J. P. Kimmins

Modelling forest ecosystem net primary production: the hybrid simulation approach used in forecast

Abstract In order to evaluate the impacts of alternative stand-level management scenarios on long-term site productivity, forest resource managers need ecologically based forest growth models. The forecast forest ecosystem management simulation model combines the traditional bioassay modelling approach with process-based simulation modelling to provide a method of projecting future forest biomass yield as well as a variety of other ecosystem variables and social values under a range of management conditions. A review of the hybrid simulation approach to modelling forest ecosystems is provided, and the representation of stand-level net primary production and nutrient cycling in forecast is described. The major driving function in the model (shade-corrected foliage nitrogen efficiency), the concept of site quality, and the simulation of site quality change are discussed. The value of an accurate definition of the state of the simulated ecosystem at the start of a run is emphasized, and the use of the ecostate ( state of the eco system, as generated by forecast ) file in this process is described. Limitations of the forecast modelling approach are reviewed.

Evaluation of the consequences for future tree productivity of the loss of nutrients in whole-tree harvesting

Abstract The advent of fibre farming (total utilization of tree crops grown on short rotations) raises the spectre of soil impoverishment on many forest sites. Although forest fertilization may be the answer in some situations, nutrient management will almost certainly become an important aspect of forest management in the future. Evaluation of the consequences for future tree productivity of the loss of nutrients in harvested materials is difficult. It requires a variety of types of information, and in many situations we are not yet in a position to make predictions with confidence. This paper reviews some of the more important questions that must be asked and answered in arriving at reliable conclusions concerning the significance of harvest-induced nutrient losses.

Yield decline in Chinese-fir plantations: a simulation investigation with implications for model complexity

A variety of competing hypotheses have been described to explain yield decline in Chinese-fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations. The difficulty in implementing field experiments suggests ecosystem modeling as a viable option for examining alternative hypotheses. We present a conceptual model of Chinese-fir yield decline and explore its merits using the ecosystem-based FORECAST model. Model results suggest that yield decline is caused primarily by a decline in soil fertility, largely as a consequence of slash burning in conjunction with short rotations. However, as tree leaf area declines, there is a transition (over subsequent rotations) from seed rain based competition to bud bank based competition, increasing the competitive impact of minor vegetation on tree growth. Short rotations increase understory survival between rotations and may cause a gradual shift from tree dominance to shrub/herb dominance over subsequent rotations. These effects are most evident on nutrient-poor sites, but...

The use of soil organic matter as a criterion of the relative sustainability of forest management alternatives: a modelling approach using FORECAST

Abstract The purpose of this study was to evaluate the usefulness and sensitivity of parameters that describe various soil organic matter characteristics for the evaluation of harvesting impacts on ecosystem function, using an ecosystem-level, hybrid simulation model (FORECAST, a model developed from its predecessor FORCYTE-11). Four separate sets of forest management options were applied to both a simulated unmanaged Douglas-fir forest (with a high accumulation of forest floor, coarse woody debris, and the nutrients contained therein) and to a simulated Douglas-fir forest growing on a site depleted in organic matter and nutrients. These four options consisted of the combination of two rotation lengths (40 vs. 80 years), and two levels of biomass utilization (full-tree vs. conventional tree length harvest). Based on the model simulations, productivity on the previously unmanaged site dropped significantly (to 59% of the value for the original, unmanaged forest) by the end of the second 40 year rotation when subjected to an intensive management option (40 year rotation, full-tree harvesting). However, a rapid drop in the active soil pool of decomposing organic matter occurred and was not rebuilt. As a result of this reduction in the size of the active soil pool, nitrogen deficiencies (N-demand greater than N-uptake) occurred in the subsequent rotations. In contrast, the initially nutritionally degraded site responded positively to an intermediate level of management (80 year rotations, stem-only harvesting), with a steady increment in the active soil organic matter pool size over the 240 year simulation period. This response indicates that sites degraded by past activities generally have the ability to recover (aggrade) if put under a less demanding management regime. Change in the mass and dynamics of active soil organic matter provides a more rapid and sensitive parameter than tree growth when attempting to assess the sustainability of management alternatives. Examples are presented which illustrate the importance of the starting state condition, as well as the management system being applied, when attempting to assess the sustainability of management alternatives.

The choice of window size in approximating topographic surfaces from Digital Elevation Models

Quantitative surface analysis through quadratic modelling of Digital Elevation Models (DEMs) is a promising tool for automatically describing the physical environment in ecological studies of terrestrial landscapes. Fundamental topographic variables such as slope, aspect, plan and profile curvature can be simply calculated from the parameters of a conic equation fitted to a DEM window through the least-squares method. The scale of the analysis, defined by the size of the DEM window used to fit the conic equation, affects both the estimated value of the topographic variables and the propagation of elevation errors to derived topographic variables. The least-squares method is amenable to the analytical treatment of the propagation of elevation errors to the derived topographical variables. A general analytical model of error propagation is presented that accounts for the effects of window size and of spatial autocorrelation in elevation errors. The method is based on the Taylor approximation of the least-square fitting equation and allows for the presence of stationary autocorrelation in the elevation errors. In numerical simulations with DEMs from British Columbia, Canada, it is shown that increasing the size of evaluation windows effectively reduces the propagation of elevation errors to the derived topographic variables. However, this was obtained at the expense of topographic detail. A methodology is proposed to evaluate quantitatively the loss of topographic detail through a χ 2-test of the corrected residuals in the immediate neighbourhood of the evaluation point. This methodology, in combination with the analytical model of error propagation, can be used to select the scale or range of scales at which to calculate topographic variables from a DEM.

“Without bamboo, the land dies”: Biomass, litterfall, and soil organic matter dynamics of a Javanese bamboo talun-kebun system

Abstract The biomass, litterfall, and soil organic matter dynamics during a complete bamboo talun-kebun rotation cycle were studied in West Java, Indonesia. This cycle consisted of 1 year of mixed species vegetable cropping (kebun) after the removal of bamboo, followed by 1 year of cassava, and 4 years of bamboo fallow (talun): a total cycle length of 6 years. In general, fruit and pod biomass constituted the highest percentage (38–68%) of the total crop biomass of 8.4 Mg ha −1 accumulated during the first year cropping. Cassava yielded a total of 4.6 Mg ha −1 of roots and tubers during the second year cropping from a total biomass accumulation of 6.7 Mg ha −1 . An experimental second year of cassava cropping without any fertilization caused a decline in the yield of roots and tubers to 60% less than the comparable value for the first year of cassava. Weeds, which reached a maximum biomass value of 1.8 Mg ha −1 at the end of the first year cropping, were all but eliminated from the later stages of the bamboo talun. The total biomass of bamboo increased with increasing age and reached 76.6 Mg ha −1 after 6 years. The distribution of total bamboo biomass between above- and below-ground components also varied with age. At 16 months after bamboo harvest, above-ground biomass accounted for 6% of the bamboo total mass. By the end of the 4-year bamboo fallow, 6 years after the bamboo harvest, 59% of the bamboo biomass was above-ground. In the final year of the bamboo talun stage, total above-ground litterfall was estimated at 4.7 Mg ha −1 , while the forest floor mass was 13.5 Mg ha −1 . There was an increase of approximately 7 Mg ha −1 of soil organic matter in the surface 25 cm of soil during the 4-year fallow. The historical, sustained success of the system with minimal external inputs of fertilizer appears to be closely related to the growth habit and biogeochemical characteristics of the bamboo, i.e. its rapid biomass accumulation, the accumulation of its litter, and the extremely high biomass of fine roots. This study provides scientific support for the traditional saying of the local farmers: “without bamboo, the land dies”.

Above- and below-ground vegetation recovery in recently clearcut and burned sites dominated by Gaultheria shallon in coastal British Columbia

Abstract Above- and below-ground vegetation recovery was assessed 2, 4 and 8 years after logging and burning on an age sequence of sites dominated by salal ( Gaultheria shallon Pursh) on northern Vancouver Island, British Columbia. The total above-ground vegetation biomass quadrupled from 1372 kg ha −1 on the 2-year-old sites to 5574 kg ha −1 on the 8-year-old sites. These are low values for post-logging above-ground biomass when compared with many other forest ecosystems. Salal was the dominant species on these sites, representing 77%, 87% and 73% of the total above-ground biomass on the 2-, 4- and 8-year-old sites, respectively. Leaf area index increased from 0.67 to 2.31 between the 2- and 4-year-old sites, but was only 2.53 on the 8-year-old sites. The total below-ground biomass increased six times from 1908 kg ha −1 on the 2-year-old sites to 11 415 kg ha −1 on the 8-year-old sites. The proportion of fine-roots to total roots declined with increasing site age as new rhizomes were produced. The upper 15 cm of the forest floor was found to contain 56% and 74% of the live fine-roots and 64% and 49% of the new rhizomes of the Gaultheria-Vaccinium and Epilobium-Cornus species groups, respectively. The ratio of below-ground to above-ground biomass varied from 1.4 on the 2-year-old sites to 2.5 on the 8-year-old sites. The post-disturbance dominance of salal on these sites after logging and burning appears to be due to its ability to reoccupy the site rapidly and completely both above-ground and below-ground from rhizomes present before disturbance, and to resist invasion by other species by pre-empting resources (nutrients in this case). A conceptual model of the leaf and fine-root biomass of salal over a period of 60 years is presented.

Aboveground biomass and nutrient accumulation in an age sequence of paper birch (Betula papyrifera) in the Interior Cedar Hemlock zone, British Columbia

Abstract The aboveground biomass and nutrient content (N, P, K, Ca, Mg) of stands of Betula papyrifera Marsh, aged 2, 8, 15, 45, 60, 75 years were measured in the Thompson Moist Warm Interior Cedar Hemlock variant (ICHmw3) of the southern interior of British Columbia. For four of these ages (2, 8, 60 and 75 years) measurements were made on good, medium and poor sites. For the other two ages, only stands on good sites coulde be located. Allometric equations relating dry weights of stemwood, stembark, branches and leaves to tree diameter at breast height (DBH) were developed to estimate aboveground tree biomass. Equations were not significantly different among the three site qualities. Average total aboveground tree biomass for all sites increased with stand age from 1.4 t ha −1 in 2-year-old stands (varying from 0.45 to 2.1 t ha −1 on poor and good sites, respectively) to 202 t ha −1 in the 75-year-old stand (156 and 234 t ha −1 on poor and good sites, respectively). As stand age increased, an increasing proportion of annual biomass increment was allocated to stems, but nutrients were preferentially accumulated in the leaves. Nutrient content of aboveground tree biomass increased with stand age and was generally in the order of N > Ca > K > Mg > P. Average rates of nutrients accumulation in biomass were greatest in the early stages of stand development, and less marked as stands aged. The concentrations of nutrients in tissues decreased in the following order: leaf > branch > stembark > stemwood. Understory minor vegetation contributed little to the nutrient pool of these paper birch ecosystems. Mineral soil contained the largest amount of nutrients among the various ecosystem components.

Testing the performance of a forest ecosystem model (FORECAST) against 29 years of field data in a Pseudotsuga menziesii plantation

The ability of the forest ecosystem management model FORECAST to project a 29-year record of stand response to factorial thinning and fertilization treatments in a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation at Shawnigan Lake (Vancouver Island, British Columbia, Canada) was assessed. Model performance was evaluated firstly using for calibration a regional data set and secondly with site-specific data from control plots. Model output was compared against field measurements of height, diameter, stem density, component biomass (aboveground), and litterfall rates and estimates of nutrient uptake, foliar N efficiency, and understory vegetation biomass. When calibrated with regional data, results from graphical comparisons, three measures of goodness-of-fit, and equivalence testing demonstrated that FORECAST can produce predictions of good to moderate accuracy depending on the variable of interest. Model performance was generally better when compared with field measurements (e.g., top height, ...

Disturbances and the sustainability of long-term site productivity in lodgepole pine forests in the central interior of British Columbia—an ecosystem modeling approach

Abstract The ecosystem-management model FORECAST was used to compare some ecological impacts of natural disturbance (wildfire) and timber harvesting. The scientific objective of the study was to assess whether or not two types of timber harvesting at various rotation lengths would have biogeochemical and biomass implications that are within the natural range of variation caused by wildfire. The practical objective was to identify management strategies that would sustain or improve long-term site productivity in lodgepole pine forests in the central interior of British Columbia. We defined three fire severity categories (low, medium and high), three fire return intervals (40, 80 and 120 years), two utilization levels (including stem-only harvesting [SOH] and whole-tree harvesting [WTH]), and three timber production rotation lengths (40, 80 and 120 years). Differences in simulated productivity, decomposing litter mass, total available soil nitrogen and nitrogen removals were compared for all 15 combinations of the five levels of disturbance at the three frequencies. The simulated nutritional impacts of timber harvesting were within the simulated range of impacts caused by the wildfire defined in this study. They were similar to the simulated long-interval, low-severity wildfire regimes. Simulations suggest that ecological rotation lengths for long-term site productivity for lodgepole pine forests in the study area would be 80–120 years. These rotation lengths are close to the average wildfire return intervals (100–125 years) in the study area, supporting the idea that the present harvesting strategies should sustain tree growth at this frequency of harvest and severity of harvesting impacts. Both WTH and SOH are acceptable harvesting methods for the maintenance of long-term site productivity in these lodgepole pine forests if harvest intervals are 80 years or longer. However, SOH is a more nutrient conservative harvest method, and should be used instead of WTH for rotations less than 80 years. The importance of initial site quality in assessing sustainable long-term site productivity by modeling is demonstrated.