AGENTES ASOCIADOS Y SU PAPEL EN LA DECLINACIÓN Y MUERTE DE ENCINOS (QUERCUS, FAGACEAE) EN EL CENTRO-OESTE DE MÉXICO

Abstract

This study was carried out to identify biotic
and abiotic agents and to determine their
role in oak decline and death in five states
of the Mexican Republic: Aguascalientes,
Colima, Guanajuato, Jalisco and Nayarit.
Two 0.1-ha permanent plots were
established at the most representative
forested areas with tree death, dieback
and bleeding cankers in these states
and sampled during 2003, 2004 and 2005
(10 plots in total). Field observations were
made, and foliage, branch, trunk and soil
samples were collected to identify the
associated agents in each plot. Isolates from
bleeding cankers were made in situ. The
most commonly found biotic agents were:
Phytophthora cinnamomi, Pythium sp.,
Hypoxylon antropunctatum, Ganoderma
spp., Armillaria sp., Apiognomonia
quercina, the dwarf mistletoe
(Phoradendron villosum), bark borers,
insect defoliators and livestock overgrazing.
Among the abiotic stress factors commonly
found were the effects due to low
temperature-lack of water and fire. Based
on their incidence, P. cinnamomi, H.
antropunctatum and low temperature-lack
of water were considered the most important
contributors to the decline and death of the
oaks. P. cinnamomi was identified in four of
the 10 plots, mainly in Colima and Jalisco;
H. antropunctatum was detected in six
plots, particularly in Nayarit and
Aguascalientes. Stem cankers (cracks) due
to low temperature-lack of water were
observed in four plots, damaging 52% of
the trees in Aguascalientes and 24% in
Guanajuato. It was concluded that P.
cinnamomi is killing some oak species, H.
antropunctatum is colonizing weakened
trees, and low temperature-lack of water is
causing stress and in some cases death in
oaks.