Leaf Area Index - Relative Density Relationships in Even-Aged Abies Balsamea - Picea Rubens Stands in Maine

Relationships between leaf area index (LAI) and relative density (RD) were examined in even-aged stands of Abies balsntnea (L.) Mill, and Picecr rtihens Sarg. Twelve sites distributed across the Acadian forest in Maine were used to test conformance to three hypothesized models of LA1 over RD: A) constantly increasing LA1 over increasing RD, B) constant LA1 with increasing RD, or C) decreasing LA1 with increasing RD. Multivariate ordination suggested young, precommercially thinned (PCT), high-siteindex (SI) A. bul.r~rn~ea stands should be analyzed separate from older, unspaced (NOPCT), lower-site P. rubens stands. All-sided A. bnl.rnmea LAIs ranged from 6.2 14.8 over RDs of 0.23 0.60, P. rzrhens LAIs ranged from 7.5 14.9 over RDs of 0.5 1 0.76. Data for both species suggest an increase in LA1 with increasing RD. At a given RD, LA1 was strongly and positively correlated with SI. Lack of data across a complete range of RDs for both stand types limited testing of any hypothesized relationship between LA1 and RD; however, when LAIs were adjusted to a common S1, conformance to trend A was evident. INTRODUCTION The trade-off between individual tree growth and stand growth has been discussed extensively in the literature (Long and Smith 1984, Long 1985. Jack and Long 1996). One can balance these by controlling stand stocking, which regulates tree and stand leaf area (LAI, m2 rn-*). Arguably the most ecologically sound method by which to measure stocking is relative density (RD), defined as the number of observed trees of average size per unit area divided by the biological maximum number possible for the site. This metric relies on the self-thinning rule (Yoda 1963, Drew and Flewelling 1977), a pattern that has been found in many plant populations. Easily measured, RD may be a potentially useful prcdictor of LAI, but only if its relationship with LA1 is understood and quantified. Forest stands occurring on mesic sites with RD < 25% are commonly characterized as having maximum individual tree growth, but below potential maximum stand growth. At such low densities. crown closure has not been achieved and the site is still on a trajectory toward its maximum LAI. After reaching -25% RD, individual tree growth increases at a decreasing rate (Long 1985). Likely a result of the beginning of competitive interaction between trees, and the start of branch-shedding and differentiation in even-aged stands or stratification among species in mixed stands (Long and Smith 1984, Jack and Long 1996). Wilson et al. (1 999) found crown closure to occur between 1 I % and 32% of RD in stands of A. balsamea P. ruhens. Foliage biomass, as a corollary to LAI, has been observed to reach its maximum soon after crown closure, depending on initial stand density (Turner and Long 1975, Meng and Seymour 1 992). Once LA has reached its maximum, where differentiation of even-aged stands into crown classes is well underway, stand growth is at its maximum potential (Long and Smith 1984) and self-thinning is evident (Dean and Long 1985). Active intervention before the onset of self-thinning ( G O % RD) is one goal of production silviculture. A common pattern of relatively constant foliage biomass over time after canopy closure has been well-documented (Turner and Long 1975, Jack and Long 1991a). Such studies require observing many stands of differing ages as a chronosequence to interpolate possible changes through time. Less is known about the behavior of LA1 with increasing RD beyond self-thinning. If foliage is described by LA1 and RD increases with time three similar trends can be hypothesized (Fig. 1.1).