The growing demand for high-bandwidth and largecapacity memory access in data-intensive workloads has driven the development and deployment of Processing-in-Memory (PIM) architectures. However, existing DIMM-based PIM systems suffer from the severe communication bottleneck between the processing elements (PEs) near the PIM banks due to their requirement on host CPU forwarding. This bottleneck limits the efficiency of collective operations and degrades scalability and performance for workloads that require inter-PE communication. To address the communication limitation, we propose CoCoTree, a computation-capable architecture for collective communication in scalable DIMM-based PIM. CoCoTree supports direct and high-throughput inter-PE communication without host intervention. CoCoTree accelerates key collective communication using novel hierarchical binary tree topology and lightweight in-network computation support. We design and implement microarchitectures for the main building blocks: Co-Leaf and Co-Node, to efficiently handle the data packing, routing, and processing in CoCoTree. Furthermore, we also introduce a packet-based communication protocol tailored to the CoCoTree architecture, which decouples control and data through a twophase configuration-computation communication mechanism to efficiently support a wide range of collective communication operations. CoCoTree effectively mitigates inter-PE communication bottlenecks, enabling scalable PIM systems capable of meeting the demands of growing data size. Experimental results show that CoCoTree achieves up to$95.6 \times$improvement for collective operations and improves end-to-end application performance by up to$10.5 \times$across various workloads over the baseline PIM, while outperforming state-of-the-art PIM communication architectures in both performance and scalability.