Ecophysiological attributes of the native Acer saccharum and the exotic Acer platanoides in urban oak forests in Pennsylvania, USA.

When the exotic Acer platanoides L. (Norway maple) and the native A. saccharum Marsh. (sugar maple) grow together in the understories of urban Quercus forests in the eastern USA, average annual height growth increments are nearly twice as large in A. platanoides as in A. saccharum, 19.26 +/- 3.22 versus 10.01 +/- 1.69 cm. We examined several ecophysiological mechanisms that might be associated with the superior invasive ability and growth of A. platanoides in two urban oak forests in Pennsylvania. Leaf longevity was 12 days greater in A. platanoides than in A. saccharum. In addition, leaf mass/leaf area ratio was greater in A. platanoides than in A. saccharum (2.67 +/- 0.03 versus 2.32 +/- 0.02 mg cm(-2)); however, leaf thickness was significantly lower in A. platanoides than in A. saccharum suggesting that A. platanoides contains more dense palisade and mesophyll cell layers than A. saccharum. Field net photosynthesis (mass basis) and photosynthetic light response curves (area basis) indicated significantly greater carbon assimilation, and nitrogen and phosphorus use efficiencies in A. platanoides than in A. saccharum. Acer platanoides also exhibited higher water use efficiency than A. saccharum (0.88 +/- 0.12 versus 0.32 +/- 0.09 mmol CO(2) mol(-1) H(2)O). Acer platanoides exhibited significantly lower osmotic potentials than A. saccharum, but a similar relative water content at zero turgor. We conclude that A. platanoides utilizes light, water and nutrients more efficiently than A. saccharum.

Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species.

The relationship between photosynthetic capacity (A max) and leaf nitrogen concentration (N) among all C3 species can be described roughly with one general equation, yet within that overall pattern species groups or individual species may have markedly different A max-N relationships. To determine whether one or several predictive, fundamental A max-N relationships exist for temperate trees we measured A max, specific leaf area (SLA) and N in 22 broad-leaved deciduous and 9 needle-leaved evergreen tree species in Wisconsin, United States. For broad-leaved deciduous trees, mass-based A max was highly correlated with leaf N (r 2=0.75, P<0.001). For evergreen conifers, mass-based A max was also correlated with leaf N (r 2=0.59, P<0.001) and the slope of the regression (rate of increase of A max per unit increase in N) was lower (P<0.001) by two-thirds than in the broad-leaved species (1.9 vs. 6.4 µmol CO2 g-1 N s-1), consistent with predictions based on tropical rain forest trees of short vs. long leaf life-span. On an area basis, there was a strong A max-N correlation among deciduous species (r 2=0.78, P<0.001) and no correlation (r 2=0.03, P>0.25) in the evergreen conifers. Compared to deciduous trees at a common leaf N (mass or area basis), evergreen trees had lower A max and SLA. For all data pooled, both leaf N and A max on a mass basis were correlated (r 2=0.6) with SLA; in contrast, area-based leaf N scaled tightly with SLA (r 2=0.81), but area-based A max did not (r 2=0.06) because of low A max per unit N in the evergreen conifers. Multiple regression analysis of all data pooled showed that both N (mass or area basis) and SLA were significantly (P<0.001) related to A max on mass (r 2=0.80) and area (r 2=0.55) bases, respectively. These results provide further evidence that A max-N relationships are fundamentally different for ecologically distinct species groups with differing suites of foliage characteristics: species with long leaf life-spans and low SLA, whether broad-leaved or needle-leaved, tend to have lower A max per unit leaf N and a lower slope and higher intercept of the A max-N relation than do species with shorter leaf life-span and higher SLA. A single global A max-N equation overestimates and underestimates A max for temperate trees at the upper and lower end of their leaf N range, respectively. Users of A max-N relationships in modeling photosynthesis in different ecosystems should appreciate the strengths and limitations of regression equations based on different species groupings.

Ecophysiological and morphological responses to shade and drought in two contrasting ecotypes of Prunus serotina.

Photosynthesis (A), water relations and stomatal reactivity during drought, and leaf morphology were evaluated on 2-year-old, sun- and shade-grown Prunus serotina Ehrh. seedlings of a mesic Pennsylvania seed source and a more xeric Wisconsin source. Wisconsin plants maintained higher A and leaf conductance (g(wv)) than Pennsylvania plants during the entire drought under sun conditions, and during the mid stages of drought under shade conditions. Compared to shade plants, sun plants of both sources exhibited a more rapid decrease in A or % A(max) with decreasing leaf water potential (Psi). Tissue water relations parameters were generally not significantly different between seed sources. However, osmotic potentials were lower in sun than shade plants under well-watered conditions. Following drought, shade plants, but not sun plants, exhibited significant osmotic adjustment. Sun leaves had greater thickness, specific mass, area and stomatal density and lower guard cell length than shade leaves in one or both sources. Wisconsin sun leaves were seemingly more xerophytic with greater thickness, specific mass, and guard cell length than Pennsylvania sun leaves. No source differences in leaf structure were exhibited in shade plants. Stomatal reactivity to sun-shade cycles was similar between ecotypes. However, well-watered and droughted plants differed in stomatal reactivity within and between multiple sun-shade cycles. The observed ecotypic and phenotypic variations in ecophysiology and morphology are consistent with the ability of Prunus serotina to survive in greatly contrasting environments.

Vertical variation in canopy structure and CO(2) exchange of oak-maple forests: influence of ozone, nitrogen, and other factors on simulated canopy carbon gain.

Stand-level and physiological measurements were made for oak and maple species common in Wisconsin forests. Scaling relationships were identified to allow the development of a model for estimating net carbon exchange at the levels of a leaf, canopy stratum, and whole canopy. Functional relationships were determined between tissue gas exchange rates and perceived controlling variables. Vertical variation in leaf properties and in the distribution of foliage by weight, area, and species were characterized for several closed canopy forests. Forest canopies were divided into four horizontal strata to develop predictive models for canopy gas exchange. Leaf and canopy layer carbon dioxide exchange rates were predicted using leaf nitrogen concentration, leaf mass per area, ozone exposure, predawn leaf water potential, photosynthetically active radiation, and vapor pressure deficit as driving variables. Direct measurements of leaf gas exchange were used to validate the components (subroutines) of the model. Net carbon dioxide exchange was simulated for canopy layers at 5-min intervals over a diurnal time course. Simulations of canopy CO(2) exchange were made for a 30-m tall, mixed oak-maple forest under hypothetical ambient and greater-than-ambient ozone pollution regimes. Daily canopy net CO(2) exchange was predicted for seven forest stands and compared with estimates of aboveground net primary production, N availability, leaf area index, and canopy N.