Responses Of Productivity In Subtropical Forests To Nocturnal Warming And The Physiological Mechanisms

Vegetation productivity is the largest carbon（C）flux in the terrestrial C cycle,playing a key role in mitigating the rise of atmospheric CO₂ concentration and climate warming.Forest ecosystem in subtropical regions has a great potential in C sink and is intensively distributed in China.The effects of climate warming on vegetation productivity have been attached great attentions.However,these studies mainly focus on the average temperature.The non-uniform climate warming at the diurnal scale and its effects on vegetation productivity are neglected.One feature of climate warming is the greater increase in nighttime temperature than daytime temperature.It is unclear whether and how nocturnal warming affects vegetation productivity in subtropical forests.To address this issue,this study firstly analyzed the characteristics of diurnal temperature change in subtropical regions from 1951-2020.Then,this study used GPP（Gross primary productivity）and climate data to analyze the effects of nocturnal warming on GPP in subtropical forests from 1982-2016,highlighting the regulation of soil moisture on responses of GPP to nocturnal warming in subtropical forests.To further explore the physiological mechanism of responses of productivity in subtropical forests to nocturnal warming,this study chose two widely-distributed subtropical plants and conducted a greenhouse potted experiment,then analyzed the effect of nocturnal warming on photosynthesis in subtropical trees and the regulation mechanism.Finally,this study set temperature and moisture gradient in another potted experiment and further explored the regulation of drought on photosynthesis responses to nighttime warming in subtropical trees.The main results are as follows:（1）The diurnal asymmetry in climate warming is widespread in the Northern Hemisphere.The nighttime temperature rose faster than the daytime temperature.In the subtropical regions of the Northern Hemisphere,the increase in the nighttime temperature was 0.5°C decade^-1,and the decrease in the diurnal temperature range was 0.08°C decade^-1.Among the plant functional types in the subtropical regions,the magnitude of non-uniform climate warming was different.In the forest of subtropical regions,the increase in the nighttime temperature was 0.51°C decade^-1,only lower than grassland and shrubland.The decrease in the diurnal temperature range was 0.11°C decade^-1,higher than other plant functional types except for wetland.This implied that the subtropical forest experienced nocturnal warming.（2）By analyzing climate and GPP data,the negative effects of nocturnal warming on GPP were larger than the positive effects.The positive effects of nocturnal warming on GPP were mainly distributed in 0-15°C of mean average temperature,which was lower than the mean average temperature in subtropical regions.Aimed at forest ecosystems,the proportion of significant negative effects was larger than positive effects,also in the subtropical forest ecosystem.By analyzing the temporal trend in the effects of nocturnal warming on GPP,this study found the positive warming effects on GPP in the subtropical forest were shifted to negative effects,which were gradually strengthened.Soil moisture regulated the responses of GPP to nocturnal warming in the subtropical forest ecosystem.The negative effects were increased with soil moisture decreasing when it was lower than0.3 m³ m^-3.（3）A reduction in leaf photosynthesis for S.superba and C.sclerophylla was observed,although the photosynthetic rate in S.superba was increased at the beginning of the experiment.The increase in photosynthetic rates in S.superba was related to the transient increase in nighttime respiration.Owing to the fast acclimation of nighttime respiration to nocturnal warming,the increase in photosynthesis was transient.In the long-term,nocturnal warming increased leaf-to-air vapor pressure deficit,which increased water loss and downregulated stomatal conductance,eventually inhibiting photosynthesis.Hence,stomatal conductance was an important trait in regulating photosynthesis under nocturnal warming.The water loss induced by nocturnal warming was the key limiting photosynthesis in subtropical plants.（4）Drought did not alter photosynthesis in two species under nocturnal warming at the beginning of the experiment.However,persistent drought treatment significantly exacerbated the negative effects of nocturnal warming on photosynthesis.On the one hand,drought significantly downregulated daytime stomatal conductance and transpiration in two species under nocturnal warming,which exacerbated the reduction in photosynthesis.On the other hand,drought significantly reduced respiratory rates in C.sclerophylla.The increased leaf starch concentration weakened the source-sink regulation of photosynthesis.The differentiated responses of S.superba and C.sclerophylla to nocturnal warming were due to their different water-use strategy.Hence,drought altered photosynthetic responses to nocturnal warming by regulating stomatal conductance.To sum up,this study found the subtropical regions in the Northern Hemisphere experienced nocturnal warming since the middle of the 20^th century,which reduced gross primary productivity in the subtropical forests.The negative effects of nocturnal warming on gross primary productivity in subtropical forests were strengthened over the last three decades.It was related to decreasing trend in soil moisture.The negative effects of nocturnal warming on photosynthesis in subtropical trees were mainly owing to increased water loss and concomitant stomatal closure.Extreme drought exacerbated the negative effects of nocturnal warming on photosynthesis.Drought inhibited photosynthesis mainly by regulating responses of stomatal conductance to nocturnal warming.These results showed that GPP in subtropical forests faced dual threats from nocturnal warming and drought.This exacerbated the positive feedback between terrestrial ecosystems and climate changes.