The computer simulation experiments presented here explored the impact of enhanced or reduced plasticity of crown development on tree growth performance in a variety of contexts. Sensitivity analysis of crown plasticity was restricted to varying the Flexi parameter (controlling maximum deformation). Qualitatively similar results have been found for the Sensi parameter (controlling precocity of response, data not shown) and both parameters are likely to be correlated in nature.
As hypothesized, it was found that the degree of crown plasticity had an impact both at individual tree level (compare Figs and ) and population level (H1) but no impact was found at stand level. Altering the relative proportion of more or less plastic strategies did not change the above-ground accumulation rate to any measurable extent not even at early stages. Of course simulation of a more strongly contrasted growth strategy may have induced such a stand-level effect. The point remains that compensation mechanisms occurred at stand level that offset the observed impact of plasticity at population level, partially contradicting hypothesis (H1). The simulations which explored a limited range of plasticity with a modest impact on growth, only partially supported (H2) stating that competitiveness of a particular strategy is context-dependent (and notably frequency-dependent). The competitive edge of any particular strategy (i.e. its performance relative to the average performance) is expected to decrease when it becomes more frequent simply because by increasing in relative frequency it is contributing to increasing the average performance, a purely arithmetical reason which would hold in the absence of any interaction. Additionally, absolute performance of any given strategy is expected to be affected by the overall competitiveness on the environment, which modifies resource availability to individual trees. This overall competitiveness was manipulated via changes in stand composition and stand density. For composition modification, the most flexible type performed better under the least competitive stand (dominated by the least productive types). However, the decrease in individual tree performance observed with increasing competition was not limited to any particular strategy (). The lack of change in relative competitiveness of the various strategies (also found when planting density was varied; and ) can be related to the fact that the strategies being confronted were very similar: they differed only in respect to their crown plasticity and even so to a moderate extent. A mixture of more contrasted strategies may reveal complementarity in resource partitioning such as that observed for light and space between tall, light-demanding types and short, shade specialist types. In such a context, change in relative frequency of the various types in mixture would probably affect their relative competitiveness.
Negative interaction between growth rate and crown plasticity was found with the present range of plasticity and growth rate explored () contradicting (H3), which conjectured positive interaction. Instead, fast growing types gained little additional competitive advantage from being more plastic. More simulations are needed to see how systematic this response may be. But with hindsight the observed pattern certainly makes sense and this result also calls for a re-examination of hypothesis (H3). It may be that high morphological responsiveness to heterogeneity in light distribution is more directly beneficial to plants with high organ turnover rate (and fast return on investment) rather than plants with high growth rate per se.
Increased plasticity conferred a significant absolute advantage in all cases examined even though the below-ground competition tended to mitigate this advantage (). As hypothesized in (H4), changing the below-ground competition intensity affected the relative competitiveness of the different types. Lower competition for below-ground resources increased the competitive edge of more plastic trees and was accompanied by changes in structure and dynamics consistent with theoretical expectations ().
Despite the fact that no specific cost of plasticity has been included in the model, some negative feedback loops did tend to reduce the advantage of crown flexibility. The main two such negative loops were reduced crown expansion rates (smaller crowns and subsequent lower growth potential as a result of vertical stretching; as previously discussed) and reduced below-ground competitiveness.
As mentioned in the Introduction, some specific costs are undoubtedly associated with crown deformation in nature. Those could, in principle, be included in the model either in the form of reduced growth efficiency (notably related to the expected reduced LUE) or in terms of increased mortality (notably as a result of increased susceptibility to wind throw of higher, more asymmetric trees) (Young and Perkocha, 1994
). A function of susceptibility to wind throw depending on crown asymmetry, crown mass and tree height could be considered (e.g. Olesen, 2001
). However, tree morphological responses to mechanical stress associated with crown deformation (such as trunk tapering, buttresses, asymmetric stem thickening, root growth or stem bending; Fourcaud et al., 2003
; Ancelin et al., 2004
) may all affect this susceptibility as well as the overall net growth efficiency of deformation. So calibration of this type of susceptibility function is certainly a delicate task.
Neglecting the negative feedback that may occur in terms of LUE (see Introduction) may have contributed to overestimating the benefits of crown plasticity at individual tree level and hence affected the in silico experimental results presented. However, the negative effects associated with crown deformation are likely to be of significant amplitude only when deformation is large (in terms of either vertical stretching or lateral crown asymmetry), which was not the case in the simulations conducted. So it is reasonable to assume that neglecting such negative feedback probably had only a minor impact on the output of the numerical experiment presented.
Simulation scenarios were not completely realistic even where they incorporated demographic processes, notably in that they considered mixtures of a reduced set of species of similar ecology immune to invasion by other species. From an ecological standpoint, a more relevant analysis of the impact of crown shape plasticity on tree performance may, however, be attempted using the same or similar model. Simulations should then feature truly multi-species stands (i.e. combination of different successional status and adult size) and systematically incorporate the demographic processes.
Clearly not all tree species in nature show the same degree of crown plasticity. For example, Gilbert et al. (2001)
have shown that early-successional species tend to be more etiolated when shaded or exposed to low R : FR than late-successional shade-tolerant species. Muth and Bazzaz (2002)
also noted that the magnitude and precision of canopy displacement at forest edges are generally greater for earlier successional trees and hardwoods than for later successional trees and conifers, suggesting that (1) global plasticity may be higher in early-successional species and (2) conifers are globally less plastic. The latter point may be related to their fairly rigid morphogenetic development (and notably the maintenance of a dominant main stem for most conifer species almost until death of the tree). The present study examined how the growth performance of a fast-growing, light-demanding species was affected by changes in crown plasticity and found that high plasticity appeared to confer an absolute advantage. Of course such conclusions are conditional on the assumptions made in the model and on the growth characteristics of the species that was simulated (Kuppers, 1989
). In shade-tolerant species, for example, crown deformation under a suboptimal light regime may actually entail crown horizontal flattening rather than vertical stretching, through altering growth rates between leader shoot and main branches or bending of the leader shoot (Henry and Aarssen, 2001
). Growing fast towards light is not necessarily the best strategy and may not be sustainable due, for example, to excessively high maintenance costs whereas a more conservative strategy may provide better survival chances, notably in the face of rapidly fluctuating levels or extremely low levels of resources: non-morphologically plastic crowns may be favoured in shade-specialist species. Physiological adaptation to unpredictable heterogeneous incoming light in the form of reduced induction time and better use of sunflecks is apparently a common strategy of slow-growing understorey specialists (Pearcy, 1990
). Lower responsiveness of more shade-tolerant species [as repeatedly pointed out by Givnish (2002
, and references therein)] is essentially adaptive as it ‘reduces the energy overhead associated with stem construction for a plant that can maintain positive photosynthesis in shade’.
Computer simulations of tree stands using realistic growth rules and realistic crown deformation pattern and degree of flexibility were conducted. Simulation experiments showed that crown shape plasticity could provide a significant competitive advantage over less plastic trees through enhanced LCE. Hence, simulation results support the statement that the crown shape plasticity observed in many tree species is of adaptive value and more generally point at the importance of morphogenetic plasticity as a determinant of performance in trees.