Dendrochronology to understand the place and the role of disturbances in community functioning

Today, it is recognized that ecosystem heterogeneity and dynamics are undergone by disturbances and it appears essential to understand the place and the role of disturbances in the functioning of communities whatever their origin (biotic, climatic or anthropic).

By its temporal dimension with an annual resolution and the possibility to assess the magnitude of various disturbances, dendrochronology appears as a tool which cannot be overlooked in ecology. Whereas it is actually poorly used, dendrochronology seems a very promising tool to understand functional mechanisms and replace systems in spatio-temporal dynamic processes.

Based on the study of tree-rings annually formed by ligneous species, dendrochronology aims at identifying original ring patterns and correlating them to specific events (disturbances). Once external factors responsible of this specific events are identified and correlated to ring series patterns, dendrochronology explores present and past interactions. Then documenting and reconstructing history of various habitats become possible. Dendrochronology makes it possible to understand functional mechanisms and to replace disturbances in dynamic processes.

The research I did during my PhD and as a participant to various research programs deals with the capability of dendrochronology in the study of the role and the place of disturbances.

Disturbances of biotic origin

The study of such a disturbance was done in my PhD (1998-2002) included in a pluridisciplinary project (Forest ecology, forest renewal and introduced species in Haida Gwaii) of the Research Group on Introduced Species (http://www.rgisbc.com). The goal is assessing consequences of several species introduction in Haida Gwaii (British Columbia, Canada) among which Black-tailed deer (Odocoileus hemionus sitkensis Merriam) in the late 19^th century and early 20^th century.

My PhD work was about identifying, assessing and reconstructing the history of a biotic disturbance, the phytophagy induced by a large mammifere introduced on an island ecosystem, the Haida Gwaii archipelago (British Colombia, Canada).

Photo 1 : Dense understory of salal (Gaultheria shallon Pursh) on a deer-free island.

The objective was to precise fundamental mechanisms driving phytophageous-ligneous interactions and to use disturbance created by phytophagy to understand its place and role in functional mecanisms.

Photo 2 : The black-tailed deer (Odocoileus hemionus sitkensis Merriam).

Using a dendroecological approach which is a retrospective method able at the same time to assess past and current consequences, it was possible to better understand the role of phytophagy on dynamics that is to say on the functioning of the community.

Photo 3 : Deer exclosure situated in the south of the archipelago showing the destruction of shrubs and regenerating tree understory under deer browsing pressure.

By associating to the dendrochronological approach an analysis of architectural plant modification induced by browsing pressure and an analysis of plant chemical compounds which influence food choice by the phytophageous, I evidenced a cessation in shrub recruitment and an alteration in tree species recruitment ruled by physical defences or by selection of individuals capable of chemical defences. In another hand, by developing a proper methodology, I could estimate growth deficit and precise for each regenerating species the delay due to browsing pressure.

Photo 4 : Young regenerating trees heavily browsed showing a characteristic shape, compact and ramified under the browse line and normal above it.

Photo 5 : The shrub understory is replaced by mosses.

These results evidence disproportionate changes between species triggered by deer ; that induce changes in forest dynamics and in the long term that may result in changing the composition of the forest. Combining and developing various analyses on shrub understory, I also had an historical approach of the disturbance. This approach shows that although deer colonisation was rapid across the archipelago, spatial and temporal differences did occur. They are linked to geographical, anthropic and climatic factors.

Photo 6 : Fraying caused by deer on thuya (Thuya plicata D. Donn. Ex Lamb).

Photo 7 : Fraying scar on a willow showing wood discoloration and compartmentalisation which aims at stopping pathogens spread.

Finally, dating obtained by dendrochronology offered a temporal framework to interprete data of comparative studies conducted on fauna and flora. Besides interest of such an approach to understand the functioning of interactions inside ecosystems, this study offers real opportunity for natural resources management.