Individual-Tree Diameter Growth and Mortality Models for Bottomland

Forest Ecology and Management 199 (2004) 307–322 Individual-tree diameter growth and mortality models for bottomland mixed-species hardwood stands in the lower Mississippi alluvial valley Dehai Zhaoa,*, Bruce Bordersb, Machelle Wilsona aSavannah River Ecology Laboratory, University of Georgia, Aiken, SC 29802 USA bWarnell School of Forest Resources, University of Georgia, Athens, GA 30602 USA Received 19 November 2003; received in revised form 4 February 2004; accepted 16 May 2004 Abstract Individual-tree diameter growth and mortality models were developed for the bottomland mixed-species hardwood stands in the Lower Mississippi Alluvial Valley (LMAV). Data came from 5-year remeasurements of continuous forest inventory plots. Six species groups were created according to diameter structure, tree growth, mortality, recruitment and light demand of species. A 5-year basal area increment model and logistic mortality model were calibrated for species groups. Potential predictor variables at tree-level and stand-level were selected based on the available data and their biological significance to tree growth and mortality. The resulting models possess desirable statistical properties and model behaviors, and can be used to update short- term inventory. # 2004 Elsevier B.V. All rights reserved. Keywords: Individual-tree growth model; Distance-independent; Mixed-species; Bottomland hardwoods; Mortality 1. Introduction have been developed and evaluated for more complex mixed-species stands (Burkhart and Tham, 1992). Recently, active management of mixed-species Mixed-species forests with a high diversity of tree stands is becoming more prevalent and seems to be species exhibit a huge range of life forms and stem a worldwide trend. This change from pure, single sizes. In these forests, age is irrelevant as a modeling species stand management to mixed-species stand variable (Vanclay, 1995), and the DBH (diameter at management stresses the need for tools to support breast height) distributions of species (species group, decision making in such stands. Specifically, reliable or stand) may be not unimodal, and thus may not be growth models for mixed-species stands are required described by standard distribution functions. There- in practice. Although mono-specific stands have been fore, some modeling approaches used successfully for modeled extensively and rather successfully for dec- mono-specific stands have limited utility in multi- ades, relatively few models or modeling strategies specific and heterogeneous forests (mixed forests). For example, whole stand models draw on stand-level * parameters such as stocking (trees/ha), stand basal Corresponding author. Tel.: þ1 803 725 6181; fax: þ1 803 725 3309. area, and standing volume to predict stand growth or E-mail address: [email protected] (D. Zhao). yield. This well developed approach has been used 0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.05.043 308 D. Zhao et al. / Forest Ecology and Management 199 (2004) 307–322 only in stands with a limited number of species (Lynch of this study were to develop distance-independent and Moser, 1986; Murphy and Farrar, 1988). Matrix individual-tree diameter growth and mortality mod- models have been applied widely in mixed-species els for bottomland mixed-species hardwood stands forest modeling (Buongiorno and Michie, 1980; Solo- in LMAV using continuous forest inventory (CFI) mon et al., 1986; Mendoza and Setyarso, 1986; Men- data. Based on the characteristics of diameter struc- gel and Roise, 1990; Osho, 1991; Buongiorno et al., ture, growth, mortality, recruitment and light 1995; Lin and Buongiorno, 1997). However, because demand of species, tree species were grouped using of the reasonable diameter classes required and ‘principal component and cluster’ analysis proce- assumption of the uniform distribution of trees within dures. For each species group, we developed indi- classes, a matrix approach may underestimate or over- vidual-tree basal area increment and mortality estimate yields. Compared with matrix models, as modelsthatcanbeusedinastandsimulatorthat well as other stand class models, individual-tree uses a list of individual trees by species (or species growth models ensure reliable predictions over all group). Finally, the performance of the individual- tree sizes, sites, and stand conditions, thus they can tree models for stand basal area prediction was provide more detailed tree growth information. Indi- examined. vidual-tree models simulate diameter (or basal area) increment and mortality for each individual-tree, then predict the growth and yield of stands, and thus avoid 2. Data using a distribution function to describe the diameter distribution. Although stand-level simulations fit yield The data used in this study are from continuous data better than tree level simulations, as a result of forest inventory plots located in bottomland mixed cumulated model errors from tree to stand-level, tree hardwood stands in LMAV. All plots used to estimate level approaches seem most appropriate for under- the parameters of the growth model belong to the standing stand growth as affected by competition Riverfront Hardwood Timber type (Putnam et al., between individuals of different species (Porte´ and 1960), in Arkansas, Louisiana and Mississippi. This Bartelink, 2002). type has been divided into two subtypes—say A and Based on the requirement of the spatial location of B. Type A is characterized by mixed soils in the ‘A’ trees, individual-tree growth models can be sub- horizon, or clay cap less than 36 in. (91.4 cm) grouped into ‘distance-dependent tree models’ where deep. Sweet pecan, sycamore, elm and cottonwood the tree location is known and ‘distance-independent are the predominate species associated with these tree models’ where the tree location is unspecified soils. Type B will have a clay cap of 36 in. (Porte´ and Bartelink, 2002). For most inventory data, (91.4 cm) or more and is characterized by ash, hack- spatial information from mapped tree locations is not berry, nuttall oak and overcup oak. The permanent available, thus distance-independent tree models have plots were established in 1968, and were re-measured been used widely for growth and yield predictions in 1968, 1973, 1978, 1983, 1988, 1993 and 1998 at 5- (Wykoff et al., 1982; Belcher et al., 1982; Wykoff, year intervals. Each plot consists of three concentric 1990; Monserud and Sterba, 1999; Sterba et al., 2002; circles with areas 0.08, 0.02, and 0.004 ha (1/5, 1/20 Yang et al., 2003). and 1/100 acre), respectively. The trees in the saw- Bottomland hardwoods of the Lower Mississippi timber size class (dbh 27.9 cm (11.0 in.)) were Alluvial Valley (LMAV) with a wide diversity of tree measured and numbered on the 0.08 ha (1/5 acre) plot, species are an important ecological resource provid- trees in the pole class (12.7 cm (5.0 in.) dbh ing many functions and values such as wildlife 27.8 cm (10.9 in.)) were measured and numbered on habitats and timber production. Several researchers the 0.02 ha (1/20 acre) plot, and finally trees in sapling including Putnam et al. (1960), Hodges (1997), class (2.5 cm (1.0 in.) dbh 12.6 cm (4.9 in.)) were Meadows and Stanturf (1997) discussed the devel- measured and numbered on the 0.004 ha (1/100 acre) opment and management of these stands. However, plot. Dbh measurements were carried to the nearest there is no suitable growth and yield model for tenth inch. Plots that had been permanently cleared or bottomland hardwoods in this area. The objectives partly cleared were re-established as new plots. There D. Zhao et al. / Forest Ecology and Management 199 (2004) 307–322 309 was no exact information on the history of harvesting species groups were created using a cluster analysis and silvicultural operations in our available data, and procedure based on the variables describing the stand measurement records of 1973 were not available. structure, tree growth, mortality and recruitment char- Therefore, the data used to develop models here were acteristics and species functional attributes, that is, taken only from the 5-year measurement intervals average dbh (D), maximum dbh (Dmax), minimum dbh where it is known that none of harvest cut, pulp cut (Dmin), mean diameter growth increment for a 5-year or chemical treatments were employed during that growth period (G), maximum diameter growth incre- interval and only from the last four growth periods ment (Gmax), basal area proportion (PBA), mortality (1978–1983, 1983–1988, 1988–1993, 1993–1998). (PM), recruitment (PR), and shade tolerance class The numbers of plots for the four growth periods (TOL) from Burns and Honkala (1990) (Zhao, are 157, 147, 120 and 122, respectively. 2003). A detailed listing of the members of the species To estimate the individual-tree diameter growth groups is presented in Table 1. species groups 1, 2 and models and individual-tree mortality models, the data 3 belong to soft hardwoods; species groups 4 and 5 are pooled over the plots of these four periods (non- belong to hard hardwoods; some small-sized species overlapping growth intervals). The pooled 546 ‘plots’ or less-abundant species combined with non-commer- can be treated as independent plots. cial species form species group 6. For re-measured permanent plot data, serial corre- Individual-tree diameter growth and mortality mod- lation and spatial correlation may be expected. These els are developed separately for each species correlations violate the assumption of independent group. Only re-measured trees that were alive during error terms in most statistical methods, thus invalidate both inventories of the growth periods were used to the t-test, the F-test, and the confidence intervals calibrate the individual-tree diameter growth models. (Koak, 1997). Borders et al. (1987) found that for Trees alive at the beginning of the growth period and permanent plot data with more than three measure- whose mortality statuses (live or dead) taken at the end ments, temporal correlation did not occur for non- of the same growth period were used to fit individual- overlapping growth intervals, whereas obvious corre- tree mortality models.

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