In every simulation model aiming to predict some future developments you can prove that changing the parameter settings for independent variables (as usually done in scenario building) will create a cornucopia of simulation results with expanding width if you increase the simulation horizon. Sometimes more or less extreme scenarios are formulated to give min/max boundaries of this cornucopia, but it is still difficult (impossible?) to estimate probabilities for different scenarios resp. story lines within the boundaries. Models covering a simulation horizon from up to 2100 are of course very vague in predicting what will be, so even this limit of climate forecast data has limited validity. In my opinion long term simulation can provide valuable insights on potential trends and development tendencies, which could aid long term decisions, but even with simulation still future is in a 'dark zone'. Therefore, your idea of “risk-splitting” seems to be the logical answer to this dilemma.

What type of climate change are you considering with regard to long lived tree species.

You are probably concerned with the catastrophic human produced climate warming projected by the IPCC and Global Climate Models that have not been validated, and that have failed to match observations (no climate warming during the last 2 decades in spite of rising atmospheric concentrations of carbon containing greenhouse gases.)

One of many papers showing that CLIMATE WARMING CAUSES INCREASED ATMOSPHERIC CO2 CONCENTRATIONS suggests  that the IPCC thesis and the assumptions of Global Climate Models ----------- that that assume CLIMATE WARMING IS CAUSEDBY ATMOSPHERIC CO2 CONCENTRATIONS ----------------- are in serious error.... see: 

http://www.clim-past.net/8/1213/2012/cp-8-1213-2012.html

The IPCC thesis and the assumptions of Global Climate Models that that assume that

CLIMATE WARMING IS CAUSED BY  INCREASED ATMOSPHERIC CO2 CONCENTRATIONS ..........has been the basis for taxpayer funded and enormously misguided government policies and regulations designed to curb human emissions of CO2.

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QUOTING from the FULL TEXT of the paper (with somewhat 'iffy' English) concerning the dominance of the Sun's influence on the Earth's CYCLICAL warming and cooling ................. available at:

 http://www.doiserbia.nb.rs/Article.aspx?id=0354-98361500018A&AspxAutoDetectCookieSupport=1#.VRbMQPnF-ah

"The start of the Grand Minimum of TSI is anticipated approximately in cycle 27±1 approximately in 2043±11 and the beginning of the phase of deep cooling of the 19th Little Ice Age (of the Maunder Minimum type) in the past 7,500 years approximately in 2060±11, with possible duration of 45-65 years "

"The Sun is controlling and practically totally determining the mechanism of quasi-bicentennial cyclic alternations of climate changes from warming to Little Ice Age and set corresponding time-scales for practically all physical processes taking place in the Sun-Earth system. The current long practically stable levels of World Ocean and of temperature additionally reflects the current state of global warming during past 17 years, which are under the direct control of the quasi-bicentennial decrease of TSI. Approximately in the end of 2014 we begin the descent into 19th Little Ice Age in the past 7,500 years " 

Peter Salonius