Abstract In recent years, one of the dominant tree species of the boreal forests of North America, Populus tremuloides, has experienced increased leaf mining herbivory. On a landscape scale this leaf damage manifests as a change of the canopy from gIntroductionMethodsResultsDiscussionAcknowledgementsReferencesLeafminer Impacts on Photosynthetic Ability of Populus tremuloides Kaarle Strailey Abstract In recent years, one of the dominant tree species of the boreal forests of North America, Populus tremuloides, has experienced increased leaf mining herbivory. On a landscape scale this leaf damage manifests as a change of the canopy from green to silver in late spring. The catalyst for this change is predation of the mesophyll tissue of these leaves by the newly hatched cohort or generation of leaf “mining” insect larvae. While no studies have been published investigating the effects of this mining on leaf function it has been assumed that damage caused by these miners inhibits the plants overall ability to photosynthesize. Historically, outbreaks of these larvae have not been frequent enough to cause significant die back of P. tremuloides. However, a recent series of warm winters has allowed more adult insects to overwinter, leading to a number of high intensity outbreaks. I found a reduction in photosynthetic ability of mined leaves of P. tremuloides in aspen stands surrounding Fairbanks, Alaska. A loss of up to 75% of leaf photosynthetic ability was detected for highly damaged leaves. Mining on undersides of leaves was found to have a more detrimental impact than mining on the top surface. This is attributed to the tissue structure of leaves and possible damage to stomata and changes to volume of interstitial space in spongy mesophyll tissue in lower parts of the leaf that may increase photorespiration. The impact of sustained leaf damage on stand health has yet to be determined. However, prolonged high intensity herbivory may contribute to tree death, and climate projections for continued warming suggest this stress may intensify in the future.Introduction Populus tremuloides Michx (quaking aspen) is a deciduous tree distributed widely across most of North America from Mexico to the Arctic (Perala 2003). A common observation of the foliage of P. tremuloides a few weeks after leaf out in early to late spring is a change of color from bright green to silver. This change in leaf color results from a type of defoliation inflicted by the larvae of various insects of the order Lepidoptera, most prevalently Phyllocnistis populiella (Hennon et al. 2001). Commonly known as leafminers, these larvae burrow between the epidermal tissue layers of leaves “mining” chlorophyll rich mesophyll tissue within the leaf (Borror and DeLong 1971). This mining creates the visible paths across the leaf surfaces, which are wider than the burrowing insects themselves and cover much more area than the leaf cells that were actually consumed. This impact has not historically been observed to be a significant direct cause of mortality for affected P. tremuloides, but it has been assumed to affect photosynthetic ability of mined leaves (DeFoliart 2003, pers. comm.), may make mined leaves more susceptible to subsequent attack by other insects or pathogens, and require the trees to alter their uptake and allocation of nutrients to compensate (Kaitaniemi et al.1999, Zangerl et al. 2002, Ruess 2003, pers. comm.). Throughout the cold northern regions of the P. tremuloides range, the intensity of leaf miner and other insect defoliator attacks has been observed to be increasing drastically in recent years (DeFoliart 2003, pers. comm.). Temperatures in interior Alaska have increased by as much as 5˚C over the past three decades (Lachenbruch 1994, Chapin et al. 2000), and the common assumption is that this allows more adult insects to survive the winters than was typical previously (DeFoliart 2003, pers. comm.). It is also suspected that consistent defoliation of trees can lead to increased tree mortality (Kaitaniemi et al.1999). To date, however, there have not been any studies published estimating the actual loss of photosynthetic ability of P. tremuloides when damaged by leafminer herbivory. I have attempted to quantify the impact of mining on photosynthetic ability of individual leaves of Populus tremuloides in relation to the surface area affected by mining activity. These results may be useful in projecting the cost of global warming to the fitness of P. tremuloides in this biome.Methods Site Selection Three stands of P. tremuloides were located in the greater area of Fairbanks, Alaska. These sites were selected to be representative of different stand densities and severity of attack. The first site selected was in a very open, isolated stand and its individuals appeared to exhibit the least amount of foliar damage. The second site selected was a narrow strip of higher density that ran adjacent to a black spruce stand. This site exhibited a high amount of foliar damage. The third site contained the highest density of trees, also along the periphery of a mixed species stand, and also appeared to exhibit the most foliar damage. Tree Selection Within each stand three trees were selected for sampling. Each of these individuals was chosen to belong to different size/age groups. One tree from each site was under 1.5m tall and young, one was between 2m and 3m, and one was a fairly mature tree, greater than 3m in height. This selection process was somewhat randomized by sampling the third tree found that fit into each size category at each site. Leaf Selection Between four and six leaves were selected for analysis from each of these trees. Leaves were selected in pairs from opposite sides of the same branch to ensure that they were also of the same cohort (grown at the same time under very similar biological conditions). These pairs were selected to meet two criteria: that both members of the pair be free of visible non-miner related damage or necrosis, and that differing amounts of surface area covered by miner trails be evident. Data Collection Measurements of photosynthesis, stomatal conductance, intra-stomatal CO2 concentration, and changes in leaf and surrounding temperature were taken with a LI-COR LI 6400 Portable Photosynthesis System (LI-COR, inc., 4421 Superior Street, Lincoln, NE 68504) using a 6cm2 leaf chamber attachment. The LI 6400 calculates
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