Differences in stem radial variation between Pinus pinaster Ait. and Quercus pyrenaica Willd. may release inter-specific competition

Species complementarity by morphological and physiological trait differences could cause distinct temporal and spatial use of resources. Accordingly, mixed stands may enhance production, biodiversity and/or provide a better adaptation to future climate conditions. We aim to identify species differences in intra-annual stem radial variation patterns, and to recognize species-specific responses to contrasting weather conditions at key intraannual growth phases. Stem radial variation was recorded from high temporal resolution point dendrometers (2012-2014) installed on twelve dominant maritime pine and Pyrenean oak trees in two mixed stands in central Spain. Species differences in stem radial variation were analyzed by synchrony statistics, intra-annual pattern modelling, and evaluating the dependence of main intra-annual growth phases on climate conditions. Both species showed important differences on intra-annual radial increment pattern despite general stem radial variation synchrony. Radial increment onset was earlier for maritime pine during the spring and rainy autumns. Species-specific responses to weather indicate that stem radial variation increased with autumn temperature in maritime pine, but decreased in Pyrenean oak trees. However, summer vapor pressure deficit may reduce stem radial variation for maritime pine. Therefore, maritime pine would adapt more efficiently to warmer temperatures associated with climate change, although summer water stress may reduce this competitive bonus

Relationships between Environmental Factors and the Growth of Above-Ground Biomass in Boreal Forest

This study investigates the influence of shortwave radiation (albedo is calculated to characterize the absorption of shortwave radiation), temperature and relative humidity on biomass growth of two coniferous species in boreal forest. Stem circumferences are measured for calculating daily biomass growth rate and calculated growth rate is analysed by statistical method for revealing its possible correlations to environmental factors (shortwave radiation, temperature and relative humidity). Comparisons between biomass growth rate and environmental factors are also made for finding correlation. Temperature sets lower limit for biomass growth. Biomass growth rate is found dependent on the values of albedo, meaning absorption of shortwave radiation dominates growth. Relative humidity is found negatively dependent on temperature. However, there is no statistical dependence of growth rate found on temperature and relative humidity, although some extreme temperatures and relative humidity are noticed affecting growth rate through evaporation (temperature affects negatively and relative humidity affects positively). The model on the relationship between values of albedo and temperature in the process of glucose absorption is also revealed and albedo is regarded to dominate such a process. Connections among these environmental factors are found and the affecting mechanism is established finally. Besides, species-specific difference of response to shortwave radiation between Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L.) is revealed

Interrogating tree response to climate forcing via high resolution stable carbon isotope (delta13C) analysis of Pinus sylvestris L. and eddy covariance measurements.

Tree-rings are natural archives of the climate variation experienced by trees during their life span. Because trees actively discriminate against 13C versus 12C in atmospheric carbon dioxide due to environmental forcing inducing higher assimilation of 12C in optimal conditions for growth, the ratio between these two isotopes represents a record of climate variability occurring at the time of the wood formation. Therefore, the delta13C can be used to reconstruct palaeoclimate from when the trees sampled were living. As the instrumental period from meteorological methods to measure climate is relatively short (last 150 years), the calibration between delta13C time series and climatic data can be used to improve the understanding of climate variability through longer periods. The main objective of this study is to produce a consistent methodology for the reconstruction of climate and environmental forcing on trees from the high resolution delta13C time series over the period 1997-2009 at three study sites in Europe where extensive eddy covariance measurements have been conducted. This atmospheric measurement technique permits a very fine resolution to observe the exchange of gases between the atmosphere and a land surface with additional applications in meteorology. At Hyytiala, dendrometer and microcore measurements were used to test if a methodology can be developed to predict the occurrence of radial growth during years when dendrological data are missing. The modeling of the intra-annual rate of growth is then performed via the Gompertz equation. Integration of cell life time to match climate information with time delta13C series is also assessed. The delta13C signature from both inter- and intra-annual resolution are tested against three weather variables: photosynthetically active radiation, temperature and precipitation. The results show stronger linear responses from the delta13C time series of the most northern site (Hyytiala, Finland) to the weather variables while the two other study sites (Norunda, Sweden and Loobos, the Netherlands) did not show significant linear relationships at both annual and intra-annual resolution. The approach developed in this study represents a first step in developing a generic method accessible for non-specialists in dendroclimatology for the use of delta13C time series as climate archives

Watching Trees Grow: Observations of Radial Tree Growth Across Multiple Temporal Scales in Northern Labrador

This research assesses whether a refinement of the temporal resolution of tree-ring data can improve our understanding of the radial growth-climate relationship. Two study sites in Northern Labrador were chosen, one coastal (Nain), and the other inland (Kamestastin). In Nain, microcore samples were taken weekly from the same five white spruce (Picea glauca) trees over the 2014 growing season. After cross sections were made and stained, the resulting 10µm thin radii provided a direct view of active ring development. In coastal Labrador, radial growth was initiated during the last week of June 2014, and ceased by August 25th. Circumference band dendrometers were installed on white spruce trees at both the Nain and Kamestastin sites. The dendrometers were used to measure micrometre-scale changes over the 2014 growing season. Analogous records of temperature were collected with equal temporal resolution, from an Environment Canada climate station (#8502800), and via a programmable data logger (UX120-006M, Onset HOBO). Correlation function analysis determined the relationship between daily temperature variables and daily variations in stem size. A strong relationship was found between minimum daily temperature and daily stem size at both sites over the eight week long growing season. Traditional dendrochronological sampling methods were utilized to retrieve tree cores from white spruce and eastern larch (Larix laricina) in Nain and Kamestastin. Site-specific master growth chronologies were created using crossdating and standardization techniques. After establishing long term records of monthly temperature and accumulated growing degree-days (GDD) at both study sites, a linear regression analysis was undertaken to determine the suitability of these two variables as predictors of annual-radial growth. An accumulated June/July GDD index was identified as an overall better predictor of annual ring-width than mean monthly temperature variables in northern Labrador. Exploring radial growth on an intra-annual scale helped to improve our understanding of the complex radial growth-climate relationship in Labrador. This allows for a strengthening of tree rings as a proxy climate indicator in remote regions of the northern boreal forest. The findings from this thesis provide the tools necessary to improve upon long-term climate reconstruction and forecasts of boreal forest structure in the face of climate change

The environmental drivers of white spruce growth and regeneration at Arctic treeline in a changing climate

As a temperature-delineated boundary, Arctic treeline is predicted to shift poleward and tree growth is expected to increase in response to rapid warming. The massive scale of the Arctic treeline magnifies these changes to impact energy balance, carbon balance, and climate-related feedbacks at local, regional, and global scales. Yet, not all sections of the Arctic treeline are reporting growth, suggesting factors other than temperature may be becoming more limiting as the climate continues to change. This dissertation investigates how water availability and tree size may modify the response to climate change of a dominant conifer species (white spruce, Picea glauca) growing at an Arctic treeline site in the Brooks Range, Alaska, USA. The first chapter examines the influence of temperature and water availability on population regeneration and individual tree growth during the 20th century. A climatic shift towards a warmer and drier climate after 1975 caused divergent responses of sapling regeneration and mature tree growth, suggesting that, while individuals have grown, this section of treeline has remained relatively stationary. The second chapter explores the present-day relationships between tree size, temperature, moisture availability, and tree growth by examining the response of intra-annual radial stem growth rate to changing environmental conditions at the Arctic treeline. Tree size and water availability play important roles in moderating the growth response to increasing temperature. Finally, in the third chapter, the environmental cues which trigger the onset of radial stem growth in spring are identified. The results suggest a combination of winter chilling and subsequent spring heat accumulation initiates onset, like trees growing at lower latitudes. However, the chilling and heating thresholds at this Arctic treeline site were far colder than those identified at lower latitudes, suggesting local adaptation to harsh Arctic winters and springs. Through these new findings, this dissertation advances our understanding of Arctic treeline dynamics and will help to predict the future of the Arctic treeline more accurately in a rapidly changing climate

Intra-annual to multi-decadal xylem traits in a tropical moist semi-deciduous forest of Central Africa

A witness of a tree’s past conditions is the wood itself. The main research question of this dissertation is how to assess and reveal the driving forces of the patterns of wood traits on pith-to-bark cross-sections in tropical trees. Cambial and leaf phenology was monitored in the Luki Reserve (Mayombe forest, D.R. Congo). Furthermore, X-ray CT densitometry was explored to assess traits in a reliable way, for multiple species. Variability in phenology is observed for T. superba, which requires traits of individual trees to be fixed on a time axis. Furthermore, X-ray CT is a suitable method for assessing traits in a fast way. Cambial activity of understory trees has shown to be species-specific, whilst many trees show zero xylem growth. Finally, 66 years of tree growth was analysed, while trait analysis revealed a median ring count of only 32, thus implying many non-periodical rings. This work presents methodological improvements to measure traits as continuous variables from pith to bark, but also acknowledges that phenology still remains a key aspect in order to fix traits on a time axis. Key words: leaf phenology, wood anatomy, stable isotopes, Mayombe, dendrochronology, X-ray CT densitometr

Influence of site factors and climate on timber properties of Sitka spruce (Picea sitchensis (Bong.) Carr.)

In plantation grown Sitka spruce, timber density is an important quality concern. Currently Sitka spruce timber meets the requirements for C16 strength grading which is the minimum requirement for construction uses. However, the margin is not exceeded by much and a small reduction in density could lower the log grading. Therefore it is essential to understand how timber density is impacted by site factors and climate in order to predict the potential effects of climate change on timber quality in Sitka spruce in the future. This has important economic implications for UK forestry. To assess the influence of site factors and climate on growth and resulting timber properties in Sitka spruce, three experiments were established; a large scale wood quality survey (“The Benchmarking experiment”), a detailed inter-site density study (“The Level II experiment”) and a continuous growth monitoring experiment (Dendrometer experiment). In the ‘Benchmarking’ experiment, increment cores were collected from 68 sites over a geographically wide area in Scotland and Northern England in order to quantify the effect of selected site factors on density. These samples were not suited for climate analysis at annual ring level and therefore disks that allowed reliable dating of the tree rings were collected from three Level II sites. The Level II samples were also used in an acoustic velocity study and an assessment of the within-tree patterns in density. Material from both Benchmarking and Level II sites was used for modelling radial density. In addition to this the Level II data functioned as a test dataset for the different density models that were developed on the Benchmarking data. Density and other tree ring variables were measured with an ITRAX X-ray densitometer and WinDENDRO software. Some acoustic measurements were also carried out with a purpose built ultrasonic scanner to allow calculation of radial variation in stiffness. A dendrometer experiment was established to monitor short term growth variation in Sitka spruce in real time. The aim was also to use the growth data to date density profiles and hence identify causes for the density differences. Data analysis was carried out in R mainly using linear and non-linear mixed effects models, the dendrochronology software package dplR and methods of time series analysis. It was found that the largest part of the variation in density was between trees within each site. Both density and stiffness were mostly influenced by the growth rate or by another variable describing the tree vigour. Fast growth decreased density both by increasing earlywood proportion as well as decreasing the density of both earlywood and latewood. Models for the radial behaviour of density and stiffness could be fitted to the data, but random tree effects remained large. This limits the applicability of these models to new sites as reparameterisation would be required, which requires data from time consuming density and acoustic velocity measurements. The possibility of modelling the model coefficients from easily measurable stand and tree variables was investigated but the results were not promising. The Dendrometer experiment indicated that annual growth in Sitka spruce was initiated in late May and terminated by mid September. Ring widths detected by the dendrometers, micro core measurements and X-ray density based tree ring analysis differed, which caused difficulties in dating the density profile. To decrease these problems in the future the microcore sample storage protocol was revised. The dendrometer data indicated that the growing season was divided into several sections between which the growth rate differed. Latewood density and maximum density had the strongest correlations with the climatic variables. At one site the correlations included temperature and rainfall in April whereas at the other precipitation throughout the growing season or during the May-August period were important. Temperature was correlated positively with density variables and rainfall negatively

Study of the interactions between snowpack and forest cover in the Aragonese Pyrenees and their eco-hydrological implications

En las montañas de latitudes medias los bosques y la nieve constituyen recursos naturales prioritarios, tanto desde el punto de vista medioambiental como del económico. Los Pirineos no son una excepción, dado que en esta cordillera ambos elementos coexisten e interaccionan complejamente en el rango altitudinal comprendido entre los 1600 y los 2300–2500 m s.n.m., ocupado en su mayor parte por el piso bioclimático subalpino. Sin embargo, las interacciones que tienen lugar entre los bosques de montaña y el manto de nieve en esta región no habían sido estudiadas en profundidad hasta el momento. Y menos aún se sabía acerca de la respuesta de las interacciones bosque–nieve al proceso de calentamiento que está aconteciendo en esta cordillera y que seespera se vea acelerado en las próximas décadas. De manera que, el principal objetivo de la presente Tesis Doctoral es avanzar en el conocimiento existente sobre las interacciones que se producen entre los bosques de montaña y el manto de nieve en los Pirineos, desde una perspectiva eco–hidrológica. La misma, se presenta como un compendio de cuatro publicaciones científicas, en las que diferentes objetivos específicos son evaluados.La mayor parte de este estudio interdisciplinar tuvo lugar en un valle de montaña (Baños de Panticosa) localizado en el Pirineo central. El diseño experimental incluyó cuatro bosques de Pinus uncinata de diversas características ambientales, entre las cuales cabe mencionar su diferente elevación (desde 1674 a 2104 m s.n.m.), exposición, estructura forestal y microclimatología debido a la compleja topografía de este enclave montañoso. En estos bosques se monitorizó intensivamente, entre 2015 y 2020, la evolución del manto de nieve, las condiciones climáticas y del suelo, la fenología de los pinos, su xylogénesis, la variación intra–anual del radio de su tronco y la concentración de carbohidratos no estructurales presentes en su albura y acículas jóvenes. En este marco se contextualizó la primera publicación científica que compone esta Tesis, la cual informó con detalle sobre los efectos que la cubierta forestal produce en la dinámica del manto de nieve en esta región, destacando las similitudes y diferencias que existen entre áreas cercanas y entre distintas temporadas de invierno. La segunda publicación científica, identificó por primera vez una señal nival en el crecimiento radial inter–anual de P. uncinata. Esta investigación se contextualizó en un contexto espacio–temporal más amplio, analizando dendrocronológicamente 36 bosques de P. uncinata localizados en las principales cordilleras montañosas del NE Peninsular y las condiciones nivales acontecidas en los mismos en las últimas décadas. La tercera publicación científica, describió cómo la dinámica estacional del manto de nieve es capaz de modificar ciertas condiciones microclimáticas de los bosques estudiados en el valle de Baños de Panticosa, y demostró cómo esta influencia nival determina en buena parte el crecimiento radial intra–anual de P. uncinata. La cuarta publicación científica, exploró cómo los futuros cambios del clima pirenaico podrían afectar a las actuales interacciones bosque–nieve que tienen lugar en el valle de Baños de Panticosa. Para ello, se simularon los cambios que experimenta la dinámica nival en los bosques estudiados bajo varios grados de forzamiento climático.Los resultados obtenidos en esta Tesis demuestran, por tanto, que el manto de nieve y los bosques de P. uncinata interactúan en ambos sentidos en las áreas de montaña analizadas, si bien, tales interacciones están sujetas a importantes fuentes de variabilidad espacial y temporal. Por una parte, la cubierta forestal, principalmente debido a la intercepción que producen las copas de los pinos y a la alteración que produce en el balance de energía, determina la distribución del manto de nieve, su magnitud y su temporalidad. Por otra parte, el manto de nieve, principalmente mediante las modificaciones que produce en el régimen de temperaturas del suelo, influye en el crecimiento radial intra e inter–anual de P. uncinata, independientemente del ampliamente conocido efecto que tiene la temperatura del aire durante la temporada de crecimiento en la formación de sus anillos. Además, esta Tesis sugiere que la cubierta forestal puede tener un importante rol en la sensibilidad del manto de nieve ante los futuros cambios que se esperan en el clima de los Pirineos.Las cuestiones abordadas en esta Tesis, titulada "Estudio de las interacciones entre el manto de nieve y la cubierta forestal en el Pirineo Aragonés y sus implicaciones eco–hidrológicas", son de gran interés científico, pero también proporcionan una valiosa información de gran aplicabilidad en la presente y futura gestión de los recursos hídricos y forestales del Pirineo.<br /