Walter Oberhuber

Impact of drought on the temporal dynamics of wood formation in Pinus sylvestris

We determined the temporal dynamics of cambial activity and xylem cell differentiation of Scots pine (Pinus sylvestris L.) within a dry inner Alpine valley (750 m a.s.l., Tyrol, Austria), where radial growth is strongly limited by drought in spring. Repeated micro-sampling of the developing tree ring of mature trees was carried out during two contrasting years at two study plots that differ in soil water availability (xeric and dry-mesic sites). In 2007, when air temperature at the beginning of the growing season in April exceeded the long-term mean by 6.4 degrees C, cambial cell division started in early April at both study plots. A delayed onset of cambial activity of c. 2 weeks was found in 2008, when average climate conditions prevailed in spring, indicating that resumption of cambial cell division after winter dormancy is temperature controlled. Cambial cell division consistently ended about the end of June/early July in both study years. Radial enlargement of tracheids started almost 3 weeks earlier in 2007 compared with 2008 at both study plots. At the xeric site, the maximum rate of tracheid production in 2007 and 2008 was reached in early and mid-May, respectively, and c. 2 weeks later at the dry-mesic site. Since in both study years more favorable growing conditions (i.e., an increase in soil water content) were recorded during summer, we suggest a strong sink competition for carbohydrates to mycorrhizal root and shoot growth. Wood formation stopped c. 4 weeks earlier at the xeric compared with the dry-mesic site in both years, indicating a strong influence of drought stress on cell differentiation. This is supported by radial widths of earlywood cells, which were found to be significantly narrower at the xeric than at the dry-mesic site (P < 0.05). Repeated cellular analyses during the two growing seasons revealed that, although spatial variability in the dynamics and duration of cell differentiation processes in P. sylvestris exposed to drought is strongly influenced by water availability, the onset of cambial activity and cell differentiation is controlled by temperature.

Photoinhibition of photosynthesis under natural conditions in ivy (Hedera helix L.) growing in an understory of deciduous trees

We investigated to what extent south-exposed leaves (E-leaves) of the evergreen ivy (Hedera helix L.) growing in the shadow of two deciduous trees suffered from photoinhibition of photosynthesis when leaf-shedding started in autumn. Since air temperatures drop concomitantly with increase in light levels, changes in photosynthetic parameters (apparent quantum yield, Φi and maximal photosynthetic capacity of O2 evolution, Pmax; chlorophyll-a fluorescence at room temperature) as well as pigment composition were compared with those in north-exposed leaves of the same clone (N-leaves; photosynthetic photon flux density PPFD< 100 μmol · m−2 · s−2) and phenotypic sun leaves (S-leaves; PPFD up to 2000 μmol · m−2 · s−1).In leaves exposed to drastic light changes during winter (E-leaves) strong photoinhibition of photosynthesis could be observed as soon as the incident PPFD increased in autumn. In contrast, in N-leaves the ratio of variable fluorescence to maximum fluorescence (FV/FMm) and Φi did not decline appreciably prior to severe frosts (up to -12° C) in January. At this time, Φi was reduced to a similar extent in all leaves, from about 0.073 μmol O2 · μmol−1 photons before stress to about 0.020. Changes in Φi were linearly correlated with changes in fv/fm (r = 0.955). The strong reduction in FV/FM on exposure to stress was caused by quenching in FM. The initial fluorescence (F0), however, was also quenched in all leaves. The diminished fluorescence yield was accompanied by an increase in zeaxanthin content. These effects indicate that winter stress in ivy primarily induces an increase in non-radiative energy-dissipation followed by “photoinhibitory damage” of PSII. Although a pronounced photooxidative bleaching of chloroplast pigments occurred in January (especially in E-leaves), photosynthetic parameters recovered completely in spring. Thus, the reduction in potential photosynthetic yield in winter may be up to three times greater in leaves subjected to increasing light levels than in leaves not exposed to a changing light environment.

Woody biomass production lags stem-girth increase by over one month in coniferous forests

Wood is the main terrestrial biotic reservoir for long-term carbon sequestration(1), and its formation in trees consumes around 15% of anthropogenic carbon dioxide emissions each year(2). However, the seasonal dynamics of woody biomass production cannot be quantified from eddy covariance or satellite observations. As such, our understanding of this key carbon cycle component, and its sensitivity to climate, remains limited. Here, we present high-resolution cellular based measurements of wood formation dynamics in three coniferous forest sites in northeastern France, performed over a period of 3 years. We show that stem woody biomass production lags behind stem-girth increase by over 1 month. We also analyse more general phenological observations of xylem tissue formation in Northern Hemisphere forests and find similar time lags in boreal, temperate, subalpine and Mediterranean forests. These time lags question the extension of the equivalence between stem size increase and woody biomass production to intra-annual time scales(3, 4, 5, 6). They also suggest that these two growth processes exhibit differential sensitivities to local environmental conditions. Indeed, in the well-watered French sites the seasonal dynamics of stem-girth increase matched the photoperiod cycle, whereas those of woody biomass production closely followed the seasonal course of temperature. We suggest that forecasted changes in the annual cycle of climatic factors(7) may shift the phase timing of stem size increase and woody biomass production in the future.

A meta-analysis of cambium phenology and growth: linear and non-linear patterns in conifers of the northern hemisphere.

BACKGROUND AND AIMS Ongoing global warming has been implicated in shifting phenological patterns such as the timing and duration of the growing season across a wide variety of ecosystems. Linear models are routinely used to extrapolate these observed shifts in phenology into the future and to estimate changes in associated ecosystem properties such as net primary productivity. Yet, in nature, linear relationships may be special cases. Biological processes frequently follow more complex, non-linear patterns according to limiting factors that generate shifts and discontinuities, or contain thresholds beyond which responses change abruptly. This study investigates to what extent cambium phenology is associated with xylem growth and differentiation across conifer species of the northern hemisphere. METHODS Xylem cell production is compared with the periods of cambial activity and cell differentiation assessed on a weekly time scale on histological sections of cambium and wood tissue collected from the stems of nine species in Canada and Europe over 1-9 years per site from 1998 to 2011. KEY RESULTS The dynamics of xylogenesis were surprisingly homogeneous among conifer species, although dispersions from the average were obviously observed. Within the range analysed, the relationships between the phenological timings were linear, with several slopes showing values close to or not statistically different from 1. The relationships between the phenological timings and cell production were distinctly non-linear, and involved an exponential pattern. CONCLUSIONS The trees adjust their phenological timings according to linear patterns. Thus, shifts of one phenological phase are associated with synchronous and comparable shifts of the successive phases. However, small increases in the duration of xylogenesis could correspond to a substantial increase in cell production. The findings suggest that the length of the growing season and the resulting amount of growth could respond differently to changes in environmental conditions.

Effects of climate variables on intra-annual stem radial increment in Pinus cembra (L.) along the alpine treeline ecotone

Abstract• Within the alpine treeline ecotone tree growth is increasingly restricted by extreme climate conditions. Although intra-annual stem growth recorded by dendrometers can be linked to climate, stem diameter increments in slow-growing subalpine trees are masked by changes in tree water status.• We tested the hypothesis that intra-annual radial stem growth in Pinus cembra is influenced by different climate variables along the treeline ecotone in the Austrian Alps. Dendrometer traces were compared with dynamics of xylem cell development to date onset of cambial activity and radial stem growth in spring.• Daily fluctuations in stem radius reflected changes in tree water status throughout the treeline ecotone. Extracted daily radial increments were significantly correlated with air temperature at the timberline and treeline only, where budburst, cambial activity and enlargement of first tracheids also occurred quite similarly. A close relationship was detected between radial increment and number of enlarging tracheids throughout the treeline ecotone.• We conclude that (i) the relationship between climate and radial stem growth within the treeline ecotone is dependent on a close coupling to atmospheric climate conditions and (ii) initiation of cambial activity and radial growth in spring can be distinguished from stem re-hydration by histological analysis.Résumé• La croissance des arbres est fortement limitée par les conditions climatiques extrêmes de la limite altitudinale de la forêt. Les variations interannuelles de croissance radiale enregistrées par des dendromètres de tige peuvent être reliées au climat. Mais la croissance radiale peut être masquée par des changements de l’état hydrique des arbres.• Nous avons testé l’hypothèse selon laquelle les variations intra-annuelles de croissance radiale des troncs de P. cembra étaient contrôlées par différents facteurs climatiques le long de l’écotone de la limite altitudinale de la forêt dans les Alpes autrichiennes. Les enregistrements obtenus à partir de dendromètres ont été comparés à des dynamiques de développement des cellules cambiales, afin de détecter avec précision la date de départ de la croissance cambiale au printemps.• Les variations diurnes de diamètre du tronc reflétaient des variations de l’état hydrique des arbres le long de l’écotone. Les incréments quotidiens en diamètre extraits de ces signaux étaient corrélés à la température de l’air à la limite forestière, mais ne l’étaient plus à des altitudes supérieures, alors que le débourrement des bourgeons, la reprise d’activité cambiale et l’expansion des premiers trachéides se produisaient pratiquement en même temps le long de l’écotone. Une forte corrélation a été détectée tout au long de l’écotone, entre la croissance radiale et le nombre de trachéides en expansion.• Nous concluons que : (i) la croissance radiale le long de l’écotone de la limite altitudinale de la forêt est fortement couplée aux conditions climatiques dans l’atmosphère, et que : (ii) l’initiation de l’activité cambiale et de la croissance radiale au printemps peut être distinguée de la réhydratation des troncs en mobilisant des analyses histologiques.

Effects of atmospheric and climate change at the timberline of the Central European Alps

Abstract• This review considers potential effects of atmospheric change and climate warming within the timberline ecotone of the Central European Alps. After focusing on the impacts of ozone (O3) and rising atmospheric CO2 concentration, effects of climate warming on the carbon and water balance of timberline trees and forests will be outlined towards conclusions about changes in tree growth and treeline dynamics.• Presently, ambient ground-level O3 concentrations do not exert crucial stress on adult conifers at the timberline of the Central European Alps. In response to elevated atmospheric CO2Larix decidua showed growth increase, whereas no such response was found in Pinus uncinata. Overall climate warming appears as the factor responsible for the observed growth stimulation of timberline trees.• Increased seedling re-establishment in the Central European Alps however, resulted from invasion into potential habitats rather than upward migration due to climate change, although seedlings will only reach tree size upon successful coupling with the atmosphere and thus loosing the beneficial microclimate of low stature vegetation.• In conclusion, future climate extremes are more likely than the gradual temperature increase to control treeline dynamics in the Central European Alps.Résumé• Cette étude examine les effets potentiels des changements atmosphériques et du réchauffement climatique au sein de l’écotone que constitue la limite supérieure de la forêt dans les Alpes centrales en Europe. Après avoir mis l’accent sur les effets de l’ozone (O3) et de l’augmentation des concentrations atmosphériques du CO2, les effets du réchauffement climatique sur le bilan de carbone et le bilan hydrique des arbres et des forêts à la limite supérieure de la forêt seront présentés en vue de tirer des conclusions sur l’évolution de la croissance des arbres et sur les dynamiques de la limite supérieure de la forêt.• Actuellement, les concentrations en O3 de l’air ambiant au niveau du sol n’exercent pas un stress critique sur les arbres adultes à la limite supérieure de la forêt dans les Alpes centrales en Europe. En réponse à des concentrations élevées en CO2Larix decidua a montré une augmentation de la croissance, alors qu’une telle réponse n’a pas été trouvée chez Pinus uncinata. Globalement, le réchauffement climatique apparaît être le facteur responsable de la stimulation de la croissance observée chez les arbres à la limite supérieure de la forêt.• Toutefois, l’augmentation de la réinstallation des semis dans les Alpes centrales en Europe est le résultats de l’invasion d’habitats potentiels plutôt qu’une migration en altitude due aux changements climatiques, bien que les semis atteindront seulement la taille des arbres après un couplage réussit avec l’atmosphère et donc perdant le microclimat favorable d’une végétation basse.• En conclusion, les phénomènes climatiques extrêmes futurs sont plus susceptibles de contrôler les dynamiques de limite supérieure de la forêt, que l’augmentation progressive de la température dans les Alpes centrales en Europe.

Effects of environmental conditions on onset of xylem growth in Pinus sylvestris under drought

We determined the influence of environmental factors (air and soil temperature, precipitation, photoperiod) on onset of xylem growth in Scots pine (Pinus sylvestris L.) within a dry inner Alpine valley (750 m a.s.l., Tyrol, Austria) by repeatedly sampling micro-cores throughout 2007-10 at two sites (xeric and dry-mesic) at the start of the growing season. Temperature sums were calculated in degree-days (DD) ≥5 °C from 1 January and 20 March, i.e., spring equinox, to account for photoperiodic control of release from winter dormancy. Threshold temperatures at which xylogenesis had a 0.5 probability of being active were calculated by logistic regression. Onset of xylem growth, which was not significantly different between the xeric and dry-mesic sites, ranged from mid-April in 2007 to early May in 2008. Among most study years, statistically significant differences (P<0.05) in onset of xylem growth were detected. Mean air temperature sums calculated from 1 January until onset of xylem growth were 230 ± 44 DD (mean ± standard deviation) at the xeric site and 205 ± 36 DD at the dry-mesic site. Temperature sums calculated from spring equinox until onset of xylem growth showed somewhat less variability during the 4-year study period, amounting to 144 ± 10 and 137 ± 12 DD at the xeric and dry-mesic sites, respectively. At both sites, xylem growth was active when daily minimum, mean and maximum air temperatures were 5.3, 10.1 and 16.2 °C, respectively. Soil temperature thresholds and DD until onset of xylem growth differed significantly between sites, indicating minor importance of root-zone temperature for onset of xylem growth. Although spring precipitation is known to limit radial growth in P. sylvestris exposed to a dry inner Alpine climate, the results of this study revealed that (i) a daily minimum air temperature threshold for onset of xylem growth in the range 5-6 °C exists and (ii) air temperature sum rather than precipitation or soil temperature triggers start of xylem growth. Based on these findings, we suggest that drought stress forces P. sylvestris to draw upon water reserves in the stem for enlargement of first tracheids after cambial resumption in spring.

Quantifying ozone uptake at the canopy level of spruce, pine and larch trees at the alpine timberline: an approach based on sap flow measurement.

Micro-climatic and ambient ozone data were combined with measurements of sap flow through tree trunks in order to estimate whole-tree ozone uptake of adult Norway spruce (Picea abies), cembran pine (Pinus cembra), and European larch (Larix decidua) trees. Sap flow was monitored by means of the heat balance approach in two trees of each species during the growing season of 1998. In trees making up the stand canopy, the ozone uptake by evergreen foliages was significantly higher than by deciduous ones, when scaled to the ground area. However, if expressed per unit of whole-tree foliage area, ozone flux through the stomata into the needle mesophyll was 1.09, 1.18 and 1.40 nmol m(-2) s(-1) in Picea abies, Pinus cembra and Larix decidua, respectively. These fluxes are consistent with findings from measurements of needle gas exchange, published from the same species at the study site. It is concluded that the sap flow-based approach offers an inexpensive, spatially and temporally integrating way for estimating ozone uptake at the whole-tree and stand level, intrinsicly covering the effect of boundary layers on ozone flux.

Intra-annual dynamics of stem CO2 efflux in relation to cambial activity and xylem development in Pinus cembra.

The relationship between stem CO(2) efflux (E(S)), cambial activity and xylem production in Pinus cembra L. was determined at the timberline (1950 m a.s.l.) of the Central Austrian Alps, for 1 year. The E(S) was measured continuously from June 2006 to August 2007 using an infrared gas-analysis system. Cambial activity and xylem production were determined by repeated microcore sampling of the developing tree ring, and radial increment was monitored using automated point dendrometers. Besides temperature, the number of living tracheids and cambial cells was predominantly responsible for E(S), and E(S) normalized to 10 degrees C (E(S10)) was significantly correlated to the number of living cells throughout the year (r(2) = 0.574; P < 0.001). However, elevated E(S) and missing correlation between E(S10) and xylem production were detected during cambial reactivation in April and during transition from active phase to rest, which occurred in August and lasted until early September. Results of this study indicate that (i) during seasonal variations in cambial activity, nonlinearity between E(S) and xylem production occurs and (ii) elevated metabolic activity during transition stages in the cambial active-dormancy cycle influences the carbon budget of P. cembra. Daily radial stem increment was primarily influenced by the number of enlarging cells and was not correlated to E(S).

Photosynthetic temperature adaptation of Pinus cembra within the timberline ecotone of the Central Austrian Alps.

Abstract• Temperature is suggested to determine the upper limit of tree life. Therefore, future climate warming may be of importance for tree distribution within the European Alps, where low temperatures limit carbon metabolism.• We focused on the effects of air and soil temperature on net photosynthesis (Pn) of Pinus cembra an evergreen climax species of the timberline ecotone of the Central Austrian Alps. Light response and temperature response curves were estimated along an altitudinal gradient ranging from the forest limit up to the krummholz limit in both summer and fall.• In general, Pn was significantly lower in fall as compared to summer. Nevertheless, independent from season mean Pn values tended to increase with elevation and were positively correlated with root zone temperatures. The specific leaf area by contrast declined with increasing elevation. Furthermore, the temperature optimum of net photosynthesis declined with increasing elevation and was positively correlated with the mean maximum air temperature of the 10 days prior the date of measurement.• Thus, our findings appear to reflect a long-term adaptation of the photosynthetic apparatus of Pinus cembra to the general temperature conditions with respect to elevation combined with a short term acclimation to the prevailing temperature regime.Résumé• La température est supposée déterminer la limite supérieure de la vie de des arbres. Par conséquent, le réchauffement climatique futur, peut avoir de l’importance pour la distribution des arbres dans les Alpes européennes, où les basses températures limitent le métabolisme du carbone.• Nous nous sommes concentrés sur les effets de la température de l’air et du sol, sur la photosynthèse nette (Pn) de Pinus cembra une espèce sempervirente climaxique de l’écotone de la limite supérieure de la forêt des Alpes centrales autrichiennes. Les courbes de réponse à la lumière et à la température ont été déterminées, en été et en automne, le long d’un gradient d’altitude allant de la limite de la forêt jusqu’à la limite des arbres rabougris (krummholz).• En général, Pn a été significativement plus faible en l’automne qu’en été. Néanmoins, indépendamment de la saison, les valeurs moyennes de Pn ont eu tendance à augmenter avec l’altitude et ont été positivement corrélées avec les températures de la zone racinaire. En revanche, la surface foliaire spécifique a diminué avec l’augmentation de l’altitude. En outre, la température optimale de la photosynthèse nette a diminué avec l’augmentation de l’altitude et a été positivement corrélée avec la température maximale moyenne de l’air des 10 jours précédant la date de mesure.• Ainsi, nos résultats semblent refléter une adaptation à long terme de l’appareil photosynthétique de Pinus cembra aux conditions générales de température à l’égard de l’altiude, combinée avec une courte durée d’acclimatation au régime thermique courant.