Reducing Solar Heat Gain during Winter: The Role of White Bark in Northern Deciduous Trees

ABSTRACT. Deciduous tree species throughout the boreal forest of North America have lighter-coloured bark than do species restricted to more southern forests. We tested the hypothesis that light-coloured bark minimizes the thawing and freezing of cambium tissue during winter that could contribute to sunscald injury. During mid-winter, maximum midday cambium temperatures of south-exposed bark of white birch (Betula papyrifera Marsh.) near Timmins, Ontario, were higher for brownpainted bark (+1.6 ˚C) than for natural bark (-9.4 ˚C) and white-painted bark (-12.1 ˚C). Rates of temperature decrease after trees were shaded at midday were more rapid for brown-painted bark (0.06 ˚C/min) than for natural bark (0.03 ˚C/min) and whitepainted bark (0.03 ˚C/min). When stems of white birch, trembling aspen (Populus tremuloides Michx.), yellow birch (B. alleghaniensis Britton), and largetooth aspen (P. grandidentata Michx.) were illuminated and subsequently shaded at-10 ˚C ambient air temperature, maximum cambium temperatures and rates of cambium cooling increased with decreasing measures of whiteness. For trembling aspen in the southwest Yukon, we found that after two years, brown-painted trees had a higher incidence (35%) of wounding that resembled sunscald injury than did white-painted trees (2.5%) and natural trees (4.5%). Therefore, we suggest that light-coloured bark reduces the risk of winter sunscald injury, probably by protecting the cambium from solar heat gain in subfreezing temperatures. This physical mechanism for reducing sunscald risk may explain why the deciduous trees at the northern limit of tree growth are those with highly reflective bark. Key words: bark colour, white birch, cambium temperatures, trembling aspen, sunscald, boreal forest, deciduous tree

Reproduction at all costs: the adaptive stress response of male arctic ground squirrels

We tested the hypothesis that adult male arctic ground squirrels (Spermophilus parryii plesius) exhibit an adaptive stress response during the mating period that may compromise their survival, whereas males at other times (nonreproductive adult males and juvenile males) have a normal functional stress response. We assessed the physiological responsiveness of the stress axis, of energy mobilization, and of the immune response by subjecting adult breeding males, adult nonbreeding males, and juvenile males to a hormonal challenge and an immunocompetence challenge. At the onset of the breeding season in spring, only 25–30% of the population were males, and of those present during the mating period, half disappeared soon thereafter, and 82% were not replaced by immigrants. Adult breeding males had the highest levels of free cortisol, the lowest maximum corticosteroidbinding capacity, slight dexamethasone resistance, the lowest hematocrit, the lowest number of white blood cells, the highest number of eosinophils, and the poorest ability to respond to the foreign antigen challenge in comparison with the other two male classes. All of these characteristics were indicative of chronic stress in breeding males that may directly compromise their survival. Juvenile males in mid-August also showed many, but not all of these characteristics, indicative of a prolonged period of stress, presumably associated with the period of dispersal. Testosterone levels remained high irrespective of age or breeding condition, decreased when dexamethasone was injected, and increased when ACTH was injected. These latter results are unique in mammals. High testosterone levels and their augmentation with stressors may play a key role in maintenance of aggressive behavior. We conclude that breeding male arctic ground squirrels exhibit an adaptive stress response in which they trade off survival for reproduction. We hypothesize that similar stress responses may have evolved in other species with comparable life histories

The interactive effects of food and predators on reproduction and overwinter survival of arctic ground squirrels

1. We examined the effects of food and predators on population limitation in the arctic ground squirrel (Spermophilus parryii plesius Richardson) in the boreal forest of the south-western Yukon. We focused on ground squirrel reproduction and overwinter survival. 2. Squirrel populations were monitored by live-trapping and radio-telemetry from 1993 to the spring of 1996 on four control and four experimental areas (one preda- tor exclosure treatment, two food addition treatments, and one predator exclosure plus food addition treatment). 3. Predator exclusion increased body condition, percentage lactating, percentage weaning litters, litter size, and doubled population density relative to controls, but had no effect on juvenile growth rate, overwinter survival, or juvenile emergence date. 4. Food addition advanced juvenile emergence date and increased adult body con- dition, percentage lactating, percentage weaning litters, litter size, population den- sity relative to controls (4-7 fold), but had no effect on juvenile growth rate or overwinter survival. 5. Predator exclusion combined with food addition increased adult body condition, percentage lactating, percentage weaning litters, and population density relative to controls (19-fold). 6. We conclude that arctic ground squirrels in the boreal forest are limited by an interaction between food and predation, acting primarily through changes in repro- duction, and that their impact on density was multiplicative

Experimental manipulation of predation and food supply of arctic ground squirrels in the boreal forest

We examined whether arctic ground squirrel (Spermophilus parryii plesius) populations in northern boreal forest in the Yukon Territory, Canada, were limited by food, predators, or a combination of both, during the decline and low phases of a snowshoe hare cycle. From 1990 to 1995, populations were monitored in large-scale (1 km2) experimental manipulations. Squirrels were studied on eight 9-ha grids: four unmanipulated control grids, two foodsupplemented grids, a predator-exclosure grid, and a predator-exclosure + food-supplemented grid. Population density was measured on all grids by livetrapping and active-season survival was measured using radiotelemetry. Population densities were lowest in 1992 and 1993 (2 years after the snowshoe hare population decline). Rates of population change were negative from 1991 to 1993, when predation pressure was most intense after the snowshoe hare decline, and positive from 1993 to 1995, when hares and predators were at low densities. Predation accounted for 125 of 130 mortalities (96%) of radio-collared squirrels. Adult survival was significantly lower in 1992 and 1993 than in 1994 and 1995, and was a strong predictor of annual rates of population change in arctic ground squirrels. Treatments were ranked as follows in their effect on adult survival: predator exclosure + food-supplemented > food-supplemented > predator exclosure > controls. Juvenile survival was lowest in 1992, and food addition and predator removal separately increased juvenile survival. On average, predator exclusion increased population densities twofold, food supplementation increased densities fourfold, and food supplementation and predator removal together increased densities 10-fold. We conclude that food and predation interact to limit arctic ground squirrel populations in the boreal forest during the decline and low phases of the snowshoe hare cycle. The snowshoe hare cycle may indirectly create a lagged secondary fluctuation in arctic ground squirrel populations through shared cyclic predators

Mutual mate choice for olorful traits in King Penguins

While studies of mate choice based on male color pattern are ubiquitous, studies of mate choice based on ornamental color traits in sexually monomorphic species are less common. We conducted manipulative field experiments on two color ornaments of king penguins (Aptenodytes patagonicus), the size of auricular patches of orange feathers and degree of UV reflectance from beak spots, to determine how the degree of ornamentation influenced pairing rate. In a reduction of auricular patch size, females paired significantly more quickly than males in both control and experimental samples. When this bias was taken into account statistically, pairing of individuals with reduced auricular patches was significantly delayed. We also reduced, but did not eliminate, UV reflectance from beak spots by applying a UV filter; no sex difference in pairing rate was evident in this experiment. Treated birds paired significantly more slowly than untreated control individuals, taking more than a week longer to pair on average than their unmanipulated counterparts, a result that was significant for males and approached significance for females. Our results may indicate mutual mate choice via UV reflectance of the beak spot. Given that this is a species where breeding is extremely slow and considerable investment by both males and females is required for successful reproduction, our results support the hypothesis that in such species, sexual selection might act on the same ornament in both sexes