Decomposition rates for red spruce (Picearubens Sarg.) and balsam fir (Abiesbalsamea (L.) Mill.) boles on the forest floor were determined for midelevation forests of the White Mountains from a chronosequence of previously logged stands. Density changes in wood and bark were described using a negative exponential model, yielding decay constants of 0.033 and 0.029/year for spruce and fir wood, respectively. The two species were not statistically different in terms of mass loss. Bole diameter had no influence on the decay rate of red spruce. Fir boles in midelevation forests decayed significantly faster than those in high-elevation forests measured in another study. Net accumulation of N, P, Ca, and Mg occurred in the wood of both species. N accumulated in bark, but P, Ca, and Mg behavior was variable. Na and K behavior was similar in the wood and bark of both species, with Na release concomitant with mass loss, while K was lost faster than mass. C:N ratios declined, and N:P ratios converged on a value of ca. 20, in the wood and bark of both species.

According to computer energy balance simulations of horizontal thin leaves, the quantitative effects of stomatal distribution patterns (top vs. bottom surfaces) on transpiration (E) were maximal for sunlit leaves with high stomatal conductances (gs) and experiencing low windspeeds (free or mixed convection regimes). E of these leaves decreased at windspeeds > 50 cm s−1, despite increases in the leaf‐to‐air vapour density deficit. At 50 cm s−1 wind‐speed, rapidly transpiring leaves had greater E when one‐half of the stomata were on each leaf surface (amphistomaty; 10.16 mmol H2O m−2 s−1) than when all stomata were on either the top (hyperstomaty; 9.34 mmol m−2s−1) or bottom (hypostomaty; 7.02 mmol m−2s−1) surface because water loss occurred in parallel from both surfaces. Hyperstomatous leaves had larger E than hypostomatous leaves because free convection was greater on the top than on the bottom surface. Transpiration of leaves with large g, was greatest at windspeeds near zero when ∼60–75% of the stomata were on the top surface, while at high windspeeds E was greatest with, 50% of the stomata on top. For leaves with low gs, stomatal distribution exerted little influence on simulated E values. Laboratory measurements of water loss from simulated hypo‐, hyper‐, and amphistomatous leaf models qualitatively supported these predictions.

No abstract

No abstract

This study combined an ecophysiological model and dendroecological analyses to evaluate potential effects of global warming on the physiology, growth, and mortality of white oak (Quercusalba L.) and black oak (Quercusvelutina Lam.) in the Ohio River region. The model integrated data for ecophysiology of oak species, site attributes, and daily temperature and precipitation to model nonlinear responses of stomatal conductance (gs), net photosynthesis (Pnet), and woody respiration (Rw) to variations in temperature and soil water content. Relationships between modeled physiological response indices and actual white and black oak annual radial growth indices were evaluated by regression analyses, using growth and weather data for the period 1900–1987 for seven upland oak–hickory forests. Modeled physiological response indices explained 40–60% of variation in radial growth indices. To evaluate the effects of global warming, daily temperature values for the period 1900–1987 were increased by 2 or 5 °C, without changing precipitation values, and physiological response indices were computed. Model indices generated in warming simulations were entered into dendroclimatic regression models calibrated under conditions without any warming to predict radial growth under warming scenarios. Under the warming scenarios, OAKWBAL predicted a substantial increase in growing season Rw, but little change in growing season Pnet. Warming merely shifted the period of near-maximal Pnet earlier in the growing season, without changing its duration. However, this result was somewhat dependent upon the ability of leaf-out phenology to track changes in temperature regime. The net effect of increased Rw, with little change in Pnet, was a reduction in radial growth and a higher frequency of years with climatic conditions stressful to oaks on upland sites. A historical association between severe drought and increased incidence of oak growth decline and mortality indicated that global warming could increase the incidence of decline and mortality in oak populations on upland sites similar to those in this study.