I noticed a couple problems with your usage of d's. Namely the ((dC+L+S)-J)/dt bit.
Let's see if I can fix the formula...
let D = density
let M = max trees
let A = area
let T = trees cut
let G = trees grown
let G0 = initial trees on map
let C = base cutting rate
let L = skill level modifier
let S = stat modifier (Agility)
let Rl = base rate of skill gain per trees cut = dL/dT
let Rs = base rate of stat gain per trees cut = dS/dT
let Rsj = base rate of stat gain when doing things other than cutting trees
let Rt = base rate of tree growth
let J = percentage of time actually spent with tree cutting job (including traveling, so if there was no other job than cutting, this would be 1.00 or 100% )
then,
dT/dt = J*(S*D+C*L)
D = A/(G0+G-T)
dG/dt = Rt*(M-(G+G0-T))
dL/dt = J*Rl*dT/dt
dS/dt = J*Rs*dT/dt + (1-J)*Rsj
We can at least do a bit of work on this.
let L0 = initial level
let S0 = initial skill
L = J*Rl*T+L0
S = J*Rs*T + t*Rsj*(1-J)+S0
So can reduce it to two simultainuous differential equations.
dT/dt = J*(A*(J*Rs*T + t*Rsj*(1-J)+S0)/(G0+G-T)+C*(J*Rl*T+L0))
dG/dt = Rt*(M-(G+G0-T))
let Q = current trees standing = G+G0-T
then we can write the equations simply as:
dT/dt = J*(A*(J*Rs*T + t*Rsj*(1-J) + S0)/Q + C*(J*Rl*T+L0))
dG/dt = Rt*(M-Q)
next, differentiate Q
dQ/dt = dG/dt-dT/dt
factor, as much as possible, each equation into terms of T, Q, and G
dT/dt = (J*J*A*Rs*T+J*A*Rsj*t-J*J*Rsj*t+J*S0)/Q + (C*J*Rl)*T +C*L0
Q*dT/dt = T*(J*J*A*Rs+(C*J*Rl)*Q)+t*(J*A*Rsj-J*J*Rsj+(C*L0))+Q*(C*L0)+J*J*A*S0
we end up with something of the form:
Q*dT/dt = (k1+k2*Q)*T+k3*t+k4*Q+k5
differentiate both sides:
QT''+Q'T' = k1T'+k2QT'+k2Q'T+k4*Q'+k3
QT''+Q'T' = (k2T')Q+(k2T+k4)Q'+k1T'+k3
further solving is left as an exercise for the student.