We study the complexity of finding communication trees with the lowest possible completion time for rooted, irregular gather and scatter collective communication operations in fully connected, $k$-ported communication networks under a linear-time transmission cost model. Consecutively numbered processors specify data blocks of possibly different sizes to be collected at or distributed from some (given) root processor where they are stored in processor order. Data blocks can be combined into larger segments consisting of blocks from or to different processors, but individual blocks cannot be split. We distinguish between ordered and non-ordered communication trees depending on whether segments of blocks are maintained in processor order. We show that lowest completion time, ordered communication trees under one-ported communication can be found in polynomial time by giving simple, but costly dynamic programming algorithms. In contrast, we show that it is an NP-complete problem to construct cost-optimal, non-ordered communication trees. We have implemented the dynamic programming algorithms for homogeneous networks to evaluate the quality of different types of communication trees, in particular to analyze a recent, distributed, problem-adaptive tree construction algorithm. Model experiments show that this algorithm is close to the optimum for a selection of block size distributions. A concrete implementation for specially structured problems shows that optimal, non-binomial trees can possibly have even further practical advantage.