Is light availability related to Choristoneura fumiferana defoliation effects on seedling storage compounds in the Eastern Canadian boreal forest?
Choristoneura fumiferana (SBW) is a major disturbance of North American boreal forest that defoliates coniferous species. Abies balsamea (AB), a very shade tolerant species, and Picea mariana (PM), an intermediate tolerant species, are its main hosts. During outbreak periods, that last 5-15 years, SBW not only defoliates on current year shoots of mature trees but also on seedlings. Seedlings are important to forest recovery after disturbances. Stored carbohydrates are key compounds used by plants to maintain basal metabolism during periods of stress. Carbohydrate storage responses can help explain how seedlings of species with different shade tolerance strategies cope with defoliation under contrasted light conditions. We hypothesize that according to their shade tolerance strategies; AB carbohydrate storage is more affected by defoliation in canopy gaps while PM carbohydrate storage is more affected in the understory. To analyze the effects of defoliation on carbohydrate storage of seedlings of both species under contrasted light conditions, soluble sugars (SS), starch (ST) and total non-structural carbohydrates (TNSC) were considered. Seedlings of both species were collected in two contrasted light environments (understory vs gaps) following a gradient of defoliation, from class 1:0-20% to class 5: 81-100%. Multiple linear regressions models were used to test the hypothesis.
Defoliation intensity (from class 1 to class 5) reduced starch and TNSC concentration significantly in needles, stems and roots of both species while soluble sugar concentration did not. Concerning carbohydrate concentration in AB, starch concentration in needles decreased faster along the defoliation intensity gradient in seedlings growing in the understory than in seedlings growing in gaps. Although starch concentration in stems and roots of AB seedlings decreased with defoliation intensity there were no differences between seedlings growing in gaps and those growing in the understory. Concerning carbohydrate concentration of PM, starch and TNSC concentration in stems were more affected by defoliation in gaps than in the understory. In fact, reduction of starch and TNSC concentration in stems in the understory were not significantly related to defoliation. In conclusion, starch and TNSC were significantly reduced by defoliation in seedlings of AB and PM. However, in contrast to our hypothesis, AB carbohydrate storage was less affected by defoliation in canopy gaps while PM carbohydrate storage was more affected in gaps suggesting that light availability in the boreal forest understory is related to defoliation effects on seedling storage compounds.