Research On Species-Specific Inversion Model For Moisture Content Of Living Wood Based On Time-Domain Reflectometry

Water is an important component of living wood and an important factor affecting the growth and development of living wood.It is of great significance to measure the moisture content of trees(wood)in real time and accurately,whether it serves for the rational management and scientific protection of forests or for the consideration of efficient processing and utilization of wood.Time Domain reflectometry(TDR)is a method based on dielectric constant.It is one of the most potential techniques for moisture content measurement at present.However,the calibration equation between TDR test parameters(electromagnetic wave propagation time)and moisture content needs to be established in the laboratory in advance.In order to improve the ability of practical application of TDR for testing moisture content of living wood,the influence of wood moisture content should be considered for electromagnetic wave propagation,studied the advantages and disadvantages of different probe insertion depths and build the northeast typical species of the calibration equation,which provide theoretical guidance and scientific basis for accurate and real-time moisture content detection of typical tree species in northeast China based on TDR method.In this paper,TDR method was used to obtain the electromagnetic wave propagation time data of four tree species,including basswood,spruce,ash and birch,under different probe insertion depths(2cm,4cm,6cm,8cm,10cm,12cm)and different moisture content conditions through laboratory tests.The influence of wood moisture content on electromagnetic wave propagation time was analyzed.The influence of probe insertion depth on TDR test results was investigated,and the inversion equations of electromagnetic wave propagation time-moisture content of basswood,spruce,ash and birch were constructed.On the basis of laboratory experiments,basswood,spruce,ash and birch,as well as Korean pine and larch,which are common species in the test area,were selected in the field.The moisture content of the six species of living wood were monitored and analyzed,to verify the effectiveness of TDR species-specific moisture content inversion model.The main research contents are as follows.(1)The influence of wood moisture content on electromagnetic wave propagation time was analyzed.Under the maximum probe insertion depth(12cm),the electromagnetic wave propagation time of basswood,spruce,ash and birch in saturated state increased by 71.6%,47.6%,76.9%and 55.1%,respectively,compared with that in dry state.The electromagnetic wave propagation time increased with the increase of moisture content,and there was a significant linear correlation.Under the same moisture content,the propagation time of electromagnetic wave increases with the increase of insertion depth.Meanwhile,the propagation of electromagnetic wave in wood was affected by the radial difference of wood moisture content,and the propagation time of probe insertion into heartwood increased more than that of sapwood with the same length,which was more obvious in birch and spruce.For different tree species,TDR has a timely response to the change of moisture content,indicating that using electromagnetic wave propagation time can effectively invert wood moisture content.(2)Species-specific and general inversion models of electromagnetic wave propagation time and wood moisture content were constructed.The regression analysis of electromagnetic wave propagation time and moisture content at different insertion depths was carried out by various mathematical models,and the 8cm probe depth was determined to be the optimal depth(R~2>0.95).Based on linear,quadratic and logarithmic functions,a species-specific inversion model of electromagnetic wave propagation time and wood moisture content was established at probe depth of 8cm.On this basis,a general moisture content inversion model for the four tree species was established,but the determination coefficient was low(R~2=0.65).Further analysis shows that the electromagnetic wave propagation time of different tree species with the same moisture content is correlated with air-dry density.Based on this,spruce-basswood and basswood-birch models with similar air-dry density were built,and the determination coefficient was close to the species-specific model,indicating that it is feasible to build a general model among species with similar air-dry density.(3)The inversion model of moisture content of living wood was verified.It was found that the predicted value of TDR moisture content was significantly higher than the actual value of living wood moisture content in the field.Birch has the lowest average difference of 10%,and the average difference of basswood,spruce and ash was more than 20%.The correlation between moisture content of living wood and electromagnetic wave propagation time is not as obvious as that of wood disk,and the goodness of fit of basswood,ash and larch is lower(R~2<0.30).Except for ash,8cm depth was better than 4cm depth.The correlation between moisture content and electromagnetic wave propagation time in the living wood test was lower than that in the laboratory test.The spruce and Korean pine with more samples had relatively high goodness of fit(R~2>0.47)at 8cm insertion depth.The regression equation function image was consistent with the laboratorial model.The regression equation images of ash,birch and larch at 8cm insertion depth also showed a linear increase trend with the increase of electromagnetic wave propagation time.The results show that using the laboratorial model to prediction the moisture content of living wood is reasonable in the variation rule between electromagnetic wave propagation time and moisture content of living wood.But the experimental error of living wood test is large and the number of individual tree species is small.It is necessary to improve the test method of moisture content and electromagnetic wave propagation time of living wood and increase the number of individual tree species.