Can a natural experiment be used as a tool to design partial cutting regimes? The decline of Austrocedrus chilensis forests, an example

Abstract

The forests of Austrocedrus chilensis in Argentina suffer mortality from what is locally known as 'mal del cipre´s'. This disturbance usually appears as aggregations of dead trees of varying sizes. The mortality process seems unpredictable, and regeneration establishment is variable under these conditions. The highly variable mortality rates results in a natural experiment that allows the correlation of overstory mortality with regeneration rate. This correlation should then make it possible to estimate the regeneration success which would follow after different levels of partial cutting, allowing the purposeful direction of the regeneration process. We collected data from eight Austrocedrus chilensis-dominated stands and reconstructed overstory mortality over time and correlated it to the regeneration recruitment. Mortality showed two patterns: increasing 'continuous' mortality over time, and 'pulses' of important basal area loss in short periods. Overstory mortality had an effect on the recruitment and composition of the regeneration by allowing different species establishment depending on the degree of mortality. By quantifying mortality basal area and recruitment over time, we calculated estimates that can provide partial cutting guidance. Partial cuttings would then allow the purposeful direction of the regeneration process. Stand reconstruction studies provided a useful tool to help designing regeneration systems using a natural experiment. Some specific management implications for symptomatic A. chilensis forests are given.

Stand development patterns as a consequence of the mortality in Austrocedrus chilensis forests

“…While previous studies have examined mortality patterns in A. chilensis forests affected by 'mal del ciprés' (Calí, 1996;Loguercio et al, 1998;Relva et al, 2009;Amoroso and Larson, 2010), this study quantifies and describes long-term patterns of overstory mortality at a larger number of stands over several decades using highquality ring-width data. Mortality in A. chilensis forests appears to be unpredictable and variable in time.…”

“…The presence of this disturbance suggests that these forests could become uneven-aged, but previous studies have shown that the establishment of new individuals in forests under these conditions is highly variable (Loguercio et al, 1998;La Manna et al, 2008b;Relva et al, 2009;Amoroso and Larson, 2010). Successful establishment of A. chilensis in healthy stands occurs in small openings in the canopy and this species is described as having a fine-scale gap-phase regeneration mode (Veblen et al, 1995;Gobbi and Schlichter, 1998;Amoroso and Larson, 2010).…”

“…In some stands, the transition from stem exclusion to understory reinitiation may be accelerated by windthrow and the creation of gaps, favouring the establishment of regeneration in the understory (Veblen et al, 1992). Large gaps would favour the more shade intolerant N. dombeyi while smaller gaps would favour A. chilensis (Veblen et al, 1995;Dezzoti, 1996;Gobbi and Schlichter, 1998;Suarez and Kitzberger, 2008;Amoroso and Larson, 2010). If dense stands on mesic sites are not subjected to such disturbances, they continue growing, and as the understory reinitiation stage is reached, stands enter a phase where the regeneration occurs in gaps of small size .…”

“…Successful establishment of A. chilensis in healthy stands occurs in small openings in the canopy and this species is described as having a fine-scale gap-phase regeneration mode (Veblen et al, 1995;Gobbi and Schlichter, 1998;Amoroso and Larson, 2010). Although canopy mortality in symptomatic A. chilensis forests appears unpredictable and variable (Loguercio et al, 1998;Relva et al, 2009;Amoroso and Larson, 2010), we predict that, in the absence of other disturbances (e.g. grazing or logging), tree deaths will provide opportunity for successful recruitment of A. chilensis in the understory of symptomatic forests.…”

Feasibility of silviculture for complex stand structures: designing stand structures for sustainability and multiple objectives

“…These approaches emulate natural disturbances in a boreal forest in Canada. Amoroso and Larson (2010) report that the dynamics of stand structure and regeneration in Austrocedrus chilensis stands suffers from cypress sickness (mal del cipre´s). Iijima and Shibuya (2010) clarify suitable regeneration sites for Picea jezoensis on fallen logs accompanied by natural disturbances in sub-boreal forests in Hokkaido, Japan.…”