In a large pot of boiling, salted water, cook the pasta until al dente. Drain, reserving 1 cup pasta cooking water.
In a large skillet, melt the butter over medium-high heat. Add the carrots and onion, season with salt and pepper and cook, stirring, until crisp-tender, about 7 minutes. Add the zucchini and cream and bring to a boil. Lower the heat and simmer, stirring, until the sauce is thickened, about 5 minutes; stir in the tarragon. Add the pasta, reserved cooking water and pecans; toss.