Process-Based Modeling to Assess the Effects of Recent Climatic Variation on Site Productivity and Forest Function across Western North America

Abstract

A process-based forest growth model, 3-PG (Physiological Principles Predicting Growth), parameterized with values of soil properties constrained by satellite-derived estimates of maximum leaf area index (LAI max ), was run for Douglas-fir (Pseudotsuga menziesii) to contrast the extent to which site growth potential might vary across western North America between a cool, wet period and a more recent, generally warmer and drier one (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009). LAI max represents a surrogate for overall site growth potential, as demonstrated from a strong correlation between the two variables, with the latter based on the culmination of mean annual increment estimates made at 3356 ground-based U.S. Forest Service survey plots across the states of Oregon and Washington. Results indicate that since 2000, predicted LAI max has decreased more than 20% in portions of the Southwest USA and for much of the forested area in western Alberta. Similar percentage increases in LAI max were predicted for parts of British Columbia, Idaho and Montana. The modeling analysis included an assessment of changes in seasonal constraints on gross primary production (GPP). A general reduction in limitations caused by spring frost occurred across the entire study area. This has led to a longer growing season, along with notable increases in summer evaporative demand and soil drought for much of the study area away from the maritime influence of the Pacific Ocean.

Assessment of age bias in site index equations

“…Most authors agree that observed age trend in site index estimates are the result of changing site conditions (Nothdurft et al 2012, Pretzsch et al 2012, Waring et al 2014, Yue et al 2014. However, in this paper more localized site index models developed for the Carpathians did not revealed correlations with the age of stands.…”

Does overshoot in leaf development of ponderosa pine in wet years leads to bark beetle outbreaks on fine-textured soils in drier years?

“…In western coniferous forests, LAI varies from < 1 to 12 m 2 ?m −2 (Runyon et al 1994;Waring et al 2014). If growing conditions become less favorable, the LAI will decrease, often abruptly (Pook 1984), which can result in increased tree mortality.…”

“…The soil receives a fraction of the precipitation for each month, and the soil water that is not released through evapotranspiration is added to the next month? s water balance or allowed to run off (Landsberg and Waring 1997;Waring et al 2014). ASW is defined as the difference between field capacity (soil moisture held at 1/3 bar tension) and wilting point (soil moisture held at 15 bar tension).…”

“…Remote sensing data may not be useful in estimating LAImax for densely-forested regions, because satellite sensors saturate from the high reflectance (Zhao et al 2010;Waring et al 2014). Comparing greenness values for several years can help analysts estimate reasonable values for LAImax for model calibration, but disturbances may be difficult to ascertain due to coarse spatial or spectral resolution or cloud interference (Hilker et al 2009).…”

Decomposition and carbon loss in lodgepole pine (Pinus contorta var. latifolia) wood following attack by mountain pine beetle (Dendroctonus ponderosae)

“…In process-based models, carbon sequestration is computed from empirically derived relationships between physiological parameters such as photosynthesis, and environmental factors such as atmospheric and growing condition variables (Waring et al 2014). By responding to external control factors, process-based models inform how changes in environmental conditions may mechanistically influence changes in forest carbon stocks and fluxes.…”