Forage quality and moose (alces alces) nutrition in a logged landscape

Forest dynamics are increasingly being influenced by forest management practices such as logging and salvage operations. These practices alter forest landscapes by shifting forest patches from mature forest dominated by coniferous trees to younger seral stages represented by grasses, forbs, shrubs, and deciduous trees. These changes affect the conditions (e.g. sunlight, water, soil nutrients) under which the regenerating plants grow, further influencing species composition and plant nutritional quality. Of note is the fact that many plants growing in full sunlight can produce more tannins (secondary metabolites produced to defend against herbivory) than plants growing in shade. Tannins decrease the digestible protein available in plants, reducing the nutritional value for herbivores such as moose. Many moose populations in central interior British Columbia (BC) have been declining in tandem with a dramatic increase in salvage logging that followed a mountain pine beetle epidemic in the 1990s and 2000s. Cutblocks are typically thought to benefit moose, as the early-seral plant regrowth provides abundant forage. However, moose populations have declined despite an increase in this habitat type. Hypothetically, these declines may be due at least in part from forage plants growing in sun-rich cutblocks producing more tannins and thus affording relatively lower digestible protein to the moose. Following this, I studied whether individual moose that foraged more in cutblocks would have poorer body condition and lower rates of pregnancy and calf survival than individuals that foraged more in unlogged forests. Working near Logan Lake, BC, I used a cohort of collared adult female moose (n=16) to select areas for forage sampling. I analyzed key forage species in cutblocks, cutblock edges, and unlogged forests over two summers (2021 and 2022) and found that plants in cutblocks did indeed produce more tannins and had lower digestible protein than those in forests. I also found that forages provided insufficient protein to support lactating moose throughout most of the summer. There were significant differences in plant nutritional quality year to year, with plants being of relatively poorer quality after the hot, dry growing season of 2021. I also followed the cohort of collared moose over the course of the study and repeatedly measured their body condition. I found that all individuals were classified as mildly to moderately nutritionally limited, but that poor body condition of adult females was not associated with depressed pregnancy or calf survival rates. Finally, I quantified each individual’s use of cutblocks and burned habitats during the summer, and counter to the results of my nutritional work, I found no effect of cutblock use on adult female moose condition or reproduction. Use of burned habitats by moose, however, was linked to calf survival. Differences in adult female moose body fat were best explained by year, with individuals being in relatively poorer condition after the hot, dry spring and summer of 2021, likely due to a combination of reduced forage quality and increased thermal stress. In summary, the cutblocks in my study area provided forage of poorer nutritional quality than forests, with cutblock edges providing forage of intermediate quality. Moreover, this landscape provided insufficient digestible protein to support lactating moose for most of the summer. However, these differences in nutritional quality between cutblocks and forests did not seem to translate into differences in adult female moose body condition or reproduction. Further work should explore whether other factors such as climate change or moose harvest are more directly linked to recent population declines in this area.