Indicadores del decaimiento en bosques de Nothofagus pumilio en el norte de la Patagonia, Argentina

Abstract

El decaimiento forestal está frecuentemente asociado a la mortalidad parcial o total de las copas en un alto porcentaje de individuos de un rodal. Si bien el decaimiento ha sido documentado a escala global, las relaciones entre las condiciones externas de los árboles y su crecimiento radial raramente han sido exploradas. El presente trabajo relaciona la intensidad del decaimiento con el crecimiento radial en 294 árboles de Nothofagus pumilio en el norte de la Patagonia. Los indicadores externos del decaimiento analizados fueron la mortalidad de la copa, la sanidad de la corteza, la incidencia de insectos barrenadores y pájaros carpinteros, así como la presencia de plantas hemiparásitas, hongos y líquenes. Los resultados indican que elevados porcentajes de mortalidad de copa están significativamente relacionados con la disminución del crecimiento radial promedio, siendo esta relación más notoria cuando se usa como medida del crecimiento radial el incremento en área basal en lugar del ancho de los anillos. El deterioro de la corteza y la abundancia de cavidades producto de las actividades de insectos barrenadores y de pájaros carpinteros también estuvieron significativa e inversamente relacionados con el crecimiento. Por el contrario, el crecimiento y la presencia de hemiparásitas, hongos o líquenes no mostraron relaciones significativas. A partir de estos resultados, se propone utilizar en la estimación de la sanidad forestal los indicadores externos (1) mortalidad de copa, (2) deterioro de la corteza y (3) cavidades de insectos y/o pájaros carpinteros, como una medida integral del decaimiento forestal de Nothofagus pumilio en la Patagonia.

Radial Growth Patterns Associated with Tree Mortality in Nothofagus pumilio Forest

“…In northern Patagonia, N. pumilio forests are exposed to insect outbreak events related to dry and warm springs [89]. In previous studies we also observed that declining trees tended to show signs of damage from bark beetles and woodpeckers, but other biotic agents present in the stem or crown such as hemiparasitic plants, fungi, and lichen did not differ between declining and healthy individuals [90]. The presence of woodpecker cavities in declining vs. healthy N. pumilio trees has also been reported in other study also relating tree dieback and growth decline to regional droughts [91], but the presence of birds may be also related to the availability of wood-boring larvae [92].…”

“…This lack of evidence could be related to the relatively low representation of broadleaf species in the literature [4,6,64,94,95]. Likewise, studies with South American deciduous species are comparatively scarce [33,[37][38][39]. To the best of our knowledge, this paper provides the first evidence for longstanding negative trends prior to death, as well as low levels of growth six years prior to death, for a deciduous subalpine angiosperm species in the Southern Hemisphere.…”

“…Extreme droughts and decreasing precipitation in northern Patagonia have induced extensive forest dieback concurrent with negative trends in radial growth of Austrocedrus chilensis, Nothofagus dombeyii and Nothofagus pumilio [33][34][35][36][37]. In previous work [38], we recorded differences in tree growth trends among N. pumilio individuals with varying degrees of crown dieback in low-elevation forests. The trees with most crown dieback showed the steepest long-term negative trends in radial growth triggered by extreme drought events in the eastern-dry edge of the N. pumilio distribution [38].…”

“…The trees with most crown dieback showed the steepest long-term negative trends in radial growth triggered by extreme drought events in the eastern-dry edge of the N. pumilio distribution [38]. The percentage of trees showing sustained declining BAI lasting two to six decades was around 44%, ranging from 22% to 88% [38]. In these forest stands, visited during the period 1998-2012, we recorded that, on average, 17% of standing trees were dead (ranging from 5% to 33%, DBH > 10 cm) [39].…”

“…In previous work [38], we recorded differences in tree growth trends among N. pumilio individuals with varying degrees of crown dieback in low-elevation forests. The trees with most crown dieback showed the steepest long-term negative trends in radial growth triggered by extreme drought events in the eastern-dry edge of the N. pumilio distribution [38]. The percentage of trees showing sustained declining BAI lasting two to six decades was around 44%, ranging from 22% to 88% [38].…”