Photosynthetic performance of invasive Pinus ponderosa and Juniperus virginiana seedlings under gradual soil water depletion

Drought Tolerance and Competition in Eastern Redcedar (Juniperus virginiana) Encroachment of the Oak-Dominated Cross Timbers

On the dry, western edge of the eastern deciduous forest of the USA (Cross Timbers), the drought-tolerant, evergreen eastern redcedar (Juniperus virginiana) is encroaching into post oak- (Quercus stellata) dominated woodlands. The overall goal of this study was to examine whether the drought tolerance strategies of eastern redcedar provide it a competitive advantage over post oak and whether this is a key attribute facilitating its successful establishment in the Cross Timbers. Specifically, we assessed xylem water potential and leaf gas exchange of these two species growing in single-species stands and in a mixed-species stand. We found that both species exhibit a similar degree of isohydry and close their stomates to the same extent in response to declining xylem water potentials. Both species had similar relative reductions in gas exchange in response to drought, despite differences in xylem anatomy. However, post oak had leaf-level gas exchange rates approximately 5× greater than eastern redcedar during periods of high moisture availability. Therefore, it did not appear that eastern redcedar encroachment into an oak-dominated forest is facilitated by growing season differences in carbon gain, although evergreen eastern redcedar can conduct gas exchange year-round when conditions are favorable while post oak is deciduous. We found that volumetric soil water content (0-45 cm) was lower in the pure eastern redcedar stand than the mixed-species or pure post oak stand which may indicate that eastern redcedar may experience favorable soil moisture conditions when encroaching into open oak woodlands. Moreover, water potentials in eastern redcedar tended to be more negative in pure stands compared to the mixed stand. Our results suggest the two species may be using water from different depths, reducing competition. Overall, our findings indicate that eastern redcedar encroachment into formerly oak-dominated Cross Timbers forests likely will continue under moderate drought, in the absence of fire, with consequences for water budgets, carbon cycling, grazing forage, wildlife habitat, and wildfire risk.

Two parthenogenetic populations of Chara canescens differ in their capacity to acclimate to irradiance and salinity

The parthenogens of Chara canescens (Charophyceae) occupy broader geographical and ecological ranges than their sexual counterparts. Two possible hypotheses explain the ubiquity of parthenogens: the occurrence of one or several parthenogens with wide niches, or of many parthenogens that are restricted to narrow ecological niches. For the purposes of this study, C. canescens individuals from two neighbouring populations of the Baltic Sea (Bodstedter Bodden = BB; Salzhaff = SH), which differed significantly in water transparency and salinity, were investigated for significant differences in physiological capacity. Individuals of both habitats acclimated quickly to daily changes in irradiances in the field, but the photosynthetic efficiency of PS II showed a significant decrease with increasing daily irradiance in the habitat BB, which has lower levels of salinity and water transparency. In addition to the field study, individuals were reared under different levels of environmental factors in the laboratory: four irradiances (70-600 μmol m(-2) s(-1)) and five salinity levels (0-24 psu). The individuals of both habitats grew almost equally well at intermediate salinity levels. Growth under the artificial light supply was highest at levels corresponding to the in situ conditions for each population. Total chlorophyll was highest at intermediate salinities (BB), or hardly changed with salinity (SH). The physiological capacity for individuals from SH clearly depends upon changing growth irradiance, whereas the capacity for individuals from BB was relatively independent of salinity and irradiance. These findings indicate that both parthenogenetic C. canescens populations are locally adapted to light. However, to test adaptive potential of the parthenogens, more than two populations should be tested in future.