Tree and Stand Dynamics: Unravelling the Different Effects Of Site and Competition The impossible dream or an agenda for more research?

Outline Explore some issues and questions relating to site, competition and the development of tree height and diameter with time. Use a systems view to illustrate how we might begin to address the issues. Draw some conclusions. Focus on growth, not mortality.

Issues Needing A Better Explanation In Growth Models Weyerhaeuser effect – coastal Douglas-fir Small vs. large tree dichotomy Height repression – lodgepole pine Mixed species / uneven-aged interactions Height and diameter responses to thinning Our concepts of site index as an indicator of productivity and how it is used in defining competition are at the centre of these issues.

Is site index a biologically meaningful concept or an overly-simple, yet convenient solution to a complex problem? If it is the latter … A Fundamental Question … we have much more work to do

What is the true biological relationship between individual tree height, diameter …

… and age? How much of what we think is due to competition is due to within site, productivity variation? This is difficult to answer unless the effects of competition are explicitly accounted for in our assessment of productivity - rather than simply assumed away as is necessary to justify the notion of site index.

Tree height, including maximum tree height is best explained by diameter in combination with at least 1 additional variable – perhaps age or height to live crown.

Is height growth in fact stimulated by increased competition … up to a maximum? The challenge in answering these questions … Would such a stimulation effect operate through changes in crown characteristics? To illustrate the point lets look at some time series data. To develop a systems view using a minimum number of variables and equations.

Tree Height Development Height is strongly related to diameter. There is a maximum height given diameter. With increasing levels of competition, height growth may in fact be stimulated. Perhaps it is necessary to integrate the effects of both site and competition into one equation involving height, diameter and age.

So What? We need to develop and test a model that encompasses these phenomena. The fundamental criteria of whether or not such a model is better than ones already in existence is: - The degree to which it addresses more completely the issues and questions that were introduced at the beginning, with a minimum number of variables, parameters and functions.

The Weyerhaeuser Effect: How Might This be Explained? A system of equations ht = f(dbh, hlc) dbh = f(age, ht, hlc, site) hlc = f(ht, baph) mortality = f(baph, dbh)

dbh ht hlc tph / surv ++ ++, max ht | dbh ++ Tree and stand dynamics – a systems point of view. W site productivity + baph - - Time interactions S , self shading

dbh ht hlc tph / surv ++ ++, max ht | dbh ++ Tree and stand dynamics – an alternative systems point of view. site productivity + baph - - Time interaction S X

Conclusions “The time has come the Walrus said” to explore alternative explanations (models) of how site and competition influence tree and stand development patterns. Although not discussed – multi-level or hierarchical modelling is an important tool in this process. Based on my experience so far, finding a system of equations to complete the systems view remains a challenge.

Thank you