A hybrid model for intensively managed Douglas-fir plantations in the Pacific Northwest, USA

Abstract

Recent advances in traditional forest growth models have been achieved by linking growth predictions to key ecophysiological processes in a hybrid approach that combines the strengths of both empirical and process-based models. A hybrid model was constructed for intensively managed Douglas-fir plantations in the Pacific Northwest, USA, by embedding components representing fundamental physiological processes and detailed tree allometrics into an empirical growth model for projecting individual tree and stand development. The simulated processes operated at a variety of scales ranging from individual branches to trees and stands. The canopy structure submodel improved predictions of leaf area index at the stand level when compared to allometric and other empirical approaches (reducing mean square error by 30-42%). In addition, the hybrid model achieved accuracy in short-term volume growth prediction comparable to an empirical model. Biases in 4-year stand growth predictions from the hybrid model were similar to those from the empirical model under thinning, fertilization, and the combination of these treatments; however, volume growth predictions in unmanaged plantations averaged approximately 36% less bias. These improvements were attributed to detailed information on crown structure (i.e. size, location, and foliage mass of primary branches), simple representation of key physiological processes, and improved site characterization. Soil moisture, temperature, and nitrogen mineralization predicted by the hybrid model also agreed closely with observed values from several previous studies. Overall, the model framework will be helpful for future analyses as it can lend insight into the influence of weather and site edaphic factors on growth, help identify mechanisms of response to silvicultural treatments, and facilitate the design of sound management regimes for Douglas-fir plantations across the Pacific Northwest region.

Climatic influences on needle cohort survival mediated by Swiss needle cast in coastal Douglas-fir

“…This variability in cohort survival and associated stomatal occlusion complicates the usual patterns in photosynthetic efficiency and introduce some additional variation in growth that has not been accounted for by correlating growth with average foliation retention (e.g., Maguire et al 2011). The ability to predict survival rates of individual needle cohorts or age classes over time may be particularly useful for refining mechanistic or hybrid models that simulate ecophysiological processes, total carbon fixation, and stem growth and yield (e.g., Mäkelä et al 2000;Schwalm and Ek 2004;Weiskittel et al 2010). Such models combine conventional empirical data with mechanistic elements such as climate-and soil-driven water availability, water uptake and evapotranspiration, nutrient uptake, foliar nutrient dynamics, and net photosynthesis.…”

δ15N constraints on long-term nitrogen balances in temperate forests

“…Bulk litterfall was collected monthly using ten traps (0.26 m 2 each) per site, with subsamples of Douglas-fir needle litter separated from bulk litterfall in March and October 2005 for 15 N analysis. All stems [5 cm diameter were inventoried in 0.02-ha plots in 2006 and converted to stand biomass using the Douglas-fir Hybrid Growth System Model, an allometry-based growth model developed at these sites (Weiskittel et al 2010). Biomass estimates were multiplied by N concentrations to determine N pools in foliage, branches, and stemwood.…”

Extending a physiological forest growth model by an observation-based tree competition module improves spatial representation of diameter growth

“…Also, dimensional change was eventually not consistently related to biomass growth (Baldwin et al 2001). Weiskittel et al (2010), Kirschbaum (1999) and Korol et al (1996) have addressed the issue of mass conservative stand NPP distribution by a weight that was related to the estimated proportion of light acquired by a tree's crown. Dimension growth was calculated via allometric equations from the individual tree volume.…”

“…Both MoBiLE and the interpolation plug-in are coupled in both directions on the annual timescale: the controlling variable provided by MoBilE is the cohort volume growth, and the ones returned back are the new dimensions of the cohort representative tree. Our method is innovative as compared to earlier approaches of model coupling (Baldwin et al 2001;Milner et al 2003;Henning and Burk 2004) because it combines many of their different benefits, (a) extends them by most recent physiological concepts, (b) implies a bidirectional control between individual growth and stand development on a timescale of one year, which is also sensitive to management actions and (c) uses a mass conservative algorithm to calculate individual tree dimensional growth from cohort total stem biomass increase: the approach is related to the work of Weiskittel et al (2010), Kirschbaum (1999) and Korol et al (1996) in that it aims to scale the biomass increment simulated by a stand-level physiological model down to the individual tree level. However, it is different in that it uses an individual tree growth prediction by a distance-dependent empirical model as the weighting criterion.…”