Functional xylem anatomy of aspen exhibits greater change due to insect defoliation than to drought.

The study of tree rings can reveal long-term records of a tree's response to the environment. This dendroecological approach, when supplemented with finer-scale observations of the xylem anatomy, can provide novel information about a tree's year-to-year anatomical and hydraulic adjustments. Here we use this method in aspen (Populus tremuloides Michx.) to identify xylem response to drought and insect defoliation. Surprisingly, we found that precipitation influenced vessel diameter mostly in the trees' youth, while this correlation was less pronounced at maturity. This is likely due to a reduction in stress the stand experiences as it ages, and reflects an ability to mediate drought stress as trees mature. Defoliation events caused consistent and profound changes in fiber anatomy likely leading to reduced structural support to vessels. We therefore expect that in years of defoliation trees may be vulnerable to drought-induced cavitation when leaf area recovers. This study highlights how the inclusion of cellular level measurements in tree ring studies provides additional information on how stress events may alter tree functioning through alterations in structure.

Growth-climate relationships vary with height along the stem in lodgepole pine.

This study tests the hypothesis that ring growth in the upper stem portion of trees is affected by climatic conditions differently than rings formed at breast height (1.3 m). A total of 389 trees from a network of 65 lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) sites in Alberta were examined using detailed stem analysis in order to examine interannual patterns of basal area increment and volume increment at different positions along the stem. Growth at lower sections of the bole was mainly driven by temperature and moisture conditions in the seasons prior to the growing season in the year of ring formation, while upper stem growth was more related to conditions during the year of growth, i.e., temperature in the early summer, or moisture in late winter to early spring. This translates into increased allocation of wood to the lower stem when prior late summer conditions are cool and wet, prior winters are mild (warm with little snow) and early summer conditions in the year of ring formation are hot and dry.

Responses of trembling aspen and hazelnut to vapor pressure deficit in a boreal deciduous forest.

The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen (Populus tremuloides Michx.) and hazelnut (Corylus cornuta Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). During the same period, daytime measurements of stomatal conductance (g(s)) and leaf water potential (Psi(leaf)) were made on these species, and sap flow was monitored in aspen stems by the heat pulse method. Weather conditions during the study period were similar to the long-term average. Despite moist soils, both species showed an inverse relationship between daytime g(s) and vapor pressure deficit (D) when D was > 0.5 kPa. Daytime Psi(leaf) was below -2 MPa in aspen and near -1.5 MPa in hazelnut, except on rainy days. These results are consistent with the hypothesis that stomatal responses are constrained by hydraulic resistance from root to leaf, and by the need to maintain Psi(leaf) above a minimum threshold value. Reductions in g(s) on sunny afternoons with elevated ambient D (maximum 2.3 kPa) were associated with a significant decrease in photosynthetic rates. However, day-to-day variation in mean carbon assimilation rate was small in both species, and appeared to be governed more by solar radiation than D. These results may be generally applicable to healthy aspen stands under normal midsummer conditions in the southern boreal forest. However, strong reductions in carbon uptake may be expected at the more extreme values of D (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.

Sap flow in trembling aspen: implications for stomatal responses to vapor pressure deficit.

We monitored sap flow and estimated diurnal changes in transpiration rates of two trembling aspen (Populus tremuloides Michx.) stands, located in the southern boreal forest and aspen parkland of Saskatchewan, Canada. In both stands during the peak growing season (June and July), sap flow during the day (0700-1700 h local time) increased linearly with vapor pressure deficit (VPD) from 0 to about 1 kPa, but then remained remarkably constant at VPD > 1 kPa (up to 4.8 kPa in the aspen parkland stand). The results imply an inverse relationship between stomatal conductance and VPD under well mixed conditions, for VPD > 1 kPa. We postulate that the stomata are operating to maintain leaf water potentials above a critical minimum value, which in turn places a maximum value on the rate of sap flow that can be sustained.

Seasonal changes in shoot regrowth potential in Calamagrostis canadensis.

A series of laboratory experiments was conducted to examine seasonal change in shoot regrowth potential following disturbance in Calamagrostis canadensis. On several dates during the 1988 and 1989 growing seasons, soil cores were collected from field sites dominated by this grass. Shoot regrowth from cores after clipping at the soil surface was monitored under dark or light laboratory conditions at 20°C. seasonal changes in field concentrations of total nonstructural carbohydrate and nitrogen in rhizomes largely accounted for the observed seasonal change in etiolated regrowth potential of shoots in laboratory experiments. In contrast, shoot regrowth potential in the light showed a very different seasonal pattern. The ratio of shoot biomass regrowth 20 d after clipping in the light versus dark treatment showed a gradual seasonal decrease from 12:1 in the early May experiment to near 1:1 in the September experiment. However, the rate of photosynthesis of regrowing shoots in the light was highest in experiments conducted late in the growing season. This may indicate a strong seasonal decrease in the proportion of current photosynthate of regrowing shoots that is allocated to new shoot growth. Alternatively, mobilization of rhizome carbohydrate reserves for shoot regrowth may have been inhibited during the re-establishment of photosynthesis in the light treatment. Either mechanism would explain why shoot regrowth in the light is poorly correlated with levels of belowground carbohydrate reserves, even under controlled laboratory conditions.