Dwarf Mistletoe on Fir in the Sierra Nevada

Description

Dwarf Mistletoe on Fir in the Sierra Nevada

Introduction:

The purpose of this project was to establish a system of permanent plots to monitor growth, mortality, and dwarf mistletoe infection in previously harvested, regenerating dwarf mistletoe infested stands dominated by red fir (Abies magnifica) and white fir (Abies concoloris), under thinned and unthinned treatments.

In the late 1970’s, thirty-two plots were established in the Klamath, Lassen, Sequoia, and Stanislaus National Forests and the La Tour Demonstration State Forest.  Plots varied in size from 0.16-2.02 hectares, depending upon site uniformity. Site selection was restricted to dwarf mistletoe infected stands composed of at least fifty percent red and/or white fir.  Stands varied in elevation from 1,646 to 2,286 m.  Thinning treatments in twenty-three of the plots resulted in stands with roughly 3.66 meter spacing between trees, leaving approximately 740 trees per hectare.  Plots were surveyed prior to treatment applications in 1979-1981 and post-thinning in 1981, 1990, 2001, 2004, and 2006.  In the pre-thinning survey, data recorded for each tree included dbh to the nearest 0.25cm, the Hawksworth dwarf mistletoe rating (dmr), and location along the strip.  Data recorded post thinning included the same data collected during the pre-thinning survey, along with an additional record of mortality and the live crown ratio of each tree.  Additional surveys for mortality alone were conducted in 2004 and 2006.

Results:

Over the course of the study, cumulative mortality across all forests was 22.7% (Range 11.4-41.0%). When all trees across all forests were considered, the general trend was for trees with higher infection ratings to have greater mortality rates and reduced survival times, with trees with low to severe infections (dmr of 2 or greater) in 1981 being significantly more likely to die over the course of the study than initially uninfected trees. Across all forests, trees with higher initial dwarf mistletoe ratings had greater risks of mortality and lower radial growth rates, while trees with higher live crown ratios and initial dbh’s had lower mortality risks and greater radial growth rates.  While the trend was for trees in thinned plots to have longer survival times than trees in unthinned plots, this effect was not significant (p=0.085).  However, thinning did increase radial growth rates.  Thinning also influenced changes in mistletoe infection intensity and spread.  At the start of the study, infection ratings of trees in thinned and unthinned stands did not differ significantly (p=0.23). However, over the course of the study, the rate of change for infection intensity differed between treatments (p<0.0001), with infection ratings increasing in thinned plots and decreasing in unthinned plots.  Additionally, at the start of the study the proportion of infected trees did not differ between treatments (p = 0.32) but in all forests except the LaTour Demonstration State Forest and Stanislaus National Forest, initially uninfected trees in thinned plots were significantly more likely to become infected over time than initially uninfected trees in unthinned plots.

The results of this study confirm the negative effects of dwarf mistletoe infection on the survival and radial growth of young true firs over time, justifying the development of silvicultural techniques for reducing mortality and stimulating growth in infected stands managed for timber production. However, findings regarding the efficacy of pre-commercial thinning for reducing mistletoe associated losses were variable, with thinning treatments successfully promoting radial growth but not significantly reducing mortality risk, and stimulating the intensification of infections on individual trees and increasing the rate of spread within a stand.