RESUMO
Using Granier-type thermal dissipation probes (TDP), we measured stem xylem sap flow of the natural dominant species Quercus liaotungensis and a reforestation species Robinia pseudoacacia from July to September in 2016 in the semiarid loess hilly region. Meteorological factors and soil water content were simultaneously monitored during the study period. Using cross-correlation analysis, time lag between diurnal patterns of sap flux density and vapor pressure deficit (VPD) was quantitatively estimated. Differences in the time lag between the two species and possible influence by different diameter classes and soil water contents were analyzed. The results showed that the diurnal courses of sap flux density were similar to those of meteorological factors, with daily peaks ear-lier than VPD. The peak of VPD lagged behind the sap flux densities of Q. liaotungensis and R. pseudoacacia 118.2 min and 39.5 min, respectively. The peak of PAR lagged behind the sap flux density of Q. liaotungensis 12.4 min, but was 68.5 min ahead of that for R. pseudoacacia. Time lag between sap flux density and VPD significantly varied between tree species and was affected by soil water content. Those during higher soil water content period were about 32.2 min and 68.2 min longer than those during the period with lower soil water content for the two species, respectively. There was no correlation between time lag and tree diameter classes. The time lag between VPD and sap flux density for R. pseudoacacia was about 21.4 min longer in smaller diameter trees than in larger trees, which was significantly different under the lower soil water content. Our results suggested that the time lag effect between VPD and sap flux densities in the two species reflected their sensitivities to driving factors of transpiration, and that higher soil water content was favorable to sap flux density reaching its peak early. The lower soil water content might lead to lower sensitivity of the trees to meteorological factors. R. pseudoacacia was more sensitive to changes of soil water content.
Assuntos
Transpiração Vegetal/fisiologia , Quercus/fisiologia , Robinia/fisiologia , China , Solo/química , Árvores , Água/análiseRESUMO
To examine the characteristics of sap flow in Quercus liaotungensis and their response to environmental factors under different soil moisture conditions, Granier-type thermal dissipation probes were used to measure xylem sap flow of trees with different sapwood area in a natural Q. liaotungensis forest in the loess hilly region. Solar radiation, air temperature, relative air humidity, precipitation, and soil moisture were monitored during the study period. The results showed that sap flux of Q. liaotungensis reached daily peaks earlier than solar radiation and vapor pressure deficit. The diurnal dynamics of sap flux showed a similar pattern to those of the environmental factors. Trees had larger sap flux during the period with higher soil moisture. Under the same soil moisture conditions, trees with larger diameter and sapwood areas had significantly higher sap flux than those with smaller diameter and sapwood areas. Sap flux could be fitted with vapor pressure deficit, solar radiation, and the integrated index of the two factors using exponential saturation function. Differences in the fitted curves and parameters suggested that sap flux tended to reach saturation faster under higher soil moisture. Furthermore, trees in the smaller diameter class were more sensitive to the changes of soil moisture. The ratio of daily sap flux per unit vapor pressure deficit under lower soil moisture condition to that under higher soil moisture condition was linearly correlated to sapwood area. The regressive slope in smaller diameter class was larger than that in bigger diameter class, which further indicated the higher sensitivity of trees with smaller diameter class to soil moisture. These results indicated that wider sapwood of larger diameter class provided a buffer against drought stress.
Assuntos
Transpiração Vegetal , Quercus/fisiologia , Solo/química , China , Árvores , ÁguaRESUMO
Based on the data of the field investigation and laboratory and the database of the 8th national forestry inventory, ecosystem carbon density, storage amount, and sequestration potential of tree layer were estimated for five typical plantation ecosystems (Robinia pseudoacacia, Populus spp., Pinus tabuliformis Pinus armandii, which were grouped as one kind of ecosystems, Larix principis-rupprechtii, and Picea asperata) in Gansu Province. The results showed that the average carbon density and total carbon storage of the five typical plantation ecosystems were 139.65 t·hm-2 and 85.78 Tg, respectively. Ecosystem carbon density varied among ecosystems. It followed the sequence of premature (250.70 t·hm-2) > mature (175.97 t·hm-2) > middle-aged (156.92 t·hm-2) > young (117.56 t·hm-2) forest. Meanwhile, carbon storage in these plantations ranked in the order of young (45.47 Tg) > middle-aged (19.54 Tg) > mature (11.84 Tg) > pre-mature (8.93 Tg) forest. Specifically, young and middle-aged plantations contributed the most and accounted for 75.9% of the total carbon storage. The realistic carbon sequestration potential (CPr) by tree layer of the five typical plantation ecosystems in Gansu Province was estimated as 7.27 Tg. The two largest contributors toCPr were R. pseudoacacia (2.49 Tg) and Populus spp. (2.10 Tg). Young plantations (3.78 Tg) showed the largest CPr, followed by middle-aged plantations (2.04 Tg), and the value of premature plantations (0.45 Tg) was the smallest. The maximum carbon sequestration potential (CPmax) might be up to 27.55 Tg, the CPmax with different plantations ranked in the order of R. pseudoacacia (9.42 Tg)> L. principis-rupprechtii (6.22 Tg) ≈ P. asperata (6.36 Tg) > Populus spp. (3.18 Tg) >P. tabuliformis P. armandii (2.37 Tg). The CPmax of young and middle-aged plantations was estimated as 18.48 and 6.89 Tg, respectively, which accounted for 92% of the total maximum carbon sequestration potential.
