Physiological ecology of forest invaders along experimental light and nutrient gradients: Does invader advantage depend on disturbance?
Non-native invasive species in disturbed environments often exhibit ‘early successional’ traits associated with high resource use, suggesting invasion success depends on resource availability. However, shade-tolerant woody species invade mid- to late-successional forests in the Eastern US, despite presumably strong light and nutrient-related growth constraints. Common garden studies have found these invasive species to exhibit strategies that are both more productive (C gain) and efficient (C gain per resource cost) compared to natives, but it is unclear whether such differences are common in conditions typical to secondary forests.
In a two-year factorial manipulation of light and soil nitrogen in a deciduous forest in central NY, we tested 1) whether invasive species adjust their traits along resource gradients more than natives; 2) how potential trait shifts relate to carbon gain and productivity, and 3) whether invasive species maintain physiological advantages over natives under low light and nutrient availabilities. We implemented a hierarchical Bayesian model that incorporated leaf trait data into a classic photosynthesis model to test the influence of trait shifts on C gain and productivity in three invasive and six native woody species. We predicted greater functional trait-environment responses by invaders, with largest growth advantages under high resource availability.
Invasive species were more responsive to light additions, with photosynthetic traits (maximum carboxylation rates, electron transport rates) increasing linearly with light availability. Native species were much less responsive to resource additions, with lower photosynthetic rates overall. Invasive species’ traits were also more coordinated compared to natives, including a stronger influence of leaf N on carboxylation rates. All species displayed increased leaf mass per area in response to light, indicating the importance of leaf structural adjustments. Surprisingly, we found no trait responses to N fertilization in any species. These results match previous studies that emphasize light over nitrogen limitations in many Eastern US forests. However, there was a positive light-by-nitrogen interaction on carbon gain for native species, and a negative interaction in invasive species. Mean net primary productivity was generally higher in invasive species under high light and predicted by differences in carbon assimilation. Our results suggest shade-tolerant understory invaders are not less productive than natives under low resources but are more responsive to limiting resources.