Summary of Proposed Activities with NSF-ROA Supplementary Funding Request 2005:

In collaboration with Dr. Jose-Luis Machado at Swarthmore College, I propose for research activities that expand the scope of my ongoing project “Functional Bases for the Trade-Off
Between Growth and Survival of Tree Seedlings” (Award Number 0093303 PI: Kaoru Kitajima). The overarching goal of this on-going project is to elucidate the functional basis for species differences in shade tolerance of tree seedlings, by evaluating the role of physical defenses and carbohydrate storage on seedling survival and growth (allocation-based trade-off hypothesis). The proposed activities (sub-project title, “Maintenance of Positive Carbon Budget in Understory Seedlings with Long Leaf Life Span”) will take advantage of the on-going experiments in Panama, Central America, and will contribute to the overall goal of the project by evaluating the light-use efficiency hypothesis as an alternative to the allocation-based trade-off hypothesis. More than 100 woody species encompassing a wide range of seedling shade tolerance are being compared in shade house and common garden experiments to quantify survival, growth rates, biomass allocation, biomechanical traits, and carbohydrate storage. In addition, leaf production and survival are monitored with monthly censuses, because leaf life span is a key functional trait that correlates well with suites of life history and physiological traits, and because cost of leaf-turn over is hypothesized to be one reason why some species fail to maintain positive net carbon balance in the shaded understory. In the on-going experiments, shade tolerant seedlings routinely maintain long-lived leaves (> 3 yrs). These seedlings with marked leaves of known age present a unique opportunity to study the effect of leaf aging and self-shading on carbon balance in tree seedlings successfully surviving near the photosynthetic light compensation point. Preliminary data collected in July 2004 demonstrate that shaded seedlings exhibit significant monotonic decline of photosynthetic capacity, despite apparent lack of self-shading.

We plan to assess significance of leaf-age effects on carbon balance of shaded seedlings by 1) in-situ measurements of photosynthetic light response curves of individual leaves for six-ten species, 2) simulation of light utilization, self-shading and whole-shoot carbon balance with a three dimensional architecture program, YPLANT, and 3) measurements of whole-seedling carbon balance (both above and below ground) with a whole-plant gas exchange chamber using potted plants transferred from nursery to the understory. YPLANT analysis will allow quantitative evaluation of how photosynthetic decline with leaf age affects carbon balance at the whole-shoot level. The whole-plant gas exchange is the only way to quantify respiration by both photosynthetic and non-photosynthetic organs, and to describe ontogenetic change of balance between photosynthesis and respiration. These additional research activities will significantly expand the original scope of the project, allowing more direct tests of the carbon-balance hypothesis as the basis of shade tolerance in tree seedlings.

A broader impact is to provide research and training opportunities to Machado and two undergraduate students from Swarthmore College, an undergraduate institution. All field research activities will take place in Panama (6 weeks in June-July 2005 by Machado and Kitajima with students, 2 weeks in December 2005 for follow-up data collection by the two PIs), while some data analysis (e.g., computer simulation with YPLANT) will continue during the fall semester at Swarthmore College to provide further research opportunities to undergraduate students.

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