A review of the roles of forest canopy gaps

Adele Muscolo · Silvio Bagnato · Maria Sidari · Roberto Mercurio.

REVIEW ARTICLE

A review of the roles of forest canopy gaps

Adele Muscolo · Silvio Bagnato · Maria Sidari · Roberto Mercurio.

Received: 2014-05-19; Accepted: 2014-06-20

? Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2014

Treefall gap, canopy opening caused by the death of one or more trees, is the dominant form of disturbance in many forest systems worldwide. Gaps play an important role in forest ecology helping to preserve bio- and pedo-diversity, influencing nutrient cycles, and maintaining the complex structure of the late-successional forests. Over the last 30 years, numerous reviews have been written describing gap dynamics. Here we synthesize current understanding on gap dynamics relating to tree regeneration with particular emphasis on gap characteristics considered critical to develop ecologically sustainable forest management systems and to conserve native biodiversity. Specifically, we addressed the question: how do gaps influence forest structure? From the literature reviewed, the size of gaps induces important changes in factors such as light intensity, soil humidity and soil biological properties that influence tree species regeneration and differ in gaps of different sizes. Shadetolerant species can colonize small gaps; shade-intolerant species need large gaps for successful regeneration. Additionally, gap dynamics differ between temperate, boreal, and tropical forests, showing the importance of climate differences in driving forest regeneration. This review summarizes information of use to forest managers who design cutting regimes that mimic natural disturbances and who must consider forest structure, forest climate, and the role of natural disturbance in their designs.

biodiversity; gap cutting; gap dynamic; forest management; forest restoration

Introduction

The importance of natural disturbances in shaping landscapes and influencing ecosystems is now well recognized in ecology. Disturbance, defined as “any relatively discrete event in time that disrupts ecosystem, community or population structure and changes resources, substrate availability, or the physical environment” (White and Pickett 1985), plays an important role in all natural ecosystems. Both small-scale (death of one or few trees) and large-scale disturbances (fires, wind storms, insect outbreaks and others) can create gaps in forest canopies that are often ideal locations and conditions for rapid plant reproduction and growth. Perhaps the most thoroughly studied impact of gap formation is how increased light helps to maintain floristic richness. Denslow (1987) theorized that the rich species diversity in tropical systems exists because each species is competitively superior for a portion of its life. Since most trees have long life spans, they exist in a variety of microenvironments as they grow. The death of a nearby tree dramatically changes light, temperature, soil moisture, and available nutrients. These conditions will favor some species, but not all. As the gap is filled, the microclimate and nutrient status slowly return to pre-disturbance levels and the resulting conditions will tend to favor a different suite of species. If a growing tree is competitively superior for a portion of its life, it will persist (Denslow 1987; Wright 2002).

On recent decades, increasing emphasis has been placed on designing silvicultural treatments that mimic natural disturbance to restore forests in a natural way, preserving biodiversity and ecosystem function while optimizing harvest and yield (Lindenmayer and Franklin 2002; Abrari Vajari et al. 2012). Gap based silviculture has recently been included in proposals for managing forests across the world and gap dynamics has been described in many temperate evergreen broadleaf (Yamamoto, 1992, 1994; Rebertus and Veblen 1993), temperate deciduous broadleaf (Runkle 2000; Yamamoto 1996), temperate coniferous (Spies et al. 1990), subalpine (e.g., Kanzaki 1984; Foster and Reiners 1986; Lertzman and Krebs 1991; Lertzman 1992; Yamamoto 1995) and boreal (Kneeshaw and Bergeron 1998) forests. In tropical forests, most gap research was based on short-term dynamics (i.e., years to decades) using large-scale permanent plot studies (e.g., Condit 1995) or on long-term dynamics (i.e., centuries to millennia) through paleoecological studies (e.g., Bush and Colinvaux 1994). In tropical forests, where the number of species is high and the range of life-history traits is broad, small-scale heterogeneity in post-disturbance microsite conditions greatly limits the ability to anticipate future stand composition and structure (Baker et al. 2005).

The impact of gap regimes on plant population dynamics is of interest to ecologists (Naaf and Wulf 2007). Despite numerous studies carried out to identify which statistical distribution best describes gap sizes (Foster and Reiners 1986; Lertzman and Krebs 1991; Yamamoto et al. 2011), whether a gap opening is a random or clumped process in space and time (Brokaw 1985; van der Meer and Bongers 1996a; Nuske et al. 2009), if the use of a large number of gaps positively affects the forest regeneration (Cuevas 2003; Kathke and Bruelheide 2010), and how gap shapes influence microclimates and species colonization Brown 1993; van Dam 2001), numerous questions related to gap frequency, size and shape remain unanswered. In addition, more precise gap descriptions will be useful to test predictions of forest models (Chave 1999; Dubé et al. 2001; Robert 2003) and to design silvicultural systems that aim to mimic natural gap disturbance regimes (Lundquist and Beatty 2002; Schliemann and Bockheim 2011). In this review, we synthesize current understanding of gap dynamics related to tree regeneration with emphasis on gap characteristics that are considered critical to develop an ecologically sustainable forest management system. Specifically, we investigated the following questions: (1) does gap size influence forest restoration? (2) does gap size differ according to the dynamics of different forest types? Do gap shape, age and distribution affect forest regeneration dynamic? The aim is to provide insight into the capacity of gap-based silvicultural regimes to guide forest biodiversity conservation, which might be useful in legal and policy environments and to forestry managers.

Gap dynamics

In temperate forests, gap opening is the major process determining regeneration development (Runkle 1982; Sapkota et al. 2009) and a vast body of literature exists on the effects of canopy gaps on tree recruitment patterns (Yamamoto 2000; Harcombe et al. 2002; Kwit and Platt 2003; Bottero et al. 2011).

When one or few canopy trees die in a forest, mainly due to natural disturbance, this creates a hole in the canopy called a 'gap'. The term gap or canopy gap is generally used to refer to such empty areas within forest canopies. Spatial heterogeneity of canopy structure means that gaps of different size and shape exist throughout a forest stand. Over time, these spaces are filled with other trees (Watt 1947; Whitmore 1989). Gaps, once formed, do not remain static but become localized sites of regeneration and subsequent growth where tree regeneration is usually a result of released advance regeneration or recruitment from buried or dispersed seed. This phenomenon, termed 'gap dynamics' (Van der Maarel 1988; Brokaw and Busing 2000; Kimmins 2004), attracted many forest scientists and ecologists, because gap dynamics is closely related with practical forest applications such as forest conservation and natural regeneration as well as with niche partitioning and species adaptation (Yamamoto 2000). The death of a single canopy tree or several neighboring trees introduces environmental heterogeneity to the forest ecosystem, such as changes in light levels, nutrient availability, litter depth, and regeneration microsites associated with snapped or uprooted trees. This heterogeneity plays an important role in maintaining the structure and composition in gap dynamic forests (Grubb 1977; Denslow 1980; Whitmore 1989). The components of gap dynamics, consisting of spatial and temporal factors and of magnitude, are also applicable to disturbance dynamics. Comparing the results of research on gaps and on dynamic disturbances in different geographical locations is a useful tool to predict the impact of different types of disturbance in different forest ecosystems.

Bottero et al. (2011) investigated gap dynamics in mixed beech (Fagus sylvatica L.), silver fir (Abies alba Miller), and Norway spruce (Picea abies L., Karst.) old-growth forests of Lom in the Dinaric Mountains of Bosnia and Herzegovina. Their results suggested that gaps were mainly formed by endogenous senescence of single canopy trees. Exogenous disturbance agents, most likely related to wind and snow, acted mainly as secondary agents in breaking weakened trees and in expanding previously established gaps. Although the findings were partially consistent with other studies of gap disturbance processes in similar oldgrowth forests in central Europe, the observed gap dynamic placed the Lom core area at the end of a gradient that ranged from forests controlled by very small-scale processes to those where large, stand-replacing disturbances predominated.

Rugani et al. (2013) studied two beech forest reserves in southern Slovenia. They examined the structural characteristics of the two forest reserves based on data from sample plots and complete inventory obtained from four previous forest management plans. To gain a better understanding of disturbance dynamics, they used aerial imagery to study the characteristics of canopy gaps over an 11-year period in the Kopa forest reserve and a 20-year period in the Gorjanci forest reserve. Their results suggested that these forests were structurally heterogeneous over small spatial scales and exhibited relatively high annual rates of coverage by newly established (0.15 and 0.25%) and closed (0.08 and 0.16%) canopy gaps. New gap formation was dependant on senescent trees located throughout the reserve, leading to the conclusion that these stands were not even-sized, but rather unevenly structured. This was due to the fact that the disturbance regime was characterized by low intensity, small-scale disturbances. Fox et al. (2000) and Herwitz et al. (2000) showed changes in gap dynamics over time, concluding that the total gap area had a clear decreasing trend: smaller gaps vanished in the course of time and the larger ones tended to shrink. Meyer et al. (2000) showed that mature beech stands close gaps via vertical growth of gap-neighboring and understorey trees. The decline of the number of canopy gaps is well reflected in gap density rather than in spatial distribution (Spellmann, H. 1991).

Gap ecology

The ecological characteristics of a gap differ from those of thesurrounding forest. Gaps are brighter and warmer due to increased irradiance, and their surface soils contain more water due to the reduction in plant transpiration (Denslow 1987). Treefall gaps exhibit substantial changes in soil composition, due to differences in microclimate that affects soil microbial biomass amount and activities which in turn change soil chemical and physical properties (Denslow 1987). The degree of gap opening is correlated with many abiotic (e.g., light availability, soil and air temperature, air vapor pressure deficit, soil nutrient and water content) (van Dam 2001) and biotic factors (e.g., humus quality, micro-flora and micro-fauna, herbivore); all variables may in turn influence tree seedling establishment and development, gap floristic composition and structure (Zhang et al. 2013). Gaps also play an important role in forest ecology helping to maintain bioand pedo-diversity and influencing nutrient cycling. Gap ecology retrospective studies have been carried out mainly by Platt and Strong (1989), Denslow and Spies (1990), Coates and Burton (1997) and McCarthy (2001). With respect to models for simulating processes in treefall gaps, recent research has been reported by Schliemann and Bockheim (2011) showing that large openings tend to have microclimates and return intervals significantly different than those of smaller treefall gaps and surrounding intact forest area, and these affect forest ecology and soil ecosystems.

Soil and air temperature

The increase in direct radiation from canopy opening increases air temperature close to the ground which in turn can increase soil temperature (Malcolm et al. 2001), causing the mortality of young tree seedlings when the topsoil temperature is ＞50 °C (Waring and Schlesinger 1985; Helgerson 1990). The relationship between gap size and soil temperature needs further study. Several studies in conifer stands showed no significant differences in mean air temperature during growing season between gaps of different sizes (Gray et al. 2002; Muscolo et al. 2007 a, b). In contrast, many other authors demonstrated that soil temperature increases with increasing gap size, both in coniferous stands in the Pacific Northwest, USA (44?45° N) (Gray et al. 2002), and in Southern Apennines, Italy (38° N) (Gugliotta et al. 2006; Muscolo et al. 2007 a, b). Sariyildiz (2008) found in oak, beech and chestnut stands in northeast Turkey (41° N) an inverse correlation between gap size and temperature increase. Ritter et al. (2005) and Gugliotta et al. (2006) showed that soil temperature was higher during growing season in the centre compared to the edges of gaps. But an increase in soil temperature during summer was also observed in the northern part of gaps (Bauhus 1996; Wright et al. 1998; Gray et al. 2002) in mature, deciduous and coniferous forests. These results, even if in some case controversial, show a relationship between gap size, air temperature and soil temperature.

Soil moisture

Soil moisture is a crucial factor for tree regeneration in many areas (e.g. in Mediterranean forests, Giacobbe 1958), and it is strictly related to alterations of rainfall patterns caused by climate change (IPCC 2007). The impact of gap opening is particularly significant on soil moisture content, which is higher inside gaps than under the surrounding closed canopy as observed in a wide variety of forest types: tropical forests (Vitousek and Denslow 1986; Veenendal et al. 1996; Ostertag 1998; Denslow et al. 1998); pine forests (Brockway and Outcalt 1998; Zhu et al. 2003); temperate conifer forests (Malcom et al. 2001; Gray et al. 2002; Cutini et al. 2004; Albanesi et al. 2005); and temperate hardwoods (Bauhus and Bartsch 1995; Ritter et al. 2005; Ritter and Vesterdal 2006; Gálhidy et al. 2006; Scharenbroch and Bockheim 2007a; Sariyildiz 2008).

The higher soil moisture content in gaps than in surrounding forest was probably due to an increase in rainfall and to a decrease in transpiration (Zirlewangen and von Wilpert 2001; Zhu et al. 2003).

Soil water conditions vary greatly according to gap sizes (Ochiai et al. 1994). In conifer forests in the Southern Apennines, soil moisture during the growing season differed between gaps of different sizes, showing the highest value in the small gaps (185-380 m2) (Muscolo et al. 2007 a, b). In temperate hardwoods, differences in soil moisture between gaps of different size were not statistically significant (Sariyildiz 2008), suggesting that location is important in determining soil moisture and temperature differences between gaps and surrounding canopy, and contributing to the differences observed in air temperature and rainfall between temperate and tropical forests.

No differences were reported between soil moisture in gap centres and gap edges in pine stands (Palik et al. 1997; McGuire et al. 2001; Gagnon et al. 2003; Gugliotta et al. 2006). In contrast, soil moisture was greater in gap centres and decreased towards gap edges in Douglas-fir forest (44?45° N) (Gray et al. 2002), in silver fir stands (38° N) (Albanesi et al. 2005), in beechdominated forest (55° N) (47.9° N) (Ritter et al. 2005; Mihók et al. 2004) and in mixed hardwood–white pine stands (47° N) (Raymond et al. 2006), presumably due to differences in rooting density (Silver and Vogt 1993; Denslow et al. 1998).

Solar radiation

Many studies of tree regeneration have focused on the relationships between gap size, light levels and regeneration success. Poulson and Platt (1989) and Messier (1996) reported that at any geographic location, the amount of light entering the gap depends on the size and topographic position of the gaps, on the location within a gap, on the height of the surrounding canopy, and also on the sun angle and sky conditions. Numerous studies (McGuire et al. 2001; Diaci 2002; Gray et al. 2002; Zhu et al. 2003; Mihók et al. 2005; Albanesi et al. 2005; Gugliotta et al. 2006; Gálhidy et al. 2006; Muscolo et al. 2007 a , b; Raddi et al. 2009; Diaci et al. 2012) supported these findings, showing that the amount of solar radiation reaching the ground in a gap is directly related to the size of the canopy opening and also to the gap position. Higher PAR (photosynthetically active radiation) transmittance values were detected in gap centres rather than at gap edges (Canham et al. 1990; Brown 1996; Gray and Spies1996; Palik et al. 1997; McGuire et al. 2001; Diaci 2002; Gagnon et al. 2003; Ritter et al. 2005; Gálhidy et al. 2006). A gradient of increased light from the southern to the northern edge of gaps was also reported by many researchers in the northern hemisphere during the growing season (Coates 1998, 2000; Wright et al. 1998; McGuire et al. 2001; Gray et al. 2002; Gagnon et al. 2003; Mihók et al. 2005; Raymond et al. 2006; Diaci et al. 2008). Diaci et al. (2008) and Caquet et al. (2010) found an interesting positive relationship between seedling height and light availability: with increasing light intensity and duration of light exposure, seedling height increased.

Nutrient cycling and soil organic matter turnover

In forest gaps there is a high rate of soil organic matter decomposition and mineralization, leading to increased levels of nutrients (Collins and Pickett 1987; Parsons et al. 1994; Zhang and Liang 1995; Poulson and Platt 1996; Palik et al. 1997; Denslow et al. 1998; Zhu et al. 2003; Prescott et al. 2003; Ritter 2005; Ritter and Vesterdal 2006; Muscolo et al., 2007b; Scharenbroch and Bockheim 2007a, 2008). According to Scharenbroch and Bockheim (2008) in situ surface C efflux was significantly greater in gaps, likely in response to increased solar radiation and soil temperature which, in turn, increased the mineralization of organic matter. An increase in soil temperature is generally positively related to an increase in soil microbial activities that regulate nutrient cycles in soil. Increments in ground nitrogen levels were also observed in artificially created gaps, and this may be strictly related to the enhanced growth rates of pioneer tree species (Denslow et al. 1998).

Studies of the effects of gap sizes on litter decomposition rates showed conflicting results. Lower decomposition rates in large gaps compared to small gaps or to soil under a closed canopy were observed by Zhang and Zak (1995), Prescott et al. (2003), Ritter (2005) and Sariyildiz (2008). Conversely, Denslow et al. (1998) found no significant relationships between gap size and litter decomposition rate. Other studies carried out over 4 years after gap formation in gaps of different sizes in conifer stands in the Apennines indicated that C/N ratio, an index used to monitor the decomposition of litter (Taylor et al. 1989), was greatest in small gaps (where humification processes prevailed) and lowest in large gaps (where mineralization processes prevailed) (Muscolo et al. 2007a, b; 2011). This finding suggested that small gaps were more favorable for late-successional species whereas large gaps encouraged early-successional species as Pinus. Considering that soil microbial biomass is either a source or sink of available nutrients and plays a critical role in nutrient transformation in terrestrial ecosystems (Singh et al. 1989), any change in microbial biomass has a direct influence on ecosystem stability and fertility (Smith et al. 1993). For these reasons, microbial biomass is always used for assessing soil quality under different types of vegetation (Groffman et al. 2001; Zeng et al. 2009) as well as for evaluating soil perturbation, restoration (Ross et al. 1982; Smith and Paul 1990) and changes induced in soils by forest management. Therefore, information on variations in soil microbial biomass is needed to improve our understanding of the effect of soil management on soil nutrient availability in gaps of different sizes compared to sites under canopy cover. Muscolo et al. (2007b) demonstrated that the greatest amount of microbial biomass and largest populations of bacteria and fungi in small gaps in pine forests contributed to more rapid and balanced turnover of organic matter and nutrients, indicating that creation of small gaps represents a silvicultural practice with minor environmental impact. QBS-ar index (biological soil quality, Parisi 2001) is based on the abundance of micro-arthropods in the soil and was measured to compare soil quality and fertility in gaps of different sizes. Blasi (2010) demonstrated that this index did not vary in soil inside gaps compared with soil under a beech canopy, suggesting that gaps did not influence micro-arthropod abundance in the context of forest ecology, and that creation of gaps as a silvicultural method was ecologically sustainable.

Biodiversity and vegetation dynamics

Gaps increase habitat diversity, structural complexity, fauna and flora species diversity (Runkle 1982, 1991; Denslow 1987; Levey 1988; Whitmore 1989; Attiwill 1994; Tews et al. 2004; Obiri and Lawes 2004; Pedersen and Howard 2004; Schnitzer et al. 2008; Wang and Liu 2011; Gray et al. 2012).

Higher species diversity was recorded in gaps than in closed forest (Denslow 1980; Sipe and Bazzaz 1995; Busing and White 1997; Schumann et al. 2003; Kumar and Ram 2005; Van Couwenberghe et al. 2010) and species exhibited niche differentiation along a gap-size gradient (Wang and Liu 2011). Zhu et al. (2003) reported that density of seedlings older than 1 year increased with increasing gap size or canopy openness (OP), suggesting that microclimate changes (light, soil water, and airflow exchange), are important in alleviating seedling competition in gaps created by thinning. This was also reported for studies of 540 m2gaps created in the Central Apennines, Italy (43° N). In 80-100 year-old silver fir (Abies alba Mill.) stands, shadeintolerant species were normally replaced by shade-tolerant species (Mercurio and Spampinato 2001), thus broadleaved species successfully regenerated in gap centres, while silver fir grew at gap edges (Mercurio 1994, 2000; Cutini et al. 2004).

In natural forests biodiversity probably increased shortly after gap creation and decreased with canopy closing, because gaps resemble the natural forest (in species composition) over time owing to plant succession. Several studies showed the importance of gaps in maintaining the diversity and regeneration of species within old-growth forests in subtropical (Barik et al. 1992) and southern tropical regions of India (Chandrashekara and Ramakrishnan 1993, 1994), China (Li et al. 2005; Zang and Wang 2002; Zang et al. 2005) and in temperate (Vetaas 1997) and subtropical forests of Nepal (Sapkota et al. 2009) and of North Carolina (Xi et al. 2008). In plantation forests composition and structure in gaps may be distinct from the rest of the forest, even after canopy closing. A characteristic of plantation forests is their consistently low species diversity (Coates and Burton 1997). Gaps in plantation forests can, however, provide habitat for native species to germinate, survive and grow. In plantation forest, when native tree species become canopy dominants in gaps, thespecies diversity of both canopy and understory increase significantly in comparison to the original forest, potentially leading to an overall increase in biodiversity (Nakamura et al. 2005; Dupuy and Chazdon 2008). Dietze and Clark (2008) quantified the abundance, competitive ability, and interspecific variability of vegetative reproduction in 18 replicated experimental gaps in the southern Appalachians and Carolina Piedmont, USA to assess the potential role of sprouting in driving gap dynamics. In a fouryear study they monitored annual rates of damaged adult survival, sprout initiation, growth, and mortality, and compared these to the performance of gap-regenerating saplings. Recruitment from sprouts constituted 26–87% of early gap regeneration and formed the dominant pathway of regeneration for some species. Sprouts from recently damaged trees also grew significantly faster than the saplings with which they competed. For all measured demographic rates (damaged tree survival, sprout initiation, number, growth, and survival) differences among species were large and consistent across sites, suggesting that vegetative reproduction was an important and non-neutral process in shaping community composition. Sprouting ability did not correlate strongly with other life-history trade-offs, thus sprouting potentially provided an alternate trait axis in promoting diversity. Rouvinen and Kouki (2011) examined the effects of gap size and canopy openness (experimentally manipulated) on within-gap variation and within-gap microhabitat variability (disturbed vs. undisturbed forest floor) in field settings of Pinus sylvestris L. dominated forest. Natural and artificial (direct seeding of silver birch Betula pendula Roth) tree regeneration and development was monitored both on disturbed (scarified soil patches) and undisturbed forest floor during three growing seasons. Results showed that gaps can be valuable in diversifying stand structure but to be successful and rapid, tree regeneration needs disturbed forest floor. Pine regenerated abundantly but birch had clearly lower regeneration, especially in small gaps.

Thus, considering the regeneration pattern, it is possible to predict with high accuracy which species can successfully establish and grow in a given canopy gaps (Qin et al. 2011).

Gap origins, characteristics and geometry

Gap origins

Natural gap formation is generally attributed to wind, snowfall, insects, diseases, acid deposition, drought, climate change, and fires. However, several studies identified other factors responsible for gap formation, including soil properties such as shallow depth to bedrock, high water table and poor drainage, and presence of coarse fragments (Liu and Hytteborn 1991; Bockheim 1997; Lin et al. 2004; Woods 2004; Scharenbroch and Bockheim 2007b), tree species characteristics such as dbh (diameter at breast height), flat-rooted pattern (e.g., Tsuga, Picea, and Abies), and tap-rooted pattern (e.g., Pinus, Quercus, and Acer) (Liu and Hytteborn 1991; Clinton and Baker 2000; Lin et al. 2004).

In natural gaps the tree generally remains in the gap and, as a consequence of wind throw, often with the root ball exhumed from the soil, but attached to the tree.

Artificial gaps result from silvicultural treatments. Stumps and root systems are normally left in the ground. Therefore soil disruption and biomass removal are different in man-made and natural gaps. Felling a single tree results in a gap formation similar to the natural situation. But usually more trees are felled in a small area and thus the gaps are larger, resulting in far higher light levels.

Gap size

Gap size is often used as an indicator of environmental heterogeneity and resource sequestration in gaps. Gap size reflects the magnitude of the disturbance (i.e. the type, number and size of falls, Ogden et al. 1991; Midgley et al. 1995; van der Meer and Bongers 1996; Lima et al. 2008), which has a direct influence on gap microclimate and understory damage levels (Zhu et al. 2007).

The size of a gap can strongly influence vegetation growth, nutrient cycling (Zhang and Zak 1995; Gray et al. 2002; Muscolo et al. 2007b) and can have considerable effect on a number of biological processes. McCarthy (2001) reported that gap disturbance determines forest structure and processes to a greater extent than previously assumed, showing that boreal forests dominated by the shade-tolerant fir (Abies) – spruce (Picea) complex are particularly well-adapted to the development of long-term, old-growth continuity in the absence of large-scale disturbance. Coates (2002) showed that seedling recruitment success in multispecies northern latitude forests varied as a function of mature tree canopy cover, gap size and position in a gap. Recruitment was abundant within canopy gaps across a wide range of gap sizes (20?5000 m2), but recruit numbers dropped off rapidly under the closed forest canopy and in the open conditions of clearcuts. Inside canopy gaps, recruitment was similar in each gap position in small gaps (＜300 m2). Conversely, in larger gaps in northern latitude forests, recruitment increased from the sunny northern to the shaded southern portions of gaps. This was true for all tree species regardless of their shade tolerance. There was no evidence of gap partitioning by any of the tree species during the regeneration phase, suggesting that adaptation to the subtleties of gap size during early recruitment were not well developed in these tree species. Favorable locations for emergence and early establishment of germinates were less favorable for growth and survival of established seedlings. Tree abundance and species diversity appeared to be controlled more by differentiation between growth and survival niches than by regeneration niches. From the perspective of forest management, abundant natural regeneration of all dominant tree species of these mixed-species forests could be obtained after partial cutting

Schliemann and Bockheim (2011) suggested that the maximum gap size should be set at 1000 m2. Larger openings tended to have microclimates and return intervals significantly different from those of smaller tree-fall gaps. Subsequently, Kern et al. (2013) evaluated the influence of harvest-created gap sizes (6, 10, 20, 30, and 46 m diameter gaps and uncut references) over 12 growing seasons on planted tree seedling growth and survival for four tree species that tend to experience poor recruitment in bothmanaged and unmanaged northern hardwood forests in eastern North America. All four of their species grew taller with increasing gap size, while survival was highest in intermediate gap sizes. Although gap size had statistically significant effects on growth and survival, the magnitude of the effects were modest. This study highlights the management challenges of using gap size as a tool to influence future forest composition in forests with overly abundant deer and pervasive shrub layers, and underscores the importance of silvicultural prescriptions that include measures for reducing these impacts.

Gap size and gap age are correlated with each other and give important information for sustainable forest ecosystem management. Diaci et al. (2012) estimated that beech recruitment generally needed 100 years or more to reach 20 m in height and fill gaps. In an old Birch forest of the dinaric mountains, Diaci et al. (2012) found that gaps of 5–15 m in diameter will fill in 5–40 years while medium gaps of average size of about 200–300 m2require around 30–60 years for closure. Conversely, in temperate forests, a negative relationship was observed between seedling height and gap size, suggesting that larger gaps will be filled in less time, due to reduced seedling competition for light, nutrients and water than in small gaps (Bullock 2000). Gap size obviously differs with respect to tree size and crown dimensions, and the differences depend also on single- or multiple-tree falls, and on gap age. Zhu et al. (2014) showed that regeneration density exhibited a significantly positive correlation with gap size providing quantitative evidence of the large effect of gaps on increasing regeneration by woody species.

It has been well established that gap size is the major factor determining tree species composition. Among other factors, gap size often determines whether the available growing space will be occupied by early or late successional species (Denslow 1987). Shade-intolerant species or early-successional species are often recruited only in the larger gaps and are usually established after gap formation, or in young gaps where more light is available (Canham 1989; Yamamoto 1992; Coates and Burton 1997; Mc Carthy 2001; Obiri and Lawes 2004; Nagel et al. 2010). According to Whitmore (1982) 1000 m2represents the minimum gap size needed for successful regeneration of shade-intolerant species. In contrast, Busing (1994), studying old-growth Appalachian cove forests, determined that a gap size of 400 m2was the threshold below which intolerant species were unable to establish.

If we consider that seedling growth and density of shadeintolerant species increased significantly with increasing gap size in Eucalyptus regnans (Van de Meer et al. 1999) and in Pinus thumbergii (Zhu et al. 2003) the statement of Ciancio et al. (2006) that a pionier species like Pinus laricio could survive in gaps of 60–100 m2is inconsistent. Indeed, Zhu et al. (2003) reported that although P. thunbergii seeds germinated in small gaps even under a closed canopy, a minimum gap ratio ≥1.0 (gap diameter to stand height) was required for survival and development to seedling stage, and a gap ratio ≥1.5 was needed for further development to sapling stage. Garbarino et al. (2012) reported that early successional and shade-intolerant species such as sycamore maple (Acer pseudoplatanus) and rowan (Sorbus aucuparia), were present only in larger gaps in a Bosnian old-growth forest. Shade-tolerant species or late-successional species grew better in small or older gaps (Canham 1989; Obiri and Lawes 2004; Nagel et al. 2010; Wang and Liu 2011). In general, shade-tolerant species, usually existing as advanced regeneration, have a great chance to respond to small gap openings. The physiological and morphological plasticity of shade-tolerant species allows rapid response to increased light environments. Smaller gaps in stands of pionier species favor late successional species that are frequently native (Van der Meer et al. 1999; Gugliotta et al. 2003, 2006; Kint et al. 2006; Muscolo et al. 2011). Large openings (＞1000 m2), created through cataclysm (fires, cyclones, downdrafts, mass earth movements), have differenct characteristics that treefall gaps. In particular, very large openings reduced shading from surrounding trees and consequently had higher solar radiation and soil temperature than small openings (Schliemann and Bockheim 2011).

The ratio between gap diameter and the height of the tallest surrounding trees (d/h ratio) should not exceed 1-1.5 (Cappelli 1988, Piussi 1994; Del Favero 2010). The seedlings of different tree species require different amounts of light to grow successfully. Shade-tolerant species such as western hemlock can survive under relatively dense forest canopies where lightdemanding species such as larches cannot. Malcom et al. (2001) related gap size, in particular d/h ratio, to the regeneration of species with different sensitivity to light. Their results showed that a d/h ratio ＞ 2 is optimal for the regeneration of shadeintolerant species (Larix sp. and Pinus sp.), a d/h ratio ranging between 1.0 and 2.0 was ideal for intermediate shade-tolerant species (Douglas fir) while a ratio ＜1.0 was required for shadetolerant species (Abies sp.). Nyland (2002) suggested that gap openings should be 1 or 2 times wider than the height of the surrounding trees for a better tree regeneration.

In short, the above findings suggest that gap size had a strong influence on tree species regeneration, especially through its affects on light intensity and soil humidity. Tree species regeneration is different in gaps of different sizes and species-specific survival and growth strategies are related to gap size and type of vegetation. Thus, gaps of different sizes can be considered one of the most important mechanisms for the maintenance of tree species diversity in forests.

Gap shape

Gap shape distributions are important descriptors of the forest disturbance regime. Gap shape substantially influences gap microclimate and can reflect the direction and the architecture of the falling tree (Brown 1993; Salvador-Van Eysenrode et al. 1998). Gap shape is very important in determining site resource availability (Canham et al. 1990; Lertzman and Krebs 1991). In general, irregular narrow gaps will receive far less p…. a….. r….. (PAR) at ground level than circular gaps of the same size. Numerous shapes have been recognized, including dumb-bell or chablis (Oldeman 1978), ellipse (Runkle 1981), and triangle (Salvador-Van Eysenrode et al. 1998).

However, gaps often are irregularly shaped (Lertzman and Krebs 1991; Battles et al. 1996; Gagnon et al. 2004). Kotanen(1997) suggested that species with poor dispersal ability were slower to colonize larger or rounder gaps than smaller or less circular gaps. Conversely, dispersive and seed-banking species were less sensitive than poor dispersers to gap size and shape, and less confined to gap edges. He found that species reproducing largely by clonal growth (bulbs and perennial graminoids) were initially most sensitive to gap size and (to a lesser extent) to shape, reaching their greatest abundance in small and (or) rectangular openings. Species relying on seed dispersal (annual grasses) also tended to do the best in smaller gaps, but were less concentrated near gap edges. Species relying on seed dormancy were least sensitive to gap size, shape, and distance from an edge. These results suggest that species respond to gap size and shape in ways consistent with their reproductive biology, highlighting the importance of the dimensions of gaps in driving plant community composition.

When gap shapes are more irregular, the effects of edge on the inner-gap environment become more pronounced because of the increased competition for both aboveground (light) and belowground (water, nutrients) resources (Gagnon et al. 2004). However, even if natural gaps are irregularly shaped (Spies et al. 1990; Lertzman and Krebs 1991; Battles et al. 1996; Gagnon et al., 2004; Schliemann and Bockheim 2011) most are elliptical or circular (Brokaw 1985; Clinton et al. 1993; Goldblum 1997) and fewer are dumbbell-shaped “chablis” (Oldeman 1978). Garbarino et al. (2012) confirmed that gap geometry was related to regeneration composition, showing that early successional and shade-intolerant species, such as sycamore maple and rowan, were positively associated with large (area and perimeter) and elongated gaps, while numbers of European beech saplings were not influenced by gap size but were weakly associated with gap filler basal area probably because this species is shade-tolerant only in the first stages of its life. The different pattern observed for rowan seedlings and saplings was probably due to the fact that this species is shade-tolerant only in the first stages of its life. Based on these results related to the observation of natural regeneration in natural gaps of different shapes, the main manmade gap shapes adopted in silvicultural management are:

―circular (De Philippis 1948; Perrin 1954; Cappelli 1988; Piussi 1994; Malcom et al. 2001; Del Favero 2010);

―elliptical, more appropriate to limit the effects of wind turbulences (Runkle 1981; Del Favero 2010); and

―square (Cappelli 1988; Piussi 1994; Del Favero 2010).

Gap age

Over time, microsite characteristics within gaps slowly revert to those of a closed forest and consequently gap age, strictly correlated with gap size, is an important parameter to be considered in forest management. Forest dynamics are driven by continuous disturbances and are characterized by quasi-equilibrium structure. Thus, gap age is an important driver of forest change prior to a complex stage of development.

Many authors reported positive correlation between regeneration density and gap age in mature stands, and also a negative correlation in old-growth stands that reflected differences in under-story light conditions and forest structure. In mature stands where tree saplings are sparse, gaps can continue to be sites of new seedling establishment because understory resources are still available in gaps surrounded by relatively open upper and mid canopy layers. Conversely, in old-growth stands, where tall trees surround many gaps, light and below ground resources may be so low that the first seedlings established in the gaps preempt resources and retard further seedling establishment. These results were confirmed in Cameroon forest by Bongjoh and Mama (1999), who found a positive correlation between gap age and regeneration, suggesting that among 5 forest blocks of aged classes (1, 3, 6, 9, 12 years ) species and seedling densities were highest in 1 year-old gaps. Thus, in relatively young forests, gaps provide the mechanisms for stands to develop complex structures, and can be used to explain patterns of shifting species composition in secondary forests (Hart and Grissino-Mayer 2009). Young gaps, considered small-scale disturbances, regulate species replacement patterns and create transitional forests. In summary, forest productivity decreases in the mature gaps due to increased competition for limiting growth resources, while productivity increases in newly formed gaps. Newly formed microsites initially offer competitor-free space, but with increasing gap age, the establishment of species is precluded by resource competition. In conclusion, small-scale disturbance is recommended in forest management because it produces forest gaps which increase understory heterogeneity improving forest health and the dynamics of mature and old growth forest (Caron et al. 2009; Kirchner et al. 2011).

Spatial and temporal distribution of gaps

In managed forests, gap density is reported as the number of gaps created per hectare, while in unmanaged forests it is the number of gaps intersected by transects (Fajardo and de Graaf 2004).

Gaps can be geometric-systematic (one gap every × m along straight lines), or irregularly spaced in relation to stand age, structure and dynamics, or to the distribution of forest roads. Frequency of gap creation has important consequences for species composition and forest structure. The rate of gap openings in natural mature temperate forests ranges from 0.5 to 2.0% per year (Brokaw 1985; Runkle 1985). The interval of gap disturbances in northeastern North America is usually in the range of 50-200 years (Runkle 1982; Seymour et al. 2002). Gap turnover time (mean time between successive creations of gap area at any one point in the forest, Brokaw 1985) is dependent on the rate of gap infilling, or the recruitment and growth of seedlings in the gaps. One rule should be observed: never open a new series of gaps if the previous ones are not completely regenerated. In temperate conifer stands, average seedling density below 0.5/m2is considered insufficient to ensure stand regeneration and supplemental planting is indicated. Marthews et al. (2008) showed that spatial gap distributions determine direct light regimes in time and space, increasing germination and emergence of seedlings. Sapkota et al. (2009) investigated gap spatial distribution, advanced regeneration and stand structure of five Shorea robustadominated forests in 25 1-ha plots subject to disturbances of dif-ferent intensities. The overall stand density changed quadratically across the disturbance gradient. A strong inverse relationship was found between overall stand density and diameter class in the least disturbed and moderately disturbed forests. Individual species showed different responses to disturbance ranging from ‘tolerant’ (Shorea robusta, Lagerstroemia parviflora and Symplocos spp.) to ‘sensitive’ (Trewia nudiflora, Adina cardifolia and Terminalia alata). They suggested that moderate disturbance intensity not only ensures high stand density, but also enhances the advanced regeneration of important tree species

Conclusions

Forest management that approximates nature appears to be a flexible toolbox in which creation of gaps is a useful tool to secure sustainable forest development. This tool mimics natural openings of various sizes that follow moderate disturbance events. This review provides information needed to design a management system that mimics natural disturbance regimes, focusing on the importance of gap size, shape and age, and also the influence of canopy composition and structure on gap size, shape, and frequency, and noting that gap characteristics affect seedling establishment and thus future canopy composition.

Future gap research should consider the primary cover type and the aerial extent of gaps in the system. Gap-cutting silviculture might yield higher diversity in tree species composition and better forest structure. From an ecological point of view, the range of different gap sizes can create diverse habitats from which fauna can also benefit. In addition, gap-cutting systems might provide higher landscape values by scheduling cuts over time and space. A gap-cutting system is particularly suited to the restoration of forest stands in protected areas. Further investigation of soil processes, including organic matter trends, composition and activity of microbial biomass, and soil characteristics, would greatly improve overall understanding of gap dynamics and their impacts on the forest as a whole.

Acknowledgments

This study was supported through funds provided by Regione Calabria within the project “Robinwood Plus”- Interreg IV C. The authors thank Dr Thomas D. Dahmer for revising the language.

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DOI 10.1007/s11676-014-0521-7

Project funding: This study was supported through funds provided by Regione Calabria within the project “Robinwood Plus”- Interreg IV C.

The online version is available at http://www.springerlink.com

Fax: 0965/312827; e-mail: amuscolo@unirc.it

Corresponding editor: Chai Ruihai