Conservation value of disturbed and secondary forests for ferns and lycophytes along an elevational gradient in Mexico

Abstract

Questions: How do species richness and composition of fern assemblages change with elevation and, within elevational belts, in differently impacted forest habitats? Is there a relationship between fern assemblages and microclimate, both along gradients of elevation and disturbance? Which species are most sensitive to habitat disturbance and microclimatic changes?
Location: The transect starts at sea level close to the Gulf of Mexico and ends 81 km away in a direct line on the eastern slopes of the Cofre de Perote at 3500 m, in central Veracruz, Mexico.
Methods: We studied the richness and composition of fern assemblages in 120 study plots at eight elevations at 20-3500 m in three forest types: natural forest (NF), disturbed forest (DF) subjected to timber extraction and grazing, and secondary forest (SF) regrown after total clearance 15-20 years ago. In addition, we measured microclimatic conditions in the three forest types at five elevations over a year.
Results: Fern richness peaked in humid montane forests at mid-elevations and was low in the dryer habitats at the ends of the gradient. Humid montane forests were most sensitive to disturbance, showing increases in mean annual temperatures by about 1°C and reduction in relative air humidity by about 20% in DF and SF compared to NF. This went along with a reduction of fern species richness by 5-60% and marked changes in species composition. In contrast, drought-deciduous forests at low elevations and coniferous forests at high elevations already had low humidity and high light intensity in NF and were less affected by human impact: Their microclimatic conditions and fern assemblages did not change markedly in DF and SF.
Conclusions: The conservation of much of the humidity-dependent biota (ferns and presumably also groups such as bryophytes and amphibians) in humid montane forests depends on the protection of natural fragments without human disturbance. In contrast, the naturally open forests at the ends of the gradient can be subjected to some exploitation while conserving much of their fern flora as long as a general forest structure is maintained.