Vegetation Seasonality Simplified
Going north from the temperate latitudes (approximately 45N), climatic (sunlight and heat) and soil (water and nutrients) conditions become increasingly unfavorable for plant growth. Thus lush broadleaf forests transition to cold-tolerant but less productive needleleaf forests (Boreal forests) and ultimately to tree-less grass-like vegetation called Tundra.
Plant growth thus decreases with increase in latitude (left panel in the figure above). The opposite of plant growth may be taken as a measure of vegetation seasonality.