Water stress and temperature impose fundamental limits to forest productivity. Stresses caused by fluctuating or extreme temperatures or limited water availability vary both seasonally and from year to year. The role of these stresses should be considered when investigating the causes of declines in forest productivity. Forest growth, or carbon gain, can be related quantitatively to three components: leaf area, rate of net photosynthesis and rate of respiration. This paper examines effects of temperature and water supply on these components with particular reference to loblolly pine (Pinus taeda L.), a species of economic importance in the southeastern United States where declines, of unknown cause, in forest productivity have been reported.

Net photosynthesis, transpiration and stomatal conductance of two-year-old Pinus taeda L. seedlings were compared under various environmental conditions. Responses to air temperature, irradiance, ambient CO(2) concentration, absolute humidity deficit and xylem pressure potential were examined. The seedlings exhibited little response to a wide range of absolute humidity deficits (7 to 16 g m(-3)) and temperatures (20 to 35 degrees C), but were sensitive to changes in water deficit, irradiance and CO(2) concentration. Net photosynthesis and stomatal conductance were linearly related under all of the environmental conditions measured. However, the gas phase limitation to photosynthesis was generally small (20 to 30%). It was concluded that although the stomatal response was closely coupled to changes in photosynthesis, internal limitations, rather than the rate of gaseous diffusion of CO(2), were primarily responsible for limiting photosynthesis.

Seedlings from 23 seed sources representing the natural range of black locust (Robinia pseudoacacia L.) were electrophoretically analyzed at 40 structural loci representing 18 enzyme systems. Genetic diversity was high for an insect-pollinated, early-successional species. On average, 71% of the loci were polymorphic (99% criterion). Average and effective numbers of alleles per locus were 2.61 and 1.58, respectively. Mean expected heterozygosity was 0.291, ranging among the seed sources from 0.250 to 0.322. Most of the genetic diversity (88%) resided within seed sources. Few geographic patterns were evident. The most differentiated sources were located in disjunct sites in Georgia and the Ozark Mountains of Arkansas and Missouri, and in Pennsylvania. The widespread planting of native and European black locust seed sources may have contributed to the lack of geographic patterns. Based on this and other studies, it was concluded that initial selections in black locust breeding programs should concentrate on intrapopulational variation.

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When conducting tree breeding experiments, geneticists often assume that individuals from open-pollinated families are halfsibs. The reliability of this assumption was tested using data from enzyme electrophoresis to estimate the genetic relatedness among progeny within 22 open-pollinated families of Robinia pseudoacacia L. (black locust) and 34 open-pollinated families of Gleditsia triacanthos L. (honey locust) from natural stands. An algorithm employing population estimates of fixation indices, pollen allele frequencies, and selfing rates was used to calculate the mean expected number of alleles in common across loci under assumptions of either full-sib (i.e., a single pollen parent) or half-sib (i.e., random mating) relationships. For each open-pollinated family, the average coefficient of relationship among progeny was calculated by linear interpolation from the observed number of alleles in common. For most families of both species, coefficients were significantly higher than 0.25 (half-sib relation), but were significantly lower than 0.50 (full-sib relation). These results suggest that the assumption of a half-sib relationship among progeny of open-pollinated families is violated for these tree species. More critical to the estimation of heritabilities and the prediction of genetic gains was the observation that estimates of the coefficient of relationship varied widely among open-pollinated families (for R. pseudoacacia r 0=0.20-0.43, mean=0.34; for G. triacanthos r 0=0.29-0.55, mean=0.36).

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Leaf conductance at three absolute humidity deficits (AHDs) (7, 14 and 21 g m(-3)), hydraulic conductance and components of tissue water potential were measured in one-year-old loblolly pine seedlings from six origins representing the geographic range of the species. Measurements were made on seedlings grown (a) with ample water (moist regime) and (b) with recurring severe drought (dry regime). However, all seedlings were well-watered prior to and during measurements. Seedlings grown in the moist regime had greater mean leaf conductances (0.30 versus 0.13 cm s(-1)) and greater responses to AHD than seedlings grown in the dry regime. They also exhibited greater hydraulic conductances (0.53 versus 0.35 microg cm(-2) s(-1) MPa(-1), less negative osmotic potentials (-1.45 versus -1.57 MPa) and higher relative water contents at turgor loss (0.72 versus 0.65). Seed source differences in water relations characteristics were detected only in seedlings grown in the moist regime. In these, trees from the three interior origins had greater mean leaf conductances than those from the three coastal sources (0.32 versus 0.28 cm s(-1)), but no differences in response to changing AHD were observed. Seedlings from North Carolina had lower osmotic potentials at turgor loss than those from Florida, Georgia or Texas. These differences in water relations characteristics are not clearly related to the observed greater survival ability of trees from interior origins compared with those from coastal origins.

Natural population of black locust maintains very high levels of genetic diversity wIthin populations without any noticeable geographic patterns. In order to assess the species' potential for manipulation through breeding programs, more detailed spatial pattern analyses of fine-scale population structure was attempted in the two study sites