Forests are an essential part of our biosphere, regulating climate, acting as a sink for greenhouse gases, and providing numerous other ecosystem services. However, they are negatively impacted by climatic stressors such as drought or heat waves. In this paper, we introduce FORTE, an open-source system for environmental monitoring with the aim of understanding how forests react to such stressors. It consists of two key components: (1) a wireless sensor network (WSN) deployed in the forest for data collection, and (2) a Data Infrastructure for data processing, storage, and visualization. The WSN contains a Central Unit capable of transmitting data to the Data Infrastructure via LTE-M and several spatially independent Satellites that collect data over large areas and transmit them wirelessly to the Central Unit. Our prototype deployments show that our solution is cost-effective compared to commercial solutions, energy-efficient with sensor nodes lasting for several months on a single charge, and reliable in terms of data quality. FORTE's flexible architecture makes it suitable for a wide range of environmental monitoring applications beyond forest monitoring. The contributions of this paper are three-fold. First, we describe the high-level requirements necessary for developing an environmental monitoring system. Second, we present an architecture and prototype implementation of the requirements by introducing our FORTE platform and demonstrating its effectiveness through multiple field tests. Lastly, we provide source code, documentation, and hardware design artifacts as part of our open-source repository.