The causes of natural durability in larch

Laich is characterized by fast juvenile growth, strong wind firmness, low susceptibility to pests and particularly high quality of its timber, including high wood density, superior mechanical strength`and natural durability. The goals of this project break new scientific ground in giving answers about causes and function of wood extractives, capture the variability within-trees, between trees and sites, and understand better the genetic control of natural durability. With the strongest dataset ever compiled from this species a rapid and accurate spectroscopic method to determine natural durability is developed. The project emphasizes an three major goals: The first goal is about the chemistry and natural durability variation in European and Japanese laich. Previous research indicated that Japanese laich has higher durability than European laich. Relationships between natural durability and structural wood properties are drawn at different resolution levels to understand the connection between tree growth and natural durability of wood. Results are important to tree growers and industry in order to meet better end-user requirements (high durable wood). The second goal is an the development of a rapid and non-destructive method to determine the natural durability of laich. With strong reference data advanced chemometrical modelling will result in`reliable calibration models, based an the broadest sample euer collected for laich. This opens opportunities for commercial dissemination at the industrial level. The third goal is an the evaluation of genetic control of heartwood extractives and natural durability. Unique sample material is made available through a twenty-years old full-sib progeny laich trial, established at two sites in France. The experimental design allows the evaluation at the interspecific level (hybrid) and the genetic control of selected wood parameters. The rapid method to measure wood extractives and natural durabilityallows the processing,of hundreds of wood samples for their extractive content, lignin and natural durability. The results are critical in tree breeding and wood quality improvement programs.

Because of its pleasant heartwood colour, high density and natural durability larch wood has been an appreciated species since centuries. There is a high variability of almost all wood quality traits among species, sites and even within the trees. Using Larch heartwood for exterior purposes is problematic because of the high variability of the natural durability. There is no process of classifying boards due to the natural durability available at the moment. The goals of this project were to give answers about causes and function of extractives in heartwood of different larch species, to capture the variability within-trees, between trees and sites; to investigate the genetic control of natural durability and to develop methods for rapid estimation of extractive content and resistance to fungicidal deterioration. Analysis were performed on a sample set that consisted of Hybrid larch (Larix eurolepsis), Japanese larch (Larix kaempferi), Siberian larch (Larix sibirica) and European larch (Larix decidua var. alpica, var. sudetica, var. polonica) from different sites across Europe. Natural durability to brown rot fungi Coniophora puteana, Gloeophyllum trabeum and Postia placenta of these specimens was determined according to EN350-1 and EN 113. To develop methods for rapid prediction of natural durability measurements were taken by use of near infrared (NIR) and colour (VIS) - spectroscopy. Further microscopic and wet chemical analysis, UV- microspectrophotometry, x- ray densidometry and artificial weathering tests were carried out. Natural durability shows huge variations between different sites, species and stem position. Within outer heartwood, covering a stem length of 2m, resistance to brown rot ranged between three durability classes according to EN 350-1. Natural durability increases with tree age, due to a change of extractive content. The acetone extractives and the phenols raises between 1.2 % and 2.2 % per 100 rings increment, double concentrations are reached with 104 and 175 rings. Double concentration of hot water extractives is reached within a period over 250 tree-rings. The resistance of Siberian and Austrian larch to Coniophora puteana and to Gloeophyllum trabeum mainly depends on the content of hot water extractives (n = 100, r = -0.81for C.P.; r =- 0.79 for G. P.). No significant correlations of the x-values to acetone extractives were found for these fungi. Postia placenta showed no sensitivity to hot water extractives, but correlated significantly with acetone extractives (n = 100, r = -0.40). Therefore, relationships between natural durability and contents of different extractives seem to depend on the species of the decaying fungi. During heartwood formation different species of the genus Larix deposit different amounts of various extractives within the xylem tissue. Japanese larch shows higher amounts of phenols and Siberian larch has more hot water extractives (arabinogalactan) than European larch. Investigations on the distribution of phenols within single cells and layers revealed highest concentration within parenchyma cells and middle lamellae. Natural durability of larch heartwood increases with density and latewood content and decreases with ring width. Comparing microscopic features like resin duct density, number of rays and parenchyma cells, just revealed a significant correlation for resin duct density. Resistance seems to be higher in wood with less resin duct. This might be caused by a negative correlation of phenols and resin ducts. Investigations on the relationship between resistance to artificial weathering and natural durability revealed a positive correlation between the x-values of Coniophara puteana and Gloeophyllum trabeum and mass loss caused by 12 weeks weathering. The relationship between high durability and high mass loss is related to the amount of hot water extractives, which show high correlation to wood density. Due to high variations there is need for rapid prediction of natural durability. Although strong relationships were found between colour of larch heartwood, especially red hue a*, and natural durability, models established with FT- NIR spectra showed higher prediction quality than colour models. FT- NIR spectroscopy proved to be a reliable and accurate method to determine natural durability and extractive content of solid heartwood.