Gradients of light availability and leaf traits with leaf age and canopy position in 28 Australian shrubs and trees

Abstract

Abstract. Light availability generally decreases vertically downwards through plant canopies. According to optimisation theory, in order to maximise canopy photosynthesis plants should allocate leaf nitrogen per area (N area ) in parallel with vertical light gradients, and leaf mass per area (LMA) and leaf angles should decrease down through the canopy also. Many species show trends consistent with these predictions, although these are never as steep as predicted. Most studies of canopy gradients in leaf traits have concerned tall herbaceous vegetation or forest trees. But do evergreen species from open habitats also show these patterns? We quantified gradients of light availability, LMA, leaf N and phosphorus (P), and leaf angle along leaf age sequences and vertical canopy profiles, across 28 woody species from open habitats in eastern Australia. The observed trends in LMA, N area and leaf angle largely conflicted with expectations from canopy optimisation models, whereas trends in leaf P were more consistent with optimal allocation. These discrepancies most likely relate to these species having rather open canopies with quite shallow light gradients, but also suggest that modelling the co-optimisation of resources other than nitrogen is required for understanding plant canopies.

Optimisation of photosynthetic carbon gain and within-canopy gradients of associated foliar traits for Amazon forest trees

“…Especially when considered in conjunction with the mathematical simplicities that ensue (Farquhar, 1989) this idea of "optimization" is conceptually attractive, even being incorporated into some canopy gas exchange models Sands, 1995;Sellers et al, 1996). But it is also now clear that although the decline in photosynthetically important elements such as nitrogen and phosphorus within plant canopies can be considerable, and sometimes even impressive, this decline is never to the same extent that it matches the reduction in Q (De Jong and Doyle, 1985;Carswell et al, 1980;Meir et al, 2002;Anten, 2005;Wright et al, 2006; see also supplementary information: http://www.biogeosciences.net/ 7/1833/2010/bg-7-1833-2010-supplement.pdf).…”

“…Various hypotheses have been proposed to account for this apparent "non-optimality". These include that plants do not grow as isolated individuals but rather in competition with others (Anten, 2005), that it might be related to direct versus diffuse radiative transfer (Buckley et al, 2002;Alton and North, 2007) or not all nitrogen being related to photosynthesis (Hikosaka, 2005); that there may be optimisation of N to light gradients within leaves as well as canopies (Terashima et al, 2005); that the required very high nitrogen concentrations at the top of the canopy may place leaves at strong risk of herbivory (Stockhoff, 1994); that there may be considerable costs of retranslocating nutrients within the plant (Field, 1983;Wright et al, 2006), that plants may over invest in Rubisco in order to cope with temporal variabilities in their environment (Warren et al, 2000) and, especially as gradients in nutrients and photosynthetic capacity are generally driven by gradients in M A rather than by variations in dry-weight nutrient concentrations (Reich et al, 1998;Ellsworth and Reich, 1993;Evans and Poorter, 2001), that there may be a practical lower limit to the minimum M A and hence N A that any species can achieve (Meir et al, 2002).…”

“…Nevertheless, one regular observation in tree canopies is that vertical gradients in photosynthetic capacity are much less than that associated with the optimal distribution maximising individual plant carbon gain (e.g. Hollinger, 1996;Kull and Niinemets, 1998;Meir et al, 2002;Wright et al, 2006).…”

Role of plant functional traits in determining vegetation composition of abandoned grazing land in north‐eastern Victoria, Australia

“…For the Delatite Peninsula however, we expect any impact from differences in light availability between land-use types to be limited due to the shallow light gradients typical of eucalypt woodlands (Wright et al 2006).…”

Divergências funcionais e estratégias de resistência à seca entre espécies decíduas e sempre verdes tropicais

“…O alto investimento aumenta a resistência à difusão do CO 2 e, consequentemente, reduz a capacidade fotossintética em espécies sempre verdes (Ishida et al 2008). Portanto, duas estratégias ecológicas são identificadas, espécies decíduas que constroem folhas pouco longevas com alta capacidade fotossintética e, espécies sempre verdes que possuem folhas longevas e elevada massa foliar, porém, com baixa capacidade fotossintética (Wright et al 2006).…”