Viviana Horna

Phenology and stem-growth periodicity of tree species in Amazonian floodplain forests

To study the impact of the annual long-term flooding (flood-pulse) on seasonal tree development in Amazonian floodplains, the phenology and growth in stem diameter of various tree species with different leaf-change patterns were observed over a period of 2 y. The trees of the functional ecotypes, evergreen, brevi-deciduous, deciduous and stem-succulent showed a periodic behaviour mainly triggered by the flood-pulse. Trees have high increment during the terrestrial phase. Flooding causes a shedding of some or all leaves leading to a cambial dormancy of about 2 mo and the formation of an annual ring. Studies carried out in tropical dry forests verify a strong relationship between the phenological development and the water status of the trees, strongly affected by seasonal drought. The comparison of the phenology and the diameter growth of the corresponding ecotypes in floodplain forest and a semi-deciduous forest in Venezuela shows a displacement of at least 2 mo in the periodicity, except for stem-succulent tree species. For stem-succulent trees it remains unclear which factors influence phenology and stem diameter growth.

Leaf water status and stem xylem flux in relation to soil drought in five temperate broad-leaved tree species with contrasting water use strategies

Abstract• Five temperate broad-leaved tree species were compared with respect to their water consumption strategies under ample and restricted water supply. We measured synchronously leaf conductance (gL) in the sun canopy, xylem sap flux (Js) and leaf water potential (predawn, ψpd and noon, ψnoon) in adult trees in a mixed stand and related them to the fluctuations in vapor pressure deficit (D) and soil moisture.• Maximum gL was particularly high in F. excelsior, C. betulus and T. cordata and revealed a higher D sensitivity. ψpd remained constantly high in A. pseudoplatanus, C. betulus and F. excelsior, but decreased in T. cordata and F. sylvatica with decreasing soil moisture.• Jsddecreased linearly with decreasing soil matrix potential in all species except for F. excelsior. Apparent hydraulic conductance in the soil-to-leaf flow path (Lc) was higher in A. pseudoplatanus than in the other species.• F. sylvatica maintained a low maximum gL and reduced Jsd markedly upon drought, but faced severe decreases in ψpd and ψnoon. F. excelsior represents an opposite strategy with high maximum gL and stable ψpd.• The species drought sensitivity increases in the sequence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica.Résumé• Les stratégies de consommation d’eau de cinq espèces d’arbres feuillus tempérés ont été comparées sous approvisionnement en eau suffisant ou limité. De façon synchrone nous avons mesuré la conductance hydraulique des feuilles (gL) dans la partie du couvert exposée au soleil, le flux de sève xylémique (Js) et le potentiel hydrique foliaire (potentiel de base (ψpd) et potentiel minimum (ψnoon)) d’arbres adultes en peuplement mixte et nous les avons reliés aux fluctuations du déficit de pression de vapeur (D) et à l’humidité du sol.• gL maximum était particulièrement élevée chez F. excelsior, C. betulus et T. cordata et a révélé une plus grande sensibilité à D. ψpd est resté constamment élevé chez A. pseudoplatanus, C. betulus et F. excelsior, mais a diminué chez T. cordata et F. sylvatica lorsque l’humidité du sol diminuait.• Jsd a diminué linéairement avec le potentiel matriciel du sol pour toutes les espèces excepté F. excelsior. La conductivité hydraulique apparente du trajet sol-feuille (Lc) était plus élevée chez A. pseudoplatanus que dans les autres espèces.• F. sylvatica a maintenu une faible gL maximum et a réduit sensiblement Jsd face à la sécheresse, mais a connu de graves diminutions de ψpd et ψnoon. F. excelsior présentait une stratégie opposée avec une gL maximum élevée et un ψpd stable.• La sensibilité des espèces à la sécheresse augmente selon la séquence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica.

Seed and seedling survival of the palm Astrocaryum murumuru and the legume tree Dipteryx micrantha in gaps in Amazonian forest

The role of regeneration strategies in structuring tropical tree communities has been a recent focus of attention of tropical ecologists. Canopy discontinuities created by tree-falls are considered to be critical to many rain forest trees for establishing and completing their life cycles. It was investigated whether it is advantageous for two common large-seeded and shade- tolerant Amazonian tree species, the palm Astroca?yum murumuru and the legume, Dipteryx micrantha, to have their seeds dispersed to gaps. The dispersal process was simulated by placing experimental seeds and seedlings in gaps and in the shaded understorey, and then following their performance. Seeds and seedlings from the two species were used. The Cox proportional hazard model was used to estimate the survival functions and compare survivorship curves. Survival of Astroca?yum seeds, and of Dipteryx seeds and seedlings, was higher in two gap zones (crown and bole) than in the shaded understorey (both near and far away from a conspecific adult). A higher proportion of seedlings of both plant species survived within gaps than in the shaded understorey over an 18-wk period. One year after the seedlings had been transplanted into gaps these patterns still held. Naturally occurringAstrocaryum seeds survived better beneath conspecific adult trees located within 30 m of gaps than beneath trees located 100 m from gaps. Variation in gap size was significant for Dipteryx seeds, but not for Dipteryx seedlings and Astroca?yum seeds and seedlings. Dipteryx seeds survived longer in smaller than in larger gaps. By colonizing tree-fall gaps, these two species improved their chances of completing their life cycles.

Stem water storage in five coexisting temperate broad-leaved tree species: significance, temporal dynamics and dependence on tree functional traits

The functional role of internal water storage is increasingly well understood in tropical trees and conifers, while temperate broad-leaved trees have only rarely been studied. We examined the magnitude and dynamics of the use of stem water reserves for transpiration in five coexisting temperate broad-leaved trees with largely different morphology and physiology (genera Fagus, Fraxinus, Tilia, Carpinus and Acer). We expected that differences in water storage patterns would mostly reflect species differences in wood anatomy (ring vs. diffuse-porous) and wood density. Sap flux density was recorded synchronously at five positions along the root-to-branch flow path of mature trees (roots, three stem positions and branches) with high temporal resolution (2 min) and related to stem radius changes recorded with electronic point dendrometers. The daily amount of stored stem water withdrawn for transpiration was estimated by comparing the integrated flow at stem base and stem top. The temporal coincidence of flows at different positions and apparent time lags were examined by cross-correlation analysis. Our results confirm that internal water stores play an important role in the four diffuse-porous species with estimated 5-12 kg day(-1) being withdrawn on average in 25-28 m tall trees representing 10-22% of daily transpiration; in contrast, only 0.5-2.0 kg day(-1) was withdrawn in ring-porous Fraxinus. Wood density had a large influence on storage; sapwood area (diffuse- vs. ring-porous) may be another influential factor but its effect was not significant. Across the five species, the length of the time lag in flow at stem top and stem base was positively related to the size of stem storage. The stem stores were mostly exhausted when the soil matrix potential dropped below -0.1 MPa and daily mean vapor pressure deficit exceeded 3-5 hPa. We conclude that stem storage is an important factor improving the water balance of diffuse-porous temperate broad-leaved trees in moist periods, while it may be of low relevance in dry periods and in ring-porous species.

Changes in wood density, wood anatomy and hydraulic properties of the xylem along the root-to-shoot flow path in tropical rainforest trees

It is generally assumed that the largest vessels are occurring in the roots and that vessel diameters and the related hydraulic conductance in the xylem are decreasing acropetally from roots to leaves. With this study in five tree species of a perhumid tropical rainforest in Sulawesi (Indonesia), we searched for patterns in hydraulic architecture and axial conductivity along the flow path from small-diameter roots through strong roots and the trunk to distal sun-canopy twigs. Wood density differed by not more than 10% across the different flow path positions in a species, and branch and stem wood density were closely related in three of the five species. Other than wood density, the wood anatomical and xylem hydraulic traits varied in dependence on the position along the flow path, but were unrelated to wood density within a tree. In contrast to reports from conifers and certain dicotyledonous species, we found a hump-shaped variation in vessel diameter and sapwood area--specific conductivity along the flow path in all five species with a maximum in the trunk and strong roots and minima in both small roots and twigs; the vessel size depended on the diameter of the organ. This pattern might be an adaptation to the perhumid climate with a low risk of hydraulic failure. Despite a similar mean vessel diameter in small roots and twigs, the two distal organs, hydraulically weighted mean vessel diameters were on average 30% larger in small roots, resulting in ∼ 85% higher empirical and theoretical specific conductivities. Relative vessel lumen area in percent of sapwood area decreased linearly by 70% from roots to twigs, reflecting the increase in sclerenchymatic tissue and tracheids in acropetal direction in the xylem. Vessel size was more closely related to the organ diameter than to the distance along the root-to-shoot flow path. We conclude that (i) the five co-occurring tree species show convergent patterns in their hydraulic architecture despite different growth strategies, and (ii) the paradigm assuming continuous acropetal vessel tapering and decrease in specific conductance from fine roots towards distal twigs needs reconsideration.

The Carbon Balance of Tropical Mountain Forests Along an Altitudinal Transect

Not much is known about the role of tropical mountain forests in the global carbon cycle. This chapter summarises a decade of research on C pools and C fluxes in Andean mountain forests of the San Francisco region along an elevation transect from 1,000 m to 3,000 m a.s.l. based on measurements in 5 (3) intensively studied stands at five elevations and supplementary data collected in additional 54 forest plots at three elevations covering different topographic positions at these altitudes. With ecosystem C pools in the range of 320–370 Mg C ha−1, these forests store equally large, or even larger, amounts of C than neotropical lowland forests, despite the decrease in aboveground biomass with elevation. Gross and net primary production (NPP) and net ecosystem production all decrease largely with elevation while fine root production seems to increase. Our results show that tropical mountain forests are playing an important, yet underestimated, role as C stores.

Diverging temperature response of tree stem CO2 release under dry and wet season conditions in a tropical montane moist forest

It is commonly presumed that plant respiratory CO2 release increases with increasing temperature. However, we report on very contrasting stem CO2 release (RS)–temperature relationships of trees in a species-rich tropical montane forest of southern Ecuador under dry and wet season conditions. Rates of RS were low and completely uncoupled from the dial temperature regime during the humid season. In contrast, during the dry season, RS was generally higher and temperature sensitivity of RS differed greatly in degree and even in the direction of response, indicating that temperature might not be the only determinant of RS. In order to explain the heterogeneity of RS, we related RS to vapour pressure deficit, wind speed and solar radiation as important abiotic drivers influencing transpiration and photosynthesis. Stepwise multiple regression analyses with these meteorological predictors either were biased by high collinearity of the independent variables or could not enhance the ability to explain the variability of RS. We assume maintenance respiration to dominate under humid conditions unfavourable for energy acquisition of the tree, thus explaining the pronounced uncoupling of RS from atmospheric parameters. In contrast, the drier and hotter climate of the dry season seems to favour RS via enhanced assimilatory substrate delivery and stem respiratory activity as well as elevated xylem sap CO2 imports with increased transpiration. In addition, tree individual differences in the temperature responses of RS may mirror diverging climatic adaptations of co-existing moist forest tree species which have their distribution centre either at higher or lower elevations.

The hydraulic performance of tropical rainforest trees in their perhumid environment Is there evidence for drought vulnerability

In a species-rich perhumid tropical rainforest of Central Sulawesi, Indonesia, we studied the hydraulic properties of eight representative tree species and searched for evidence that analyzing hydraulic patterns at the species level provides valuable information to understand the role of abiotic drivers and structural parameters in controlling plant water consumption.We investigated the relationship between xylem hydraulic properties and tree size with the aim to determine possible hydraulic plasticity in response to the vertical variation in environmental conditions in an otherwise constantly humid forest.We found leaf-specific (LSC) and sapwood-area specific (kS) hydraulic conductivity of twigs to significantly increase with tree height across species. The marked increase of LSC and kS with tree height was closely coupled with an increase in mean vessel diameters and this trend was consistent for both, stem and twig xylem. Rates of xylem sap flow (J) of all tree species were strongly positively related to atmospheric vapor pressure deficit (VPD) and radiation (R), when evaluated on a daily basis. However, J started to level off at VPD values of about 0.4 kPa in trees of all size classes. We therefore concluded that the stomatal response of tropical moist forest trees is very sensitive to changes in the atmospheric evaporative demand.

Sap Flow and Stem Respiration

The effects of seasonal flooding on stem xylem flow and carbon release were investigated on common tree species of the Amazonian floodplain forests locally known as Varzea. The annual flooding lasts several months, reaches five to eight meters and drives the phenology of most forest species. Leaf shedding of deciduous trees starts at the onset of flooding and new leaves are produced after the peak of flooding in July. For evergreen species leaf shedding and new leaf production occur simultaneously during flooding. It has been generally assumed that these phenology patterns are associated to physiological stress during flooding. Here the focus is on tree and species functioning during water stress by flooding. Measurements of stem xylem flux and stem carbon release were taken to monitor changes in species ecophysiological behavior during flooding.