Collaborative research: Linking leaf and root traits to ecosystem structure and function in a common garden study of 14 temperate tree species

P. Reich1, D. Eissenstat3, J. Oleksyn1 2, S. Hobbie1, J. Chorover4, M. Tjoelker5, O. Chadwick6

1Univ. Minnesota, 2Polish Academy of Sciences, 3Penn State, 4University of Arizona, 5Texas A&M, 6UC Santa Barbara

June 1, 2002 - May 31, 2006 NSF DEB-0128944 Ecosystems Program

We propose to test a series of hypotheses about the mechanisms whereby woody plant species alter ecosystem processes and properties. Plant traits have long been considered key controls of ecosystem functioning and can influence nutrient cycling, decomposition, productivity, and pedogenesis. However, most of our current understanding of plant effects on ecosystem structure and function comes from comparisons among species or communities where separating effects of climate, soils, or stand age from those of vegetation is difficult. Furthermore, these comparisons have focused primarily on the aboveground component of the ecosystem. To understand the connections between aboveground and belowground processes and the role of the species in controlling ecosystem structure and function requires common garden experiments. Yet common garden experiments with long-lived species are rare, and those that do exist contain too few species to allow robust generalizations. We propose a study of a unique, replicated common-garden experiment of 33-year old monoculture stands of 14 temperate tree species in Poland. Preliminary data show that in just 30 years plants have caused major changes in soil properties such as pH, and forest floor and mineral soil calcium (Ca), nitrogen (N) and carbon (C). The large number of species, the stand age, the replicated experimental design, as well as established collaborations with Polish scientists, make this site uniquely suited to testing important hypotheses related to interspecific variation in leaf and root traits and how these traits influence ecosystem structure and function. We propose to test several interwoven hypotheses about the links between tissue and ecosystem level properties and processes: (1) Plant species are characterized by predictable "syndromes" of interconnected leaf and fine root traits such that species with low metabolic rates (e.g., low photosynthesis, respiration, and nutrient uptake) have dense structure (e.g., low specific leaf area and low specific root length), low nutrient concentrations, and low turnover rates, and vice versa. (2) Species differences in tissue-level traits translate into large differences among species in ecosystem structure (e.g., canopy and root system mass, forest floor accumulation). For example, species with dense, long-lived, low-nutrient fine roots and foliage will have large aboveground and belowground biomass (i.e., large canopies and root systems) and large forest floor organic matter accumulation. In contrast, species with thin, short-lived, high-nutrient fine roots and foliage will have low canopy and root system biomass and accumulate little forest floor mass. (3) Species with small canopies and root systems promote rapid rates of ecosystem processes (production, decomposition and N mineralization) per unit mass of tissue, while species with large canopies and root systems have low rates of ecosystem processes per unit mass of tissue. Therefore, despite large species effects on ecosystem structure, species will converge in their effects on ecosystem processes when those processes are measured on a per unit area basis. (4) Plant species impose long-term constraints on biogeochemical cycling and pedogenesis by their direct impact on litter chemistry and decomposition, soil Ca and Al pools, and soil pH. High pH promotes microbial biomass and activity, stimulates decomposition of litter and fast-cycling soil organic matter (SOM), but stabilizes slow-cycling SOM. Low pH increases the rate of SOM leaching and podzolization, and shifts the plant-available pool of lithogenic cations from Ca to Al. To test these hypotheses, we will examine: 1) above- and belowground tissue and ecosystem physiology, structure and productivity; 2) litter and soil C and nutrient cycling; and 3) soil chemistry and pedogenesis. Our planned measures in 33-year-old tree species monocultures over a multi-year period will provide a rare opportunity to link individual species traits (above- and belowground) and ecosystem processes such as decomposition, productivity, nutrient cycling and soil pedogenesis in a controlled, replicated experiment with a large number of long-lived species.