Pathogens and insects in a pristine forest ecosystem: the Sierra San Pedro Martir, Baja, Mexico1
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چکیده
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 ≥50 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. Résumé : Nous avons déterminé l’incidence des organismes pathogènes et des insectes dans des peuplements mixtes de conifères de la Sierra San Pedro Martir située dans le nord de la région de Baja, au Mexique, pour évaluer le rôle des ravageurs dans un écosystème forestier vierge. Nous avons aussi déterminé la distribution spatiale des deux ravageurs les plus communs, le gui (Phoradendron pauciflorum Torrey) et le scolyte du sapin (Scolytus ventralis LeConte), chez le sapin concolore (Abies concolor (Gord. & Glend.) Lindl.) dans une zone quadrillée de 25 ha pour évaluer leur dispersion ainsi que les relations entre les variables qui caractérisent les hôtes et les ravageurs. Dans ces peuplements ouverts semblables à des clairières, la densité moyenne atteignait 160 arbres/ha, dont 58% étaient des arbres de plus de 20 cm au diamètre à hauteur de poitrine (dhp). Dans ces peuplements inéquiennes de faible densité, nous avons constaté que les diverses espèces de conifères étaient bien représentées sans qu’aucune ne soit complètement dominante. Le pourcentage cumulatif de mortalité dans la Sierra San Pedro Martir était de 12,7%, variant de 2 à 24%, et la plus forte mortalité survenait dans les classes de plus forts diamètres, les arbres avec un dhp ≥50 cm. L’analyse de régression linéaire multiple montre que 78% de la mortalité observée est expliquée par les organismes pathogènes et les scolytes (r2 = 0,78, P = 0,0001, F = 84). L’incidence moyenne des ravageurs était respectivement de 21, 88 et 2% pour le pin de Jeffrey (Pinus jeffreyi Grev. & Balf. in A. Murray), le sapin concolore et le pin à sucre (Pinus lambertiana Dougl.). Nous avons trouvé plusieurs relations entre les variables qui caractérisent les hôtes et les ravageurs de même qu’une interaction entre organismes pathogènes et insectes. De plus, nous avons trouvé que, dans l’ensemble de la Sierra San Pedro Martir, les espèces non-hôtes peuvent interférer avec certaines interactions hôtes–ravageurs. Les patrons spatiaux obtenus à partir de la zone quadrillée de 25 ha révèlent que l’incidence de P. pauciflorum et de S. ventralis est généralisée. Phoradendron pauciflorum n’a démontré aucune structure spatiale dans la zone quadrillée de 25 ha, mais S. ventralis a un certain degré de structuration spatiale dans la zone inventoriée. Nous avons également trouvé que la sévérité des dommages causés par le gui est négativement corrélée avec la régénération du sapin. Can. J. For. Res. 32: 448–457 (2002) DOI: 10.1139/X01-219 © 2002 NRC Canada 448 Received 5 April 2001. Accepted 30 November 2001. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 1 March 2002. P.E. Maloney2 and D.M. Rizzo. Department of Plant Pathology, University of California, Davis, CA 95616-0655, U.S.A. 1Although the information in this document has been funded in part by the U.S. Environmental Protection Agency, it may not necessarily reflect the views of the Agency and no official endorsement should be inferred. 2Corresponding author (e-mail: [email protected]). I:\cjfr\cjfr32\cjfr-03\X01-219.vp Tuesday, February 26, 2002 9:30:13 AM Color profile: Generic CMYK printer profile Composite Default screen
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تاریخ انتشار 2002