their leaves all year round

Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest. This pattern of growth is related to the climatic conditions, growth normally ceasing when conditions are either too cold or too dry. In readiness for the inactive period, trees form buds to protect the meristem, the zone of active growth. Before the period of dormancy, the last few leaves produced at the tip of a twig form scales. These are thick, small and closely wrapped and enclose the growing point in a waterproof sheath. Inside this bud there is a rudimentary stalk and neatly folded miniature leaves, ready to expand when the next growing season arrives. Buds also form in the axils of the leaves ready to produce new side shoots. A few trees, such as the eucalyptus, have "naked buds" with no protective scales and some conifers, such as the Lawson's cypress, have no buds but instead have little pockets of meristem concealed among the scale-like leaves.[57]
When growing conditions improve, such as the arrival of warmer weather and the longer days associated with spring in temperate regions, growth starts again. The expanding shoot pushes its way out, shedding the scales in the process. These leave behind scars on the surface of the twig. The whole year's growth may take place in just a few weeks. The new stem is unlignified at first and may be green and downy. Palm trees[clarification needed] have their leaves spirally arranged on an unbranched trunk.[57] In some tree species in temperate climates, a second spurt of growth, a Lammas growth may occur which is believed to be a strategy to compensate for loss of early foliage to insect predators.[58]
Primary growth is the elongation of the stems and roots. Secondary growth consists of a progressive thickening and strengthening of the tissues as the outer layer of the epidermis is converted into bark and the cambium layer creates new phloem and xylem cells. The bark is inelastic.[59] Sooner or later[which?] the growth of a tree slows down and stops and it gets no taller. As long as the crown remains in balance with the roots[clarification needed] the tree should remain healthy but its ability to defend itself against fungal attack is diminished. If damage occurs the tree may in time become hollow

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Trees are much taller than herbaceous or shrubby plants[citation needed]and ensuring the upper-most leaves are supplied with water originating in the root system requires that water is drawn up through the xylem from the roots by the suction produced as it evaporates from the leaves.[clarification needed] If insufficient water is available the leaves will die.[61]
The leaves of trees come in a wide range of shapes and sizes which have evolved in response to environmental pressures including climate and predation. They can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen. The needles of coniferous trees are compact but are structurally similar to those of broad-leaved trees. They are adapted for life in environments where resources are low or water is scarce. Frozen ground may limit water availability and conifers are often found in colder places at higher altitudes and higher latitudes than broad leaved trees. In many cases, their branches hang down at an angle to the trunk which decreases the likelihood of them breaking when weighed down by snow.[clarification needed] Broad leaved trees in temperate regions have a different[clarification needed] strategy for dealing with winter weather. When the days get shorter and the temperature begins to decrease, the leaves no longer makes new chlorophyll and the red and yellow pigments already present in the blades become apparent.[62] Synthesis in the leaf of a plant hormone called auxin also ceases. This causes the cells at the junction of the petiole and the twig to weaken and sooner or later the joint breaks and the leaf floats to the ground. In tropical and subtropical regions, many trees keep their leaves all year round. Individual leaves may fall intermittently and be replaced by new growth but most leaves remain intact for some time. Other tropical species and those in arid regions may shed all their leaves annually at a particular time of year.[citation needed] Often this will coincide with the onset of the dry season or some other climatic event.[clarification needed] Many deciduous trees flower before the new leaves emerge.