by Daniel Brouse
November 18, 2025
For decades, we have studied the trees of Pennsylvania, tracking shifts in canopy structure, species resilience, and environmental stressors. Our long-term field data reveal a profound and accelerating decline. Since 2003, old-growth trees have consistently lost about 40% of their foliage over multi-year intervals, leading to premature mortality. During the same period, canopy height has fallen by roughly one-third, a drastic structural collapse in forests that once represented stability and ecological longevity. These changes are not isolated -- they mirror global patterns of forest decline -- but Pennsylvania offers one of the clearest windows into the cascading pressures reshaping ecosystems worldwide.
The primary drivers of this decline are not singular. Chronic ozone exposure weakens leaf tissues, reduces photosynthetic capacity, and lowers defenses. Warming temperatures, heat stress, hydrological whiplash (extreme swings between drought and deluge), and the spread of pests and disease compound these vulnerabilities. Together, they form a multifactorial stress landscape in which even historically resilient species cannot recover. The number of tree species capable of surviving under modern conditions is shrinking rapidly.
As the canopy thins, an unexpected force rises: vines. In Pennsylvania's forests, species like wild grape, kudzu, and bittersweet take advantage of the increased sunlight penetrating the weakened upper layers. Vines, once confined to understory limits, now climb higher each year. When they reach the diminished canopy, they smother, shade, and eventually kill the already stressed trees. This vine-driven mortality accelerates the loss of old growth and destabilizes the entire forest system -- affecting biodiversity, soil retention, and wildlife habitat. What was once a finely balanced ecosystem now tilts toward collapse.
This local unraveling echoes a global trend. Across Canada, much of the northern United States, and parts of Europe, old-growth forests face similar failures. The wildfire-ravaged forests of Canada highlight the challenge: replanting the same species no longer guarantees success when temperature and moisture regimes have shifted beyond their tolerance. Trees that would have thrived for centuries can no longer survive long enough to mature. Reforestation guided by historic baselines is now a losing strategy; adaptive management that anticipates the climate of 2050-2100 is essential.
In Pennsylvania, our work increasingly focuses on identifying species suited to emerging conditions. Trees with rapid regrowth, high heat tolerance, and broader ecological plasticity show the strongest potential. Among them:
Red maple (swamp/soft maple): Highly adaptable, tolerant of wetter soils and shifting precipitation patterns.
Kousa dogwood: Resistant to many regional diseases affecting native dogwoods.
Cherry (black cherry): Regenerates vigorously and may outcompete slower species under stress.
By contrast, species such as many pines, which cannot regenerate from damaged tissues, fare poorly in the new climate regime.
Honey locust trees (Gleditsia triacanthos) are dioecious, meaning male and female flowers occur on separate trees. This distinction is essential for natural reproduction: male trees produce pollen, while female trees produce the flowers and, when pollinated, the seed pods. For successful fertilization, male and female trees must be within several hundred feet of each other so wind and insects can transfer pollen. If only one sex is present, the trees cannot reproduce sexually, and any new trees must be produced vegetatively, typically through clonal propagation such as cuttings or grafting.
Pennsylvania's forests are not simply changing -- they are undergoing a structural transformation driven by the combined forces of chemistry, physics, biology, and climate. The decline of old-growth trees is not just an ecological signal; it is a warning. Without careful, science-guided species selection and adaptive reforestation strategies, vast stretches of eastern forests may transition into degraded ecosystems dominated by invasive vines, weakened understory growth, and fragmented canopy cover.
The future of the region's forests hinges on acknowledging these realities and acting accordingly. The old equilibrium is gone. The question now is whether we can build a new one robust enough to survive what comes next.
ALSO SEE: The Decline of Penn’s Sylvania Brouse (2024)