Variable retention harvesting and climate variations influence over natural regeneration dynamics in Nothofagus pumilio forests of Southern Patagonia

Abstract

Natural regeneration is a key process to sustain structure, function, and composition in harvested forests over time. Variable retention harvesting system is a strategy that maintains a high level of heterogeneity in managed stands for multiple ecosystem services, which provides different micro-environmental conditions for natural regeneration that respond differently according to climate variations. The objective was to determine the influence of remnant over story on the dynamics of Nothofagus pumilio natural regeneration in forests managed under variable retention in the medium term (0 to 16 years-after-harvesting, YAH) and its interaction with climate variables. Recruitment period, survival, seedling bank of pre- and post-harvesting periods, regeneration age and heights were determined at permanent plots under three different harvested forest conditions (1: AR = aggregated retention, 2: DRI = dispersed retention under the influence of AR, 3: DR = dispersed retention without influence of AR) and one primary forests (PF) as a control (4 treatments × 6 replicates × 3 areas × 16 YAH). Height growth rates were associated to combined occurrence of hemispheric modes (ENSO and SAM) that influence over climate variations. Results showed different regeneration dynamics according to remnant forest over story: (i) a recruitment stage occurred during the first 4 YAH; (ii) 67,000 and 33,000 individuals ha- 1 for DRI and DR established respectively, with 15,000 individuals ha- 1 higher than 130 cm height at 16 YAH; (iii) equal density proportion of pre- and post-harvesting saplings between treatments (DRI and DR); (iv) significant differences in regeneration heights among treatments from 3 to 16 YAH, which were higher at DRI and DR, reaching to 130 cm height at 16 ± 1 years of age. Saplings height growth reacted favourably to higher mean temperatures associated with positive SAM and ENSO. These results reflect a successful and resilient natural regeneration process for N. pumilio where variable retention system to obtain timber products is below the thresholds of no-return, reacting differently according to climatic drivers by the presence of different structures of remnant over story. This provides useful information for forest management practices in the medium term, where the maintenance of stand heterogeneity (e.g., different types of over story retention) ensures successful and resilient regeneration to recover the stand original structure within a context of climate change.