Alterations in Douglas-fir crown structure, morphology, and dynamics imposed by the Swiss needle cast disease in the Oregon Coast Range

Abstract

Plants respond to defoliation in many different and complex ways, depending on their growth habit and form as well as the extent and duration of the defoliation. Tree crowns have been shown to be quite sensitive to disturbances such as defoliation, however quantitative relationships have rarely been developed, making the true biological meaning of crown condition assessments quite difficult to decipher. The sudden emergence of Swiss needle cast (SNC) in the Oregon Coast Range prompted investigation of the response of Douglas-fir crown structure, crown morphology, and foliage dynamics to extended defoliation. Using data from permanent plots and 82 destructively sampled trees, hypotheses regarding the response of trees to defoliation were tested with linear and nonlinear models. Responses of crowns were investigated at multiple levels including the needle, branch, tree, and stand scales. At the individual-needle level, SNC has resulted in foliage that is smaller in length, width, and projected area; lower in dry mass; and higher in specific leaf area. The disease has influenced the foliage age structure by increasing the proportion in the younger age classes with greater SNC severity. The disease has also resulted in crowns that have a greater proportion of their current and 1-year-old needles located higher in the crown than Abstract approved Douglas A. Maguire normal, while the 2-, 3- and 4-year-old needles are shifted towards the crown base relative to healthy trees. At the branch-level, the disease has modified growth patterns as well as dry matter production and allocation. The number of secondary lateral branches on a primary branch declined in response to the disease, as did the foliated branch length and diameter for a given position in the crown. The ratio of branch length to branch diameter, however, increased with disease severity, which suggests altered elongation patterns. Branches in the lower portion of the heavily diseased crowns were elongating faster than normal, while elongation of branches in the mid-crown was slower. Elongation of branches in the upper crown was unaffected. The disease significantly reduced branch foliage dry matter and area for a given position in the crown and diameter. Branches, however, allocated more dry matter to higher order branches, but less dry matter to primary branch elongation. At the tree-level, the disease has led to crowns that are shorter than normal, while the largest measured standing width remains unaffected. Crown radii and maximum branch diameter profiles suggested that changes within the crown might be occurring at different levels due to variations in SNC damage within the crown. The number of primary interwhorl branches decreased with greater disease severity. SNC also significantly reduced total foliage and branchwood dry matter. Overall, the vertical distribution of foliage dry matter was less skewed and more uniform with increased disease severity. At the stand-level, SNC has significantly increased crown recession rates, woody litterfall, the specific leaf area of the litter, and growth efficiency. The disease caused a decline in foliage litterfall rates as well as leaf area index. In addition, the seasonal distribution of foliage litterfall was altered, with a greater amount occurring in the summer than normal. Crown condition, as assessed by foliage retention and the crown sparseness index, varied with SNC and other site factors. Within-tree variability of foliage retention was significantly higher than between-tree or between-plot variability. Although assessments of foliage retention were found to be highly variable, the sample size required to attain a sufficiently precise estimate for a given tree and stand are lower than the sample sizes currently being collected. The crown sparseness index was found to be significantly less variable than foliage retention and was primarily influenced by stand factors such as age and stand density. Defoliation caused significant changes at the individual needle, branch, tree, and stand levels, complicating efforts to accurately predict growth responses to defoliation. This detailed analysis of crown and foliage dynamics helps to establish links among current SNC studies. For example, growth losses associated with SNC are due to the reduction of foliage area, but also to changes in the size and vertical distribution of the needles. In addition, crown assessment indices such as foliage retention and crown length to sapwood area ratio represent different aspects of crown condition. Crown attributes are sensitive to the direct and indirect effects of SNC, which have important implications for tree growth and stand management. Important direct effects include the premature loss of foliage, while important indirect effects of SNC include changes in the within-crown light environment. Integrating measures of crown condition into forest models, therefore, represents an important step towards incorporating physiological-mechanisms into models for predicting growth responses to environmental changes such as tree disease and for understanding the complex responses of tree morphology and growth.