Picea crassifolia and P.wilsonii,commonly used for afforestation in northern China,are increasingly likely to be subjected to high temperatures and soil drought stress as a result of global warming.However,little is known about the effects of these stresses on foliar photosynthesis in the two species.To investigate how photosynthetic characteristics and sensitivity respond to prolonged high temperatures and soil drought,foliar gas exchange and other closely related parameters were recorded from four-year-old seedlings of both species.Seedlings were grown under two temperature treatments(25/15 and 35/25 °C) and four soil water regimes [80,60,40 and 20% of maximum field capacity(FC)] for 4 months.Although all treatments significantly reduced photosynthetic rates(Pn) of both species,P.crassifolia exhibited greater photosynthetic acclimation than P.wilsonii.Differences in photosynthetic acclimation were mainly related to variations in stomatal conductance(Cond) and the maximum quantum yield of PSII(Fv/Fm) between treatments.Indeed,higher Cond and Fv/Fmin all treatments were shown for P.crassifolia than for P.wilsonii.Moreover,photosynthesis in P.crassifolia exhibited inherently lower temperature sensitivities(broader span for the temperature response curves; lower b) and higher thermostability(invariable b between treatments).Further,severe drought stress(20% FC) limited the survival of P.wilsonii.Our results indicate that P.wilsonii is more susceptible to high temperatures and soil drought stress.Planting P.crassifolia would be more expected to survive these conditions and hence be of greater benefit to forest stability if predicted increases in drought and temperature in northern China occur.
Aims With a close association with plant water availability,foliar δ^(13)C had been investigated extensively in alpine regions;however,foliar δ^(15)N has rarely been concurrently used as an indicator of plant nitrogen availability.Due to the positive correlations between leaf nitrogen con-tent and foliar δ^(13)C andδ15N found in previous studies,we expected that they should show consistent patterns along an altitudinal gradient.Methods To test our hypothesis,we measured foliar δ^(13)C andδ15N in conjunc-tion with multiple key leaf functional traits of Quercus aquifolioides,a dominant species of alpine forest on the eastern slopes of the sygera mountains,southeastern Tibetan Plateau from 2500 to 3800 m.Important findings(i)Contrary to our hypothesis,foliar δ^(13)C exhibited a significant pos-itive linear relationship with altitude;in contrast,foliarδ15N initially increased and subsequently decreased with altitude,the change in trend occurring around 3300 m.(ii)our analyses indicated that leaf internal resistance and stomatal conductance,rather than photosynthetic capacity indicated by leaf N concentration,appar-ently explained the altitudinal variation in foliar δ^(13)C,while differ-ences in foliar δ^(15)N were likely the result of soil N availability.(iii)Principal component analysis revealed a clear association between δ^(13)C and a tradeoff between water loss and carbon gain,indicated by traits related to gas exchange such as leaf thickness,density,sto-matal properties.In contrast,the second axis was associated withδ15N and nitrogen acquisition strategy in Q.aquifolioides across its altitudinal distribution,represented by traits related to nitrogen concentration and stomata per gram of leaf nitrogen.
