Effects of environmental factors on gas exchange in deciduous trees: focus on photosynthetic water-use efficiency
Date
2015-07-06
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Abstract
Keskkonnatingimused on ajas pidevas muutumises, mistõttu käesoleva sajandi jooksul atmosfääri CO2 kontsentratsioon ja Maa keskmine temperatuur tõenäoliselt tõusevad. Samuti prognoositakse ekstreemsete ilmastikuolude sagenemist. Regionaalses skaalas ennustatakse Põhja-Euroopas vihmasadude sagenemist ja suuremat õhuniiskust. Samas puudub meil teadmine, kuidas mõjutab suurenev õhuniiskus puude kasvu ja metsaökosüsteemide seisundit. Selle tühimiku täitmiseks on Ida-Eestis käivitatud Metsaökosüsteemi Õhuniiskusega Manipuleerimise Eksperiment (ingl. FAHM), kus uuritakse kasvava õhuniiskuse mõju lehtmetsale. Käesolevas töös analüüsitakse, kuidas mõjutab kõrgemas õhuniiskuses kasvamine kahe puuliigi arukase ja hübriidhaava lehe tasemel toimuvaid protsesse ja seeläbi puude juurdekasvu. Selgub, et õhuniiskuse mõju sõltub aastast ehk siis kasvuperioodil valitsevatest keskkonna-, eriti just ilmastikutingimustest. Niiskemal aastal mõjutab suurenenud õhuniiskus puu mineraalset toitumist ja vähenenud toitainete omastamise tõttu jääb ka puude kasv kõrgemas õhuniiskuses väiksemaks. Kuival suvel leevandab õhuniiskuse tõus põua mõju puudele ja seeläbi nende juurdekasv pigem suureneb. Läbiviidud eksperimendid näitasid, et kohanemine õhuniiskusega muudab lehe vedela ja gaasilise faasi juhtivusi veedefitsiidi korral ebaproportsionaalselt, mis suurendab ekstreemsete ilmastikuolude ilmnemisel niiskes õhus kasvanud puudes hüdraulilise düsfunktsiooni riski. Katsete tulemustest selgus, et kõrgema õhuniiskusega kohanenud taim on veevaeste olude ilmnedes võimeline õhulõhede avatust kiirtemini reguleerima ja see omadus peaks parandama taimede toimetulekut ebasoodsates tingimustes. Samas on kõrgema õhuniiskusega kohanemisel negatiivne mõju ja äkilise põua saabudes on niiskemas õhus kasvanud puudel suurem veetranspordi häirumise oht. Seega võivad kasvava CO2 ja temperatuuri eeldatavat positiivset mõju puude juurdekasvule tasakaalustada suureneva õhuniiskusega kaasnevad negatiivne efektid.
Environmental factors are steadily changing over time, while the atmospheric CO2 concentration and the Earth’s average temperature are predicted to increase during this century. Extreme weather events are also likely to become more frequent. On a regional scale, more frequent rainfall and higher air humidity are predicted for Northern Europe. At the same time, there is a lack of knowledge of how increasing air humidity affects forest ecosystems. To fill this gap, the Free Air Humidity Manipulation (FAHM) experimental facility has been built in Eastern Estonia in order to investigate the impact of increased air humidity on forest ecosystem functioning. In the present thesis, acclimation of leaf-level processes to higher air humidity and its ultimate effect on tree growth was studied in two deciduous tree species. Our results reveal that effects of air humidity vary among the study years, depending on prevailing weather conditions. In rainy years, the high air humidity reduces soil nutrient acquisition and therefore tree growth rate declines. In drier years, high air humidity alleviates the effects of summer drought and the trees’ growth is rather stimulated. We also found that trees acclimated to higher air humidity show disproportionate changes in sensitivity of gaseous versus liquid-phase conductance to water deficit, which may impose a great risk of hydraulic dysfunction during weather extremes. Trees grown under increased air humidity are able more effectively to adjust their stomatal conductance and this should improve their capability to cope in adverse environmental conditions. On the other hand, the leaf acclimation to higher humidity have a negative impact on tree growth in humid years and the trees show a greater risk of hydraulic failure at high atmospheric evaporative demand. Therefore the positive effect on tree growth induced by the rising CO2 concentration and temperature may be smaller due to the negative effect of co-occurring increasing air humidity.
Environmental factors are steadily changing over time, while the atmospheric CO2 concentration and the Earth’s average temperature are predicted to increase during this century. Extreme weather events are also likely to become more frequent. On a regional scale, more frequent rainfall and higher air humidity are predicted for Northern Europe. At the same time, there is a lack of knowledge of how increasing air humidity affects forest ecosystems. To fill this gap, the Free Air Humidity Manipulation (FAHM) experimental facility has been built in Eastern Estonia in order to investigate the impact of increased air humidity on forest ecosystem functioning. In the present thesis, acclimation of leaf-level processes to higher air humidity and its ultimate effect on tree growth was studied in two deciduous tree species. Our results reveal that effects of air humidity vary among the study years, depending on prevailing weather conditions. In rainy years, the high air humidity reduces soil nutrient acquisition and therefore tree growth rate declines. In drier years, high air humidity alleviates the effects of summer drought and the trees’ growth is rather stimulated. We also found that trees acclimated to higher air humidity show disproportionate changes in sensitivity of gaseous versus liquid-phase conductance to water deficit, which may impose a great risk of hydraulic dysfunction during weather extremes. Trees grown under increased air humidity are able more effectively to adjust their stomatal conductance and this should improve their capability to cope in adverse environmental conditions. On the other hand, the leaf acclimation to higher humidity have a negative impact on tree growth in humid years and the trees show a greater risk of hydraulic failure at high atmospheric evaporative demand. Therefore the positive effect on tree growth induced by the rising CO2 concentration and temperature may be smaller due to the negative effect of co-occurring increasing air humidity.
Description
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Keywords
gaasivahetus, veekasutus, fotosüntees, keskkonnategurid, keskkonnamõjud, heitlehised puud, gas exchange, water use, photosynthesis, environmental factors, environmental impacts, deciduous trees