Springer Old Growth Forests - Chapter 6 ppt

Based on an extensive review of the literature on old-growth forests in boreal, temperate and tropical biomes, this chapter discusses 1 the distinctstructural and compositional features

Functional Relationships Between

Old-Growth Forest Canopies, Understorey

Light and Vegetation Dynamics

Christian Messier, Juan Posada, Isabelle Aubin, and Marilou Beaudet

6.1 Introduction

age), considerable amounts of large pieces of dead wood and a complex horizontaland vertical structure (see Chap 2 by Wirth et al., this volume) These threeelements create a unique understorey environment, including light, that differssomewhat from earlier successional or second-growth (i.e forests that haveregrown following harvesting) forests Identifying factors that influence variation

in light availability within forested ecosystems represents an important component

in our understanding of the complex determinants of understorey vegetationdynamics Based on an extensive review of the literature on old-growth forests

in boreal, temperate and tropical biomes, this chapter discusses (1) the distinctstructural and compositional features that are likely to influence the understoreylight environment in old-growth forests, (2) the particular understorey light condi-tions found under such forests, and (3) the unique understorey vegetation assem-blage that can develop in old-growth forests We focus, as much as possible, onshared trends among all three biomes, but we also discuss some of the fundamentaldifferences that differentiate them Comparisons with second-growth forests arealso often made to highlight the uniqueness of old-growth forests

6.2 Structural and Compositional Features of Old-Growth

Old-growth forests possess distinct structural and compositional features thatinfluence understorey light environment and vegetation growth and dynamics(Chap 2 by Wirth et al., this volume) Since such forests are generally foundwhere small-scale disturbances predominate, their disturbance regime tends to becharacterised by gap dynamics [see Chaps 2 (Wirth et al.) 10 (Bauhus), 13 (Graceand Meir), and 19 (Frank et al.), this volume] Old-growth forests generally have

canopies that are heterogeneous horizontally due to the presence of gaps, and highlystructured vertically due to variable tree heights and multiple layers of vegetation.Trees in old-growth forests often have longer longevity but are not necessarily

mid-shade-tolerant species of eastern North America, can live as long (380 years) as any

grandifolia (300 years) Similarly, Pseudotsuga menziesii, a rather shade-intolerantconifer on the west coast of North America, normally lives longer than the asso-

Contrary to popular belief, old-growth forests are not necessarily composed of

‘‘giant trees’’ Indeed, some of the largest trees in the world are early-successional

Eucalyptus regnans in Australia However, old-growth forests generally have olderand larger trees than managed forests simply due to the length of time they have had

to grow and develop since the last catastrophic disturbance

Also, old-growth forests do not necessarily contain more tree and other plantspecies than earlier successional or second-growth forests In fact, mid-successionalforests tend to be more species rich because they often contain both early- and later-successional species (Connell 1978) Over time, mid-successional forest havinglow disturbance will tend to have a more uniform environment, which allows thecoexistence of some fairly specialised plant species (Hubbell et al 1999) How it isthat some low disturbance old-growth tropical forests can contain several hundredtree, shrubs, vine, and epiphytic species in relatively small areas still remainssomewhat of a mystery This question has recently triggered an intense debateabout the veracity of the niche theory and a new theory, the neutral theory (Bell2000; Hubbell 2001), has been proposed In brief, the neutral theory states that thehigh species diversity found in some ecosystems is not due primarily to a highnumber of niches or highly specialised species, but rather to a stochastic processes

of extinction, immigration and speciation (Hubbell 2001) Many papers haveargued for or against the new theory (Volkov et al 2003; Chase 2005), but Gravel

et al (2006) suggested an elegant explanation where both theories (niche andneutral) can be reconciled They suggest that niche theory better explains thedistribution of species when species richness, niche overlap and dispersal capabil-ities of species are low, whereas the reverse is true for the neutral theory

Because old-growth forests favour shade-tolerant or late-successional treespecies that can become established and develop in the understorey, they tend tohave a more uneven or complex structure This structural complexity is neither wellunderstood nor studied Traditionally, foresters have simplified old-growth foreststructure as uneven aged, with a regular inverse J shape age or diameter classdistribution In reality, this is not always the case and old-growth forests exhibit atremendous variability of structures and compositions

The vertical distribution of foliage in the understorey has been shown to differbetween second-growth and old-growth forests, in boreal (Aubin et al 2000),northern temperate deciduous (Angers et al 2005), and tropical (Montgomeryand Chazdon 2001) forests Old-growth forests tend to have less vegetation near

the forest floor and a more continuous distribution of foliage vertically than youngforests (Brown and Parker 1994; Montgomery and Chazdon 2001) In a comparison

of old-growth northern hardwood forests with forests logged 10 30 years before (throughselection cuts and diameter-limit cuts), Angers et al (2005) observed the presence of adense and uniform sub-canopy foliage layer in forests that have been partially logged.They suggested that this layer resulted from the recruitment of pre-establishedshade-tolerant regeneration following simultaneous creation of numerous canopyopenings during partial harvesting Similar development of trees or shrubs afterhuman disturbance has been observed worldwide (Royo and Carson 2006).Due to the presence of large trees, with large crowns, single tree gaps tend to belarge in old-growth forests (Dahir and Lorimer 1996), while smaller trees withslender crowns that die in second-growth forests (often because they becomeovertopped) create smaller gaps or sometimes only sub-canopy gaps below thecanopy layer (Connell et al 1997) The creation of large gaps can be enhanced intropical forests where vines attaching tree crowns together cause simultaneous treefalls (Strong 1977) Also, due to sparser understorey vegetation in closed canopyparts of old-growth forests, canopy openings that extend to the forest floor are morelikely to occur than in second-growth forests (Montgomery and Chazdon 2001).Canopy gaps may be filled rapidly by already established vegetation, and if thatvegetation comprises mainly shade-tolerant trees, the gap will fill rapidly vertically.However, if the gap is occupied mainly by low-stature plant species such as lowshrubs or ferns, the gap may not fill quickly vertically, providing an opportunity formore shade-intolerant species to become established Therefore, in terms of pre-established vegetation at the moment of gap formation, the initial conditions arevery important to the future dynamic of that gap (Poulson and Platt 1996; Beaudet

et al 2004; Royo and Carson 2006)

Gaps in temperate deciduous forests can also be filled relatively quickly by crownexpansion of surrounding trees (Runkle and Yetter 1987; Frelich and Martin 1988;Young and Hubbell 1991; Brisson 2001) An average lateral growth of 18 cm peryear has been reported for temperate deciduous trees (Runkle and Yetter 1987).Lateral filling is however very limited in conifer forests (Umeki 1995; Stoll andSchmid 1998), which would explain why some old-growth conifer forests tend toremain open longer following gap formation (see Fig 6.1)

6.3 Understorey Light Environments and Dynamics

Old-growth forests present a complex, changing, and heterogeneous light ronment Understorey light availability varies depending on the type of vegetation,size and orientation of gaps, penumbral effects, leaf movement, cloud distributionand movement, atmospheric aerosols, topography, height of the canopy, seasonaltrends in plant phenology and seasonal and diurnal movement of the sun (Baldocchiand Collineau 1994; Gendron et al 2001) It is generally recognised that ground-levelmean light availability is not sufficient to capture the complexity of forest light

envi-environments (Nicotra et al 1999; Moorcroft et al 2001; Beaudet et al 2007).Other aspects of the light environment (variance, frequency distribution, spatialautocorrelation, shape of vertical profile, etc.) need to be taken into account, andoften better differentiate forest types (e.g old-growth vs second-growth forests).Forest canopies not only attenuate the quantity, but also modify the quality oflight that reaches the understorey In terms of light quality, they attenuate more thephotosynthetically active radiation, between 400 and 700 nm, than the far-redbetween 700 and 800 nm, which causes the red (655 665 nm) to far-red (725

735 nm) ratio to decrease under forest canopies Changes in this ratio have beenshown to affect many growth and morphological variables, especially in shade-tolerant plants (Lieffers et al 1999; Ballare´ 1999) However, light quality is known

to fluctuate in the same manner as light quantity (Lieffers et al 1999)

Compared to second-growth forests, understorey light availability in old-growthforests tends to vary much more, both horizontally and vertically, than at any other

Fig 6.1 Comparison of percent light transmittance near the forest floor and above the understorey vegetation among boreal, temperate and tropical biomes (mean data for the calculation listed in Table 6.1) Means at the forest floor and above the understorey vegetation among biomes were

the forest floor in the boreal biome compared to the other two biomes, the value was not

particular point in the natural succession of a forest (Nicotra et al 1999; Bartemucci

et al 2006) or compared to managed forests (Beaudet and Messier 2002) Althoughabsolute mean light levels at the forest floor and above the main understorey vegeta-tion layer tend to increase from tropical to boreal forests (Table 6.1, Fig 6.1), overallthey generally range from less than 5% in closed forests to a maximum of 65% fullsunlight in large recent gaps in boreal forests Spatially, the frequency distribution

of understorey light levels is often markedly right-skewed, with most micrositeshaving low light conditions, and a few microsites with higher light levels (Fig 6.2)

In the darkest areas of old-growth forests, the extremely low light conditions limitboth the survival and growth of the understorey vegetation, even of the most shade-tolerant species In understorey microsites, where the canopy transmits more thanapproximately 5 10% of full sunlight, the vegetation tends to be highly structuredvertically (Bartemucci et al 2006) In larger gaps, light levels as high as 65% above

layer, which tends to develop after gap formation (Beaudet et al 2004) This verticaland horizontal heterogeneity in light levels is also extremely dynamic (Aubin et al.2000) For instance, following a canopy disturbance caused by an ice storm thatincreased light near the forest floor from 1% to 20%, it took as little as 3 years forthe light level to recover to pre-gap conditions (Beaudet et al 2007) In fact, lightlevels often tend to become, at least momentarily, even lower than before theopening of the main canopy, due to development of a dense understorey layer(Beaudet et al 2004, 2007) Furthermore, constant understorey vegetation growthand dieback, tree mortality and large branch breakages continuously create afluctuating light environment Smith et al (1992) found little year-to-year correlation

in light environment in a mature lowland moist tropical forest of Panama, indicatingthe need for frequent assessment of the light environment for long-term studies ofplant responses Beaudet et al (2007) also found little correlation in a maturetemperate forest before and after a severe ice-storm, but good correlation thereafter.Becker and Smith (1990) found a very weak positive spatial autocorrelation (2.5 m)

in a mature tropical forest of Panama in a typical year, but autocorrelation up to22.5 m in a very dry year where leaf fall was severe

While average light availability reaching the forest floor might not differ greatlybetween old-growth and second-growth forests, the understorey vertical profile isoften quite different For instance, results reported by Montgomery and Chazdon(2001) indicate a stronger light attenuation between 9 m and 1 m in second-growthcompared to in an old-growth tropical forest (suggesting the presence of a denserunderstorey vegetation layer in second-growth) Similar results were found intemperate forests between managed and mature unmanaged forests (Beaudet

et al 2004) Such a sub-canopy sapling layer is expected to homogenise lightconditions near the forest floor in managed compared to old-growth forests (wheregaps are not all created simultaneously, hence greater heterogeneity) (Angers et al.2005) Accordingly, spatial autocorrelation between light measurements indicatesthe presence of larger patches with higher light in old-growth than in second-growthtropical forests (Nicotra et al 1999)

Fig 6.2 Frequency distribution of light levels (%PPFD) at 1 m above the forest floor of tropical (open bar: second growth, solid bar: old growth) (redrawn from Nicotra et al 1999), temperate (open bar: 1 yr after a 30% selection cut, grey bar: 13yrs after a 30% selection cut, and solid bar:

Beaudet et al 2004; old growth data from Beaudet et al 2007) and boreal forests (open bar: aspen stands, grey bar: mixed stands, solid bar old forests) (redrawn from Bartemucci et al 2006) Light availability in old growth forests increases from tropical to the boreal forests In all three figures, we can see that the frequency distribution of light is similar, except for one year after a 30% selective cut However, in all three biomes, we tend to find microsites with relatively high

%PPFD (> 10%) only in older or old growth forests.

Finally, understorey light at any particular point varies also temporally within ayear due to various phenological events In most of the tropics, alternating wet anddry seasons cause various patterns of leaf fall, and different tree species havedifferent timing and extent of leaf fall, thus creating more variability In temperatedeciduous forests, the seasonal variations in light are related to temperature varia-tion that makes deciduous trees shed their leaves in the autumn and grow them back

in the spring However, there exists a 2 4 week difference among species in terms

of timing of leaf production in the spring and leaf abscission in the autumn, andsuch differences can be used by some understorey plants that flush early or keeptheir leaves late in the season to gain some additional days of photosyntheticproduction As such, understorey plants can survive in extremely variable lightenvironments through acclimatisation of the form and function of foliage andcrown, or the timing of sprouting from rhizome and roots to capture the better litmicrosites that are constantly created Although all of these characteristics can befound to some extent in earlier successional stages, they tend to be more acute inold-growth forests Like the spatial distribution of light, frequency distributions

of light in old-growth forests are generally right-skewed and the understorey isexposed to low light most of the time and only occasionally to high light events(Oberbauer et al 1988) Note that, despite being rare, these high light events or

‘sunflecks’ can be crucial for plant survival in the shade (Chazdon 1988)

An important difference between tropical, temperate and boreal forests is that inthe former the sun passes near the sky zenith most of the year, while at higherlatitudes the sun tends to be at angles below the zenith for extended periods of time

respectively) the sun never reaches the sky zenith (Campbell and Norman 1998) As

a result, light in the tropics tends to be more vertically distributed and have a higherflux in the middle of the day than at higher latitudes This vertical distributionreduces the surface area of shadows projected to the forest floor and can contribute

to the development of a more complex vertical forest structure (but cf Chap 17 byGrace and Meir, this volume)

The major differences in and around gap light regimes among close matureforests are largely a function of canopy height, gap size, latitude and sky conditions(Canham et al 1990; Fig 6.3) A study by Gendron et al (2001) has demonstratedthe very complex variability in light conditions throughout the growing season andamong various types of microsites in secondary deciduous forests Some forests,such as 25-year-old Norway spruce, may have a greater net photosynthetic gainunder overcast days compared to sunny days, due in part to the higher penetration ofdiffuse light within the canopy (Urban et al 2007) The incredible complexity inboth spatial and temporal variability in the understorey light environment calls for are-assessment of the ‘‘gap’’ versus ‘‘non-gap’’ characterisations of the understoreyenvironment in most forests, particularly old-growth forests (Lieberman et al 1989;Beaudet et al 2007)

Simple measures of forest structure such as estimated aboveground biomassand leaf area index (LAI) are not correlated with average light transmittance(Brown and Parker 1994) Information about the vertical arrangement of the canopy

(e.g variation in leaf area density) provides better predictions of light transmission(Brown and Parker 1994) Although the spatial and temporal variability in under-storey light dynamic is hard to predict, there have been many recent attempts atmodelling it in various forest types (the different approaches are reviewed inLieffers et al 1999) Most models report a good correlation between simulatedand measured light values at any point above the understorey vegetation (Beaudet

et al 2002; Piboule et al 2005) These models incorporate some allometric andgeometric measures of trees, some values of tree species canopy transmittance, andpositioning of each individual tree Sensitivity analyses showed that the crownradius of individual trees has a large impact on predicted light transmission atthe stand-level (Beaudet et al 2002; Ru¨ger et al 2007) Light below the mainoverstorey canopy is related to tree density, basal area, length of the canopy, andtransmittance value of the canopy In contrast, light near the forest floor is influ-enced through complex interactions among canopy, subcanopy, and understoreyvegetation (Montgomery and Chazdon 2001; Beaudet and Messier 2002; Beaudet

et al 2004; Montgomery 2004; Bartemucci et al 2006) Aubin et al (2000)suggested different light extinction coefficients that take into account understoreyvegetation characteristics and accurately estimate understorey light transmission

6.4 Consequences for Understorey Vegetation

Composition and Dynamics

Little is still known about the understorey vegetation characteristics of old-growthforests As stated by McCarthy (2003), most definitions of old-growth forests arebased essentially on tree species composition, forest structure and disturbancecharacteristics Studies on understorey vegetation were essentially descriptivebefore 1980, and were mainly for the purposes of classification Matlack (1994a)pointed out the need for further research to fill ‘‘our monstrous ignorance about theunderstorey dynamics of forest communities’’ Numerous gaps in our knowledge ofunderstorey vegetation communities still exist For instance, long term or geograph-ically broad studies are missing This lack of basic knowledge impedes the devel-opment of a sound understanding of the understorey functional and structuralcharacteristics of old-growth forests

The many attempts at relating understorey vegetation diversity and dynamics

to light conditions have failed, probably due to the highly dynamic nature of thelight environment and to the importance of other factors such as soil properties,micro-topography, presence of dead wood, and time since last disturbance Ineffect, two recent studies have found that the spatial patterning of understoreyspecies groups under early-successional and old-growth forests is influencedmainly by factors other than light, including disturbance history, chance and neigh-bourhood effects such as clonal reproduction (Frelich et al 2003; Bartemucci et al.2006) Furthermore, since increases in light conditions might not be followed by

an increase in soil resource availability (see Chap 10 by Bauhus, this volume),

understorey plants need to be able to establish and develop in a belowgroundenvironment where belowground competition levels are high.

Although the overall alpha diversity (i.e number of species) of old-growth forests

is generally not greater than in forests of other successional stages, old-growth forestsare still characterised by fairly unique plant species or assemblages (Table 6.2) Some

of the uniqueness of ‘‘old-growth’’ species assemblages can be related to threecharacteristics: (1) the fact that those forests have been there for long periods oftime, allowing particular plants to establish and prosper; (2) the special understoreylight conditions created by the small-scale disturbance regime; and (3) the presence

of a very heterogeneous micro-topography of mounds and pits created by theuprooting of large trees (Beatty 2003) This spatial microsite heterogeneityprovides a wide range of environmental conditions and may ‘‘serve to segregatespecies that might otherwise out-compete one another’’ (Beatty 2003) Manyspecies are associated with windthrow mounds or bases of large trees (Rogers 1982),while others need nurse logs for their establishment (Scheller and Mladenoff 2002).Many studies have shown that the understorey of old-growth forests is dynamicand not in a steady-state (e.g Brewer 1980; Davison and Forman 1982) Theselong-term changes in understorey herbaceous communities might also be attribut-able to disturbances that happened centuries ago Brewer (1980) was still observingchanges in the understorey caused by a major disturbance that occurred 150 yearsago Oren et al (2001) suggested that herb diversity should be highest in youngstands, lowest in mature stands, and increase again in old growth stands

6.4.1 Traits of the Understorey Vegetation

To be successful in old-growth forests, understorey plant species need to be adapted

to survive for prolonged periods with a low availability of resources In old-growthforests, the amount of time a tree spends in the shade could last up to severalhundreds of years because of the small and irregular disturbance characteristic of

balsamea,which stays small by bending and thus can survive for almost 100 years

in very deep shade In the following section, we present a general list of traits thatcan favour plant survival in the shade We have separated them into traits related toplant form and function, and those related to resource allocation

6.5 Acclimatisation of Plant Form and Function to Low