Relationships between climate, forest practices and incidence of Dothistroma septospora.

Cedar Welsh, MSc Candidate

PROJECT PURPOSE AND IMPLICATIONS

Until recently, Dothistroma needle blight has been uncommon and of little concern in lodgepole pine stands in western North America. During the past decade, however, prevalence of the disease has increased (Bradshaw 2004). In particular, northwestern British Columbia has reported severe damaged to managed and natural stands of lodgepole pine. For instance, recent low-level aerial surveys conducted over 40,000 ha of lodgepole-dominated managed stands showed 92% to be suffering varying degrees of damage. The foliar disease is now so prevalent and chronic that entire plantations of lodgepole pine are failing, and the severity of the disease is such that mature pine trees are also succumbing (Woods 2003).

The situation in British Columbia is unique. Dothistroma needle blight is internationally considered a serious forest pathogen only in exotic plantations in the southern hemisphere (Gibson 1972; Bradshaw et al. 2000). However, the damage being reported in British Columbia is an example of disease severity in the northern hemisphere, where hosts are native, and mature stands are affected. In addition, the most serious impacts have been usually associated with the retardation of growth due to defoliation, rather than to mortality (Bradshaw 2004). It is imperative that changes in the extent and nature of Dothistroma needle blight outbreaks be determined to avoid future epidemics and develop comprehensive strategies for management of lodgepole pine in British Columbia.

Dothistroma needle blight poses a significant threat to the growth and yield of lodgepole pine (Pinus contorta var. latifolia) in northwestern British Columbia. More numerous and longer reconstructions of past Dothistroma needle blight are clearly needed to improve our understanding of this disturbance agent. The purpose of this research is to compare the influence of climate on the extent and nature of past Dothistroma needle blight outbreaks to understand the spatial and temporal variations of the disease in forests of northwestern British Columbia. The main objective is to reconstruct both outbreak history and climate through dendrochronological techniques to determine the relationship between climate and the historical outbreak dynamics of the disease. Knowledge of the relationship between climate and outbreak history of Dothistroma needle blight will help to quantify the effects of climate change on disease spread and development which will allow better predictions of future impacts of climate change on forest health.

PROJECT START DATE AND LENGTH

The 2004/2005 year was the first year of the three-year project (2006/2007).

METHODOLOGY OVERVIEW

1. Geographical Area of Study
The main study area is located within the Fort St. James, Nadina, Skeena-Stikine, and Kalum Forest Districts (Fig.1.). The sites fell within three biogeoclimatic variants (ICHmc1, ICHmc2, and SBSmc2). In addition, two sites were sampled in the Prince George Forest District.

2. Field Methods for Outbreak Reconstruction

Site Selection: Reports from the Canadian Forest Service, Forest Insect and Disease Survey (FIDs) were used to identify stands in northwest British Columbia and the Prince George Forest District where Dothistroma had historically been observed. A total of six stands with known historic outbreaks were identified. Four were identified in the northwest and two in the Prince George Region.

In addition, 20 sites were located within the Fort St. James, Nadina, Skeens-Stikine, and Kalum Forest Districts in the northwest to develop a long-term and widespread record of outbreak.

Tree Selection: A total of 20 host and 20 nonhost spruce increment cores were collected from each site. Trees were selected based on evidence of longevity and diameter (dbh, >15cm). All trees were cored at 0.3m, unless the tree had substantial butt rot, in which case it will be cored at 1.3m height. Where available, discs from standing fire-killed trees were sampled in order to extend the chronology back in time.

2. Field Methods for Climate Reconstruction

Site Selection: A climate site was selected within each of the biogeoclimatic zones to cover the area sampled. Areas sensitive to climate fluctuations, such as rocky outcrops and valley slopes, were selected as sites for our climate reconstructions. Two sites were identified within the SBSmc2 to expand its range. Two additional climate sites were established to correspond to the known outbreak sites outside the general sampling area and biogeoclimatic variants.

Tree Selection: Pine cores were collected from each of the six climate sites. At each site 20 trees were cored, two cores per tree. Where available, discs from standing fire-killed trees were sampled in order to extend the chronology back in time.

3. Stand Composition Plots: Three plots were established in each study site using a systematic layout with a random start. In each plot, species composition was determined using a 5.64-m circular quadrat. All live and dead trees 1.3m tall or more, with stems inside the quadrate were tallied by species. Downed trees were included if the root collar is within the plot boundary. Diameter at breast height (1.3m DBH) was recorded for each tree in the plot. Aspect and elevation was recorded at each sample site.

4. Tree Ring Analysis

All cores and discs were mounted and sanded following the procedures of Stokes and Smiley (1968). Annual rings-widths have been measured to the nearest 0.001mm using the Velmex "TA" System in conjunction with MeasureJ2X (1999-2004).

Live and dead materials from each site were cross-dated using the computer program COFECHA (Holmes 1983). This technique is use to detect measurement and visual cross-dating errors. COFECHA tests for errors by computing correlation coefficients between individual series for each species in a stand, ensuring that each ring width is placed in its proper time sequence (Veblen et al. 1991). Cores containing such errors were corrected or removed from the data set.

Cross-dated series were then standardized with the program ARSTAN (Cook and Holmes 1984) to produce a master chronology for each species in each site. Standardization (or detrending) creates dimensionless ring indices with a mean of 1.0 by removing age-related growth trends. The specific standardization technique depends on which type of variation is to be extracted from the ring-widths (i.e. high-frequency or low-frequency variation).

Tree Ring Analysis for Outbreak Reconstruction: Pine and spruce cores for our outbreak reconstructions were detrended by fitting either a negative exponential or linear regression function to the ring-widths. We chose this conservative approach to preserve the ring-width variation due to disturbance.

In order to distinguish outbreaks events from climate contained in our host index series, we removed (or "corrected") the host index through subtraction of the nonhost (spruce) index series. This technique rests on the assumption that, if nonhost and host trees respond in a similar manner to climate variations, then the difference between them will primarily reflect non-climatic environmental variables in the host, such as the effects of a defoliation event. Graphical plots, correlations, and response function analyses were used in order to assess the host-nonhost climate response. Response function analysis uses principal components to estimate the response of tree growth to climate variables. The climate variables used were monthly total precipitation and monthly average temperature (BIOCLIM model, Canadian Forest Service). The resulting monthly climate response coefficients of the host and nonhost chronologies were directly compared.

For our six known outbreak sites, we compared the timing and duration of ring-width suppressions in our corrected chronologies with the historical records. Those sites showing strongly demarcated negative growth responses during the known periods were used in our signal development. This Dothistroma-specific signal was then used to infer past outbreak occurrences throughout the chronology and the individual 20 chronologies by assigning values to parameters in the program OUTBREAK (Holmes and Swetnam 1996).

Tree Ring Analysis for Climate Reconstruction: The specific standardization technique for our climate reconstructions has not been determined at this time. However, gradual trends in growth due to age and site changes will be removed to obtain a time series showing only the tree-growth climatic responses.

Total monthly precipitation and minimum, maximum, and mean monthly temperature variables will be tested using the computer program PRECON (Fritts et al. 1971). PRECON (for PRECONditioning) is a response function analysis used to identify those monthly climate variables that have significant associations with annual radial growth of the species chronology. The method uses multivariate statistics and eigenvector techniques to identify associations (Gedalof and Smith 2001). As a result, climate variables for particular monthly intervals are identified that significantly related to variations in annual growth.

The modeled relationships will be confirmed using various statistical verification techniques. A verification scheme is often used to test for "prediction bias" of a regression model by observing the predictive power of the model against a set of the climate data (Cropper 1982). A statistically significant verification indicates the validity of the regression model for accurately predicting the climate variable from annual growth. We will then use a least squares regression to reconstruct the specific climate variable back in time.

5. Disease Dynamics and Climate: To understand the spatial and temporal variations of the disease we will compare outbreak patterns among the sites. We will also group the sites into their respective biogeoclimatic variants to see if differences exist between the groups. We anticipate that outbreak patterns may be different between the variants because of the differences not only in climate patterns, but with ecological communities and land-use histories. We will then compare the historical outbreak patterns to determine if temporal changes may be related to favourable climate variations.

6. Host Availability: To investigate whether spatial and temporal patterns of the disease are related to changes in host availability, we will compare patterns between heavily managed areas with areas relatively free of human influence. These comparisons will provide evidence that changes in forest composition and structure has altered the dynamics of the disease.

PROJECT SCOPE AND REGIONAL APPLICABILITY

This project links shifts in climate with the historical dynamics of Dothistroma needle blight for the purpose of forecasting forest health responses to climate change. It will enable development of more comprehensive strategies for management of lodgepole pine in British Columbia.

The regional applicability of the project is those areas within the natural range of lodgepole pine, particularly wetter environments in central to northern British Columbia.

INTERIM CONCLUSIONS

We have found a distinctive ring-width signature attributable to a Dothistroma needle blight defoliation event. In all documented outbreaks the signature started with thin latewood and a 40% decrease in annual radial growth during the first year, followed by a sharply reduced growth for at least 3 years or more.

Known Dothistroma needle blight outbreaks were successfully detected in the ring-width series. Also, preliminary results indicate that a number of past unrecorded outbreaks (not in survey records) are present in some of the host chronologies from both northwest British Columbia and the Prince George Region. The timing of these unrecorded outbreaks corresponds with those results found in Woods et al. 2005. These outbreaks were observed during times of documented peaks in precipitation during the 60's, 80's, and mid-to-late 90's. From a regional-scale perspective the late 70's to early 80's appears to be marked with a period of spatial synchrony of Dothistroma needle blight activity among the sites.

CONTACT INFORMATION

Kathy J. Lewis
UNBC
250-960-6659
lewis@unbc.ca