[R] Partial Plot

[Morio]

Hi everybody,
I'm using RF for ecological modelling. In particular I'm interested in
defining crop suitable area.
At the moment I have calibrated RF using a number of pedological and
climatological data as predictor of presence/absence of crops.
When plotting partial dependence of presence/absence versus a
predictor, in some cases I obtained the graph as you see in file
section (T_AUG.JPG) that represents the partial dependance on august
mean temperature. It is not clear to me why the left side of the plot
is flat and relatively high. Consider that  in the dataset I used the
presence of the crop has a minimum at 21°C. In the partial plot I
would expect minimum values for temperature below 21°C.
Could you help me?
Thanking in advance
Marco & Bob