CHAPTER IV: DETERMINATION OF NUTRITIONAL STANDARDS
2.3 Diagnosis of jack pine stands
As for black spruce, only a partial diagnosis of jack pine stands could be made due to the small number of standards developed. A consistent diagnosis could be made for the generally excessive Mg (also true of other species) but not for N. The application of our CND standards to nutrient diagnosis of jack pine stands revealed a high proportion of stands with excess supply of nutrients. The number of stands diagnosed as having excessive amounts of nutrients is especially large when considering jack pine CND6 indices, which did not identify any of the 24 selected stands as deficient in any of the studied nutrients (Fig. 8c). This general diagnosis of excess nutrients with CND standards coupled with the fact it did not agree with the diagnosis established with N concentration and the fact that jack pine generally responds positively to N fertilization (Foster and Morrison 1983; Weetman et al. 1987) suggests that our CND6 standards are questionable.
Therefore, the computed CND6 optimum ranges for jack pine should not be used for diagnosis and further research is required to validate these values.
On the other hand, the high proportion of stands diagnosed as having deficient concentrations of Mn suggests that the computed optimum ranges for both black spruce and jack pi ne are too high. This is in agreement with the high Mn concentrations observed at the Beardmore stands. Such high Mn concentrations may be caused by Mn-rich soil parent materials. These stands were located upland and on well-drained upland sites, thus water-logging could not have caused the high Mn availability.
Nitrogen diagnosis based on our concentration model identified a relatively high proportion of stands showing N deficiency (40%). This is in agreement with the common positive response of jack pi ne to nitrogen fertilization (Camiré and Bernier 1981; F oster and Morrison 1983; Weetman et al. 1987; Morrison and Foster 1995). Still, more than half of the jack pine stands used for nutrient diagnosis were identified as having optimum foliar N concentrations. This could suggest that N is not always limiting in jack pine stands. This hypothesis would be in agreement with a study done by Foster and Morrison (2002), who conc1uded that nitrogen was not the main limiting nutrient in a productive jack pine stands in Ontario.
In regard to Mg diagnosis based on nutrient concentrations, only a small number of jack pine sites were found to be Mg deficient. Approximately 30% of the stands diagnosed were identified as having excess Mg. This seems realistic given the high number of white spruce stands found to have high Mg and it would agree with the hypothesis of the widespread occurrence of Mg-rich soil parent materials in the eastern part of the Canadian boreal forest. However, sorne of the selected jack pine stands may be growing on soils that are very poor in Mg. Such Mg-poor sandy soils have been found to limit the growth ofred pine in Quebec (Truong 1975a).
CHAPTERVI CONCLUSION
The results of this research show that the use of a statistically conservative but conceptually robust boundary line approach can be suitable to determine optimum nutritional ranges for nutrient concentrations and CND scores of boreal trees species, given that sampling was spread over more than one year and covered multiple sites.
However, results from white spruce second-year needles show that relationships between growth and nitrogen concentration is not as strong in older foliage, therefore limiting the potential diagnostic applications. The few standards obtained for species with lower nutrient requirements (black spruce and jack pine) indicate that a greater sampling effort will be needed to ob tain significant models. Hence, this study clearly demonstrates that the amount of variation in tree growth and foliar nutrient concentrations required for computing optimum nutrient concentrations and CND scores for aIl major nutrients can only be obtained by significantly increasing the sample size far above the initial expected levels and by carefully selecting sites to be sampled.
Results obtained with white spruce were sufficient to determine that nutritional disorders in the southeastem boreal forest are no longer restricted to nitrogen. The use of CND was found to be more sensitive than CV A and provided valuable information on the nature of nutritional problems in white spruce stands. The limited number of suitable data for nutrient diagnosis prevented the application of multivariate statistics which could have provided further information on the nature of nutrient interactions in white spruce stands.
Yet, preliminary investigation with principal component analysis revealed possible antagonistic relationships between N and Mg. As these preliminary results are in agreement with atmospheric N deposition effects, CND could become an essential tool to assess complex nutrient interactions caused by anthropogenic stresses.
The small number of standards obtained for black spruce and jack pine only allowed a partial assessment of the nutrient status stands of these species. Nevertheless, nutrient diagnosis for the latter species enabled us to draw conclusions for sorne nutrients and highlighted important concems with the validity of the computed optimum eND ranges for these species. The inconsistency between CNDs and CND6 strongly suggests
that the inclusion of Mn in CND computations can significantly alter the computation of optimum nutritional ranges and the nutrient diagnosis results. The CND method requires further assessment before being applied to situations where micronutrients are included.
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