expected to be a better metric of vine
capacity than pruning weight. It is
assumed that exposed leaf area
reflects light interception, but in the
vineyard it is difficult to estimate the
exposed leaf area (how much exposure
is “exposed”?) and average leaf
function due to the complexity of the
grape canopy.
Alain Carbonneau developed a
practical method in 1995 to estimate
vine capacity based on the surface area
of varying trellis systems.4 However,
this approach is limited by its assumptions
of uniform photosynthetic functionality
across the exposed canopy.
In practice, most grapegrowers and
viticulturalists discuss “crop load,” i.e.
the ratio of vine capacity to the
demand of the crop, rather than “vine
balance.”

Measuring crop load requires estimations
of both vine capacity and crop
demand. “Yield/pruning weight” or
“leaf area/g of fruit” address this balance
and are useful expressions for
estimating crop load. However, they
are limited for the aforementioned reasons
and do not address seasonal
dynamics.
Integrating the many complex
processes in a dynamic environment
requires dynamic models. Sunlight
energy absorption, carbon production
and utilization are the bestunderstood
processes, and they have been
successfully modeled in many plants.
The lead author has developed a simplified
carbon balance model to evaluate
the seasonal dynamics of the carbon
component of vine capacity and
demand for carbon.8,9,10

Seasonal dynamics of vine
balance and crop load
A simplified dynamic carbon balance
model, developed initially for
apple trees, was adapted for grapevines
using STELLA®, an easytouse
autoprogramming software. The
model (VitiSim) uses a daily time step
to simulate seasonal leaf area development,
vine light interception, canopy
photosynthesis, canopy respiration,
total dry matter accumulation, and
dry matter distribution to competing
vine organs (leaves, roots, fruit, etc.).
From this data we can: a) develop
seasonal curves of the growth, vine
capacity, and demands of various
organs and estimate balances between
their supplies and demands;
