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Autonomous weeders for Christmas tree plantations - a feasibility study 4 Weed control requirements in Christmas tree ProductionBent Keller The Danish Forest and Landscape Institute, Department of Forestry This chapter present results from an ongoing experiment that has the purpose of investigating the influence of different spatial weeding intensity on Christmas tree growth and survival on former agricultural land. 4.1 MethodThe species in the experiment are Nordmann fir (Abies nordmanniana). Two parallel test areas were established; one in Randbøl National Forest District – at Tønballegård, and one in Wedellsborg Forest District. The experimental area at Tønballegård is on a clay loam soil and the area at Wedellsborg is on a light sandy soil. Planting was done with machines. Around each tree a circle is weeded mechanically. The following treatments are included in the test (the percentage area is calculated on the basis of a planting distance, which leaves a circular growth space of app. 1.2 m2 for each tree. The actual planting distance in the trial is wider):
The trial is on singletree basis with 30 trees per treatment, for a total of 210 trees plus 10 empty planting spots. The trees are inspected for shoot development at the start of each growing season. Plant survival, health, height, leader shot length, number of leader shots, number of branches in the top branch whorl, root collar diameter are measured/registered after each growing season. In 10 circles of each treatment, the soil moisture content in the ploughing layer (25 cm) is measured using the TDR method. In each circle the soil moisture content is measured five times in increasing distance from the centre: 15, 30, 45, 60 and 75 cm. The soil water content is expressed in terms of percent of field capacity. Field capacity is determined by measurements of the soil water content in winter about 2 days after the last rainfall. 4.2 Results4.2.1 The average height of the trees.The average height of the trees can be seen in table 4.1 and 4.2. Table 4.1.
On the solid soil of Tønballegård the trial shows that there has been no unambiguous effect of the treatment on the average height of the trees. The weeded trees have obtained a slightly higher average height, which in year 2000 only in one single case is significant. Table 4.2.
In the Wedellsborg trial on the poor sandy soil the picture is considerably different. From growing seasons 1997 to 2000 all the mechanical treatments had considerably larger height than the untreated (0 %). Apparently the 20 % treatment differs from the other treatments with a somewhat lower average height in year 2000, however, the difference is not significant. The chemical treatment does not at any time differ significant from the untreated. On the other hand the average height in year 2000 is significant lower then the 40, 60, 80 and 100 % treatments (p-values 0,0004 0,0001 0,0011 and 0,0001). 4.2.2 Root collar diameter of the treesThe root collar diameter of the trees can be seen in table 4.3 and 4.4. Table 4.3.
It is evident that all the mechanical treatments have given a significantly larger root collar diameter from 1997 to 2000. The 60, 80 and 100 % treatments have already even given a significant larger root collar diameter after the first growing season. The chemical weeded trees do not differ significantly from the untreated in the years 1999 and 2000. Table 4.4.
From 1997 and further on all the treatments have given significant larger root collar diameters than the untreated. In the year 2000 the chemical weeded trees have a significant smaller root collar diameter than all the mechanical treated (p-values 0,0151 0,0001 0,0001 0,0001 0,0001). 4.2.3 Health score of the living trees.The state of health of the trees can be seen in table 4.5 and 4.6. Table 4.5.
It is evident that no immediate evidence of effects of the treatment on the state of health of the trees on the rich soil. There are significant manifestations in the years 1996, 1997, 1998 and 1999. In 1996 an improved healthiness has been obtained by weeding. The other years the health of the weeded trees has been poorer than the health of the untreated trees. Table 4.6.
On the light sand soil there are significant effects of the treatments in the years of 1997, 1999 and 2000. In all cases there are positive effects due to the weeding. 4.2.4 Soil moistureSoil moisture is presented in the following figures as percent of the field capacity. This means that the curves will have a very characteristic course. Before the start of the growth season the moisture will be close to 100%, which is equal to the field capacity. During the growth season the level will fall rapidly in the vegetation-covered areas and somewhat slower in the weeded areas because of the evapo transpiration. In drought periods the level will equalise almost to the point of wither limit. Figures 4.1 and 4.2 show the soil moisture in a normal and a dryer situation. Figure 4.1. In the normal situation in figure 4.1 it can be seen, that there is a distinct effect of the treatments. There is in the mechanically weeded trees a clear gradient, which reflects the effect of treatment, showing that the soil moisture has a tendency to fall away from the weeded area close to the plant. Figure 4.2 The corresponding curves for the Wedellsborg trial can be seen in figures 4.3 and 4.4. Figure 4.3. Figure 4.4. The gradient in the normal situation in figure 4.3 is much more distinct and reflects the effect of the treatment. In the dry situation the level of moist in the mechanical weeded areas near the plants has come down to almost the same level as in the not weeded areas. 4.3 Other investigationsIn each growing season the time of bud breaking, frequencies of leader shots and mean number of branches in the top whorl were recorded. These parameters, however, were not affected significantly by the different treatments. 4.4 DiscussionChristmas Tree producers are basing their cultivation systems on tradition and practical experience, which has indicated that the Nordmann’s fir is responding to the weeding with better prosperity, growth and quality. Traditionally a 100 % weeding is therefore carried out in the Christmas tree plantations. In the trials reported here the trees have appeared to be surprisingly little influenced by the degrees of weeding above 40 %. Thus it can be expected that fully satisfactory cultivation can be maintained with a considerably reduced level of weeding. On both study sites the development of height has only been slightly influenced by degrees of weeding above 40%. The root collar diameter, which notoriously is a parameter very sensitive to treatment, only responds very little on degrees of weeding above 40%. The general state of health of the trees has hardly been affected by degrees of weeding above 20%. Regarding the architecture of the trees the trials show that the number of trees with leader shot defects does not seem to be influenced by the degree of weeding, just as the type of defect also seems to not be influenced. This last issue is very important as trees with more than one leader shot can be corrected by cutting off the extra leader shoots, and thus get into perfect shape. In contrast trees with no leader shot at all are very difficult to repair. Also the number of branches in the branch whorl does not seem to be influenced by the degree of weeding. The missing influence of the degree of weeding on the architecture of the trees is in good accordance with the fact that the time of bud breaking is also not influenced by the degree of weeding in the trials. In this connection it is important to remember that the risk of spring night frost damages partly depends on the time of bud breaking and partly on the locality. The trials are placed on coastal areas and are thus not very likely to be exposed to spring night frost damages. Regarding the soil moisture the TDR measurements from the trial shows that under normal circumstances in he areas with graduated mechanical weeding, there is more plant available water close to the trees than in areas with untreated soil. The TDR data also show that the mechanical weeding can be regarded as an insurance because soil drying out in drought situations occurs more slowly in the mechanical weeded soil than in the vegetation covered soil. 4.5 Sub conclusionTo date, the he trials show that fully satisfactory cultivation results can be achieved on light as well as relatively heavy soils with area graduated mechanical weeding with a weeding degree of 40%, corresponding to a weeded circle
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