Mark E. Kubiske

Leaf structural characteristics of 31 hardwood and conifer tree species in central Wisconsin: influence of light regime and shade-tolerance rank.

Abstract Leaf structural characteristics were examined in understory and open-growing individuals of 26 hardwood and five conifer tree species, representing a range of shade-tolerance classes, in central Wisconsin. Sun leaves of open-growing hardwoods generally had greater thickness, specific mass and stomatal density than shaded leaves in the understory. In contrast, the conifer species exhibited few consistent differences in sun-shade needle length and specific leaf mass. Guard-cell length was not consistently different between sun and shade leaves of the hardwood species. Oaks had greater leaf thickness and stomatal density but lower guard-cell lengths compared to other hardwoo. Shade-intolerant hardwood species generally had greater sun- and shade-leaf thickness, specific leaf mass and guard-cell length than more-tolerant species. These differences in hardwood leaves suggest that acclimation to light regime exists for species representing a broad range of tolerances, and that differences in sun-shade leaf structure among species may vary with shade tolerance.

Rehydration effects on pressure-volume relationships in four temperate woody species : variability with site, time of season and drought conditions

SummarySeasonal pressure-volume (P-V) analyses were conducted on rehydrated and non-rehydrated leaves of Quercus rubra, Q. ilicifolia, Q. prinus, and Fraxinus americana in central Pennsylvania, U.S.A., to test the hypothesis that rehydration-induced shifts in P-V parameters occur in woody species from a non-arid region, and that the magnitude of these shifts increases with species drought tolerance and drought conditions. The species from a xeric ridge (Q. ilicifolia and Q. prinus) displayed increases of about 0.4–0.6 MPa in the osmotic potentials at full and zero turgor and a concurrent loss of symplastic solutes following 12 h and 24 h rehydration, particularly during a late-season drought. In contrast, the mesic, valley species (Q. rubra and F. americana) did not display significant shifts in osmotic parameters with rehydration at any time. In several instances, the relative water content at zero turgor (RWC0) increased by about 6% (e.g., from 85% to 91%) and the bulk elastic modulus (ε) decreased by about 4.0 MPa following rehydration and correction for the plateau effect; the magnitude of these shifts was greatest in the xeric species. However, when data were not corrected for the plateau effect, RWC0 decreased by about 4% in some of the species/date combinations. Plateaus were also responsible for some of the decrease in ε with rehydration, but not for the shifts in osmotic potentials. The largest increases in osmotic potentials corresponded with decreases in tissue osmotic solute content. Rehydration-induced shifts in P-V parameters were responsible for masking or reducing most of the species and seasonal differences exhibited in nonrehydrated samples.

Ecophysiological analysis of woody species in contrasting temperate communities during wet and dry years

This study employed an intensive sampling regime in which leaf gas exchange and tissue-water relations were measured simultaneously on the same leaf at midday on 19 tree species from three distinct forest communities during wet (1990) and dry (1991) growing seasons. The study sites were located on a xeric barrens, a misic valley floor, and a wet-mesic floodplain in central Pennsylvania, United States. The xeric, mesic, and wetmesic sties had drought-related decreases in gravimetric soil moisture of 53, 34 and 27%, respectively. During the wet year, xeric and mesic communities had high seasonal mean photosynthetic rates (A) and stomatal conductance of water vapor (gwv) and low midday leaf water potential (ψ), whereas the wet-mesic community had low A and gwv and high midday ψ. The mesic and wet-mesic communities had dry year decreases in predawn ψ, gwv and A with the greatest drought effect occurring in the mesic community. Regression analysis indicated that species from each site that exhibited high wet-year A and gwv tended to have low midday ψ. This trend was reversed only in the mesic community in the drought year. Despite differences in midday ψ, all three communities had similar midday leaf turgor pressure (ψp) in the wet year attributable to lower osmotic potential at zero turgor (ψπ0) with increasing site droughtiness. Lower wet year ψπ0 in the xeric community was due to low symplast volume rather than high solute content. Species with the lowest ψπ0 in the wet year often did not have the lowest ψπ100 possibly related to differences in tissue elasticity. Moreover, increased elasticity during drought may have masked osmotic adjustment in ψπ100 but not in ψπ0, via dilution of solutes at full hydration in some species. Despite the sampling regime used, there were no relationships between gas exchange and osmotic and elastic parameters that were consistently significant among communities or years. This result questions the universal, direct effect of osmotic and elastic adjustments in the maintenance of photosynthesis during drought. By including a large number of species, this study provided new insight to the ecophysiology of contrasting forest communities, and the community-wide impact of drought on contrasting sites.

Seasonal Tissue Water Relations of Four Successional Pennsylvania Barrens Species in Open and Understory Environments

Seasonal tissue water relations were measured in co-occurring saplings of Quercus velutina Lam., Quercus prinus L., Sassafras albidum (Nutt.) Nees, and Acer rubrum L. from adjacent open and understory sites in the central Pennsylvania barrens. Open-growing plants exhibited greater and earlier phenological shifts in osmotic potentials under moist conditions, whereas understory plants had greater osmotic adjustment during a mild, late-season drought. Sassafras albidum was an exception, exhibiting steadily declining osmotic potentials at full and zero turgor over the course of the season on both sites. Elastic modulus (ε) steadily increased for all species on the open site, while A. rubrum showed a decrease and S. albidum and Q. velutina showed an increase in ε in the understory. Relative water content at zero turgor (RWC0) was similar in the understory and open sites except during the drought period when understory plants had lower values. Quercus prinus generally exhibited the lowest RWC0 values, although A. rubrum, a later successional species, had a seasonal decrease in RWC0 at both sites. Thus, each species exhibited somewhat unique combinations of seasonal osmotic and elastic adjustment, which acted in concert to balance tissue water loss with turgor maintenance under changing environmental conditions. These results indicate that a variety of ecophysiological mechanisms operate to allow species of different successional rank to tolerate open and understory barrens environments.