Résumé
1. The degradation of the Andean cloud forest raises strong biological
conservation issues and threatens the sustainability of a crucial water
resource. The idea that nurse-based restoration can accelerate the
recovery of these forests is underexplored, despite its promise as a
restoration technique. Recent conceptual models predict that facilitation
among plants may be an important mechanism, but there is a lack of strong
empirical support. We gathered experimental data to test this prediction
and explore the relevance of using nurse-based forest restoration in these
environments. 2. A 20-month factorial experimental design in the Andean
tropical cloud forest was established. We measured the survival and
estimated the biomass production of transplanted seedlings of a keystone
canopy forest species, Ceroxylon echinulatum (Arecaceae), in a deforested
area in the presence/absence of herbivory, a potential nurse plant (the
tussock grass Setaria sphacelata, Poaceae), and artificial shade. 3. The
joint effects of deforestation and herbivory led to the death of all
seedlings, whereas most seedlings survived in the adjacent forest, which
was used as the control. The presence of nurse plants led to significantly
higher survival and growth of Ceroxylon seedlings throughout the
experiment, regardless of herbivore presence. 4. The nurse effects were
explained by a reduction of the relative abiotic stress experienced by the
seedlings outside the forest, i.e. the consistently decreasing maximum
vapour pressure deficit. Furthermore, nurse tussocks delayed and reduced
the effects of herbivory by offering physical protection and a refuge for
seedlings against detection by herbivores. However, the effects of
herbivory and abiotic stress on facilitation were not additive. 5.
Synthesis and applications: Facilitation in degraded cloud forest can be
intense as soon as the beneficiary plants are driven away from their
physiological optimum (relative abiotic stress) and/or are confronted by
herbivory. Using pre-established exotic tussock grasses as a nurse-based
restoration technique in degraded cloud forest is a low-cost,
non-detrimental (to biodiversity) option, especially in the absence of
nurse trees and shrubs. The success of this method requires transplanting
seedlings at the base of tussocks.