Andreas Gruber

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.

Frost drought in conifers at the alpine timberline: xylem dysfunction and adaptations

Drought stress can cause xylem embolism in trees when the water potential (psi) in the xylem falls below specific vulnerability thresholds. At the alpine timberline, frost drought is known to cause excessive winter embolism unless xylem vulnerability or transpiration is sufficiently reduced to avoid critical psi. We compared annual courses of psi and embolism in Picea abies, Pinus cembra, Pinus mugo, Larix decidua, and Juniperus communis growing at the timberline vs. low altitude. In addition, vulnerability properties and related anatomical parameters as well as wood density (D(t)) and wall reinforcement (wall thickness related to conduit diameter) were studied. This allowed an estimate of stress intensities as well as a detection of adaptations that reduce embolism formation. At the alpine timberline, psi was lowest during winter with corresponding embolism rates of up to 100% in three of the conifers studied. Only Pinus cembra and Larix decidua avoided winter embolism due to moderate psi. Minor embolism was observed at low altitude where the water potentials of all species remained within a narrow range throughout the year. Within species, differences in psi50 (psi at 50% loss of conductivity) at high vs. low altitude were less than 1 MPa. In Picea abies and Pinus cembra, psi50 was more negative at the timberline while, in the other conifer species, psi50 was more negative at low altitude. Juniperus communis exhibited the lowest (-6.4 +/- 0.04 MPa; mean +/- SE) and Pinus mugo the highest psi50 (-3.34 +/- 0.03 MPa). In some cases, D(t) and tracheid wall reinforcement were higher than in previously established relationships of these parameters with psi50, possibly because of mechanical demands associated with the specific growing conditions. Conifers growing at the alpine timberline were exposed to higher drought stress intensities than individuals at low altitude. Frost drought during winter caused high embolism rates which were probably amplified by freeze-thaw stress. Although frost drought had a large effect on plant water transport, adaptations in hydraulic safety and related anatomical parameters were observed in only a few of the conifer species studied.

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.

Human immunodeficiency virus type 1 protease inhibitor attenuates Candida albicans virulence properties in vitro.

The putative virulence factor secreted aspartyl proteinase (SAP) of Candida albicans and the human immunodeficiency virus type 1 (HIV-1) protease both belong to the aspartyl proteinase family. The present study demonstrates that the HIV-1 protease inhibitor Indinavir is a weak but specific inhibitor of SAP. In addition, Indinavir reduces the amount of cell bound as well as released SAP antigen from C. albicans. Furthermore, viability and growth of C. albicans are markedly reduced by Indinavir. These findings indicate that HIV-1 protease inhibitors may possess antifungal activity and we speculate that in vivo SAP inhibition may add to the resolution of mucosal candidiasis in HIV-1 infected subjects.

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.

Human Immunodeficiency Virus Type 1 gp160 and gp41 Binding to Candida albicans Selectively Enhances Candidal Virulence In Vitro

Previously, it has been shown that human immunodeficiency virus (HIV)-1 envelope proteins gp160 and gp41 bind to Candida albicans. Whether this interaction affects candidal virulence in vitro was investigated. HIV-1 gp160 or gp120 treatment of C. albicans significantly altered neither growth nor phospholipase activity of the fungus. However, treatment of C. albicans with gp160, but not with gp120, led to an elevation of free and cell-bound aspartate proteinase. In addition, culture supernatants obtained from C. albicans treated with gp160 or gp41, but not with gp120, showed a strong increase in proteinase activity. Finally, C. albicans viable yeast cells treated with gp160 or gp41 and serum were phagocytosed by polymorphonuclear leukocytes to a lesser extent than was C. albicans treated with gp120 and serum or serum alone. These findings suggest that the interaction between HIV-1 gp160 and C. albicans may promote the virulence of C. albicans in HIV-1-positive patients.

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.

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).