No abstract

No abstract

In this study we use a demographic approach to analyse the differing abundance of three congeneric columnar cacti: Neobuxbaumia macrocephala (the rarest), Neobuxbaumia tetetzo (intermediate), and Neobuxbaumia mezcalaensis (the most common). Populations of these species were studied in the Tehuacan Valley (Central Mexico) over a 3‐year period. We employed traditional models and life table response experiments (LTRE) to explore the association between particular demographic traits and the degree of rarity of each species. Most matrices showed population growth rate (λ) values close to unity; the only exception was N. mezcalaensis in 2001–2002 (λ=1.091±0.088). In the three species the highest elasticity values corresponded to entries referring to the stasis of pre‐reproductive plant and small adults. However, the LTRE indicated that most important differences in the λ values between years and species were associated with variation in fruit production and seedling recruitment. N. mezcalaensis yielded the highest mean λ due to its high fecundity in 2001–2002. This result suggests that the variation in demographic behaviour observed between species is sufficient to explain the higher abundance of N. mezcalaensis compared to the rare N. macrocephala. The high fecundity values and recruitment success observed in the former, even if they occur only seldom, may result in higher average λ values accounting for its high population densities.

Forest conversion to agriculture and grassland has been widespread in south-eastern Mexico. The productivity, functioning and carbon dynamics of secondary forests growing after abandonment of agricultural fields are expected to differ from those of primary forests. This study analysed whether forest age and seasonal variations affect the amount and temporal distribution of litterfall and associated nutrient transfer. Litterfall was measured across a chronosequence of semi-evergreen tropical forest in Calakmul, Yucatan peninsula, Mexico, and an index was created to evaluate the effect of land use intensity on litterfall collected in 16 stands from October 2012 to September 2014. Total litterfall ranged from 5.2 ± 0.6 to 7.1 ± 0.3 Mg ha -1 year -1 and peaked in secondary forest aged 10-20 years. Leaves contributed 84-91 % of total litterfall. The associated transfer of carbon ranged from 2.3 ± 0.3 to 3.2 ± 0.1 Mg ha -1 year -1 and of nitrogen from 62 ± 7 to 84 ± 4 kg ha -1 year -1 . Carbon and nutrient accumulation in the organic horizon (Oa) increased significantly with forest age. However, carbon in mineral soil (down to 0.30 m depth) did not increase over time. Peaks in monthly litterfall coincided with the dry season, with higher peaks in a year with lower rainfall in the dry season. Peaks were also higher in secondary forests than in primary forests, due to changes in species composition. Higher land use intensity reduced carbon and nutrient transfer through litter in regenerating secondary forests. Longer-term research is required to analyse the climate sensitivity of litter dynamics in these tropical forest frontiers.

No abstract