Identifying the main drivers for the production and maturation of Scots pine tracheids along a temperature gradient

Even though studies monitoring the phenology and seasonal dynamics of the wood formation have accumulated for several conifer species across the Northern Hemisphere, the environmental control of tracheid production and differentiation is still fragmentary. With microcore and environmental data from six stands in Finland and France, we built auto-calibrated data-driven black box models for analyzing the most important factors controlling the tracheid production and maturation in Scots pine stem. In the best models, estimation was accurate to within a fraction of a tracheid per week. We compared the relative results of models built using different predictors, and found that the rate of tracheid production was partly regular but current and previous air temperature had influence on the sites in the middle of the temperature range and photosynthetic production in the coldest ones. The rate of mature cell production was more difficult to relate to the predictors but recent photosynthetic production was included in all successful models.Peer reviewe

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

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

Historical growth rates and changing climatic sensitivity of boreal conifers

This thesis is concerned with the expression of relatively long-timescale growth forcing in tree-ring chronologies. The operation of different standardisation techniques, used in dendroclimatology to remove internal, non-climate related growth trends in measured series of ring-widths, is explored with an emphasis on the efficiency of the Regional Curve Standardisation (RCS) technique. The approach adopted here makes extensive use of concepts taken from tree-growth models and is based on the assumption that common external growth forcing operates through its influence on photosynthesis. A definition, of the growth rate of trees in terms of the carbon production by unit foliage, is the rationale that underlies this work and leads to the use of a multiplicative model for processing individual tree and chronology indices. The presence of a “common signal” in series of tree measures can lead to the distortion of the shape of detrending curves and a problem with bias in chronologies. Problems of the RCS technique are identified which are associated with tree age and diameter-related bias, arising from the use of ring-width to establish tree growth rates, regardless of tree diameter. These problems are manifest as “end effects” in chronology development and are most significant in the most recent century. Alternative, significant modifications of the RCS approach are proposed: the Multiple RCS (MRCS) and the Size-Adjusted RCS (SARCS) methods which greatly mitigate these problems. These are made possible by the introduction of two new concepts in dendroclimatology: the “best fit means” method and the use of “signal-free measures”. The concept of the mechanical strength of trees is used to simulate tree growth from series of ring-width measures and a “process based standardisation” (PBS) model is developed. The PBS model is tested and shown to be a feasible alternative to existing standardisation techniques

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

The role of hydraulic strategies in understanding the response of fynbos to drought

Includes bibliographical references.This dissertation is an investigation into the relevance of the hydraulic framework (sensu McDowell et al., 2008) in understanding the response of diverse communities to drought. This framework distinguishes species that respond to dehydration by rapidly decreasing stomatal conductance (gs) (isohydric), thereby maintaining relatively constant midday minimum water potential, from those that maintain relatively high gs (anisohydric), thereby maintaining carbon assimilation albeit at the cost of hydraulic dysfunction. However, currently, the importance of the hydraulic framework in explaining drought response in diverse communities is poorly understood. Drought-related plant mortality is of particular concern in South Africa’s fynbos biome, a global biodiversity hotspot. This Mediterranean-type region is predicted to experience an increase in drought, with unknown consequences for the endemic flora. We asked whether drought affected this diverse ecosystem in a predictable manner and if these patterns fit with existing frameworks of plant drought-mortality mechanisms

SPATIAL VARIABILITY IN FOREST FUELS: SIMULATION MODELING AND EFFECTS ON FIRE BEHAVIOR

Forests in the western United States and elsewhere face a growing crisis arising from global warming, changes in fuel beds and an increasing human population. Fire management policy emphasizes fuel treatments, such as thinning and prescribed burning, to remedy this situation because fuels are the one component of the problem that we can directly affect through management action. At present, however, the tools we have for the evaluation of fuel treatments are inadequate because they do not describe the fuel bed, or effects of modifications to the fuel bed on fire behavior in sufficient detail. The work described here presents a system that has potential to address the shortcomings of current approaches. In the first chapter, to improve our ability to represent wildland fuels, a three dimensional spatially explicit fuel model, FUEL3D, is presented which represents fuels at a level of detail comparable to what we can actually measure: stands as collections of individual trees, with branches and foliage. In conjunction with new, physical fire models, detailed fire behavior simulations can be carried out using fuels represented with FUEL3D as inputs. This system thus comprises a simulation laboratory which will greatly enhance our capabilities to evaluate fuel treatments and strengthen our understanding of fire and fuel interactions. In the second chapter, this system is demonstrated in an exploratory simulation study which examines the impact of spatial variability within an individual tree crown on fire behavior. Results demonstrate that the distribution of fuel within a tree crown significantly affects the rate of fuel consumption, as well as the timing, duration and magnitude of heat produced. This suggests that modeling of both crown fire initiation and propagation would benefit from more detailed description of crown fuels. In third chapter a replicated series of stand scale fire simulations is carried out to examine variability in forward spread rate; accelerated spread rates endanger fire fighters. Substantial variability is observed to arise from fine scale fuel-atmosphere-fire interactions which are not easily predicted beforehand. A new strategy is proposed in which physical fire models are used to quantify the potential drivers of variability in fire behavior

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

Cyclical succession in semi-arid savannas revealed with a spatial simulation model

Patch-dynamics is a new scale-explicit mechanism explaining the coexistence between woody species and grasses in savannas including asynchronous cyclical successions at the patch-scale. In this dissertation, I developed, implemented, validated, and analysed the spatially-explicit individual-based simulation model SATCHMO for a semi-arid savanna patch to investigate whether cyclical succession emerge from a realistic parameterization to support the applicability of patch-dynamics to savannas. Model analyses revealed significant shrub cycles at a period of 33 years that were driven by precipitation and not by fire. I suggest that shrub cycles occur in three phases which is very well supported by field data from the study site in South Africa and model results. In the patch-dynamic context, the ecological-economic problem of shrub encroachment is a natural, transient phase in the cycle, so that large-scale rotational schemes are an appropriate option for livestock management. -Patch-dynamics ist ein neuer, skalenexpliziter Mechanismus zur Erklärung der Koexistenz von Holzpflanzen und Gräsern in Savannen, bei dem in mosaikartigen Patchen jeweils zyklische Sukzessionen asynchron ablaufen. In dieser Dissertation wurde das räumlich-explizite, individuenbasierte Simulationsmodell SATCHMO für einen semi-ariden Savannenpatch entwickelt, implementiert, mit eigens aufgenommenen Felddaten aus Südafrika validiert und analysiert, um herauszufinden, ob bei einer realitätsnahen Parametrisierung zyklische Sukzessionen aus dem Modell hervorgehen, um die Anwendbarkeit von Patch-dynamics auf Savannen zu untermauern. Die Modellanalyse ergab signifikante Strauchzyklen mit einer Periode von 33 Jahren, die von Bodenfeuchte und nicht von Feuerdynamik getrieben werden. Es wird vorgeschlagen, dass die Zyklen in drei Phasen ablaufen, die gut durch die Modell- und Felddatenergebnisse belegt sind. Das ökologisch-ökonomische Problem der Verbuschung ist in diesem Zusammenhang eine natürliche, vorübergehende Phase in den Zyklen, dem mit groß-skaligen Rotationsweiden beigekommen werden kann