Different xylogenesis responses to atmospheric water demand contribute to species coexistence in a mixed pine–oak forest

Seasonal patterns of wood formation(xylogenesis)remain understudied in mixed pine-oak forests despite their contribution to tree coexistence through temporal niche complementarity.Xylogenesis was assessed in three pine species(Pinus cembroides,Pinus leiophylla,Pinus engelmannii)and one oak(Quercus grisea)coexisting in a semi-arid Mexican forest.The main xylogenesis phases(production of cambium cells,radial enlargement,cell-wall thickening and maturation)were related to climate data considering 5-15-day temporal windows.In pines,cambium activity maximized from mid-March to April as temperature and evaporation increased,whereas cell radial enlargement peaked from April to May and was constrained by high evaporation and low precipitation.Cell-wall thickening peaked from June to July and in August-September as maximum temperature and vapour pressure deficit(VPD)increased.Maturation of earlywood and latewood tracheids occurred in May-June and June-July,enhanced by high minimum temperatures and VPD in P.engelmannii and P.leiophylla.In oak,cambial onset started in March,constrained by high minimum temperatures,and vessel radial enlargement and radial increment maximized in April as temperatures and evaporation increased,whereas early wood vessels matured from May to June as VPD increased.Overall,15-day wet conditions enhanced cell radial enlargement in P.leiophylla and P.engelmannii,whereas early-summer high 15-day temperature and VPD drove cell-wall thickening in P.cembroides.Warm night conditions and high evaporation rates during spring and summer enhanced growth.An earlier growth peak in oak and a higher responsiveness to spring-summer water demand in pines contributed to their coexistence.

Evidence of Hydropassive Movement in Stomatal Oscillations of Glycyrrhiza inflata under Desert Conditions

Stomatal conductance was found to change from steady-state to a slate of oscillations during daytime when vapour pressure deficit (VPD) increased to a value of 1 kPa in Glycyrrhiza inflata Batalin grown under the conditions of arid desert in north-west China. The injected metabolic inhibitors (NaN3 or carbonyl cyanide-m-chlorophenyl-hydrazone (CCCP)) slightly reduced the stomatal conductance but did not significantly decrease the intensity of stomatal oscillations (amplitude/average). The oscillation intensity was found to he significantly correlated with VPD and root resistance, but not with the respiration rate. There might exist a minimum threshold of VPD (0.8 kPa) and root resistance (1/4 relative value) that induced stomatal oscillations. These results suggested that stomatal oscillations induced by atmospheric drought stress and root resistance were mainly a type of hydropassive movement.

Regulation of the water status in three co-occurring phreatophytes at the southern fringe of the Taklamakan Desert

Aims We investigated the regulation of the water status in three predominant perennial C3 phreatophytes(Alhagi sparsifolia,Populus euphratica,Tamarix ramosissima)at typical sites of their occurrence at the southern fringe of the hyperarid Taklamakan Desert(north-west China).Methods In the foreland of the river oasis of Qira(Cele),we determined meteorological variables,plant biomass production,plant water potentials(WL)and the water flux through the plants.We calculated the hydraulic conductance on the flow path from the soil to the leaves(kSL)and tested the effects of kSL,WL and the leaf-to-air difference in the partial pressure of water vapour(Dw)on stomatal regulation using regression analyses.Important Findings Despite high values of plant water potential at the point of turgor loss,all plants sustained WL at levels that were high enough to maintain transpiration throughout the growing season.In A.sparsifolia,stomatal resistance(rs;related to leaf area or leaf mass)was most closely correlated with kSL;whereas in P.euphratica,~70%of the variation in rs was explained by Dw.In T.ramosissima,leaf area-related rs was significantly correlated with WL and kSL.The regulation mechanisms are in accordance with the growth patterns and the occurrence of the species in relation to their distance to the ground water.