New study shows that Guam鈥檚 endangered cycad and legume trees deliver ecosystem services in biogeochemistry

Technician Rosalita Ca帽ete holds leaf litter samples from the Mariana Islands legume tree known as Serianthes nelsonii tree (left) and the Micronesian Islands cycad tree known as Cycas micronesica (right). The leaf litters from these two endangered trees exert profound influences on spatial differences in biogeochemical cycling patterns.

 

The study combined two experimental approaches to show that the slowly decomposing leaf litter from Cycas micronesica can modify decomposition patterns of the litter from co-occurring trees by decreasing the speed with which they decompose. This tree is a member of the plant group known as cycads. In contrast, the rapidly decomposing litter from Serianthes nelsonii can modify decomposition patterns of the same mixed trees by synergizing the speed with which they decompose. This tree is a member of the legume plant family. Both trees are endangered with looming extinction risks due to a confluence of human-induced threats.

鈥淚sland forest restoration initiatives and endangered tree species conservation efforts depend on local evidence to inform policy and management decisions,鈥� said author Thomas Marler, a retired professor of the university鈥檚 College of Natural and Applied Sciences. 鈥淥ne form of evidence that benefits policy-makers involves new knowledge about contributions of individual threatened species to ecosystem-level health.鈥�

The newly published information sheds light on the risks of losing these endangered tree species. Not only do these two species substantially modify the biogeochemical cycling patterns of plant litter decomposition in Guam鈥檚 forest communities, they do so in opposite directions.

鈥淲e already knew that Guam鈥檚 cycad leaf litter exhibits chemical traits that predict sluggish decomposition and that the legume tree鈥檚 leaf litter exhibits chemical traits that predict swift decomposition,鈥� Marler said. 鈥淲e also knew that these chemical traits accurately predicted the decomposition dynamics when the leaf litter of each species is decomposed without being mixed with other plant litters.鈥�

What was not known until now is that adding the litter from these two endangered tree species to a mixture of leaf litter types modifies the decomposition dynamics of the other mixed litters.

The widespread coconut tree may provide an example. This native palm species is an important component of Guam鈥檚 forest communities. When the coconut tree鈥檚 litter accumulates on the forest floor without mixing with other tree litters, a baseline speed of decomposition occurs, which releases nutrients to the soil and carbon dioxide to the atmosphere. When the same coconut tree鈥檚 litter is mixed with Cycas micronesica litter, the nutrients within the coconut leaf are released more slowly and the net result is an increase in the amount of humus and organic matter that accumulates on the forest floor. Yet when the same coconut tree鈥檚 litter is mixed with Serianthes nelsonii litter, the nutrients within the coconut leaf are released more rapidly to increase the available nutrition of the soil for supporting growth of other plants. Within a small footprint of the forest floor, micro-niche habitats are, therefore, created by these litter mixtures and the response is an increase in biodiversity in the food chain.

Micronesian island conservation efforts have been hindered by the scarcity of empirical studies that generate local findings. Yet sustainable programs aimed at restoring degraded habitats and conserving endangered species cannot proceed in a meaningful way in the absence of locally produced new knowledge. This new publication describes the manner in which these two trees contribute to spatial heterogeneity of litters on the forest floor to create spatial patches of more diverse food resources for soil organisms and more diverse habitat for forest-dwelling animals. The university鈥檚 College of Natural and Applied Sciences continues to provide scientific guidance for global restoration efforts by contributing ecological case studies within the under-studied Micronesian region.