COS 67-1
Coordination of growth and hydraulic transport within and across woody plant species
Coordination of growth and hydraulic transport within and across woody plant species
Wednesday, August 13, 2014: 8:00 AM
Bataglieri, Sheraton Hotel
Background/Question/Methods
A key assumption of many plant water transport studies is that increasing water transport or the efficiency of transport should lead to greater carbon allocation and growth. Water and carbon fluxes are intrinsically linked through their shared stomatal pathways and studies indeed show a general correlation between growth and water transport. However, these studies do not address how growth and water transport scale within and across species. The allometry between growth (G) and hydraulic conductance (K) indicates how directly water transport and carbon fixation and allocation are coordinated.
We hypothesized that G vs K scaling would be isometric and invariant across co-occurring species. Larger species, by nature, must grow more, so we predicted steeper G vs mass scaling in larger species. If G vs K isometry exists, then K vs mass scaling should also be steeper in larger species.
We tested these hypotheses using six co-occurring species of shrubs and trees. For each species, we selected 13 to 22 individuals. We obtained whole plant hydraulic conductances from sapflow and xylem pressure measurements. We estimated aboveground mass and annual growth from species-specific allometries.
Results/Conclusions
We found a general convergence across species in growth vs hydraulic conductance. Most scaling exponents did not differ from isometry and at a given K, different species tended to have similar G. As we predicted, there were no significant effects of species stature on the scaling relationships. In growth vs mass scaling, we found that larger species had greater growth for a given mass and growth tended to increase at a faster rate (i.e. steeper scaling exponents). Hydraulic conductance vs mass was expected to show the same trends due to observed G vs K isometry, but this was not supported. This discrepancy likely occurred because although G vs K exponents agreed with isometry, most were somewhat shallower which indicates ontogenetic variation in water use efficiency and/or carbon allocation.
A key assumption of many plant water transport studies is that increasing water transport or the efficiency of transport should lead to greater carbon allocation and growth. Water and carbon fluxes are intrinsically linked through their shared stomatal pathways and studies indeed show a general correlation between growth and water transport. However, these studies do not address how growth and water transport scale within and across species. The allometry between growth (G) and hydraulic conductance (K) indicates how directly water transport and carbon fixation and allocation are coordinated.
We hypothesized that G vs K scaling would be isometric and invariant across co-occurring species. Larger species, by nature, must grow more, so we predicted steeper G vs mass scaling in larger species. If G vs K isometry exists, then K vs mass scaling should also be steeper in larger species.
We tested these hypotheses using six co-occurring species of shrubs and trees. For each species, we selected 13 to 22 individuals. We obtained whole plant hydraulic conductances from sapflow and xylem pressure measurements. We estimated aboveground mass and annual growth from species-specific allometries.
Results/Conclusions
We found a general convergence across species in growth vs hydraulic conductance. Most scaling exponents did not differ from isometry and at a given K, different species tended to have similar G. As we predicted, there were no significant effects of species stature on the scaling relationships. In growth vs mass scaling, we found that larger species had greater growth for a given mass and growth tended to increase at a faster rate (i.e. steeper scaling exponents). Hydraulic conductance vs mass was expected to show the same trends due to observed G vs K isometry, but this was not supported. This discrepancy likely occurred because although G vs K exponents agreed with isometry, most were somewhat shallower which indicates ontogenetic variation in water use efficiency and/or carbon allocation.
From these data, we conclude that growth and hydraulic conductance are indeed closely coordinated within and across co-occurring species but there is a certain disconnect between the two which leads to different relationships between species. More detailed measurements of carbon dynamics could help explain this apparent disconnect.