Phenological Studies Of Evergreen Broadleaf Quercus Pannosa In The Upper And Lower Range Limits In Hengduan Mountains

Exploring the establishment processes of species distributions and deeply understanding the underlying mechanisms that biotic and abiotic factors drive and influence such boundaries has always been one of the core issues in evolutionary biology.In the past decades,the important role of the key phenological events of plants in shaping distribution ranges of species has attracted extensive attentions and in-depth reports from researchers.The process-based phenological research has become a new method to solve the puzzles in the distribution ranges of species,and has achieved remarkable progresses and breakthroughs.In some special natural habitats,however,the studies on the relationships between the phenology of some important plant species and their distribution ranges are still very scarce,some of w-hich are even blank,including evergreen broad-leaved tree species in temperate East Asia and the upper and lower limits of the distribution of these critical tree species.In this study,Quercus pannosa,a typical evergreen broad-leaved tree species in Baima Snow Mountains,Hengduan Mountains,was investigated during 2016 to 2018.Via high-frequency phenological observations,growth data measurements and combining climatic data in situ,we properly studied and discussed the potential relationships between the development patterns of phenology,the growth dynamics of vegetative organs(shoots and leaves)and environmental characteristics such as temperature and precipitation in the upper and lower limits of Q.pannosa,contrasting with the middle of Q.pannosa population.Main results and conclusions are summarized as follows:(1)In the upper limit of Q.pannosa population(4270m a.s.1).the growth of Q.pannosa trees was significantly restricted by low temperature.The lengths of shoots and leaves were the smallest among three experimental sites as well as the specific leaf area(SLA)values,but the longevity of leaf was the longest there.O.pannosa trees experienced a rather short growing season in the upper limit,only 150 days.The onset of spring phenology was around June 25 in every year,which may be controlled by photoperiod.And the shoot phenology belonged to the single flush type.Because there was less time for maturation,tiny apical leaves would be forced to stop growing immediately and enter the maturation stage,even though the sizes of these apical leaves were just half of other normal leaves.It could be an adaptation of Q.pannosa to balance their biomass with low temperature,short growing season in high elevation.(2)In the middle of Q.pannosa population(3443m a.s.1),the onset of bud opening was the earliest(nearly on May 13)comparing to another two experimental sites,which may be controlled by temperature in spring.The shoots showed a single flush type,too.Due to optimal climate conditions and a long physiological growing season there,the lengths of both shoots and leaves were much longer than other individuals in the upper or lower range limits.Although the leaf area was very large in this area,the values of the specific leaf area were very close to the upper limit,which was probably owing to nearly half of the growing season and a large amount of resources were invested in the maturation stage of new leaves,namely a high construction cost for ensuring that leaves can obtain a high resistance for environmental stress.(3)Iin the lower limit of Q.pannosa population(2420m a.s.l),the onset of bud opening was largely determined by rainfall,and the growth processes of vegetative organs were highly consistent with the timing of monsoon.The time of bud opening of Q.pannosa varied with the onset of rainy season and the intensity of precipitation,therefore the bud opening has demonstrated a high plasticity.The phenological pattern of shoots belonged to a single flush type.And contrasting with the other two sites.Q.pannosa trees living in this habitat reached the highest SLA value but with the shortest longevity of leaf.The results showed that the phenological characteristics and growth patterns of evergreen broad-leaved tree species represented by Q.panno.s'a were quite different in different natural habitats.which reflected the high adaptability of plants to the dominant environmental stress factors in habitats,and also proved that plant phenology played a key role in determining the distribution ranges of plant species.