Congruence of the spatial pattern of light and understory vegetation in an old-growth, temperate mixed forest

Highlights

• Understory light had aggregated spatial patterns at scales of 10 × 10 and 25 × 25 m.

• Herbs’ and bryophytes’ pattern was associated to the finer-scale pattern of light.

• The pattern of woody seedlings related to the coarser-scale aggregation of light.

• Herbs and seedlings were classified according to their response to light pattern.

• Heterogeneous light conditions are needed to maintain understory biodiversity.

Abstract

Light is one of the most important drivers of understory vegetation in forests, influencing the patterns of total cover as well as the abundance of individual species.

Based on a multi-scale approach, the relationships between the amount and pattern of relative diffuse light and forest understory were studied in an old-growth, temperate mixed forest (Hungary). The recorded vegetation variables were the cover of the vascular understory (herbs, woody seedlings), the bryophyte layer, and some selected vascular understory species.

The pattern of light showed aggregations at two scales: 10 × 10 and 25 × 25 m. Both vascular understory and bryophyte cover had significant positive correlations with light availability, and their spatial pattern was related to it. The pattern of seedlings displayed the strongest relationships with that of light at a coarser scale (25 × 25 m) than herbs and bryophytes (10 × 10 m). At the species level, Festuca heterophylla, Fragaria vesca and Poa nemoralis were characterized as light-demanding herbaceous species (their spatial pattern was congruent with light), Brachypodium sylvaticum and Carex pallescens were transitional, while some species proved to be shade-tolerant (e.g. Ajuga reptans, Dryopteris carthusiana, Viola reichenbachiana). Regarding seedlings, the patterns of Betula pendula, Carpinus betulus, Pinus sylvestris and Quercus petraea were related to the pattern of light.

According to our observations, diversity and composition of vascular forest understory and bryophytes were related to heterogeneous light conditions. Forest management should maintain continuous shelter on the stand level; however, smaller gaps are necessary for the survival of light-demanding forest herbs and bryophytes, and larger gaps for tree seedlings.

Introduction

The heterogeneous spatial distribution of limiting environmental factors often creates peculiar patterns of vegetation (Fortin et al., 2002). Light is one of the most important explanatory variables in forests (Whigham, 2004, Neufeld and Young, 2014). Besides its amount and quality, its heterogeneous pattern is also a determining factor for the cover and diversity of understory vegetation (Canham et al., 1994). Understory light is largely determined by stand structure, tree species composition and the pattern of the overstory layer (i.e. regular or aggregated pattern of trees; presence and spatial arrangement of gaps in the canopy, Martens et al., 2000, Valladares and Guzmán, 2006).

Light distribution at the ground level of forests varies on several scales. There are pronounced and well demonstrated differences between the light regimes of various forest types, due to different stand structure and management (Bartemucci et al., 2006). Also within a single stand, light conditions may be remarkably heterogeneous due to gaps, especially in forests dominated by deciduous, shade-tolerant species (Muscolo et al., 2014). Finally, light availability also has a fine-scale spatial pattern within mature, heterogeneous, albeit closed stands, which originates in the structural and compositional heterogeneity of the overstory layer. Tree pattern, age distribution, physical damage of leaves and branches, herbivory, disease, crown geometry and the species-specific features of trees all add to the variability of canopy and light conditions (Canham et al., 1994).

The light requirements of the understory species is variant, which results their different responses to contrasting situations, such as various stand types (Verstraeten et al., 2014, Márialigeti et al., 2016) or gap formation in homogeneous, closed forests (e.g. Collins et al., 1985, Gálhidy et al., 2006, Kern et al., 2014). However, measuring the fine-scale relationships between irradiance and understory vegetation under a heterogeneous but closed canopy is complicated. Some surveys investigated the drivers of understory species richness, composition, or cover within such stands. Light usually proved to be a key factor, from many environmental variables (Mrotzek et al., 1996, Chávez and Macdonald, 2010, Ádám et al., 2013, Yu and Sun, 2013, Neufeld and Young, 2014, Sabatini et al., 2014). This implies that forest understory species are indeed sensitive to the fine-scale variability of light conditions under heterogeneous canopies.

Not only the composition and the amount, but also the spatial pattern of the understory can be related to light. Furthermore, the light-response of the particular components of the understory may manifest itself at different spatial scales. Thus, to acquire information about the congruence between the pattern of understory and light, and to unfold most of these relationships, spatially explicit, multi-scale pattern analysis methods are needed (Whigham, 2004). However, there are very few studies regarding the spatial pattern of forest understory (Campetella et al., 1999), and especially the spatial patterns of individual herbaceous species, or their environmental drivers within a near-natural, unmanaged stand (Miller et al., 2002, Scheller and Mladenoff, 2002, Gazol and Ibáñez, 2010). Besides light, other drivers (microhabitats, substrates, soil moisture, etc.) can also determine the understory pattern (Gazol and Ibáñez, 2010). Understanding the spatial scale of the relationships between light (or other environmental factors) and understory may help to maintain the proper scale of habitat heterogeneity in forests.

More studies concentrate on the drivers of the survival, growth, and spatial pattern of woody seedlings than those of herbs, as seedlings directly determine the structure of the next generation of trees. The amount and pattern of light is also crucial for the seedlings, but the strength of this effect depends on the shade-tolerance of the species, and is also influenced by environmental heterogeneity (Getzin et al., 2008). Besides the effect of light and other abiotic factors, the influence of biotic factors (interactions between species) is also important. The relative importance of light and the biotic interactions may depend on the successional stage of the stand, the investigated guilds (trees or shrubs, shade-tolerants or light-demandings, Lin et al., 2014), and the age class of the seedlings (Yan et al., 2015). Kuninaga et al., 2015, Petritan et al., 2015 revealed that, because of density dependent mortality, the initially clumped spatial pattern of seedlings turns to random or regular distribution. However, only a few studies examine the effects of the light pattern on the spatial pattern of woody seedlings (Scheller and Mladenoff, 2002, Raymond et al., 2006).

Forest-dwelling bryophytes are traditionally regarded as shade-tolerant species (Proctor, 1982). Kubásek et al. (2014) showed that the photosynthetic apparatus of bryophytes is adapted for the efficient utilization of light, the intensity of which is dynamically changing in the forest understory. It allows forest bryophytes to exist under the extreme ecophysiological circumstances formed by the canopy shade. Among more favorable light conditions they may be outcompeted by more productive, light-demanding vascular species (Bergamini et al., 2001, Virtanen et al., 2000). However, results about the relationship of bryophytes and vascular understory are contradictory. Other surveys showed positive interactions between bryophytes and vascular plants (Márialigeti et al., 2009), because their environmental demands can be similar (Lee and La Roi, 1979), and herbs are also able to modify the microclimate to be more favorable for bryophytes (Aude and Ejrnaes, 2005). However, it is logical that since bryophytes live in an environment where light intensity is limited, in laboratory experiments they respond to ameliorating light conditions with an increased biomass (Rincón, 1993). According to Márialigeti et al., 2009, Tinya et al., 2009a -besides the density of trees and litter cover -, light influences the cover of bryophytes, especially that of species inhabiting mineral soils. However, little is known about whether the pattern of bryophytes is related to the pattern of light, and about the spatial scale of this possible connection.

This study investigates the relationships between the spatial pattern of light and the vascular understory vegetation (herbs, woody seedlings) and the bryophyte layer, within a temperate mixed forest stand, at different spatial scales. We focused on the following questions:

• At what spatial scale is the light pattern aggregated in a temperate mixed forest with a heterogeneous and species-rich canopy layer?

• To what extent is the cover and spatial pattern of the vascular understory and the forest-floor bryophyte layer related to light?

• To what extent are the cover and the spatial pattern of particular vascular understory species related to irradiance? Is it possible to classify them based on their responses to light (light-demanding, transitional, shade-tolerant)?