Tales from the crypt: the curse of an old growth tree in a fragmented forest

Background/Question/Methods

Habitat loss and fragmentation has been recognized as a major source of decreasing biodiversity and species extinction. While reproductive success has been well documented for plant populations in fragmented landscapes, little is known about the effects on offspring performance, especially in terms of seed quality. Field studies describing effects of forest fragmentation in plant populations are rarely concerned with late successional tree species. Therefore, I investigated population dynamics of an old growth tree species, Poulsenia armata (Moraceae), in a Mexican fragmented forest. I hypothesized that P. armata viability and persistence is in jeopardy as a result of severed forest fragmentation at Los Tuxtlas forest. In the last 40 years, Los Tuxtlas forest has lost almost 84% of the original cover as a result of the conversion of forest to grassland for cattle ranching. The remaining continuous forest persists in a 640 ha biological station. A growth experiment was conducted to examine whether the high levels of fragmentation of los Tuxtlas resulted in lower seed quantity and quality of P. armata in the fragmented forest when compared to seeds from the continuous forest.

Results/Conclusions

Forest fragmentation resulted in decreased reproductive success and reduced seed germination and seedling performance. Seedlings from the continuous forest accumulated greater biomass, and allocated more resources to roots and stems, while seedlings from forest fragments allocated a higher percentage of biomass to leaves. A soil quality analysis revealed that trees from fragmented forest had fewer resources in the soil than trees in the continuous forest. With fewer resources available, reproductive trees experience a trade-off between quantity versus quality in fragmented populations. Indeed, seeds from the continuous forest had two times more nitrogen and carbon than seeds from forest fragments. Additionally, results suggest that while maternal investment seems to be affected by environmental conditions, seedling survival and growth appear to be severely impacted by maternal effects rather than soil conditions.

While forest fragmentation acted consistently in reducing P. armata reproductive success and seedling performance, population size remained unaffected during the three years of census.Habitat fragmentation is a relatively recent phenomenon in the area of Los Tuxtlas and the effects are not sufficiently strong to reduce the size of fragmented populations. However, it is only a matter of time until population reduction since offspring fitness seems to be already affected by forest fragmentation, which is a serious indication of reduced population viability.