Typically, watershed Si export studies have focused on geochemical factors (e.g. geology, runoff) – ignoring the role of terrestrial biology. Terrestrial vegetation however, is a critical, yet poorly, understood regulator of Si export from land. In fact, land plants take up between 55 - 200 Tmol Si yr-1, an amount up to 40 times greater than the annual continental export of Si to the coastal ocean. Understanding the mechanisms driving the uptake and release of Si from watersheds is critical to determining Si availability in downstream receiving waters and the ecological processes that depend upon this Si (e.g., phytoplankton composition, food web dynamics, carbon cycling). Here I will discuss ongoing and future Si related research at Harvard Forest. In particular, I will focus on the role trees play in altering Si export. Sap Si concentrations from dominant tree species in Harvard Forest will be discussed in terms of seasonal changes and potential downstream impacts.