Patterns of wood carbon dioxide efflux across a 2,000-m elevation transect in an Andean moist forest

During a 1-year measurement period, we recorded the CO2 efflux from stems (RS) and coarse woody roots (RR) of 13–20 common tree species at three study sites at 1,050, 1,890 and 3,050 m a.s.l. in an Andean moist forest. The objective of this work was to study elevation changes of woody tissue CO2 efflux and the relationship to climate variation, site characteristics and growth. Furthermore, we aim to provide insights into important respiration–productivity relationships of a little studied tropical vegetation type. We expected RS and RR to vary with dry and humid season conditions. We further expected RS to vary more than RR due to a more stable soil than air temperature regime. Seasonal variation in woody tissue CO2 efflux was indeed mainly attributable to stems. At the same time, temperature played only a small role in triggering variations in RS. At stand level, the ratio of C release (g C m−2 ground area year−1) between stems and roots varied from 4:1 at 1,050 m to 1:1 at 3,050 m, indicating the increasing prevalence of root activity at high elevations. The fraction of growth respiration from total respiration varied between 10 (3,050 m) and 14% (1,050 m) for stems and between 5 (1,050 m) and 30% (3,050 m) for roots. Our results show that respiratory activity and hence productivity is not driven by low temperatures towards higher elevations in this tropical montane forest. We suggest that future studies should examine the limitation of carbohydrate supply from leaves as a driver for the changes in respiratory activity with elevation

Contribution of previous year's leaf N and soil N uptake to current year's leaf growth in sessile oak

The origin of N which contributes to the synthesis of N reserves of in situ forest trees in autumn and to the growth of new organs the following spring is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20-year-old sessile oaks were ¹⁵N labelled by spraying ¹⁵NH₄¹⁵NO₃: (i) on leaves in May, to label the N pool remobilized in the autumn for synthesis of reserves, (ii) on soil in the autumn, to label the N pool taken up from soil and (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of ¹⁵N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of ¹⁵N into the soil–tree system; more than 30 % of the administered ¹⁵N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage ¹⁵N (73 %) and to a lesser extent from soil ¹⁵N (27 %). The following spring, ¹⁵N used for the synthesis of new leaves came first from ¹⁵N stored during the previous autumn, mainly from ¹⁵N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, ¹⁵N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %)

Drought and photosystem II activity in two Mediterranean oaks

The responses of photosystem II (PS II) to drought were analysed on two Mediterranean oak species, Quercus ilex and Q pubescens, using the chlorophyll fluorescence pulse-amplitude-modulation technique. The maximal PS II photochemical efficiency (Fv/Fm) of the evergreen Q ilex and the deciduous Q pubescens oaks was only affected when leaf predawn water potential was lower than -4 MPa. This value is rarely observed on mature trees growing in the field, but can be undergone by young seedlings during drought periods, hence confirming the stability of PS II. Whatever the irradiance, drought resulted, in both species, in lower values of PS II photochemical efficiency in a light-adapted state (ΔF/F m'), due to stomatal closure and/or a direct inhibition of the dark reactions of photosynthesis. Diurnal decreases of Fv/Fm of 30 min dark-adapted leaves were greater for lower predawn water potential; a recovery was observed in the late afternoon. The reversible decreases in the diurnal time-courses of maximal fluorescence led us to assume the onset of protective mechanisms from permanent photodamages in Q ilex and, to a lesser extent, in Q pubescens.Sécheresse et activité du photosystème II chez deux chênes méditerranéens. La réponse du photosystème II (PS II) à la sécheresse a été analysée sur deux espèces de chênes méditerranéens, Quercus ilex et Q pubescens par la technique de la fluorescence chlorophyllienne pulsée. L'efficacité photochimique maximale du PS II (Fv/Fm) de Q ilex (sempervirent) et de Q pubescens (caducifolié) n'a été affectée que pour un potentiel hydrique de base inférieur à -4 MPa. Une telle valeur n'est pratiquement jamais atteinte en conditions naturelles sur des arbres adultes, mais peut être subie par de jeunes plantules lors des sécheresses estivales, confirmant ainsi la résistance du PS II. Quel que soit l'éclairement, la sécheresse a induit, chez les deux espèces, des valeurs plus faibles de l'efficacité photochimique du PS II à la lumière (ΔF/Fm'), dues à une fermeture des stomates et (ou) une inhibition directe des réactions sombres de la photosynthèse. Les décroissances journalières de Fv/Fm après 30 minutes d'adaptation des feuilles à l'obscurité, étaient plus prononcées pour les potentiels de base les plus faibles ; une récupération a été observée en fin d'après-midi. Les diminutions diurnes, reversibles, de la fluorescence maximale traduisent la mise en place de mécanismes de protection contre les dommages résultant de niveaux lumineux élevés chez Q ilex et, à un degré moindre, chez Q pubescens

Effects of elevated carbon dioxide on leaf gas exchange and growth of cork-oak (Quercus suber L) seedlings

Leaf gas exchange and growth were determined on cork-oak (Quercus suber L) seedlings which were grown from acorns for periods of up to 4 months in greenhouses at ambient (350 μmol mol-1) and at elevated (700 μmol mor-1) concentrations of carbon dioxide. In well-watered conditions, daily maximum photosynthesis (15 μmol m-2 s -1) and stomatal conductance (440 mmol m-2 s-1) of plants grown and measured at 700 μmol mol-1 CO2 did not differ from those of plants grown and measured at 350 μmol mol-1. In conditions of moderate drought, net CO2 assimilation was at least twice as great in elevated CO2, but stomatal conductance was unchanged. Elevated CO2 affected total biomass production, the average increase being 76 and 97% at 3 and 4 months, respectively. Shoot biomass, root biomass, stem height and total leaf area were increased by elevated CO2. Root and stem ramification were also enhanced by elevated CO2, but no change in root/shoot ratio was observed.Effets d'une augmentation du CO2 atmosphérique sur les échanges gazeux et la croissance de plantules de chêne-liège (Quercus suber L). Des mesures de croissance et d'échanges gazeux ont été menées sur des plantules de chêne-liège (Quercus suber L) de 3 et 4 mois qui ont grandi avec une concentration en dioxyde de carbone de 350 μmol mol-1 ou de 700 μmol mol-1. Dans des conditions non limitantes en eau, la photosynthèse (15 μmol m-2 s-1) et la conductance stomatique (440 mmol m-2 s-1) maximales journalières, mesurées avec la concentration de CO2 de croissance, n'étaient pas différentes entre les deux traitements. En conditions de stress hydrique modéré, la photosynthèse nette était deux fois plus élevée en CO2 double, alors que les conductances stomatiques sont restées égales entre les deux traitements. La biomasse des jeunes chênes-lièges était plus élevée quand ils ont poussé à 700 μmol mol-1, le gain étant de 76 et 97 % à trois et quatre mois respectivement. La biomasse des tiges, des racines, la longueur de la tige principale et la surface foliaire totale ont été augmentées en CO2 double. Les ramifications des tiges et racines étaient plus nombreuses en CO 2 élevé mais aucune variation du rapport racine/tige n'a été observée