Catherine Potvin

THE STATISTICAL ANALYSIS OF ECOPHYSIOLOGICAL RESPONSE CURVES OBTAINED FROM EXPERIMENTS INVOLVING REPEATED MEASURES

Physiological ecologists often analyze the responses of physiological or bio- chemical traits to environmental factors such as temperature, irradiance, water potential, or the concentrations of CO2, 0, and inorganic nutrients. The data for such a response curve typically are gathered by sequential sampling of the same plant or animal, and their analysis should explicitly allow for this repeated-measures design. Unfortunately, the sta- tistical analysis of response curves in ecology generally has either been ignored or incorrectly done. In an effort to encourage rigorous analysis of response data, we address statistical treatment of response curves and illustrate the correct alternatives that are available. Four different statistical methods for analyzing response curves are considered: analysis of vari- ance with repeated measures (ANOVAR), multivariate analysis of variance with repeated measures (MANOVAR), a nonparametric split-plot analysis (NP split-plot) and parametric comparison of models fitted to the data by nonlinear regression. Analyses of the CO2 dependence of photosynthesis in the C4 grass Echinochloa crus-galli following chilling are used as examples of these different methods. ANOVAR, potentially the most powerful analysis, makes stringent assumptions about the variance-covariance structure of the data. Within limits these assumptions can be relaxed and a corrected significance level used. When the variance-covariance structure badly violates the ANOVAR assumptions, MANOVAR or NP split-plot are viable alter- natives. In physiological ecology, however, the use of MANOVAR frequently is limited by small sample sizes and the tendency for the number of levels of the treatment factor to exceed the sample size. Greater attention to experimental design can avoid this problem. The NP split-plot is based on simple assumptions and could be widely used. The comparison of curves fitted by nonlinear regression is also distribution free and provides an interesting alternative when the responses are amenable to fitting. For any of these analyses to be viable the thoughtful choice of experimental protocols and design is essential.

BIODIVERSITY AND ECOSYSTEM FUNCTIONING: IMPORTANCE OF SPECIES EVENNESS IN AN OLD FIELD

Changes in land use, habitat fragmentation, nutrient enrichment, and environmental stress often lead to reduced plant diversity in ecosystems. However, it remains controversial whether these reductions in diversity will affect energy flow and nutrient cycling. Diversity has two components: species richness, or the number of plant species in a given area, and species evenness, or how well distributed abundance or biomass is among species within a community. We experimentally varied species evenness and the identity of the dominant plant species in an old field of Quebec to test whether plant productivity would increase with increasing levels of evenness, and whether relationships would be invariant with respect to species identity. Total and belowground biomass increased linearly with increasing levels of evenness after one growing season. These relationships did not depend on the identity of the dominant species. Relationships between aboveground biomass and evenness varied and depended on the identity of...

Biodiversity enhances individual performance but does not affect survivorship in tropical trees

We developed an analytical method that quantifies the relative contributions of mortality and individual growth to ecosystem function and analysed the results from the first biodiversity experiment conducted in a tropical tree plantation. In Sardinilla, central Panama, over 5000 tree seedlings were planted in monoculture and mixed-species plots. After 5 years of growth, mixed-species plots yielded, on average, 30-58% higher summed tree basal area than did monocultures. Simulation models revealed that the increased yield of mixed-species plots was due mostly to enhancement of individual tree growth. Although c. 1500 trees died during the experiment, mortality was highly species-specific and did not differ consistently between biodiversity treatments. Our results show that the effects of biodiversity on growth and mortality are uncoupled and that biodiversity affects total biomass and potentially self-thinning. The Sardinilla experiment suggests that mixed-species plantings may be a viable strategy for increasing timber yields and preserving biodiversity in tropical tree plantations.

Neighborhood effects and size-asymmetric competition in a tree plantation varying in diversity.

A plantation of native trees was established in Panama in 2001 to study the relationship between biodiversity and ecosystem functioning. Five years later, mixed-species plots had experienced enhanced tree growth compared with monocultures. Searching for underlying mechanisms, we developed a neighborhood model isolating size and identity effects. We found that the size of neighbors is, by far, the largest source of variation in individual-tree diameter and height. Size-asymmetric competition appears as a structuring factor in the plantation. The relative growth rate of small trees was significantly lower than that of large trees, and their height and basal diameter were most variable. The 50 smallest trees of the plantation suffered a disproportionate amount of death, and the proportion of small trees was highest in monoculture. Increased biomass allocation to branches for trees growing in three-species plots suggests that competition for light might be taking place. Clearly, local neighborhood plays a central role in determining productivity, suggesting that scale needs to be incorporated in the theoretical development and analysis of biodiversity and ecosystem functioning.

Can we predict carbon stocks in tropical ecosystems from tree diversity? Comparing species and functional diversity in a plantation and a natural forest

• Linking tree diversity to carbon storage can provide further motivation to conserve tropical forests and to design carbon-enriched plantations. Here, we examine the role of tree diversity and functional traits in determining carbon storage in a mixed-species plantation and in a natural tropical forest in Panama. • We used species richness, functional trait diversity, species dominance and functional trait dominance to predict tree carbon storage across these two forests. Then we compared the species ranking based on wood density, maximum diameter, maximum height, and leaf mass per area (LMA) between sites to reveal how these values changed between different forests. • Increased species richness, a higher proportion of nitrogen fixers and species with low LMA increased carbon storage in the mixed-species plantation, while a higher proportion of large trees and species with high LMA increased tree carbon storage in the natural forest. Furthermore, we found that tree species varied greatly in their absolute and relative values between study sites. • Different results in different forests mean that we cannot easily predict carbon storage capacity in natural forests using data from experimental plantations. Managers should be cautious when applying functional traits measured in natural populations in the design of carbon-enriched plantations.

LONG‐TERM CO2 ENRICHMENT OF A PASTURE COMMUNITY: SPECIES RICHNESS, DOMINANCE, AND SUCCESSION

The present study addresses responses of a pasture community to CO2 en- richment in situ. It focused on two levels of organization. We examined changes in both community properties and species-specific responses during long-term exposure to high CO2 concentration. The underlying hypothesis is that CO2 enrichment could change com- munity composition. At the community level, we observed higher species richness and lesser dominance under enriched than ambient CO2. Two species were apparently central in explaining our results, Agropyron repens and Plantago major. The cover of this first species increased only under ambient CO2. Conversely, the cover of the latter species decreased under ambient CO2 but remained stable under enriched CO2. Species were pooled into dicots and monocots to examine space acquisition. Changes in monocot cover through time were more tightly coupled with that of dicots under ambient than high CO2. Enrichment with CO2 appeared to have a positive effect on the early-successional species, preventing the complete dominance by late-successional species. In fact, under elevated CO2 early- and late-successional species were coexisting. Therefore, our results suggest the possibility that succession patterns might be altered by CO2 enrichment apparently because enriched CO2 stimulates the growth of dicots.

High-fidelity national carbon mapping for resource management and REDD+

BackgroundHigh fidelity carbon mapping has the potential to greatly advance national resource management and to encourage international action toward climate change mitigation. However, carbon inventories based on field plots alone cannot capture the heterogeneity of carbon stocks, and thus remote sensing-assisted approaches are critically important to carbon mapping at regional to global scales. We advanced a high-resolution, national-scale carbon mapping approach applied to the Republic of Panama – one of the first UN REDD + partner countries.ResultsIntegrating measurements of vegetation structure collected by airborne Light Detection and Ranging (LiDAR) with field inventory plots, we report LiDAR-estimated aboveground carbon stock errors of ~10% on any 1-ha land parcel across a wide range of ecological conditions. Critically, this shows that LiDAR provides a highly reliable replacement for inventory plots in areas lacking field data, both in humid tropical forests and among drier tropical vegetation types. We then scale up a systematically aligned LiDAR sampling of Panama using satellite data on topography, rainfall, and vegetation cover to model carbon stocks at 1-ha resolution with estimated average pixel-level uncertainty of 20.5 Mg C ha-1 nationwide.ConclusionsThe national carbon map revealed strong abiotic and human controls over Panamanian carbon stocks, and the new level of detail with estimated uncertainties for every individual hectare in the country sets Panama at the forefront in high-resolution ecosystem management. With this repeatable approach, carbon resource decision-making can be made on a geospatially explicit basis, enhancing human welfare and environmental protection.

Diagnosing the uncertainty and detectability of emission reductions for REDD+ under current capabilities: an example for Panama

In preparation for the deployment of a new mechanism that could address as much as one fifth of global greenhouse gas emissions by reducing emissions from deforestation and forest degradation (REDD+), important work on methodological issues is still needed to secure the capacity to produce measurable, reportable, and verifiable emissions reductions from REDD+ in developing countries. To contribute to this effort, we have diagnosed the main sources of uncertainty in the quantification of emission from deforestation for Panama, one of the first countries to be supported by the Forest Carbon Partnership Facility of the World Bank and by UN-REDD. Performing sensitivity analyses using a land-cover change emissions model, we identified forest carbon stocks and the quality of land-cover maps as the key parameters influencing model uncertainty. The time interval between two land-cover assessments, carbon density in fallow and secondary forest, and the accuracy of land-cover classifications also affect our ability to produce accurate estimates. Further, we used the model to compare emission reductions from five different deforestation reduction scenarios drawn from governmental input. Only the scenario simulating a reduction in deforestation by half succeeds in crossing outside the confidence bounds surrounding the baseline emission obtained from the uncertainty analysis. These results suggest that with current data, real emission reductions in developing countries could be obscured by their associated uncertainties. Ways of addressing the key sources of error are proposed, for developing countries involved in REDD+, for improving the accuracy of their estimates in the future. These new considerations confirm the importance of current efforts to establish forest monitoring systems and enhance capabilities for REDD+ in developing countries.

Tropical tree diversity enhances light capture through crown plasticity and spatial and temporal niche differences

Light partitioning is often invoked as a mechanism for positive plant diversity effects on ecosystem functioning. Yet evidence for an improved distribution of foliage in space or time in diverse plant communities remains scarce, and restricted mostly to temperate grasslands. Here we identify the mechanisms through which tree species diversity affects community-level light capture in a biodiversity experiment with tropical trees that displays overyielding, i.e., enhanced biomass production in mixtures. Using a combination of empirical data, mechanistic models, and statistical tools, we develop innovative methods to test for the isolated and combined effects of architectural and temporal niche differences among species as well as plastic changes in crown shape within species. We show that all three mechanisms enhanced light capture in mixtures and that temporal niche differences were the most important driver of this result in our seasonal tropical system. Our study mechanistically demonstrates that niche differences and phenotypic plasticity can generate significant biodiversity effects on ecosystem functioning in tropical forests.

Conservation of Useful Plants: An Evaluation of Local Priorities from Two Indigenous Communities in Eastern Panama

On both theoretical and practical grounds, respect for, and inclusion of, local decision-making processes is advocated in conservation, yet little is known about the conservation priorities on local territories. We employed interviews and ecological inventories in two villages in order to (1) evaluate the local perception of the conservation status of important plant resources; (2) compare patterns of plant use; and (3) compare perceived conservation status with population structure and abundance in the field. One-third of the 35 species examined were perceived to be threatened or declining. These were predominantly used locally for construction or sold commercially, but were not necessarily rare in the field. The destructiveness of harvest was the most consistent predictor of conservation status in both villages. Contrasting patterns were found in the two villages for the frequency of plant harvest and the relationship of this variable with conservation status. We suggest that local knowledge is an efficient means to rapidly assess the status of a large number of species, whereas population structure analysis provides an initial evaluation of the impact of harvest for selected species.ResumenTanto desde una perspectiva teórica como práctica el respeto e inclusión de los procesos locales de toma de decisiones es una forma en que la conservación puede ser promovida. Sin embargo, poco se sabe sobre las prioridades de conservación en territorios indígenas. En el presente estudio se emplearon entrevistas e inventarios ecológicos en dos localidades indígenas para (1) evaluar la percepción de los habitantes locales sobre el estado de conservación de recursos vegetales importantes; (2) comparar los patrones de uso de plantas; y (3) comparar la percepción del estado de conservación con la estructura de las poblaciones y la abundancia de las especies en el campo. Una tercera parte de las 35 especies estudiadas fueron percibidas como amenazadas o en proceso de declinación. Se trata de plantas utilizadas principalmente como materiales de construcción o que aportan productos que son comercializados, pero que no son necesariamente escasas en el campo. En ambas localidades la variable que predijo el estado de conservación de forma más consistente fue el grado de destrucción de las plantas asociado a las prócticas de cosecha. Encontramos patrones contrastantes entre las dos localidades con respecto a la frecuencia de cosecha y ala relación de esta variable con el estado de conservación de las plantas. Sugerimos que el uso de conocimientos locales es una forma eficiente de evaluar con rapidez el estado de un gran número de especies, en tanto que el análisis de la estructura de poblaciones aporta una evaluación inicial sobre el impacto de la cosecha para algunas especies de inters’e.