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Details below (be sure to also see The TeleBrella® oversized portable beach umbrella). See PortaBrella on CBS News: Exclusively available in Royal Blue for 2016: Features and specifications for Portabrella™ portable beach umbrella include: PortaBrella® on Waikiki Beach (renting a beach umbrella and two chairs runs $70/day!).Event Decor Direct is excited to offer these new and custom chair covers for sale at an incredible factory-direct price.Volume 327, 1 September 2014, Pages 251–264 •Canopy space filling can increase with species number.•Complementary crown shapes may increase canopy space filling.•Mixing can modify allometry and inner space filling of tree crowns.•Denser canopy space filling can increase light interception.•Denser canopies may increase stand productivity and resilience.Mixed-species forest stands are well explored in their favourable ecological, economical, and socio-economical functions and services compared with pure stands, but still poorly understood in their structure and functioning.
Canopy structure and tree morphology affect the environmental conditions within the stand, the tree growth, and by this most forest functions and services. Here, I review how canopy structure and crown morphology in mixed stands can differ from pure stands and how this depends on the selection of tree species and interactions between them. The focus is on the macrostructure of canopy and crowns derived from the trees’ positions, their convex crown hulls, and their space filling with branches.In mixed canopies the sum of the crown projection area, but not the ground coverage by crowns, mostly exceeds pure stands due to multiple crown overlaps. The interspecific differences in crown shape and allometric scaling cause a ‘selection effect’ when complementary species are combined. In interspecific environment furthermore ‘true mixing effects’ like intraspecific shifts in size, shape, and inner space filling of crowns may occur. The much denser and more plastic canopy space filling in mixed stands may increase light interception, stand density, productivity, and growth resilience to disturbances.
I discuss the relevance of interspecific interactions for forest management, model building, and theory development and draw perspectives of further research into stand canopy and crown structure.Graphical abstractLeft: In canopies of un-thinned or just moderately thinned mixed species stands (95% and 75% line, respectively) the sum of the crown cross section area mostly exceeds pure stands (100% reference line) due to multiple crown overlaps. Right: Mixing species with varying morphological traits can cause a selection effect (a–c) but beyond this may trigger a morphological plasticity which can lead to denser canopy space filling, completer light capture, and productivity gains of mixed versus pure stands (d–f).Until the middle of the 20th century the strong influence of agronomics on forestry resulted in extensive mono-specific production systems. Since then, forest practice and forest science focused on more complex mixed-species stands (Puettmann et al., 2009). Evidence is growing that mixed-species forest stands can supply many ecological, economical and socio-cultural forests goods and services in a similar or even better way as far-from-nature monocultures (Gamfeldt et al., 2013).
Tree species richness may trigger the variety of habitats and species diversity of other forest plants and animals (Gotelli and Colwell, 2001, Noss, 1990 and Paillet et al., 2010), improve humus conditions and soil fertility (Binkley, 2003 and Rothe and Binkley, 2001), the resilience to disturbances (Griess and Knoke, 2011), and the stand productivity (Morin et al., 2011, Piotto, 2007 and Paquette and MessierThese advantages may be coupled with a depletion of soil water (Schume et al., 2004), loss of wood quality (Knoke and Seifert, 2008), increase of harvesting costs (Hanewinkel, 2001), or other drawbacks of mixed compared with pure stands. Some of the pros and cons may even change spatially (Forrester, 2013) and temporally (Lebourgeois et al., 2013 and Río et al., 2014) depending on the prevailing site conditions. While research initially concentrated on comparing growth and yield between mixed and pure stands (Kelty, 1992, Pretzsch et al., 2010 and Pretzsch et al., 2013a and b), works by among others Forrester et al. (2006) and Rothe and Binkley (2001) gradually proceeded to analysing and understanding the mechanism behind mixing effects.
Especially a better understanding of the species structural and functional traits and the dependency of these traits from the environmental conditions appears indispensable for developing new resource efficient multi-species production systems (Forrester, 2013 and Richards et al., 2010).Research into pure stands provides a wealth of knowledge about the interspecific variation of structural and functional traits (e.g., Augusto et al., 2002, Larcher, 2003 and Purves et al., 2007). When cultivating tree species in mixture, complementary structural and functional traits can be useful for improving their resource efficiency and yield. Benefits can result among others from combining light demanding with shade tolerant species (Zöhrer, 1969), shallow with deep rooting species (Schmid and Kazda, 2001 and Schmid and Kazda, 2002), slim-crowned and height oriented with wide-crowned and more laterally expanding species (Pretzsch and Schütze, 2005 and Pretzsch and Schütze, 2009), or nitrogen-fixing with non-nitrogen-fixing species (Forrester et al., 2006).
Further analyses in this paper will distinguish between ‘selection effects’ and ‘true mixing effects’. Suppose species with complementary traits are mixed but each species sticks to its behaviour which is known from pure stands, the mixed stand provides hardly any surprises. In this case the performance of the mixed stand is equal to the weighted mean of the growth of the neighbouring pure stands. As this kind of mixing effect results from nothing more than selecting the species, it is called selection effect or ‘additive effect’ (Forrester, 2013 and Kelty, 1992). A ‘true mixing effect’ in contrast means that the interspecific environment triggers species traits which go beyond their behaviour known from pure stands (Forrester, 2013). Compared with the restriction in pure stands, interspecific neighbourhood may trigger abilities of crown expansion and interlocking which the species acquired by their mutual co-evolution in the past, but which are rather irrelevant, undesired by forestry, or even unknown as long as the species grow in pure stands.
However, when crowns and roots are let off the leash in mixed stands they may develop a behaviour not known from pure stands but highly relevant for understanding, modelling and predicting mixed stand dynamics. A synonymous term for the true mixing effect is ‘multiplicative effect’ (Kelty, 1992 and Rothe, 1997, pp. 4, 150).Because of their size, firm position, and longevity, tree crowns both reflect and determine many ecosystem characteristics, functions, and services (Franklin and Spies, 1991, Ishii et al., 2004 and Ozanne et al., 2003). On the one hand the crown size indicates leaf area and reflects the light interception and growing conditions of individual trees within the stand (Assmann, 1970, pp. 111–122; Binkley et al., 2013). Thus crown and canopy structures reflect the individual trees’ light interception (Sterba and Amateis, 1998 and Webster and Lorimer, 2003). On the other hand crown morphology and the resulting canopy structure determines among others the within-stand environmental conditions, the stand productivity, stand stability and resilience, habitat structure, and even the aesthetic value of a stand.