| Regeneration from seed is one of the most critical stages in the lifecycle of a plant.The regeneration phase need to overcome high mortality rates,due to biotic and abiotic stressors,operating during seed germination,seedling emergence and seedling establishment,to be able to successfully recruited into larger size classes.Successful regeneration affects community characteristics such as species diversity,abundance and distribution.Temperature and water are two key abiotic factors that determine regeneration.Global climate change is modifying both of these environmental cues,resulting in dryer warmer conditions in tropical and subtropical areas.This trend has become more evident in China.Therefore,a greater understanding of how the regeneration response is affected by water and temperature and which traits could be predictive can improve our understanding of climate resilience and adaptation.Research about what factors and which plant traits are directly associated with early regeneration is mostly focused on economically valuable species in horticulture and agriculture or the dominant species in ecological communities.The genus Ficus is important for conservation of forest function and structure as they are keystone species that provide significant ecosystem services.Some Ficus tree forms start their life cycle as hemiepiphytes where they begin as an epiphyte and later extend aerial roots to make connections with the ground.Hemiepiphytes are known to be better adapted to canopy growth and display a more drought resilient trait cluster,with studies mainly focused on leaf photosynthesis and plant water use efficiency.However,no studies have focused on assessing temperature and water stress at the seed germination,seedling emergence and seedling recruitment processes that are the earliest stages of regeneration,especially in an empirical,comparative setting.Therefore,this thesis,using 15 Ficus species--six hemiepiphytic and nine non-hemiepiphytic species--from south china,aimed at addressing two main knowledge gaps;1.assessing the differences between the regeneration responses of congeneric,hemiepiphytic and non-hemiepiphytic Ficus species to variation in temperature and water availability,and 2.assessing a plant trait-based framework for predicting regeneration responses in two divergent growth forms,i.e.hemiepiphytic and non-hemiepiphytic species.First,the effect of temperature(15/5°C,25/15°C,35/25°C;day/night diurnal variation),water availability as substrate moisture(0.00 MPa,-0.20 MPa,-0.35 MPa)and water vapor(18.5% to 99.5% relative humidity),and their interaction with growth form on seed germination,seedling emergence and survival was tested.Under conditions of high temperature(35/25°C),hemiepiphytes had higher gemination and seedling survival,achieved peak germination slower and extended germination.Greater water stress(-0.35 MPa)in growth substrate resulted in higher germination of non-hemiepiphytes,while hemiepiphytes took shorter time to complete germination,but had higher seedling emergence and survival.Hemiepiphytes germinated at 99.5% relative humidity more readily compared to non-hemiepiphytes.These findings on the response to temperature and water stress provide the first comprehensive evidence that hemiepiphytic Ficus species are better adapted to drier and warmer conditions during the critical transition from seed to seedling,through greater flexibility in achieving peak germination and duration of regeneration activity,hemiepiphytes modulated their recruitment process to be more resilient to temperature and water stress.This may allow them to be more successful in regenerating under ambient conditions that are more transient in forest canopies.Second,the ability of eight seed traits to predict seed water movement rate(WMR)and regeneration success was assessed using the same 15 Ficus species.The seed traits assessed were: seed weight,moisture content,seed length,width and coat thickness,and traits associated with seed water gap structure(WGS),i.e.WGS thickness(WGST),and length and width change during the initial imbibition period(WGSLC and WGSWC,respectively).The WMR slowed with time and was greater for hemiepiphytic species with a thicker WGS and for non-hemiepiphytes that showed a greater change in the width of the WGS.At the 99.5% RH condition,hemiepiphytes showed higher germination,which decreased with greater WGSLC and seed coat thickness.Higher hemiepiphytic germination was predicted at the 18.5% RH condition.Nonhemiepiphytic germination was reduced in species with greater seed coat thickness and increased with seed length,but had no relationship with WGSLC or WGSWC.Seedling emergence was higher in hemiepiphytes but only under high RH(99.5%)and increased with greater WGSLC.This study identified the complex water gap as an important mechanical and functional structure that modulates water entry and affect germination,in both hemiepiphytic and nonhemiepiphytic species,but with contrasting differences.Hemiepiphytes modulated their regeneration response when water was limited,as well as in the absence of water stress,by overcoming seed structural barriers such as longer WGS and thicker seed coats.These findings provide novel insights into how seed functional and structural traits predict the regeneration response in the divergent hemiepiphytic and non-hemiepiphytic growth forms in the genus Ficus.Above all,this thesis reveals the regeneration response differences to water and temperature stress between hemiepiphytic and non-hemiepiphytic Ficus species during the critical transition from seed to seedling,and found that trait coordination in the water gap structure and,seed length and width,provides a mechanistic understanding of recruitment differences between the two growth forms.These findings help to improve our ecophysiological knowledge of the regeneration ecophysiology response to climate change in the genus Ficus and provide an important scientific basis for exploring the evolution of Ficus and the dynamic changes of tropical and subtropical forests. |
PDF Full Text Request