Vegetation Trends In Whitehorse Yukon, 1984 To 2009

To assess temporal greenness trends at the landscape scale for Whitehorse, Yukon (417 km2), this study derived a Landsat normalized difference vegetation index (NDVI) time-series from 1984 to 2009. Using a greenest annual pixel approach, NDVI trend analysis revealed that 37% of studied area had significant greening (p<0.05) and that only 1% of the studied land area had significant browning. Yearly mean NDVI values declined in drought years and increased in years with greater precipitation. Greening pixels were most prevalent in white spruce (Picea glauca) and subalpine fir (Abies lasiocarpa) dominant forests, suggesting that increased amounts of precipitation and rising temperatures have benefited both species and associated shrub communities. Forests where trembling aspen (Populas tremuloides) are dominant displayed the least greening, which may be explained by the proliferation of aspen serpentine leaf miner (Phyllocnistis populiella), and drought related die-back on south-facing slopes that have become warmer across the study period.

Vegetation Trends In Whitehorse Yukon, 1984 To 2009

To assess temporal greenness trends at the landscape scale for Whitehorse, Yukon (417 km2), this study derived a Landsat normalized difference vegetation index (NDVI) time-series from 1984 to 2009. Using a greenest annual pixel approach, NDVI trend analysis revealed that 37% of studied area had significant greening (p<0.05) and that only 1% of the studied land area had significant browning. Yearly mean NDVI values declined in drought years and increased in years with greater precipitation. Greening pixels were most prevalent in white spruce (Picea glauca) and subalpine fir (Abies lasiocarpa) dominant forests, suggesting that increased amounts of precipitation and rising temperatures have benefited both species and associated shrub communities. Forests where trembling aspen (Populas tremuloides) are dominant displayed the least greening, which may be explained by the proliferation of aspen serpentine leaf miner (Phyllocnistis populiella), and drought related die-back on south-facing slopes that have become warmer across the study period.

The leaf miner Phyllocnistis populiella negatively impacts water relations in aspen

Within the North American boreal forest, a widespread outbreak of the epidermal leaf miner Phyllocnistis populiella Cham. has damaged quaking aspen (Populus tremuloides Michx.) for nearly 20 years. In a series of experiments, we tested the effects of feeding damage by P. populiella on leaf water relations and gas exchange. Relative to insecticide-treated trees, the leaves of naturally mined trees had lower photosynthesis, stomatal conductance to water vapor, transpiration, water-use efficiency, predawn water potential and water content, as well as more enriched foliar δ13C. The magnitude of the difference between naturally mined and insecticide-treated trees did not change significantly throughout the growing season, suggesting that the effect is not caused by accumulation of incidental damage to mined portions of the epidermis over time. The contributions of mining-related stomatal malfunction and cuticular transpiration to these overall effects were investigated by restricting mining damage to stomatous abaxial and astomatous adaxial leaf surfaces. Mining of the abaxial epidermis decreased photosynthesis and enriched leaf δ13C, while increasing leaf water potential and water content relative to unmined leaves, effects consistent with stomatal closure due to disfunction of mined guard cells. Mining of the adaxial epidermis also reduced photosynthesis but had different effects on water relations, reducing midday leaf water potential and water content relative to unmined leaves, and did not affect δ13C. In the laboratory, extent of mining damage to the adaxial surface was positively related to the rate of water loss by leaves treated to prevent water loss through stomata. We conclude that overall, despite water savings due to closure of mined stomata, natural levels of damage by P. populiella negatively impact water relations due to increased cuticular permeability to water vapor across the mined portions of the epidermis. Leaf mining by P. populiella could exacerbate the negative effects of climate warming and water deficit in interior Alaska.

Variable extrafloral nectary expression and its consequences in quaking aspen

Extrafloral nectaries (EFNs) are secretory glands most commonly linked to defensive mutualisms. Both a plant’s need for defense and the strength of defense provided by mutualists will vary with plant condition and local insect community. Thus, the benefit of EFNs may vary spatially and temporally. However, little attention has been paid to natural variation in the presence and abundance of EFNs within and among individuals of the same species. Quaking aspen, Populus tremuloides Michx., bear EFNs on a subset of their leaves. Here, we describe patterns of EFN expression on shoots within ramets, among ramets, and among putative clones in interior Alaska. We also examine the relationship between EFN presence and herbivory by both the very abundant aspen leaf miner, Phyllocnistis populiella Chambers, and less common chewing herbivores. The proportion of leaves bearing EFNs varied from 33% to 87% among distinct aspen stands. Within stands, short (1–2 m height) ramets had higher EFN frequency than their taller (>4 m) neighbors. Patterns of herbivory also differed between short and tall ramets. Compared with leaves without EFNs, those with EFNs suffered less mining damage on short ramets but slightly higher damage on tall ramets. Tall ramets suffered more chewing damage than short ramets, but this damage was unrelated to the presence of EFNs. Our results suggest that variable EFN expression may be explained by variation in the benefits of EFNs. Leaves with EFNs on short ramets benefit through reduction in leaf mining, but this benefit does not extend to tall ramets or other forms of herbivory.

Bionomics of the Aspen Leaf Miner, Phyllocnistis populiella Cham. (Lepidoptera: Gracillariidae)

The aspen leaf miner, Phyllocnistis populiella Cham., has recently become common and very abundant in western North America, and has been authoritatively identified only from trembling aspen, Populus tremuloides Michx. Overwintered adults lay eggs on young aspen leaves in spring, and new-generation moths emerge in about two months. Only the single epidermal cell layer is mined on the upper or lower leaf surface, most of the feeding being done by the third-instar larva in about a week. Heavy attack results in defoliation by mid-summer. Activities of breeding populations are affected by temperature (50-55°F. is optimal for copulation, 54-56°F. for feeding, and 52-58°F. for oviposirion) and developmental stage of aspen leaves. Eggs are laid singly near the apex of the leaf and tend to be evenly spaced. Distribution of eggs between leaves tends to be uniform in a particular level of the tree, although more eggs are laid in the lower crown. A female moth can develop more than 40 eggs, but only about 7 are usually laid because of limited opportunities. Mortality in the larval and pupal stages is often high; population decline usually follows mortalities above 70%. Parasitism sometimes plays an important role, but the independence of population changes between broad geographic areas suggests that numbers of P. populiella may be strongly influenced by climatic factors. Population trends and damage can be assessed conveniently for large geographic areas and for specific sites by continuing studies in permanent sample plots. Expected damage may be predicted from estimates of new-generation adult populations. Because inter-tree variation exceeds intra-tree variation, more trees and fewer branches per tree should be taken to increase sampling efficiency. Also, greater precision is attained by the use of the individual leaf surface rather than the leaf as a basic sample unit.

A Description of the Immature Stages of Phyllocnistis populiella Chambers (Lepidoptera: Gracillariidae)

The adult of Phyllocnistis populiella was described by Chambers in 1875. No detailed descriptions of the immature forms of this species have appeared in publication, although descriptions of other species in the genus have been published. Sich (1902), in his paper on P. suffusella Zell., presents a critical discussion of earlier works on the genus and gives a comprehensive account of the general features of the immature forms, although he omits details on setation and some other features of taxonomic interest. Certain aspects of larval and pupal morphology have been treated by Grandi (1932), Kozlovsky (1955) and Trägårdh (1913), while Hering (1951) and Needham (1928) have discussed structure as related to habit.