Planted pioneer trees function as nurses after a decade in the mountains of Central Argentina

Abstract

Early enrichment of non-pioneer tree species in forest restoration sites is challenging, especially where nurse trees and soil nutrients have been lost promoting sapling dieback and death from sun, wind, and frost exposure. We hypothesized that pioneer tree stand age, microsite and fertilization would enhance performance of non-pioneer species by reducing dieback, with fertilization being most effective when planting at open microsites or with younger nurse trees. Our study was performed in the first montane forest restoration in Central Argentina, where pioneer trees were successfully planted but Maytenus boaria, a late-successional species, struggled with dieback during dry/cold seasons. We experimentally planted M. boaria saplings in a three factor design: Pioneer tree stand age (2, 6, and 11 years), microsite (closed and open), and NPK fertilization (with or without). We monitored M. boaria for 15 years. Survival was almost double at closed (69%) compared to open microsites (37%), and marginally higher at the 11-year-old stand. Final height was over double at the 11-year-old stand compared to the 2- and 6-year-old stand. Fertilization increased height only in the 11-year-old stand, both in closed and open microsites. Contrary to our expectations, fertilization and proximity to trees slightly increased dieback. We conclude pioneer trees act as nurses for M. boaria but not due to protection from dieback causing agents. When natural regeneration does not occur, a good practice would be to plant non-pioneers a decade after planting the pioneer trees, at closed microsites and applying fertilization.