Carbon and water relations of juvenileQuercusspecies in tall‐grass prairie

Abstract

Abstract. In ecosystems where environments are extreme, such as deserts, adult plant species may facilitate the establishment and growth of seedlings and juveniles. Because high temperatures and evaporative demand characterize tall‐grass prairies of the central United States (relative to forests), we predicted that the grassland‐forest ecotone, by minimizing temperature extremes and moderating water stress, may function to facilitate the expansion of Quercus species into undisturbed tall‐grass prairie. We assessed the carbon and water relations of juvenile Quercus macrocarpa and Q. muhlenbergii, the dominant tree species in gallery forests of northeast Kansas, in ecotone and prairie sites. To evaluate the potentially competitive effects of neighboring herbaceous biomass on these oaks, juveniles (< 0.5 m tall) of both species also were subjected to either: (1) removal of surrounding above‐ground herbaceous biomass, or (2) control (prairie community intact) treatments. Herbaceous biomass removal had no significant effect on gas exchange or water relations in these oak species in either the prairie or the ecotone environment. Although the ecotone did alleviate some environmental extremes, photosynthetic rates and stomatal conductance were ca. 20 % higher (p < 0.05) in both oaks in prairie sites vs. the ecotone. Moreover, although leaf temperatures on average were higher in oaks in the prairie, high leaf temperatures in the ecotone had a greater negative effect on photosynthesis. These data suggest that the grassland‐forest ecotone did not facilitate the growth of Quercus juveniles expanding into this grassland. Moreover, the carbon and water relations of juvenile oaks in the prairie appeared to be unaffected by the presence of the dominant C4 grasses.

The role of photosynthesis in the recruitment of juvenile Quercus gambelii into mature Q. gambelii communities

“…The average value below the canopy at our site during the monsoon was 40 6 46 mmol m 22 sec 21 and after the monsoon it was 124 6 109 mmol m 22 sec 21 which was at the lower end of the range found below the P. ponderosa community (Naumburg and De-Wald 1999). In a prairie areas adjacent to a Q. macrocarpa (bur oak) and Q. muhlenbergii (Chinquapin oak) gallery forest, the daily range of PFD (,240 to 1900 mmol m 22 sec 21 ) was similar to the daily range found in our open grassland community during the monsoon period (237 to 1769 mmol m 22 sec 21 ); while the light reduction in the ecotone between the grassland and Quercus gallery forest was dependent on the time of day, but was as much as 50% of that in the grassland (Danner and Knapp 2001b). Studies of Q. rubra (northern red oak) showed light gradients from 2.6% of full sunlight under the canopy to 69% full sunlight in small associate clearings (Kaelke et al 2001).…”

Abiotic constraints on the establishment ofQuercusseedlings in grassland

“…Mean (+1 SD) PFD values at 10 cm above ground level in all plots combined for the growing season were significantly altered by the treatments (control 806 + 310 mmol m À2 s À1 ; grass removal 1468 + 266 mmol m À2 s À1 ; shade 987 + 238 mmol m À2 s À1 ). Mean PFD in the shade treatment was similar to the mean PFD along the prairie-forest ecotone (881 + 47 mmol m À2 s À1 ; Danner & Knapp, 2001a). The shelter was open-sided to minimize microclimatic alterations, however, there was a significant effect of both shade and grass removal on spatially averaged canopy temperatures (control < shade < grass removal; range 2±13 8C) measured with an infrared thermometer (Cole Palmer Instrument Co., Vernon Hills, Illinois, USA).…”

“…In the shade treatment, aboveground grass and herbaceous biomass were removed and an opensided shelter with fiberglass screen at the top was placed over the plot to simulate the lower light environment and reduced grass and herbaceous cover of the prairie-forest ecotone (Bragg et al, 1993;Greenlee & Callaway, 1996;Danner & Knapp, 2001a). Shoot emergence was observed in five to seven plots within each block by May for a total of 18 plots.…”

“…To assess the impacts of high PFD and leaf temperature on seedling success, one of three experimental treatments was randomly assigned to the plots within each block: (1) plant community intact (control), (2) grass removal (all aboveground grass and herbaceous biomass in the plot removed by harvesting aboveground tissue), and (3) shade. In the shade treatment, aboveground grass and herbaceous biomass were removed and an opensided shelter with fiberglass screen at the top was placed over the plot to simulate the lower light environment and reduced grass and herbaceous cover of the prairie-forest ecotone (Bragg et al, 1993;Greenlee & Callaway, 1996;Danner & Knapp, 2001a). The shelter consisted of a 1 Â 1 Â 0.75 m 3 PVC frame, and the shade screen decreased PFD by ca.…”

Survival and growth of three endangered oak species in a Mexican montane cloud forest

“…It is likely that the larger acorn in Q. insignis provided greater reserves and allowed more rapid early growth, while the smaller acorns of Q. sartorii and Q. xalapensis likely provide lower reserves for early growth, as it has been docuAverage a) height and b) root collar diameter growth rates registered in three Quercus seedling species under two different environments: light gap and closed canopy during the experimental period mented for other Quercus in America (Danner & Knapp 2001).…”