSIONSnThe results reveal the clear distinctions in stem hydraulic traits and leaf water-stress tolerance between the co-occurring evergreen and deciduous angiosperm trees in an Asian dry karst forest. A novel pattern was demonstrated linking leaf longevity with stem hydraulic efficiency and leaf water-stress tolerance. The results show the correlated evolution in drought tolerance between stems and leaves.

Ecology of hemiepiphytism in fig species is based on evolutionary correlation of hydraulics and carbon economy

Woody hemiepiphytic species (Hs) are important components of tropical rain forests, and they have been hypothesized to differ from non-hemiepiphytic tree species (NHs) in adaptations relating to water relations and carbon economy; but few studies have been conducted comparing ecophysiological traits between the two growth forms especially in an evolutionary context. Using common-garden plants of the genus Ficus, functional traits related to plant hydraulics and carbon economy were compared for seven NHs and seven Hs in their adult terrestrial tree-like growth phase. We used phylogenetically independent contrasts to test the hypothesis that differences in water availability selected for contrasting suites of traits in Hs and NHs, driving evolutionary correlations among functional traits including hydraulic conductivity and photosynthetic traits. Species of the two growth forms differed in functional traits; Hs had substantially lower xylem hydraulic conductivity and stomatal conductance, and higher instantaneous photosynthetic water use efficiency. Leaf morphological and structural traits also differed strikingly between the two growth forms. The Hs had significantly smaller leaves, higher leaf mass per area (LMA), and smaller xylem vessel lumen diameters. Across all the species, hydraulic conductivity was positively correlated with leaf gas exchange indicating high degrees of hydraulic-photosynthetic coordination. More importantly, these correlations were supported by correlations implemented on phylogenetic independent contrasts, suggesting that most trait correlations arose through repeated convergent evolution rather than as a result of chance events in the deep nodes of the lineage. Vatiation in xylem hydraulic conductivity was also centrally associated with a suite of other functional traits related to carbon economy and growth, such as LMA, water use efficiency, leaf nutrient concentration, and photosynthetic nutrient use efficiency, indicating important physiological constraints or trade-offs among functional traits. Shifts in this trait cluster apparently related to the adaptation to drought-prone canopy growth during the early life cycle of Hs and clearly affected ecophysiology of the later terrestrial stage of these species. Evolutionary flexibility in hydraulics and associated traits might be one basis for the hyper-diversification of Ficus species in tropical rain forests.

Differentiation in light energy dissipation between hemiepiphytic and non-hemiepiphytic Ficus species with contrasting xylem hydraulic conductivity

Hemiepiphytic Ficus species (Hs) possess traits of more conservative water use compared with non-hemiepiphytic Ficus species (NHs) even during their terrestrial growth phase, which may result in significant differences in photosynthetic light use between these two growth forms. Stem hydraulic conductivity, leaf gas exchange and chlorophyll fluorescence were compared in adult trees of five Hs and five NHs grown in a common garden. Hs had significantly lower stem hydraulic conductivity, lower stomatal conductance and higher water use efficiency than NHs. Photorespiration played an important role in avoiding photoinhibition at high irradiance in both Hs and NHs. Under saturating irradiance levels, Hs tended to dissipate a higher proportion of excessive light energy through thermal processes than NHs, while NHs dissipated a larger proportion of electron flow than Hs through the alternative electron sinks. No significant difference in maximum net CO2 assimilation rate was found between Hs and NHs. Stem xylem hydraulic conductivity was positively correlated with maximum electron transport rate and negatively correlated with the quantum yield of non-photochemical quenching across the 10 studied Ficus species. These findings indicate that a canopy growth habit during early life stages in Hs of Ficus resulted in substantial adaptive differences from congeneric NHs not only in water relations but also in photosynthetic light use and carbon economy. The evolution of epiphytic growth habit, even for only part of their life cycle, involved profound changes in a suite of inter-correlated ecophysiological traits that persist to a large extent even during the later terrestrial growth phase.

Responses of hydraulics at the whole-plant level to simulated nitrogen deposition of different levels in Fraxinus mandshurica

Nitrogen (N) deposition is expected to have great impact on forest ecosystems by affecting many aspects of plant-environmental interactions, one of which involves its influences on plant water relations through modifications of plant hydraulic architecture. However, there is a surprising lack of integrative study on tree hydraulic architecture responses to N deposition, especially at the whole-plant level. In the present study, we used a 5-year N addition experiment to simulate the effects of six different levels of N deposition (20-120u2005kgu2005ha(-1)u2005year(-1)) on growth and whole-plant hydraulic conductance of a dominant tree species (Fraxinus mandshurica Rupr.) from the typical temperate forest of NE China. The results showed that alleviation of N limitation by moderate concentrations of fertilization (20-80u2005kgu2005ha(-1)u2005year(-1)) promoted plant growth, but further N additions on top of the threshold level showed negative effects on plant growth. Growth responses of F. mandshurica seedlings to N addition of different concentrations were accompanied by corresponding changes in whole-plant hydraulic conductance; higher growth rate was accompanied by reduced whole-plant hydraulic conductance (Kplant) and higher leaf water-use efficiency. A detailed analysis on hydraulic conductance of different components of the whole-plant water transport pathway revealed that changes in root and leaf hydraulic conductance, rather than that of the stem, were responsible for Kplant responses to N fertilization. Both plant growth and hydraulic architecture responses to increasing levels of N addition were not linear, i.e., the correlation between measured parameters and N availability exhibited bell-shaped curves with peak values observed at medium levels of N fertilization. Changes in hydraulic architecture in response to fertilization found in the present study may represent an important underlying mechanism for the commonly observed changes in water-related tree performances in response to N deposition.

Is hemiepiphytism an adaptation to high irradiance? Testing seedling responses to light levels and drought in hemiepiphytic and non‐hemiepiphytic Ficus

The epiphytic growth habit in many Ficus species during their juvenile stages has commonly been hypothesized to be an adaptation for avoiding deep shade in the forest understory, but this has never been tested experimentally. We examined growth and ecophysiology in seedlings of three hemiepiphytic (Hs) and three non-hemiepiphytic (NHs) Ficus species grown under different irradiance levels. Both Hs and NHs exhibited characteristics of high light requiring species, such as high plasticity to growth irradiance and relatively high maximum photosynthetic assimilation rates. Diurnal measurements of leaf gas exchange showed that Hs have much shorter active photosynthetic periods than NHs; moreover, leaves of Hs have lower xylem hydraulic conductivity but stronger drought tolerance as indicated by much lower rates of leaf diebacks during the drought treatment. Seedlings of NHs had 3.3- and 13.3-fold greater height and biomass than those of Hs species after growing in the nursery for 5u2009months, indicating a trade-off between growth and drought tolerance due to the conflicting requirements for xylem conductivity and cavitation resistance. This study does not support the shade-avoidance hypothesis; rather, it suggests that the canopy regeneration in Hs is an adaptation to avoid alternative terrestrial growth-related risks imposed to tiny Ficus seedlings. The NHs with terrestrial regeneration reduce these risks by having an initial burst of growth to rapidly gain relatively large seedling sizes, while in Hs seedlings more conservative water use and greater drought tolerance for surviving the canopy environment are intrinsically associated with slow growth.

Microenvironment in the canopy rivals the host tree water status in controlling sap flow of a mistletoe species

Mistletoes absorb water from the vascular system of their hosts and thus the water use of mistletoes can be influenced by the water status of their hosts besides abiotic environmental conditions; however, there is a lack of studies on the dynamics of mistletoe water utilization in relation to both types of controlling factors. By building a canopy platform at 20 m above the ground, we monitored the dynamic changes of sap flow of Viscum coloratum (Kom.) Nakai (Loranthaceae) in combination with continuous measurements of microclimatic variables and volumetric water content (VWC) of its host tree branch xylem. We found that the host tree VWC exhibited substantial fluctuations during sunny days but lower VWC of the host did not negatively affect the sap flow of V. coloratum. Hourly and daily mean transpiration rates (Esap) of V. coloratum calculated from sap flow measurements showed strong positive correlations with photosynthetic photon flux density (PPFD) and vapor pressure deficit (VPD) measured in close vicinity to the point of mistletoe attachment. The mean Esap of V. coloratum was substantially higher than that of their host during clear days (4.55 ± 0.54 vs 2.01 ± 0.15 kg m-2 day-1). Moreover, the mistletoe-to-host transpiration ratio was not constant but became increasingly larger with the increase of PPFD or VPD on both hourly and daily bases, suggesting a weaker control of water loss in the mistletoe in comparison to its host species. The strong dependence of mistletoe Esap on micrometeorological variables and its decoupling from the host tree xylem water status suggests that the development of dense tree canopy functions as a potential mechanism for the host trees in reducing the competitive water use of mistletoes. These findings have important implications for the interactions between mistletoe species and their host trees in temperate forests.

Hydraulics play an important role in causing low growth rate and dieback of aging Pinus sylvestris var. mongolica trees in plantations of Northeast China

The frequently observed forest decline in water-limited regions may be associated with impaired tree hydraulics, but the precise physiological mechanisms remain poorly understood. We compared hydraulic architecture of Mongolian pine (Pinus sylvestris var. mongolica) trees of different size classes from a plantation and a natural forest site to test whether greater hydraulic limitation with increasing size plays an important role in tree decline observed in the more water-limited plantation site. We found that trees from plantations overall showed significantly lower stem hydraulic efficiency. More importantly, plantation-grown trees showed significant declines in stem hydraulic conductivity and hydraulic safety margins as well as syndromes of stronger drought stress with increasing size, whereas no such trends were observed at the natural forest site. Most notably, the leaf to sapwood area ratio (LA/SA) showed a strong linear decline with increasing tree size at the plantation site. Although compensatory adjustments in LA/SA may mitigate the effect of increased water stress in larger trees, they may result in greater risk of carbon imbalance, eventually limiting tree growth at the plantation site. Our results provide a potential mechanistic explanation for the widespread decline of Mongolian pine trees in plantations of Northern China.

Elevated ozone concentration decreases whole-plant hydraulic conductance and disturbs water use regulation in soybean plants

Elevated tropospheric ozone (O3 ) concentration has been shown to affect many aspects of plant performance including detrimental effects on leaf photosynthesis and plant growth. However, it is not known whether such changes are accompanied by concomitant responses in plant hydraulic architecture and water relations, which would have great implications for plant growth and survival in face of unfavorable water conditions. A soybean (Glycine max (L.) Merr.) cultivar commonly used in Northeast China was exposed to non-filtered air (NF, averaged 24.0xa0nlu2009l-1 ) and elevated O3 concentrations (eO3 , 40u2009nlu2009l-1 supplied with NF air) in six open-top chambers for 50u2009days. The eO3 treatment resulted in a significant decrease in whole-plant hydraulic conductance that is mainly attributable to the reduced hydraulic conductance of the root system and the leaflets, while stem and leaf petiole hydraulic conductance showed no significant response to eO3 . Stomatal conductance of plants grown under eO3 was lower during mid-morning but significantly higher at midday, which resulted in substantially more negative daily minimum water potentials. Moreover, excised leaves from the eO3 treated plants showed significantly higher rates of water loss, suggesting a lower ability to withhold water when water supply is impeded. Our results indicate that, besides the direct detrimental effects of eO3 on photosynthetic carbon assimilation, its influences on hydraulic architecture and water relations may also negatively affect O3 -sensitive crops by deteriorating the detrimental effects of unfavorable water conditions.