The Effects Of Population Dynamics On Maintaining Tree Distribution Patterns And Diversity

Species distribution patterns and diversity are central topics in community ecology and macroecology.Demographic strategies represent a series of adaptive tactics that species have formed through long-term coupling with environments,such as demographic compensation(opposite responses among vital rates such as growth,survival,and recruitment along the same environmental gradient)or demographic buffering(low temporal variability of vital rates).These strategies enable species to reduce their sensitivity to environmental changes and persist under different environmental conditions.However,we know little about how population dynamics affect species distribution patterns and diversity through demographic strategies at large spatial scales and over long time series.To clarify the role of plant population dynamics in maintaining species distribution patterns and diversity,this study uses forest monitoring data at continental scales and over long time series,and employs research methods such as integral projection models to study the spatiotemporal patterns and dynamic mechanisms of tree population dynamics.Furthermore,the study analyzes the role of demographic compensation in maintaining tree species distributional boundaries and the role of demographic buffering in maintaining tree species diversity.The main results are as follows:(1)Population dynamics and maintenance mechanisms of marginal tree populations.The dynamics of edge populations are primarily driven by the mean warmest month temperature and the survival of large-diameter individuals.Almost half of the 81-tree species’edge populations maintained a stable state,with population growth rate(λ)close to 1.The spatial variation inλacross tree species’ranges was best explained by the variation in survival rate(61%).The survival of individuals,particularly those with large diameters,was crucial to the persistence of edge populations.The sensitivity ofλof edge populations to climate was higher than that of local competition for both southern and northern distribution edge populations,with the highest sensitivity to the mean warmest month temperature,which does not support the species interactions-abiotic stress hypothesis.(2)The role of demographic compensation in stabilising marginal tree populations.The occurrence of demographic compensation within the 81-tree species is infrequent,and if present,is insufficient to rescue marginal tree populations.When negative correlations between contributions of vital rates were detected,they mostly involved the size distribution of recruits.While demographic compensation reduces the spatial variation inλeffectively,some species still exhibit a potential retraction from their southern edges.Moreover,simulated climatic warming caused a significant decline inλfor most species and weakened the effectiveness of demographic compensation in stabilizing their ranges.(3)Population dynamics and maintenance mechanisms of tree species in a fluctuating environment.The population dynamics in fluctuating environments are primarily driven by the temporal variability of dry-season precipitation and individual survival rate.The magnitude ofλtemporal variation of the 208-tree species is related to their abundance,with higher abundance corresponding to lower variation.While the survival of individuals was crucial to theλ,the temporal variation inλwas best explained by the number of recruits,accounting for 64%of the variation.Both temperature and precipitation variability had significant effects onλ,but increased variability in dry season precipitation reducedλof most tree species,indicating that dry season precipitation variability is the primary environmental factor limiting population growth of tree species.(4)Differently buffered or labile life histories promote tree diversity.Demographic buffering effects vary among the 208-tree species and promote the maintenance of species diversity.Tree species with longer lifespans tend to have higher buffering effects,while those with lower abundance or stronger recovery abilities are prone to have lower buffering effects.The inter-species differences in buffering effects reduced the differences in long-term population growth rates(λ_s),indicating that inter-species differences in demographic buffering effects promote tree species diversity maintenance.However,the increase in temperature and precipitation variability leads to a decrease in theλ_s of most tree species and an increase in the inter-species differences inλ_s,which has a negative impact on diversity maintenance.In conclusion,demographic compensation is uncommon to occur in distribution ranges of tree species,and thermal stress may outweigh the buffering effect of compensatory changes.Demographic buffering,on the other hand,promotes the maintenance of tree species diversity,but its effectiveness weakens with increasing climate variability.The study serves as a cautionary note on the growing demographic vulnerability of forests with increasing climate change,offering crucial scientific references for the conservation and management of biodiversity in the context of global change.