Ecological Archives M071-008-A2

P. R. Moorcroft, G. C. Hurtt, and S. W. Pacala. 2001. A method for scaling vegetation dynamics: the ecosystem demography model (ED). Ecological Monographs 71:557-585.

Appendix B. Below-ground Limitation of Leaf Physiology.

In our model, low soil moisture and low available soil nitrogen limit leaf physiological performance of individuals causing their leaf-level carbon gain and water loss ($A_n$ and $\Psi_{}$) to move along a path between ($A_o,\Psi_o$) and ($A_c,\Psi_c$). The precise shape of this path will depend on the mechanisms of soil water and nutrient limitation within a plant. We thus follow Foley et al. (1996) and adopt a simple phenomenological interpolation scheme:

$\displaystyle A_n(\bar{{\bf r}},t,c^*) = c^*A_o(\bar{{\bf r}},t) + (1-c^*)A_c(\bar{{\bf r}},t)$     (B.1)
$\displaystyle \Psi(\bar{{\bf r}},t,c^*) = c^*\Psi_o(\bar{{\bf r}},t) + (1-c^*)\Psi_c(\bar{{\bf r}},t)$      

where the degree of limitation, $c^*_{}$, ranges from zero to one and depending on soil-water and nitrogen availability.

Each plant tissue in our model has a fixed $\mbox{C:N}_{}$ ratio. All the active pools ( $B_a = B_r + B_{sw} + B_l$) have a common ratio $\mbox{(C:N)}_a$ that differs among functional types (see Plant Functional Diversity section below), and structural stem ($B_s$) has $\mbox{(C:N)}_s = 150$. Plants take up $N_{}$ from the available pool in the soil so as to maintain these ratios. If there is no available $N_{}$, then $c_{}^*$ switches abruptly to zero, stopping carbon production. To avoid numerical problems, this switch is smoothed slightly as available $N_{}$ becomes very small.

Given available soil nitrogen, the value of $c^*_{}$ is : $c_{}^* = 1/(1+(D:S)_\Psi)$, where $(D:S)_\Psi$ is the ratio of water demand to water supply. Thus $c_{}^* \approx 1$ if water supply greatly exceeds demand and $c_{}^* \approx 0$ if the reverse is true.

The potential demand by a plant with leaf biomass $B_l$ is $\Psi_oB_ll({\bf x})$, where $l({\bf x})$ is specific leaf area (m$^2$ kg$^{-1}$). We assume that supply is $K_W W B_r$, where $K_W$ is a constant, $W$ is available soil water, and $B_r$ is root biomass. Thus; $(D:S)_\Psi= \Psi_oB_ll({\bf x})/(K_WWB_r)$. The constant $K_W$ is set to $80_{}$, chosen to cause stomatal closure near observed wilting points.


[Back to M071-008]