How Well Understood Are the Processes that Create Dendroclimatic Records? A Mechanistic Model of the Climatic Control on Conifer Tree-Ring Growth Dynamics - Dendroclimatology - page 40

A further challenge to modeling the environmental control on tree growth is that
our understanding of the nature of the relationship between the concentration of car-
bohydrates in the developing xylem and the eventual ring width as mediated by the
external environment (climate, photoperiod, exogenous ecological factors) remains
incomplete (Savidge 2000 a , b). Earlier research found the amount of carbohydrates
in the cambium was positively correlated with the period of greatest tracheid pro-
duction (Parkerson and Whitmore 1972 ) . However, Sundberg et al. ( 1993 ) revealed
that the relative concentration of sucrose could not explain ring width differences
between two populations of Pinus sylvestris . Similarly, a study of the effect of prun-
ing Eucalyptus grandis trees revealed that the loss of stems and foliage does not
reduce tree diameter (Thomas et al. 2006 ) . Even extreme insect defoliation has
been observed to not result in reduced-diameter growth or tree-ring width (Jones
et al. 2004 ) , even in one case despite a 50% reduction in the number of leaves, pre-
sumably because stored starches can compensate to some degree for the immediate
loss of photosynthetic capacity (Hoogesteger and Karlsson 1992 ) . Dickson et al.
( 2000 ) have hypothesized that the export of photosynthate could be generally sink
limited as opposed to excess source mediated; that is, environmental constraints
may reduce developing tissues' demand for metabolic products prior to the point in
time at which photosynthesis or total carbohydrate availability becomes truly lim-
iting. Körner ( 1998 ) examined potential environmental controls on alpine tree-line
position and found that trees in these environments were not limited by carbon avail-
ability and photosynthetic rate, but rather apparently directly by temperature. In a
recent paper, Körner ( 2003 ) is even more direct:
It is concluded that, irrespective of the reason for its periodic cessation, growth does not
seem to be limited by carbon supply. Instead, in all the cases examined, sink activity and its
direct control by the environment or developmental constraints restricts biomass production
of trees under current ambient CO 2 concentrations.
3.1.2 The Cambial Control Hypothesis
A different understanding of the environmental control on the characteristics of
annual tree rings is that cellular processes in the cambium are directly influenced by
environmental conditions. It is recognized that these cellular processes of growth,
division, differentiation, maturation, and death, which do ultimately give rise to
the structure and size of the annual tree ring, are controlled by gene expression
and hormonal stimulus, and that they can respond to both external (environmen-
tal) and internal (developmental) influences (Savidge 2000 a ; Plomion et al. 2001 ;
Schrader et al. 2003 ) . Rossi et al. ( 2007 ) concluded that the minimum thresh-
old temperature for growth they observed in tree-line larch, pine, and spruce was
directly limiting secondary growth at the cellular level, and that below this temper-
ature the sink for nonstructural carbohydrates in the secondary xylem was reduced.
Numerous experiments have shown that direct heating of stems can initiate cambial
activity after the onset of winter dormancy (Gricar et al. 2006 ) . Abe et al. ( 2003 )
found that cell expansion rate in the cambium decreases as a direct consequence