|Abstract or Summary
- We determined the incidence of pathogens and insects across mixed-conifer stands in the Sierra San Pedro
Martir (SSPM) of northern Baja, Mexico, to assess the role of pests in a pristine forest ecosystem. We also determined
the spatial distribution of the two most common pests, mistletoe, Phoradendron pauciflorum Torrey, and the fir engraver,
Scolytus ventralis LeConte, of white fir (Abies concolor (Gord. & Glend.) Lindl.) across a 25-ha grid to assess
spread and what host and pest variables were related. In these open parklike stands the mean tree density was 160
trees/ha, of which 58% were trees >20 cm diameter at breast height (DBH). In these low-density, mixed-aged stands
we found that mixed-conifer species were well represented with no one species being completely dominant. Percent cumulative
mortality for the SSPM was 12.7%, ranging from 2 to 24%, with the greatest amount of mortality occurring
in the larger size classes, trees ³50 cm DBH. Multiple linear regression analysis showed that 78% of the mortality we
observed was explained by pathogens and bark beetles (r2 = 0.78, P = 0.0001, F = 84). Mean pest incidence for
Jeffrey pine (Pinus jeffreyi Grev. & Balf. in A. Murray), white fir, and sugar pine (Pinus lambertiana Dougl.) was 21,
88, and 2%, respectively. We found a number of relationships among host and pest variables, as well as a pathogen–
insect interaction, and across the SSPM we found that nonhost species may be interfering in certain host–pest interactions.
Spatial patterns from the 25-ha grid survey revealed that both P. pauciflorum and S. ventralis incidence were
widespread. Phoradendron pauciflorum showed no spatial structure across the 25 ha but S. ventralis showed some degree
of spatial structuring across the survey area. We also found that mistletoe severity was negatively correlated with
regeneration of white fir. In pristine forests, pathogens and insects influence mortality and regeneration success, affecting
stand structure and composition.
- Maloney, P. E., & Rizzo, D. M. (2002). Pathogens and insects in a pristine forest ecosystem: the Sierra San Pedro Martir, Baja, Mexico. Canadian Journal of Forest Research, 32, 448-457.