PnET is a simple, lumped-parameter, monthlytime-step model of carbon and water balances of forests built on two principal relationships: 1) maximum photosynthetic rate is a function of foliar nitrogen concentration, and 2) stomatal conductance is a function of realized photosynthetic rate. Monthyly leaf area display and carbon and water balances are predicted by combining these with standard equations describing light attenuation in canopies and photosynthetic response to diminishing radiation intensity, along with effects of soil water stress and vapor pressure deficit (VPD). PnET has been validated against field data from 10 well-studied temperate and boreal forest ecosystems, supporting our central hypothesis that aggregation of climatic data to the monthly scale and biological data such as foliar characteristics to the ecosystem level does not cause a significant loss of information relative to long-term, mean ecosystem responses. Sensitivity analyses reveal a diversity of responses among systems to identical alterations in climatic drivers. This suggests that great care should be used in developing generalizations as to how forests will respond to a changing climate. Also critical is the degree to which the temperature responses of photosynthesis and respiration might acclimate to changes in mean temperatures at decadal time scales. An extreme climate change simulation (+3° C maximum temperature, −25% precipitation with no change in minimum temperature or radiation, direct effects of increased atmospheric CO2 ignored) suggests that major increases in water stress, and reductions in biomass production (net carbon gain) and water yield would follow such a change.

A generalized, lumped-parameter model of photosynthesis, evapotranspiration and net primary production in temperate and boreal forest ecosystems.

https://www7.nau.edu/mpcer/direnet/publications/publications_h/files/Hanson_PJ_Weltzin_JF_Drought_disturbance_from.pdf

Drought disturbance from climate change: response of United States forests.

Over the last 4 years, two data sets have emerged which allow increased accuracy and resolution in the definition and validation of a photosynthesis model for whole forest canopies. The first is a greatly expanded set of data on the nitrogen-photosynthesis relationship for temperate and tropical woody species. The second is a unique set of long-term (4 year) daily carbon balance measurements at the Harvard Forest, Petersham, Massachusetts, collected by the eddy-correlation technique. A model (PhET-Day) is presented which is derived directly from, and validated against, these data sets. The PnET-Day model uses foliar nitrogen concentration to calculate maximum instantaneous rates of gross and net photosynthesis which are then reduced for suboptimal temperature, photosynthetically active radiation (PAR), and vapor pressure deficit (VPD). Predicted daily gross photosynthesis is closely related to gross carbon exchange at the Harvard Forest as determined by eddy-correlation measurements. Predictions made by the full canopy model were significantly better than those produced by a multiple linear regression model. Sensitivity analyses for this model for a deciduous broad-leaved forest showed results to be much more sensitive to parameters related to maximum leaf-level photosynthetic rate (A max) than to those related to light, temperature, VPD or total foliar mass. Aggregation analyses suggest that using monthly mean climatic data to drive the canopy model will give results similar to those achieved by averaging daily eddy correlation measurements of gross carbon exchange (GCE).

/pdf/extrapolating-leaf-co2-exchange-to-the-canopy-a-generalized-1e4eb673up.pdf

Extrapolating leaf CO2 exchange to the canopy: a generalized model of forest photosynthesis compared with measurements by eddy correlation.

Limited mesophyll diffusion conductance to CO2 (gm) can significantly constrain plant photosynthesis, but the extent of gm-limitation is still imperfectly known. As gm scales positively with foliage photosynthetic capacity (A), the CO2 drawdown from substomatal cavities (Ci) to chloroplasts (CC, Ci‐CC¼A/gm) rather than gm alone characterizes the mesophyll diffusion limitations of photosynthesis. The dependencies of gm on A, foliage structure (leaf dry mass per unit area, MA), and the resulting drawdowns across a dataset of 81 species of contrasting foliage structure and photosynthetic potentials measured under non-stressed conditions were analysed to describe the structure-driven potential photosynthetic limitations due to gm. Further the effects of key environmental stress factors and leaf and plant developmental alterations on gm and CO2 drawdown were evaluated and the implications of varying gm on foliage photosynthesis in the field were simulated. The meta-analysis demonstrated that gm of non-stressed leaves was negatively correlated with MA, and despite the positive relationship between gm and A, the CO2 drawdown was larger in leaves with more robust structure. The correlations were stronger with mass-based gm and A, probably reflecting the circumstance that mesophyll diffusion is a complex three-dimensional process that scales better with mesophyll volume-weighted than with leaf area-weighted traits. The analysis of key environmental stress effects on gm and CO2 drawdowns demonstrated that the effect of individual stresses on CO2 drawdowns varies depending on the stress effects on foliage structure and assimilation rates. Leaf diffusion limitations are larger in nonsenescent older leaves and also in senescent leaves, again reflecting more robust leaf structure and/or nonco-ordinated alterations in leaf photosynthesis and gm. According to simulation analyses, in plants with a larger part of the overall diffusion conductance from the ambient atmosphere to the chloroplasts in the mesophyll, photosynthesis is less sensitive to changes in stomatal conductance. Accordingly, in harsher environments that support vegetation with tougher long-living stress-tolerant leaves with lower gm, reductions in stomatal conductance that are common during stress periods are expected to alter photosynthesis less than in species where a larger part of the total diffusion limitation is determined by stomata. While structural robustness improves plant performance under environmental stress, low gm and inherently large CO2 drawdown in robust leaves limits the photosynthesis of these plants more severely under favourable conditions when stomatal conductance is high. The differences in overall responsiveness to environmental modifications of plants with varying gm need consideration in current large-scale ecosystem productivity models.

/pdf/role-of-mesophyll-diffusion-conductance-in-constraining-35wi8g07mp.pdf

Role of mesophyll diffusion conductance in constraining potential photosynthetic productivity in the field

In herbaceous dominated patches and ecosystems, tree establishment is influenced partly by the ability of woody seedlings to survive and grow in direct competition with herbaceous vegetation. We studied the importance of season long wet and dry spells on the competitive interactions between herbaceous vegetation and oak seedlings along a light and nitrogen gradient in an infertile secondary successional grassland in central North America. We conducted a field experiment in which seedlings of bur oak (Quercus macrocarpa) and northern pin oak (Q. ellipsoidalis) were exposed to two levels of light (full sun and 80% shade), three levels of nitrogen input (0, 5, 15 g m−1 yr−1), and three levels of water input (low, medium and high). In addition, seedlings were grown with and without the presence of surrounding herbaceous vegetation under both light and all three water levels. Seedling survival, growth, and rate of photosynthesis were significantly affected by competition with herbaceous vegetation and these effects varied along the multiple resource gradient. Overall, seedling survival of both species was significantly greater in wetter and shaded plots and when surrounding herbaceous vegetation was removed and was lower in nitrogen enriched plots. We found that soil water was significantly affected by varying inputs of water, light, and the presence or absence of herbaceous vegetation, and that seedling survival and rate of photosynthesis were highly correlated with available soil water. Our findings show that the impact of season long wet and dry spells on tree seedling success in grasslands can be affected by light and soil nitrogen availability.

Survival, growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient

Red oak (Quercus rubra), a mesic species, and chestnut oak (Quercus prinus), a xeric species, were grown in a greenhouse with and without fertilizer (F+ and F-, respectively) and subjected to a 10-week drydown (W-) or kept well watered (W+). In both species, fertilized seedlings exhibited greater reductions in mean net photosynthesis (A), leaf conductance (g(wv)), leaf water potential (Psi(leaf)) and water use efficiency (WUE) during the drydown than unfertilized seedlings. In the W- treatments, red oak showed greater reductions in A, g(wv) and Psi(leaf) than chestnut oak. Differential fertilization of the seedlings of both species had a greater effect on tissue water relations than differential watering. During the latter weeks of the drydown, there was no osmotic adjustment in red oak, but chestnut oak in the F+/W- treatment had significantly lower osmotic potentials at full and zero turgor than seedlings in any of the other treatments. The results indicate that high nutrient availability does not improve the drought tolerance of these two oak species.

The impact of water and nutrient deficiencies on the growth, gas exchange and water relations of red oak and chestnut oak

Field and laboratory studies were used to evaluate net photosynthesis and plant and tissue water relations in saplings of co-occurring xeric (Quercus prinus and Q. ilicifolia) versus mesic (Q. rubra and Castanea dentata) hardwood species during a droughty summer. Mean daily net photosynthesis, leaf conductance, leaf water potential, and water use efficiency were reduced in all four species during the peak drought period. Throughout the study, net photosynthesis and leaf conductance were on average high in Q. ilicifolia and Q. prinus, intermediate in Q. rubra, and low in C. dentata. (...)

Ecophysiological responses in mesic versus xeric hardwood species to an early-season drought in central Pennsylvania

In the northeastern United States, forest stands on xeric sites, such as ridgetops and steep upper slopes, generally experience more gypsy moth, Lymantria dispar (L.), defoliation than those on mesic lowland sites. To address whether foliage quality from a site could contribute to forest stand susceptibility, we reared gypsy moth larvae on chestnut oak, Quercus prinus (L.), and northern red oak, Quercus rubra (L.), foliage collected from two xeric and two mesic forest sites. Food quality of foliage was measured as protein and phenolics (hydrolyzable and condensed tannins, total phenolics and protein binding capacity) and was related to measures of gypsy moth success. Chestnut oak foliage had greater measures of phenolics than red oak and produced heavier male and female pupae and more fecund females. Foliage from xeric sites was likely to have greater measures of phenolics, but only male pupal weights were greater for larvae reared on xeric site foliage. Larval development times (days to pupation) were shorter on chestnut oak foliage than on red oak foliage for females and shorter on foliage from mesic sites than from xeric sites for males. Female pupal weights and fecundities were negatively correlated with measures of red oak hydrolyzable tannins, but there was no relationship between chestnut oak phenolics and gypsy moth success. The greater frequency of gypsy moth outbreaks on xeric sites may be more a function of tree species composition than site induced differences in foliage quality.

Forest Stand Susceptibility to the Gypsy Moth (Lepidoptera: Lyman triidae): Species and Site Effects on Foliage Quality to Larvae

We tested the hypothesis that soil nitrogen availability could alter the efficacy of the cry1A(a) d-endotoxin of Bacillus thuringiensis (B.t.) through variation in phytochemistry. Extracts of the c...

Effect of nitrogen availability on the growth and phytochemistry of hybrid poplar and the efficacy of the Bacillus thuringiensis cry1A(a) d-endotoxin on gypsy moth

Variation in phenolic leaf traits was measured for red oaks (Quercusrubra L.) and chestnut oaks (Quercusprinus L.) growing in a resistant stand (on a mesic lowland) and in a susceptible stand (on a...

Karl W. Kleiner

Papers

The impact of water and nutrient deficiencies on the growth, gas exchange and water relations of red oak and chestnut oak

Ecophysiological responses in mesic versus xeric hardwood species to an early-season drought in central Pennsylvania

Forest Stand Susceptibility to the Gypsy Moth (Lepidoptera: Lyman triidae): Species and Site Effects on Foliage Quality to Larvae

Effect of nitrogen availability on the growth and phytochemistry of hybrid poplar and the efficacy of the Bacillus thuringiensis cry1A(a) d-endotoxin on gypsy moth

Variation in leaf quality of two oak species: implications for stand susceptibility to gypsy moth defoliation