What is the main point of the lecture? Why does John talk about the length of time it takes for a tree to grow?

Listen to part of a lecture in an urban planning class. (P = Professor, J = John, K = Kay)
P: So far in this semester we focus on major issues that city planners have to deal with, like transportation and providing affordable housing. But there are other less obvious factors that need to be considered. And these can have a significant economic impact on the city and affect the quality of life. As you remember, we began discussing one of these factors last class, the urban forest. Remember, an urban forest refers to all the trees within the city, trees in residence yards, trees lining city streets, or trees in parks. We started talking about the idea of assigning a monetary value to trees, and this can influence how many and what kinds of trees are planted. As I mentioned last class, it may seem odd to put a price tag on a tree, but many stake-hoiders like city officials, business owners or private citizens, they want to see a dollar figure before decisions are made about planting new trees or maintaining trees. OK. So continuing with approaches used to determine a tree’s worth, can someone describe the one we discussed last class? John?
J: We’ve talked about the sales comparison approach which involves looking at what houses sell for and trying to determine what different variables they are worth. So with trees, if two houses are exactly alike except one house has a few trees at the back yard and the other doesn’t, we can determine the trees’ value based on any difference in the sale price of the houses. So if the house without trees sells for $100,000, and the one with trees sells for $105,000, then the trees are worth $5,000. If everything else is the same.
P: Exactly. That last part is very important. If everything else is the same. The chances of finding two homes’ status exactly the same, same location, same size, same amenities, ah, things like a big swimming pool, are pretty slim. So to figure out what the trees, for instance, are worth, appraisers, people who determine the value of a house, estimate the values after adjusting for the absence or the presence of different amenities. So the houses and the trees can be compared. This acyustment depends on the skill of the appraiser and it may vary depending on who’s doing the appraising. OK, so that’s the sales comparison approach. Today, I wanna look at two other approaches, the benefit-based approach and the cost-based approach. So the benefit-based approach. Kay, what do you think this involves?
K: Well guessing from the meaning, I guess determining how trees increase things, like providing shade which saves energy because people can use less air-conditioning to cool their home.
P: Good. Kay just gave us an example of one benefit trees can provide, and one we can convert into a dollar figure. Now there are other several benefits that this approach can take into account. For example, trees can also protect the pavement on streets from the sun, protected from deteriorating. So streets with trees don’t need to be repaved this often as streets without trees. And these save city’s money.
J: Can this approach really be acted? I mean, trees take a long time to grow. So if we try putting a value on a tree that’s not fully grown, like if we determine how much shade will it provide to save energy, that figure will be different if we check again a few years later.
P: You’re right. If we plant a tree we may not see all the benefits they can provide for several years. So calculations for this are made based on the expected tree growth, so that’s another element planners need to consider. OK, what else goes into the benefit approach? Well, we also consider the amount of storm water run-off that’s reduced by trees. Trees absorb rain water and this is especially useful during heavy rain, because it can reduce flooding. Cities can spend millions of dollars a year managing storm water. So trees help reduce this cost. Yes, Kay?
K: Is it really possible to account for all the benefits of trees? I mean, with a monetary value? What about, like social benefits? I read somewhere that trees can actually make people happier, which can increase workers’ productivity, and they can even make community relationship stronger, I mean between neighbors.
P: Good point. Societal benefits like the one you just mentioned, well, are harder to calculate. You can’t just put a monetary value on them, and we’ll talk about it later. But first, let’s move on to the cost-based approach.