Tropical dry forests are critically endangered, as 97% of their area is threatened by human disturbances. The soils of these forests are highly degraded and tend to have low phosphorus available impacting plant performance. Therefore, the lack of precipitation and soil nutrient depletion make necessary the employment of active interventions including large-scale planting of native seedlings and management strategies such as phosphorus fertilizers and irrigation. To test the efficacy of these practices we established a large-scale experiment in south-western Colombia on abandoned pastures where we planted 5,691 seedlings of 11 native species. Our plantings were coupled with three treatments: phosphorus + water, water, and a control to determine: 1) if N-fixing seedlings have faster growth and higher survival rates compare to non-N-fixing seedlings, and 2) if N-fixing and non-N-fixing seedlings respond differently to watering and phosphorus addition. The 11 species planted are all able to associate with arbuscular mycorrhizae, but only for four of them can associate with N-fixing bacteria. We monitored the survival, growth, and resprouting rates of our 5,691 seedlings for two years. After 2 years of monitoring our plots, N-fixing seedlings had 87% survival rate across our three treatments, while non-N fixers had 67% survival rate. Growth rates were similar for both N-fixing and non-N-fixing seedlings. Seedlings that received extra water when planted had higher survival rates, however, after 2 years no differences among treatments were found. Finally, seedling’s growth rates were similar across the control and the two treatments, suggesting that phosphorus is not the only nutrient limiting plant growth. Our data show that overall N-fixing seedlings had higher survival rates, but growth rates of both N-fixers and non-N-fixers were similar. Water was key for early seedling establishment but fertilization with phosphorus did not increase seedlings growth more than watering after seedlings were established. These results suggest that planting a mix of both N-fixers and non-N-fixers is necessary to ensure the success of restoration projects and that extra management practices like phosphorus addition are not always critical for seedlings establishment and growth.
Tropical dry forest, N-fixing-seedlings, phosphorus, watering, growth, survival, restoration