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Chapter 4 Dynamics of multiaged stands 4.1 Introduction consist of multiple canopy strata, upper strata will receive plentiful sunlight whereas lower strata will Understanding stand dynamics is prerequisite for be in much poorer light environments. The term silviculture and particularly for the silviculture of light regime is used to describe the dynamics of complex stands. The simultaneous growth and de- light quality, duration, and intensity on a daily, sea- velopment of multiple age classes of trees compli- sonal, or annual basis. A common means of describ- cates our interpretations of within-stand growth ing the light regime in understory canopy strata is and competition. This is particularly true in the the ratio of below-canopy light to above-canopy understory, where small trees are important future light. This is often expressed as the percent of above- overstory trees but may exist in variable states of canopy light (PACL) or percent of photosynthetical- competition for light or moisture. Some of these un- ly active radiation (PAR). The PACL typically varies derstory trees must be able to persist and grow to inversely with the light extinction characteristics maintain vigor and form and to advance their posi- and number of the canopy layers overhead. Thus, tion in the stand. Growth and persistence of under- a single-stratum overhead might have a steep PACL story trees in multiaged stands is therefore a process gradient as shown in Figure 4.1. As stand structure of obtaining adequate resources while maintaining increases in complexity, the light regime would also an overstory. This chapter discusses the dynamics become more complex (Figure 4.1). PACL is a useful of overstory/understory relations through the lim- tool for expressing the relative light intensity under itations of light and moisture in complex stands. a forest canopy; however, it does not provide much Shade tolerance is presented as a dominant factor indication of the quality of the light that is received. that limits options in multiaged stands, and tree Light quality can vary with type of foliage over- architecture is presented as a diagnostic variable. head, the amount of diffuse and direct light, and the This information is used to define tree growth pat- quality of the light. terns in multiaged stands as trees move through the Most vascular plants increase their growth with canopy and respond to increases in growing space. increasing light intensity until their light needs More detail on stand dynamics, and particularly are met. An asymptotic or saturation curvilinear the dynamics leading to the formation of complex relationship is often used to describe these rela- stand structures, is available in Oliver and Larson tions (Figure 4.2; Coates and Burton 1999; Stan- (1996). Gap dynamics are presented in Chapter 5. cioiu and O’Hara 2006c; O’Hara et al. 2007b). The implication of this form is that plant growth in- 4.2 Dynamics of light-limited systems creases with light intensity until some maximum. Above this maximum, growth is generally unaf- 4.2.1 Light regimes fected by increases in light intensity. The shape of The reduced light in the lower strata of multiaged this light-response relationship would vary with stands has a dominant influence on growth and form the species in both the understory and overstory, of regeneration. Because multiaged stands typically and their shade tolerance. Multiaged Silviculture. Kevin L. O’Hara. © Kevin L. O’Hara 2014. Published 2014 by Oxford University Press. DyNAmIcs Of mULTIAGED sTANDs 23 complex structure Height Seedling sprout height single-stratum structure PACL PACL Figure 4.2 Seedling or sprout height with increasing percent above- Figure 4.1 Percent above-canopy light (PACL) as a function of canopy light (PACL). height in a simple single-stratum structure with a mature overstory (solid line) and in a complex structure with multiple canopy strata (dashed line). A key component of multiaged silviculture is find- stands will only be possible at low overstory stock- ing the appropriate stocking relationship that pro- ing levels. Acker et al. (1998) and Zenner et al. (1998) vides for a healthy overstory and adequate growth describe the growth trade-offs for two-strata stands of the understory. This is often viewed as a simple in the Pacific Northwest, US. The slower growth of trade-off that is somewhat representative of the more understory trees has been cited as a reason multiaged complex trade-offs in stocking control of stands stands are inherently less productive than even-aged with multiple canopy strata. For a two-strata stand, stands (Assmann 1970), although such reasoning ig- ­Figure 4.3 demonstrates that an expectation for rapid nores the potential benefits of low overstory stock- understory growth can only be achieved with a cor- ing on overstory tree growth. responding decrease in the overstory stocking or In some forests, clumpy patterns of overstory growing space occupancy. Understory growth that is trees may be necessary to provide adequate light comparable to growth of trees growing in even-aged to the understory. Gersonde et al. (2004) used a high high overstory growth Figure 4.3 Trade-offs between age classes Overstory growth or canopy strata in a simple two-aged or Understory growth two-strata stand. An allocation of growing space to one age class reduces the potential of the other and represents a trade-off understory growth between providing enough space for the low low older age class to grow and ensuring that 02040 60 80 100 there is enough space for the younger age Overstory growing space occupied (%) class to persist (adapted from O’Hara 1998). 24 MULTIAGED SILVIcULTURE light model to characterize the light regimes in Sunflecks are brief periods of irradiance under multiaged conifer forests in California, US, and relatively closed forest canopy conditions (Chaz- found irregular canopies allowed more light to don and Pearcy 1991). They are an important part intermediate canopy strata. Battaglia et al. (2002) of the understory light regime in many forests be- described the enhanced light regime in stands of ag- cause they supplement diffuse light energy, which gregated patterns of longleaf pine overstory trees. may be more generally present. In some forest Although their treatments left similar amounts of types, sunflecks may account for up to 90% of the residual basal area, the light regimes were very dif- total PAR, but usually they are considerably less ferent because of the variable patterns of residual (Lieffers et al. 1999). Sunflecks are also highly dy- trees (Figure 4.4). The effect of these treatments is namic in that they may last only seconds and can to have different parts of the stand in different light vary daily and seasonally in both light quality and regimes or different positions in the diagram shown duration (Baldocchi and Collineau 1994). The con- in Figure 4.3. Some areas may be to the left, where trasts in light intensity from the sunfleck area to the understory light is more available, whereas others general forest floor can also be quite variable, and are further to the right, where the overstory density all sunflect radiation may not be used (Wayne and is higher. Whereas aggregated spatial patterns may Bazzaz 1993). As a result, trees vary in their ability create favorable light regimes in some situations, in to react to sunflecks due to differences in patterns of other cases, other variables may be important. For stomatal closure and other adaptations—such as in example, Peck et al. (2012) found no clear patterns tree architecture—to light regime. with three pine species in Minnesota, US. Other fac- Light quality is another important variable affect- tors, such as latitude, or variations in shade toler- ing understory dynamics in multiaged stands. At ance among species, may override the advantages one level, light quality might refer to the contrasts of having gaps overhead. Loftis (1990a, b) conclud- between direct beam light, diffuse light through an ed lower and mid-story shade was more important overhead canopy or high shade, or the combination in affecting oak regeneration in the southern Appa- of diffuse and direct light near the forest canopy or lachians, US. low shade (Chapman 1945; Oliver and Larson 1996; 150 100 120 90 Y 50 Y 60 30 0 0 0 60 120 050 100 X X Single Tree Large Group Figure 4.4 Spatial patterns for light in longleaf pine stands in Georgia, US. The figure on the left shows the result of a single tree selection treatment and on the right, that of a large group treatment. Dark patterns represent areas in poorer light regimes and light areas, the best light regimes (from Battaglia et al. 2002. The effect of spatially variable overstory on the understory light environment of an open-canopied longleaf pine forest. Can. J. For. Res., 32, 1984–1991, © Canadian Science Publishing or its licensors). DyNAmIcs Of mULTIAGED sTANDs 25 Leiffers et al. 1999). Light quality may also refer to the Table 4.1 Shade tolerance classes for selected North America proportion of the light spectrum available to plants. species (from Daniel et al. 1979). The structure of the forest can lead to a variety of Tolerance class Species different light qualities depending on whether the light passes through a forest canopy or through large Very tolerant American beech or small gaps (Endler 1993). The forest canopy can Balsam fir therefore affect the range of the light spectrum that Eastern hemlock is transmitted and thus benefit certain trees over oth- Sugar maple ers. For example, Lieffers et al. (1999) showed how Western hemlock the light quality—as represented by the ratio of red Western redcedar to far-red wavelengths—varied with light intensity to give conifers a general advantage at some light Tolerant Coast redwood intensities but not at others.

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