On November 5, 2013 during the "midi de la foresterie" conference organized by the Chaire AFD, we had the pleasure of listening to guest speaker Dr. Patrick James, PhD, Professor at Université de Montréal. Professor James discussed his current research on the spruce budworm in a presentation entitled "Intégrer les hypothèses «ennemi naturel» et «sylvicole» de la dynamique des épidémies de la tordeuse des bourgeons de l'épinette".
Although many years of scientific research have led to a better understanding of spruce budworm (Chonsfoneura fiimitbrana) and forest ecosystem interrelationships, Professor James believes that many opportunities exist to better understand the factors that govern the periodic outbreaks of this economic and ecologically important defoliating species.
Forest ecosystems are inherently complex because they are made up of a number of abiotic and biotic entities and processes that interact over multiple spatial and time scales which are difficult to predict. Structural complexity in terms of attributes, species composition, vigor and diversity are recognized to be important factors in maintaining the stability and resiliency of forest ecosystems. Forest structure and species diversity are thus important factors in influencing the susceptibility of stands to insect epidemics.
Insect Epidemics in Canada
Insect epidemics are an important natural disturbance in Canadian boreal forests, affecting a greater area of forest than fires and harvesting. Although insect damage does not result in the same magnitude of mortality, insect epidemics influence ecological interactions and therefore largely impact ecosystem processes. Insect disturbances create and are affected by the spatial structure of the landscape. The interaction between the insect disturbance and the spatial structure make it difficult to distinguish between the causes and effects of epidemics, because they are integrated in each other. One of the key questions that may help in addressing this issue is: What role does insect movement and dispersion play in epidemics?
The Spruce Budworm
The spruce budworm is a major defoliating, endemic insect, found throughout the boreal and boreal mixedwood forests of Canada. It is particularly prevalent in eastern Canada, because the forests in this part of the country contain a high abundance of tree host species including balsam fir (Abies basamea), white spruce (Picea glauca) and black spruce (Picea mariana). Severe population outbreaks are cyclic and occur Indicates forest areas in Canada that have been affected at least once by spruce budworm approximately every 35 years, resulting in high tree mortality continuing over a period of about 10 years. Outbreaks are a natural phenomenon and though they inflict widespread damage to economically valuable tree species, they strongly affect boreal forest dynamics and play an important role in shaping the natural forest mosaic.
Three major spruce budworm epidemics occurred throughout the 20th century and evaluations of these outbreaks suggest that the extent and severity of damage has increased over time. This is evident in Quebec, where the majority of the province was affected by that last epidemic and the severity of defoliation across the province increased significantly from 1962 to 1992. In certain parts of the province such as Côte-Nord, defoliation damage by local budworm populations in 2013 is considered to be quite severe. This area was less affected in during the previous outbreak period which warrants many questions as to what factors are responsible for this latest outbreak. Is it random or have ecological factors changed in Côte-Nord since the last outbreak, making it more vulnerable to attack?
Forest areas in Canada that have been affected at least once
by spruce budworm (from Peltonen et al. 2002)
The Silvicultural Hypothesis
The silvicultural hypothesis suggests that human induced changes to forest ecosystems through harvesting, have resulted in homogenous forest structures across the landscape, which may influence the susceptibility and vulnerability of stands to spruce budworm attacks. Previous studies focused on the proportion of mature balsam fir in the stands and the importance of food supply as the primary cause of outbreaks. Recent studies however, have indicated that biologically diverse stands tend to show lower levels of defoliation than homogenous stands thus, the structural and compositional complexity of stands may indeed be a more important factor affecting stand vulnerability. The mechanism through which this occurs remains largely unresolved but is likely related to the role of parasitoid predators. The hypothesis is that a diverse forest will maintain a high diversity and abundance of parasitoids which will control the growth of spruce budworm population.
The Bird Feeder Effect
Current research suggests that the ability of parasitoid predators to respond to changes in budworm densities could be essential for stabilizing food webs. The bird feeder effect occurs when high local prey densities of spruce budworm attract high levels of parasitoid predators from the surrounding forest, minimizing the severity of outbreaks. The dispersal capacity of predators however, may be negatively affected by forest fragmentation whereby weak connectivity and long distances between stands inhibits predator movement, resulting in weaker control of the outbreak. Furthermore, heterogeneity and structural diversity of surrounding forests maintains a high density and diversity of natural enemies which is important for controlling spruce budworm populations. Homogenization of surrounding forests may then diminish predator density and the capacity of the ecosystem to control the outbreak.
- What are the relative effects of dispersion, parasitoids and the forest on the spruce budworm?
- What is the role of composition and connectivity of the forest and the landscape on:
- The diversity and abundance of parasitoids?
- The dispersal ability and gene flow of spruce budworm and parasitoids?
- How does connectivity affect the dynamics of epidemics and ecosystem stability?
Integrated Landscape Genetics
To address these research questions, integrated landscape genetics will be used to determine how environmental factors and landscape elements affect insect dispersion. Insect populations separated over long distances by ecological features such as lakes and mountains, and subjected to various biotic interactions have slight genetic differences. These differences can then be used to determine dispersal patterns between populations and across the landscape.
In the summer of 2012 sampling was conducted at 22 sites in Côte-Nord, in order to identify resident spruce budworm populations. Larval and moth DNA will be extracted and allele frequencies will be calculated in order to compare the genetic structure of spruce budworm populations between locations. Furthermore, the diversity of parasitoids at each site was also characterized and the genetic structure of key parasitoid species such as Glypta fumiferanae was characterized in function of the landscape.
Preliminary results indicate that there is some spatial variation in the rate of parasitism between different parasitoid predators. In northern latitudes the rate of parasitism was elevated for Glypta fumiferanae whereas the rate of parasitism for Apanteles fumiferanae was elevated in southern latitudes. However, it is not yet known what factors (such as forest type, climate, host tree density etc.) contribute to this variation.
The BIOSIM model was used to predict the periods in which moths could be found during collection periods in the months of July, August and September. Some moths were found in sites outside of the model predicted periods, suggesting that these moths are migrant, coming from outside populations. The genetic structure of these moths will be used to determine the location from which they came. From this information, dispersion models will be created. Sampling was repeated in 2013 and collections will be repeated again in 2014 in order to create a spatiotemporal model of spruce budworm dispersal.
The objective of this research is to use integrated landscape genetics to better understand the dynamics of spruce budworm epidemics. This will be determined using environmental factors and their effect on the dispersal and abundance of spruce budworm and parasitoids, as well as how they affect interactions between species. Finally, this will lead to a better understanding of how forest management practices can be ameliorated to aid in the control of spruce budworm outbreaks.
Presentation Summary: Jessica Smith, étudiante M.Sc. biologie, UQAT
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