Effects of introduced ungulates on forest understory communities in northern Patagonia are modified by timing and severity of stand mortality

Abstract

Natural disturbances such as fires, windstorms, floods, and herbivory often act on plant communities, affecting their structure and the abundance and composition of their species. Most research has focused on the effects of single disturbances on plant communities whereas the synergistic effects of several disturbances have received less attention. In this study, we evaluated how timing and severity of tree mortality modified plant use by introduced deer and early post-mortality successional trajectories in northern Patagonian conifer forests. We sampled understory composition and deer use in Austrocedrus chilensis (ciprés de la cordillera) forest stands undergoing varying timing and severity of forest mortality as reconstructed using dendroecological techniques. In addition, we evaluated the effect of fallen logs on plant composition and deer use of plants by monitoring areas of massive dieback where fallen logs had been removed for fire hazard reduction, and nearby control areas not subjected to such removal. Stepwise regression analyses showed that history and severity of tree mortality strongly influence plant composition and deer use of plants. For deer use (with pellet counts and browsing index as response variables), results showed a positive relationship with degree of stand mortality and a negative relationship with cover of fallen logs. Similarly, cover of unpalatable shrub species was explained by canopy mortality history, whereas cover of palatable shrub species was positively associated with severity of canopy mortality. In areas where fallen logs had been removed, pellet counts were six times higher than those in control areas. Though total shrub species cover was similar between log removal and control areas, proportion of unpalatable shrubs increased in areas where fallen logs had been removed. In conclusion, deer use of plants was strongly limited by tall fallen logs, allowing palatable species to establish and grow. Fallen log removal accelerated deer entrance and changed understory composition toward more browse-resistant and unpalatable species. These results underscore the importance of considering the dynamics (timing, severity, and extent) of fallen woody debris influencing understory herbivory and post-disturbance succession. In addition, experimental results underpin the importance of maintaining snags and large woody debris in disturbed landscapes where salvage logging is a routine procedure.

Modelling Phytophthora disease risk in Austrocedrus chilensis forests of Patagonia

“…Thinning may increase the danger of wind throw (Relva et al 2008) and may ensure the introduction of root rot, as it occurs with Chamaecyparis lawsoniana forests affected by P. lateralis (Zobel et al 1985). Previous to the identification of the biotic causal agent of A. chilensis disease, salvage cuttings were carried out, even though this treatment did not achieve disease control (Loguercio et al 1999).…”

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).…”

“…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.…”

Introduced ungulate herbivore alters soil processes after fire

“…Similarly, the introduction of beaver (Castor canadensis) in Chile has resulted in great shifts in riparian plant communities (Wallem et al this issue), reduces macroinvertebrate richness and diversity by 50% but results in a 3-5-fold increase in secondary production in streams relative to areas that have not been impacted by beaver (Anderson and Rosemond 2007). Such results suggest that much more study and experimentation is required across species and ecosystems to determine both the extent and mechanisms of the effects of introduced herbivores on ecosystem processes as well as how introduced herbivores interact with other disturbance regimes (sensu Relva et al 2008).…”