Six-week-old half-sib seedlings of Ulmus americana L. were subjected to different amounts of flexure daily for 3 weeks under controlled greenhouse conditions. The daily flexure treatments were: no flexing in a staked stem, minimal flexing in a non-staked stem, and five, 10, 20, 40, and 80 flexures. Seedling height and diameter growth and average leaf area were determined before and after the treatments. The ratio of the change in height growth (DeltaH) to the change in diameter growth (DeltaD; (DeltaH:DeltaD)) before and after the 3-week treatments were calculated. At the end of the 3-week experiment, staked seedlings were significantly taller and had smaller stem diameters than all of the flexed seedlings. Height growth tended to decrease exponentially with increased flexure, with significant differences between the extremes of treatment. All of the flexure treatments significantly increased stem diameter compared to staked seedlings. The DeltaH:DeltaD ratio exhibited an exponential function in response to increased flexure. Average leaf area decreased with increased flexure, and seedlings in the 40x and 80x flexure treatments had significantly less leaf area than seedlings in all of the other treatments. These data are similar to the dose responses previously observed in herbaceous species. The finding that trees exhibit greater sensitivity to low doses of flexure than to high doses of flexure indicates that slight exposure to wind may result in a large initial alteration in stem morphology, producing a thigmomorphogenetic effect. Trees will continue to respond to increasing amounts of mechanical stress, but at an exponentially declining rate. Declining leaf areas in response to increasing amounts of mechanical stress may result in a decrease in available photosynthate, resulting in a tree of smaller stature compared to trees exposed to lower amounts of mechanical loading.

To identify hybrid-specific differences in developmental response to mechanical perturbation (MP), we compared the effects of stem flexure on several morphological and mechanical properties of two Populus trichocarpa Torr. & A. Gray x P. deltoides Bartr. ex Marsh. hybrids, 47-174 and 11-11. In response to the MP treatment, both hybrids exhibited a significant increase in radial growth, especially in the direction of the MP (47-174, P = 0.0001; 11-11, P = 0.002), and a significant decrease in height to diameter growth ratio (P = 0.0001 for both hybrids), suggesting that MP-treated stems are more tapered than control stems. A direct consequence of the MP-induced increase in radial growth was a significant increase in flexural rigidity (EI, N mm(2)) in stems of both hybrids (47-174, P = 0.0001; 11-11, P = 0.009). Both control and MP-treated stems of Hybrid 47-174 had significantly greater height to diameter ratios and EI values than the corresponding stems of Hybrid 11-11 (11-11 stem ratios and EI values were 85 and 76%, respectively, of those of 47-174). In Hybrid 47-174, Young's modulus of elasticity (E, N mm(-2)), a measure of stem flexibility, for MP-treated stems was only 80% of the control value (P = 0.0034), whereas MP had no significant effect on E of stems of Hybrid 11-11 (P = 0.2720). Differences in flexure response between the hybrids suggest that Hybrid 47-174 can produce a stem that is more tolerant of wind-induced flexure by altering both stem allometry and material properties, whereas Hybrid 11-11 relies solely on changes in stem allometry for enhanced stability under MP conditions.

Effects of trunk girdling on seasonal patterns of xylem water status, water transport and woody tissue metabolic properties were investigated in ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) trees. At the onset of summer, there was a sharp decrease in stomatal conductance (g(s)) in girdled trees followed by a full recovery after the first major rainfall in September. Eliminating the root as a carbohydrate sink by girdling induced a rapid reversible reduction in g(s). Respiratory potential (a laboratory measure of tissue-level respiration) increased above the girdle (branches and upper trunk) and decreased below the girdle (lower trunk and roots) relative to control trees during the growing season, but the effect was reversed after the first major rainfall. The increase in branch respiratory potential induced by girdling suggests that the decrease in g(s) was caused by the accumulation of carbohydrates above the girdle, which is consistent with an observed increase in leaf mass per area in the girdled trees. Trunk girdling did not affect native xylem embolism or xylem conductivity. Both treated and control trunks experienced loss of xylem conductivity ranging from 10% in spring to 30% in summer. Girdling reduced xylem growth and sapwood to leaf area ratio, which in turn reduced branch leaf specific conductivity (LSC). The girdling-induced reductions in g(s) and transpiration were associated with a decrease in leaf hydraulic conductance. Two years after girdling, when root-to-shoot phloem continuity had been restored, girdled trees had a reduced density of new wood, which increased xylem conductivity and whole-tree LSC, but also vulnerability to embolism.

Our primary objective was to present and test a new technique for in vitro estimation of respiration of cores taken from old trees to determine respiratory trends in sapwood. Our secondary objective was to quantify effects of tree age and stem position on respiratory potential (rate of CO2 production of woody tissue under standardized laboratory conditions). We extracted cores from one to four vertical positions in boles of +200-, +50- and +15-year-old Pinus ponderosa Dougl. ex Laws. trees. Cores were divided into five segments corresponding to radial depths of inner bark; outer, middle and inner sapwood; and heartwood. Data suggested that core segment CO2 production was an indicator of its respiratory activity, and that potential artifacts caused by wounding and extraction were minimal. On a dry mass basis, respiratory potential of inner bark was 3-15 times greater than that of sapwood at all heights for all ages (P < 0.0001). Within sapwood at all heights and in all ages of trees, outer sapwood had a 30-60% higher respiratory potential than middle or inner sapwood (P < 0.005). Heartwood had only 2-10% of the respiratory potential of outer sapwood. For all ages of trees, sapwood rings produced in the same calendar year released over 50% more CO2 at treetops than at bases (P < 0.0001). When scaled to the whole-tree level on a sapwood volume basis, sapwood of younger trees had higher respiratory potential than sapwood of older trees. In contrast, the trend was reversed when using the outer-bark surface area of stems as a basis for comparing respiratory potential. The differences observed in respiratory potential calculated on a core dry mass, sapwood volume, or outer-bark surface area basis clearly demonstrate that the resulting trends within and among trees are determined by the way in which the data are expressed. Although these data are based on core segments rather than in vivo measurements, we conclude that the relative differences are probably valid even if the absolute differences are not.

Summary• A technique for measuring in vitro respiration was investigated to understand why rates were higher than those reported in vivo and to elucidate trends within mature Pseudotsuga menziesii (Douglas-fir) trees.• Extracted increment cores were divided into 3 -4 radial depths and a gas chromatograph was used to compare respiration rates radially and vertically within stems.• Respiration of inner bark was 2-3 times greater than sapwood, and 50 -70% higher in outer than inner sapwood. Inner bark and outer sapwood released > 40% more CO 2 at treetops than at bases. Trends were robust for CO 2 production on a core dry-mass, volume, or total carbon basis. By contrast, CO 2 production on a nitrogen basis showed almost no significant variation.• This in vitro technique provided an effective index for relative differences in respiration within tree stems. Discrepancies between in vitro and in vivo measurements might be related to the gaseous environment in stems. The estimated within-stem gradients in respiration were possibly determined by enzyme quantity and availability and could be useful in scaling to whole-trees.