Size mediates ageing: merging architectural complexity with cellular senescence M. Mencuccini School of GeoSciences University of Edinburgh (UK)

Structure of the talk Our perspective on plant hydraulics Our perspective on plant hydraulics The theoretical framework The theoretical framework The approach The approach Some results Some results Preliminary conclusions on our take of the age-related declines Preliminary conclusions on our take of the age-related declines

Our perspective. I: Interspecific convergence and significance of stature. Almost isometric scaling If life forms are not considered, scaling is not isometric (WBE b=1.0) Significant intercept shifts predicted by model of biomass allocation. (Mencuccini, 2003)

Our perspective. II: Significance of stature during stand development. (Mencuccini & Grace, 1996) Decline in NPP in Scots pine paralleled by decline in hydraulics and leaf area index

Schulze et al (1999) Global Change Biology 5, Siberian forest, pine chronosequence after fire Law et al. (2004) Global Change Biology Our perspective. III: Ecosystem processes

Theoretical framework. I: Hydraulics and architectural complexity (West et al., Nature 400, ) a i-1 l i-1 a i l i The plant as a fractal-like structure of order N

Theoretical framework. II: Ageing processes in plants Maturation: progressive changes in the habitual behaviour of meristems leading up to adult reproductive maturity Senescence: progressive loss of function accompanied by decreasing fertility and increasing mortality rate with age (population and individual) Cellular senescence: irreversible changes in leaf physiological activity and growth triggered by gene expression with age

the state of the art Ryan et al., 2004 NPP Ag = GPP - Resp. - TBCA Results:  GPP decreases dramatically.  TBCA, if anything, decreases.  Respiration, if anything, decreases.  Hydraulic limitation hypothesis only partially supported. Controlled study quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated Eucalyptus plantations in Hawaii (final tree height ~ 25 m).

We need new approaches, experimental manipulations

True senescence in monocarpic plants Inherent changes in meristems do occur: in pea plants, shoot meristems have a limited growth potential; true senescence does occur (Lockhart & Gottschall, 1961).

Different ages, different sizes Different ages, same size Scots pine sycamore ash poplar Different sizes, same age Same age, same size Different ages, different sizes Different ages, same size Different sizes, same age Same age, same size

Our results: RGR (RGR=NAR x SLA x LMR) Mencuccini et al. (Ecology Letters, in press), Martinez-Vilalta et al. (submitted)

Our results: NAR Mencuccini et al. (Ecology Letters, in press), Martinez-Vilalta et al. (submitted)

Our results: SLA Mencuccini et al. (Ecology Letters, in press), Martinez-Vilalta et al. (submitted)

Our results: g s Mencuccini et al. (Ecology Letters, in press), Martinez-Vilalta et al. (submitted)

Species/ Param. Acer pseudoplatanus Fraxinus excelsior Pinus sylvestrisPopulus trichocarpa x deltoides graftsdonorsgraftsdonorsgraftsdonorscuttingsdonors A net ns-0.15**ns-0.24**n/a-ns %N ns-0.003*ns-0.07 s ns-0.18**ns  13 C ns 0.10*ns0.14**ns 0.06**0.04*ns Results Mencuccini et al. (Ecology Letters, in press), Martinez-Vilalta et al. (submitted)

Size mediates ageing: merging architectural complexity with cellular senescence Ryan et al., 2004 If genetic control is present at the beginning of the life cycle, it is not unreasonable to expect it to be present more or less throughout, and especially, at the end of the life cycle If genetic control is present at the beginning of the life cycle, it is not unreasonable to expect it to be present more or less throughout, and especially, at the end of the life cycle or very young plants there is always a clear decline in growth rates which is stable upon grafting. For very young plants there is always a clear decline in growth rates which is stable upon grafting.

A mechanism for the decline in hydraulic conductance: size-dependent hydraulic tapering (West et al., Nature 400, ) a i-1 l i-1 a i l i The plant as a fractal-like structure of order N

Do vessel and hydraulic tapering depend on size and/or age? Petit et al. (in prep.) Apexsycamore base1.4msycamore base26msycamore

Synthesis The “age-related” decline in growth is not an effect of age per se (i.e., cellular senescence) Effect of tree height on tree hydraulics and water use possibly on all four species, but it does not seem to be the only driver of the decline in growth. …a single cause for the “size-related” decline? We have a potential mechanism to explain effects of height on tree hydraulics. Can we reconcile WBE with the data? Do trees EVER age?