Elevation and shade interact to affect survival and growth of native tree seedlings in an Andean forest restoration field experiment

Andean tropical montane forests are increasingly threatened by human activities and global environmental change. Restoration efforts are limited by insufficient understanding of the factors that influence native seedling establishment during the early stages of restoring degraded exotic pasturelands. Although elevation, light incidence and competition with exotic grasses have been pointed out as barriers to forest recovery in tropical montane regions, the performance of native tree species in grasslands—and their response to variation in these environmental factors—varies greatly. Here, we conducted a field experiment to evaluate the effects of shade cover and mowing on the initial survival and growth of native tree species with contrasting elevational distributions across the Andean region. We planted seedlings of five species in three degraded pasture sites dominated by exotic grasses along a 900-m elevational gradient, using a full-factorial design with shade cover and mowing treatments replicated in eight blocks per site. Over 17 months, we observed marked variation in survival and growth rates among species along the elevational gradient. The effect of elevation on seedling survival was closely linked to species' elevational preferences: species with low-elevation affinities experienced significant declines in survival at higher elevations, whereas high-elevation species performed better at higher sites. Increasing elevation also had a consistent negative effect on growth rates across all species. These elevational patterns were driven by underlying differences in temperature and solar radiation, two key factors shaping seedling performance. Furthermore, survival depended on the interaction between shade cover and elevation, with all species exhibiting higher survival under shade at the highest elevations. In contrast, responses to mowing and any interactions with elevation and shade were species-specific and transient. Synthesis and applications. Our experimental study showed that restoration efforts should consider species-specific elevational preferences, with shade substantially enhancing seedling survival at high-elevation sites. In contrast, the widely adopted practice of mowing to control non-native grass species in restoring Andean forests may be beneficial only to certain species. Given the generally low survival and slow growth rates observed at the highest elevations, conservation efforts should prioritize adaptive protection of remnant forests in these areas to complement restoration strategies.