Preventing crown collisions increases the crown cover and leaf area of maturing lodgepole pine

Abstract

Summary 1Crown collisions induced by tree sway are hypothesized to reduce crown closure and leaf area in maturing cold temperate forests. These declines are thought to lead to the decline in productivity when a stand ages. 2 We tethered groups of lodgepole pine ( Pinus contorta Dougl. Ex Loud. Var. latifolia Engelm.) trees in a web pattern at 10 m height, in four 15-m tall stands in western Alberta, Canada, to determine whether preventing crown collisions would increase crown cover and leaf area. 3 The stands all had less than 65% crown closure at the beginning of study. Photographs of the canopy were taken in each control and webbed plot in 1998 and at the same point in 2004. Six years after webbing, crown cover had increased by 14.4%, compared to a 2.1% increase for the control plots. 4 Webbing also resulted in significant increases in mean branch length, leaf area per branch and foliage density of individual branches from top and middle sections of the crown. Polishing of branches, caused by chronic contact with adjacent trees, was three times as common on control trees compared to webbed trees. The mean leaf area per tree was larger for the webbed trees. 5 Crowns of webbed trees were more symmetrical than those of control trees. Trees from webbed plots, however, had a decline in leaf area density. The branches of control trees were typically curved upward with twigs pointed inward, making the crowns more compact compared to the outwardly expanding crowns of trees from the webbed plots. 6 The fact that crowns expanded laterally after webbing, despite little change in light regime, provides strong evidence against the hypothesis that loss of crown closure in maturing stands is caused by a lack of light. 7 The study indicates that the decline in crown closure and leaf area in maturing and tall stands is at least partly related to wind-induced sway of trees abrading the edges of crowns.

Neighbor identity and competition influence tree growth in Scots pine, Siberian larch, and silver birch

“…Nonmeasured variables, in turn, may include direct quantitative differences in traits, such as the level of light interception or root competition (Strand et al, 2006). Plants can also use wind velocities modified by neighbors as proximity signals (Aphalo and Ballaré, 1995), or experience direct crown collisions, which may decrease crown cover, branch length and shoot density of individual branches in the crown (Meng et al, 2006). Furthermore, the nutrient acquisition strategies of root systems may differ among boreal tree species (Kalliokoski et al, 2009).…”

Crown architecture and its role in species interactions in mixed boreal forests

“…wavelengths (Aphalo and Ballaré 1995;Smith 2000), mechanical crown collision (Meng et al 2006) and chemical signalling mainly with volatile organic compounds (Baldwin et al 2006;Heil and Karban 2010). Even when all the above-mentioned sources of communication signals between plant individuals have been blocked, seedlings have sensed their neighbours suggesting that unknown communication channels still exist, sensing e.g.…”

“…Generally, a growing pattern producing dense and spatially limited crowns seems an efficient use of growing space (Day and Greenwood 2011), which limits physical contact between individuals, thus preventing damage caused by crown collision (Meng et al 2006). The suggested growing pattern also allows high biomass allocation to photosynthesising foliage at the expense of branches (Borchert and Tomlinson 1984).…”

“…These effects are currently not fully understood (Gagliano et al 2012), but recognised signals include differences in the spectral composition of light reflected from the foliage of various neighbouring species (Smith 2000), chemical signals (Heil and Karban 2010) and differences in species-specific crown architecture affecting wind velocities that trees van sense as proximity signals (Aphalo and Ballaré 1995). Crown collision through swaying is a more direct wind effect, which has been reported to decrease the crown cover, branch length and shoot density of individuals (Meng et al 2006).…”

“…A plastic response in tree growth and architecture may be dependent on the neighbouring species, because species-specific differences exist e.g. in growth allocation, light interception structures such as crown profile (Horn 1971;Yokozawa et al 1996), individual leaf size, spectral leaf characteristics and leaf density (Jahnke and Lawrence 1965;Gates 1980), in resource uptake timing and the ratio of needed resources (Goldberg 1987) and in the crown characteristics causing mechanical disturbance due to wind sway (Meng et al 2006).…”