The Holocene Ecology of Pinus sylvestris in Mountain Treelines of the Northern Cairngorms: A Synthesis of Contemporary Ecological Theory and Holocene Research

Palaeoecology is limited to indirect observation of the past, which means that research into Holocene mountain treelines in the Northern Cairngorms depends on contemporary ecology. Much of this contemporary research has not yet been applied in palaeoecology, or does not consider Holocene timescales. The resultant lack of a Holocene framework for treeline ecology is an obstacle for palaeoecological modelling, and the use of Holocene research in contemporary management. To resolve that problem, this study synthesises a framework of theory and evidence using an approach that is novel in Holocene research, based in artefactual constructivism. This adapts ecological treeline theory to the Holocene context of Pinus sylvestris in the Northern Cairngorms using temporal ecology, tree biology, and palaeoenvironmental evidence. In this framework, mountain treeline ecotones are the result of compound stresses, shelter effects, organism responsiveness to stress, and feedbacks with soil evolution. Compound stress regimes are formed by interacting factors, rather than single variables. Evidence collated in this research shows that stresses varied throughout the Holocene in compound patterns, often described by trends in oceanicity; low temperatures, precipitation, and wind intensity were especially important. These compound stresses affect the resilience of wider treeline ecosystems via their effects on the stress susceptibility and stress responses of trees. Longer term (e.g. elevation-forced) trends in stress predisposition therefore affect the consequences of shorter term (e.g. anthropogenic) stresses, and vice versa. This framework explains the Late Holocene fragmentation and retreat of mountain treelines in the Northern Cairngorms, and their earlier expansion to a Middle Holocene maximum. It also suggests that mechanisms affecting stress susceptibility were important in Pinus range dynamics throughout the Holocene, and will yet be important in long-term woodland management.