Factors limiting regeneration of an endangered conifer in the highlands of Guatemala

The relative importance of seed and microsite limitation during regeneration has become a crucial issue for conservation of rare plant species. Abies guatemalensis Rehder (Pinaceae) is an endemic conifer in the upper montane forests of Mexico and northern Central America. The species suffers from unsustainable usage but little is known about its natural regeneration including any limiting factors. To improve conservation of the remnant populations, natural regeneration was evaluated, and seedling emergence and establishment were examined in a factorial litter removal, sowing and transplant experiment. Forty 100 m² plots were randomly placed in six forest stands in the highlands of Guatemala, and regeneration was studied over seven months during the rainy season. Natural regeneration was patchy and sparse (320 recruits ha^-1); it was positively correlated with altitude and soil pH, but negatively with upper tree canopy cover and leaf area index (LAI). In the sowing experiment no emergence was observed in control subplots, whereas 0.7% of the sown seeds produced seedlings. Litter removal had no influence on seedling emergence but a positive effect on transplant survival. Seedling emergence was positively correlated with cover of the upper tree canopy, LAI and altitude. Soil pH, base concentrations, total nitrogen and organic matter were positively correlated with transplant survival. These findings indicate that regeneration of A. guatemalensis is both seed- and microsite-limited, with some variation among the regeneration stages. Conservation management could increase regeneration by selective canopy thinning, and litter removal might increase seedling survival. The populations at lower altitude are particularly endangered due to reduced natural regeneration.