Timber Harvesting Model - Cubic Spline Approximation

Profit maximizing owner of a commercial tree stand must decide when to clearcut the stand.

In [13]:
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec
from compecon import DPmodel, BasisSpline, demo, NLP

Model Parameters

Assuming that the unit price of biomass is $p=1$, the cost to clearcut-replant is $\kappa=0.2$, the stand carrying capacity $s_{\max}=0.5$, biomass growth factor is $\gamma=10\%$ per period, and the annual discount factor $\delta=0.9$.

In [2]:
price = 1.0
kappa = 0.2
smax  = 0.5
gamma = 0.1
delta = 0.9

State Space

The state variable is the stand biomass, $s\in [0,s_{\max}]$.

Here, we represent it with a cubic spline basis, with $n=200$ nodes.

In [3]:
n = 200
basis = BasisSpline(n, 0, smax, labels=['biomass'])

Action Space

The action variable is $j\in\{0:\text{'keep'},\quad 1:\text{'clear cut'}\}$.

In [4]:
options = ['keep', 'clear-cut']

Reward function

If the farmer clears the stand, the profit is the value of selling the biomass $ps$ minus the cost of clearing and replanting $\kappa$, otherwise the profit is zero.

In [5]:
def reward(s, x, i , j):
    return (price * s - kappa) * j

State Transition Function

If the farmer clears the stand, it begins next period with $\gamma s_{\max}$ units. If he keeps the stand, then it grows to $s + \gamma (s_{\max} - s)$.

In [6]:
def transition(s, x, i, j, in_, e):
    if j:
        return np.full_like(s, gamma * smax)
        return s + gamma * (smax - s)

Model Structure

The value of the stand, given that it contains $s$ units of biomass at the beginning of the period, satisfies the Bellman equation

\begin{equation} V(s) = \max\left\{(ps-\kappa) + \delta V(\gamma s_{\max}),\quad \delta V[s+\gamma(s_{\max}-s)] \right\} \end{equation}

To solve and simulate this model, use the CompEcon class DPmodel.

In [26]:
model = DPmodel(basis, reward, transition,

S = model.solve()
Solving infinite-horizon model collocation equation by Newton's method
iter change       time    
   0       3.1e-01    0.0000
   1       1.1e-01    0.0186
   2       1.1e-02    0.0221
   3       4.6e-03    0.0221
   4       1.9e-16    0.0378
Elapsed Time =    0.04 Seconds

The solve method retuns a pandas DataFrame, which can easily be used to make plots. To see the first 10 entries of S, we use the head method

In [27]:
biomass value resid j* value[keep] value[clear-cut]
0.000000 0.000000 0.067287 1.387779e-17 keep 0.067287 -0.132713
0.000250 0.000250 0.067320 -7.491474e-09 keep 0.067320 -0.132462
0.000500 0.000500 0.067353 -5.953236e-09 keep 0.067353 -0.132212
0.000750 0.000750 0.067387 -1.332984e-09 keep 0.067387 -0.131962
0.001001 0.001001 0.067420 7.309628e-10 keep 0.067420 -0.131712


To find the biomass level at which it is indifferent to keep or to clear cut, we interpolate as follows:

In [23]:
scrit = NLP(lambda s: model.Value_j(s).dot([1,-1])).broyden(0.0)[0]
vcrit = model.Value(scrit)
print(f'When the biomass is {scrit:.5f} its value is {vcrit:.5f} regardless of the action taken by the manager')
When the biomass is 0.30669 its value is 0.17402 regardless of the action taken by the manager

Plot the Value Function and Optimal Action

In [37]:
fig1 = plt.figure(figsize=[8,5])
gs = gridspec.GridSpec(2, 1,height_ratios=[3.5, 1])

S[['value[keep]', 'value[clear-cut]']].plot(ax= plt.subplot(gs[0]))
plt.title('Action-Contingent Value Functions')
plt.ylabel('Value of Stand')

ymin,ymax = plt.ylim()
plt.vlines(scrit,ymin, vcrit,linestyle=':')
demo.annotate(scrit, ymin,'$s^*$',ms=0)

plt.title('Optimal Action')
<matplotlib.legend.Legend at 0x23f3f129588>

Plot Residuals

In [34]:
S['resid2'] = 100*S.resid / S.value

fig2 = demo.figure('Bellman Equation Residual','','Percent Residual')
<matplotlib.collections.LineCollection at 0x23f3eeb40f0>


The path followed by the biomass is computed by the simulate() method. Here we simulate 32 periods starting with a biomass level $s_0 = 0$.

In [33]:
H = model.simulate(32, 0.0)

fig3 = demo.figure('Timber harvesting simulation','Period','Biomass')
[<matplotlib.lines.Line2D at 0x23f3ee5e5c0>]
In [ ]:
#demo.savefig([fig1, fig2, fig3])