Date of Award

Fall 2018

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Theodore Howard

Second Advisor

Heidi Asbjornsen

Third Advisor

Mark Ducey

Abstract

In recent years, there has been a trend towards reforesting previously degraded areas by planting trees. While plantations offer an opportunity to restore ecosystem functions and diversity, most reforested plantations currently consist of monocultures, or single-species systems. Originally planted to provide wood for both global and regional markets, monocultures provide only a few goods and services and reduce plant biodiversity in comparison to multi-species systems, like forests. An alternative to the current plantation design is planting mixed species systems. Beyond diversifying a plantation, mixed species stands provide an opportunity to enhance ecosystem services that include non-timber forest products, carbon sequestration, and increased soil fertility. There is also evidence that mixed species stands tend to be more resilient to disturbances and climate change than monocultures.

Both anthropogenic and environmental stresses put pressure on tropical forests. Plantations, however, provide a means of decreasing anthropogenic pressures on forests by providing timber products, among other goods and services. Despite the increase in popularity of plantations, research still does not fully understand how certain plantation species might affect resource use (i.e., nutrient abundance and water quantity). While a growing body of research has begun to include studies on how resource use might change when species are planted in monocultures and mixtures, additional research is needed in areas where soils are severely degraded. In the seasonally dry tropics of Panama, how species in monocultures and simplified mixed species systems respond to changes in water availability is also of importance because survival of the dry season is central to the longevity of the species and the stand.

In Panama, the Agua Salud Project offers a unique opportunity to explore the dynamics of reforested areas that were previously degraded, with a specific emphasis is on understanding ecosystem services provided by forests and how these services change with land use change. Like most deforested areas, the Agua Salud Project plantations are planted on sub-marginal lands with poor soil. In 2008, native species plantations were established in two blocks which had previously been cleared 40 years before. The spatial arrangement of the species in the mixed species stands allows for isolation of interactions between species so we can test both interspecific interactions and how species strategies vary in monocultures and simplified mixed systems. Understanding the link between species diversity and water use dynamics is a crucial first step toward proper selection of species that balance the tradeoffs between growth and transpiration.

In addition to selecting species that regulate water well in this region, choosing species that are economically valuable in such a way that they can compete financially with the commonly planted non-native Tectona grandis (teak), is necessary to transition away from planting non-natives and towards planting native species. In addition to ecophysiological characteristics of these species when planted in different combinations, we provide information about how the native species will compete financially against teak and whether mixtures or monocultures are more lucrative operationally.

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