Duncan S. Wilson

Sources of within- and between-stand variability in specific leaf area of three ecologically distinct conifer species

Specific leaf area (SLA) is an important ecophysiological variable, but its variability within and between stands has rarely been simultaneously examined and modeled across multiple species. Extensive datasets on SLA in coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco), hybrid spruce (Picea engelmannii Parry × Picea glauca (Moench) Voss × Picea sitchensis (Bong.) Carr.), and ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) were used to estimate variability of SLA within a canopy and its relationship to tree- and stand-level covariates, and to predict SLA at various locations in tree crowns. Also, in the case of hybrid spruce, variation in SLA due to different relative horizontal lengths from the bole was examined. In all species, SLA systematically increased from tree tip to crown base and decreased with foliage age class. Cardinal direction did not have a highly significant influence in either Douglas-fir or hybrid spruce, but SLA did significantly decrease from branch tip to bole in hybrid spruce. Tree- and stand-level (e.g. density, site index) factors had relatively little influence on SLA, but stand age did have a significant positive influence. For ponderosa pine, a significant relationship between canopy mean current-year SLA and carbon isotope discrimination was also found, suggesting the importance of water stress in this species. An equation was fitted to estimate SLA at various points in the canopy for each species and foliage age class using absolute height in the canopy, relative vertical height in the tree, and stand age.RésuméLa surface spécifique des feuilles (SLA) est un paramètre écophysiologique important mais sa variabilité intra et inter-peuplements n’a jamais été examinée et modélisée sur des gammes larges d’espèces. Des jeux de données très détaillés de SLA de Douglas côtiers [Pseudotsuga menziesii var. menziesii (Mirb.) Franco], d’épicéas hybrides (Picea engelmannii Parry × Picea glauca (Moench) Voss × Picea sitchensis (Bong.) Carr), et de pins ponderosa (Pinus ponderosa Dougl. ex P. & C. Laws.) ont été mobilisés pour évaluer la variabilité de SLA dans une canopée. Les relations entre SLA et des covariables à l’échelle de l’arbre ou du peuplement ont été précisées, un modèle prédictif de SLA à différents niveaux dans les couronnes a été construit. Dans le cas de l’épicéa, l’impact de la distance de branche entre l’aiguille et le tronc a également été testé. Dans toutes les espèces, SLA augmentait systématiquement du sommet des arbres à la base de la couronne, et diminuait avec la classe d’âge des aiguilles. La direction cardinale n’avait guère d’influence sur SLA ni dans le cas du Douglas ni dans celui de l’épicéa; mais SLA diminuait systématiquement depuis l’extrémité des branches vers le tronc. Les facteurs arbre et peuplement (comme la densité, l’indice de productivité de la station) n’avaient que peu d’impact sur SLA alors que l’âge du peuplement avait un effet significatif et positif. Pour le pin ponderosa, une relation significative a été détectée entre la valeur moyenne de SLA des aiguilles de l’année et la discrimination isotopique du carbone, ce qui suggère l’impact des contraintes hydriques pour cette espèce. Un modèle de prédiction de SLA à différentes positions dans la canopée a été ajusté sur les données de chaque espèce et classe d’âge, en se basant sur la hauteur dans la canopée, la hauteur relative dans l’arbre et l’âge du peuplement.

Environmental basis of soil–site productivity relationships in ponderosa pine

Understanding the environmental basis for soil-site quality relationships requires that we connect the environmental factors important to resource availability to the physiological processes influencing tree productivity. The nitrogen productivity concept provides this link by relating nitrogen uptake rate to plant growth, although the concept has been verified almost exclusively by laboratory experiments on tree seedlings. We tested the nitrogen productivity concept in a field setting by relating foliage production to nitrogen mineralization rate in 19 mature ponderosa pine (Pinus ponderosa) stands across a moisture gradient in central Oregon, USA. Models developed following the nitrogen productivity concept predicted annual foliage production precisely and adequately represented the different influences of nitrogen and water stress. Current-year foliage production was proportional to older foliage nitrogen content (R 2 ¼ 0.82), and a model including a water stress index (stable carbon isotope ratio, d 13 C) further explained 95% of the variability. A direct link between soil nitrogen availability and canopy nutrition was less clear. Annual foliage production was positively, but weakly, correlated with soil-estimated N-uptake (estimated in situ), likely because annual nitrogen uptake was small relative to nitrogen retained in the canopy. Foliage nitrogen was highly conserved with a mean retention time of 10.5 yr, which was 2.2 times longer than foliage retention. Annual nitrogen uptake amounted to 0-11% of total canopy N. Multiyear estimates of cumulative N fluxes are needed to adequately assess N availability. Soil nutrient pools were poorly correlated with nutrient uptake and were not useful for predicting stand productivity.

Bayesian Small Area Models for Assessing Wildlife Conservation Risk in Patchy Populations

Species conservation risk assessments require accurate, probabilistic, and biologically meaningful maps of population distribution. In patchy populations, the reasons for discontinuities are not often well understood. We tested a novel approach to habitat modeling in which methods of small area estimation were used within a hierarchical Bayesian framework. Amphibian occurrence was modeled with logistic regression that included third-order drainages as hierarchical effects to account for patchy populations. Models including the random drainage effects adequately represented species occurrences in patchy populations of 4 amphibian species in the Oregon Coast Range (U.S.A.). Amphibian surveys from other locations within the same drainage were used to calibrate local drainage-scale effects. Cross-validation showed that prediction errors for calibrated models were 77% to 86% lower than comparable regionally constructed models, depending on species. When calibration data were unavailable, small area and regional models performed similarly, although poorly. Small area estimation models complement wildlife ecology and habitat studies, and can help managers develop a regional picture of the conservation status for relatively rare species.