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 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 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 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 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 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

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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 a general advantage at some light Tolerant Coast redwood intensities but not at others. Clearly, there is more Grand fir to light regimes than simply the light intensity, and Red maple these light regime dynamics cannot be represented species with simple stocking or stand-­density measures that Tanoak are used to approximate overstory light interception. White fir Instead, the light environment for the understory of a multiaged stand is highly complex, and, in the ab- Yellow-cedar sence of detailed data about light quantity and qual- Intermediate Coast Douglas-fir ity, the performance of trees on the site may be the Eastern white pine best indication of the present light regime. Radiata pine Red oaks 4.2.2 Shade tolerance Sugar pine Western white pine The relative shade tolerance of trees is often a lim- iting factor for multiaged silviculture, because it White ash determines the structural arrangement of trees in White oak mixed-species stands. Physiologically, shade tol- Yellow birch erance is the result of relative light compensation Intolerant Black cherry points for different species. The light compensation Eastern redcedar point is the light level where respiration and photo- Loblolly pine synthesis are equal. Below this light level, the plant Lodgepole pine is unable to produce enough carbon to sustain itself, but above this level, it produces a surplus. Species Noble fir vary in their compensation points and their ability Ponderosa pine to tolerate low-light regimes. Species also vary in Shortleaf pine their structure in relation to shade tolerance Yellow poplar and light regime (Abrams and Kubiske 1990). Trees Very intolerant Aspen species may therefore have both physiological and morpho- Black locust logical responses to shade (Canham 1989). Rank- Cottonwood species ing species’ shade tolerances is often used to show the relative ability of different species to endure Jack pine shade as as would be encountered in a multiaged Larch species system (Malcolm et al. 2001). The primary ranking Longleaf pine for many North American species can be found in a table produced by Daniel et al. (1979) that places species in five shade tolerance categories (Table 4.1). 26 Multiaged Silviculture

A shade tolerance ranking can be conceptualized species. Similar patterns are common in other forest for multiaged stands by determining which species types. This is one of the major impediments to the can endure the low light that penetrates the foli- use of multiaged systems in managing mixtures of age of another species in an overstory position. If a shade-intolerant and shade-tolerant species. species endures this light level, that species is more Whereas shade tolerance rankings generally fo- tolerant than the overstory species. Hence, the rank- cus on the ability of a tree to survive or endure the ing represents a guide for how species might be shade of another, growth rates under various light vertically arranged in multiaged structures. Species levels are also an important and more complex with low tolerance to shade could be above more consideration. Understory trees generally have as- tolerant species but not below (Figure 4.5). These ymptotic growth relationships with increasing light types of structures demonstrate the limitations (Figure 4.2). These light-response relationships that differences in shade tolerance place on mixed vary with species. Figure 4.6 shows the probabil- stands. A mixture with a more shade tolerant spe- ity of mortality for several Himalayan species, and cies dominating the understory is likely to shift in Figure 4.7 shows growth patterns for three species species composition to that more tolerant species. in eastern Europe. In both cases, different patterns In a ­50-year study on eastern North American hard- were evident by species. However, these relation- woods, Schuler (2004) reported that multiaged treat- ships generally correspond to shade tolerance rank- ments encouraged less desirable, shade-tolerant ings, with more tolerant species showing greater growth at low light levels. But the crossing of these relationships indicates stand tolerance is more com- plicated than a simple ranking. Other factors may affect a species response to its light regime. Site quality was shown to be a factor with western redcedar and coast Douglas-fir in Brit- ish Columbia, Canada (Drever and Lertzman 2001). For young broadleaf seedlings in eastern North America, allocation to carbohydrate reserves varies

0.6 Tsuga dumosa Abies densa 0.5 Picea spinulosa Pinus wallichiana Larix griffithiana 0.4

0.3 p

0.2

0.1

0 0.05 0.5 0.01 0.11.0 GSF

Figure 4.6 Probability of mortality (p) over a two-year period as a function of light availability (GSF) for five Himalayan species. The highest risk of mortality was for the shade-intolerant Larix and Pinus species (from Gratzer et al. 2004, Interspecific variation in the Figure 4.5 Relative shade tolerance of Norway spruce and Scots response of growth, crown morphology, and survivorship to light of pine in southern Finland. The Norway spruce crown in the center six tree species in the conifer belt of the Bhutan Himalayas. Can. J. extends to the ground, whereas the Scots pine crowns are all quite For. Res. 34, 5, 1093–1107, © Canadian Science Publishing or its high. licensors). Dynamics of multiaged stands 27

Figure 4.7 Relative annual height growth for silver fir, Norway spruce, and European beech in Romania as a function of percent above-canopy light (PACL). Patterns are shown for each species with standard error bands. At the lowest light intensities, the more shade-tolerant silver fir grows the most and Norway spruce the least. But at higher light intensities, the relative increment patterns vary (from Stancioiu and O’Hara 2006c). with light levels (Canham et al. 1999), and this al- the former, stands may appear closed, but available location, in turn, affects survival and growth (Webb soil moisture may limit complete canopy closure. In 1981; Kobe and Coates 1997). Likewise, Kneeshaw these situations, light may limit the development of et al. (2006) found changes in shade tolerance rank- some species, and moisture may limit others within ing with increasing size in small boreal trees in east- a single stand. This is demonstrated in Figure 4.8, ern Canada. Ranking shade tolerance is therefore a which shows how oaks in the eastern US become simplification of relatively complex relationships more competitive on dryer sites regardless of the describing relative tree or plant growth in a range light regime. In contrast, on moist sites, they are of light environments. most competitive under only a narrow range of light Despite its complexity, relative shade tolerance regimes. The phenomenon depicted in Figure 4.8 is still a guiding concept in multiaged silviculture. is representative of the strong interaction between Species vary in their ability to endure low light light and soil moisture that exists for many species levels. Some will be more competitive in complex and affects competition between trees and other structures than others, and managers need to be vegetation. It may be a seasonal limitation or a limi- aware of these relationships and how they might tation that occurs during only a few years. Likewise, vary with site quality, tree age, or tree size. limitations of nutrients or other soil conditions, such as depth or pH, may favor one species over another. 4.3 Dynamics of moisture-limited On much dryer sites, where stands may be open systems (Figure 4.9), the limiting factor is primarily soil moisture or some other below-ground limitation. The limiting factor for some multiaged forests is soil It is difficult to generalize about these sites, which moisture. The dynamics and recruitment of new may be called woodlands or savannah wood- trees and competition for growing space is consider- lands (Helms 1998) or have local names, such ably different when soil moisture is limiting instead as chaparral in California, US, dehesas in Spain, of light. At one extreme, you have complex stands, or maquis around the Mediterranean. They may where moisture limitations may affect only some have open spaces between trees in a form of open of the species. At the other, you have open stands, stem exclusion (O’Hara et al. 1996). Trees may be where leaf area index (LAI) and stocking may be relatively short and small in these structures, and low and the plant communities are quite open. In below-ground competition may be intense. Age 28 Multiaged Silviculture

Figure 4.8 Relationship between moisture and light for oaks in the eastern US (from Johnson et al. 2009; reproduced with permission from CABI).

Figure 4.9 Dryland juniper—pine woodland in California, US. structures may be diverse due to highly variable dis- a convergence of high seed production with the turbance histories and episodic regeneration peri- creation of growing space by disturbance may be ods correlating with climatic events or disturbances necessary for regeneration. (Tyler et al. 2006; Shinneman and Baker 2009). A va- Many climate-change scenarios predict warmer, riety of adaptation strategies favor different types dryer climates, which would lead to an increase in of plants on these moisture-limited sites. These these moisture-limited forest or woodland types. In strategies may be related to rooting depth, resist- some cases, their present existence may be due in ance to desiccation, or regeneration properties that part to anthropogenic activities (Martín Vicente and allow plants to establish quickly and exclude other Fernández Alés 2006; Blondel 2006). Although not plants. Unlike many light-limited systems, where traditional multiaged forests managed for timber regeneration may be common or even continuous, production, these moisture-limited forests may be on the more extreme of these moisture-limited sites, an expanding proportion of the world’s forests. The Dynamics of multiaged stands 29 management of these types will be based, in part, predictable ways (Oliver and Larson 1996). Coni- on their age structure and use many of the con- fers with strong epinastic control—the ability of the cepts related to the dynamics of more light-limited terminal bud to control the angle and length of systems. lateral branches—­tend to form conical or excur- rent crown shapes in full sunlight but transition to 4.4 Tree and stand architecture flat, “umbrella-­shaped” crowns with greater shade ­(Figure 4.10). The difference is generally a shorten- The architecture of individual tree crowns is strong- ing of the internode distance and a lengthening of ly affected by the light regime where they develop. the branches in conifer trees in greater shade. Species For small trees, tree architecture can be diagnostic as with weaker epinastic control or decurrent growth to potential changes in light environment that may forms, such as many broadleaved species, will also affect tree vigor and form. Understory tree growth vary in crown form in different light regimes, but the rates vary with light regime in relatively predict- differences typically are not as pronounced as those able ways. Crown form also varies in relatively in trees with strong epinastic control (Figure 4.11).

Figure 4.10 Flat-topped grand fir in Washington State,U S, showing the umbrella form which is characteristic of trees with strong epinastic control in shaded environments.

Figure 4.11 Decurrent growth pattern of European beech in a multiaged stand in Slovenia. 30 Multiaged Silviculture

The maintenance of a central leader results in a the shape dependant upon the epinastic control tree with a single bole, but the tendency in poor light of the species and the degree of change. Changes regimes is for young trees to form flat tops with mul- in crown form are indicative of deteriorating light tiple leaders. For trees with both weak and strong regimes as a tree becomes more suppressed. Like- epinasty, control grows stronger as the light regime wise, improvement in the light regime results in improves. There is also an interaction with shade trends toward more conical or excurrent forms, or tolerance, as species with greater tolerance have crowns with a more dominant leader. For conifers, greater variation in crown form or crown plasticity improvement in the light regime often leads to a in different light regimes (Givnish 1988; Chen et al. lengthening of the leader relative to branch length 1996; Williams et al. 1999). An intolerant pine has lit- and the development of a more conical crown form tle variation in form in different light regimes, but (Tucker et al. 1987). Figure 4.12 shows this for hiba, a tolerant fir would display a great range in form. a Japanese cypress species, as it changes from a These morphological characteristics related to flat-topped, suppressed form to a conical form. For light regime have been quantified in a number of broadleaved species with weak epinastic control, studies for small trees. For example, trees in low- trees often do not return to a single-bole form and light regimes have larger crown width-to-length ra- instead may become forked trees with a strongly de- tios than trees in better light regimes (Messier et al. current growth form (Stancioiu and O’Hara 2006b). 1999; Ruel et al. 2000). The terminal-to-lateral ratio Management of multiaged stands can therefore compares the lengths of vertical and lateral exten- affect light regimes and the form of understory sions and is greater in trees in better light regimes trees. Maintaining adequate light will prevent the (Canham 1988a; Messier et al. 1999; Stancioiu and extreme suppression that leads to top dieback or O’Hara 2006b). Leaf morphology may also vary mortality, or to the stem forking that may make a (Abrams and Kubiske 1990). Specific leaf area, or tree less desirable for timber. Although many stud- the ratio of leaf surface area to leaf mass, is greater ies have successfully studied light regimes in rela- in trees grown in poorer light regimes (Williams et tion to small-tree morphology (e.g., Mitchell and al. 1999; Grassi and Bagnaresi 2001). Conifer needle Arnott 1995; Chen et al. 1996; Sprugel et al. 1996; morphology also varies with greater stomatal den- Messier et al. 1999; Williams et al. 1999; Grassi and sity in trees from better light regimes (Youngblood Bagnaresi 2001; Stancioiu and O’Hara 2006b) or and Ferguson 2003). for growth responses of trees to management (e.g., A reduction in the quality of light regime with Tucker et al. 1987; Lieffers et al. 1993; Ruel et al. time may result in a tree changing from a conical 2000), information about light thresholds is lacking form or a form dominated by a central leader to a for most species. The relationships between small- flat-topped form or one with multiple leaders, with tree crown form and light are probably similar to

Stem Elongation

Bowl-type Dome-type Conical

Figure 4.12 Crown development of hiba in Japan after release showing elongation of the leader and a complete change from a flat-topped to a conical crown form (from Hitsuma et al. 2006; reproduced with permission from Springer Japan and the Japanese Forest Society). Dynamics of multiaged stands 31 those for small-tree growth and light. For example, For stands, leaf area index (LAI) is a measure of Stancioiu and O’Hara (2006b, 2006c) reported that the ratio of leaf surface area to ground area covered light levels above approximately 30% or 40% were by the canopy. An LAI of 3.0 indicates an average of necessary to maintain growth and crown form for three layers of foliage over an area. LAI provides a Norway spruce, European beech, and silver fir. measure of the light resources used by a stand and The crown architecture of large trees in multiaged therefore provides a measure of occupied growing stands is less affected by suppression and is similar to space (O’Hara 1988). Following a stand-replace- trees in even-aged stands. Although the architecture ment disturbance, LAI increases gradually and, of these large crowns is much less studied than for barring further disturbance, eventually reaches understory trees, there is likely a gradual transition a maximum for a site. LAI is a common driver in from trees with understory features to upper canopy ecosystem process models (Running and Coughlan trees. Tree leaf area increases dramatically with in- 1988; Landsberg and Waring 1997; Battaglia et al. creasing dominance in multiaged stands (O’Hara 2004) and has been used to describe growth efficien- 1996). Additionally, specific leaf area decreases with cies of individual trees and age classes in multiaged increasing dominance, as larger trees have less leaf stands (O’Hara 1996). LAI is not generally compa- surface area per unit of leaf mass (O’Hara and Nagel rable between different species: a unit of LAI for a 2006). However, O’Hara and Nagel found the range shade-intolerant species will have different light re- in specific leaf area was less in multiaged stands than ception and production relationships than a unit of in comparable even-aged ponderosa pine stands. LAI for a shade-tolerant species. This suggests some subtle differences in crown ar- The distribution of biomass in any stand struc- chitecture in larger trees between multiaged and ture is highly variable due to species, age, and site even-aged stands that have not been fully explored. characteristics. Figure 4.13 displays the vertical

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Palms etc. 1.3 0.3 0.20.1 0 0.20.1 01020 Leaf area density Leaf biomass Wood biomass density ha ha–1 m–1 density t ha–1 m–1 t ha–1 m–1

Figure 4.13 Vertical biomass profiles for tropical forest in Malaysia (from Kira 1978; reproduced with permission from Cambridge University Press). 32 Multiaged Silviculture distribution of biomass in a tropical forest in Malay- These within-cutting cycle growth curves are sig- sia. In this forest, which reaches nearly 60 m above moid in shape, based on the assumption that their ground, most foliage and branch biomass is at the growth is greatest early in the cutting cycle when mid-height of the main canopy, and there is very growing space is most available but decline later in little in the top 15 m. Stem biomass is primarily near the cutting cycle as available growing space declines. the ground. The presence of foliage biomass in the Actual growth relationships are, of course, more lowest 5 m suggests there are regenerating trees, be- complex. Normal variations in cutting cycle length, sides the palms and lianas, but this diagram does unequal volume removals, variations in age class not reveal which species are present. Stand-level ar- densities, or different species composition would chitecture is difficult to interpret with height profile lead to variable growth patterns. What is clear is diagrams. However, the distribution and density of that trees left too long in suppression or under ex- foliage does provide information on the light envi- treme states of suppression may not respond to in- ronment in the lower canopy. creased growing space. The term release has two related meanings in silviculture. It commonly refers 4.5 Growth patterns to treatments that free young trees from competition (Helms 1998), but it also is used to describe the re- Tree growth patterns in multiaged stands are vari- sponse effect of the subject trees. Both definitions are able from one age class to another and are also in- pertinent to discussions of growth relationships in fluenced by variations in the canopy around them. multiaged silviculture. Each cutting cycle treatment Oliver and Larson (1996) described the idealized de- essentially represents a release of remaining age class- velopment of a stand containing four cohorts or age es, and the remaining trees should show a release as classes as a series of increasing height growth curves they respond to the improved competitive environ- with advancement in the structure (Figure 4.14). ment. This latter type of release is critical to many Trees in lower canopy positions struggle to develop types of multiaged silvicultural applications, from in poor light environments or with limited growing shelterwood removals to more complex systems. space (Figure 4.10), whereas trees in upper canopy Many studies have attempted to predict release positions are vigorous in an environment with rela- from tree characteristics. These are useful approach- tively little competition. Over succeeding cutting cy- es because tree crown architecture is strongly affect- cles, each age class advances in rank and, with these ed by suppression, and the degree of suppression idealized relationships, assumes a growth rate like corresponds to the subsequent response or release to that of the next older age class until it is harvested. improved conditions. Generally these studies have

COHORTS feet meters A B C D 20 cutting cutting cutting 60

40

10 ee height

Tr E 20 F Figure 4.14 Idealized growth patterns for G a multiaged forest with four age classes and repeated 30-year cutting cycles (Oliver and 0 03060 90 120 Larson 1996; reproduced with permission from Time in years John Wiley & Sons). Dynamics of multiaged stands 33 focused on advance regeneration that may have po- vigorous state. Smaller trees also are more likely to tential for release into a future overstory position. respond to release than larger trees. These trees can be completely released to become In contrast to evergreen conifers, broadleaved part of an overstory after a shelterwood removal, or understory trees have more variable growth habits. part of a developing stratum in a multiaged stand. These trees not only have a different architecture For evergreen conifers, studies have found that than conifers do but also have different life strate- prerelease growth rates correspond well to postre- gies for response to disturbance and release (Bond lease growth in many forest types (Ferguson and and Midgley 2001). The most significant factor is Adams 1980; Seidel 1985; Helms and Standiford that many of these broadleaved species can repro- 1985; Tesch and Korpela 1993; Metslaid et al. 2007). duce through stump or seedling sprouts. Postre- Trees with larger crowns (greater internode lengths) lease growth rates are affected by prerelease vigor, or narrower crowns generally have greater vigor with trees with high vigor responding more rapidly and respond better to release. A released tree sud- than trees with low vigor. When suppression results denly exposed to a better light regime may have to in poor form, release often results in retention of the convert foliage to adapt to the new light conditions poor form characteristics of the released tree. In (Tucker and Emmingham 1977; Tucker et al. 1987), these cases, sprouting trees may be cut at the base but a tree in a better prerelease light environment so that they resprout, forming a new stem or stems will have less foliage to convert (Oliver and Larson with better form (Johnson et al. 2009). 1996). Deciduous trees respond more rapidly be- cause foliage conversion is more rapid. Larger trees 4.6 Mortality may actually exhibit slower growth after release than smaller trees, as larger trees have a greater res- Tree mortality is an important process for any piration sink that requires a large part of photosyn- stand, including complex stand structures such as thetic production of the released tree (Figure 4.15; multiaged and mixed-species forests (Franklin et al. Oliver 1976; Ferguson and Adams 1980; Kneeshaw 1987). However, tree mortality patterns are poorly et al. 2002). This observation agrees with the report understood in any stand structure. Most research by Messier et al. (1999) that there is a maximum sus- has focused on even-aged stands, but some infor- tainable height for boreal species in Canada, where mation is available on complex stands. A primary larger trees are less able to respond to release. The focus of many studies is the change in the density of best trees for release in either overstory removal these stands over time. For even-aged forests, these operations or in more complex multiaged stands density trends are well documented as self-thinning are trees with crown characteristics indicating a curves (Figure 4.16). Similar limitations on size

16 0.33 m 14

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6 5.0 m Figure 4.15 Height increment for grand fir trees following release in north Idaho, US. Trees that were 5.0 m in height at

Annual height increment (cm) 4 the time of release (solid line) grew rapidly the first year and then slowed. Trees that were 0.33 m in height (dotted line) 2 grew slowly at first but increased their growth rapidly over 0 246 81012 the 10 years following release (adapted from Ferguson and Years since release Adams 1980). 34 Multiaged Silviculture

Mean Tree Volume (m3) (ft3) 150 4.0 3.0 100

2.0 Maximum size-density relationship 50 [In v (ft3) = 12.644–1.5 in ρ(TPA)] 1.0

0.5

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5 0.1 Figure 4.16 Self-thinning relationship for coast Douglas-fir in the Pacific Northwest, US. Lines represent repeated measurements of permanent plots that generally move upward and to the left on the diagram and are limited 1 by the maximum size-density relationship. 100 200 400 600 800 1000 2000 Vertical movement on the diagram represents (trees/acre) an increase in average tree size. Movement to the left represents mortality, and movement to 300 500 1000 1500 2000 3000 5000 the right is ingrowth (from Drew and Flewelling (trees/hectare) 1979; reproduced with permission from the Density Society of American Foresters).

and density must surely exist for complex strands. These distributions are discussed in more detail However, whereas these self-thinning relationships in Chapter 7. A diameter distribution with a nega- indicate that there is a limiting condition for aver- tive exponential shape is a common form used to age size and density, they do not give insight into describe multiaged stands and as a target for man- the dynamics of mortality. agement. The assumption imbedded in the use of For even-aged stands, the process of differen- these models is that trees in small classes eventually tiation, or separation of trees into crown classes replace trees in large classes and that, consequently, (O’Hara and Oliver 1999), is the first step in the the decline in the number of trees with increasing suppression and mortality of small trees. Vigor- diameters is indicative of a high rate of mortality. ous trees become dominant, and less vigorous trees But these negative exponential diameter distribu- fall into subordinate positions. For all trees but the tions may be poor representations of mortality in dominants, there is a general decline in status, even natural stands. In northern hardwoods in North though they may be increasing in size, and the America, Goff and West (1975) concluded mortality number of dominant trees declines with time. In was high in small trees, low in the main canopy, and young, even-aged stands experiencing stem exclu- high again for the largest trees. Mortality in the un- sion, mortality is concentrated in smaller trees as a derstory was high because of the large numbers of result of competition and various forms of mechani- trees and the poor light conditions. The mortality in cal damage (see Lutz and Halpern 2006). large trees was due to senescence and the reduced In older and more complex stands, mortality pat- vigor in the oldest trees. terns become more complex. Diameter frequency Because of the abundance of understory trees in distributions are often used to describe the structure some forest types—either to due to ease of regener- of complex stands, particularly multiaged stands. ation or intentional management—most mortality Dynamics of multiaged stands 35 in multiaged stands occurs in the understory (Har- When Casperson (2006) measured background combe 1987). Many young seedlings never become mortality in managed and unmanaged northern fully established, succumbing to an assortment of hardwood stands in eastern North America, they biotic or abiotic factors such as desiccation, herbivo- found that although the rate of postharvest mortal- ry, insufficient solar radiation, etc. (Swaine and Hall ity was relatively minor, it was highest in smaller 1983; Kitajima and Augspurger 1989; Collet and trees and thus could lead to unexpected changes Moguedec 2007; Keyes et al. 2009). In addition, mor- in stand structure and development. In a study of tality rates may be lower for some species because North American boreal forests, postharvest mortal- of their shade tolerance or preferential grazing by ity was high in the first year after harvest but ta- herbivores. For example, Petritan et al. (2007) found pered over the next 10 years to background levels that in Germany, the more shade-tolerant European (Thorpe et al. 2008). Windthrow, a major cause of beech had a higher survival rate than the less shade- postharvest tree mortality, is greater following more tolerant maple and ash did. In Slovenia, Klopcic severe treatments that result in greater exposure to and Boncˇina (2011) attributed a decline in the num- residual trees (Gardiner et al. 2005). Mortality will bers of silver fir to ungulate browsing. Similarly, occur even in stands managed well below densities in tropical north Queensland, Australia, seedlings where competition-related mortality usually oc- protected from browsing had a much higher rate of curs. This noncompetition mortality is due to log- survival than unprotected seedlings did (Osunkoya ging damage to roots or stems, greater exposure of et al. 1992). Understory trees in boreal, temperate, trees to wind, or simply background mortality fac- or tropical forests experience similar barriers to de- tors that are independent of harvest activities. velopment, and mortality rates may be very high. As trees develop, they experience an evolving set 4.7 Synthesis of stresses that lead to mortality. For saplings, shade tolerance is a key variable in light-limited systems. The dynamics of multiaged and other complex For example, in coastal British Columbia, Canada, stand structures govern many of the management mortality was directly related to shade tolerance, options for these stands. These dynamics indicate with the shade-intolerant paper birch experiencing the importance of recruiting and maintaining regen- a rate of mortality that was more than 10 times that eration in an understory canopy position. In some of the more tolerant western redcedar (Kobe and situations, light limitation may be a factor, whereas Coates 1997). In addition, for a group of boreal spe- moisture or nutrient limitations may be important cies in eastern Canada, Kneeshaw et al. (2006) con- in others. Moreover, in any situation, understory cluded that size was also a factor affecting sampling development is controlled, at least in part, by over- mortality because of the greater respiration costs of story competition, which may itself limit light or larger trees. However, on sites with moisture limi- moisture. Shade tolerance is a dominant factor lim- tations, shade and drought tolerances may be con- iting options to design stand structures; for exam- flicting factors and may lead to circumstances in ple, it would be counterproductive to design a stand which less shade-tolerant species are more likely to with shade-tolerant species in the overstory and in- survive (Caspersen and Kobe 2001). tolerant species in the understory. Similarly, mortal- Managed multiaged stands will typically be man- ity is also a function of these overstory/­understory aged at considerably less than maximum density trade-offs but can be reduced by maintaining total through their cutting cycles (O’Hara and Valappil stand density at less than maximum levels. 1999). At lower densities, well below the maximum, Growth patterns of multiaged stands may fol- competition-related mortality should be minimal. low the idealized pattern shown in Figure 4.14, or, But older trees may die from pathogens, wind, or more likely, follow countless other variations. For general senescence. Moreover, management op- example, subtle changes in overstory density will erations can contribute to mortality. Trees can be affect understory growth. Likewise, variations in damaged by harvesting operations or as a response understory density will affect understory growth to sudden changes in environmental conditions. when the overstory is held constant. Species vary in 36 Multiaged Silviculture their ability to respond to increased growing space the growth of understory cohorts, thereby con- and in their ability to endure low levels of resource centrating growth in either the understory or the availability. overstory. They can also modify overstory density Although the multiaged stand dynamics present- to affect species composition and control mortal- ed in this chapter highlight the limitations of man- ity. Understanding the dynamics of complex stand aging multiaged stands, they also demonstrate the structures is not only important for determining the many options available for managing these stands. limitations of managing these structures but also Managers have the flexibility to slow or accelerate for discovering the possibilities.